Supply Chain - Precast Concrete - SuDS Products

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.

The Hill View Road Combined Sewer Overflow (CSO) is located within a constrained residential catchment in Brimington, Chesterfield. Historic performance data identified spill frequencies approaching 100 events per annum, significantly exceeding current regulatory expectations. As part of Yorkshire Water’s Storm Spill Reduction Programme (SSRP), the scheme aimed to reduce spill frequency to a maximum of ten events per year through the provision of additional storage capacity, within a site footprint of approximately 0.1ha and in close proximity to existing residential and commercial infrastructure. Hydraulic & civil solution

The selected solution comprised the design and construction of a new concrete shaft tank providing offline storm storage. During storm events, excess flows from the existing CSO are intercepted and diverted into the new structure. Once flows subside, stored volumes are returned to the sewer network via pumped discharge, utilising available downstream capacity.

The shaft measures approximately 7.5m internal diameter and up to 12.5m depth, providing over 300m³ of effective storage, exceeding the minimum hydraulic requirement of 270m³. A fully passive overflow route allows flows to revert to the existing CSO arrangement during exceedance events or power failure, ensuring resilience and preventing upstream surcharge.

[caption id="attachment_30225" align="alignnone" width="1200"] (left) Revit model and (right) digital visualisation - Courtesy of Galliford Try[/caption] Ground risk & shaft foundation design

The principal technical challenge was managing geotechnical uncertainty associated with historic coal mining activity. Early desk studies and intrusive ground investigation indicated the potential presence of coal seams or voids at, or close to, shaft formation level, introducing uncertainty around bearing capacity and uplift resistance.

An initial worst-case solution was developed incorporating a substantial mass concrete flotation plug at the shaft base. To support efficient optioneering, the design team employed parametric shaft design tools, enabling rapid assessment of shaft diameter, base thickness, collar geometry and flotation performance as assumptions evolved.

As excavation progressed, further inspection confirmed that the shaft location was clear of coal seams at formation level. This allowed the foundation design to be re-evaluated and optimised, removing the mass concrete beneath an underreamed base.

The revised solution satisfied all stability and flotation requirements while removing approximately 80m³ of concrete. This reduced excavation volumes, muck away, lifting operations and programme duration, while delivering a measurable reduction in embodied carbon.

Digital design & constructability

Outputs from the parametric design tools were directly integrated into a three dimensional BIM model developed in Revit. The model was used to coordinate reinforcement, penetrations and construction sequencing, improving buildability and reducing the risk of late-stage design changes.

Particular attention was given to the shaft collar and cover slab arrangement. Traditional approaches often require significant temporary works and post-installation modification to accommodate service penetrations. At Hill View Road, penetrations for ventilation, cabling and chemical dosing were sleeved and cast integrally during construction, eliminating secondary works and reducing manual handling, cutting and vibration risks.

The cover slab was designed without beams or post installed openings, simplifying construction and long-term maintenance.

[caption id="attachment_30230" align="alignnone" width="1200"] (left) Commencement of shaft sinking and (right) excavation to formation - Courtesy of Galliford Try[/caption] Working within a constrained footprint

The restricted site footprint presented a further challenge, requiring accommodation of the shaft, valve chamber, lifting operations, welfare facilities, materials storage and access routes within a tightly defined boundary.

Digital site visualisation and construction sequencing models were used to verify the safe and efficient layout of all elements. Applied later in the project, they confirmed that storage areas, pedestrian segregation, plant positions and exclusion zones were robust and fit for purpose.

This approach resulted in a stable and predictable construction sequence, minimising congestion, rehandling and interface risk despite the constrained site.

Trenchless installation & environmental constraints

The rising main route between the existing CSO and the new shaft was constrained by mature trees and environmental sensitivities. Open cut installation would have required excavation within root protection areas, posing a risk to established vegetation.

A trenchless installation methodology was therefore adopted, enabling installation without disturbing sensitive areas. This reduced surface disruption and simplified reinstatement while maintaining alignment accuracy and construction quality.

[caption id="attachment_30227" align="alignnone" width="1200"] Cover slab placement - Courtesy of Galliford Try[/caption] Screening & operational performance

To protect downstream assets and enhance operational reliability, flows entering the shaft pass through an existing 6mm two-dimensional mechanically cleaned screen prior to storage. This prevents ingress of solids and debris, reducing maintenance requirements and the risk of pump blockage.

Ultrasonic level monitoring controls the pumped return of stored flows to the sewer network once storm conditions subside. The system is configured to maximise use of available downstream capacity while safeguarding against surcharge.

