Supply Chain - Storage Tanks - Precast Concrete

Garnswllt Wastewater Treatment Works (WwTW) serves a predominantly domestic catchment south of Ammanford, receiving flows from Ammanford, Llandybie, Glanaman, Brynamman, Capel Hendre, Saron, Tycroes and Pen-y-Groes, with a total population equivalent of approximately 30,000. To meet the requirements of the Water Framework Directive (WFD) and reduce phosphorus concentrations within the final effluent discharged to the Afon Llwchwr, a capital investment project was necessary to enhance both the treatment performance and hydraulic capacity of the works. Existing works

Garnswllt WwTW is an activated sludge plant providing carbonaceous treatment, nitrification and phosphorus removal. Incoming flows gravitate to the inlet works, where flows above the existing 360 l/s pass-forward flow (PFF) are diverted to storm treatment.

Primary treatment is provided via two primary settlement tanks (PSTs), followed by a three-lane activated sludge process and four final settlement tanks, with final effluent discharged to the River Loughor.

[caption id="attachment_32000" align="alignnone" width="1200"] (left) Temporary scaffold under 132kV HV line for the FST works, (top right) existing ecological pond drained to allow the construction of the new PST and (bottom right) start of the installation of the PST precast panels - Courtesy of Mott MacDonald Bentley[/caption] Project summary

Mott MacDonald Bentley (MMB) was commissioned by Dŵr Cymru Welsh Water (DCWW) to deliver an improvement scheme at Garnswllt WwTW.

The improvement works will deliver both environmental and operational benefits by increasing the site’s ability to manage higher flows associated with future population growth by increasing treatment capacity from 360 l/s to 431 l/s. The improvements will also reduce the frequency of spills to storm storage and minimise environmental impacts.

Scope of works

To address the new regulatory drivers, Dŵr Cymru Welsh Water and Mott MacDonald Bentley jointly developed a single, cost-effective solution to ensure compliance and improve the resilience of the treatment process.

The following main elements at the WwTW were delivered by MMB:

Modification of inlet works to allow an increase in pass-forward flow (PFF). Construction of a new 25m primary settlement tank (PST) to meet the gap in hydraulic retention at the existing PSTs, which would otherwise have worsened with the increase in PFF. Construction of a 5m diameter x 5m deep PST feed pumping station. New return activated sludge (RAS) and surplus activated sludge (SAS) pumping for existing Final Settlement Tanks 3 & 4 to improve performance and provide future resilience. Replacement of siphon scraper bridges on Final Settlement Tanks 1 & 2 (FST1 & FST2) to improve performance and reliability. Power upgrades, including new motor control centres, kiosks and new emergency generator. New access road to the PST and feed pumping station. New ecological pond for improvement of habitat quality. [caption id="attachment_31999" align="alignnone" width="1200"] Garnswllt WwTW inlet works channel - Courtesy of Mott MacDonald Bentley[/caption] Garnswllt WwTW: Supply chain - key participants

A strong collaboration between the client, Dŵr Cymru Welsh Water, the project delivery partner, Mott MacDonald Bentley, and its supply chain was required to ensure a successful project:

Main designer & main process/M&E contractor: Mott MacDonald Bentley Civils contractor: JN Bentley Ground investigation: Spencer Quantum Electrical installation: Zone Electrical Ltd Mechanical installation: Whitland Engineering MCC & switchgear: RSE Control Systems (GPS) Main distribution board kiosk: Industrial GRP Emergency generator: A&M Generators Ltd Concrete installation: Concrete Structures & Floors Ltd PST scraper bridge: DD Engineering Ltd PST precast tank system: Naylor Concrete Products Ltd PST feed pumping station precast panels: FP McCann PST feed pumping station submersible pumps: Xylem Water Solutions IQM actuators: Rotork FST siphon bridges: Colloide PST desludge, FST RAS & SAS PC pumps: SEEPEX UK Ltd Dewatering wells: Stuart Wells Ltd Manual valves: AVK UK Ltd Actuated eccentric plug valve: MJ Wilson Group Ltd Trace heating & lagging: Waste Water Manufacturing Level transmitters: Siemens Temporary treatment units: Siltbuster Group Grass cell paving: Groundtrax Systems Ltd [caption id="attachment_32002" align="alignnone" width="1200"] Construction of the new PST- Courtesy of Mott MacDonald Bentley[/caption] Primary settlement tank

The Mott MacDonald Bentley project team utilised the precast concrete system from Naylor Concrete Products Ltd for the 25m diameter PST. Through collaboration with other MMB framework teams working with Severn Trent and United Utilities, as well as the supplier, lessons learned were applied to deliver the 4m high tank ‘right first time’. The team more than doubled the anticipated installation rate per day, meeting the tight 5mm tolerance between the 75 precast units.

As a result of applying shared knowledge, the tank was designed with mitigations to support efficient installation and was constructed in record time. The tank passed all tests and commissioning first time, with the supplier adopting the Garnswllt team’s construction methods due to their efficiency and accuracy.

