Supply Chain - Treatment Process Technologies - Pile Cloth Filters

Anglian Water tasked its @one Alliance with identifying an effective and innovative solution to this issue. The conventional approach, iron dosing, would have needed major civil works, a new concrete base, modifications to the existing process, and an ongoing supply of iron chemicals. This would create ongoing costs, increased carbon emissions from deliveries, and higher operational complexity.

Instead, the team chose to challenge convention by developing Anglian Water’s first Integrated Constructed Treatment Wetland (ICTW) as a nature-based alternative. The final design created a three‑cell wetland covering 18,320m² (around 2.5 football pitches).

To build a wetland of this size, Anglian Water purchased the required land from a local farmer who owned the field downstream of the existing works. The existing outfall pipeline ran parallel to the farmer’s field and Tempsford Road, carrying the treated wastewater out towards the River Ivel. To feed the wetland, the team bypassed the start of that pipeline, diverting the water through the gravity-fed wetland before cutting back into the existing pipeline at the new wetland outfall location.

Over a six-month construction and planting period, the team excavated the three wetland cells and planted more than 110,000 native plants, including:

Emergent wetland plant species

Marginal wetland plant species

Like many capital projects, the Everton Integrated Constructed Treatment Wetland faced several challenges, including:

As the UK’s first ICTW, the project has drawn international attention. In November 2025, Anglian Water hosted officials from the Mexican Embassy and Government during a UK trade mission exploring innovative water management approaches. Grant Tuffs, Regional Engagement Manager, noted that the visit strengthened international collaboration and highlighted the value of nature‑based solutions in modern water infrastructure.

The wetland has also been visited throughout the earlier stages of the project by Richard Fuller, local Bedfordshire Conservative MP, showcasing the positive and widespread attention this project has received throughout its lifecycle.

Because no new dosing plant was needed, the existing site infrastructure and power usage remained unchanged. The planning and land acquisition process took three years, reflecting the complexity of delivering a first‑of‑its‑kind solution. This allowed the team to develop a robust, repeatable design for future wetlands planned across AMP8.

The Everton Integrated Constructed Treatment Wetland Project eliminated the need for chemical dosing, reducing lorry movements, operational carbon, and long‑term chemical costs. A deeper pond area was also created to encourage biodiversity and support local wildlife.

The Everton Wetland was completed in October 2025. With planting now established, full growth and system maturity are expected by Spring 2027. The plants have an indefinite lifespan but is likely to be between 30 and 50 years or more.

Overall, the project achieved an 84% reduction in carbon emissions, cutting the carbon footprint from 654 tCo2e to 104 tCo2e, largely due to balanced cut‑and‑fill earthworks that avoided imports and exports and the removal of 99% of anticipated concrete use.

The Everton Wetland Project demonstrates how nature-based design can deliver regulatory compliance while providing long-term environmental, operational, and carbon benefits. By replacing chemical dosing with an ICTW, Anglian Water has reduced costs, cut emissions, increased biodiversity, and created a blueprint for future schemes.

As part of the £37m wetland programme, the lessons from the Everton Integrated Constructed Treatment Wetland Project will shape the next generation of wetlands to be delivered across AMP8. This project represents a significant milestone in the UK water industry’s shift toward sustainable, resilient infrastructure.

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.

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:

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

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.

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.

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.

New primary settlement tank No. 4

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.

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

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.

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.

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.

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:

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.

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

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:

The existing works comprises of a screened inlet process along with an inlet flume with flows conveyed by gravity into dual primary settlement tanks. From there, flows pass into two trickling filters and subsequently into dual humus tanks, before continuing to a lagoon. The final effluent then passes through a sampling chamber prior to discharge to the watercourse.

The consented flow to full treatment (FFT) for Hoylandswaine STW remains unchanged at 8 l/s (691 m³/d). Flows in excess of three times dry weather flow (3 x DWF), overflow to the storm tanks via existing overflow weirs on the inlet channel.

One of the key challenges was the extremely constrained footprint. Space limitations were so significant that the welfare facilities had to located off-site during the construction phase. This constraint also drove the increased use of off-site manufacture and Design for Manufacture and Assembly (DfMA), which in turn reduced on-site construction time and lowered health and safety risks.

The purpose of this project was to reduce the phosphorus concentrations in the final effluent without exceeding a total iron concentration.

The new requirements were as follows:

To meet the tightened permits, the following scope of work was necessary:

Inlet works

At the existing inlet works, a fine screen and duty-only screenings handling unit were installed. The screenings handling system was upgraded to a Series 591 ACE Screener® from Haigh Engineering.

SIRIS Environmental Ltd was engaged to verify the design of the inlet flow measurement. The existing inlet flume was assessed and confirmed to be MCERTS‑compliant. A new ultrasonic level monitor was installed to meet the UMON4 driver so as to achieve compliant measurement of flow to full treatment

Dosing

To comply with the new phosphorus discharge permit, primary and secondary stage coagulant dosing was required. This was provided in the form of a poly aluminium chloride (PACl) dosing kiosk, complete with internal storage and a service water booster pump.

Due to the nature of the chemical dosing system, a dedicated off-loading area was also required. This comprised a bunded delivery apron, an interlocked delivery chamber, and an emergency drench shower from Aqua Safety Showers International Ltd.

The new primary and secondary stage coagulant (PACl) storage and dosing system was initially designed for ferric sulphate dosing; however, on-site trials identified PACl as a more suitable solution to achieve the phosphorus discharge consent.

The site layout and chemical delivery bund area were designed to accommodate the future addition of a primary stage sodium hydroxide (caustic) storage and dosing system, allowing for future site growth. The chemical dosing and storage system from Chemdose Ltd, was manufactured and tested off-site and includes 14 days of chemical storage capacity.

The primary stage dosing system was designed to deliver coagulant directly into the first compartment of the inlet works outlet chamber downstream of the inlet flume via a dosing tray. It was noted that no stable standing wave exists at low flows downstream of the flume, resulting in insufficient natural mixing.

While the original design proposed air mixing to provide the required mixing energy, site observations confirmed that the turbulence generated by raw water entering the distribution chamber provided adequate mixing. This allowed the air mixing system to be designed out, removing the need to modify the inlet works final chamber and delivering both CAPEX and OPEX savings for the client.

The second stage chemical dosing location was trialled and optimised, with the most suitable position identified as the inlets to each of the existing humus tanks.

Tertiary solids removal (TSR)

Tertiary solids removal represents the final, advanced stage of wastewater treatment, designed to remove any remaining suspended solids, nutrients, and microorganisms following secondary treatment. This process typically utilises physical filtration, such as cloth membrane technology, to achieve the high-quality effluent required for compliance with discharge standards.

At Hoylandswaine, a new tertiary solids removal disc filter was installed downstream of the existing humus tank distribution chamber, providing the final treatment stage for removing solids prior to discharge in line with the new phosphorus permit. A bypass pipe has been incorporated to allow for planned maintenance of the TSR unit.