Hill View Road CSO: Supply chain - key participants Main contractor & temporary works: Galliford Try Designer: GHD Shafts, tunnelling & civil engineering: KIAMM Ltd Electrical installation: Main Electrical Ltd MCC & kiosks: Bridges Electrical Engineers Ltd Pumps: Xylem Water Solutions Water treatment plant: Siltbuster Group Shaft sections & cover slabs: FP McCann Ltd Access covers: Steelway Brickhouse Site security & construction timelapse: Wireless CCTV Ltd Generator hire: Power Electrics Plant hire: Morris Leslie Plant Hire Programme optimisation

To support efficient delivery, design acceptance was split into two phases. The civil design for the shaft and associated structures was approved ahead of completion of the mechanical and electrical design.

This enabled excavation and structural works to commence earlier, while detailed MEICA design progressed in parallel. It is estimated that this approach reduced the overall programme by approximately five weeks, without introducing additional technical or commissioning risk.

Hill View Road CSO: Construction Timelapse - Courtesy of Wireless CCTV Ltd

Outcomes

Commencing on site in April 2025 and completing in November 2025, the £2.5m Hill View Road CSO project has delivered:

Over 300m3 of additional storm storage. Approximately 85% reduction in spill frequency. Removal of around 80m3 of concrete through foundation optimisation. Reduced excavation depth and construction duration. Improved operational resilience and maintainability. Safe and efficient delivery within a constrained urban site. Conclusion

The Hill View Road CSO project demonstrates how early engagement with ground risk, supported by parametric digital design tools and informed engineering judgement, can deliver efficient and robust infrastructure solutions.

Despite its relatively modest scale, the scheme required careful management of geotechnical uncertainty, site constraints and constructability. The use of digital optioneering, integrated modelling and design for construction detailing enabled reductions in material use, programme and risk, while achieving a significant improvement in environmental performance.

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

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

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

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

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

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

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

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

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

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

This passive approach:

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

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

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

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

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

Two primary techniques were deployed:

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

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

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

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

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

Cost savings

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

Further efficiencies were realised through:

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

These measures ensured improved outcomes while maintaining affordability for customers.

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

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

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

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

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

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

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

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

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

Health & safety and customer engagement

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

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

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

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

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

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

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

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

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

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

Owned and operated by Dŵr Cymru Welsh Water, Corwen Wastewater Treatment Works (WwTW) is located north of Corwen town centre in north Wales. The works serves a predominantly rural catchment with a current population equivalent (PE) of around 2000 and comprises inlet pumping (of both foul and storm flows), primary settlement, stone media trickling filters, and humus settlement, with final effluent discharge into the River Dee. The River Dee is of high environmental importance and includes designated sensitive areas, including Sites of Special Scientific Interest (SSSI). Project background

Phosphorus in treated wastewater can lead to oxygen depletion, deterioration of habitat quality, and adverse impacts on sensitive aquatic ecosystems. As such, phosphorus control is a critical component in protecting the ecological status of receiving waterbodies, particularly those designated for environmental conservation.

With the River Dee designated as a Site of Special Scientific Interest, the regulatory direction set by Natural Resources Wales (NRW) is to progressively reduce permitted phosphorus discharges to support the achievement of ‘good ecological status’ under environmental water quality objectives. This has resulted in the introduction of tighter total phosphorus consents; in this case, a limit of 1.0 mg/l.

The introduction of the tightened total phosphorus consent required targeted upgrades to maintain compliance with the revised conditions. Dŵr Cymru Welsh Water undertook an assessment of the necessary modifications through its process design, defining the scope of works required to achieve compliance at Corwen while maximising the reuse of existing assets where practicable.

The scheme also presented an opportunity to address ageing infrastructure, improve hydraulic performance across the works, and enhance operational resilience within a site constrained by flood risk.

[caption id="attachment_31276" align="alignnone" width="1200"] Biofilter distribution bed - Courtesy of Eric Wright Water Ltd[/caption] Existing works & drivers

The primary driver for the scheme was to maintain regulatory performance in line with revised consent conditions. To achieve this, chemical precipitation using ferric sulphate dosing was required, integrated into the new inlet pumping main. Additional drivers included:

Ageing infrastructure, including inlet screening, the existing Dortmund-type primary settlement tank, the existing humus settlement tank, half-bridge scrapers and the biofilter distribution arms. Existing hydraulic constraints affecting distribution efficiency. Opportunities to improve operational flexibility within the inlet and settlement processes. Site flooding risk due to proximity to the River Dee. Design & delivery approach

The scheme was delivered under a design and build contract, with Eric Wright Water Ltd acting as principal contractor. Waterco Datrys undertook the multidisciplinary design, encompassing civil, mechanical, and electrical disciplines, supported by FBW Engineering.

A collaborative delivery model was adopted, with weekly coordination meetings held between Dŵr Cymru Welsh Water Process, Operations, and the delivery team. This approach ensured that operability, maintainability, and commissioning requirements were fully embedded within the developing design.

