Supply Chain - Stormwater Storage

To meet AMP7 Water Industry National Environment Programme (WINEP) requirements, Northumbrian Water appointed Esh Stantec to deliver a programme to increase storm tank capacity at several sewage treatment works. The scope and delivery of the projects were strongly influenced by Environment Agency regulatory deadlines. The works comprised a range of solutions, including buried in situ and precast reinforced concrete tanks, caisson shafts, and reuse of existing assets. Across a four-year period, Esh Stantec progressed six projects from concept design through to detailed design and construction, delivering a combined additional storm storage volume of 590m3 with an estimated programme value of £10m.

This article focuses on two contrasting schemes at Greatham STW in Hartlepool and Great Ayton STW in North Yorkshire, with particular emphasis on Great Ayton, where a constrained site and unlikely ground conditions required a collaborative and adaptive construction response.

Greatham STW

At Greatham STW, the scope was to provide an additional 55m3 of storm capacity. In parallel, the Greatham Wetlands Project was also progressing, driven by requirements for nitrogen removal and habitat creation. This created an opportunity to coordinate more closely across project teams and identify efficiencies for the client at an early stage.

[caption id="attachment_31021" align="alignnone" width="1200"] Rebar for the base of the Greatham STW above-ground tank - Courtesy of Esh Stantec[/caption]

With the future wetlands project in mind, optioneering for the storm tank project initially focused on several forms of short asset life storage solutions, to reduce CAPEX and carbon. For example, a pillow/bladder tank was proposed, as commonly used in the agricultural industry. However, this was ultimately discounted due to its unsuitability for providing the settled storm storage required by the Environment Agency.

As such, the selected solution was an above-ground glass-coated steel tank from Balmoral Tanks Ltd, which provided the required volume within a compact footprint, could be installed relatively quickly, and could be re-purposed as a detention tank for the future wetlands project. A high-level radial weir and central combined inlet/outlet arrangement were used on the storm tank to promote even flow distribution and settlement, with the system operating completely under gravity to minimise operational complexity, power demand, and whole-life cost and carbon.

In collaboration with the Greatham Wetlands team, data collection requests were aligned to avoid duplication and improve the value of early survey and site information for both projects. The layout was also developed to protect future pipe corridors, accommodate changes in forecast flows, and allow simpler future pipework connections, reducing the likelihood of abortive works for Northumbrian Water as the wider site strategy progressed.

[caption id="attachment_31016" align="alignnone" width="1200"] Completed Greatham STW storm tank - Courtesy of Esh Stantec[/caption] Great Ayton STW

At Great Ayton STW, the original requirement was to provide 317m3 of additional settled storm storage, later increased to 321m3 during delivery. The site was heavily constrained by existing treatment assets, buried services, and geotechnical risk, all of which influenced the chosen solution.

Optioneering considered an above-ground tank with pumped feed, a buried reinforced concrete tank, and a deep shaft solution. The pumped option was discounted because it introduced reliance on mechanical plant at the inlet works and increased the risk of screen blinding, blockage, and power resilience issues during storm conditions. A shallow buried tank required a larger footprint, greater excavation, and more disruption. The preferred solution was therefore a precast concrete caisson shaft, where the required volume could be provided vertically within a constrained area.

A key element of the civil design was the new inlet and overflow pipework to the shaft. Early definition design identified a pinch point between existing DN500 surface water drains, other buried services, and the trickling filters, which were considered sensitive to settlement. This area was treated as a significant risk, and therefore data collection focused here to define viable pipeline corridors. The original arrangement of separate gravity inlet and overflow pipes was ultimately rationalised into a combined inlet/overflow pipe, reducing excavation, simplifying the shaft tie-in, and lowering service conflict risk while still satisfying the settled storm performance requirement.

[caption id="attachment_31083" align="alignnone" width="1200"] Great Ayton STW: (top left) formwork and rebar for inlet interception chamber, (top right) group of boulders removed from shaft, (bottom left) precast concrete segmental ring building, and (bottom right) shaft construction with jacking system engaged - Courtesy of Esh Stantec[/caption] Northumbrian Water WINEP Storm Tank Programme: Supply chain - key participants Principal Designer & Principal Contractor: Esh Stantec Specialist shaft designer: COWI Ground investigation: Ian Farmer Associates Temporary works: JC Consulting Temporary works: MGF Ltd MEICA: Retroflo Ltd Shaft & tunnelling contractor: Terra Solutions Ltd Shaft formwork: Doka Core drilling: A19 Drilling Limited PCC shaft segments: FP McCann Ltd Systems Integration: IDEC Group Ltd - Aureos Temporary dewatering: Project Dewatering Temporary overpumping: United Rentals Water treatment plant: Siltbuster Group OSMA UltraRib structured wall pipework: Wavin Glass-coated steel tank: Balmoral Tanks Ltd Ductile iron pipework: Saint Gobain PAM UK Modular pipe seal: Hauff-Technik Return pump: Xylem Water Solutions Metalwork fabrications: ADL Fabrications Asbestos removal: Lucion Environmental Ltd Rebar/reinforcements: Midland Steel Shaft construction

Following main contract award, Esh Stantec appointed Terra Solutions Ltd as the specialist shaft contractor, with COWI UK Limited acting as specialist shaft designer. Construction of the shaft began in January 2025.

Ground investigation identified granular strata with groundwater encountered in upper and lower horizons, with monitored levels typically between about 1.7m and 2.5m below ground level. Given the groundwater table and sensitivity of nearby structures to drawdown, the shaft was designed to be sunk using a wet caisson methodology rather than a dry excavation with extensive dewatering.

The caisson method relied on maintaining water balance within the shaft as excavation progressed, allowing internal water pressure to counter surrounding groundwater and reduce the risk of basal instability or piping. Precast concrete rings were built progressively at surface level and sunk using a reinforced concrete collar and jacking system.

The intended sequence was to reach formation, place an underwater concrete plug to resist flotation, sump pump internally, and then construct the reinforced concrete base slab and benching. This approach avoided dependence on an abstraction licence and reduced the risk of settlement to adjacent assets.

Shaft ovality issue & resolution

During shaft sinking, much larger cobbles and boulders were encountered than had been indicated by the pre-construction ground investigation. At around 5m below ground level, these obstructions prevented further shaft sinking. Material was found underneath the cutting edge and larger particles were also believed to be bearing against the external face of the shaft lining.

Once the main obstruction had been removed and sinking resumed, the uneven resistance, caused by remaining material on the outside of the shaft, was considered to have initiated ovalisation in the lower shaft rings, which became visually apparent following further sinking and dewatering.

[caption id="attachment_31014" align="alignnone" width="1200"] Great Ayton STW: Ovality first observed - Courtesy of Esh Stantec[/caption]

The situation was managed through a highly collaborative response, with close coordination between the principal contractor, specialist shaft contractor, designer and temporary dewatering contractors, with safety and asset protection treated with critical importance. External dewatering, which had previously been avoided because of settlement and permitting concerns, was introduced on a controlled temporary basis to allow inspection and obstruction removal, supported by settlement monitoring of nearby structures.

Laser scanning was then used to quantify the shaft geometry and confirm the degree of ovality. Rather than abandon the shaft, the team worked through a series of design and construction reviews to develop a recoverable solution that maintained the required storage volume while reducing residual risk.

This included:

Revising the shaft from the original 13-ring arrangement to an 11-ring shaft with part of the storage volume recovered below the cutting ring within the plug zone. The internal base and benching arrangement were modified to reduce concrete volume to free up more storage volume. Pouring the stabilising plug underwater to maintain positive pressure inside the shaft. Designing and installing a fixed diameter in situ reinforced concrete structural liner, acting as an independent structure to the shaft. The upper three rings were demonstrated to remain structurally stable without secondary lining, preserving useful storage volume. Pouring was complete in stages. Dewatering was then used in a tightly controlled manner to empty the shaft and install the liner before being removed as soon as practicable to limit further ground movement. [caption id="attachment_31012" align="alignnone" width="1200"] Great Ayton STW: RC liner works complete - Courtesy of Esh Stantec[/caption]

The Great Ayton Storm Tank Project showed both the value and the limitation of pre-construction information on a constrained operational site. The ovality issue became one of the defining challenges of the project, and its resolution depended on close collaboration between contractor, designer and client teams to understand the risk, adapt the design and complete the asset safely.

Pipe penetrations

At Great Ayton, Hauff-Technik GKD modular seals were adopted for selected pipe penetrations through the shaft wall to provide a more controlled watertight detail than traditional grout arrangements. This also reduced installation risk around circular penetrations and time spent working from height. This was a particularly suitable application for the combined inlet/overflow pipe.

This product was trialled here as an innovative approach and has since been proposed as a more standard solution across the Esh Stantec joint venture for similar applications. Note, the detail was not considered acceptable for the rising main penetration due to the potential thrust forces generated during pressure testing.

[caption id="attachment_31023" align="alignnone" width="1200"] Great Ayton STW: (left) 600mm core through shaft wall and (right) DN500 inlet/overflow pipe, with modular seal installed - Courtesy of Esh Stantec[/caption] Conclusion

The WINEP Storm Tank Programme showed that storm storage delivery on operational wastewater sites requires solutions tailored to site-specific constraints rather than a single standard approach. Greatham provided a compact and efficient above-ground solution, while Great Ayton required a deep shaft and a carefully managed redesign during construction in response to unlikely ground conditions. In particular, the resolution of the shaft ovality issue demonstrated the importance of collaborative working and adaptive engineering in reducing risk and achieving a compliant final asset for Northumbrian Water.

Crucially, all works were completed in time to meet the regulatory deadlines and secure the AMP7 WINEP output, with additional storm storage delivered through solutions matched to operational need, buildability, and long-term performance.

The requirement of the Burnley Integrated Catchment Scheme is the River Water Quality Improvement for the Water Framework Directive (WFD) driver identified in the Water Industry National Environmental Programme (WINEP) to achieve a ‘Good’ status in the defined reaches of Pendle Water and the River Calder. The project, which commenced in 2020, is one of United Utilities’ largest AMP7 investments and is being delivered by Advance-plus, a joint venture between MWH Treatment, J Murphy & Sons (JMS) and Stantec UK. In previous articles published by Water Projects, United Utilities has shared the project journey from concept, solution design development, and construction. This case study focuses on the project team’s experience in achieving effective project outcomes. Project requirements & summary

To meet ‘Good’ status in Pendle Water and the River Calder, the treatment capability at Burnley Wastewater Treatment Works (WwTW) had to be enhanced to achieve compliance with the new treated effluent permit standards, coupled with management of storm discharges in the defined reaches, to achieve compliance with the WFD 99%ile wet weather intermittent standards. There are eight regulatory drivers attached to this WFD element of the scheme.

The project also includes two additional regulatory drivers for U_MON3 spill monitoring and U_MON4 flow monitoring.

Through complex river water quality and network modelling, the lowest whole life cost integrated catchment solution was determined and developed.

The Integrated Catchment Solution comprised four sites, which are all inter-dependent in delivering the regulatory outputs.

[caption id="attachment_26729" align="alignnone" width="1200"] Modification of existing PSTs 6, 7, 8 and 9 - Courtesy of United Utilities[/caption] Burnley WwTW

Serving a domestic population equivalent of approximately 107,000 plus trade load, Burnley WwTW has undergone a significant upgrade during this AMP7 project with the scope comprising the design, construction, and commissioning of:

A new Evoqua BioMag® activated sludge plant (ASP) from Xylem Water Solutions. The BioMag® system enhances conventional biological wastewater treatment by using magnetite (an iron oxide powder) to ballast the microbiological floc, creating an increase in mixed liquor suspended solids (MLSS) in the ASP and enhanced performance in the final settlement tanks (FSTs). A new 64-panel, 12,000m3 detention tank constructed by A-Consult Ltd with associated feed pumping station. A new additional (4th) primary settlement tank. A new 2500 l/s interstage pumping station (pass forward of FtFT and RAS flows). Refurbishment of the four existing FSTs. A new surplus activated sludge (SAS) thickening process. A new ferric and caustic chemical dosing plants. New MCCs, SCADA upgrade and power upgrade. U_MON3 event duration monitoring on spills to storm tanks. U_MON4 flow monitoring.

The final effluent quality for the new works at Burnley WwTW is to comply with both WRA (spot samples) and UWWTD (composite samples):

Design Permit WRA Parameter Average 95% ile Upper Tier Limit BOD 9 mg/l 50 mg/l Solids 40 mg/l Ammonia 2 mg/l 12 mg/l Total P 0.25 mg/l Iron 4 mg/l 8 mg/l Design Permit UWWTD Composite Samples Parameter Compliance Requirement Upper Tier Limit BOD 25 mg/l (or 70% removal) 50 mg/l COD 125 mg/l (or 75% removal) 250 mg/l Total P 2 mg/l (or 80% removal) [caption id="attachment_26728" align="alignnone" width="1200"] Evoqua BioMag® activated sludge plant - Courtesy of United Utilities[/caption] Burnley WwTW Catchment Strategy: Supply chain - key participants Client: United Utilities Construction Delivery Partner: Advance-plus JV MWH Treatment (Advance-plus jv) J Murphy & Sons (Advance-plus jv) Stantec UK (Advance-plus jv) Civil design detention tank: COWI UK Ltd Physical modelling: Hydrotec Consultants Ltd Dewatering design: OGI Groundwater Specialists Ltd Dewatering: Alba Dewatering Services Ltd Concrete construction: STAM Construction Ltd Detention tank shaft: Joseph Gallagher Ltd Pump station shaft: EJ Kelly Sheet piling: Sheet Piling (UK) Ltd PCC driven piling: Van Elle Concrete cutting: Holemasters Temporary works/shoring: MGF Ltd Precast post tensioned tanks: A-Consult Ltd Precast roof sections: Macrete (Ireland) Ltd Precast shaft segments: FP McCann Blower building: Saredon Steel Buildings Ltd BioMag® building: Gallaway Construction Ltd Access metalwork: Tushingham Steel Fabricators Evoqua BioMag® process: Xylem Water Solutions BioMag® mechanical installation: Alpha Plus Ltd [caption id="attachment_26724" align="alignnone" width="1200"] Detention tank water test - Courtesy of United Utilities[/caption] Civils & pipework: SGC Civil engineering Ltd Pumping station mechanical installation: Fluid Sealing & Engineering (FSE) Transformers: Winder Power Electrical installation: PICOW Engineering Group MCCs: Technical Control Systems Ltd Systems integration: Tata Consultancy Services SAS glass coated steel tank: Stortec Engineering Ltd Dosing systems: NPS Engineering Group PST fixed scraper bridge: EPS Water Epoxy coated carbon steel pipework & wall starters: Freeflow Pipesystems Ltd SAS drum thickeners: Huber Technology Odour control system: Air-Water Treatments Ltd PST odour covers: Corporate Engineering Ductile iron pipework: Saint Gobain PAM UK Ductile iron pipework: Electrosteel Castings (UK) Ltd FST scraper bridge refurbishment: Jacopa Ltd Blowers: Sulzer Pumps Wastewater Ltd Aeration system: Suprafilt Ltd SAS tank air mixing system: Utile Engineering Submersible pumps: Xylem Water Solutions Valves: AVK UK Ltd Standby generators: DTGen Kiosks: Industrial GRP Ventilation: Air Technology Systems Ltd Lindred Road CSO (PEN0056)

Work at this existing combined sewer overflow (CSO) site comprised the construction of an online 1,950m3 detention tank to store storm waters during times of heavy rainfall and reducing discharges to the watercourse. New pumps within the tank return the effluent to the sewer for onward treatment at Burnley WwTW. A new MCC and power supply were installed for this facility.

[caption id="attachment_18286" align="alignnone" width="1200"] Lindred Road detention tank during construction - Courtesy of United Utilities & Advance-plus[/caption] Altham Outfall Fennyfold CSO (BUR0026)

This existing CSO site is located in the local town of Padiham and on the sewer network that transfers flows to Hyndburn WwTW for treatment. The solution at this site was to construct a new chamber over the downstream sewer, install a new ‘real time’ controlled actuated penstock valve that regulates and optimises flows passed forward to Hyndburn, whilst reducing spills from the existing CSO. A new control kiosk was also provided.

Hyndburn WwTW

Value engineering and engagement with the Environment Agency enabled a no-build solution to be accepted at Hyndburn WwTW site, with only the optimisation of existing storm tank emptying system necessary for compliance.

Process optimisation: update to previous report

New primary settlement tank No. 4

New primary settlement tank (PST) No. 4 desludge pump run/dwell timers have been optimised to provide primary sludge concentration to a target of ~3.5% ds w/w. Daily PST No. 4 sludge blanket dips are undertaken by ADP and UU Operations undertook routine measurement of primary sludge concentration to inform any occasional minor changes to desludge pump run/dwell timers. The set-points for the run/dwell timer of the PST No. 4 desludge pumps typically range from 9-12 minutes for the run phase and 45-60 minutes for the dwell phase. These set-points will be adjusted in the future to accommodate changes in operational conditions at the site, including those related to the current sludge treatment center. [caption id="attachment_26733" align="alignnone" width="1200"] PST4 with odour control cover - Courtesy of United Utilities[/caption] Interstage pumping station The new interstage pumping station (IPS) has been commissioned with close consideration of the operating level set-points as determined during detailed hydraulic design. Further optimisation of the recycled activated sludge (RAS) control flow rates was undertaken following an exceptionally dry and warm weather period during April-June 2025 which has then resulted in supplementary further optimisation of the IPS control set-points. To maintain a stable hydraulic gradient for control of RAS (via gravity) from the final settlement tanks, the IPS top water level has been further optimised at 2.5m set-point with an operating level set-point control band optimised across a 0.5m depth (as per design). The IPS pumps minimum/maximum operating speeds (32.5Hz and 50Hz respectively) have been further optimised to provide stable control to level set-point across the 0.5m operating level band and avoiding, as much as possible, pump start/stops during dry weather flow periods where pump minimum performance exceeds received flows into the pumping station. The IPS has operated generally in accordance with the initial duty/assist/assist cascading level control with PID control loop tuning set-points remaining generally unchanged since original turn of flows in early October 2024. Further optimisation of the IPS process control set-points is not envisaged in the future. [caption id="attachment_26725" align="alignnone" width="1200"] Interstage pumping station - Courtesy of United Utilities[/caption]

Process air blowers: The new process air blower package was initially commissioned with support from the supplier, Sulzer Water & Wastewater. The majority of operational set-points within the blower package system are standard defaults and did not require further optimisation. The assist blower start/stop changeover set-points were optimised during a subsequent Sulzer site visit to ensure stable operational transition, avoid unnecessary noise and prevent nuisance under/over pressure process trips.