Following installation, MMB began developing internal standard design guidance and construction methodology using Garnswllt as a reference to drive efficiencies on future settlement tanks when using the Naylor Concrete system.

A half-bridge scraper system from DD Engineering Ltd was installed at the PST to allow sludge to be collected from the hopper. A duty/standby desludge progressive cavity pump set up was directly connected to the hopper, eliminating the need for an intermediate desludge chamber.

[caption id="attachment_31998" align="alignnone" width="1200"] Construction progress of the PST - Courtesy of Mott MacDonald Bentley[/caption]

Given the tank depth of approximately 5m from hopper to top water level, visual inspection of sludge would have been difficult. Instead, a sludge density monitor was installed at ground level next to the pump to provide operators with information on sludge composition.

Due to space constraints on site for construction, the new 25m diameter PST was located away from the existing PSTs, making gravity-only flow arrangements difficult to maintain. Early in the design, interstage pumping was identified as necessary. The proposed PST location also presented challenges due to existing alluvium and a high groundwater table associated with the adjacent river. To reduce dewatering and excavation requirements, the PST was partially raised above ground approximately 3m high.

PST feed pumping station

An interstage pumping station was designed with duty, assist and standby submersible pumps to accommodate flows ranging from approximately 80 l/s to 240 l/s.

Flows were fed by gravity from the downstream grit trap extension chamber and controlled via an actuated eccentric plug valve and flowmeter. This ensured that the 50%/25%/25% flow split between PST3 (the new PST), PST2 and PST1 remained accurate. The eccentric plug valve was selected to minimise solids build-up, as its geometry clears the flow path during operation.

A modulating Rotork IQM actuator was specified to provide high cycle capability and precise control, accommodating up to 1,200 starts per hour. This was selected to allow the control system to respond quickly to changes in upstream flow and maintain the required distribution across all PSTs.

[caption id="attachment_31996" align="alignnone" width="1200"] New PST feed pumping station - Courtesy of Mott MacDonald Bentley[/caption] FST siphon bridges

Another key element of the scope was the installation of new siphon bridges from Colloide, serving the two 22m diameter FST1 and FST2. These tanks are flat-bottomed rather than conical, requiring a different desludging approach.

The system uses a pump-primed siphon to initiate flow through the desludge pipe, after which gravity maintains continuous siphoning. Plunger valves regulate sludge withdrawal during startup, enabling operators to control draw-off rates and stabilise the process. Typically, outer valves provide lower draw-off rates, while the central valve accommodates higher sludge accumulation.

The existing systems had deteriorated over time, affecting operability and overall performance of the tanks, and were replaced following close collaboration between MMB, DCWW’s process, operations and asset teams, and the siphon bridge subcontractor.

Installation was complex due to 132 kV overhead power lines running directly above the tanks, preventing the use of mobile cranes. An engineered scaffold solution incorporating runway beams, a crash deck and lever hoists enabled safe and controlled installation without cranage.

[caption id="attachment_31997" align="alignnone" width="1200"] PST bypass & outlet pipework (foreground) and PST desludge PC pumps (background) - Courtesy of Mott MacDonald Bentley[/caption] Dewatering

Groundwater control was critical for both the new PST construction and the FSTs refurbishment due to the depth involved and the high groundwater table. The existing FSTs feature perimeter flotation valves, allowing groundwater to enter the tanks when empty and preventing structural uplift. MMB had to ensure groundwater levels remained below these valves before emptying the tanks and start the replacement of the bridges.

An initial dewatering design was completed to support a discharge and abstraction licence application to Natural Resources Wales. A series of dewatering wells were proposed around each existing FSTs and new PST excavation.

Further investigation on site identified an existing underdrain network beneath the existing tanks, which was refurbished and used successfully for dewatering around the FSTs. Testing confirmed effective groundwater drawdown while maintaining compliance with licence conditions (i.e., 122 l/s maximum abstraction flow rate), saving approximately £100,000 and four weeks compared to the initially proposed dewatering wells.

DfMA MCC

Two new motor control centres (MCC7 and MCC8) from RSE Control Systems (GPS) were installed on site. MCC7 served the PST scraper bridge, feed pumping and desludge pumping station, and MCC8 supplied the RAS/SAS pumping stations for FST3 and FST4.

[caption id="attachment_31994" align="alignnone" width="1200"] The new elevated DfMA MCC kiosk - Courtesy of Mott MacDonald Bentley[/caption]

A design for manufacture and assembly (DfMA) approach was adopted with the supply chain, with kiosks, drives and steel platforms preassembled offsite in a factory-controlled environment.

This approach improved quality, reduced programme duration and enhanced health and safety performance by minimising onsite fabrication and installation activities.

Carbon reduction

Carbon reductions were achieved across the scheme. A 600m2 access road was constructed using a grass-cell system from Groundtrax Systems Ltd rather than reinforced concrete, reducing embodied carbon by approximately 80% (around 50 tCO2e) and saving approximately £40,000.

