A multi-phase £17m project has improved the resilience and operational efficiency of this key Water Recycling Centre in Milton Keynes.
Background
Anglian Water’s Cotton Valley Water Recycling Centre (WRC) treats flows from Milton Keynes and the surrounding area. The wastewater treatment facility serves a substantial population, treating wastewater to stringent regulatory standards before discharge. Over time, the facility has encountered various operational challenges that hindered its efficiency and compliance. These operational challenges included:
- limitations in post-digestion dewatering capacity
- recurring pollution incidents during high storm surges
- the imposition of a stringent phosphorus standard
- inadequate stormwater storage capacity
Addressing these challenges became imperative to ensure the facility’s long-term sustainability and resilience.
The project
Skanska, working as principal contractor at Cotton Valley as a member of Anglian Water’s @one Alliance, was tasked with identifying the issues affecting the Cotton Valley WRC and developing solutions. These were narrowed to two key areas:
- Pollution incidents and stormwater management: In recent years, pollution incidents, particularly during high storm surges, posed significant environmental and regulatory concerns as the scrutiny of the water industry increases. Inadequate stormwater storage capacity and outdated pumping infrastructure has increased the impact of these incidents in the Milton Keynes area. These persistent pollution incidents stem from the incoming flows received at the treatment plant station.
- Phosphorus standard compliance: A newly imposed phosphorus standard mandated a reduction in phosphorus levels to be discharged from the WRC. Prior to the new phosphorus investment, the existing chemical dosing infrastructure lacked the capacity to meet the stringent requirements of the new standard.
Solutions
Working with Anglian Water it was agreed that four schemes would be completed, with a total investment of £17m. These ran from 2022 to 2025.
Digester upgrade scheme: Two new centrifuges were installed with enhanced dewatering capabilities to replace the outdated units. A modern conveyor system to streamline sludge transport and optimise operational efficiency was also part of the scope. In addition, a new polymer dosing system was installed, including a dedicated polymer silo to improve sludge dewatering performance. A new motor control centre (MCC) within the existing infrastructure was required to facilitate centralised monitoring and control of the upgraded digester system.
Stormwater storage scheme: The biggest financial investment was in the construction of two additional glass-coated steel stormwater storage tanks – increasing capacity from 5,795m3, to 20,502m3, achieving Environment Agency requirements. In addition, new side weir storm overflows and modulating penstocks to regulate stormwater flow and optimise management were installed. Advanced storm feed pumps and gravity collection system tanks, to facilitate efficient stormwater transfer and storage during peak flows, were also part of the scope.
Phosphorus standard compliance: To comply with new legislation, two 30m3 ferrous chloride storage tanks were installed, equipped with instrumentation for precise dosing control. This element of the project included installation of a new tanker delivery point and chamber to allow for chemical replenishment and ensure uninterrupted operation when required; a new kiosk and ducts/draw pits to accommodate advanced chemical dosing equipment.
Terminal Pumping Station (TPS) resilience: Two duty/assist pumps with a capacity of 675 litres/second were installed in the TPS inlet well with automatic controls to operate the assist unit. These pumps are connected to the existing bypass pipework to ensure continuous operation during peak flows and emergencies.
Downstream pumps with a capacity of 550 l/s were also installed. These pumps were connected to a duty-only 12mm bar screen, which was essential for consent compliance. The discharge is directed into Storm Tank 1, providing additional capacity to manage excess flows during high storm surges. An unscreened bypass was also installed to enhance the operational flexibility, with screenings directed into a 6.1m3 skip. A new MCert flow meter was also installed to monitor peak flow and screen performance, ensuring regulatory compliance.
Challenges & outcomes
In completing the works the team managed a range of challenges. This included management of high-risk activities during the winter period, when conditions were cold and wet. The works coincided with the Covid-19 pandemic when extended material and component lead-times were commonplace across the global supply chain, leading to challenges with the sequencing of the delivery programme.
Working as a collaborative team, including the extended supply chain, the £17m project was successfully completed.