The process air blower discharge header control set-point has been further optimised to 640 mbarg in attempt to minimise dissolved oxygen depletion during storm events. The occasional depletion of dissolved oxygen remains an issue during the extraordinary rate of change of incoming flows and loads often associated with these storm events.

The addition of alternative ‘Storm Mode’ process control may ultimately be required to fully remedy the situation. Further optimisation of the process air blower process control set-points may therefore be required in conjunction with implementation of future ‘Storm Mode’ process control philosophy.

New primary ferric sulphate dosing: Extensive and routine jar-testing to monitor the optimal primary ferric dose molar ratio dose to achieve target residual orthophosphate and Total Iron concentrations was undertaken during May and June 2025.

The primary ferric molar ratio dose providing optimal phosphate and iron control (~1mg/l orthophosphate and <20mg/l Total Iron concentrations in PST effluent), as determined during the Summer Completion Test, was 4.15 MR. For reference, the ‘as-commissioned’ diurnal orthophosphate base load profile is shown below.

[caption id="attachment_26740" align="alignnone" width="1200"] Burnley WwTW: Orthophosphate diurnal load profile - primary ferric LUT set-points (as commissioned November 2024) - Courtesy of United Utilities[/caption]

A summary of initial and subsequent primary ferric dosing MR dose ‘optimisation’ is provided below:

28 November 2024 Dosing commences at 5MR 11 December 2024 Dosing increased to 6MR to target 1mg/l orthophosphate in PST effluent 20 December 2024 Dosing reduced to 5MR in response to ASP ammonia breakthrough spikes 24 December 2024 Dosing reduced to 4MR in response to ASP ammonia breakthrough spikes 2 January 2025 Dosing paused in response to observed ASP ammonia breakthrough spikes 9 January 2025 Dosing recommenced at 2MR 11 February 2025 Dosing increased to 3MR to target 1mg/l orthophosphate in PST effluent 25 February 2025 Dosing increased to 3.5MR to target 1mg/l orthophosphate in PST effluent 6 March 2025 Dosing increased to 3.75MR to target 1mg/l orthophosphate in PST effluent 17 March 2025 Dosing reduced to 3.5MR in response to ASP ammonia breakthrough spikes 24 April 2025 Dosing increased to 3.75MR to target 1mg/l orthophosphate in PST effluent 30 April 2025 Dosing paused in response to observed FST iron breakthrough spikes 3 May 2025 Dosing recommenced at 4.5MR following jar testing 8 May 2025 Dosing reduced to 4.15MR in response to jar test results N/A No primary ferric MR change since 8/5/25

Routine, minor, operational changes to the primary ferric dosing set-points will remain a business-as-usual activity associated with normal wastewater treatment works operation in the future; and most notably, in response to envisaged seasonal variation and catchment characteristics.

Caustic dosing: An initial set-point of 7.1 pH units was adopted for interstage pH process control upon commencement of the Summer Completion Test. This control set-point has been further optimised on occasion and in agreement with/request by United Utilities Operations to 7.15 pH units to either reduce chemical consumption or in consideration of prevailing final effluent alkalinity measured value.

Routine, minor, operational changes to the interstage pH setpoint considering prevailing final effluent pH and alkalinity will remain a business-as-usual activity associated with normal wastewater treatment works operation in the future.

[caption id="attachment_26726" align="alignnone" width="1200"] Chemical dosing area - Courtesy of United Utilities[/caption]

Liquid polyelectrolyte dosing: The ‘sweetening’ dose set-point of 25 l/h per FST has been retained and a storm event %FtFT trigger setpoint of 75% (in line with Evoqua recommendation). During periods of low FtFT, the 25 l/h ‘sweetening’ dose set-point results in the liquid poly system batching around four times per day; ensuring that a fresh, but matured, make-up solution is available at all times in readiness for flash increase in FtFT or prolonged storm event.

The poly dose proportional set-point has been gradually reduced to the current value of 0.6 mg/l as active product. It is noted that the product selection and amount of liquid polyelectrolyte dosed is subject to EA permitting and, as such, strict limits on maximum poly dose rate applies (currently 3.2 mg/l maximum as active product).

Further optimisation of the liquid poly ‘proportional’ dose process control set-points is not envisaged as necessary in the future.

[caption id="attachment_26734" align="alignnone" width="1200"] New assets at Burnley WwTW - Courtesy of United Utilities[/caption]

Magnetite addition & recovery: Magnetite, an iron oxide powder, is added to the activated sludge process via the BioMag® system to provide mixed liquor enhanced settleability in the final settlement tanks by microbiological floc ‘ballasting’.

The new works was operated during Q1 of 2025 to maintain the magnetite : MLSS ratio at the target of approximately 2:1, which was necessary to achieve the necessary mixed liquor settleability performance as measured by Specific Stirred Volume Index (SSVI).

Control to the target ratio is effected by management of magnetite ‘top-up’ addition, and in consideration of daily surplus activated sludge wastage volume, which relies upon having a disposal route for SAS. Mixed liquor settleability performance matured naturally during Q2 of 2025 and the magnetite : MLSS ratio was coincidentally reduced to target of 1:1 by the reduction of magnetite ‘top-up’ addition to the design anticipated amount of 500 to 600 kg/d.

The historic magnetite : MLSS ratio data is shown in the graph below. It can be seen that magnetite : MLSS ratio has trended down since the Summer Completion Test and as such, the ‘top-up’ magnetite loading set-point has been increased slightly to compensate.

Routine operational changes to the daily magnetite loading rate will therefore remain a business-as-usual activity associated with normal wastewater treatment works operation in the future. [caption id="attachment_26722" align="alignnone" width="1200"] MLSS minus magnetite (true MLSS) and true MLSS : Magnetite ratio on-site tests and laboratory results - Courtesy of United Utilities[/caption]

Recycled activated sludge control: Further optimisation of the RAS control set-points was undertaken following the extended period of exceptionally dry and warm weather period during April-June 2025. During this dry weather period the RAS flow rate was maximised to design limitation of 1,040 l/s to best manage potential for denitrification within the FSTs and avoid rising sludge and scum formation where possible. RAS control to ‘FtFT Mode 1’ was adopted for control of RAS flows during the Summer Completion Test in accordance with a three-tiered banded philosophy as follows:

FtFT flow (l/s) RAS flow (l/s) <600 400 >600 <1000 600 >1000 750

The RAS control system has operated stably for many months, and no significant further optimisation is envisaged. However, routine operational changes to the RAS control rate or mode of operation will remain a business-as-usual activity associated with normal wastewater treatment works operation in the future.

Activated sludge plant: Routine control of MLSS concentration has remained by the conventional method of monitoring food to micro-organism ratio (F/M) to the design target ~0.08d-1, by assessment of sludge age against target of ~14 days and then by manual adjustment of the surplus activated sludge daily wastage volume.

Routine monitoring of the biological load on the ASP and as-required operational changes to the daily SAS wastage volume to maintain F/M ratio and sludge age within an acceptable range will remain a business-as-usual activity associated with normal wastewater treatment works operation in the future. The historic F/M ratio and sludge age data is shown below.

[caption id="attachment_26739" align="alignnone" width="1200"] F to M ratio and sludge age - Calculated using on-site tests and laboratory test results - Courtesy of United Utilities[/caption]

Similarly, routine microscopic analysis has been undertaken by UU labs to monitor and track filamentous (and other indicator bacteria) abundance.

Summary/conclusion

The Summer Completion Test was carried out by Advance-plus over a 28-day period between 21 July and 18 August 2025. From the test results, it is concluded that the new works has successfully demonstrated compliance with the requirements of:

CT1: Final effluent water quality compliance with site WRA Regulatory Permit limits for BOD, TSS, ammonia, Total Phosphorus, Total Iron and pH. The final effluent water quality adequate compliance with asset standard for alkalinity residual (noting routine pH set-point change made during completion test in response to slight reduction in background alkalinity). CT2: Final effluent water quality compliance with site UWWTD Regulatory Permit limits for BOD and COD. CT3: Final effluent water quality compliance with site UWWTD Regulatory Permit limits for Total Phosphorus. CT4: Magnetite recovery rate of the BioMag® process.

As water companies across the UK respond to increasing regulatory expectations, climate change and growing environmental pressures, investment in stormwater management infrastructure has become a critical component of long-term resilience planning. For Anglian Water’s @one Alliance, this challenge is being addressed through their extensive stormwater storage programme across AMP8. This follows on from the successful completion of an extensive AMP7 programme, with construction well underway on the £47m investment that will deliver 35 new storm tanks across the East of England. Background

The programme forms part of Anglian Water’s wider £11 billion AMP8 investment plan and builds directly on the achievements of AMP7, where more than £96 million was invested in the construction and upgrade of 112 storm tanks across the region.

Together, these programmes represent a significant step forward in improving stormwater resilience, reducing environmental risk and supporting the long-term performance of wastewater treatment assets across one of the UK’s most environmentally sensitive operating regions.

[caption id="attachment_31098" align="alignnone" width="1200"] Storm tank construction at Canwick Water Recycling Centre in Lincolnshire - Courtesy of @one Alliance[/caption] Regional challenges

The East of England presents a unique set of challenges for wastewater network operators. Despite being recognised as the driest region in the UK, the area remains highly vulnerable to flooding and extreme weather events. Low-lying landscapes, flat catchments and increasingly unpredictable rainfall patterns can create significant pressure on wastewater infrastructure during short-duration, high-intensity storms.

Many communities continue to be served by combined sewer networks, where surface water and wastewater are conveyed through the same system. During periods of exceptional rainfall, flows entering the network can increase dramatically, placing pressure on pumping stations, transfer systems and water recycling centres (WRCs).

Without adequate stormwater storage capacity, treatment processes can become overwhelmed, increasing the risk of storm overflows operating and potentially impacting receiving watercourses.

And for Anglian Water, protecting rivers, streams, coastal waters and ecologically sensitive environments remains a key operational priority. These water bodies support biodiversity, agriculture, recreation and local communities, making their protection fundamental to both environmental stewardship and regulatory compliance.

At the same time, water recycling centres must remain capable of safely treating flows during increasingly variable weather conditions. Maintaining resilience within these assets is therefore essential to ensuring reliable service for customers while protecting the environment.

[caption id="attachment_31097" align="alignnone" width="1200"] Inside the Canwick WRC storm tank - Courtesy of @one Alliance[/caption] The role of storm tanks

Storm tanks play a vital role in managing peak flows during severe weather events. They provide temporary storage during periods of heavy rainfall and help to balance hydraulic loading across treatment processes, reducing the risk of flows exceeding the available treatment capacity.

And when storm conditions subside, the stored water can be returned gradually to the treatment process, allowing it to receive full treatment before being discharged back into the environment. This approach reduces the operational risk, improves treatment performance during peak events and contributes to the reduction of pollution incidents and environmental impacts.

AMP8: Establishing a proven delivery model

The AMP8 programme has been heavily influenced by the success of the AMP7 Storm Tank Programme. Between 2020 and 2025, Anglian Water’s a @one Alliance delivered 112 storm tank projects across the region, representing a capital investment of more than £96 million.

The scale of delivery required a consistent and repeatable approach while maintaining flexibility to respond to the unique constraints presented by individual sites.

Although the overall objective remained the same across the programme, no two locations were identical. Each site brought its own set of engineering, operational and environmental challenges, including restricted footprints, complex hydraulics, existing asset interfaces, operational continuity requirements and varying ground conditions.

To address this, the alliance developed a standardised delivery model that could be adapted to suit local circumstances while maintaining consistency in design, construction and operational performance. The approach enabled efficiencies to be realised across multiple projects, reducing design time, improving constructability and accelerating delivery without compromising quality or safety.

[caption id="attachment_31099" align="alignnone" width="1200"] The new storm tank at Purleigh WRC in Essex - Courtesy of @one Alliance[/caption] Innovation & sustainable design

Innovation was a key feature throughout AMP7 and continues to shape delivery during AMP8. One of the most successful solutions was the widespread adoption of above-ground glass-coated steel (GCS) storm tanks. These structures offered several advantages over traditional construction methods, including reduced construction time, improved health and safety performance and lower embodied carbon.

Detailed hydraulic modelling was undertaken during design development to ensure each tank was appropriately sized and optimised for operational performance. The programme also demonstrated how existing assets could be re-purposed to maximise value and minimise environmental impact.

At Cotton Valley Water Recycling Centre in Milton Keynes, two redundant primary settlement tanks were successfully converted into permanent stormwater storage assets. By reusing existing infrastructure rather than constructing entirely new facilities, the project delivered significant savings in both capital expenditure and embodied carbon while achieving the required storage capacity.

Now building on these successes, the AMP8 Storm Tank Programme is now progressing across multiple sites throughout the Anglian Water region. The £47 million programme will deliver 35 new storm tanks designed to increase storage capacity, improve operational resilience and support compliance with increasingly stringent environmental regulations.

Once completed, the new assets will provide additional capacity to manage stormwater surges generated by extreme rainfall events, helping to reduce flood risk and minimise the likelihood of untreated wastewater entering rivers and coastal waters. Importantly, the lessons learned during AMP7 have been embedded within AMP8 delivery strategies, enabling the alliance to continue improving efficiency, safety and sustainability outcomes.

[caption id="attachment_31096" align="alignnone" width="1200"] New storm tank at Weston WRC - Courtesy of @one Alliance[/caption] Prioritising health, safety & environmental performance

Safety has remained a primary consideration throughout programme development and delivery. Above-ground tank solutions continue to be favoured where appropriate, largely due to the health and safety benefits they provide during construction. Tanks can be assembled progressively from ground level, significantly reducing the need for working at height and lowering overall construction risk.

Extensive planning and design-stage optioneering have also been undertaken to identify opportunities for material reuse, waste reduction and carbon savings. Where practical, existing assets are being retained, re-purposed or incorporated into new designs. Material selection is also being assessed against whole-life carbon and operational performance criteria, ensuring that decisions made during construction continue to deliver value throughout the asset lifecycle.

This focus on sustainability aligns with Anglian Water’s broader commitment to achieving net zero operational carbon while delivering infrastructure that remains resilient for future generations.

Collaborative delivery through the @one Alliance

A defining factor behind the success of both the AMP7 and AMP8 programmes has been the collaborative delivery model employed by the @one Alliance.

Unlike traditional client-contractor arrangements, the alliance model encourages early collaboration, shared objectives and collective decision-making. Designers, constructors, operational specialists and project teams work together from the earliest stages of project development, enabling challenges to be identified and resolved before they impact programme delivery.

This integrated approach has consistently driven improvements in efficiency, innovation and safety performance while delivering better outcomes for customers and the environment. And by creating a culture focused on shared success rather than individual organisational objectives, the Alliance has established a delivery model that continues to attract recognition across the infrastructure sector.

[caption id="attachment_31095" align="alignnone" width="1200"] Completed storm tank at Alford WRC in Lincolnshire - Courtesy of @one Alliance[/caption] Looking ahead

Stormwater management is expected to remain a major focus area for water companies throughout the coming decades. Climate change projections indicate that intense rainfall events will become increasingly common, placing greater pressure on existing wastewater systems. At the same time, environmental expectations from regulators, stakeholders and communities continue to rise.

Against this backdrop, proactive investment in resilient stormwater infrastructure is becoming essential. Through the successful delivery of AMP7 and the ongoing implementation of the AMP8 Storm Tank Programme, Anglian Water and the @one Alliance are demonstrating how targeted investment, collaborative delivery and innovative engineering can work together to address these challenges.

The programme is not simply increasing storage capacity. It is helping to strengthen operational resilience, protect sensitive environments, reduce pollution risk and improve the performance of critical wastewater infrastructure across the East of England.

As construction progresses over the coming years, the benefits of these investments will be felt across rivers, coastlines, communities and ecosystems throughout the region, providing a lasting legacy of environmental protection and resilience for generations to come.

Bishops Lydeard Water Recycling Centre (WRC) falls within the South Somerset Council area, serving the adjoining villages of Bishops Lydeard and Greenway via a gravity sewer. The site discharges to Back Stream and on to the River Tone, serving a total population equivalent of approximately 3,056 PE (2040 forecast).  The WRC was identified by the Environment Agency under WINEP as requiring process improvements, including reduction of phosphorus levels in treated wastewater prior to release. Project drivers

Regulatory obligations included:

New MCERTS flow measurement installed at inlet works. New phosphorus consent (WFD_IMPg): 1.0 mg/l with AMP8 stretch target of 0.8 mg/l Total P. Iron (Fe): 3 mg/l 95%ile | 8 mg/l absolute maximum (ABS). Aluminium (Al): 1.5 mg/l ABS. Storm storage (WINEP U_IMP6): Increase in volume from 147m3 to 228m3. EDM (U_Mon3): Monitoring to and from the storm tank. [caption id="attachment_31173" align="alignnone" width="1200"] Site under construction - Courtesy of Envolve Infrastructure[/caption] Proposed solution

To meet the new consent permits, the following elements were required.