Approximately 6,000 tonnes of excavated material was reused to construct a replacement ecological pond, in accordance with a Materials Management Plan verified under the CL:AIRE Definition of Waste Code of Practice. This avoided off site disposal, saving approximately 20 tCO2e and £200,000. The existing ecological pond had to be drained and removed to allow the construction of the new PST.

[caption id="attachment_32003" align="alignnone" width="1200"] Grass-cell access road to PST - Courtesy of Mott MacDonald Bentley[/caption] Conclusion

Despite challenges including ground conditions, flooding, weather and operational constraints, the Garnswllt WwTW scheme was delivered on programme and within budget. The site can now treat a PFF of 431 l/s in line with WFD requirements.

The two existing final settlement tanks (FST1 & FST2) are now significantly easier to operate and maintain, with improved performance following desludging upgrades. They are now able to receive full flows, resolving previous operational issues.

Desludging performance and resilience at FST3 & FST4 have also been enhanced through the installation of new RAS and SAS pumping systems. The addition of the new primary settlement tank has significantly improved hydraulic performance across the primary treatment stage, reducing previous overloading.

Finally, the electrical distribution system was simplified with a new main distribution board, and overall resilience was enhanced through the addition of an emergency generator.

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.

Newport Wastewater Treatment Plant is located approximately 1.5km south-west of Newport Town, close to the county boundary between Tipperary and Limerick. The facility is an asset of Uisce Éireann and is operated by Tipperary County Council under an existing Service Level Agreement. The original plant was constructed in the 1980s on a greenfield site. Prior to the upgrade, the plant was non-compliant with the requirements of the Urban Wastewater Treatment Directive (UWWTD) and the applicable Wastewater Discharge Authorisation standards (WWDA). Existing works

The existing plant was designed to treat flows from a population equivalent (PE) of 1,900, and to achieve effluent discharge standards of 25 mg/l biochemical oxygen demand (BOD5), 35 mg/l suspended solids (SS) and 125 mg/l chemical oxygen demand (COD).

Treated effluent and overflow stormwater from the plant discharge to the Newport/Mulcair River via two separate adjacent outfalls located within the existing site. One outfall is dedicated to treated effluent, while the second discharges stormwater only.

The original treatment process comprised preliminary treatment by screening followed by secondary treatment consisting of a single oxidation ditch and a single clarifier. Sludge treatment facilities on site included 12 existing sludge drying beds.

[caption id="attachment_31193" align="alignnone" width="1200"] (top left) Overview of existing works, (bottom left) existing clarifier tank and (right) the exiating oxidation ditch - Courtesy of Ward & Burke[/caption] Scope of works

The principal objectives of the Newport WwTP upgrade were to provide sufficient treatment capacity to accommodate planned development and population growth within the Newport agglomeration, while also delivering the improvement works required to achieve compliance with the Urban Wastewater Treatment Directive and the applicable wastewater discharge authorisation standards.

The existing treatment capacity of Newport WwTP was 1,900 PE. The upgraded process stream was designed to provide a 10-year design horizon capacity of 4,200 PE, while the proposed inlet works and underground structures were future-proofed for a 25-year design load of 4,500 PE. This approach reflected the different service lives associated with civil structures and replaceable mechanical and electrical plant, including pumping systems, aeration equipment and blowers. Provision for future expansion of the mechanical and electrical systems will be subject to updated load projections, detailed design and a subsequent planning application. The upgrade works comprised the construction of a new below-ground inlet works structure incorporating 6mm fine screens, a 19mm bypass screen, grit removal and classification equipment, screenings handling facilities, grease removal plant and an overflow chamber designed to limit flow to three times dry weather flow before discharge to the process tanks. [caption id="attachment_31180" align="alignnone" width="1200"] Newport WwTP process overview - Courtesy of Ward & Burke[/caption]

The inlet works was sized for flows of up to 70 l/s. The contract also included decommissioning of the existing inlet works and associated storm overflow, extension of the gravity sewer to the new inlet works, and provision of a partially below-ground stormwater tank with associated pumps and pipework.

Additional treatment capacity was delivered through the construction of a new stormwater storage tank, a new oxidation ditch and a new final settling tank, both configured to maintain the existing hydraulic profile.

The scheme further incorporated ferric dosing for ortho-phosphate removal, underground splitter chambers upstream of the biological and settlement stages, a new motor control centre for the additional treatment stream, telemetry and control integration with the existing infrastructure, SCADA upgrades to reflect both new and decommissioned assets, and the connection of the new process stream to the live plant to enable phased testing and commissioning.

Permanent energy monitoring equipment was installed to support monitoring and verification of plant performance during operation.

[caption id="attachment_31185" align="alignnone" width="1200"] New inlet works - Courtesy of Ward & Burke[/caption] Newport WwTP: Supply chain - key participants

Ward & Burke was appointed by Uisce Éireann in December 2022 as main contractor for the upgrade of the existing plant.

The contract included design, civil engineering, MEICA installation and plant operations during the commissioning and handover period.

