Supply Chain - Precast Concrete - Tank Wall Panels

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.

Located near New Inn, South Wales, this innovative nature-based solution (NbS) project addresses the water quality impact from a storm overflow (SO) discharging into the Afon Lwyd during wet weather. Water quality modelling classified the existing high spilling asset as having a ‘Severe Impact’ on the river which under the Storm Overflow Assessment Framework (SOAF), prompted investigation, optioneering and construction of the most cost-effective solution to deliver a ‘No Impact’ outcome. Using a multi capitals assessment, the team evaluated cost, carbon and long term value to both nature and society. This showed that prioritising a nature-based solution delivered substantial a cost saving (£43m) and carbon savings (half the embodied carbon) against its grey equivalent solution, a 33,500m3 storage tank, alongside wider community and environmental benefits. Root cause and optioneering stage

During the optioneering stage, a range of interventions were assessed to address the storm overflow impact, including catchment‑scale rainfall disconnection, increased pass‑forward flow, nature‑based treatment and conventional storage. Catchment disconnection was discounted due to the high levels of customer disruption required, while increasing pass‑forward flow was not feasible due to downstream hydraulic constraints. Given the overflow’s location adjacent to open land by the river, a nature‑based solution was identified as the most effective and deliverable option.

[caption id="attachment_29039" align="alignnone" width="1200"] The field pre-construction used for occasional grazing, with the existing storm overflow situated within the trees to the right - Courtesy of Arup[/caption]

A two-stage approach was chosen to maximise success, with an aerated reed bed providing initial intensive solids capture followed by two Integrated Constructed Wetland (ICWs) in series, providing effluent polishing and an opportunity for the industry to research wetland treatment in this application.

The favourable topography enabled a gravity system, eliminating the need for lifting flows and in turn, reducing operational energy demands. The field’s proximity to New Inn’s residents also allowed for place-making features for the local community, such as footpaths, an outdoor classroom and natural play areas, enhancing community engagement and educational opportunities.

The indirect monetary value of these wider benefits was captured in the decision-making stage via the use of CIRIA’s B£ST estimation tool, which estimated £12m in realised benefits over the next 40 years across social, natural and other capitals; something none of the grey solutions offered.

[caption id="attachment_28938" align="alignnone" width="1200"] Site layout - Courtesy of Arup[/caption] Design stage: New compound

The existing storm overflow (SO) was located amongst trees on the edge of the field. The incoming pipe laid in the eastern corner of the field meant the SO could be relocated into a new compound, which will contain the following chambers and plant:

Screening chamber: The screening chamber houses an orifice flow control and a new mechanical screen that removes solids from excess flows. Only flows above the permitted pass‑forward rate are diverted to the reed bed, ensuring that the treatment process receives appropriate, pre‑screened volumes. Bypass chamber: This chamber is designed to divert the highest flows from the catchment before they enter the nature‑based solution. The flow threshold is set so that around 95% of the total discharge volume still passes through the NbS, with the bypass only operating during the most dilute, peak‑flow conditions. MCC kiosk: New kiosk to house the motor control centre. Aeration blower kiosk: An acoustically insulated kiosk to house the blowers for the aerated reed bed.

Design for Manufacture and Assembly principles were adopted to several elements of the compound. The screening and bypass chamber were fabricated off site by FP McCann Ltd, reducing the need to cast these chambers in situ.

Both the kiosks were prefabricated off-site by Morgan Marine Ltd to save time and reduce on-site activities in the construction phase.

[caption id="attachment_28921" align="alignnone" width="1200"] The new compound (March 2025) - Courtesy of Welsh Water[/caption] Design stage: Reed bed & integrated constructed wetlands

The design team carried out a catchment Flow & Load Survey to ensure a better understanding of the design parameters required to achieve the ‘No Impact’ outcome. The survey gave insight into the challenges around pollutant loads in intermittent discharges, with varying concentrations of biochemical oxygen demand (BOD) and ammonia found in the excess diluted sewage; especially the difference between accommodating the ‘first flush concentration’ and weaker dilutions as a storm event continued.

Working collaboratively with the reed bed and ICW suppliers early in this stage, increased confidence in achieving the necessary pollutant removal in the final effluent.

A Flood Consequence Assessment (FCA) was completed as part of the planning application and the findings required the reed bed and wetlands to be moved further away from the river, reducing the size of the originally allocated footprint. The revised layout however, still provided sufficient treatment area (3,000m2 for the reed bed and a combined ICW treatment area of 2,400m2) and created additional space for the public to enjoy.

Planning & stakeholder engagement

The planning application was critical to the success of this project, which was well received by local councillors and relevant bodies, in part, due to the extensive up-front work done by Arup’s planning team. Arup specialist teams carried out predictive odour and noise modelling of the solution to address concerns raised by the planning authority. The project team also engaged extensively with the local community through both in-person and online events, ensuring the design met local needs and preferences.

To align with the Sustainable Approval Body (SAB) requirements as part of planning approval, the design team implemented sustainable drainage features across the site. The maintenance track and vehicle areas in the compound were constructed from permeable TruckCell™ paving, made from recycled plastic, whilst vegetated swales and depressions were incorporated alongside the impermeable access track and footpaths. This reinforced the project’s low‑carbon and sustainable design ethos.