Phosphorus removal: Chemical dosing to be added to the existing treatment stream, including a ferric sulphate dosing system and also a sodium carbonate dosing system for alkalinity adjustment. Storm storage: A new 228m3 above-ground modular steel tank to be constructed on a reinforced concrete base with mechanical flushing bell. Key elements will include a storm return pump station, Copasac and sampling chamber, pump mixers, interconnecting pipework, penstock modifications, new MCC and control systems, and EDM instrumentation. Additional sludge storage: An additional 160m3 of sludge storage via a modular steel tank on a reinforced concrete base, together with a sludge transfer pumping station, interconnecting pipework, removal of the old RC sludge storage tanks, a new bunded tanker discharge point, and associated MCCs, control systems and instrumentation. Design & early contractor involvement (ECI)

Through early contractor involvement during the design phase, Envolve Infrastructure and Trant Engineering used their specialist sector knowledge to guide key constructability decisions across safety, quality, time and cost.

Envolve Infrastructure worked closely with all stakeholders to develop a construction methodology and programme sequencing that maintained uninterrupted existing processing throughout. Close coordination and collaborative working between the delivery teams, designers and Wessex Water Operations was necessary to deliver the scheme successfully, on time and without incident.

Bishops Lydeard WRC: Designers, consultants & contractors Project delivery: YTL Construction Design consultant: Sweco Principal & civil contractor: Envolve Infrastructure MEICA contractor: Trant Engineering Automation & commissioning: YTL Construction Pumping station: Matt Durbin Associates Wellpoint dewatering: Dewatering Services Ltd Temporary works: MGF Ltd [caption id="attachment_31165" align="alignnone" width="1200"] Multi media filtration plant pump station under construction - Courtesy of Envolve Infrastructure[/caption] Bishops Lydeard WRC: Suppliers of process plant & equipment Multi-media filtration plant: Bluewater Bio Ltd Moving bed biofilm reactor plant: Siltbuster Group Ferric chemical dosing: WES Ltd Alkalinity plant: Rospen Industries Above-ground storage tanks: Stortec Engineering Ltd Flushing bells: Eliquo Hydrok Ltd MCC, panel & kiosk: RSE Control Systems (GPS) Pipework: Saint Gobain PAM UK Pipework: Frazer Utilities Actuated penstocks: Glenfield Invicta Storm return pumps: Xylem Water Solutions Storm mixer pumps: KSB Ltd Valves: AVK UK Ltd Steelwork: Woodhams Group Precast headwall: Althon Ltd Site access bridge: Retro Bridge Ltd Coring: Drillcut (UK) Ltd ABB WaterMaster MCERTS flow measurement: ABB Pulsar Ultra 4 EDM monitoring: Pulsar Measurement [caption id="attachment_31161" align="alignnone" width="1200"] Storm tank under construction - Courtesy of Envolve Infrastructure[/caption] Challenges & mitigation

Ground conditions

Ground conditions across the site were inherently poor, with a high-water table and flood zone adjacent to the watercourse. Working through proposals during design and undertaking further site investigation, YTL Construction, Envolve Infrastructure and Sweco reached a value-engineered solution, replacing piled foundations with an increased thickness of structural granular fill, delivering cost, programme and environmental benefits.

Existing assets & site constraints

The limited space and positioning of existing assets required a phased construction approach to maintain access for both construction traffic and operational vehicles. A joint phased programme was developed alongside design development, ensuring the new storm tank, sludge tank, MMF plant and chemical dosing units were constructed and commissioned without impacting the existing works.

[caption id="attachment_31171" align="alignnone" width="1200"] New HV route into site - Courtesy of Envolve Infrastructure[/caption]

HV diversions

To create sufficient space for the new elements of work, the existing HV cable feeding the WRC, including a section of overhead cable, required diverting. Working alongside the power provider, Envolve Infrastructure undertook the necessary civils works to reroute the cable and clear space for the new MCC kiosk, sludge tank, alkalinity unit and pumping station.

Access bridge

The existing access bridge spanning the adjacent watercourse was found to be unfit for ongoing construction and tanker traffic. As the only means of access to the works, and the sole access point for an adjacent resident, the bridge could only be closed for vehicle access up to 7 days. Through careful planning and weekend working, Envolve Infrastructure successfully installed a new steel bridge with reinforced concrete abutments, minimising disruption to local residents and maintaining full operational continuity of the WRC.

[caption id="attachment_31164" align="alignnone" width="1200"] New access bridge installation - Courtesy of Envolve Infrastructure[/caption] Efficiencies, sustainability & environment

A collaborative approach was adopted throughout design to improve buildability, involving YTL Construction, design consultants, technical specialists and early contractor involvement. The YTL Construction Offsite Build (OSB) in-house team provided the design, manufacture and supply of pre-assembled standardised equipment, including:

Chemical dosing kiosks. Point of application boxes. Emergency showers. Washwater booster sets. Lighting columns.

This methodology delivered improved efficiency, reduced programme, enhanced health and safety, and reduced carbon through fewer vehicle movements and shorter on-site construction durations.

In addition to the existing biological treatment process, a new Moving Bed Biofilm Reactor (MBBR) from Siltbuster Group was installed to further improve biological wastewater treatment.

[caption id="attachment_31162" align="alignnone" width="1200"] Newly completed MBBR - Courtesy of Envolve Infrastructure[/caption]

Environmental mitigation measures included:

Engagement with the Environment Agency to plan works through classified watercourses. Use of HVO fuel for all plant and equipment. Replanting of hedgerows and trees with native species. Preservation and protection of local habitats and tree protection zones. Re-use of excavated material on site, reducing landfill and vehicle movements. Off-site manufacturing to reduce on-site carbon impact. Electrical & mechanical works

The Trant Engineering Wessex Water delivery team undertook the detailed electrical design, full M&E installation, testing and commissioning works for the scheme. The M&E scope encompassed the installation and integration of a new multi-media filtration (MMF) tertiary treatment plant from Bluewater Bio Ltd, MMF feed pumping station, wash water booster set, ferric sulphate dosing system, front end ferric dosing system, alkalinity dosing system, alkalinity dosing chamber, new sludge storage and transfer pumping station, decant tree, works return pumping station, storm storage tank, storm return and final effluent monitoring, together with all associated process pipework and instrumentation.

[caption id="attachment_31166" align="alignnone" width="1200"] Multi-media filtration plant - Courtesy of Envolve Infrastructure[/caption]

The project also included a substantial site-wide electrical infrastructure upgrade incorporating a new 200kVA power upgrade, new MCC kiosk, standby generator installation, DNO metering arrangements, power distribution, telemetry integration, site lighting and control systems. Trant Engineering undertook all LV electrical design, cable routing, containment, power and control installations, instrumentation integration and commissioning activities across the operational treatment works.

As part of a subsequent side-stream enhancement package delivered towards the close of the scheme, Trant Engineering additionally provided all M&E installation and LV detailed design for the MBBR plant and blowers, MBBR feed pumping station and an associated MCC to serve the additional side-stream works. The design also incorporated provision for future expansion, with the site configured to accommodate two future DAF units with minimal additional modification requirements.

Key project outputs delivered by Trant Engineering included:

Storm storage capacity increase to approximately 310m3. Installation of EDM monitoring on the inlet and outlet of the storm tank. New tertiary multi-media filtration plant from Bluewater Bio Ltd for improved final effluent quality. Ferric sulphate dosing from WES Ltd, and alkalinity dosing from Rospen Industries for phosphorus removal. New sludge handling, transfer and return pumping assets. Site-wide electrical infrastructure and standby power upgrades. New MCCs, telemetry and process control integration. Installation and commissioning of MBBR side-stream treatment systems from Siltbuster Group. Future-proofed layout to facilitate potential DAF installation.

The successful delivery of these works enhanced treatment performance, increased operational resilience and ensured long-term compliance with tightening environmental discharge consents, demonstrating Trant Engineering’s capability to deliver complex integrated M&E and electrical design solutions within live operational wastewater environments.

[caption id="attachment_31172" align="alignnone" width="1200"] Sludge storage tank - Courtesy of Envolve Infrastructure[/caption] Project completion

Works commenced on site in October 2023, with all regulatory outputs achieved on 20 December 2024, representing the collaborative approach to project management, construction and commissioning across all partners. At the time of writing (May 2026), the newly completed works at Bishops Lydeard WRC are in full operation and continuing to deliver on scheme objectives for the local environment and the residents it serves.

This complex and highly successful project provides a significant improvement to the treatment process and quality of final effluent and delivers additional robustness against future storm events. It was delivered to the highest quality and health and safety standards.

Northumbrian Water commissioned the Coundon Burn Pollution Risk Reduction scheme to mitigate pollution risk to Coundon Burn, a tributary of the River Wear in County Durham. The scheme was driven by hydraulic incapacity within the local sewer network, which was contributing to frequent pollution events, alongside the deterioration of an existing pipe bridge. Traditional solutions would have required significant capital investment, extended programmes, and major environmental disruption. Instead, through innovative design, collaborative value engineering, and efficient delivery, the project achieved over £1m in savings, reduced costs by 25% against target, and was completed four months ahead of schedule, demonstrating a more sustainable and effective approach to infrastructure delivery. Project background

The Coundon Burn Pollution Risk Reduction Scheme is a strong example of a high-performing project that has delivered against key milestones while creating lasting value for Northumbrian Water, its customers, and the wider community.

The scheme addressed both hydraulic constraints and asset deterioration through a carefully optimised design combining underground storage, targeted sewer diversions, and over 1km of sewer rehabilitation.

[caption id="attachment_31428" align="alignnone" width="1200"] (left) The existing brick culvert which runs through the 10m high and (right) the Grade II listed former railway arch - Courtesy of Esh Stantec[/caption]

Rather than defaulting to large-scale replacement, the team focused on maximising existing assets and minimising environmental impact. Early collaboration and detailed hydraulic modelling enabled rapid refinement of the solution, ensuring capacity improvements could be achieved with reduced intervention.

A key challenge for the Esh Stantec project team was navigating a highly constrained site, including:

An existing 6m high pipe bridge. Ancient woodland. A 10m high embankment. An existing brick culvert. Grade II listed former railway arch. Project planning/solution selection

The initial solution posed significant environmental risks and buildability concerns including the need for deep excavations and significant ground disturbance. These risks were greatly reduced by implementing trenchless construction techniques and minimising replacement through extensive rehabilitation of the existing pipe network.

At the heart of the scheme is a gravity-driven attenuation system, designed for both performance and simplicity.

Storm flows are captured within a new FP McCann precast concrete storage tank, regulated by a Hydro-Brake® from Hydro International, enabling the system to fill and empty naturally, without the need for pumping.

This passive approach:

Eliminates operational energy demand. Reduces maintenance requirements. Enhances long-term reliability.

The tank design was further optimised by increasing depth to reduce footprint and positioning it near existing access points to improve maintainability.

[caption id="attachment_31433" align="alignnone" width="1200"] (left) Overview of site during tank construction and (right) start of the construction of the precast concrete tank - Courtesy of Esh Stantec[/caption] Coundon Burn Pollution Risk Reduction: Supply chain - key participants Client: Northumbrian Water Project delivery: Esh Stantec Main contractor: Esh Construction Main designer/consultant: Stantec UK Auger boring: Active Tunnelling Temporary works/shoring: National Trench Safety UK Horizontal directional drilling: Ken Rodney Construction Storage tank: FP McCann Ltd Flow control: Hydro International Ltd Vegetation clearance: W Marley Agricultural Contractors Wall reinstatement: Neville White Construction Innovations

Innovation was central to delivery, particularly in navigating environmental and physical constraints.

Approximately 400m of trenchless installation was delivered through sensitive areas, including ancient woodland and beneath critical infrastructure.

Two primary techniques were deployed:

Auger boring: On the northern side of the site, auger boring was undertaken by Active Tunnelling Ltd to navigate the aforementioned ancient woodland, the 10m high embankment, and the Grade II listed former railway arch. Horizontal directional drilling: On the southern side of the site, horizontal directional drilling was by Ken Rodney Construction Ltd to eliminate the need for extensive temporary works, reducing the environmental impact by avoiding significant mature trees, mitigating health and safety risks, and reducing the programme duration compared to traditional construction methods, all whilst working close to the existing watercourse. [caption id="attachment_31430" align="alignnone" width="1200"] Auger boring for the installation of vitrified clay pipes - Courtesy of Esh Stantec[/caption]

In addition, over 1km of cured-in-place-pipe (CIPP) relining works was installed within the existing sewer network throughout the ancient woodland as an alternative to replacing sections of pipework that were cracked and disjointed.

This provided future resilience to the existing network and improved hydraulic performance, whilst minimising intrusive works in an extremely environmentally sensitive area. To facilitate this relining works, a comprehensive over pumping arrangement was installed to manage sewer flows.

Extensive up-front surveys and early engagement with technical specialists informed a comprehensive construction mitigation strategy, addressing protected species including bats, otters, and waxcap fungi to name but a few. Collaboration with Natural England, the Environment Agency, Durham County Council, and the Auckland Project ensured compliance and stakeholder confidence.

Notably, the project achieved zero environmental incidents during construction, and the mitigation strategy is now being adopted by Northumbrian Water as a template for future AMP8 schemes, both in terms of procedurally and culturally.

Cost savings

The project demonstrated exceptional performance against programme and budget. Delivered four months ahead of schedule through proactive planning, collaborative working, and effective risk management, the value engineering-led approach generated over £1m in savings compared to the initial £4.5m project cost.

Further efficiencies were realised through:

Optimising the storage solution from pipe systems to a compact precast structure. Careful sequencing of construction activities. Reusing materials, including aggregates from nearby projects.

These measures ensured improved outcomes while maintaining affordability for customers.

[caption id="attachment_31429" align="alignnone" width="1200"] (left) Horizontal directional drilling rig and (right) the accompanying steering heads available to use on site - Courtesy of Esh Stantec[/caption] Carbon reduction

From the outset, carbon management was embedded at the heart of decision-making. As PAS2080-aligned designers and delivery partners, Esh Stantec adopted a TOTEX-based approach, assessing capital, operational, and embodied carbon across all design options. This enabled informed, data-driven decisions that prioritised lower carbon solutions without compromising performance or resilience.

A key achievement of the project was exceeding the initial 25% carbon reduction target against the baseline design. This was realised through a series of innovative interventions focused on reducing and replacing carbon-intensive activities. Most notably, the team challenged traditional construction methods by adopting trenchless technologies. These techniques avoided extensive excavation within ancient woodland, significantly reducing plant use, material volumes, and associated carbon emissions, while also protecting sensitive habitats.

Through a focus on reducing, replacing, and removing carbon, the team achieved a significant reduction in whole-life carbon, from 310 tCO2e to 226 tCO2e, delivering a high-value, low-carbon solution that aligns with Northumbrian Water’s long-term sustainability goals.

The decision to rehabilitate over 1km of existing sewer through cured-in-place-pipe lining, rather than full asset replacement, delivered substantial embodied carbon savings. By maximising the use of existing infrastructure, the project avoided the need for new pipe manufacture, transport, and installation, demonstrating a clear commitment to carbon reduction through reuse.

The project also removed carbon through smarter design of new infrastructure. The attenuation tank was engineered as a gravity-based system, eliminating the need for mechanical pumping and ongoing energy consumption. This passive solution significantly reduces operational carbon over the asset’s lifecycle while also lowering maintenance requirements. Furthermore, the tank design was optimised to minimise its physical footprint and material usage, reducing embodied carbon.

[caption id="attachment_31434" align="alignnone" width="1200"] Construction of the precast concrete tank - Courtesy of Esh Stantec[/caption]

Material selection played a critical role in lowering carbon impact. The specification of concrete with ground granulated blast-furnace slag (GGBS) content reduced the embodied carbon associated with cement, while maintaining durability and performance. Carbon hotspots identified during the design phase, such as deep shafts and a new pipe bridge were actively designed out, demonstrating a proactive approach to carbon elimination.

Beyond delivery metrics, the project has created a positive and enduring impact. The scheme has significantly reduced pollution risk to Coundon Burn, improving water quality and protecting the wider River Wear catchment.

Health & safety and customer engagement

A positive health and safety culture was actively promoted across the supply chain from design through to the specialist subcontractors employed to deliver the scheme. This was achieved by holding regular collaborative planning sessions where design and delivery was openly discussed and challenged for ways to improve both efficiency and safety.

This positive teamwork attitude enabled early identification of challenges or pinch points in delivery and allowed alternative options to be considered. This process allowed all the supply chain contractors and suppliers to take part in construction rehearsals where overlap and integration could be planned.

As the site access is off the busy A688 highway, it was recognised that safety of the public and workforce would be improved by implementing a temporary speed limit on this section of road for access on and off site. This was implemented collaboratively with the Local Authority and was positively received as a means of improving safety for the workforce and the community.

An excellent standard of customer engagement was upheld throughout the scheme, with regular liaison between the site team and landowners/local residents/businesses to minimise disruption and provide progress updates. When working on farmland, a coordinated access route was included to allow the farmer to navigate live-stock to both fields during construction works.

[caption id="attachment_31435" align="alignnone" width="1200"] Auger boring trenchless technology was used to avoid disruption to the areas of ancient woodland - Courtesy of ESH-Stantec[/caption] Conclusion

Northumbrian Water wanted to reduce pollution risk to Coundon Burn, a small but environmentally sensitive tributary of the River Wear in County Durham and improve the resilience of the wastewater network.

The Esh Stantec team designed plans to increase sewer capacity and renew the existing sewer pipeline to reduce storm overflow into Coundon Burn and improve river quality. Through hydraulic modelling, option appraisal and design development, underground storage, sewer diversions away from the watercourse, and the rehabilitation of the existing combined sewer to extend the life of existing assets were successfully implemented.

The Coundon Burn Pollution Risk Reduction Scheme has significantly improved water quality and reduced pollution risk within the River Wear catchment, while strengthening the resilience of the local wastewater network.

Through a combination of trenchless construction, asset reuse, and efficient design, the project was delivered ahead of programme, under budget, and with significant carbon savings.

The scheme demonstrates how integrated design and delivery can achieve resilient, low-impact infrastructure solutions, particularly within environmentally constrained locations. The approaches developed during the project provide a basis for future schemes within Northumbrian Water’s AMP8 programme and beyond.