Client: Uisce Éireann Principal designer, contractor & project manager: Ward & Burke Employer representative: Tetra Tech RPS Resident engineer: Nicholas O’Dwyer Ltd PSDP H&S coordinator: TOBIN Ecologist: Flynn Furney Environmental Consultants Ltd Precast tanks: Shay Murtagh Precast Precast chambers: Tracey Concrete Screening & grit removal: Jacopa Ltd Lifting equipment: T Allen Engineering Services Ltd Diffused aeration system: SSI Environmental Pumps, mixers & fine bubble diffusers: Sulzer Pumps Wastewater Ltd Aeration blowers: AERZEN Machines Scraper bridges: Keltec Engineering Site metalwork: D&E Welding Washwater booster pumps: Campion Pumps PVC pipe: Total Pipeline Specialist Ductile pipe & fittings: Fusion Pipeline Products Actuators: AUMA Actuators Ltd Valves: Valve & Actuator Solutions Talis penstocks: AVK UK Ltd Odour control: CMI John Cockerill Environmental Ultrasonic meters: Pulsar Measurement PLC software & commissioning: Ward & Burke Kiosks: Gleeson Steel & Engineering Flow measurement: Siemens Flow switches: IFM Electronics Level switches: CPS Access covers: EJ Ireland Formwork: Mitchell Brothers Ready mix concrete: Kellys of Fantane Aggregates: O’Connell Quarries [caption id="attachment_31189" align="alignnone" width="1200"] Newport WwTP showing existing (red) and new (green) assets - Courtesy of Ward & Burke[/caption] Civil engineering works

The principal civils work and the corresponding construction methodologies adopted for the upgrade were as follows:

Work element Construction Methodology Storm tank Below-ground precast chamber Aeration tank Below-ground precast chamber Final settlement tank Below-ground precast chamber Returns pump station Below-ground precast chamber Washwater pump station Above-ground precast chamber Inlet works & splitter chambers Below ground in situ concrete [caption id="attachment_31187" align="alignnone" width="1200"] Cofferdam construction for new aeration tank - Courtesy of Ward & Burke[/caption] Process technologies

Process technology selection focused on maintainability, operational efficiency and reduced water and energy demand.

The inlet works incorporated 6mm step screens, eliminating the requirement for dedicated process washwater and thereby reducing operational water consumption and associated carbon impacts. An air-lift grit removal system was adopted, comprising a stainless-steel coarse-bubble diffuser arrangement and stainless-steel grit lift pipework. Because the system contains no submerged moving parts, routine maintenance can be carried out without tank entry; improving both safety and maintainability.

Biological treatment efficiency was supported through the use of 9-inch fine-bubble diffusers supplied by SSI Environmental, selected to provide high oxygen transfer efficiency and energy-efficient aeration.

In addition, a final effluent service water system was installed to reuse treated final effluent for washwater duties at the inlet works launders, grit classifier and screenings handling equipment. This system comprised a buffer tank, booster pumps, valves and associated pipework, and removed the need to use potable water for these process functions. [caption id="attachment_31184" align="alignnone" width="1200"] New aeration tank - Courtesy of Ward & Burke[/caption] Key challenges

Project delivery was complicated by the need to undertake the works within an operational wastewater treatment plant while maintaining continuous treatment performance. Significant construction risk was also associated with the installation of large underground structures to depths of up to 5m in silts and gravels, with variable groundwater conditions influenced by the proximity of the adjacent river.

These challenges were further intensified by the requirement to complete the design-and-build scheme, including commissioning, within a programme duration of 550 days.

Conclusion/summary

The upgraded plant was commissioned in June 2025, with operation scheduled to transfer from Ward & Burke to Uisce Éireann in June 2026. Overall, the project delivered a substantial improvement in treatment performance, hydraulic capacity and operational resilience for Newport Town.

The upgraded works provide improved effluent quality through the replacement of inadequate existing treatment assets, enhanced protection of receiving water quality in the Newport/Mulcair River, and additional treatment capacity to support forecast population growth and planned development within the agglomeration.

The integration of new civil, mechanical, electrical, instrumentation, control and automation systems has also improved maintainability and operational robustness, while moving the plant closer to full compliance with the Urban Wastewater Treatment Directive and EPA wastewater discharge licensing requirements.

Northumbrian Water’s WINEP Kirklevington Transfer scheme demonstrates the value of challenging the obvious asset-level answer. Rather than adding further treatment capacity at a constrained legacy works, the project team developed and delivered a network-scale solution where flows are transferred to a larger downstream treatment works with available capacity, supported by a new pumping station, balancing storage, rising main and targeted reuse of existing assets. The result was a compliant, lower-impact and more resilient solution delivered through a complex school, highways and strategic water-main interface. Background & project drivers

The driver for the Kirklevington Transfer Scheme was a WINEP requirement to achieve a stringent 0.3 mg/l phosphorus compliance in the final effluent. While the challenge could have been viewed narrowly as a local treatment capacity constraint at the existing sewage treatment works, this would likely have led to adding to or upgrading processes within an already constrained site, bringing increased construction risk, operational complexity, and limited long-term flexibility.