BE JV engaged Eliquo Hydrok Ltd to supply a pile cloth filter tertiary solids removal unit, complete with an upstream coagulant flocculation tank. Eliquo Hydrok provided their SF6/30 unit, which comprises six rotating disc filters that are periodically cleaned, triggered by an increase in differential head, using filtered effluent.

Suction pumps draw filtered final effluent back through the rotating cloths, removing the build-up of solids on the influent side of the media. This process reduces head loss and allows filtration to continue efficiently. The resulting washwater is returned to the existing liquors return pumping station, from where it is conveyed back to the primary tanks for co-settlement and subsequent manual draw-off to the existing on-site sludge storage tanks.

Due to the site topography at Hoylandswaine, ground levels were reduced and retaining walls constructed to enable gravity flow to the TSR, eliminating the need for interstage pumping.

Final effluent dosing chamber

Flow from the TSR discharges by gravity into a new in situ reinforced concrete, below-ground final effluent sampling chamber. This chamber incorporates a steel baffle to create a stilling zone, enabling accurate measurement by the new outlet turbidity probe.

Power supply

Power availability at the site was limited and, following feasibility assessments for a supply upgrade, it was necessary to  design a system that could operate within the constraints of the existing 400V / 60A supply.

A new Miniform-type switchboard was installed within a GRP kiosk, back-feeding the existing switchboard located in the control building. This arrangement distributes power to the new site assets while maintaining the existing works on the original switchboard, supplied from the existing LV board.

The new LV switchboard kiosk facilitates connection to a mobile generator, providing resilience in the event of a mains power failure.

The upgraded works successfully met the compliance deadline, with all process-critical improvements completed by 31 March 2025. The full project is scheduled for completion in mid-2026.

The Hoylandswaine WwTW upgrade scheme has enhanced the site’s ability to treat incoming flows while stabilising throughput, significantly improving operational resilience within the local Yorkshire Water network for the benefit of future generations.

Leyhill WRC currently treats flows through an inlet works, primary settlement tanks, filtration and humus tanks, with sludge storage and handling facilities supported by a motor control centre, standby generator and mains incoming supply.

The agreed solution includes a new moving bed biofilm reactor (MBBR) tertiary treatment process, together with associated electrical infrastructure upgrades and a dedicated pump station to manage flows above the capacity of the MBBR unit.

Design for the scheme ran from June 2024 to January 2025, with construction starting in February 2025, and commissioning from November to March 2026.

Key drivers and requirements included:

The scheme comprises the following new assets, delivered during the scheme.

Moving bed biofilm reactor feed pump station

Flow is prioritised through the existing recirculation and washwater pumping station, with an upsized rising main installed from the existing manhole. The new station comprises a precast concrete wet well and valve chamber fitted with duty and standby submersible pumps, level monitoring and an electromagnetic flow meter on the rising main to monitor and cap flows to the new tertiary treatment unit, with a hydraulic bypass to the MBBR bypass flow pumping station for flows above this capacity.

Siltbuster Moving Bed Biofilm Reactor 30 tertiary treatment unit

A moving bed biofilm reactor from Siltbuster Group, together with a duty/standby AERZEN Machines air blower skid, provides the new tertiary treatment process, removing organic substances and assisting denitrification ahead of final discharge.

A single duty unit has been installed on an asphalt base, with space reserved alongside for a future second unit, should additional capacity be required.

Moving bed biofilm reactor bypass flow pump station

To manage flows above the maximum throughput of the MBBR30 unit of 20.4 litres per second, a dedicated MBBR bypass flow pumping station has been constructed.

The station comprises a precast concrete wet well housing duty/standby Flygt Concertor pumps from Xylem Water Solutions, each rated at around 25 l/s, operating on a simple start-and-stop control. Flows are pumped via a dedicated rising main to the final effluent sample chamber, where they are measured by an electromagnetic flow meter before joining the main final effluent stream for sampling. This arrangement allows peak flows, which can exceed 30 l/s, to be safely managed without risk of surcharging or flooding the downstream humus settlement tanks.

Electrical infrastructure

The existing 50kVA single pole transformer has been upgraded to a twin pole mounted 200kVA transformer with an authorised supply capacity of 100kVA, meeting a future estimated peak demand of 92kVA. A new 200kVA mains distribution panel, cutout switch and metering have been installed in a new kiosk, with a terminal draw pit and duct runs to the existing site distribution.

A new motor control centre has been provided in an eight-metre kiosk on raised steelwork, sized to accommodate future expansion of the treatment process without further cost. In addition, control of the existing recirculation and wash water pumping system has been updated to initiate at low incoming flow, maintaining optimum wetting rates across the existing filtration process.

Degassing, monitoring & sampling chamber

A new three-section chamber provides degassing through a stilling baffle, ammonia and temperature monitoring, and final effluent sampling through a V-notch weir compliant with the Environment Agency’s Monitoring Certification Scheme (MCERTS). A small sump and pump remove any residual flow, with monitoring signals routed via PVC cable tray to the adjacent sampling kiosk.

Sampling kiosk

A prefabricated kiosk sits adjacent to the degassing and sampling chamber and houses the instrumentation for the ammonia and temperature monitoring of the final effluent. Monitoring signals are routed from the chamber into the kiosk, where they are displayed and transmitted to the site telemetry system. The kiosk has been sized to accommodate a sample bath and additional monitoring equipment in future, providing Wessex Water with a facility already configured to extend its effluent quality monitoring capability without the need for further civil or structural works.

New standby generator

A new standby generator from Addicott Electrics Ltd provides backup power for the new tertiary treatment plant and associated electrical infrastructure. The generator sits on a reinforced concrete plinth with a concrete surround, positioned alongside the existing ferric dosing kiosk so that it can share the kiosk’s chemical containment bund. An external fill point cabinet is located adjacent to the bund, with dual contained pipework, comprising a flexible line within a rigid outer pipe, running from the fill cabinet to the generator.

Leyhill is being used by Wessex Water as a trial site for two ground engineering techniques with the potential to reduce cost and carbon across future AMP8 schemes.

Soil stabilisation

Around 900m2 of ground at the north-west of the site has been improved using in situ soil stabilisation. This area underlies the new motor control centre, the tertiary treatment plant, the degassing and sampling chamber and the MBBR bypass flow pumping station.

By strengthening the existing ground in place, the technique avoided the need to remove and replace large volumes of material as muck away, reducing lorry movements, disposal costs and the associated carbon footprint of the scheme.

A minimum clearance of 500mm was maintained between the underside of new assets and the lowest level of the stabilisation works, and ground conditions were monitored throughout and after the process to confirm long term stability. The works were carried out by specialist contractor JJ Mac Ltd in May 2025, supervised by YTL Construction.

Asphalt hardstanding

Rather than constructing concrete bases beneath all of the new plant, asphalt hardstanding has been used wherever practical, including beneath the Siltbuster MBBR30 unit and blower skid and as the base for the new motor control centre.