Corwen WwTW: Supply chain - key participants Client: Dŵr Cymru Welsh Water Principal Designer & Contractor: Eric Wright Water Ltd Civil & MEICA design: Waterco Datrys Piling contractor: Van Elle EICA design: FBW Engineering Services Ltd EICA contractor: Lloyd Morris Electrical Ltd Mechanical contractor: Whitland Engineering Ltd Precast concrete: Shay Murtagh Precast Ltd Half-bridge scrapers: Eric Wright Water Ltd Inlet screen & screenings handling: Jacopa Limited Imports screen: Huber Technology Ferric dosing tank & associated equipment/kiosk: Colloide Safety shower & hose reels: Hughes Safety Showers Ltd Stainless steel sludge tanks: Stortec Engineering Ltd Sludge tank mixers: Xylem Water Solutions Pumps: Watson Marlow Ltd Pumps: Grundfos Pumps: KSB Ltd Pumps: Xylem Water Solutions Washwater booster pumpset: Dutypoint Ltd Static mixer: Statiflo International Ltd Access steelwork fabricator: Whitland Engineering Ltd [caption id="attachment_31279" align="alignnone" width="1200"] Jacopa inlet screen - Courtesy of Eric Wright Water Ltd[/caption] Inlet works & screening upgrade

Raw flow enters the works via a gravity sewer to the inlet pumping station wet well. The inlet works were upgraded to improve screening performance and operational resilience. A new 6 mm, 2D mesh Jacopa bandscreen was installed within the wet well, replacing the existing screening arrangements and maceration equipment. The new screen and screenings handling system provide enhanced removal of gross solids and rag material prior to pumping to downstream treatment processes and storm storage.

A high-level bypass arrangement, incorporating a manually raked screen, was also provided to ensure flow continuity in the event of mechanical failure. The bypass is set above known flood levels, maintaining operability during extreme events.

Inlet pumping main & ferric dosing

The pass-forward inlet pumping arrangement was reconfigured to discharge directly to full treatment, replacing the previous high-level channel. The new inlet main incorporates:

An MCERTS-compliant flow meter. Connections for sludge supernatant and humus return flows. Valved rodding points for cleaning and inspection. Ferric sulphate dosing via a static mixing arrangement. [caption id="attachment_31275" align="alignnone" width="1200"] Colloide ferric dosing tank and access platform - Courtesy of Eric Wright Water Ltd[/caption]

A 10m3 ferric sulphate storage tank has been installed, along with a dedicated dosing kiosk designed and supplied by Colloide. The dosing installation includes a bunded chemical delivery off-loading area with a contained drainage system, ensuring that any spills are safely managed in accordance with operational and environmental requirements.

The containment area is controlled via an actuated valve, configured to maintain functionality and resilience during flood events.

Primary settlement upgrades

A new radial primary settlement tank was constructed adjacent to the existing Dortmund-type tank. The new tank comprises a precast, above-ground circular structure and is fitted with a half-bridge scraper mechanism, an automated scum removal system, and a V-notch weir arrangement. Sludge draw-off is directed to the existing sludge handling infrastructure.

The existing primary settlement tank has been retained to provide operational flexibility and can be brought back into service during maintenance or outages of the new tank.

[caption id="attachment_31271" align="alignnone" width="1200"] Primary settlement tank - Courtesy of Eric Wright Water Ltd[/caption] Biofilters & hydraulic improvements

Downstream biological treatment is provided by two existing stone media biofilters. Upgrades were undertaken to improve hydraulic distribution and optimise treatment consistency.

Initial investigations identified hydraulic surcharge as being associated with the existing dosing siphon arrangement. This was addressed through removal of the siphon and the installation of a new inlet pipework configuration.

The scope of works included increasing the media depth, replacing the distributor arms, modifying the inlet pipework arrangement, and investigating hydraulic back-up between the primary settlement tank and the biofilters.

Humus settlement

A new humus settlement tank was constructed adjacent to the existing unit to increase overall settlement capacity and provide operational flexibility. The new tank comprises a precast, above-ground structure fitted with a half-bridge scraper mechanism, an automated scum removal system, and a V-notch weir arrangement.

New peristaltic pumps were installed to serve both humus settlement tanks, returning humus sludge to the primary settlement tank for co-settlement.

[caption id="attachment_31273" align="alignnone" width="1200"] Humus settlement tank - Courtesy of Eric Wright Water Ltd[/caption] Final effluent & washwater systems

A new final effluent kiosk was installed, housing online monitoring instrumentation for phosphorus, iron, suspended solids, and pH.