Instead, the project team re-framed the challenge at a network level, identifying an alternative strategy to decommission the local treatment process and transfer flows to a larger treatment works with sufficient capacity to meet regulatory requirements.

[caption id="attachment_32214" align="alignnone" width="1200"] Excavation works to prepare for pre-fabricated tank installation - Courtesy of Esh Stantec[/caption] This approach required robust technical evidence, close engagement with Northumbrian Water and the Environment Agency, and clear evidence that hydraulic performance, treatment capacity and storm overflow resilience would be maintained without transferring risk elsewhere in the system. The solution

The Kirklevington Transfer Scheme replaced the treatment function at Kirklevington STW with a new sewage pumping station and 380m3 balancing tank, supported by a valve chamber, MCC kiosk and chemical dosing arrangement. Calcium nitrate dosing from OMEX Environmental Ltd was incorporated to manage septicity risk within the transferred flows.

Existing assets were reused where they remained appropriate: the inlet screen was modified to accommodate increased inflow, existing storm tanks were retained, and the existing power supply and kiosk were reused where practicable.

Flows will be conveyed via approximately 1.7km of 225mm PE100 SDR17 rising main, discharging to a bespoke chamber before continuing through an upsized combined sewer. A key element of the solution was the adoption and upgrade of existing private sewer infrastructure at Conyers School, avoiding a longer route and removing the need for a more disruptive alignment, including a potential trenchless railway crossing.

Engineering judgement & innovation

The technical significance of Kirklevington is not a single isolated detail; it is the sequence of engineering judgements that changed the project outcome. The team followed a TOTEX-led hierarchy to avoid unnecessary new build, re-use assets where technically defensible, optimise existing network capacity, and only construct new infrastructure where it added clear system value.

[caption id="attachment_32219" align="alignnone" width="1200"] Illustrative visualisation of the proposed works. Base imagery from Google Earth; imagery © 2026 Airbus. Proposed infrastructure has been digitally rendered by Esh-Stantec for illustrative purposes only and do not depict existing site conditions or the final constructed design[/caption]

This hierarchy materially changed the pipeline route. By establishing that the downstream network and the existing private sewer could accept the transferred flows, subject to targeted upsizing and adoption, the team reduced the length of new pipework by approximately half. This reduced material use, land disturbance, highway interface, programme exposure and community disruption.

The balancing tank also reflected practical delivery intelligence. The use of a modular precast concrete tank supported off-site fabrication, reducing site activities, improving installation certainty and shortening the period of open excavation. On a constrained live site, these choices reduced both construction risk and operational interface risk.

Kirklevington Transfer: Supply chain - key participants Principal designer & contractor: Esh Stantec Designer: Stantec UK Contractor: Esh Group Archaeological services: Archaeological Services Durham University Temporary works design: JC Consulting Temporary works cofferdam: MGF Ltd Fencing & groundworks: Stuart Marley Ltd Piling works: Interdrive Piling Ltd Directional drilling installation: Pipeline Drillers Ltd Pipelaying: DMJ Civils Ltd Tanks & chambers: Carlow Tanks (PPR Environmental Ltd) MEICA installation & commissioning: Retroflo Ltd Pipework & fittings: Saint Gobain PAM UK Pumps: Xylem Water Solutions MCC: CEMA Ltd Chemical dosing: OMEX Environmental Ltd Chemical dosing: Yara UK [caption id="attachment_32222" align="alignnone" width="1200"] Construction of the 380m3 balancing tank - Courtesy of Esh Stantec[/caption] Collaboration & project delivery

Kirklevington was delivered through intensive collaboration between Esh Stantec, Northumbrian Water, local stakeholders and specialist subcontractors. Collaborative planning sessions were used to hold the programme against a hard regulatory date. Weekly design and site meetings allowed design development and construction planning to progress in parallel, while regular client progress meetings gave visibility of risks and decisions.

The local authority highways team was engaged to avoid clashes with nearby contractors and to coordinate at pinch points. Regular communication with local schools enabled safe working during term time, reduced disruption, and ensured reinstatement was completed before the start of the academic year. Design engineers maintained regular site attendance, supporting fast resolution of buildability issues and reducing the risk of design intent being lost during construction.

Trenchless crossing & delivery constraints

One of the most sensitive construction interfaces was the 65m horizontal directional drilling crossing below a main road and a large-diameter raw water main.

The crossing avoided a disruptive road closure and reduced the direct interface with the strategic water asset. It required close collaboration between the site team, lead designers, geotechnical specialists, the HDD contractor, Northumbrian Water network operatives and other stakeholders.

The works were planned during the school summer holiday to reduce the exposure of pupils, parents and staff to construction movements. While this was the correct social and safety decision, it compressed the available programme and in doing so, increased the premium on planning, contingency and rapid decision-making. Joint risk meetings were used to agree the accuracy requirements, escalation strategy and contingency approach before the crossing was undertaken.