Anglian Water are investing £10m to upgrade Ramsey Water Recycling Centre in Cambridgeshire to address regulatory compliance, improve treatment capacity and enhance environmental protection and water quality in local watercourses, including the nearby Ramsey High Lode. The project combines two critical schemes; storm tank capacity upgrades and increased flow to full treatment (FtFT), ensuring the site meets future demand, stringent environmental standards, and safeguards aquatic ecosystems during high-intensity rainfall events. The issues

The primary challenge driving this project was the inadequate storm volume capacity at Ramsey WRC. The existing storm tank could only hold 260m³, far below the required capacity set by the Environment Agency (EA).

Failure to increase the on-site stormwater storage capacity would result in a consent failure leading to the risk of environmental harm and regulatory penalties for Anglian Water.

Additionally, the site’s existing flow to full treatment infrastructure was outdated and in need of major repair and upgrade. The current system could not handle the additional flows required to meet future population growth and environmental obligations. Without intervention, Ramsey WRC faced operational inefficiencies and potential breaches of discharge permits.

To overcome these issues, Anglian Water tasked the @one Alliance to design a dual-scheme solution to address the stormwater storage and FtFT issues.

[caption id="attachment_28313" align="alignnone" width="1200"] Construction of the reinforced concrete slab for the new storm tank - Courtesy of @one Alliance[/caption] Storm tank upgrade

The new storm tank designed to significantly increase storage capacity and reduce overflow risk during heavy rainfall events. The new tank is 11.1m in diameter and 8.47m high. This provides an additional 693m3 of storage capacity bringing the total volume of stormwater storage to 953m3. This upgrade ensures compliance with Environment Agency requirements and future-proofs the site against climate change impacts and population growth.

To improve operational efficiency and safety, the design incorporates external dry well submersible pumps for automated cleaning. This innovation eliminates the need for manual cleaning and access platforms, reducing health and safety risks for maintenance teams.

Furthermore, the stormwater storage tank has been carefully integrated into the existing infrastructure, optimising the limited site footprint and minimising disruption during construction. This approach allowed the project team to deliver a high-capacity solution without extensive civil works or compromising ongoing operations.

[caption id="attachment_28307" align="alignnone" width="1200"] Construction of new Storm Tank 4 - Courtesy of @one Alliance[/caption] Flow to Full Treatment (FtFT) upgrade

The second element of the project focused on increasing the site’s ability to treat incoming flows during peak conditions.

Modular side-stream: Anglian Water implemented a modular side-stream solution supplied by Siltbuster Group, which offers flexibility and scalability within the constrained site layout. This system includes:

Three Lamella clarifiers for primary solids removal. Two moving bed biofilm reactors (MBBR) complete with blower skids for biological treatment. Three dissolved air flotation (DAF) units to polish effluent and remove residual solids.

These technologies work in harmony to deliver robust treatment performance even under challenging conditions.

Supporting this advanced treatment process are associated pumps, flow splitters, and access platforms, ensuring smooth operation and ease of maintenance.

Tertiary solids removal: To further enhance effluent quality, the project has incorporated tertiary solids removal (TSR) through twin-stream pile cloth filters and disc filters from FLI Water Ltd, which provide an additional layer of protection for local watercourses.

Power supply: Recognising the increased power demands of these systems, the upgrade also includes a 315kVA electrical power enhancement, installation of a new LV switchboard kiosk, and integration of telemetry systems for real-time monitoring and control.

Collectively, these improvements enable Ramsey WRC to handle an additional 20 litres per second (l/s), bringing the total FFT capacity to 89.4 l/s, while ensuring compliance with stringent environmental standards.

[caption id="attachment_28314" align="alignnone" width="1200"] DAF tanks and access steelwork - Courtesy of @one Alliance[/caption] Challenges

Delivering this complex project posed several significant challenges that required innovative solutions and careful planning.

Space constraints: The existing site footprint offered very limited options for new installations, making it difficult to accommodate additional infrastructure without major disruption. The project team’s decision to adopt modular side-stream units provided the flexibility to position equipment efficiently within the restricted area. This approach minimised the need for extensive civil engineering works and allowed the team to make the most of the available space. Integration with existing assets: Maintaining continuous operations during construction was critical to avoid service interruptions. The team addressed this by retaining key components of the existing system, such as inlet works, screens and grit removal facilities. This strategy not only reduced costs and complexity but also ensured that the site remained fully functional throughout the upgrade process. Regulatory compliance and timeline management: With a design horizon extending to 2025, another significant hurdle was for the project needed to align with NEP FLOW Driver U_IMP6 and WINEP obligations. To meet these requirements, the team engaged stakeholders early and implemented a phased construction plan. Integration of multiple advanced technologies: The project faced technical complexity due to the integration of multiple advanced technologies, including lamella clarifiers, moving bed biofilm reactors (MBBR), dissolved air flotation (DAF) units, and tertiary solids removal (TSR) systems.

Coordinating these systems demanded robust design and commissioning strategies. The team mitigated risks by collaborating closely with specialist suppliers, ensuring seamless integration and reliable performance across all components.

[caption id="attachment_28309" align="alignnone" width="1200"] MBBR tanks - Courtesy of @one Alliance[/caption] Ramsey WRC: Supply chain - key participants Client: Anglian Water Project delivery: @one Alliance Principal contractor: Skanska Civils installation: Hercules PLC Stormwater storage tank: Hayes GFS Ltd Modular side-stream solution: Siltbuster Group TSR disc filter: FLI Water Ltd Mechanical & electrical installation: Waveney Pumps Ltd Air blowers: AERZEN Machines Access steelwork: Steelway Power upgrade: UK Power Networks MCC & kiosk: Max Wright Ltd Pumps: Xylem Water Solutions Gorman-Rupp pump supply: Hydromarque Ltd Verderflex peristaltic pumps: Verder Ltd Pipework: Saint Gobain PAM UK Site security & construction timelapse: Wireless CCTV Ltd Asbestos removal: Oracle Solutions Coordination & collaboration

The successful delivery of the Ramsey WRC upgrade was underpinned by a highly coordinated and collaborative supply chain strategy. Given the complexity of the dual-scheme approach, combining storm tank capacity upgrades with advanced flow to full treatment (FFT) enhancements, the project required contributions from multiple Tier 2 contractors and specialist suppliers, each playing a critical role in achieving the technical and regulatory objectives.

[caption id="attachment_28312" align="alignnone" width="1200"] Lamella settlement and access steelwork - Courtesy of @one Alliance[/caption] Conclusion

Work commenced in August 2024 and at the time of writing (January 2026) is scheduled for completion in the upcoming months; ensuring that all regulatory deadlines are met without delays.

The Ramsey Water Recycling Centre upgrade stands as a clear demonstration of Anglian Water’s commitment to resilience, regulatory compliance and environmental stewardship. By investing £10m into advanced treatment technologies and storm capacity improvements, the project not only addresses immediate operational and regulatory challenges but also future-proofs the site for AMP8 and beyond.

This strategic investment ensures that Ramsey WRC can continue to meet the growing demands of population increase and the tightening environmental standards that govern water recycling operations.

The flow to full treatment element of the investment is on track to achieve an 77% reduction against Anglian Water’s capital carbon baseline, which is key to supporting Anglian Water’s commitment to reaching a net zero organisation by 2030.

North-west of Nottingham and separated by the River Erewash, two of Severn Trent’s sewage treatment works are situated within 2.5km of each other; part of the legacy of the times of the public statutory undertakers. Heanor Milnhay STW in Derbyshire has a dual-stream process, with part of the flows going through trickle filters and the rest being treated in a conventional activated sludge plant. It treats up to 258 l/s from a population equivalent of 41,000 from the Heanor catchment. Newthorpe STW in Nottinghamshire, which also serves a PE of approximately 41,000, is a filter treatment process with a maximum capacity of 324 l/s. Project objectives The objectives of the project are to improve the quality of the effluent at Heanor Milnhay STW and Newthorpe STW to meet the Water Framework Directive (WFD) requirements set out in the National Environmental Programme (NEP). This is to be achieved through the transfer of flow to full treatment (FtFT) from Heanor Milnhay STW to Newthorpe STW for combined treatment. As part of the NEP commitments to meet the obligations of the WFD in AMP7, a new phosphorus (P) permit of 0.2 mg/l will be introduced on 22 December 2024. The target price of the project is £35.8m. Project summary Heanor Milnhay: Heanor Milnhay will be decommissioned once construction at Newthorpe is complete, with the site only retaining its storm tanks. The storm capacity will be increased from 2,000m3 to 7,000m3 by the transformation of its two primary settlement tanks into new storm tanks and the construction of an extra blind tank. A combined sewage overflow chamber will split the storm flows from the flows to be treated at Newthorpe, which will be pumped through a dual rising main to the new works. [caption id="" align="alignnone" width="1200"] Google Maps image showing general scheme overview - Courtesy of Mott MacDonald Bentley[/caption] Newthorpe: At Newthorpe, the secondary and tertiary treatment will be decommissioned. Only the preliminary treatment (screening and grit removal) and the primary settlement tanks will be retained as part of the new process stream. The extension, which will be built on the field adjacent to the existing works, will use an Enhanced Bio-Phosphorus Removal (EBPR) process, in a modified University of Cape Town (UCT) configuration. This process aims to reduce chemical consumption using polyphosphate-accumulating organisms (PAO) to eliminate phosphorus. Newthorpe STW extension Newthorpe STW will see a complete transformation. The new process will be capable of treating double the flow in a similar footprint to the existing works. The extension will consist of: New inlet works to treat the flows transferred from Heanor Milnhay STW: While the Newthorpe flows will still be treated at the existing inlet works on site, Heanor Milnhay flows, which will arrive at the site crude, will go through a 2D 6mm escalator fine screen provided by Longwood Engineering and a Crossflow grit detritor provided by Tuke & Bell, before entering the new treatment. Combination chamber with a high flow overflow: The new chamber will combine the flows coming from Newthorpe, which have been settled in the existing primary settlement tanks, with the flows coming from Heanor Milnhay. Heanor Milnhay flows will not be settled before entering the secondary treatment to provide the EBPR with the right level of load. However, the process is protected from highly diluted load flows with a high-flow overflow, that will allow flows above 66% of FtFT to bypass the anaerobic and anoxic section and go directly to the aerobic treatment. [caption id="" align="alignnone" width="1200"] Newthorpe STW extension Revit model - Courtesy of MMB[/caption] An anaerobic and anoxic tank, with a capacity of 2,438m3 per section (4,876m3 in total): To provide the right level of tank in series and mixing in the system, each tank consists of three equal pockets with submersible mixing. The recirculated active sludge (RAS) and the liquors from the thickening of the surplus active sludge (SAS) will join the process in an intermediate chamber located between the anaerobic and the anoxic sections. The tank is a 40m long, 25m wide and 6.5m high structure. A 4-lane activated sludge plant (ASP) holding a total process volume of 20,000m3 (5,000m3 per lane): This is the biggest structure on site; 90m x 42m and a height of 6.5m. Each line has a swing zone that turns 3,000m3 of volume from aerobic to anoxic in summer. Sulzer are providing the blowers and Xylem Water Solutions provides the aeration grid and pipework for the tank. Final settlement tanks: 3 (No.) 33m diameter final settlement tanks designed and built by Tank Consult Ltd, with full scraper bridges designed by EPS Water. Mixed media filtration: Consisting of four horizontal vessels supplied by Bluewater Bio Ltd, this is the largest plant of this kind for the treatment of wastewater installed in the UK. Sludge thickening: Comprising Alfa Laval gravity belt thickeners aided by polymer dosing by Richard Alan Engineering. Chemical dosing: Alkalinity dosing to limit ammonia from Lintott Control Systems and ferric dosing to trim P to meet the new tightened permit of 0.2mg/l from Colloide. [caption id="" align="alignnone" width="1200"] Anaerobic & anoxic tank construction progress - (July 2023) - Courtesy of Mott MacDonald Bentley[/caption] Newthorpe STW: Supply chain - key participants Client: Severn Trent Principal designer & contractor: Mott MacDonald Bentley FRC: Bell Formwork Services Ltd Design/build of FRC final settlement tanks: Tank Consult Ltd Rising main HDD: Johnston Trenchless Solutions (JTS) Tertiary solids removal: Bluewater Bio Ltd Gravity belt thickeners: Alfa Laval Inlet screens: Longwood Engineering Company Ltd Grit removal plant: Tuke & Bell Scraper bridges: EPS Water Glass-coated steel tanks: Goodwin Tanks Aeration diffusers for ASP: Xylem Water Solutions MCCs: CEMA Ltd Kiosks: Morgan Marine Coated pipework & wall starters: George Green (Keighley) Ltd Polymer dosing: Richard Alan Engineering Alkalinity dosing: Lintott Control Systems Ferric dosing: Colloide Mixers, blowers & submersible pumps: Sulzer Pumps Wastewater Ltd Progressive cavity pumps: NOV Booster pumps: Grundfos Valves: AVK UK Ltd Lifting equipment: T Allen Engineering Services Ltd Steelwork: Galliford Try Fabrications Fencing: Town & Country Fencing (Midlands) Ltd Carbon reduction One of the main aims for the Newthorpe delivery team has been minimising the carbon footprint of the project, using the PAS2080 carbon hierarchy. Build nothing: The original scope of works included new inlet works to treat Newthorpe STW flows. However, after a structural inspection of the existing inlet works, and given that the design flows were not increased, it was proposed to reuse the existing structure, with some minor modifications, just replacing the mechanical equipment. The estimated carbon saving was circa 250 tonnes of CO2. Build less: A joint effort of the Mott MacDonald Bentley process and structural team lead to a big carbon saving in the ASP structure. The process team identified the opportunity of reducing one lane from the outline design, with no detriment to the process. At the same time, the structural team developed a solution with variable wall and slab thickness to optimise the concrete. In total, it was estimated that the embodied carbon of the structure was reduced by 400 tonnes. [caption id="" align="alignnone" width="1200"] Final settlement tanks construction progress July 2023 - Courtesy of Mott MacDonald Bentley[/caption] Build smart: Due to the depth of the final settlement tanks into the ground, flotation was the limiting factor in the design. In the preliminary calculations, the estimated slab thickness required for the tank to float in a maintenance scenario was above 1000mm. The project team, therefore, challenged themselves to try to achieve a more sustainable solution. The outcome was to install an active drainage system that, in case of maintenance, would lower the groundwater level to 2m below the final ground level. That reduced the slab thickness to 400mm, without having a major impact on the operation of the plant, as this drainage system will only be required during maintenance scenarios where the tank needs to be emptied. Build efficiently: To carry out the earthworks for the new site, the site team has used GPS-intelligent plant that, utilising a 3D model of the ground, reduced the plant-operator interface on site, improving H&S as well as efficiency. The excavated ground has been re-used to screen the site and provide biodiversity net gain, instead of having to transport it off-site. Progress to date Construction started in the summer of 2022 and at the time of writing, July 2023, is progressing well: The ASP slab is complete and the ASP walls are 70% complete and there is similar progress on the anaerobic/anoxic tank. Both structures will be ready to start hydrostatic testing and installation of M&E equipment by early Autumn. [caption id="" align="alignnone" width="1200"] ASP Tank construction (July 2023) - Courtesy of Mott MacDonald Bentley[/caption] FST1 is now completed and undergoing hydraulic test and FST2 and FST3 are under construction. Construction of the inlet works for the Heanor Milnhay flows and the MCC basement are due to start shortly. The first section of the rising main connecting Heanor Milnhay and Newthorpe are underway with a planned completion by the end of 2023. The rising main design has been challenging due to the number of features and a large number of external stakeholders requiring complex management by MMB. While half of the rising main will be installed by open trench methodologies, almost 1.2km will be built using directional drill, in two separate sections. The first section will cross the River Erewash, the Erewash Canal, and the disused Nottinghamshire Canal. The second section will follow the alignment of Newmanleys Road and will avoid any need for road closures or traffic management in the area. Particularly challenging is the section closer to the works, which has old mine works recorded, and therefore needs special measures to minimise the risk of encountering unknown mining features. In Heanor Milnhay STW, the redundant assets in the area where the new assets will be built have been demolished, however, the construction on site is subject to the completion of the first directional drill shot and has not yet progressed. [caption id="" align="alignnone" width="1200"] (left) Ground modelling to explore coal seams and (right) detail of rising main welding area - Courtesy of Mott MacDonald Bentley[/caption] Summary Newthorpe and Heanor Milnhay STWs are being merged together in one of Severn Trent's major AMP7 projects, with Mott MacDonald Bentley as principal designer and contractor. The expansion of Newthorpe STW will make it one of the largest enhanced bio-phosphorus treatment sites in the region, covering a population equivalent of 100,000; achieving a tightened permit of 0.2 mg/l while minimising the use of chemicals to remove phosphorus. The construction of the project is ongoing with the biggest structures set to be completed in Autumn 2023. The expected completion date of the project, including the biological maturation of the EBPR is December 2024.

Located in south-east Staffordshire, Lichfield Sewage Treatment Works is a medium sized works operated by Severn Trent. The site treats wastewater from a population equivalent of approximately 40,200 and ensures that the effluent is suitable for discharge into the Full Brook. The investment into the sewage treatment works is driven by both the tightening of environmental permits, and projected population growth within the catchment. By 2038, Severn Trent’s forecasts that the population equivalent will rise to around 56,000 - an increase of roughly 30%. Existing works

The Existing Lichfield STW employs a biological treatment process. Flows entering the site pass through the inlet works before proceeding to three rectangular primary settlement tanks (PSTs). Here, sludge settles out before being pumped to four sludge tanks where it is thickened via decanting before being transported off-site.

Effluent from the PSTs is then distributed evenly between twelve biofilters, before passing through seven hummus settlement tanks (HSTs). The partially treated effluent then passes through a moving-bed biofilm reactor (MBBR), deep bed sand filter (DBSF) and four Mecana cloth filters before the treated effluent is discharged to the Full Brook.