[caption id="attachment_28924" align="alignnone" width="1200"] Maintenance track constructed out of TruckCellTM - Courtesy of Arup[/caption] Pont-y-felin Lane: Supply chain - key participants Main designer/contractor: Morgan Sindall Infrastructure Lead designer & planning team: Arup Reed bed: Natural Wastewater Solutions (NWS) Integrated constructed wetland: Salix River and Wetland Services Vesi Environmental Ltd Aeration blowers: AERZEN Machines Precast screening & bypass chambers: FP McCann Ltd Chamber covers: Steelway Brickhouse Mechanical screens: Huber Technology Mechanical installation: Industrial Pipework Services MCC: Lloyd Morris Electrical Ltd MCC kiosk: Morgan Marine Ltd TruckCell™ paving: ABG Geosynthetics Construction stage

Construction started in January 2024, with Morgan Sindall Infrastructure mobilising to bring the nature-based solution system online before the regulatory deadline of 31 March 2025. The team navigated challenges including invasive species within the compound area and persistent groundwater during the large excavations, requiring an extensive dewatering activity (under approved Abstraction Licences).

Working with the sub-contractors, Natural Wastewater Solutions (NWS) for the reed bed, and Salix River and Wetland Services and Vesi Environmental Ltd for the integrated constructed wetland, 33,904 tonnes of soil were removed and the ground re-profiled. 30,000 tonnes was re-used off site, whilst 3,000 tonnes was recycled and re-used on site, with the final 904 tonnes taken to landfill, due to the presence of Japanese knotweed.

[caption id="attachment_29040" align="alignnone" width="1200"] Site drone photos throughout construction (January 2024 - June 2025) - Courtesy of Welsh Water[/caption]

Due to the impermeable nature of the soil, the reed bed and constructed wetland both required under-draining to minimise the impact on the groundwater table. Once the under-drains were installed, the basins were constructed, which included the following:

12,000 plants throughout the three basins, including: Phragmites planted for the reed bed and; Lesser pond sedge, reed sweet grass, lesser reedmace and water mint for the ICWs. 6,100m2 of liner across the three basins. 17,500m of aerated pipeline installed. 3,710 tonnes of stone.

The site team pushed to see the first flows through the system just 12-months later, ahead of the 31 March deadline and handled the heavy rainfall from Storm Eowyn on 24 January without any issue.

Monitoring

A robust sampling regime is currently underway to understand the performance of the individual stages and final effluent from the nature-based solution. This will inform future permitting of these types of solutions in cooperation with the environmental regulator, Natural Resources Wales (NRW).

The initial sampling results are promising, with significant reductions in BOD, ammonia, total suspended solids (TSS) and phosphorus in the final effluent. River water quality sampling upstream and downstream of the outfall is also being carried out to understand the impact of the scheme on the Afon Lwyd.

[caption id="attachment_28926" align="alignnone" width="1200"] Integrated constructed wetlands (October 2025) - Courtesy of Arup[/caption] Research

Welsh Water is participating in the Chemical Investigation Programme (2025–2030) with UKWIR, where samples from the wetlands are currently being used to help understand bio-accumulation of emerging substances such as pharmaceuticals in intermittent discharges.

Conclusion

With the site due to open to the public soon, Pont-y-felin Lane exemplifies a paradigm shift in storm overflow management, demonstrating how nature-based solutions can deliver technical excellence, environmental restoration, and community enrichment, all while setting a precedent for low-carbon infrastructure in the water sector. The forward‑thinking optioneering, combined with the client’s commitment to selecting a nature-based solution, over traditional ‘grey’ options, has resulted in a scheme that proves natural solutions can deliver more sustainable outcomes, for less.