[caption id="attachment_32216" align="alignnone" width="1200"] Balancing tank nearing completion - Courtesy of Esh Stantec[/caption] Sustainability & carbon

The main carbon benefit came from changing the asset strategy rather than adding a visible low-carbon product late in the process. By transferring flows to an existing downstream works with capacity, the scheme avoided a more substantial treatment upgrade at Kirklevington and reduced the whole-life operational burden associated with a small local treatment site.

The revised route reduced the new pipeline length by around 50%, cutting the volume of pipe, fittings, excavation, reinstatement and temporary works required. The project material records a 126 tCO2 saving from route selection. The use of existing assets, adoption of private sewer infrastructure, off-site precast construction and local subcontractors further reduced waste, site movements and construction exposure.

There was also a direct environmental outcome. The receiving watercourse now receives no final effluent from the former local treatment works, rather than receiving effluent treated to a lower phosphorus concentration. Archaeological Services Durham University undertook pre-construction archaeological surveys, and the pipeline alignment was selected to avoid working within the flood zone where practicable.

[caption id="attachment_32215" align="alignnone" width="1200"] New hardstanding and access covers to the balancing tank and below-ground pumps - Courtesy of Esh Stantec[/caption] Health, safety & welfare

Safety management was integrated into planning and methodology rather than treated as a separate construction control. Toolbox talks were used for the key construction activities, remote-control plant was adopted where practical, and stakeholder interfaces were managed collaboratively across the delivery team and supply chain.

The decision to programme the school-interface works during the summer holiday reduced public interface risk. Operational safety was also improved through the automation of de-ragging logic, reducing the need for direct human intervention during future operation and maintenance.

Social value & community benefit

The project’s social value was most visible in how it handled its immediate community interfaces. Working around the school calendar reduced disruption to pupils, parents and staff. The revised route and use of existing infrastructure reduced working in the carriageway and lowered the impact on local traffic. The scheme also reduced the need for tanker access through narrow residential streets to the existing works.

The team retained community value in smaller decisions as well as in headline engineering decisions. An ornamental tree carving of an owl was created instead of simply removing a tree, and football strips were provided to a local youth football team. These actions were modest individually, but they helped turn an intrusive infrastructure project into one with visible local benefit.

[caption id="attachment_32218" align="alignnone" width="1200"] Ornamental tree carving of an owl - Courtesy of Esh Stantec[/caption] Outcomes

The scheme achieved operational completion ahead of the regulatory deadline and delivered a significant saving of 25% against the agreed target cost. It modernised critical wastewater infrastructure, reduced the need for future operational interventions at a constrained local works, and delivered a more resilient network-level arrangement.

Its strongest lesson is that the best infrastructure solution is often created before detailed design begins; by asking whether the project has been framed at the right asset boundary.

Located in St Helens in the north-west of England, this project was undertaken to address a longstanding issue associated with a Combined Sewer Overflow (CSO) at Gaskell Street. Under storm conditions, the existing outfall regularly discharged stormwater into Sankey Brook, posing both environmental and regulatory challenges. The scheme was therefore driven by the requirements of the Environment Agency’s Storm Overflow Discharge Reduction Plan and delivered as part of United Utilities’ AMP8 Better Rivers programme, which seeks to significantly reduce the frequency and impact of storm overflows across the network. Project drivers

The Gaskell Street CSO & Detention Tank Project was developed to address several key environmental improvement drivers:

EnvAct_IMP2: Reducing spill frequency to no more than an average of 10 rainfall events per year. EnvAct_IMP4: Ensuring no local adverse ecological impact. WINEP screening driver: 6mm solids separation up to the 1 in 5-year design storm critical flow rate with 2050 climate change allowance. [caption id="attachment_31809" align="alignnone" width="1200"] Casagrande C6 XP2 rig used by Van Elle for grouting of fissures & voids - Courtesy of Ward & Burke[/caption] Stakeholder collaboration

With a challenging regulatory deadline of 31 March 2026 driving an accelerated 11‑month delivery programme, the scheme was undertaken within a complex risk environment.

Ground conditions presented significant uncertainty, including a suspected former brickworks, areas of historic landfill, and legacy coal mine workings. These constraints required a proactive and risk-informed approach throughout design and construction. Against this backdrop, close and continuous collaboration with the project delivery team, St Helens Borough Council, local stakeholders, key suppliers, and the Environment Agency was critical to successful delivery. This integrated approach enabled risks to be effectively managed, solutions to be aligned with stakeholder expectations, and project momentum to be maintained despite the challenging conditions.

The chosen solution

To address these requirements, the preferred solution brought together several complementary civil, mechanical and hydraulic interventions designed to improve screening performance, provide additional storm storage capacity and upgrade the existing overflow infrastructure.