An asphalt base generally carries a smaller carbon footprint than an equivalent concrete slab, while still providing a durable surface for the loads imposed by the new plant. Where higher structural loading is required, such as beneath the standby generator, a reinforced concrete plinth and surround have been retained.

Both techniques are being monitored for performance and cost, with the results expected to inform their wider adoption across future Wessex Water AMP8 schemes.

Construction followed a carefully sequenced approach to protect the soil stabilisation works and maintain site operations throughout. Distribution network operator works and other preliminary enabling activities were carried out first, followed by the deeper below-ground elements that needed to be in place before stabilisation could begin.

This included the pipework connecting the MBBR feed pumping station’s hydraulic bypass to two new manholes, with excavations reaching up to 6m below ground level, and the MBBR bypass flow pumping station itself, which extends to around 5.5m below ground level, avoiding the risk of later excavation compromising the treated ground.

Specialist contractor JJ Mac Ltd then carried out the soil stabilisation works, supervised by YTL Construction. Material arising from the works was reused on site, including to raise the level of an existing final effluent chamber that required repair due to its poor condition, and asphalt hardstanding was laid over the stabilised ground to receive Siltbuster Group’s MBBR30 unit, AERZEN Machines’ blower skid and motor control centre.

During construction, the existing works remained fully operational, with flow prioritised through the existing recirculation and washwater pumping station to maintain treatment performance. The new MBBR feed pumping station has been commissioned to deliver a maximum of 20.4 l/s to the MBBR30 unit, with any excess flow directed via the hydraulic bypass to the MBBR bypass flow pumping station and on to the final effluent sample chamber.

Site access is constrained by high voltage overhead lines crossing the main entrance, beneath which all construction traffic must pass, and height restriction barriers were installed to manage this safely. Ground investigation also identified buried structures and services across the site, including foundations and cabling not shown on existing records, which were considered during detailed design and excavation planning.

A CCTV survey of the site drainage identified that an existing French drain discharges close to the works return pumping station, and that a manhole previously connected to a dedicated drainage headwall had been buried. Surface water entering at the site entrance also contributes to this pumping station being frequently overwhelmed during heavy rainfall, with the primary settlement tank weirs known to drown as a result.

The new works have been designed so that only a small additional area drains to the works return pumping station, limiting any worsening of the existing situation, with further improvements to the wider site drainage arrangement identified for future consideration.

The Leyhill WRC AMP8 Improvements Scheme has met the tightened ammonia permit limit of 3.8 mg/l ahead of the March 2026 deadline, with effluent ammonia already measured at 0.67 mg/l; an improvement of around 67.5% on previous performance and comfortably within the new limit.

This reduction will help protect fish and aquatic invertebrates from ammonia toxicity, support stable dissolved oxygen levels in the receiving watercourse and reduce the risk of eutrophication and algal blooms downstream, while demonstrating Wessex Water’s continued commitment to environmental stewardship and regulatory compliance.

The soil stabilisation and asphalt techniques trialled at Leyhill WRC have reduced muck away, cost and carbon, with results expected to inform future Wessex Water AMP8 schemes.

The scheme also incorporates space and capacity for future growth within several key assets, giving Wessex Water flexibility to respond to further requirements without significant additional construction.

Rather than relying on conventional chemical dosing or energy-intensive tertiary plants, the scheme has been designed to demonstrate how compact, modular, prefabricated technology can provide a chemical-free and carbon-efficient alternative. Once operational, the Bolnhurst system will act as a reference site for future phosphorus removal schemes in Anglian Water’s AMP8 programme.

Under WINEP, Bolnhurst WRC is required for the first time to achieve an effluent phosphorus concentration of 0.5 mg/l. This is a stringent limit, particularly for a rural treatment works with no existing phosphorus removal capability. The site’s existing assets were an inlet wet well, followed by a series of RBCs, that were at the end of their lifespan, and a small lagoon that provided settlement.

Without intervention, Bolnhurst would have been unable to meet the new consent, exposing Anglian Water to regulatory penalties and increasing nutrient loading in the receiving watercourse. Achieving the new standard required a complete wastewater treatment package, instead of just a tertiary treatment process. The challenges on site included:

Anglian Water tasked the @one Alliance to provide a robust solution that provides a variety of benefits to the local communities and environment. Initially, a range of potential treatment options were considered during the feasibility design, however following a multi-criteria assessment covering cost, carbon, footprint, operability and resilience, an innovative algae-based photo-bioreactor technology was selected as the preferred option.

Supplied by ALGAESYS S.A., the central feature of the upgrade is the 200m2 algae-based photo-bioreactor system. This is a low energy, chemical-free system for primary, secondary and tertiary treatment, capable of removing solids, organic matter, nitrogen and phosphorus, as well as reducing pathogens. At Bolnhurst WRC, this robust decarbonised technology harnesses naturally occurring algae to capture dissolved phosphorus and other nutrients from secondary effluent. The algae assimilates phosphorus as it grows, enabling P-removal without the need for chemical precipitation.

The bioreactors are designed to operate in modular tanks, equipped with LED lighting to help out in the depths of winter, and controlled aeration to keep the algae alive. Once established, the algal biomass can be periodically harvested, with the potential for beneficial reuse as an agricultural soil conditioner. There is also potential for the wastewater to be used for data centre cooling, instead of precious potable water; as AI data centres grow in the UK.

Given the restricted site footprint, the bioreactor plant was designed as a compact, modular system. Prefabricated units were assembled off-site and delivered to Bolnhurst for rapid installation. This approach minimises site disruption, lessens health and safety risks, reduces construction carbon, and allows future expansion by adding additional modules if required.

To integrate the algae system into the existing WRC, a number of supporting works were delivered:

With the regulatory deadline preceding full commissioning of the algae plant, a temporary lamella clarifier with ferric dosing from Siltbuster Group was installed. This interim system is already in place and is expected to secure compliance ahead of the statutory deadline, ensuring Anglian Water avoids permit breaches while final commissioning of the permanent works is completed.

Once fully commissioned, the Bolnhurst phosphorus removal project is expected to deliver a broad set of benefits:

The constrained nature of the site necessitated careful planning of construction logistics, lifting operations, and traffic management. Prefabrication was central in reducing working at height, hot works, and confined space entries.

Project delivery relied on a wide network of supply chain partners coordinated through the @one Alliance:

During commissioning, strict controls are in place to manage the co-existence of temporary and permanent treatment streams, ensuring no risk of environmental non-compliance. Operator training is being carried out in parallel to ensure that the new processes are understood and embedded before handover.

Once the algae-based system is fully operational, Bolnhurst WRC will be one of the first sites in the UK to meet phosphorus consents through a large-scale, chemical-free process. The project is expected to demonstrate the feasibility of algae systems as a mainstream alternative to chemical dosing, while provide real-world performance data for AMP8 phosphorus removal planning. This will also potentially reduce Anglian Water’s dependency on chemical supply chains & finally, showcase how modular prefabrication can overcome footprint constraints on small rural works.

The broader significance lies in proving that regulatory drivers can act as catalysts for innovation, driving adoption of lower-carbon and more sustainable wastewater technologies.