Ashbourne Sewage Treatment Works (STW) in Derbyshire serves a population equivalent of 48,259 from both domestic customers and a high trade load, primarily from poultry processing. There has been a works in this location since the 1930s, with existing processes including inlet pumping, 2mm fine screening, grit removal, primary settlement, secondary treatment through an oxidation ditch and final settlement. The drivers for enhancement at the works are a predicted 8% growth in domestic population of over the next 10 years, and quality improvements to meet both the Water Framework Directive (WFD) and Fisheries Directive respectively requiring an increase in treated flow to full treatment from 80 l/s to 175 l/s, and discharge permit changes including ammonia limit reduction to 5 mg/l and a total phosphorus limit of 0.5 mg/l. The existing conventional treatment process would be unable to meet the new permit conditions due in December 2021. Design solution The key enhancement is the addition of a new process stream, developed as the lowest TOTEX (capital and whole life operational cost) comprising of: Two new primary settlement tanks (PSTs) with a new secondary distribution to split flows between the new (60%) and existing (40%) process streams. New 5376m3 activated sludge plant (ASP) operating in enhanced biological phosphorus removal (EBPR) mode with two new final settlement tanks (FST). Refurbishment of two existing oxidation ditches to operate in an EBPR double ditch configuration. Tertiary treatment to the full flow including a 30m3 ferric sulphate dosing rig and tertiary solids removal plant. A new outfall pipeline with a new headwall. Digital focus MWH Treatment (MWHT) are committed to using digital processes to drive value throughout project delivery. The project commenced with visual project initiation, an approach which included a full digital survey of the site and utilised drone images to reduce the number of site visits required during design development. The added value continued through the life of the project e.g. digital information was included as part of the online induction process which gives all visitors an overview of the works and ongoing construction activities before arriving on site in addition to establishing standards and checking competencies. [caption id="" align="alignnone" width="1200"] BIM model of Ashbourne STW - Courtesy of MWHT[/caption] At the heart of the project is the collaborative working and digital delivery techniques which have utilised tools such as collaboration software (Autodesk BIM360), digital planning tools including virtual reality, augmented reality and 4D planning (Synchro). Autodesk BIM360 was used for building combined federated models of MWHT internal designed assets with supplier’s model details enabling true collaboration. With Severn Trent, MWHT and supply chain members accessing the model, real time collaborative reviews of the 3D model enabled optimising layouts, removing clashes and constraints. Additionally, the team progressed 4D Synchro digital scenario development and activity rehearsals, delivering programme efficiency. The models were further used for access lifting maintenance and operability reviews with Client’s operational staff, the virtual reality giving an immersive interactive review that helped resolve ergonomic constraints for issue free handover to Operations. MWHT completed all major procurement of subcontract packages four months into the project cycle, enabling early engagement and liaison with the varying partners throughout the design phase to incorporate innovative ideas, such as assessing all structures for off-site build potential or modularisation, wherever possible. [caption id="" align="alignnone" width="1200"] Process commissioning (February 2021) - Courtesy of MWHT[/caption] Collaborative design Process design sizing of the new ASP was undertaken by Severn Trent Treatment Process Engineering (TPE) working closely with MWHT’s inhouse design team. Process flexibility to optimise biological treatment was collaboratively developed; plant flexibility provided by using a ‘swing zone’ within the ASP for nitrification and de-nitrification, and further allowing anoxic capacity reduction by turning the front-end mixers off. This level of control will allow Severn Trent to optimise for efficient operation, saving power across the year and giving rise to major OPEX savings. The use of this technology needed everyone to be bought into the benefits and was peer reviewed by Severn Trent’s Community of Practice. Ground engineering Preliminary ground investigation enabled early identification that the ground conditions where the new ASP and FST’s would be sited contained soft alluvium material in parts, unsuitable to carry the 80kPa load from the ASP. This varied ground, had the potential to result in differential settlement of up to 30mm across the 45m long ASP structure. MWHT’s design team opted for a partially piled solution, utilising an existing sandstone shelf under 45% of the base of the structure to minimise the number of piles required for the construction. [caption id="" align="alignnone" width="1200"] ASP and FST construction area – Courtesy of MWHT[/caption] Ashbourne STW : Supply chain - key participants Client: Severn Trent Full design & main contractor: MWH Treatment Ltd Civil works - general: Hollywood Civil Engineering Civil works - general: Naylors Construction FRC: Smithform Ltd Piling: Aarsleff Ground Engineering Electrical installation: Elsym Installations Mechanical installation: Hollywood Mechanical Engineering Safety showers & eyebaths: Aqua Safety Showers International Ltd MCC and ICA controls: Total Automation & Power Kiosk access stairs & platforms: Step on Safety Ltd Precast wall units: A-Consult Ltd Portal frame building: Saredon Steel Buildings Aeration system: Suprafilt Sludge thickening: Huber Technology Polymer dosing: Richard Alan Engineering Pumps: Xylem Water Solutions PST/FST bridges & chemical dosing rig: EPS Water TSR plant: Eliquo Hydrok Ltd RAM pumps: EMS Industries Ltd Modern methods of construction The use of off-site construction significantly benefits construction across a number of themes including reduced programme times as off-site manufacture runs in parallel with on-site activity and safer construction with activity in controlled factory environments away from weather impacts. Two design for manufacture and assembly value added solutions from Ashbourne are described below: Precast PSTs and FSTs: The site required two 12.5m PSTs and two 18.8m diameter FSTs. Utilising the lessons learned on the recent successful adoption of precast post stressed wall units on MWHT’s Earl Shilton ASP project, an early decision was made to proceed with this construction method for Ashbourne’s circular tanks, with proven key supply chain partner A-Consult Ltd chosen. The precast wall panels were built off-site in a controlled factory environment and simply erected on the base using a mobile crane. Each panel was lifted and placed sequentially until the tank was complete following which tank walls were then threaded with steel tendons circumferentially and post tensioned around the structure. Preformed GRP launders from the scraper bridge manufacturer, EPS provided were simply attached to the units. The precast solution saved 4 weeks per tank over a traditional in situ approach, minimising construction time and on-site risk. [caption id="" align="alignnone" width="1200"] Precast FSTs - Courtesy of MWHT[/caption] Pop-up sludge building: The increased sludge production generated by SAS in addition to primary sludges required enhancement of on-site thickening to reduce tankering and minimize OPEX. A new building was required and the team identified an efficiency to additionally include the new polymer dosing skids and the main MCC, negating the need for additional kiosks. Traditionally this would have required a 22m x 13m conventional clad portal frame construction, requiring on-site assembly with significant durations of working at height to fix steel and the cladding. MWHT share innovations nationally across their business through Process Improvement Teams (PIT) and the Ashbourne team identified a novel DfMA portal frame building solution from supplier Saredon which had been used in the Thames region. The DfMA portal frame building was delivered in sections with wall and roof structure and cladding units arriving in one piece and ‘unfolding’, before being lifted into position. The process of erecting the building and finishing works to make watertight and secure took less than two weeks, saving at least two weeks over conventional construction and minimizing assembly risks associated with erecting buildings of this type. [caption id="" align="alignnone" width="1200"] Saredon Steel Buildings ‘pop up’ building during installation - Courtesy of MWHT[/caption] Efficient project delivery The team has delivered the first phase of the project by commissioning the ASP within 19 months from site start. Achieving this programme required drive from all parties and a collaborative team from project inception. Commencing with an initial whole team meeting to define objectives, critical success factors and setting the tone, communication was effective through the life of the project. A number of specific actions have helped drive programme out-performance that is contributing to over 10% reduction in project on costs over similar activity delivered in AMP6: Programme intensity: Operating multiple work faces across the site, completing many areas in parallel, shortening the critical path and removing the risk of areas of work becoming critical later in the programme. Multiple suppliers: Traditionally the civils groundworks would have been subcontracted to a single supplier however the volume and intensity of activity made this a significant programme risk. The package was split into independent work packages which ensured each subcontractors’ resources were not overstretched enabling efficient and safe delivery. DfMA opportunities for precast tanks and pop-up buildings were progressed delivering saving on critical and parallel activities. Collaborative working: All team members, including the clients commissioning team bought into one overall programme with regular collaborative working review to generate a ‘pull’, earliest finish, programme. [caption id="" align="alignnone" width="1200"] Sludge equipment model - Courtesy of MWHT[/caption] Safety innovation - augmented intelligence Learning lessons from previous best practice helps inform safer and more effective Risk Assessments and Method Statements (RAMS). Augmented Intelligence can be used to review any RAMS produced against a database of similar RAMS, scanning for omissions and similarities to provide a report of correlation, contradiction and omissions. Ashbourne provided the base site for the trial of the first known AI system for RAMS assessment in the Water Industry with delivery partner Intuety. Many of Ashbourne RAMS were used to feed this system, machine learning used to continuously improve the system which has now been deployed, providing additional RAMS production support across MWHT projects in the Severn Trent region. Community engagement Leaving a community legacy of a job well done is a core objective, contributing to the communities in which we work. These initiatives align with both Severn Trent and MWHT’s corporate responsibility and staff wellbeing programmes. At Ashbourne, the MWHT site manager reviewed community engagement opportunities during the Covid-19 pandemic and identified an opportunity to support the local Scout group, providing essential first aid equipment that will be used once scouting activity restarts. [caption id="" align="alignnone" width="1200"] Precast PSTs - Courtesy of Hollywood Civil Engineering[/caption] March 2021 saw the new ASP (Phase 1) commissioned and it will be a significant success for Severn Trent and MWH Treatment, achieving AMP7 regulatory obligations early and leaving a legacy of high quality, low cost sewage treatment for customers for many years to come.