[caption id="attachment_30529" align="alignnone" width="1200"] Google Earth image of the existing Lichfield STW[/caption] New consent permits

The following effluent quality permits of the River Trent will be enforced by end of December 2026:

Permitted Full Flow to Treatment (FFT) of the site will be increased from 263 l/s to 373 l/s. Permits for biological oxygen demand, ammonia, phosphorus and iron will be relaxed to 20 mg/l / 3.0 mg/l / 0.5 mg/l / 4.0 mg/l respectively. Additionally, the suspended solids permit will be limited to 30 mg/l.

These permits aim to protect the sensitive habitats of the River Trent as well as ensuring the future operational capacity of the treatment works.

Due to the significantly increased catchment and the tightening effluent quality permits of the local brook, Severn Trent proposed constructing a final effluent pumping station to discharge effluent 5km north at the River Trent. Comparatively, continuing to discharge to Full Brook was not economically viable and a higher risk to meeting the required permit.

[caption id="attachment_30517" align="alignnone" width="1200"] Revit model showing the site layout - Courtesy of MMB[/caption] Early Contractor Involvement 1

During 2024, Severn Trent appointed Mott MacDonald Bentley (MMB) under an Early Contractor Involvement (ECI) agreement to review the provided outline design for the upgrade to treatment at Lichfield Sewage Treatment Works.

The proposed scope included new combined storm overflow, new storm screens, replacing the primary settlement tanks (PSTs) with three new Dortmund-style PSTs, new gravity belt thickener sludge handling, new side stream activated sludge process (ASP) and two final settlement tanks (FSTs), new final effluent pumping station and pipeline (let as a standalone Infra Project), and converting the existing PSTs into storm storage tanks.

MMB conducted a detailed review of the outline design which concluded that the original scope held excessive programme and cost challenges, which would result in failure to meet the permits regulatory date. Upon issue of the initial target price from MMB, Severn Trent posed an affordability challenge with the initial price needing to be reduced by approximately 40%.

[caption id="attachment_30528" align="alignnone" width="1200"] (left) The three existing PSTs and (right) the two existing storm tanks - Courtesy of Severn Trent[/caption] Lichfield STW: Supply chain - key participants Client: Severn Trent Principal designer: Mott Macdonald Bentley Principal contractor: JN Bentley Specialist consultant: Mott Macdonald Final effluent pumping station & pipeline (infra scheme): GEDA Construction Ground investigation: Geotechnical Engineering Ltd Helical screw piles: UKHelix Co Ltd FRC works: Bell Formwork & Civil Engineering Services Ltd Electrical installation: Main Electrical Precast panels: Naylor Concrete Products Ltd Scraper bridges: EPS Water RAM pumps: EMS Industries Ltd Storm return pumps: Sulzer Pumps Wastewater Ltd Scum pumps: NOV Mono Pumps Mecana cloth filter TSR: Eliquo Hydrok Ltd Flushing gates: Jacopa Ltd Storm screens: Spirac Ltd MCCs: BGEN Ltd Kiosks: NPS Engineering Group Valves: AFFCO Flow Control (UK) Ltd Pipes: Saint Gobain PAM UK Pipes: Aquaspira Ltd Lifting equipment: T Allen Engineering Services Ltd Access steelwork: Utiliqo Ltd Backup generator: Central Power Services Sampling: Servitech International Ltd Landscaping: Holts Tree Care [caption id="attachment_30524" align="alignnone" width="1200"] PST earthworks completed early to bring programme forward - Courtesy of MMB[/caption] Optioneering

With the affordability challenge outlined in the ECI Phase, Severn Trent initiated a renewed Optioneering and Outline Design Phase in 2025 with MMB to ensure the December 2026 permit regulation date is met within Severn Trent’s budget.

Using the Outline Design as a starting point, Mott MacDonald Bentley reviewed the existing sites capacity to understand the limitations it had, and where MMB could propose further optimisation to the existing assets and reduce the original scope down collaboratively with Severn Trent. By challenging the ways of thinking, the client’s standards and pushing the boundaries of efficient delivery.

The original storm storage solution of re-purposing the existing PSTs to additional storm storage was challenged by MMB, with an opportunity for a significantly reduced programme to be unlocked by building a new storm tank, due to not requiring the commissioning of the new PSTs to enable the removal of the existing PSTs from service. This meant that the new storm tank could be built simultaneously as the new PSTs.

This also removed significant design, and health and safety risks, which required puncturing and underpinning the existing base of the tanks which are nearly 100 years old; and due to the original programme, working during on this tank in the winter months.

[caption id="attachment_30522" align="alignnone" width="1200"] Revit model of existing inlet works and new storm screens - Courtesy of MMB[/caption]

To save capital costs, MMB proposed constructing two larger PSTs instead of three, and relocated their position to optimise hydraulics and minimise pipework requirements. Each PST was designed to take the FFT until 2038, ensuring continued operation in the event a tank needs to be taken out of service for maintenance.

MMB discovered an opportunity to remove the filter feed pumping station from the scope by optimising the hydraulics. To prove this, MMB highlighted investigation work to be conducted in ECI 2, which would confirm the feasibility. To maintain design progress, MMB and Severn Trent agreed to assume that this opportunity could be realised, and to proceed with the design at risk.

With agreement from Severn Trent to take on increased tanker loads from site, the proposed sludge gravity belt thickening system was removed on a risk-based approach. With the existing sludge decanting system deemed serviceable for the immediate term.

With inputs from Process Engineering, MMB was able to question Severn Trent on the requirement for the ASP and FSTs by challenging the limits on hydraulics through existing secondary treatment, and process optimisation in the tertiary area.

This resulted in a side stream Mecana cloth filter and a mass blend at the final effluent collection chamber before sampling. New bypasses were also required to ensure the existing assets do not exceed their process or hydraulic limits.

[caption id="attachment_30520" align="alignnone" width="1200"] Revit model showing new CSO chamber - Courtesy of MMB[/caption] Early Contractor Involvement 2

In 2025, MMB was engaged again under an ECI Contract to complete a new outline design incorporating the affordability challenge changes outlined above. In addition to the cost challenge, MMB optimised the programme to facilitate designing and building the scheme within 18 months to meet the regulatory compliance date in December 2026.

The new MMB outline design consisted of the following:

New combined storm overflow. New storm screens. A new storm tank. Two new Dortmund-style PSTs. A new side-stream Mecana cloth filter and tertiary asset bypasses. A new final effluent pumping station and pipeline; let to GEDA Construction as part of the Infra Scheme.

To minimise future delays and enable effective planning, MMB conducted extensive site investigation works during this period to confirm existing site drawings due to the nature of the tight hydraulics through the site and route proving to mitigate potential clashes with our critical gravity underground pipes. Ecology surveys were completed early to highlight all risks which required monitoring and mitigating to protect our challenging programme prior to any of the main construction works commencing.

Due to location of assets in the outline design changing, as well as the proposal of new structures, geotechnical information was missing in key locations. To address this, MMB specified additional boreholes during ECI 2 while continuing the structural design using the best available data, with the designs verified once the new information was obtained.

[caption id="attachment_30518" align="alignnone" width="1200"] Revit model showing side-stream Mecana filter and AMP8 Infra scheme pump station – Courtesy of MMB and GEDA[/caption]

Trial holes and surveys were completed to assess restricted hydraulics and ascertain whether the scope was possible. These surveys also proved useful in chasing opportunities on scope reductions with agreement from Severn Trent to reinforce the affordability challenge on this project.

To maintain project momentum, the design team started detailed design early to help enable the early excavation works at the end of 2025, which were critical to achieving programme.

This was only possible due to the close collaboration with Severn Trent and the earned trust shared between both parties.

The detailed design was developed for efficiency in delivery, with a focus on Mott MacDonald Bentley’s right first-time attitude, and use of its in-house Production Hub, to integrate standard designs that are proven to gain early wins within the design development, and programme opportunities in the design and construction programme.

The site team exploited opportunities in the programme, identifying areas which could be completed ahead of time, which may have held slight cost inefficiencies, but had significant programme gains. This included full landscape clearing of the site and the intended working areas.

With the site setup completed, significant earthworks for the PST area began to ensure construction could begin as soon as the design information was issued. This resulted in immediately pulling the construction programme ahead of schedule by approximately a month.

[caption id="attachment_30519" align="alignnone" width="1200"] Revit model showing the PSTs and storm tank - Courtesy of MMB[/caption] Collaborative working

MMB collaborated not only with Severn Trent, but also with the ongoing AMP7 Caustic Dosing Rig Scheme, and the AMP8 Infra Scheme. This collaborative approach was undertaken across both design and on‑site construction; supporting Severn Trent in meeting regulatory permit requirements.

During outline design, MMB led discussions with Severn Trent to collectively develop the scope to set up an efficient programme that is affordable and meets the process performance to ultimately determine the minimum viable product. Crucially the revised solution is now positioned to hit the regulatory permit date.

Whilst having a presence on site. MMB has ensured a constant liaison with the two other contractor parties each week in both design and on-site activities. Discussions are held on weekly basis as a minimum to manage construction interfaces between the contractors and Severn Trent as well as maintaining the Construction (Design and Management) boundaries between each party.

Deliveries and construction traffic have been planned weekly ensuring safety of site personal and minimising disruptions caused to the other parties. This proved to be essential for keeping with the one-way road system established on site.

[caption id="attachment_30527" align="alignnone" width="1200"] PST excavation works (December 2025) - Courtesy of Mott MacDonald Bentley[/caption] Conclusion

The upgrade of Lichfield Sewage Treatment Works addresses both significant forecast population growth and the need to meet tightened environmental regulations. Through Early Contractor Involvement, Mott MacDonald Bentley worked closely with Severn Trent to review and challenge the original outline design, identifying substantial cost and programme risks that threatened the December 2026 regulatory compliance date.

A focused optioneering process allowed the team to evaluate existing asset’s capacities, challenge standard design assumptions, and develop a streamlined minimum viable product solution that aligned with the relaxed River Trent discharge permits whilst remaining affordable. Early site investigations, proactive risk management, and detailed design undertaken at pace were key to improving delivery certainty.

Overall, the project demonstrates how early contractor involvement and a strong collaborative working environment can unlock value, manage risk, and deliver a solution on time, within challenging regulatory and budgetary constraints.

The existing Bromsgrove STW is a medium-sized ASP works serving the Bromsgrove area if north Worcestershire. This scheme is driven by two upcoming Environment Agency (EA) permit changes; (i) an increase in dry weather flow (DWF) associated with growth in the catchment and (ii) improving the final effluent quality through enhanced phosphorus removal. The Bromsgrove STW Growth Scheme is a flagship AMP8 project for both Mott MacDonald Bentley and Severn Trent, which will increase treatment capacity by 30% and support long-term growth to 2038, lower phosphorus in final effluent to 0.23 mg/l, and significantly increasing storm resilience. Project drivers

The DWF rate will increase from 133 l/s to 169 l/s, which is necessary due to a forecasted population equivalent (PE) increase from 72,376 to 89,183 by the 2038 design horizon. This major project increases the flow to full treatment (FFT) from 359 l/s to 463 l/s and tackles existing hydraulic challenges through a brand-new inlet works, expanded primary treatment, a new secondary treatment stream, and seven new pumping stations. Additional storm storage will strengthen resilience while helping to reduce storm spills, thereby improving local watercourse quality.

As well as the DWF and FFT increases, tighter chemical permits are being imposed with reductions to BOD (8 mg/l), ammonia (3 mg/l) and Total Phosphorus (0.23 mg/l), requiring construction of a CoMag® tertiary solids removal plant to achieve the second permit date.

[caption id="attachment_32949" align="alignnone" width="1200"] The existing Bromsgrove STW - Courtesy of Mott MacDonald Bentley[/caption] Project delivery

The £42m capital investment project has been designed and delivered by Mott MacDonald Bentley (MMB), serving as both the Principal Designer and Principal Contractor on behalf of the client Severn Trent. MMB have undertaken the detailed design and construction of the scheme, having initially been engaged under an Early Contractor Involvement (ECI) phase. The ECI phase allowed for a more detailed price build up, early engagement of supply chain opportunities, development of ground investigation understanding and identification of key constraints.

Bromsgrove STW: Supply chain: Designers, consultants & contractors Principal designer & contractor: Mott MacDonald Bentley Site & ground investigation: Dunelm Geotechnical & Environmental Precast structures: FLI Precast Solutions Formwork, reinforcement & concrete: Bell Formwork & Civil Engineering Services Ltd Formwork, reinforcement & concrete: TCL Structures (UK) Diamond drilling & hole sawing: Holemasters Electrical installation: Protocol Control Systems Ltd Access steelwork: Alpha Plus Ltd Access steelwork: Fluid Sealing & Engineering (FSE) MCC steelwork: Utiliqo Ltd Existing works & project scope

The existing treatment process at Bromsgrove STW comprises an inlet works, two 27.8m diameter primary settlement tanks (PSTs), a three-lane activated sludge plant (ASP), three 22m diameter final settlement tanks (FSTs), three storm tanks with blind tanks (equating to approximately 3300m3 of storage), ancillary pumping stations, chemical dosing and sludge handling facilities for both primary and activated sludge thickening.

[caption id="attachment_32942" align="alignnone" width="1200"] New site layout - Courtesy of Mott MacDonald Bentley[/caption]

To meet the new permits, Mott MacDonald Bentley’s initial scope of works included:

An increased capacity inlet works able to take 6 x DWF. Additional 1800m3 of storm storage. An additional 27.8m diameter PST. Additional two-lane ASP. Two additional 22m diameter FSTs. Seven new pumping stations. A CoMag® TSR plant. A standby primary sludge thickening plant. A surplus activated sludge (SAS) buffer tank. Early contractor involvement (ECI)

Presented with an outline design, MMB worked through the ECI Phase to identify key opportunities or risks, freeze the site layout, and offer cost saving opportunities such as descoping a new primary sludge thickening plant by expanding the existing capacity.

Collaboration between the design team, located both in the UK and India, and the client through early design sprints identified some key opportunities.

Reusing former filter beds

The site previously operated as a biofilter works, with ten redundant trickling filter beds and two humus tanks buried below ground; occupying almost all available space.

While outline design suggested excavating and disposing of all this material to allow part-buried construction of the new water retaining structures, MMB’s design team identified the opportunity to use the filter media as competent ground and construct the new PST, ASP, FSTs and storm tank at ground level.

This reduced resource use, vehicle movements, programme and ground risk, whilst dramatically reducing the project carbon use through the mitigation of subsequent material import. The above ground structures are fed by interstage pumping stations that were already required in outline design, so the solution only saw a marginal increase in operational expenditure. Overall, the approach allowed for a reduction in excavation volume of approximately 11,000m3.

[caption id="attachment_32946" align="alignnone" width="1200"] Google Earth image showing construction underway at Bromsgrove STW - Courtesy of Mott MacDonald Bentley[/caption]

Reworking site layout

Relocating the new secondary treatment stream to the centre of the site reduced the length of rising mains used to feed the process and created a more linear layout, which offered capital and operational savings. The relocation also allowed the team to design out the risk of interacting with an existing below-ground HV cable, and a pocket of contaminated land.

The ECI phase also allowed the team to jump-start the application process for Abstraction and Discharge Licences with the EA.

Following the identification of groundwater as a key project risk, Mott MacDonald Bentley implemented a comprehensive groundwater monitoring programme using continuous dataloggers. Groundwater level fluctuations were analysed alongside the operational behaviour of the adjacent raw water borehole, rainfall data, and local watercourse levels to establish a robust evidence base.

The resulting data played a significant role in securing the necessary licences. In parallel, the decision to raise the main structures above ground level mitigated groundwater related construction risks and delivered a programme saving of approximately seven months, allowing elements of the works to progress concurrently.

Bromsgrove STW: Supply chain: Process plant & equipment Ferric dosing: EPS Water Inlet screens: Huber Technology Scraper bridges & inlet detritor: Jacopa Ltd Submersible pumps/mixers: Sulzer Pumps Wastewater Ltd ASP launders: JBF Group ASP aeration plant: Suprafilt Ltd CoMag® TSR plant: Xylem Water Solutions Steel distribution chambers: George Green (Keighley) Ltd Weholite HDPE distribution chambers: SDS Limited HDPE with rebar ASP splitter chamber: NOV Storm tank & sludge tank: Goodwin Tanks Ltd Storm tank cleaning: Eliquo Hydrok Ltd SAS buffer tank & clarifiers: Balmoral Tanks Ltd SAS poly dosing: Richard Alan Engineering SAS thickening: Alfa Laval RAM Pumps: EMS Industries Ltd Progressive cavity pumps: SEEPEX UK Ltd Valves & bellmouths: AFFCO Flow Control (UK) Ltd Penstocks & stoplogs: Waterfront Fluid Controls MCCs: BGEN Ltd Kiosks: Industrial GRP Lifting equipment: T Allen Engineering Services Ltd Flume & MCERTS: SIRIS Environmental Detailed design

As the detailed design progressed, the project team continued to identify opportunities for optimisation across the scheme. By leveraging Mott MacDonald Bentley’s design and build model, fostering ongoing collaboration between design and site personnel, and adopting a procurement-led design approach, innovative solutions were developed that improved constructability, reduced costs, and enhanced overall project efficiency. These opportunities were systematically evaluated, refined, and incorporated into the final design.

[caption id="attachment_32947" align="alignnone" width="1200"] The two new final settlement tanks and two new activated sludge plant lanes - Courtesy of Mott MacDonald Bentley[/caption]

Semi-precast structures

The identification of a semi-precast construction method for the PST, ASP and FSTs as a preferred construction method drove key design decisions early on.

Engaging FLI Precast Solutions, design teams from both companies worked to generate a suitable solution and adequate ground improvement designs to accelerate the construction of these key water retaining structures. The benefits of this approach allowed the ASP structure to incorporate tapered walls from 600mm at the base to 300mm at the top. Over the 7m ASP height this allowed for a healthy reduction in concrete and carbon.