The Bolnhurst Water Recycling Centre Phosphorus Removal Project represents a landmark step towards sustainable nutrient management. As it moves into final commissioning, the scheme is poised to deliver compliance with stringent phosphorus limits, while simultaneously advancing Anglian Water’s goals on carbon reduction, circular economy, and resilience.

By combining an innovative photo-bioreactor system with conventional treatment, the project demonstrates how small, space-constrained WRCs can meet demanding environmental standards without reliance on chemicals or energy-intensive processes.

Once operational, Bolnhurst will stand as a reference site for the industry, providing valuable insight for future projects and reinforcing the role of the @one Alliance in delivering innovative, low-carbon infrastructure that safeguards both communities and the environment.

The Butler WTW project involved the design and construction of a fully integrated groundwater treatment facility providing a robust, flexible and fully compliant treatment solution aligned with SEW operational and regulatory standards to support long-term regional supply requirements.

The scheme forms part of South East Water’s Water Resources Management Plan (WRMP19), which identified the requirement for additional treatment capacity in the region alongside a strategic objective to reduce dependency on third-party bulk water supplies.

Raw water is sourced entirely from groundwater via a network of eight boreholes on either side of the M20. All sources discharge via a common rising main, forming the sole intake to the treatment works. This ensures resilience to meet future demand and to overcome wellfield challenges.

Murphy was appointed as the Major Projects Framework contractor by South East Water to deliver the scheme from outline design through to commissioning. The project delivery was structured to progress from outline design at RIBA Stage 3 under a Professional Services Contract, through to detailed design and construction at RIBA Stages 4 and 5 under an Engineering & Construction Contract. All stages involved collaborative design between Murphy and SEW.

The Butler WTW incorporates the following key processes, mechanical, electrical and civil elements:

The design of Butler WTW was influenced by several key constraints, most notably the uncertainty surrounding the performance and water quality of newly developed groundwater sources. While the existing boreholes had historical data available to inform early design decisions, the additional boreholes initially had limited yield and water quality information.

To address this, the treatment process was developed to operate effectively across a broad spectrum of potential conditions. Flexibility was a core design principle, influencing several key elements of the works.

The approach to sludge and GAC washwater management was developed with a focus on operational simplicity and reliability, avoiding the need for more complex treatment arrangements such as recycle streams to the head of the works. Instead, a sedimentation-based solution was adopted, providing effective solids removal through a straightforward process configuration.

This approach delivers a robust, low-maintenance system that enhances operational efficiency, reduces the level of operator intervention required, and supports long-term sustainability of the treatment works.

Design and delivery of the Butler WTW project required careful consideration of several key constraints and drivers.

Constraints associated with borehole availability, phased development of raw water sources, and integration with existing temporary treatment infrastructure required a coordinated approach to ensure continuity of supply during transition to the permanent facility. It proved to be particularly challenging to construct the project whilst keeping the temporary treatment works online.

Successful delivery depended on close engagement with a range of stakeholders. Central to this was the client, South East Water, alongside their operational and engineering teams. Collaboration also extended to key sub-contractors, suppliers and technology providers, ensuring alignment of design, construction and commissioning activities throughout the project lifecycle.

Delivery to programme was a primary objective, driven by the need to bring additional treatment capacity online due to regional demand forecasts. The project required detailed planning and sequencing to manage interfaces between borehole development, treatment process installation and commissioning activities. A structured delivery strategy ensured clear visibility of progress and risks across all stakeholders.

The project was delivered across multiple sites and presented several significant logistical and environmental constraints. Space on the main water treatment works site was particularly restricted, impacting both the permanent site layout and the management of construction activities and deliveries.

Key infrastructure elements also required reconfiguration during the works, including the relocation of the main power incomer and standby generator to an adjacent site across the road. In addition, the positions of major process units were adjusted in response to site conditions, with the sedimentation tanks relocated to mitigate risks associated with poor ground conditions and a high water table, which posed challenges for deep excavations.

The Butler WTW site was bordered by Bellingham Way, a busy road that imposed additional constraints on access and traffic management, while nearby watercourses required protection throughout the works.

Environmental considerations also included tree protection measures and the management of flood risk, with appropriate dewatering strategies implemented to enable safe excavation and construction.

The new Butler WTW provides a modern, high-capacity groundwater treatment solution that is fully aligned with South East Water’s long-term supply strategy, ensuring resilience, sustainability and the ability to meet future demand. The scheme has been designed to maximise reliability while maintaining operational simplicity, supporting both current needs and future adaptability.

Key outcomes include the delivery of up to 20 MLD of high-quality potable water, significantly strengthening local supply capacity. This reduces reliance on imported bulk water, improving network resilience and lowering operational costs associated with external supply dependencies. The works also offer increased operational flexibility, enabling the treatment process to respond effectively to variations in raw groundwater quality, which is critical for maintaining consistent output standards.

In addition, the project achieves full compliance with all relevant regulatory and operational standards, ensuring water quality, environmental protection and safe operation. Sludge and washwater management processes have been deliberately simplified, providing a robust, low-maintenance solution that enhances efficiency, reduces operator intervention, and supports long-term operational sustainability.

The Butler WTW Project represents a significant investment in water supply infrastructure within Kent. Through the integration of robust process design, flexible operation and collaborative delivery, the scheme provides a resilient and future-proofed solution to regional water demand.

The facility is well positioned to support population growth while maintaining high standards of water quality, operational reliability and regulatory compliance.

Regulatory obligations included:

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

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

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

Ground conditions

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

Existing assets & site constraints

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

HV diversions

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

Access bridge

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

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

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

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

Environmental mitigation measures included:

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

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

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

Key project outputs delivered by Trant Engineering included:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The new MMB outline design consisted of the following:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Before the upgrade, treatment at Broadway STW comprised an inlet pumping station, three Dortmund primary settlement tanks, three granite-media biofilters, four Dortmund humus tanks, 12 modular sand filters and a sludge holding tank. Temporary reed beds were also present but were offline when the original process design report was prepared.

The principal driver for the project was the need to achieve compliance with the revised discharge and flow permits. However, the design also needed to respond to an unusual characteristic of the Broadway catchment; its loading varies materially with visitor numbers. The process design was therefore developed against both average and peak theoretical loads. At the future design horizon, the theoretical peak loading was assessed as approximately:

This variation influenced the sizing and control of the biological treatment, aeration system and final settlement tanks. The new process needed sufficient capacity during peak tourism periods while retaining enough operational turndown to avoid inefficient operation or over-aeration during periods of lower loading.

The upgrade was necessary to address the following tighter final-effluent standards:

Severn Trent appointed Galliford Try to design and build a new activated sludge process and associated treatment facilities under its AMP7 Quality Water Framework. The £11m scheme replaced the vast majority of the existing treatment process with new preliminary, secondary and tertiary treatment assets.

The overall project objective was to design, construct and commission the works necessary for Broadway STW to achieve the tighter phosphorus and iron standards while accommodating the increased permitted dry weather flow.