Dearne Reach is a 4.3-hectare Integrated Constructed Wetland (ICW) in Clayton West, Huddersfield, designed to enhance water quality along 4km of the River Dearne. Delivered by Mott MacDonald Bentley on behalf of Yorkshire Water, this ICW will reduce levels of un-ionised ammonia and improve storm flow treatment in line with the salmonid fundamental intermittent standards. To deliver this nature-based solution, Mott MacDonald Bentley excavated 13 wetland cells covering over 44,000m2 (equivalent of 7 football pitches!), creating space to house more than 350,000 wetland plants. Project background

As an alternative to the traditional grey infrastructure solution, which initially would have entailed 7,000m3 of additional storm storage and a 4.5km diversion of final effluent to a lower river reach, the Dearne Reach ICW offers a sustainable approach to pollution mitigation. Funded by Yorkshire Water as part of their capital delivery programme, it is currently the largest ICW developed in the UK to improve water quality while increasing storm storage resilience.

Aligning with Yorkshire Water’s WFD drivers and net zero ambitions, this project pioneers an ICW tailored for intermittent storm overflows, distinguishing itself from conventional ICWs that require constant inflow. Its innovative design ensures effective management of stormwater variability and will contribute to driving continuous innovation in nature-based solutions across the water industry.

[caption id="attachment_25943" align="alignnone" width="1200"] Construction of the first cells - Courtesy of MMB[/caption] Integrated constructed wetlands

Constructed wetlands are engineered ecosystems that harness natural processes to treat wastewater, stormwater, and industrial effluents. Sometimes described as ‘nature’s kidneys’ because of their ability to filter and clean wastewater by mimicking natural wetlands, these systems use porous media beds and emergent plants to remove contaminants through physical, chemical, and biological mechanisms.

Various treatment wetland types exist, including free water surface wetlands - also known as integrated constructed wetlands (ICWs).

ICWs allow water to flow above the soil through aquatic plants, enabling the removal of suspended solids, nutrients, and pathogens. These passive systems, closely resembling natural wetlands, rely on processes such as adsorption, precipitation, sedimentation, and filtration to improve water quality.

[caption id="attachment_25944" align="alignnone" width="1200"] Cell construction - Courtesy of MMB[/caption] Advantages for Dearne Reach

The ICW at Dearne Reach offers numerous advantages:

Water quality improvement: Effective removal of nutrients, heavy metals, pathogens, and organic pollutants. Carbon sequestration: Capturing carbon within plant biomass and soil, contributing to achieving net zero goals. Biodiversity enhancement: Providing habitats for birds and other wildlife, supporting ecological balance. Community benefits: Creating green spaces for public enjoyment and research opportunities. Versatility: Adapting to varying wastewater loads and flows, making it highly efficient at stormwater management. Low operational costs: Passive systems with minimal maintenance and no chemical requirements over decades, unlike conventional treatment plants. Plantings

At Dearne Reach, 52 different species of plants have been designed to sit in 13 adjacent cells in which over 350,000 plants adsorb nutrients from storm overflow discharges and wastewater flowing out of the Clayton West Wastewater Treatment Works.  These have been specifically selected for the treatment capacity, and alongside the enhancement of existing hedgerows across the site, has enabled a staggering 38% biodiversity net gain in terrestrial habitats.