[caption id="attachment_31107" align="alignnone" width="1200"] 3D Revit models: (left) key structural elements of the scheme and (right) section through combined CSO & detention tank - Courtesy of Ward & Burke[/caption]

The principal elements of the scheme comprised:

A new CSO chamber and 700m3 underground detention tank constructed within Sunshine Park. A replacement 6mm screen sized for significant peak design flows of 1,481 l/s (1 in 5-year event) and 1,739 l/s (1 in 30-year event). A new overflow sewer and outfall headwall to replace undersized existing assets. Project details

At the heart of the scheme was the 12.5 m internal diameter, 14.6m external diameter, 10m deep in situ reinforced concrete (RC) sunk caisson shaft which provides the required 700m3 of storm storage volume. Flows are screened upon entry into the detention tank via a new CSO channel housing an Eliquo Hydrok 180° MecMex removable screen, 3m in length, sized to deliver 6mm solids separation for a design flow rate of up to 1500 l/s.

[caption id="attachment_31112" align="alignnone" width="1200"] liquo Hydrok MecMex screen installed on removable guide rails within CSO channel - Courtesy of Ward & Burke[/caption]

During storm conditions, screened flows are discharged to Sankey Brook via the new overflow sewer and a precast outfall headwall from Tracey Concrete constructed on the riverbank. The headwall incorporates a flap valve from Althon Ltd to prevent high river levels from backing up into the sewer network during storm events, while integrated energy dissipation and erosion protection measures were included to manage discharge velocities and protect the bed and banks of the watercourse from scour.

Following the end of a storm event, the screened effluent retained within the detention tank is returned to the foul network by a duty-standby return pump set operating at a controlled rate of 20 l/s, allowing the stored volume to be released gradually without overloading downstream assets. To maintain long-term operational performance and minimise the risk of sediment accumulation within the tank, a low-head mixer pump fitted with hydro-ejector venturi nozzles was provided to periodically mobilise and clean deposited solids.

The wider scope of works also included the construction of a series of new manholes and connecting sewers to integrate the upgraded assets with the existing network, together with dedicated instrumentation chambers housing flow switches and associated monitoring equipment. To support the operation of the site’s mechanical and electrical installations, a new motor control centre panel from Lloyd Morris Electrical Ltd was installed within a purpose-built kiosk, providing a secure and accessible location from which the screening, pumping and ancillary systems could be powered, monitored and controlled. The kiosk also accommodated a new telemetry panel, enabling operational data and alarms to be communicated directly to United Utilities’ control systems and supporting effective remote monitoring and site management. [caption id="attachment_31808" align="alignnone" width="1200"] Electrical installation within MCC kiosk - Courtesy of Ward & Burke[/caption] Supply chain: Designers, consultants & contractors Principal contractor: Ward & Burke Outfall FRAP application: JBA Consulting Ecological consulting & surveys: The Environment Partnership Tyrer Ecological Bowland Ecology Vegetation clearance: Treefellers Ltd Invasive species management: Budget Knotweed Solutions Testing services: Socotec Road safety audit: Six:TEN Highways & Traffic Utility surveys & CCTV: Lanes Group Excavation shoring: MGF Ltd Ground treatment & permeation grouting: Van Elle Electrical installation: LEC Electrical Groundworks & enabling works: JR Hind Ltd Reinforcement drawings & schedules: Paul Benham Ltd Concrete cutting services: Access Drilling Services Ltd Caisson shaft construction

Construction of the underground detention tank presented a number of significant geotechnical challenges, with the ground investigation identifying weak mudstone bedrock, a high phreatic groundwater level and potential voids beneath the working footprint in a historical coal mining area. To address these challenges, Van Elle was engaged to undertake initial ground treatment, using injection grouting to stabilise and fill the potential voids identified in the ground investigation prior to the main shaft works commencing.

[caption id="attachment_31807" align="alignnone" width="1200"] Excavation and sinking of the 12.5m internal diameter caisson shaft to formation level - Courtesy of Ward & Burke[/caption]

A sunk in situ caisson method was then selected as the preferred construction approach, providing a robust means of mitigating the risks associated with groundwater ingress, ground instability within the weak mudstone, and flotation forces generated by the high groundwater table.

As the caisson was progressively excavated and sunk, bentonite slurry was injected through ports cast into the in situ concrete shaft wall to lubricate the concrete-to-ground interface, reducing skin friction and enabling controlled sinking of the structure. The bentonite was batched on site using a bespoke mixing and storage tank, designed and manufactured in-house by Ward & Burke. The caisson was successfully sunk to formation level in just 4-weeks, to form the structure of the new underground detention tank.