Final effluent is reused for washwater applications across the site, including inlet and imports screening. A new pumping station was constructed to provide both final effluent washwater supply and outfall pumping, improving the resilience of final effluent sampling and settlement processes during periods of high river levels.

Where potable washwater is required, a new Dutypoint booster system provides a reliable supply to the chemical dosing and sludge handling areas. This arrangement ensures clear segregation between process effluent and potable systems, enhancing operational safety.

[caption id="attachment_31280" align="alignnone" width="1200"] Final effluent chamber and sampling chamber - Courtesy of Eric Wright Water Ltd[/caption] Sludge handling & storage

Sludge handling arrangements at Corwen WwTW were upgraded as part of this scheme to improve operational resilience, segregate sludge streams, and enhance overall handling efficiency.

Two new above-ground stainless steel storage tanks, supplied by Stortec Engineering Ltd, were installed to provide dedicated storage for indigenous sludge. This improves process stability and enables more controlled sludge management across the works. In addition, a dedicated imports reception facility was constructed to allow tanker deliveries of cess and sludge to be received, screened, and stored safely on site.

The imported sludge system includes an integrated screening unit, supplied by Huber Technology, to remove gross solids prior to pumped transfer into a new above-ground stainless steel imports tank, also supplied by Stortec Engineering Ltd.

[caption id="attachment_31272" align="alignnone" width="1200"] Sludge tanks & imports screen platform - Courtesy of Eric Wright Water Ltd[/caption] Electrical & ICA

Key elements of the electrical and ICA upgrades included:

Upgrade of the incoming power supply and transformer capacity. Installation of a new kiosk housing the main MCC, with provision for future expansion. Integration of PLC and HMI control systems. Installation of MCERTS-compliant instrumentation for flow and sludge monitoring. Innovations

A number of innovations were adopted for the successful delivery of the project:

The use of precast concrete settlement tanks, from Shay Murtagh Precast Ltd, reduced programme duration and construction risk. A collaborative delivery approach enabled efficient progression of a complex scheme within tight timescales. Asset reuse was maximised wherever practicable: The inlet screening solution was designed to utilise the existing wet well, avoiding the need for a new structure. Existing biofilter beds were refurbished to improve treatment capacity and performance. The existing humus tank was retained, with mechanical elements replaced while preserving the structurally sound concrete infrastructure. [caption id="attachment_31278" align="alignnone" width="1200"] Installation of the MCC kiosk - Courtesy of Eric Wright Water Ltd[/caption] Construction constraints

Flood risk

The site is located adjacent to the River Dee and is subject to regular flooding. Critical electrical and mechanical infrastructure was therefore installed on elevated platforms or protected within chambers constructed above the design flood level.

The outfall chamber was sealed and fitted with a non-return valve to allow site floodwater to drain to the outfall while preventing backflow. Process chambers and inlet works upgrades were designed to maintain operational resilience during flood conditions.

Ground conditions

Ground conditions comprised variable made ground overlying clay, gravel, and silt deposits. Due to variability in bearing capacity, and the importance of hydraulic levels across the works to ensure efficient hydraulic performance, piled foundations were adopted for key structures, including settlement tanks and sludge storage tanks.

Live site operation

The project was delivered within a fully operational treatment works. Maintaining compliance throughout construction required careful sequencing, temporary bypass arrangements, and the staged commissioning of new assets.

These activities were managed and delivered by Eric Wright Water Ltd in collaboration with Dŵr Cymru Welsh Water.

[caption id="attachment_31274" align="alignnone" width="1200"] De-sludge return pump platform - Courtesy of Eric Wright Water Ltd[/caption] Conclusion

The Corwen WwTW phosphorus upgrade scheme delivers a targeted process intensification and infrastructure renewal project within a constrained and operationally sensitive site.

The introduction of ferric dosing, combined with upgraded settlement processes and improved hydraulic control, ensures compliance with the tightened 1.0 mg/l phosphorus consent. The scheme has also significantly enhanced site resilience through the implementation of flood protection measures and electrical infrastructure upgrades.

The works were delivered while maintaining continuous treatment, demonstrating effective coordination between Dŵr Cymru Welsh Water, the design team, and the contractor. The upgraded works now provide a robust and flexible asset capable of meeting current compliance requirements while accommodating future operational demands.

In addition to achieving regulatory compliance, the scheme supports Dŵr Cymru Welsh Water’s wider ambition to protect and enhance the natural environment, particularly the rivers and catchments across Wales. By improving treatment performance, operational resilience, and long-term asset reliability, the project contributes to sustainable wastewater management and helps deliver meaningful environmental improvements within the communities it serves.

The completed works therefore represent not only an investment in infrastructure resilience, but also a positive step towards safeguarding water quality for future generations.

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.

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 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.

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.