The circular settlement tanks were designed with precast coping stones upon which the scraper bridges could run. This reduced the need for working at height risks associated with FRC activities. The solution dramatically cut the number of lifts associated with formwork and rebar, as well as the time spent working at height by our workforce leading to important health and safety benefits.

Pre-fabricated chambers

This philosophy of off-site manufacture was embraced further for Bromsgrove STW’s distribution and collection chambers. Collaboration between design, site and the scheme’s dedicated on-site buyer, led to the procurement of seven pre-fabricated chambers from three different suppliers.

[caption id="attachment_32945" align="alignnone" width="1200"] Pre-fabricated Weholite HDPE chamber from SDS Ltd for use in PST de-sludge system - Courtesy of Mott MacDonald Bentley[/caption]

Driven by MMB’s procurement-led design approach, the team completed technical bid assessments from various companies to select the most suitable solution for each scenario, namely:

HDPE chambers from SDS Limited. HDPE with pre-formed rebar for concrete surround from NOV. A bespoke self-supporting steel chamber from George Green (Keighley) Ltd.

Each instance of this shifted pressure from the construction programme earlier into the procurement phase to allow for a more streamlined and de-risked installation.

Inlet works hydraulics

A review of the inlet works hydraulics led to a complex and innovative approach to flow measurement and splitting; a solution that reduced the size of the FFT pumping station and led to a fully screened storm solution.

As a result of raising the new PST, all flows out of the inlet works would need to be lifted to the elevation of PST 3 before two thirds would ultimately gravitate down to ground level. MMB’s hydraulics engineer identified an opportunity to gravitate to existing PST1 and PST2, removing the need for unnecessary pumping.

Shifting the whole inlet works allowed for the upstream end to be cast directly against the existing reception chamber. By breaking through this wall at commissioning, the head losses associated with the proposed under-pressure tap are eliminated.

[caption id="attachment_32948" align="alignnone" width="1200"] New inlet works structure relative to the existing - Courtesy of MMB[/caption] Collaborative planning

Given the complexity of the project, the division of design resource between the UK and MMB’s Global Delivery Services (GDS) in India, and the requirement for upwards of six core engineering disciplines, a structured collaborative approach has been essential to ensure alignment across all parties. As the scheme moved into the construction phase this developed further to include site management, site engineers, QSs, buyers and subcontractors.

To achieve this, twice-weekly collaborative planning (CP) sessions are held. One session focuses on reviewing progress from the previous week, identifying any incomplete or missed tasks, categorising the reasoning, and determining whether further investigation or corrective action is required.

The following session then provides a nine-week look-ahead, ensuring all stakeholders are aware of upcoming activities across different work areas and identifying opportunities for acceleration or procurement associated lead-time risks.

An action plan is also maintained to assign tasks to specific individuals, promoting clear accountability and ownership among engineers. In addition, a high-level programme aligned with the CL32 programme and the client’s key milestones is used to visualise key dates and track overall project progress.

On site, the five-week look-ahead plans are updated weekly by site managers, providing site operatives with clear visibility of upcoming work areas, expected durations, and supporting better planning and productivity.

Innovation during construction

HP Site Print automated setting out robot

The HP Site Print automated setting-out robot was used during the project, completing 824 setting-out points, reference lines, and level surveys across the inlet works, PST, ASP, and both FSTs.

[caption id="attachment_32956" align="alignnone" width="1200"] (left) HP Site Print autonomously setting out ASP PCC unit positions and - Courtesy of Mott Macdonald Bentley[/caption]

This printer completes setting-out significantly faster than manual marking and reduces engineers’ time on site, allowing them to focus on other activities and ultimately improving efficiency. There is no need to manually interpret drawings, as the layout is taken directly from CAD models, reducing the risk of misreading drawings or using incorrect dimensions.

The most dramatic benefit came when paired with the semi-precast construction approach. After pouring the foundation on which the units were placed, the setting-out robot then surveyed the cast slab to provide a level survey before marking the depth of packing needed under each unit. The four corners were set out for each of the hundreds of panels, whilst also marking the extents of the shuttering needed for the concrete ‘stitches’. Each panel code was annotated on the slab to ensure the correct unit was placed.

The HP Site Print, also provides health and safety benefits by reducing the number of people required in high risk areas and minimising time spent around high energy hazards. Additionally, it improves quality control by reducing reliance on individual skill levels and ensuring consistency across the works.

Roborigger ARM1500-50

Bromsgrove STW is the first project in the UK water sector to use the Roborigger ARM1500-50; a load control system based on the ARM1500 module, fitted with a 50 tonne WLL spreader beam. It added huge value to the project by increasing the lifting efficiency of the lifting team by an average of 15%. This remotely controlled lifting device, based on drone technology, removes the need for tag lines and keeps operatives outside the immediate lifting zone, significantly improving safety. Its inertia-based orientation control enhances lift efficiency in challenging weather conditions, allowing operations to continue safely when lifting activities would normally be stood down. The unique remote orientation control also enabled the installation of precast baffle walls, weighing up to 14 tonnes, into slots with only 25mm tolerance, without the need for manual intervention.

Furthermore, its cloud-based data capture supports real-time logistics monitoring and continuous optimisation of lifting operations, helping to identify opportunities to accelerate planned lifting activities.

[caption id="attachment_32955" align="alignnone" width="1200"] Roborigger ARM1500-50 lifting attachment installing baffle walls in ASP2 - Courtesy of Mott Macdonald Bentley[/caption]

OpenSpace

OpenSpace technology has helped the whole project team work together more effectively. It allows people to track progress, spot issues, and compare what is built on site with the design in real time. Regular 360° photos and OpenSpace Air 3D models have improved design checks, sped up RFIs, and helped teams solve problems faster. With nearly 40% of the design team based in Mumbai, the easily accessible current 360° and drone captures have aided understanding of the construction works and helped to avoid buildability issues. On site, this tool is also used in the planning of activities and in communication with stakeholders from the client and supply chain.

Outcomes

The culmination of the aforementioned innovations has meant that all major structures are complete and water-tested, with the project team currently working to achieve the DWF permit date two months ahead of schedule. Works in support of the later phosphorus permit change have started early, and MMB are targeting achieving this new consent limit up to three years ahead of the date imposed by the Environment Agency.

Successes at Bromsgrove STW have already been recognised at a regional scale, winning the Innovation and Technical Excellence award at the ICE West Midlands. With the scheme also awaiting the outcome of shortlists for further national competitions.

Leamington Spa is a historic spa town with a growing population and significant development pressures. The River Leam, which flows through the town centre, is a focal point for recreation, including wild swimming, boating, and public parks. The sewerage catchment includes Leamington, Warwick, and surrounding villages, all draining to Longbridge Sewage Treatment Works (STW). Six CSOs were identified for improvement. This requirement was designed to safeguard public health and ensure compliance with the Bathing Water Directive and related water quality with the aim to reduce spill volumes and spill frequency with a variety of interventions; to give an insight into the best approach for bathing waters. Endorsed by Ofwat in 2021, the scheme is part of a broader strategy to support post-pandemic economic recovery through environmental infrastructure investment. The Leamington scheme exemplifies innovation in hydraulic modelling, and collaborative delivery within tight regulatory timeframes. Project overview

The Green Recovery Leamington Spa Sewerage Scheme is a flagship initiative under Severn Trent’s Green Recovery programme, aimed at moving a stretch of the River Leam towards bathing quality. This ambitious project addresses the performance of six critical Combined Sewer Overflows (CSOs) in Royal Leamington Spa, Warwickshire, through a suite of technically advanced designs that test a variety of complementary solutions.

Few projects manage to balance technical complexity with community benefit as seamlessly as the Leamington Spa Sewerage Scheme with live public functional spaces is technically complex but also requires attentive and agile stakeholder engagement, delivering large measurable environmental improvements.

[caption id="attachment_26765" align="alignnone" width="1200"] Surface water separation - Courtesy of Forkers Ltd[/caption]

While early guidance targeted storm overflows with direct discharges into these areas, the programme has since expanded to include overflows that pose wider environmental and public amenity impacts. This includes high amenity areas such as recreational waters and areas of interest to businesses and other organisations, even if they are not formally designated as bathing waters.

The shift reflects a more holistic approach to water quality management, prioritising interventions based on environmental sensitivity, public usage, and the potential impact of overflows. This broader targeting ensures that improvements are aligned with community expectations and environmental stewardship goals.

Hydraulic modelling & option development

The project leveraged the Longbridge Strategic Sewerage Growth model, enhanced with updated flow monitoring and surface water network refinements. Hydraulic solutions were identified early through iterative hydraulic modelling and refined through feasibility and Early Contractor Involvement (ECI) with Forkers Ltd.

By integrating advanced hydraulic modelling and real-time control, the scheme exemplifies the kind of forward-thinking infrastructure needed to meet the challenges of climate resilience and urban growth.

Preferred interventions included surface water separation, pumped and gravity storage, and network optimisation.

[caption id="attachment_26764" align="alignnone" width="1200"] Shaft storm tank precast concrete segments - Courtesy of Forkers Ltd[/caption] Leamington Spa Sewerage Scheme: Supply chain - key participants Deliver contractor: Forkers Ltd Designer: WSP Tunnelling/shafts/timber headings: Active Tunnelling Ltd Electrical/mechanical: STAL (UK) Ltd Ground support & shoring: MGF Ltd Vacuum excavation: VAC UK (Kilkern Ltd) Drainage sub-contractor: Arthur Civil Engineering Ltd Emergency temporary pumping: Selwood Precast concrete pipe & rings: Marshalls Civils & Drainage Precast concrete rings: FP McCann Ltd Plastic storage pipes: Aquaspira Ltd GEOlight® storage tank: SDS Limited Concrete & aggregates: Aggregate Industries | Cemex Plant hire: Flannery Plant Hire (Oval) Ltd Traffic management: RouteOne Traffic Management Ltd Carriageway surfacing: JMC surfacing Ltd Surface water separation (SWS)

A major surface water separation scheme was implemented between Campion Road and Milverton School, delivering significant benefits in surface water management. The project successfully disconnected impermeable areas from the combined sewer network, reducing the volume of stormwater entering the system. Additionally, it incorporated below-ground storage to help manage flood risk, enhancing the area’s resilience to heavy rainfall events and contributing to improved environmental outcomes.

[caption id="attachment_26763" align="alignnone" width="1200"] Surface water separation - Courtesy of Forkers Ltd[/caption] Pumped & gravity storage

The Lower Avenue Combined Sewer Overflow (CSO) scheme involved the installation of an 880m3 shaft tank with a return flow capacity of 30 l/s, located in the Station Approach car park. The shaft tank was designed and constructed by Active Tunnelling Ltd. At Princes Drive 19B, the scheme increased the flow into the existing storm tanks by an additional 75 l/s. Both storage systems were equipped with real-time control (RTC) logic to optimise return flows and minimise the risk of spills into the river, ensuring more efficient and environmentally responsible stormwater management.

Climate change & resilience

To ensure long-term resilience and performance under changing climate conditions, the design rainfall was uplifted by 15% to reflect projected 2050 scenarios. Spill frequency assessments were carried out using Severn Trent’s Technical Standard Rainfall Dataset, representing worst-case conditions to ensure robust system performance. All conveyance infrastructure was sized accordingly to accommodate future climate impacts, supporting sustainable and reliable operation over the asset’s lifetime.

Construction & delivery

The project was delivered under tight AMP7 deadlines, requiring early contractor involvement and agile design development. Forkers Ltd and WSP played key roles in feasibility, site investigation, and outline design.

Key construction innovations included the use of GEOlight® blocks from SDS Limited for modular storage. GEOlight® blocks offer a significant advantage in large-scale infrastructure projects due to their lightweight, corrosion-resistant design, which simplifies installation and reduces long-term maintenance costs compared to concrete or steel alternatives.

[caption id="attachment_26767" align="alignnone" width="1200"] GEOlight® modular storage tank - Courtesy of Forkers Ltd[/caption] Environmental & community benefits

Reducing the impact from CSOs has had significant social implications. Access to clean and safe recreational waters promotes public health and well-being, encouraging outdoor activities and fostering a connection with nature. This initiative does not just solve a problem, it redefines what is possible in urban water management, setting a precedent for future projects across the UK and beyond.

Spill reduction. Flood risk: No detriment demonstrated; some areas show betterment particularly through the implementation of surface water separation in the north-east, the construction of stormwater storage tanks in the south, and comprehensive improvements along the River Leam corridor aimed at enhancing water quality and resilience. Education: Engagement with Milverton School on drainage and sustainability. Broader significance & replicability

The Green Recovery Leamington Spa Sewerage Scheme stands as a testament to the power of innovative engineering and sustainable practices in addressing modern environmental challenges. This project not only resolves immediate issues related to CSOs but also sets a benchmark for future initiatives aimed at enhancing urban wastewater management.

The Leamington Spa Surface Water Separation Scheme, delivered as part of Severn Trent’s Green Recovery Programme, exemplifies a forward-thinking approach to sustainable water management.

Central to the project was the installation of a two megalitre GEOlight® attenuation tank from SDS Limited beneath a playing field adjacent to Milverton Primary School. The tank, connected to newly laid surface water sewers across several roads in Leamington Spa, significantly increases stormwater storage capacity within the network. This enhancement plays a critical role in protecting downstream sewage treatment infrastructure from overload and reducing the frequency of combined sewer overflows into the River Leam.

[caption id="attachment_26768" align="alignnone" width="1200"] Construction of the 2 megalitre attenuation tank - Courtesy of Forkers Ltd[/caption]

The project not only supports Severn Trent’s ‘Get River Positive’ commitments but also aligns with local and national planning policies. By integrating innovative technology with sustainable construction practices, the scheme delivers measurable environmental benefits. It demonstrates how targeted infrastructure investment can contribute to long-term river health, climate resilience, and community wellbeing; setting a precedent for future water management initiatives across the region.

Its success lies not only in the numbers (reduced spills, increased storage, improved water quality) but in the way it brings together engineering excellence, ecological sensitivity, and community engagement. The project’s legacy will be felt not just in the River Leam’s improved health, but in the model, it provides for sustainable, scalable, and socially conscious infrastructure.

Conclusion

The Green Recovery Leamington Spa Sewerage Scheme is a model of integrated water management, combining hydraulic engineering and community recreational benefit. It demonstrates how ambitious environmental goals can be achieved through technical excellence, collaboration, and innovation.

In a sector where impact is often measured in compliance, this scheme goes further; delivering outcomes that resonate with local communities, regulators, and the environment alike.

The project not only safeguards the River Leam for recreational use but also sets a precedent for inland bathing water schemes across the UK and beyond.

Uffculme Wastewater Treatment Works (WwTW) is located on the outskirts of the village of Uffculme in mid-Devon, serving a population equivalent (PE) of approximately 3,000. The works are accessed primarily via an agricultural track across neighbouring fields and are situated within a semi-rural environment with sensitive downstream watercourses forming part of the local river network. Reducing storm spill frequency and improving environmental protection is a key strategic objective for South West Water, driven by regulatory pressures, environmental commitments, and public scrutiny. Existing treatment works

The existing works comprises an inlet and primary treatment arrangement, after which the process stream divides between conventional treatment units and an aerated ditch system. While robust and well established, the original configuration had limited storm storage capacity, resulting in a higher frequency of storm discharges during periods of intense or prolonged rainfall.

In address this, South West Water commissioned Galliford Try to deliver additional storm storage capacity at Uffculme WwTW. The primary objective of the project was to significantly reduce the number and duration of storm discharges to the local river system, while minimising operational disruption and environmental impact during construction. The project required careful optioneering due to site constraints, ground conditions, and the presence of existing buried services.

Project drivers & optioneering

The principal driver for the Uffculme Storm Storage Scheme was environmental compliance, specifically the reduction of storm overflow events from the treatment works. South West Water places a high level of importance on environmental stewardship and river water quality, and this scheme formed part of a wider programme of investment aimed at improving storm resilience across its treatment assets.

At the feasibility and initial design stages, Galliford Try undertook a comprehensive optioneering exercise to identify the most appropriate solution for delivering additional storm storage at the constrained Uffculme site. Three primary options were considered:

Construction of a traditional in ground storm tank shaft. An above-ground reinforced concrete (RC) extension to the existing storm tank. An above-ground stainless steel storm storage tank.

To inform the options appraisal, preliminary site investigations were carried out to assess geological conditions, groundwater levels, and the extent and condition of existing buried services. The site investigations revealed a dense network of services and challenging ground conditions, both of which would significantly increase risk during deep excavation.

As a result of these findings, the traditional shaft solution was discounted due to unacceptable construction risk and programme uncertainty. The above-ground RC extension option was also deemed unfavourable, as it would have required extensive groundworks and structural integration with the existing asset, presenting further risk and disruption to the operational works.

Following technical, environmental, and programme assessments, the above-ground stainless steel tank option was selected as the preferred solution. This option offered a reduced construction footprint, quicker installation, and lower risk profile, while still delivering the required increase in storm storage capacity.

Design overview

The chosen solution comprised the installation of a new above-ground stainless steel storm storage tank, hydraulically linked to the existing storm infrastructure at Uffculme WwTW. A key design principle was to maximise the use of gravity flows wherever possible, thereby reducing reliance on mechanical pumping and improving long term operational resilience.

The storm tank was designed to fill via gravity from the existing storm system, ensuring that the asset could perform its function even during power interruptions or equipment failure. A single-duty pump was provided to allow controlled return of stored storm flows back into the treatment process once capacity became available. To support effective tank emptying and maintenance, an external mixer jet was incorporated to assist with solids re suspension and cleaning of the tank base.

The design also considered long-term operability, access for inspection, and safe integration with existing assets. Interfaces with the existing storm tank were carefully engineered to allow controlled balancing between assets and ensure reliable operation during both dry weather and storm conditions.