[caption id="attachment_25945" align="alignnone" width="1200"] Liner installation - Courtesy of MMB[/caption] Variable stormwater discharges

The project introduced an ICW specifically designed to treat intermittent storm overflows. Unlike traditional ICWs, which are effective only with constant inflow, this innovative design can handle the variability and unpredictability of stormwater discharges. The wetland is divided into multiple cells, each optimised for maximum retention time and pollutant removal.

Advanced hydraulic controls and computational fluid dynamics (CFD) simulations were employed to model and optimise the flow through the wetland, ensuring effective treatment of fluctuating volumes and pollutant loads.

Pumping station & rising main

A new wetland pumping station has been designed to raise up to 350 l/s of flows from Clayton West WwTW to the ICW through a new rising main constructed using HDD trenchless methods. Once the water reaches the head of the wetlands, the entire system is passively flowing through two precast concrete distribution chamber structures and fourteen 1.4m wide cascades to transfer the water through each of the 13 cells.

Harnessing solar power

The innovative design harnesses solar power to operate a level sensor and flow meter chamber to successfully manage stormwater variability whilst achieving a passive system ICW with no additional power requirements. The introduction of the solar panels mean that the flow monitoring chamber could be installed to provide future quantity and quality monitoring data, which can be shared to support the evidence base of these emerging infrastructures.

As the largest ICW developed in the UK, its unique design and successful results will contribute to driving continuous innovation in nature-based solutions for storm management across the water industry. It stands as an excellent example of the power of collaboration between civil engineering and nature-based solutions to mitigate the impacts of climate change and promoting sustainable development.

[caption id="attachment_25941" align="alignnone" width="1200"] All cells constructed - Courtesy of MMB[/caption] Collaborative approach

To successfully design and build a wetland on the scale of Dearne Reach it was clear from the project outset that a collaborative design approach would be key to both a successful programme and commercial output. Therefore, collaboration between the design team, digital delivery team and geotechnical experts was fundamental. The challenges facing the site included, high shallow perched groundwater, sandy, gravelly clay requiring high degrees of compaction, and a requirement for over 13,000m3 of material for topsoil reuse. Alongside construction on a complex site within a coal mining legacy area, with archaeological potential and intersecting with a public right of way.

Dearne Reach ICW: Supply chain: key participants Principal designer & contractor: Mott MacDonald Bentley Outline design: Stantec UK Pressure testing: Ant Hire Groundworks: Lowthers Groundwater treatment: Siltbuster Group HDD rising main: Joseph Gallagher Ltd Liner suppliers: SIG Construction Accessories Precast structures: Craven Concrete Ltd Headwalls: Althon Ltd Treatment works package: HB Tunnelling Electrical package: Circle Control & Design Systems Ltd MCC installation: Technical Control Systems Ltd Communications instruments: Churchill Controls Ltd Wetland plant suppliers: Salix River and Wetland Services Grass & flower seed: Hurrells & Mclean Seeds Ltd Erosion protection: Geogrow Gate valves: Affco Flow Control (UK) Ltd Actuators: Glenfield Invicta Ltd Instrumentation: Marlec Instrumentation: IFM Electronic Ltd Flow meter: SIRIS Environmental Pipework: Aquaspira Ltd [caption id="attachment_25946" align="alignnone" width="1200"] Planting completed in all the cells - Courtesy of MMB[/caption] HDPE pipework & couplers: Westwood Pipelines Ltd HDPE pipework & couplers: Radius Systems DI pipework: George Green (Keighley) Ltd DI pipework: Electrosteel Castings (UK) Ltd DI pipework: UTS Engineering Ltd Metalwork: Fabtek Duct pits: Advanced Infrastructure Systems Pipe supports: Midland Fixings Ltd Drainage pipework: Wolseley UK ltd Drainage pipework: Myers Building Supplies Drainage pipework: Keyline Civils Specialist Ltd Pumps: Xylem Water Solutions Lifting gantry: Peter Cassidy (Leeds) Ltd Welfare cabins: Nixon Hire | Wernick Group Bolt-sets: BAPP Industrial Supplies Bolt-sets: VULCAN Industrial Fasteners RoadCem access roads: DNS Aggregates: Hague Plant ltd Concrete: Heidelberg Materials Plant hire: Lynch Plant Hire Plant hire: Grantham Industries Ltd Plant hire: Flannery Plant Hire (Oval) Ltd [caption id="attachment_25937" align="alignnone" width="1200"] Filling the first cells with water - Courtesy of MMB[/caption] Construction achievements

During construction, several notable achievements were made:

Groundwater management: As a result of high groundwater levels on the site, the system hydraulics were analysed, and cell height adjusted. This optimised the cut-fill balance and ensured no material import would be required to construct the embankments, ultimately preserving resources and reducing material waste whilst ensuring there would be no flotation risk

Bypass pipework reductions: Through the development of robust future access and maintenance plans, an opportunity was realised to eliminate unnecessary bypass pipework infrastructure. Initially, this had been proposed to allow each cell to be isolated for future maintenance. The removal of this pipework scope significantly reduced the carbon footprint and project costs.   