Supply chain: Suppliers of process plant & equipment Control panels: Lloyd Morris Electrical Ltd Kiosks: Quinshield Ltd Screen supply & install: Eliquo Hydrok Ltd Precast CSO channel & roof slab: Shay Murtagh Precast Precast manhole rings, sewer pipes & headwall: Tracey Concrete Concrete washout tanks: Kelly Tanks Ltd Pipework supply: Electrosteel Castings (UK) Ltd Coated steel dropper pipes: PMJ Mechanical Ltd Submersible return & mixer pumps: Xylem Water Solutions Flap valve supply: Althon Ltd Davit socket & lifting arm: T Allen Engineering Services Ltd Lifting equipment: Rope & Sling Specialists (RSS) Access covers: Technocover Ltd Geotextile supply: Tailor Engineering Instrumentation: Siemens Fabricated steel components: C&D Engineering GRP ladders, platforms & handrails: Chemglass Reinforcing steel supply: Dominic Lydons Steel Concrete pumps: Camfaud Concrete Pumps Ltd Concrete supply: Cemex Welfare buildings: Nixon Hire Site security services: Direct Security Ltd Muck removal & aggregate supply: Envirovue | J Phillips Grab & Tipper Hire Ltd | JD Haulage & Aggregates Plant hire: GAP Group Mobile crane services: Cork Crane Hire | AMF Lifting [caption id="attachment_31113" align="alignnone" width="1200"] Gaskell Street site following the installation of the RC precast channel - Courtesy of Shone’s Drones[/caption] Innovation & efficiencies

Integrated structure

A key innovation on this project was the decision to combine what would typically be two separate underground RC structures - a rectangular CSO chamber and a circular detention tank shaft - into a single integrated structure. The early-stage design adopted the conventional arrangement, with the CSO chamber and detention tank developed as standalone elements, each requiring its own excavation, formwork and temporary works elements.

Ward & Burke and United Utilities devised an alternative solution, in which a CSO channel structure was integrated within the detention tank and designed to be supported on rebates cast directly into the detention tank shaft concrete walls.

This integrated approach delivered a significant reduction in overall structural footprint within the park, enabling the existing football pitch to be retained in full for continued use by local residents following the works. It also eliminated the need for a second deep excavation in the challenging ground conditions, removing the associated geotechnical, groundwater and programme risks. The combined structure delivered considerable savings in construction programme and cost, alongside a meaningful reduction in embodied carbon and the overall volumes of concrete and reinforcing steel required, delivering better outcomes for the community, the client and the environment.

CSO channel

An additional efficiency involved the CSO channel itself being fabricated and installed as a single precast unit rather than being cast in situ within the shaft. Manufacturing the channel off-site ensured tighter dimensional tolerances and a higher likelihood of a higher quality finish; eliminating the need for installing complex formwork and falsework at height within the caisson.

[caption id="attachment_31109" align="alignnone" width="1200"] (left) CSO channel structure being lifted into position and (right) the precast RC CSO channel and coated steel dropper pipes installation - Courtesy of Ward & Burke[/caption]

Manufactured by Shay Murtagh Precast, the 62-tonne unit was installed in a single lift onto the rebates cast into the shaft wall, significantly reducing installation time on site and removing a sequence of high-risk activities from the confined working environment when compared with the alternative in situ construction method.

Outfall headwall Installation

Constructing a new outfall headwall directly within Sankey Brook required regular engagement with the EA and JBA Consulting to develop the ecological, technical and third-party materials necessary to support the Flood Risk Activity Permit application.

Several ecological surveys were undertaken across the affected watercourse and surrounding habitats, including assessments for water voles, otters, bats and nesting birds. Precautionary working method statements were developed in response to the survey outputs and briefed to all site personnel prior to works commencing.

A geomorphology assessment was carried out to understand existing channel behaviour, sediment transport and erosion characteristics at the discharge location, which informed the erosion protection design including a baffle wall and downstream riprap to dissipate discharge energy and protect the bed and banks against scour. A bespoke construction methodology was also developed to minimise working within the footprint of the river and to protect water quality throughout the headwall installation. [caption id="attachment_31110" align="alignnone" width="1200"] New outfall headwall installation on bank of Sankey Brook - Courtesy of Ward & Burke[/caption] Social value & community benefits

Social value and community engagement were built into the project from the outset. A tailored traffic management plan was developed with St Helens Council and local residents to coordinate construction traffic and access arrangements, minimising disruption to the surrounding community.

To support biodiversity, the team produced a tree-planting plan and ecological assessment that identified opportunities to increase planting within the park. The project also delivered a programme of volunteer-led community activities, including litter-picking and river-cleaning events in Sunshine Park, helping to improve the wider environmental health of Sankey Brook during the works.

In addition, a resident and stakeholder liaison event gave the community an opportunity to meet the project team, learn about the purpose and progress of the works, and raise questions or concerns directly. Together, these measures helped create a lasting positive impact for the people of St Helens.

Conclusion

The Gaskell Street CSO & Detention Tank scheme demonstrates how innovative engineering, close collaboration and a strong focus on environmental outcomes can deliver meaningful improvements to local river health. By combining the CSO chamber and detention tank into a single integrated structure, and by engaging early and regularly with the Environment Agency, St Helens Borough Council and the local community, Ward & Burke and United Utilities delivered this complex project on time and ahead of the 31 March 2026 regulatory deadline.

The scheme will significantly reduce the frequency and volume of wastewater spills at Gaskell Street CSO, improve water quality in Sankey Brook, and support the wider aims of United Utilities’ AMP8 Better Rivers programme: cleaner watercourses, enhanced public spaces and lasting community benefit for the people of St Helens.

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.