Uffculme Storm Storage Scheme: Supply chain - key participants Client: South West Water Principal designer & contractor: Galliford Try Ground investigation: LK Group Civils tank base & tank erection: Stortec Engineering Ltd Supporting civils: Bailey Partnerships Temporary works: Galliford Try Wave screen: Jacopa Ltd MCC & software: Lintott Control Systems Pumps: Xylem Water Solutions Pipework: Burdens Concrete pumps: Camfaud Concrete Pumps Ltd Concrete & aggregates: Holcim UK Lifting operations: Spence Cranes Tree surgery: R&R Services [caption id="attachment_31536" align="alignnone" width="1200"] (left) Base shuttering and steel in place and (right) new cast storm tank base - Courtesy of Galliford Try[/caption] Construction & enabling works

Construction activities commenced following mobilisation and site establishment. Due to the semi-rural nature of the site and its proximity to existing vegetation, significant shrub and vegetation clearance was required prior to excavation works. This activity was carefully planned and executed to minimise environmental impact and was undertaken by Galliford Try’s subcontractor, R&R Services.

Excavation to formation was carried out with particular care due to the previously identified density of buried services. Vacuum excavation techniques were employed extensively to safely locate and expose live services, allowing them to be protected or diverted as required. This approach significantly reduced the risk of service strikes and prevented disruption to the operational treatment works.

Once excavation reached formation level, formation layers and blinding were installed to provide a stable base for the new tank installation. At this point, responsibility for the tank installation was handed over to specialist contractor Stortec Engineering Ltd, whose scope included the fabrication, delivery, erection, and commissioning support of the stainless steel storm tank.

During the tank installation period, Galliford Try continued to undertake a series of enabling and complementary works, including:

Installation of interconnecting pipework from the existing storm tank to the new tank boundary. Modifications to the existing storm tank to enable balancing and return flows between assets. Upgrades to access routes around the storm tank area to improve operational safety. Installation of ducting and penetrations for subsequent MEICA works.

This phased approach allowed works to progress efficiently while maintaining treatment operations and minimising disruption to the live site.

[caption id="attachment_31533" align="alignnone" width="1199"] Storm tank erected onto base - Courtesy of Galliford Try[/caption] MEICA installation & commissioning

Following completion of the main civil and tank installation works, mechanical, electrical, instrumentation, control, and automation (MEICA) fit-out activities were undertaken.

These works included installation of the return pumping system, external mixer jet, associated pipework, valves, electrical supplies, instrumentation, and control interfaces.

Final benching was completed within the new storm tank to ensure appropriate flow paths and complete drainage during emptying operations. Upon completion, the benching was allowed to cure for a minimum of seven days in line with quality and durability requirements.

A drop test was then carried out to verify the structural integrity and watertightness of the tank. The test was successfully completed, providing confidence that the asset met all design and performance criteria.

Once commissioning activities were concluded, the surrounding areas within the site perimeter were reinstated, including surfacing, fencing, and landscaping as required. The completed asset was subsequently handed over to South West Water for operational use.

[caption id="attachment_31537" align="alignnone" width="1200"] (left) New storm feed and storm overflow pipework and (right) new MCC for storm pump and mixer control - Courtesy of Galliford Try[/caption] Process description & integration

Under normal operational conditions, wastewater enters the works via the inlet works and passes through the traditional front-end treatment processes. During periods of high flow, when incoming wastewater exceeds the hydraulic capacity of downstream treatment units, excess flows are diverted to storm storage.

With the introduction of the new storm tank, additional storage capacity is now available, reducing the likelihood of uncontrolled storm discharges. Storm flows are conveyed to the tank via gravity, filling progressively during rainfall events. Once storm conditions subside and treatment capacity becomes available, stored flows are returned to the treatment process using the dedicated return pump.

The inclusion of the external mixer jet ensures that solids remain mobilised during storage and return operations, reducing the risk of deposition, odour generation, and maintenance issues. This integrated approach enhances overall treatment resilience and environmental performance.

[caption id="attachment_31534" align="alignnone" width="1200"] New storm pump and storm tank mixer - Courtesy of Galliford Try[/caption] Environmental considerations, innovation & sustainability

Environmental protection and biodiversity enhancement formed an integral part of the Uffculme Storm Storage Scheme. In addition to reducing storm spill frequency and improving river water quality, the project team implemented measures to enhance local habitats within the site boundary.

Hibernacula were created to provide refuge and overwintering habitats for small mammals and reptiles. These structures were carefully located to avoid operational areas while providing long-term ecological benefit. In recognition of this initiative, the project team was awarded a Galliford Try Environmental Award, highlighting the scheme’s positive contribution to biodiversity.

From a carbon and sustainability perspective, the selection of an above-ground stainless steel tank provided several benefits. The solution reduced the need for deep excavation and extensive concrete pours, thereby lowering embodied carbon and construction risk. The gravity-based design further minimised operational energy requirements over the asset’s life.

Health, safety & delivery performance

Health and safety performance was a key priority throughout the project. Works were delivered without any lost time incidents, demonstrating effective planning, risk management, and collaboration between Galliford Try, subcontractors, and South West Water operational staff.

Particular attention was paid to managing interface risks associated with live services, operational wastewater assets, and restricted access routes. The use of vacuum excavation, controlled work sequencing, and regular coordination meetings contributed to the scheme’s excellent safety record.

The project was delivered to a high standard of quality, completed on programme, and met all client and regulatory requirements.

[caption id="attachment_31539" align="alignnone" width="1200"] (left) Jacopa wave screen and (right) outlet/overflow chamber - Courtesy of Galliford Try[/caption] Conclusion

The Uffculme WwTW Storm Storage Scheme represents a successful example of targeted investment to improve environmental performance at a small rural wastewater treatment works. Through careful optioneering, innovative design, and effective collaboration, the project team delivered a resilient and efficient storm storage solution that significantly reduces storm overflow risk.

The scheme was completed on time, safely, and to the required quality standards, while also delivering added value through biodiversity enhancements and reduced carbon impact.

As such, it provides a positive case study for similar storm resilience projects across the UK water sector.

Guildford, the county town of Surrey, sits a commutable 30 miles south west of London and 70 miles from the south coast. Boasting a castle, a cathedral, a picturesque town centre and a university, it is surrounded by Green Belt countryside. The River Wey flows through the town centre and then northwards through water meadows where it provides the outfall for the STW. The Wey eventually discharges into the Thames. The current STW serves a population equivalent (PE) of around 107,000, but the town has a growing population which is expected to grow to circa 125,000 PE by 2041. Background

The town and immediate area are currently served by a sewage treatment works established at the turn of the last century and upgraded over the years. It is located beside the River Wey, to the north of the A3 trunk road and north of Guildford town centre. It lies just south of Slyfield Industrial Estate. The current STW has a capacity to cater for a population equivalent of around 120,000.

Guildford Borough Council’s Local Plan adopted in 2019 allocates a site to the west of the River Wey wrapping around the eastern side of Slyfield Industrial Estate for mixed use development. Part of the 40 Ha allocated site is currently occupied by the existing sewage treatment works.

Weyside Urban Village is a mixed-use regeneration project which will provide some 1,500 new homes, new community facilities, Gypsy and traveller pitches and 6,500m2 of additional light industrial and trade space. 11Ha of the site are allocated for waste management purposes. The plans include a new council depot, a public recycling centre and a new, relocated sewage treatment works.

In order to bring the development site forward, within the plan period to 2034, Guildford Borough Council (GBC) have entered into partnership with Thames Water Utilities Ltd (TWUL) to build the new STW as the first phase of the redevelopment. The new location is slightly further north on the eastern edge of the Industrial site and is a brownfield site, formerly a gravel quarry and subsequently a landfill tip.

[caption id="attachment_26867" align="alignnone" width="1200"] Figure 2: Site overview: BIM model of new Guildford STW - Courtesy of BAM Enpure JV[/caption]

The new sewage treatment works allows for an increase in capacity to serve a population of 125,000 allowing for predicted population growth in the town and surrounding area as well as more stringent effluent discharge standards.

The new site also includes areas within the new works for future expansion and increased treatment capacity as need arises.

Previously in Water Projects

The scheme to build a new sewage treatment works (STW) in Guildford is now mid-way through construction. This article provides an update to the project following on from previous articles published in Water Projects. In 2023, Enpure Ltd’s Project Delivery Director Shaun Christopher described in detail the process and assets being provided in the new works and in 2024, Dan Russell, BAM’s Operational Lead, gave an update on the progress of the project one year into the construction.

This article also covers some of the planning considerations and requirements for the scheme.

Contract

In 2021 TWUL awarded a contract to a BAM-Enpure Joint Venture to design, build and commission a new STW for Guildford, and once operational, to de-commission the existing works. BAM-Enpure Joint Venture (JV) is a non-integrated Joint Venture, formed specifically to combine experienced civil engineering contractor BAM with the process engineering MEICA specialism of Enpure, to deliver a turnkey solution for TWUL.

Construction began in the spring/summer of 2023 with BAM undertaking design and construction of the civils works and Enpure providing process and MEICA design and installation. BAM have employed Arcadis as their main civils designer and the JV has let a number of specialist sub-contractor design packages alongside.

[caption id="attachment_26869" align="alignnone" width="1200"] Figure 3: 3D design using a model hosted by Arcadis, image from Autodesk’s Navisworks viewer - BAM Enpure JV & Arcadis[/caption] The project

The project consists of the following:

A transfer tunnel. An inlet pumping station. Inlet works screening. Primary settlement. Activated sludge treatment. Final settlement. Tertiary treatment of final effluent. Chemical dosing facilities. Storm storage tanks. Import cess and sludge facilities. A new control centre. New welfare building. Outfall into the River Wey.

A detailed description of the process and assets being installed is set out in the previous Water Projects article published in 2023.

Timeline Phase 1: Ground improvement: Started summer 2023 and completed February 2025 Phase 2: Civil engineering: Started summer 2023 with completion due in late 2025. Phase 3: Mechanical & electrical engineering: Started autumn 2024 and is due to complete summer 2026. Phase 4: Commissioning: Starts winter 2025 and is due to complete in spring 2027. [caption id="attachment_26883" align="alignnone" width="1200"] Figure 4: 3D model of the sludge processing area - Courtesy of BAM Enpure JV & Arcadis[/caption] Developing the site

The new site comes with additional challenges in that it is brownfield site formerly used for gravel extraction and then landfilled through the 1960s, 70s and 80s, with mainly domestic and some commercial waste. The site is classified as a waste development such that the local planning authority is Surrey County Council (SCC).

The planning permission granted was subject to a number of conditions and commitments, not only relating to the siting and development of the STW itself such as visual impact, odour, noise and traffic generation, but also to the remediation and reuse of the landfill site. Plans and construction methods needed to ensure that there was no temporary or permanent environmental damage to the surrounding area as a result of the impact on or disturbance of the buried waste material and that operatives working on the new STW were protected from any ill effects of the landfill during operation of the site. The landfill also poses particular constraints and hazards during the construction phase.

The strategy for the site was to leave as much of the landfill waste in situ and build above and through the waste. A detailed remediation strategy formed part of the planning requirements. The plan set out how the construction and operation phases of the project would manage issues of ground water and ground gas, potential ground water contamination and leachates, contaminated solid material including asbestos, and short- and longer-term settlement of the landfill.

Dan Russell’s case study in Water Projects 2023 provided a detailed description of the use of a combination of ground improvement and foundation techniques, which consisted of:

Rapid Impact Compaction (RIC) to improve ground strength and reduce long-term settlement of the landfill. Vertical Rigid Inclusions (VRI), a displacement concrete pile technique which provided support for roads, lighter structures pipes and ducts. Driven concrete piles bedded in the London Clay below the land fill to support large structures, tanks and pipelines.

Figures 5 (below) show the historic gravel extraction map and depth of landfill determined by ground investigation - both images were extracted from the project’s Detailed Remediation Strategy.

[caption id="attachment_26880" align="alignnone" width="1200"] Figures 5: (left) Spatial variation in landfill thickness 2023 and (right) historic site layout (1976) with present day boreholes overlaid - Courtesy of Arcadis[/caption]

However, the impact of the waste material goes beyond its bearing capacity. The site was already being subject to long term monitoring for ground gas and groundwater quality by Thames Water to provide a baseline for the project. Additional boreholes were sunk and a Groundwater & Ground Gas Monitoring Plan was approved by the local authority. Daily and continuous automatic remote monitoring was undertaken during the ground improvement and excavation phases with trigger levels set and exceedances reported back to SCC and a requirement for quarterly summary reporting. Monitoring will continue during commissioning of the site at less frequent intervals.

Asbestos: Asbestos had been discovered in a third of samples taken during the ground investigation and so a monitoring regime was established. Air sampling around the perimeter of the site ensured any airborne particles were detected and works could be stopped if necessary. Proximity asbestos monitoring and identification of asbestos material in excavation has also been used extensively during excavation across the site. The project used Socotec to undertake asbestos monitoring and testing.

Ground gas: A ground gas protection design has been necessary to reduce risk of ground gas entering occupied buildings, kiosks, tanks and chambers where operational or maintenance access is undertaken. A ground gas design protection strategy was set out by Arcadis with PAGeo Contracting Ltd providing expert advice and a detailed ground gas design, PAGeo then supplied gas dispersal matting, venting components and gas membranes. Both the strategy and the detailed design for ground gas protection have been submitted to and approved by SCC.

Independent verification of the gas protection installation has been provided by MTS Ltd as verification of the installation and subsequent reporting to SCC is a planning condition.

[caption id="attachment_26872" align="alignnone" width="1200"] Figure 6: Gas protection void former being laid on stone blanket prior to tank base blinding, Connections to vents at tank perimeter - Courtesy of BAM[/caption]

Surface drainage: On-site surface drainage has been carefully designed such that new impermeable areas are positively drained to the site drainage system to remove the risk of a spill causing environmental pollution. This surface water is then fed into the inlet pumping station for processing through the works. As the permeable area of the site has been greatly reduced this also significantly reduces leachate mobilisation.

Biodiversity: Another planning condition is the provision of Biodiversity Net Gain (BNG). In conjunction with GBC, the project provides 10% net biodiversity gain by use of on-site planting and off-site enhancement of the land directly to the north of the new STW. A Site of Natural Green Space, biodiversity enhancements to an area of farmland, and a marshy meadow along the River Wey are being provided.

The Environment Agency have also constructed a new fish pass in this area allowing fish to migrate around Old Bucks Weir.

Guildford STW: Civils works supply chain - key participants

Main contractor: Bam Nuttall Ltd Planning consultant: Adams Hendry Consulting Ltd Civil engineering design: Arcadis Group Ltd Boreholes & testing: Geo-Environmental Services Ltd Asbestos monitoring: Socotec Ltd Rapid impact compaction: Cofra Ltd (Boskalis Group) Vertical rigid inclusions: Foundation Piling Ltd Concrete piling: Aarsleff Ground Engineering Ltd Temporary works specialist: Mabey Hire Ltd Temporary works specialist: MGF Ltd Tunnels & shafts: Joseph Gallagher Ltd In situ concrete works: Kelly Formwork Ltd In situ concrete works: DJ Civils Ltd Precast concrete manufacturer: FLI Precast Solutions Precast concrete manufacturer: A-Consult Ltd Pipe supplier: Electrosteel Castings (UK) Ltd Pipe supplier: Saint Gobain PAM UK Soil processing: Remediation Waste Management Services Concrete supplier: Heidelberg Materials Gas protection: PAGeo Contracting Ltd Gas protection system installation verification: MTS Ltd [caption id="attachment_26876" align="alignnone" width="1200"] Figure 7: Sludge processing area: MCC kiosk (grey in foreground), sludge blending tanks (top left), poly dosing kiosks (top right) - Courtesy of BAM[/caption] Overview of project progress

As of summer 2025, civils works are nearing completion and MEICA fit out has started.

The installation of piled supports has been undertaken, and all major structures are complete. Installation of the below-ground pipework and buried ducting is in progress and the welfare building is watertight and available for equipment installation. Moving towards installation of MEICA equipment, the water testing of structures started in the February of 2025.

Steel staircases and high-level walkways are mostly complete. The inlet screens and grit removal system are in place. GRP tanks for sludge processing are being installed and many control kiosks are in place. Electrical supply buildings are near completion.

MEICA work commenced in February 2025, overlapping with completion of civils to maximise programme efficiencies. Designed alongside the civils elements of the site, the MEICA installation has taken place apace with the first bridges on the final settlement tanks attached, the larger glass coated steel tanks for the sludge area in place, and generators and some kiosks have been installed.

BAM has started the installation of the above-ground pipework and cable pulling in buried ducts, and the fit out of control rooms will commence shortly. Construction of MCC basements has been completed and two of the three MCC kiosk, and power supply and distribution works are nearing completion.

[caption id="attachment_26878" align="alignnone" width="1200"] Figure 8: Sludge processing area: (left) the bases for elevated cake silos, (middle), sludge belt presses, and (right) the poly prep & dosing kiosks - Courtesy of BAM[/caption] Guildford STW: MEICA works supply chain - key participants Main designer/contractor: Enpure Ltd CFD analysis: The Fluid Group Cake silo: Saxlund International PST scrapers & ferric dosing plant: Colloide Inlet works & PST odour covers: Power Plastics Ltd Grit remover plant: Jacopa Escalator screens: Longwood Engineering Imported sludge Strainpress: Huber Technology Sludge thickening & dewatering: Kent Stainless Ltd Odour control plant: Odour Services International Ltd FBDA grids & blowers: Xylem Water Solutions Cloth pile filters: Eliquo Hydrok Ltd Storm tank cleaning system: Sulzer Pumps Wastewater Ltd Tank logging system (WASP): JR Pridham Services Ltd Glass lined steel tanks: Balmoral Tanks Ltd Sludge tank mixing: Landia UK Ltd Tunnel

A 1.5m diameter gravity tunnel, 1.5km long, transfers flows from the existing works and intercepts minor sewers en route. There are six shafts along the tunnel route, some of which act as junctions between the new tunnel and the existing sewers. Shafts were sunk by Joseph Gallagher using the caisson method and the tunnel was driven between them using a full-face tunnel boring machine, (named Daphne by a local school). The tunnel and shafts were completed in 2024 and the final work remaining are the completion of the large inlet pumping station (IPS), shaft (E2) at the new works and connection of Guildford’s main sewer to the head of the tunnel, which will allow all flows to be diverted to the new tunnel.