Sustainable lining solution: As the existing ground did not have a suitable impermeability to protect the underlying groundwater, the ICW had to be lined. To avoid the use of extensive plastic on site, a geosynthetic clay liner was used to create a self-sealing, resilient hydraulic barrier while minimising environmental impact.

Archaeological and geological integrity: The team ensured the coal mining legacy areas remained undisturbed, with archaeological trenching conducted site-wide to ensure any potential pre medieval features were not disturbed during construction.

RoadCem technology for sustainable access: Traditional concrete access roads were replaced with Roadcem soil stabilisation technology to reduce the required material import and associated carbon footprint with use of extensive artificial concrete.

Biodiversity commitment: 350,000 locally sourced plants were installed, arranged in seven planting zones, achieving a 38% biodiversity net gain in terrestrial habitats.

Public right of way enhancements: Social value was equally important to the project and the team collaborated with the local council to enhance the public right of way, improving accessibility with a wheelchair-friendly, 2m-wide footpath along the perimeter of the ICW, featuring timber fencing for optimal wetland viewing points to support community health and well-being.   

Carbon savings: The project has achieved a 60% reduction in embodied carbon and 40% reduction in operational carbon across its lifecycle – delivering a more environmentally friendly solution in comparison to the original grey infrastructure solution.

[caption id="attachment_25940" align="alignnone" width="1200"] Fully planted wetland - Courtesy of MMB[/caption] Innovative digital construction

From the outset, early collaboration with Mott MacDonald Bentley’s digital delivery team identified an opportunity to pilot the use of intelligent plant machinery, alongside a ‘Smart Construction Dashboard’ during the build.

Prior to mobilisation on site, the model and delivery programme was shared for upload into the Smart Construction/Earthbrain software. These platforms initially use Artificial Intelligence to generate an efficient delivery output, detailing size, type and number of machines required. They also generate a powerful earth shifting model, highlighting the most efficient distribution workflow for the cut/fill material to travel and be placed.

This was most useful to compare the tendered assumptions and check for any oversights in the planning phases. This approach improved accuracy, efficiency, and most importantly, improved health and safety standards on site. This marked a first-of-its-kind trial for deploying intelligent plant in the construction of a nature-based solution.

The site team reported significant benefits, particularly given the scale of the excavation, such as:

Model-driven excavation optimised material use, eliminating unnecessary cutting/filling and promoting a right-first-time methodology. Automated systems reduced manual surveying and minimised plant-people interaction, ensuring safer site operations. Precision excavation reduced material waste while moving 65,000m3 (117,000 tonnes) of soil, limiting reliance on imported fill. [caption id="attachment_25939" align="alignnone" width="1200"] Wetland plants developing - Courtesy of MMB[/caption] Successes & summary

The Dearne Reach ICW Project exemplifies sustainable development by delivering a low-carbon stormwater management solution. Leveraging natural processes such as sedimentation, filtration, and biological uptake, the wetland effectively removes pollutants and reduces the ammonia levels in stormwater in line with the salmonid fundamental intermittent standards. This enhances water quality and strengthens local biodiversity, aligning with global climate adaptation efforts.

The project’s innovative approach to managing stormwater discharges aligns with Yorkshire Water’s efforts to mitigate and adapt to the effects of climate change. It stands as an excellent example of the power of collaboration between civil engineering and nature-based solutions to mitigate the impacts of climate change and promoting sustainable development.

This first-of-its-kind trial has set a new benchmark for future wetland and nature-based solution projects across the water industry, with lessons learned shared to inspire continuous innovation in sustainable construction.

Comber is a small town in County Down, located on the shores of Strangford Lough, 4.5 miles from Newtownards and 12 miles from Belfast. All flows from the Comber Catchment arrive at Upper Crescent Wastewater Pumping Station (WwPS), which is the terminal pumping station that pumps flows to Ballyrickard Wastewater Treatment Works (WwTW) for treatment. A study on the drainage network of Ballyrickard/Comber was carried out in January 2010, which identified inadequacies in the system and made recommendations for improving and upgrading the network. Specifically, the study identified that Upper Crescent WwPS did not meet Northern Ireland Environmental Agency (NIEA) requirements for emergency storage volumes. It also identified that Upper Crescent WwPS had been experiencing frequent breakdowns that contributed to high operational costs. Existing works

Upper Crescent WwPS was constructed in the 1970s, and the site contains two pumping stations:

Wastewater pumping station: Pumps foul flows from the catchment to Ballyrickard WwTW in Newtownards for treatment. Storm pumping station: Services stormwater from the local catchment area during large storm events that can overwhelm the sewer network. This pumping station plays a key role in preventing out-of-sewer flooding within Comber during storm events and limits further discharges to Strangford Lough. [caption id="attachment_26824" align="alignnone" width="1200"] Existing site showing proximity of site to neighbouring properties and local park to the north - Courtesy of GRAHAM[/caption] Project team

As principal contractor and principal designer, GRAHAM Construction had overall responsibility to deliver the project on time and within budget. Enisca Brown Ltd was contracted for designing and delivering a turnkey MEICA, process design and build solution for the new works.

McAdam provided civil, structural, and geotechnical design services, including the initial enabling and investigatory works carried out at the Early Contractor Involvement (ECI) stage, as well as the detailed design for construction.