Galliford Try’s scope incorporated:

Galliford Try acted as both Principal Designer and Principal Contractor, with project management led by its delivery team and detailed multidisciplinary design undertaken by Galliford Try Engineering. The design was coordinated with the client, operational representatives, specialist process suppliers and the wider construction supply chain.

The new inlet works has to pass the increased flow to full treatment together with site returns. As the selected secondary treatment solution used a crude-sewage oxidation ditch, effective screenings and grit removal were important to protect the downstream process and limit the accumulation of grit within the ditch.

Rather than adopting a conventional reinforced-concrete inlet structure, the project developed a piped inlet-works solution using fabricated steel process tanks and interconnecting pipework. This enabled greater use of Design for Manufacture and Assembly (DfMA), reducing the amount of concrete required and transferring a proportion of the construction activity into a more controlled manufacturing environment.

In addition to reducing the scheme’s embodied carbon, the fabricated approach shortened on-site construction activities and reduced the health and safety risks associated with extensive in situ concrete work.

The inlet package incorporated screen, screenings handling, grit removal and flow-to-full-treatment control. Longwood Engineering Co Ltd supplied the screening package, while Jacopa supplied the grit-removal equipment.

The centrepiece of Severn Trent’s  Broadway STW upgrade was the new activated sludge process, comprising a selector zone, oxidation ditch, aeration package and downstream final settlement tanks.

Crude sewage and return activated sludge are mixed upstream of the selector zone before passing into the oxidation ditch. Ferric dosing is incorporated upstream of the selector and ditch to support phosphorus removal.

The original process design established a required aerobic and anoxic ditch volume of approximately 2,880m3, together with a 27m3 selector/anoxic zone. The process was developed for an operating mixed liquor suspended solids concentration of approximately 2,450 mg/l under normal conditions and up to 3,300 mg/l during peak loading.

The oxidation ditch was constructed by STAM Construction Ltd with the aeration package supplied by Suprafilt Ltd, including blowers, from Aerzen Machines, diffusers and flow inducers. The aeration system was required to operate across a wide range, from relatively low airflow during periods of reduced loading to the peak oxygen demand associated with the tourist season.

The original design basis identified a standard oxygen requirement ranging from approximately 413 kg/day to 2,225 kg/day.

This broad operating envelope was a central process consideration. Multiple aeration units and suitable control arrangements provided both peak capacity and operational turndown, allowing the process to respond to changes in catchment load without routinely over-aerating the ditch.

Two new final settlement tanks were provided downstream of the oxidation ditch; each fitted with a half-bridge scraper and scum-removal facilities.

The tanks were designed around the potential for elevated mixed liquor concentrations during peak loading. The original process design assessed two tanks of approximately 13.9m diameter, subject to selection of the appropriate standard product size.

Tank Consult Ltd undertook the structural design and construction of the final settlement tanks, with EPS Water supplying the half-bridge scrapers. A new pumping station manages return activated sludge and surplus activated sludge. Xylem Water Solutions supplied the site-wide pumps and mixers associated with the new treatment process.

The return activated sludge system was designed to support a range of return rates in response to works flow and process conditions. Surplus activated sludge is removed from the same facility and transferred to the new sludge-treatment system.

Achieving the new 0.35 mg/l total phosphorus permit required a combination of biological treatment, ferric dosing and tertiary solids removal.

The design incorporated ferric chemical dosing with provision for dosing upstream of the oxidation ditch and at a secondary location associated with final settlement or tertiary treatment. This gave the operating team flexibility to optimise chemical consumption and phosphorus performance in response to changing flows, loads and seasonal conditions.

The chemical system was designed with duty and standby facilities, flushing arrangements to reduce the risk of crystallised ferric blocking idle lines, and a dedicated delivery area. The original process basis allowed for a 15m3 chemical storage tank.

Colloide supplied the ferric chemical dosing plant, with Moor Fabrications Plastics Ltd providing the chemical point-of-application box. The design also incorporated suitable access, containment, delivery and maintenance arrangements.

Supplied by Bluewater Bio Ltd, a new FilterClear tertiary solids-removal plant was incorporated to support compliance with the stringent phosphorus permit. The plant receives settled effluent via a new feed pumping station and removes fine solids, including chemically precipitated phosphorus, before final discharge.

The original process design required the tertiary plant to accommodate average and peak suspended-solids loads and a maximum design flow that included site returns. The treatment system was developed to support a target average tertiary effluent total phosphorus concentration of no more than 0.28 mg/l, providing process headroom against the 0.35 mg/l annual-average permit.

The installation also included clean and dirty backwash storage tanks, a dirty-backwash pumping station and a washwater system. Goodwin Tanks supplied the glass-coated steel storage tanks, DutyPoint Ltd supplied the washwater and potable-water booster systems, and Bollfilter UK Ltd supplied the washwater filtration equipment.

Final-effluent monitoring and sampling facilities were included to demonstrate process performance and support ongoing operation. Servitech International Ltd supplied the final-effluent and phosphate-monitoring equipment.

Moving from the former biofilter-based process to an activated sludge process changed the character and quantity of sludge generated at Broadway. The upgraded works were expected to produce approximately 0.5 tonnes of dry solids per day of surplus activated sludge under peak conditions.

Surplus activated sludge is transferred from the RAS/SAS pumping station to a new sludge-thickening building. The facility includes polymer dosing and a Huber S-DISC sludge thickener from Huber Technology, with thickened sludge transferred to a new glass-coated steel storage tank.

The design basis allowed for a thickener throughput of approximately 13.5 m³/hour and seven days of thickened-sludge storage at 6% dry solids. Decant arrangements provide additional operational flexibility if the mechanical thickener is unavailable.

Liquors generated by thickening, decanting and tertiary-filter backwashing are returned to the head of the treatment process through the site’s works-return system.

Regular three-dimensional model reviews were held with Severn Trent’s project and operational teams as the detailed design developed. These reviews enabled operability, access, maintenance, lifting and process-interface requirements to be assessed early enough to influence the design.

This engagement was particularly important because the new process included numerous interdependent civil, mechanical, electrical and process packages. Coordination extended across the inlet works, oxidation ditch, settlement tanks, tertiary treatment, sludge facilities, chemical dosing, power distribution and site-wide infrastructure.

Specialist supplier designs were managed through Viewpoint as the project’s common data environment. This provided a controlled review and approval process and helped ensure that package designs complied with Severn Trent requirements, relevant British Standards and Water Industry Mechanical and Electrical Specifications.

Significant changes included incorporating the replacement of the existing motor control centre (MCC01) with the new MCC02 and re-purposing the existing liquor-return pumping station as a drainage pumping station for part of the site.

Reuse of the existing pumping station avoided unnecessarily abandoning a serviceable asset, although it required additional civil, mechanical and electrical design coordination.

The oxidation ditch and final settlement tank construction required an excavation approximately 30m long, 20m wide and 5m deep. Ground investigation identified a high groundwater table in this area, making excavation support and groundwater management key considerations.