[caption id="attachment_26873" align="alignnone" width="1200"] Figure 9: (top left) 3D model of the Inlet Pumping Station (IPS) - Courtesy of Arcadis, (bottom left): steel fixing to IPS internal walls - Courtesy of BAM Enpure JV, and (right): IPS shaft construction - Courtesy of BAM[/caption] Inlet pumping station

The new inlet pumping station (IPS) is a 12.5m diameter shaft with a depth of 17m, which houses six pumps with the capacity to lift flows of around 2,500 l/s from the tunnel entry, at the base of the shaft, to the inlet works where all flows are screened. The shaft is complete except for the internal walls and installation of penstocks which allow isolation of half the well at any time for maintenance of the pumps. This will be completed in the autumn of this year.

Storm tanks & inlet works

Two storm tanks provide a combined capacity of approximately 8 million litres to which flow can be diverted to temporarily hold sewage arriving at the new works should flows exceed the capacity of the works. Constructed onto in situ bases, the walls of the storm tanks were prefabricated, placed and post-tensioned by A Consult Ltd.

The inlet works and larger chambers were entirely cast in situ due to lack of repeatable sections and logistics. In situ concrete work for these was undertaken by Kelly Formwork. The inlet screens have been installed and walkways are in place.

[caption id="attachment_26874" align="alignnone" width="1200"] Figure 10: Circular precast storm tanks and Inlet works (August 2025) - Courtesy of BAM[/caption] Primary settlement, activated sludge plant & final settlement tanks

Screened flows from the inlet works will then pass through primary and secondary treatment tanks comprising rectangular primary settlement tanks (PSTs) and activated sludge plant (ASP) tanks. The walls for these tanks were designed and supplied by FLI Precast Solutions Ltd and installed by BAM and DJ Civils Ltd on in situ bases prepared by Kelly Formwork Ltd. These tanks are complete with pipework being fitted and the ASP has commenced water testing.

To ensure watertightness of the larger rectangular tanks, Heidelberg Materials and Sika’s concrete technology were used.

Concrete containing Sika WT-200 P was utilised for the PSTs and the ASP. Sika WT-200 P is a combined water-resisting and crystalline waterproofing admixture used to reduce the permeability of concrete, making it suitable for water retaining structures.

Gas protection has been laid under all large tanks. Materials were supplied by PAGeotechnial Ltd and verified by MSL Ltd. Void former mating laid on a bed of single grade 40mm stone provides a preferential escape path for ground gases and is connected to vents at the tanks periphery allowing ground gases to vent to atmosphere.

These tanks are not habitable spaces and only require a dispersal system to discourage any build-up of gas below the large tank bases.

[caption id="attachment_26875" align="alignnone" width="1200"] Figure 11: From top: Activated sludge plant, primary settlement tank, and two of the four final settlement tanks - Courtesy of BAM[/caption] Final settlement tanks

The four final settlement tanks were also precast, supplied, installed and post-tensioned by A-Consult Ltd onto in situ bases prepared by Kelly Formwork Ltd. The FSTs are complete with bridges and scrapers installed and are currently being water tested.

Tertiary treatment plant & outfall

Following final settlement, flows will pass through final filtration in the tertiary treatment plant before being discharged to the River Wey via new, twin outfall pipes. Civil construction is complete and MEICA installation of cloth pile filters and drums from Eliquo Hydrok Ltd is underway.

The outfall pipework has been fully installed across the water meadow and the headwall has been constructed at the discharge point into the River Wey.

[caption id="attachment_26877" align="alignnone" width="1200"] Figure 12: Tertiary treatment plant construction (August 2025) - Courtesy of BAM[/caption] Sludge processing

The project also provides a dewatering facility for imported sludge from other small treatment works. This area is nearing completion with the larger coated glass sludge blending tanks in place and the smaller buffer tanks being installed. Gas vent pipes have been installed below the extensive concrete slabs to allow venting of ground gases to the atmosphere.

The sludge belt presses from Kent Stainless Ltd for sludge dewatering and thickening have been installed adjacent to the bases for the elevated sludge cake silos. The silos from Saxlund International have been sized to provide storage of seven days’ worth of processed sludge, which can be discharged to trucks for export from site.

Power & control

Power supply and distribution infrastructure is now in place. The three MCCs have been constructed in sequence and the first two are being fitted out.

Welfare building

A new welfare building houses the automated systems for running the new plant and controlling effluent quality as well as office and mess space for Thames Water staff and operatives. The building is watertight with cladding being installed and the fit out of building fixtures and plant control equipment is in progress.

[caption id="attachment_26871" align="alignnone" width="1200"] Figure 13: Welfare building housing plant control systems - Courtesy of BAM[/caption] 3D modelling

Design has utilised a 3D model hosted by Arcadis with each designer adding in modelling information as their design capabilities have allowed. This has enabled precise integration of designers individual input and interrogation of the integrated design

The model is used extensively by engineers on site. 3D modelling has, for example, allowed duct chambers to be precast with precision positioning of cable openings, saving on in situ working and substantially reducing programme on these installations.

Piling & ground improvement

To enable the STW to be constructed on the site without the waste being removed a combination of ground improvement techniques have been employed including driven concrete piles, Rapid Impact Compaction, and Vertical Rigid Inclusions.

The Rapid Impact Compaction work undertaken by Cofra was completed in late summer of 2023. From July 2023 to the spring of 2024, to support the larger structures, tanks and main process critical pipelines, 3500 concrete piles were driven through the landfill into the clay. The design and installation of this piling operation was undertaken by Aarsleff Ground Engineering.

[caption id="attachment_16135" align="alignnone" width="1200"] (left) Driving precast concrete piles and (right)Vertical Rigid Inclusion installation - Courtesy of Bam Nuttall Ltd[/caption]

In areas where settlement criteria were less critical, but needed controlling (for example roadways and minor pipework), a Vertical Ridged Inclusion (VRI) piling technique was used. The system involves displacement drilling a column in the landfill and filling the void with concrete. This works by consolidating the soil around the pile, transferring load to lower layers, thus increasing bearing capacity and reducing settlement over time.

Since 2024 the installation of VRIs by Foundation Piling has been completed with 7500 inclusions installed across the site. During detailed design, the project was able to remove the need for a load transfer platform above the VRIs across much of the site. This was done by detailed analysis of the 3D model to target differing requirements for support and by reducing inclusion spacings, thereby reducing the expected landfill removal by 37,500m3.

The load transfer platform remains below roads, but pipework sits within the grid of VRIs.

The year ahead

Completion of the mechanical works is expected in the first part of 2026 with installation of electrical and control systems expected to complete in the second part of 2026. Dry and wet commissioning will be undertaken sequentially across the site throughout the coming year, ensuring the new works is ready to commence process commissioning.

The final, and arguably most challenging activity, will be to connect the existing sewer network to the new tunnel, diverting flows from the existing STW to the new STW. A series of interventions will be required to intercept the various sewers, along the route of the tunnel, all the whilst ensuring the new STW is performing within strict limits and the existing STW is able to maintain its performance during the ramp down of flows.

Yorkshire Water’s Pocklington Sewage Treatment Works is located in the East Riding of Yorkshire. The site processes wastewater from the local catchment to safely return it to the environment, discharging treated effluent into the nearby Pocklington Beck. Yorkshire Water has delivered a £7.3m project to reduce the levels of phosphorus present in the treated wastewater returned to the environment and improve the water quality of more than 2km of the river downstream of the treatment works. Project drivers

The AMP 7 WINEP drivers for Pocklington STW include a reduction in phosphorus and iron by December 2024 and a UIMP6 storm storage capacity increase by March 2025. The revised Environmental Agency Permits were:

Parameter Existing Value New Value Total P (annual average) 1 mg/l 0.25 mg/l Iron (95%ile) N/A 4 mg/l Iron (upper tier) N/A 8 mg/l UIMP6 storm capacity 658m3 766m3 Existing works

The existing works comprise a screened inlet arrangement incorporating grit removal and screenings handling, with flows conveyed by gravity to a storm overflow weir chamber. Under normal operation, the main flow passes to the primary settlement tank (PST) distribution chamber and is treated across three primary settlement tanks. Subsequent treatment proceeds through four trickling filters before discharge to the humus sludge handling system via a screw feed, and onward to dual humus tanks for secondary clarification. From here, clarified effluent is directed to a high-lift pumping station, which transfers flows to the tertiary nitrifying sand filter for final polishing. The treated effluent then passes through a sampling chamber prior to discharge to the receiving watercourse.

[caption id="attachment_31381" align="alignnone" width="1200"] Inlet works - Courtesy of Enpure Ltd[/caption] Scope of works

The objective of the project was to achieve compliance with tightened phosphorus discharge consent in the final effluent, without breaching the revised total iron limits.

To meet these requirements, a comprehensive programme of process, mechanical, electrical, and civil upgrades was undertaken across the site, including the following key elements:

Chemical dosing

The existing chemical dosing kiosk was replaced with a new primary and secondary stage chemical storage and dosing system. This comprised twin internal storage tanks with a combined capacity of 20m³, together with duty/standby dosing pumps configured for automatic changeover. The installation included primary and secondary dosing lines with defined points of application, as well as associated safety infrastructure, including a safety shower.

Nitrifying sand filters

Refurbishment of the tertiary nitrifying sand filter (TNSF) system was carried out, including replacement of the process air compressors and decommissioning of the redundant process air blowers. The works also incorporated a new backwash pipeline and drainage chamber arrangement, complete with flow measurement, together with the provision of an additional washwater connection and hydrant.

High lift pumping station

Upgrades to the high-lift pumping station included the installation of new pumps and associated pipework, along with servicing and refurbishment of the existing bollfilter to ensure reliable downstream flow transfer to the tertiary treatment stage.

[caption id="attachment_31383" align="alignnone" width="1200"] Overview of storm tanks and chemical dosing plant - Courtesy of Enpure Ltd[/caption]

Storm capacity

The site’s storm storage capacity was increased from 658m³ to 766m³ through raising the coping levels of the existing tank walls and overflow weirs. In addition, structural modifications were undertaken to facilitate the replacement of the existing half-bridge scraper within the radial blind storm tank.

Electrical & control systems

Significant electrical and control system upgrades were also delivered. These included modification of the inlet works motor control centre (MCC) to accommodate the new dosing installation and scraper replacement, and upgrades to the sand filter kiosk MCCs to integrate the new TNSF equipment and allow for decommissioning of redundant assets.

Further works included modifications to all the MCCs by CEMA Ltd to enable integration into a new ethernet-based control network, installation of additional instrumentation and asset monitoring signals, and the provision of a new telemetry interface programmable logic controller (PLC) within the storm returns pumping station kiosk.

Ancillary works included the installation of site lighting to support new plant, process areas, and access routes, together with the associated electrical, control and instrumentation (EICA) systems necessary to support the upgraded infrastructure.

[caption id="attachment_31377" align="alignnone" width="1200"] Primary settlement tank distribution chamber - Courtesy of Enpure Ltd[/caption] Pocklington STW: Supply chain - key participants Main designer & contractor: BEJV Civils: Barhale Process & M&E: Enpure Ltd Electrical installation: Circle Control Mechanical installation: JBF Group Mechanical installation: Tushingham Steel Fabricators Ltd MCCs, PLCs & SCADA: CEMA Ltd Ferric dosing: IIES (Northern) Ltd Kiosks: Morgan Marine Ltd Safety showers/eyebaths: Aqua Safety Showers International Ltd Tertiary nitrifying sand filters: Hydro International Ltd Scraper bridge refurbishment: WGM Engineering Ltd Pumps: Xylem Water Solutions Valves: Cotswold Valves Ltd Trace heating & lagging: Jade Insulation Flow meters & level transmitters: ABB Hose reels: ARCO Ltd Inlet works

The existing works inlet consists of two Haigh Engineering screens complete with screenings handling and a grit removal system on the back end, which leads into the storm chamber.

Chemical dosing

To achieve compliance with the revised phosphorus discharge consent, implementation of both primary and secondary stage coagulant dosing was required. This was delivered via a purpose-built ferric dosing kiosk complete with integral chemical storage and a service water booster system. Given the nature of the chemical handling operations, a dedicated off-loading facility was also provided, comprising a bunded tanker delivery apron, an interlocked delivery point, and an emergency drench shower to meet safety and environmental requirements.

[caption id="attachment_31373" align="alignnone" width="1200"] Chemical dosing kiosk and safety shower - Courtesy of Enpure Ltd[/caption]

The new ferric storage and dosing installation was designed to provide sufficient capacity and dosing control to consistently achieve the required phosphorus removal performance. The system incorporates approximately 10 days of on-site chemical storage and was factory-tested prior to delivery to minimise site commissioning risk. The site layout and bunded delivery arrangement were configured to allow a ‘drive-in, drive-out’ tanker operation, improving both safety and operational efficiency.

The primary dosing system was initially configured to deliver coagulant into the primary settlement tank (PST) distribution chamber. However, following site-based process optimisation trials, the dosing point was relocated to the inlet works outlet chamber to improve mixing conditions and enhance phosphorus removal efficiency.

Secondary stage dosing locations were similarly subject to on-site trials to determine the most effective point of application. The preferred primary secondary dosing location was established at the base of the humus tank screw feed, with a secondary contingency dosing point provided at the existing HST distribution chamber. Both points of application were configured with duty/standby dosing arrangements and automatic changeover to ensure operational resilience and continuity of treatment performance.

Tertiary Nitrifying Sand Filter (TNSF)

The tertiary nitrifying sand filter operates on a counter-current flow principle. Influent water enters via a distribution system located at the base of the unit and is treated as it flows upward through the granular media bed, in this case sand, prior to discharge through the filtrate outlet at the top of the reactor.

Biological nitrification is achieved by autotrophic nitrifying bacteria colonising the media. To sustain this process, adequate oxygen transfer and carbon dioxide availability are essential. This is facilitated by aeration hoods positioned within the upper region of the media bed, supplied with process air to maintain optimal dissolved oxygen concentrations and promote effective ammonia oxidation to nitrate. The resulting nitrates remain in solution within the treated effluent.

[caption id="attachment_31380" align="alignnone" width="1200"] Tertiary nitrifying sand filter - Courtesy of Enpure Ltd[/caption]

The media bed simultaneously functions as both a biological reactor and a physical filtration system, capturing suspended solids and biomass. To maintain treatment efficiency, the media undergoes continuous regeneration. Solids-laden media is drawn from the base of the unit via an airlift pump and conveyed to an integral media washing system located at the top of the filter.

Initial separation of impurities occurs within the airlift riser due to turbulent mixing. The media then discharges into a washing chamber (labyrinth), where it is subjected to a controlled counter-current flow of clean water. This process removes retained solids and sloughed biological growth.

The waste stream is discharged via the wash water outlet, while the denser, cleaned media is returned to the bed. This mechanism induces a slow, continuous downward movement of the media through the filter. Compressed air for the airlift system is supplied via a dedicated air distribution panel.

This continuous cycle of filtration and in situ media cleaning enables uninterrupted operation, eliminating the need for periodic backwashing and ensuring consistent treatment performance.

This element of the contract was awarded at an early stage whilst developing the remaining project scope. This approach allowed focused attention on this key component of the project, which involved confined space entry, along with a comprehensive condition assessment and overhaul of an existing, previously disused asset. This strategy also supported carbon savings and contributed to the circular economy.

The existing TNSF units were refurbished as part of the works. BEJV engaged Hydro International to deliver key process upgrades, including the supply and installation of two new compressors with acoustic enclosures, replacement filter media, full renewal of internal pipework, and system commissioning to restore operational performance and reliability.

[caption id="attachment_31379" align="alignnone" width="1200"] TNSF compressors - Courtesy of Enpure Ltd[/caption] Storm tanks

The original project scope proposed refurbishment of the existing storm tank, alongside the provision of additional capacity by increasing the coping level of the adjacent square storm tank.

However, during inspection under dry weather flow conditions, it became evident that the existing storm tank had deteriorated beyond economical repair due to its age and original construction. Consequently, the agreed solution was to demolish the existing structure and construct a new tank within the same footprint.

To maintain compliance with the site’s storm storage permit throughout the construction period, a temporary strategy was implemented. This comprised both temporary over-pumping arrangements and interim storm storage provision.

These works introduced additional logistical and operational constraints, including the requirement to secure temporary access to adjacent third-party land to accommodate temporary storm tanks while demolition and reconstruction activities were undertaken.

In addition, a separate ‘blind’ storm tank was incorporated into the overall strategy to supplement storm storage capacity. The initial scope included replacement of the existing scraper bridge and removal of the launder troughs, however, following a detailed condition assessment, the existing assets were found to be structurally sound and suitable for refurbishment.

The adopted solution involved raising the scraper bridge by approximately 300mm and applying a protective re-coating system. This approach delivered a cost saving of approximately £50,000 to the client while maintaining operational performance and extending asset life.

[caption id="attachment_31375" align="alignnone" width="1200"] Storm tanks - Courtesy of Enpure Ltd[/caption] Status

The upgraded works at Pocklington STW successfully met the regulatory compliance date, with all process-critical improvements completed by 31 March 2025. The overall project was subsequently finalised in March 2026 following the completion of additional works.

This scheme has significantly enhanced the treatment capability for incoming raw water, while stabilising throughput and increasing operational resilience. As a result, it delivers a robust and sustainable improvement to the local Yorkshire Water network, providing long-term benefits for the communities it serves.