The contract delivery team worked in a collaborative manner with NI Water and their project manager AECOM, as well as key suppliers and subcontractors to deliver a robust, effective and modern wastewater pumping station and storm water storage facility. This allows for future growth within the catchment area and protects the environment from discharges during storm events.

Early Contractor Involvement (ECI)

In 2022, GRAHAM undertook Early Contractor Involvement to further develop the scope and complete detailed design for the main construction works. Due to the age of the existing pumping stations, rigorous investigations were carried out to determine their operation and control philosophy, condition and potential for imposing health and safety risks to operatives and the contract team.

The completion of the detailed design, ahead of main contract award, ensured cost certainty for NI Water and lessened the risk of delays due to design changes on site.

[caption id="attachment_26822" align="alignnone" width="1200"] Early excavation works on the cofferdam - Courtesy of GRAHAM[/caption] Upper Crescent WwPS: Supply chain - key participants Client: NI Water Project manager: AECOM Civil contractor: GRAHAM MEICA designer & contractor: Enisca Browne Ltd Civil, structural & geotechnical design: McAdam Cofferdam temporary works design: McErlain Consulting Sheet piling contractor: Ward Piling PLC software & commissioning: Enisca Browne Ltd PLC software & commissioning: Profitec Solutions Ltd Metalwork design/fabrication & installation: Evans Plant Pumps & mixers: Xylem Water Solutions Static storm screen: Eliquo Hydrok Ltd Access covers: JP Corry Pipes & fittings: APP Fusion Group Backup generator: AJ Power Ltd Lifting equipment: Walter Watson Lifting equipment: Barbour Engineering & Fabrication Site security fencing & gates: Kane Fencing Ltd [caption id="attachment_26825" align="alignnone" width="1200"] Installation of the sheet piles for the cofferdam - Courtesy of GRAHAM[/caption] Project description & scope

The existing Upper Crescent WwPS in Comber involved a 2-Phase upgrade plan to complete all activities required:

Phase 1: Early procurement of items with long lead-in times and site preparation:

Early procurement of items such as generator, overflow screen, gantry crane and MCC panel. Relocation of several deep incoming sewer lines around the footprint of the cofferdam. Installation of new motor control panels for the existing Storm PS and new Wastewater PS to ensure uninterrupted operations. Relocation of overhead NIE (Northern Ireland Electricity) cables. Installation of a new upsized NIE transformer. Installation of a new 550kVA backup generator. New NIE incomer panel for connection point to the new NIE TX from the new 550KVA diesel generator.

Phase 2: Construction

Further site investigation works. Construction of new wastewater pumping station. Construction of new valve and flowmeter chambers. Installation of M&E equipment including three pumps, two mixers and static overflow screen. Construction of new inlet and overflow sewers. Construction of new rising main. Demolition of existing wastewater pumping station and associated apparatus. Landscaping and reinstatement. [caption id="attachment_26819" align="alignnone" width="1200"] Final excavation to formation level (minus 10m below ground level) within the cofferdam - Courtesy of GRAHAM[/caption] Construction

One of the biggest challenges was that the two adjacent pumping stations had to remain operational throughout the entire construction phase. The new Upper Crescent WwPS and 687m3 of stormwater storage required the construction of a 26m diameter x 10m deep cofferdam and the new WwPS could not be commissioned until the existing had been decommissioned.

Screw pumps: Each of the screw pumps within the Upper Crescent Storm Pumping Station was individually phased over to the new control panel in a lengthy process, which required a test period of each pump before the next pump could be phased over. Staged changeover was required due to the criticality of their operation during storm events. The pumps could not be taken out of service during storm events due to the potential of out-of-sewer flooding in low lying areas within the Comber Catchment.

Cofferdam construction: Once the programmed critical works within Phase 1 were complete, Phase 2 works could commence on the construction of the circular cofferdam. This was undertaken by specialist piling contractor Ward Piling Ltd with responsibility for:

Temporary works design. Sheet pile installation. Dewatering well installation. Construction of three circular concrete ring beams. Excavation of the cofferdam. Dewatering of the excavation.

A secondary 3-sided cofferdam was also constructed adjacent to the main cofferdam to facilitate the construction of a new inlet manhole.

[caption id="attachment_26828" align="alignnone" width="1200"] Completed cofferdam and 280 ton crawler crane - Courtesy of GRAHAM[/caption]

Craneage: The craneage requirements for the entire project were critical in developing the construction methodology and phasing of the works. Multiple heavy and bulky lifting operations were required to construct the new Upper Crescent Wastewater Pumping Station. These included lifting 25-ton excavators into and out of the cofferdam, lifting of the 10-ton steel plate wall shuttering/formwork used to cast the circular pumping station structure, pulling of the sheet piles with vibro hammer and installation of the large precast roof beams and slabs.

A 280-ton crawler crane was selected as the suitable crane for our project needs. The imposed loading of the crawler crane also had to be considered in the design of the cofferdam, due to the loading it would place onto the cofferdam structure. Early planning of the detailed and complex construction methodology streamlined the construction process and avoided any major delays or unforeseen complexities in the construction of the new WwPS.

Concrete pours & precast roof: Due to the proximity to Comber River/Strangford Lough and the findings from the ground investigation during the ECI, it was anticipated that a high-water table would be encountered with steady flow of groundwater infiltration. However, the reality was much better than expected. The sheet pile installation was much more effective at cutting off groundwater infiltration than anticipated, significantly reducing the dewatering resources.