Galliford Try’s in-house temporary works and geotechnical teams assessed potential solutions and supported engagement with specialist suppliers. The construction team fulfilled the role of Temporary Works Coordinator, integrating the temporary works requirements with the permanent design and construction methodology.

The teams also contributed to:

A carbon workshop was carried out during the initial design stage, with opportunities and risks recorded in the project’s Low Carbon Action Plan.

The most significant opportunity was the use of fabricated steel process tanks and a piped arrangement for the new inlet works in place of a conventional reinforced-concrete structure. This reduced concrete usage, enabled greater off-site manufacture and simplified the activities required on site.

Further efficiency and optimisation measures included:

These measures reduced site interfaces, supported programme certainty and improved the repeatability of design details that could be applied on other schemes.

The Broadway STW scheme represented a major transformation of the site’s treatment process. The new works combined new preliminary treatment, activated sludge treatment, final settlement, chemical phosphorus removal, tertiary filtration and mechanical sludge thickening in an integrated solution developed around the operational demands of a seasonally variable catchment.

Collaboration between Galliford Try, Severn Trent’s project and operational teams, specialist sub-contractors and process suppliers was central to coordinating the large number of interfaces. The use of three-dimensional model reviews, DfMA, standardised details, precast components and existing-asset reuse improved constructability and helped ensure that the final design was practical to operate and maintain.

The upgraded process was developed to provide the treatment capacity and operational flexibility required to accommodate future growth and tourist-season peaks while meeting the tighter dry weather flow, biochemical oxygen demand, ammonia, phosphorus and iron permit requirements.

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

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

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

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

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

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

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

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

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

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

The project consists of the following:

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

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

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

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

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

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

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

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

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

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

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

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

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

Guildford STW: Civils works supply chain - key participants

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The new permits aim to safeguard the future operational capacity of the treatment works and enhance the quality of effluent discharged into the River Avon, thereby protecting sensitive habitats and preventing pollution of nearby designated wildlife sites.

Permitted full flow to treatment (FFT) to the site will be increased from 60.2 l/s to 87.0 l/s by December 2026, along with a new ammonia permit of 15 mg/l. Additionally, Total Phosphorus will be limited to 0.22 mg/l, and Iron to 4 (8UT) mg/l in December 2027. Biological oxygen demand (BOD) and suspended solids (SS) permits will remain the same at 25 mg/l and 45 mg/l, respectively.

The current treatment works employs a biological treatment process. Flow enters the site and passes through the inlet works before proceeding to two 235m3 Dortmund-style primary settlement tanks (PSTs).

Here, sludge is settled out and directed to a dedicated 113m3 sludge storage tank, where it undergoes further thickening before being transported off-site for additional treatment. Partially treated effluent from the PSTs is then distributed evenly between three 28m diameter biological filters. Finally, three 235m3 Dortmund-style humus settlement tanks facilitate further sludge removal before the treated effluent is discharged into the River Avon.

In 2023, Mott MacDonald Bentley (MMB) was appointed by Severn Trent, under an Early Contractor Involvement (ECI) agreement, to review the outline design for upgrading biological treatment at Bidford-on-Avon STW. The proposed scope included replacing the inlet works, constructing new primary and humus settlement tanks, enhancing sludge processing and storage, installing two biofilters, and integrating tertiary solids removal capabilities.

Initial site investigations highlighted major constraints, including topographical, ecological and spatial challenges within the site boundary, as well as potential difficulties in achieving regulatory permit dates. The scale and cost of temporary works were substantial, and there were significant health and safety risks associated with some elements of the scope.

Following a detailed review, MMB concluded that the original scheme presented excessive buildability and programme challenges, which could potentially result in failure to meet the regulatory date for the new consent.

To maintain project momentum, the design team considered alternative options. To support this, MMB undertook enabling works, including vegetation clearance, trial holes, and topographical and utility surveys. These actions ensured early service identification, allowed for safer construction methodologies to be considered, and helped mitigate programme risk.

A renewed optioneering phase was initiated to explore alternative solutions aimed at improving constructability whilst ensuring compliance with environmental and safety standards.

One key proposal involved constructing an offline solution in the northern field, replicating the assets outlined in the original design and connecting into the existing inlet works. Although this option would have mitigated many risks associated with the original design, it introduced new ones, such as delay in securing land ownership and planning permission.

Due to its complexity and the resulting uncertainty in the programme, this approach was ultimately deemed unfeasible.

However, the northern field remained in consideration as a potential location for the temporary working area. With the existing site too constrained to safely accommodate construction welfare alongside construction activities, this alternative offered a more practical solution. Yet, after several months of planning and evaluation, the proposal was ruled unviable, due to the inability to agree temporary access to the land.

Faced with land ownership constraints in the northern field, MMB and Severn Trent considered the option of a temporary working area in the field to the east of the site. Even though this initially appeared to be a workable alternative, the complex access strategy required, using the existing access road and new routes across the eastern field, came with its own ecological and archaeological hurdles. Mitigation efforts were expected to increase costs substantially and jeopardise the first regulatory permit deadline in December 2026.

In order to address the above challenges, MMB proposed a strategic shift in scope to Severn Trent, replacing the originally proposed biological filtration (fixed film) approach with an activated sludge (suspended growth) system.

This recommendation stemmed from a robust desk study, highlighting gains in programme efficiency, affordability, and process performance. To meet the increased flow from population growth and tightening environmental permits, the revised design comprises of a new prefabricated inlet works, a racetrack-style double oxidation ditch, steel final settlement tanks, chemical dosing, and tertiary solids removal capabilities, supported by containerised sludge thickening plant and a sludge holding tank.

Following detailed collaboration and review with Severn Trent, the scope change was approved, with MMB instrumental in driving the transition. The solution exemplifies how Severn Trent, as part of AMP8, is championing small-to-medium sized oxidation ditches for their operational benefits and how MMB is at the forefront of their delivery, setting a new benchmark for process excellence.

The revised solution is now positioned to meet the regulatory date whilst also providing a safer construction environment. This is to be achieved by maximising the available working area within the existing site, by minimising the footprint of the proposed assets.

Crucially, it does so whilst eliminating the need to build over existing assets, significantly reducing risk during construction. This approach also enables the establishment of a compliant CDM setup on-site, delivering substantial health and safety benefits and supporting efficient project delivery.

The site layout has been strategically engineered to optimise gravity flow, delivering long-term savings in operational expenditure. By eliminating the need for large pumping stations, the design significantly reduces energy demand and enhances sustainability, starting with flow management at the new inlet works.

This inlet works, to be supplied as a complete prefabricated package by SPIRAC Ltd, represents another proposed installation of the SPIRAC package in the Severn Trent framework. Designed as a raised inlet works, it incorporates screening and grit removal at the initial treatment stage. Utilising a solution manufactured off-site will streamline the construction and commissioning process, leading to significant programme savings.