The new WwPS is a 15m internal diameter reinforced concrete structure with internal dividing wall, which splits the structure into the stormwater storage tank and foul pumping station wet well. The concrete structure was cast in situ in 15 steps. Concrete pours starting with the base slab and followed by five 1.35m high kicker pours to contain the internal benching. [caption id="attachment_26821" align="alignnone" width="1200"] (left) Reinforced concrete pumping station structure under construction and (right) roof beam and completed RC structure - Courtesy of GRAHAM[/caption]

The main walls were then cast; the remaining 7.5m height of the structure in five wall pours (four quarter radius pours and the internal dividing wall pour). The walls were cast using custom adjustable radius steel formwork shutters. Every effort was taken to maximise the speed and ease of construction, and the larger pours reduced the number of construction joints within the wastewater retaining structure.

The roof beams were precast on site due to their size and weight; 1m deep cross sections weighing approximately 20 tonnes each. A steel platform was constructed on site, on which each of the roof beams was cast while the tank structure was under construction.

The roof slabs were set into pockets cast into the top of the tank walls, with a H-shape beam over the wet well to accommodate large access covers for the three submersible pumps and two mixer pumps. The roof deck consisted of 22 precast slabs, which were cast off site due to insufficient space on site. The largest slab weight was 14 tonnes.

Inlet manhole: Upper Crescent WwPS receives incoming flows from the Comber Catchment via three gravity sewers entering from three corners of the site. The sewers all combine in the new inlet manhole, which was constructed adjacent to the new WwPS. Within the new inlet chamber, there are two manually operated penstocks. These can be used to direct flows into the wet well or storm tank as required, via the two independent pipelines shown. This provides NI Water operatives the opportunity to isolate the wet well during period of maintenance and utilise the storm tank to receive the incoming flows. [caption id="attachment_26820" align="alignnone" width="1200"] (left) Roof beam and completed RC structure and (right) incoming sewers - Courtesy of GRAHAM[/caption] Turn of flows

The commissioning of the new pumping main required a temporary shutdown of the WwPS to disconnect the existing pumping station and connect the new pumping station to the pumping main. This critical activity required meticulous planning and detailed coordination between the contract team and NI Water.

The work was programmed for forecasted dry days to ensure flows were reduced during the operation. Regular commissioning meetings were held with NI Water and the contract team in advance of the proposed works. Detailed discussions concerned the proposed works, programme and specific requirements in a systematic approach to confirm with relevant stakeholders that all eventualities had been considered before the works were progressed.

The final connection to the rising main was programmed to take place at night, during an 8-hour period when incoming flows would be at a minimum. The works would also be very weather dependant, as wet weather would dramatically reduce the amount of time available to complete the connection. The additional stormwater in the combined network could potentially overwhelm the on-site storage more rapidly than the team could complete the connection.

[caption id="attachment_26829" align="alignnone" width="1200"] Wet well pumps - Courtesy of GRAHAM[/caption] The existing Upper Crescent WwPS was taken out of service on 26 February at 23.00. The inlet pipe to the new WwPS was opened and the existing pipeline to the old WwPS isolated. The existing pumping station wet well was pumped down to its lowest possible point possible to provide maximum storage. Then approximately 500m of 450mm HDPE diameter rising main was drained back to the existing pumping station wet well. The new pumping station was used to provide additional on-site storage of 687m3. The team worked through the night and the rising main connection was completed by 05:00 the next morning. Commissioning

Once the pumping main connection was completed, the new Upper Crescent Wastewater Pumping Station became operational and a 28-day trial period to verify the station’s functionality/reliability to the client commenced. During the 28-day trial, the existing wastewater pumping station was methodically demolished, down to the basement levels ensuring that the new WwPS was not affected.

The excavation was backfilled with engineered fill to ground level in preparation for final hardstanding construction and landscaping, which was completed during the final weeks of construction.

[caption id="attachment_26823" align="alignnone" width="1200"] Demolition of the existing WwPS - Courtesy of GRAHAM[/caption] Community engagement

Upper Crescent WwPS is located at the end of a cul-de-sac and corner of a public park. Ahead of construction commencing on site and throughout the work, a number of public relations activities were employed to inform the many stakeholders about the project and in particular the piling activities, removal of spoil from site and turn of flows when commissioning the new pumping station. These included: regular briefings to local elected representatives; one-to-one meetings with stakeholders; letter drops; house calls; school visits to the site; press releases and social media announcements.

During the project’s construction phase, GRAHAM needed some additional lands to enable the construction of the new Upper Crescent WwPS, as the pumping station would occupy most available lands within the existing site. The construction team where therefore required to obtain a section of the neighbouring Muckers Park owned by Ards and North Down Borough Council to form a temporary site compound and provide additional land for a crane platform. The crane platform required a diversion of footpath used by the local Park Run.

Upper Crescent WwPS: Timelapse video

Early engagement with Comber Park Run enabled successful diversion of footpaths for the weekly race and patrons of the park. The diversion had to be artificially lengthened, to ensure that the length of the park run circuit was not shortened. During the construction of the new WwPS, GRAHAM provided refreshments for a number of park runs, distributing NI Water reusable water bottles and provided a cake for the Park Run’s 10th Anniversary.

On completion of the WwPS, the park was reinstated to its original configuration and handed back to the Ards and North Down Borough Council. Upper Crescent WwPS was operational from 27th February 2025 with site-wide testing and commissioning of the plant successfully undertaken for handover to NI Water in June 2025.

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.