The solution also includes a suite of other critical assets in the vicinity of the inlet works. Using pre-assembled, containerised assets where appropriate enables rapid installation and health and safety benefits during construction.

The proposed containerised sludge thickening plant exemplifies a streamlined installation process that reduces labour requirements and minimises disruption to other construction activities.

The oxidation ditch, measuring 32m in length, 28m in width, and with a capacity of 3,800m3 has been designed in a double-ditch ‘racetrack’ configuration. This layout enables efficient air distribution through an aeration grid, which is integral to the activated sludge treatment process. Following the scope change, MMB acted swiftly to engage subcontractors early, accelerating programme delivery through smart, efficient structural design.

FLI Precast Solutions, with support from MMB, has designed the precast wall units by using finite element analysis. This has enabled optimised structural design that allows the bending moments and shear forces to be calculated accurately. This approach has also allowed for the reduction of embodied carbon by reducing the amount of concrete used within the structure.

The position of the oxidation ditch has also capitalised on the natural topography that was once considered a constraint. Now, it acts as a strategic advantage to enhance flow dynamics by gravitating through the process and enhances construction efficiency by reducing the amount of excavation required.

The two 15m diameter final settlement tanks form a key stage in the treatment process, preparing flows for final polishing via tertiary solids removal. Having challenged conventional design standards, it is proposed to fabricate the final settlement tanks from stainless steel. This significantly reduces both the duration of construction and the carbon footprint.

A tertiary solids removal (TSR) stage will be delivered through Eliquo Hydrok TSR units; advanced cloth filters designed to remove suspended solids and ensure effluent quality. Once treated, the final effluent will be discharged into the River Avon.

As the site currently lacks chemical dosing infrastructure, a dedicated dosing area will be constructed as part of the upgrade. This will introduce controlled ferric dosing to promote particle coagulation, enhancing effectiveness of settlement and supporting overall process performance.

The site will benefit from a complete drainage upgrade, aligning with modern drainage standards. Potentially contaminated rainfall will be directed back to the inlet works, while clean runoff will flow into a sustainable drainage pond. Suspended solids will be captured through a cascade system at the outfall, before flows enter the pond.

The pond improves environmental resilience and presents an opportunity for Biodiversity Net Gain to be embedded into the design. It could potentially offer a new wetland habitat and provide an area for the planting of native trees and wildflowers. These areas will enhance the ecological value of the area, support local wildlife and promote long-term environmental sustainability.

The design for the Bidford-on-Avon scheme demonstrates engineering excellence through robust optioneering processes. These have resulted in bold scope change that aligns the solution with Severn Trent’s operational, financial, programme and environmental objectives, without sacrificing engineering quality.

The adoption of an activated sludge process, gravity-led flow and design for manufacture and assembly has transformed the scheme from a project at risk of missing regulatory deadlines into one on track to meet all key milestones. This turnaround has been driven by procurement-led design, early supplier engagement and a strong focus on construction efficiency. Financially, the revised design is set to deliver £multi-million savings without compromising quality or compliance.

Additionally, environmental stewardship has remained central to the design philosophy, with carbon reduction embedded across design elements and a strong emphasis being placed on Biodiversity Net Gain and sustainable drainage.

The design ensures that Bidford-on-Avon treatment works can accommodate future population growth and deliver high-quality final effluent to the River Avon.

The upgrades at Ilkley comprise significant modifications to flow conveyance between the upstream CSO network and the WwTW.

These include a new works inlet pumping station to receive all incoming site flows, a 17,500m³ storm storage tank, and new duty/standby inlet screens rated at 300 l/s. Additional enhancements include two new primary settlement tanks (PSTs), a combined secondary treatment feed pumping station to convey flows to the existing trickling filters and a new secondary aerated reed bed, and a tertiary integrated constructed wetland (ICW).

The hydraulic upgrade is significant, with the works being reconfigured to pump forward flows from a dry weather flow of circa. 60 l/s to a new Flow to Full Treatment (FFT) of 300 l/s, increasing from the existing FFT of 114 l/s. During storm events, the system is capable of receiving peak inflows of up to 3,500 l/s. Flows exceeding the 300 l/s treatment capacity are diverted by gravity to the new screened storm storage tank. Once capacity becomes available at the inlet pumping station, stored storm flows are returned and pumped forward for full treatment.

Yorkshire Water’s £41m project commenced on site in July 2024. The scheme forms a key part of wider investment to address storm overflow performance and improve the quality of inland bathing waters. This case study provides an overview of the works delivered by Ward & Burke and its specialist supply chain, focusing on the infrastructure upgrades and process enhancements implemented to achieve storm spill reduction and long-term environmental improvement.

A key technical element of the scheme is the modification of the upstream CSO Pumping Station interface and the transfer of flows towards the treatment works. The design includes both low-level and high-level connection arrangements, allowing different hydraulic conditions to be managed through separate flow paths.

A low-level pipe connection is provided from the former CSO Pumping Station to the tunnel launch shaft to convey low to average flows forward. In addition, a high-level culvert provides a route for higher flows under storm conditions. This dual-level approach allows the system to manage both average flows and larger storm flows to be passed forward under gravity without relying on a single hydraulic route that could be surcharged.

From the tunnel launch shaft, flows are conveyed through a micro tunnel and high-level culvert arrangement towards the new works inlet pumping station. The site layout to the works includes the launch shaft and reception shaft, which was converted to the works inlet pump station, with a 1200 mm concrete tunnel carrying low to average flows and a 2m wide by 1m high culvert for storm conditions. The addition of these shafts and tunnels provide an additional 3,500m3 of storage within the network further increasing the site’s resilience to storm events.

The new works inlet pumping station is the primary control point in the upgraded WwTW. It receives flows from the tunnel and culvert system and pumps flows forward to the inlet works. Under dry weather conditions, the pumping station passes forward approximately 60 l/s. As rainfall increases and additional flows are received from the upgraded network, the pump station is designed to increase its pass-forward flow up to the new FFT of 300 l/s.

This arrangement allows the works to remain hydraulically controlled during high-flow events. The treatment process continues to receive the design FFT of 300 l/s, while excess storm flows are temporarily stored and returned for treatment once capacity becomes available.

To provide additional resilience, a backup generator has been installed to support site-wide power demand up to 1000 kVA. This enables treatment and storm management assets to remain operational during a power failure, maintaining screening, pumping, and flow control capability during periods when treatment continuity is most critical.

The storm tank provides the storm storage capacity for the upgraded works. It is a large rectangular storage structure 135m in length, 25m in width, with an average depth of 5.2m across the length of the tank, allowing for a max storage of 17,500m3. The tank is hydraulically connected to the works inlet pumping station so that attenuated flows can be returned when capacity is available.

During a storm event, excess flows enter the storm tank via the high-level culvert and weir, where twin, 8.75m long, 6mm RoK1 perforated mesh storm screens from Huber Technology collect and remove rags from incoming flows.

After the storm tank has been emptied, the water retained within the flushing gate chambers is released to flush each 5m wide lane of the storm tank. The flushing system, from CSO Group Ltd, provides an automated method of cleaning the tank lanes, reducing manual intervention and reusing storm water as the cleaning medium. This results in low OPEX costs, simple maintenance, and reduced odour issues from storm settlement.

A significant design feature of the storm tank is the use of exposed sheet pile walls. Rather than constructing a concrete retaining wall system, the exposed sheet piles form part of the permanent tank structure. This offers both a carbon and cost benefit by reducing the quantity of concrete required in the tank construction. The exposed sheet pile walls have been designed with a service life consistent with the storm tank design life of 100 years.

This approach provides a practical example of carbon-conscious civil engineering. By using the sheet piles as permanent works rather than temporary works only, the scheme reduces material duplication and makes more efficient use of the installed structural system as the sheet piles were also used as a coffer dam to construct the tank floor.

Flows pumped forward from the works inlet pumping station pass to the modified inlet works, where the existing screening arrangement is being replaced with two new duty/standby 45° belt EscaMax inlet screens from Huber Technology. Each screen is capable of handling flows up to 300 l/s.

The increase in screen capacity, together with the revised hydraulic route into the inlet works required modifications to the existing inlet works structure and associated access steelwork. Sections of the existing channel walls were removed to accommodate the new flow path and the larger screen units.

Tapered channel sections and benching were installed upstream of the screens to manage velocity and improve hydraulic conditions upstream of the screening units. This helps promote even flow distribution across the screen face, reducing the risk of localised settlement and screen blinding.

The access arrangements were also upgraded as part of the works. New access steelwork has been provided to allow safer inspection, operation and maintenance of the new actuated control valves, screens and surrounding equipment.

The upgraded primary settlement stage includes two new 20m diameter primary settlement tanks (PSTs) operating alongside one existing PST.

The addition of two new PSTs increases the settlement capacity of the works and supports the increased treatment capacity. Fitted to the PSTs are steel half rotating bridges.

The new PSTs are integrated into the existing works through a distribution arrangement that allows flows to be split and recombined before secondary treatment pumping. The new PSTs are controlled on incoming flow rate and one of the three PSTs can be fully drained outside of storm conditions to avoid septic conditions.

Sludge removal from the PSTs is handled by the associated RAM desludge pumping arrangement from EMS Industries Ltd, and is fully automated by means of EMS’s adaptive pumping technology.

Following primary settlement, flows from the new PSTs and the existing PST combine and pass to the new combined secondary treatment feed pumping station.

From here, flows are split between the existing secondary treatment stream and the new aerated reed bed. The existing treatment stream includes trickling filter media beds and humus tanks which take existing flows up to 114 l/s, while the reed bed provides additional secondary treatment up to 300 l/s.

The combined secondary feed arrangement allows the works to use both existing and new assets in parallel. This provides operational flexibility and allows flows to be managed across multiple treatment routes. It also reduces reliance on a single secondary treatment process allowing large scale maintenance to be undertaken.

One of the most sustainable elements of the scheme is the use of a 4,286m2, dual-bed vertical subsurface flow aerated reed bed system as a secondary treatment process. Aerated reed beds are engineered planted gravel bed systems that use forced aeration to support aerobic biological treatment within the media.

Wastewater is distributed across the bed and passes through the media, where biofilm grows on the aggregate surface and around the plant root zone. Air is supplied into the bed by duty/standby blowers from AERZEN Machines, running constantly to maintain aerobic conditions, enabling removal of residual BOD and support nitrification within the reed bed. The reeds provide a resilient vegetated treatment environment, but the principal process mechanism is the combination of media-based biofilm treatment, hydraulic contact and controlled aeration.

The aerated reed beds offer an interesting balance of treatment performance, flow and load variation resilience, operational simplicity, environmental integration and carbon impact compared with conventional secondary treatment options. They can provide a more natural visual appearance than traditional process plant and can be integrated into sites where environmental sensitivity is a key consideration.

Following secondary treatment, flows gravitate to the Integrated Constructed Wetland (ICW) Feed Pumping Station. From there, flows are pumped under the River Wharfe by means of a directionally drilled main across to the new 16,860m2 free water surface flow treatment wetland or ICW which is made up of six treatment cells for final effluent polishing.

The integrated constructed wetland provides a nature-based tertiary treatment stage prior to discharge. This engineered treatment wetland reduces residual suspended solids, BOD, nutrients and pathogens by maximising the processes that naturally occur within wetlands, such as sedimentation, filtration, accretion and nutrient uptake. Its diversely planted, habitat-rich treatment environment also provides environmental, aesthetic and community benefits. The 29 different plant species bring resilience to its treatment capacity and enhance biodiversity.

The integrated constructed wetland has been designed around an existing pedestrian access route, allowing the scheme to retain and enhance public connectivity through the area. This provides an opportunity to return a nature-rich environment to public use, with the wetland creating a more attractive and biodiverse setting.

The wetland will provide visual and environmental amenity value in addition to its treatment function. The planted cells, open water margins and landscaped areas will create habitats for birds, insects, amphibians and other wildlife, while also improving the character of the surrounding area for walkers and local residents.

By integrating the treatment process with the existing public access route, the solution helps demonstrate how wastewater infrastructure can be delivered in a way that’s sustainable and supports both environmental performance and community value.

The project combines civil engineering, MEICA engineering, complex tie-ins, retained operational assets, new process plant and nature-based treatment. Maintaining treatment during construction is a major consideration. Temporary works, flow management, isolation planning and staged commissioning are therefore central to the delivery strategy.

The aerated reed bed and integrated constructed wetland provide a nature-based treatment solution that supports lower-carbon wastewater treatment. By using engineered planted treatment systems rather than solely relying on conventional infrastructure, the scheme reduces the requirement for high embodied carbon materials and energy intensive assets.

The approach also supports more sustainable construction by reusing suitable as-dug material within the works where practicable, reducing imported fill requirements, disposal volumes, vehicle movements and associated carbon emissions.

The Ilkley WwTW upgrade is a technically significant project, addressing UK’s first inland bathing water designation at Ilkley, combining major hydraulic conveyance works prioritising flows by gravity, storm storage, increased treatment capacity, conventional primary settlement, an innovative aerated reed bed secondary treatment process, tertiary ICW treatment and outfall relocation.

As of 31 March 2026, all proposed treatment assets and associated mechanical, electrical and control equipment with the sewage treatment works boundary are commissioned and operational. The upgraded works is now receiving and treating all flows entering the site, with the new process stream providing increased hydraulic capacity and improved treatment resilience.

The ICW is under construction, with completion currently planned for December 2026. Once complete, it will provide an additional final polishing stage before discharge through the new outfall downstream of the bathing water stretch.

A further phosphorus removal scheme is also being developed to meet new phosphorus discharge permits at Ilkley. This future phase will include ferric sulphate dosing upstream of the primary settlement tanks to support phosphorus removal, together with a new sludge storage tank to manage the increased sludge production associated with ferric addition.

Overall, the scheme provides improved resilience for the adjacent water course and bathing water.