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Mogden STW CHP, biogas & heating circuit enhancement

Mogden Sewage Treatment Works is one of Thames Water Utilities largest assets with an estimated 2,000,000 population equivalent (PE). The sewage treatment works was built around 1930 and has a capacity for treating up to 1,064 Ml/d of waste water. The sites primary treatment for Biochemical Oxygen Demand (BOD) incorporates an activated sludge process applying diffused aeration that takes place within five “batteries” of aeration lanes; These are identified as A, B, C, D & E.

Previously all process air for Batteries A, B, C and D was supplied by 8 No centrifugal blowers which are located within the Power House building. This large building also contained 4 No Combined Heat and Power (CHP) engines. The process air was conveyed to each set of batteries (Grouped as A&B and C&D) via a buried air ring main that is constructed in cast iron and glass-reinforced plastic (GRP). Batteries A&B were operating at near capacity and interruptions to the air supply of more than 45 minutes would have resulted in severe breaches of the final effluent consent. Batteries C&D are more resilient and can be maintained for longer, but any prolonged outage of the air supply would have affected all four batteries.

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The Eight2O team have created a short video to explain the project and showcase their great work, which you can watch here.


This project involved the design, manufacture and installation of various new pipework systems that had to be integrated into the existing pipework systems to facilitate the introduction of 3 No. new CHP engines within the Power House building. There is also an option to introduce a 4th CHP engine in the future should it be required.

Mogden STW has two distinctive sludge heating demands, namely, sludge pasteurisation and sludge digestion. Sludge at Mogden is pasteurised to provide a suitable product that can be used for fertiliser on farmland. Once the sludge has been through the pasteurisation process it is then anaerobically digested in 16 No individual tanks.

Franklyn Yates Engineering were awarded the contract to design, supply and install the new pipework systems for both the biogas arrangement that feeds the new CHP engines and the associated pasteurisation hot water system. Part of the contract was to design, supply and install 2 No condensate pots and 6 No heat exchangers to facilitate the whole process. We were also contracted by Eight2O with the installation of various free issued inline equipment that included a siloxane filter, 2 No coalescent filters and a pressure reducing station.


A decision was made between Eight20 and Franklyn Yates, primarily due to time constraints and the interface challenges with other contractors within the overall contract programme, to install a manufacturing facility at site. This was for the fabrication of the stainless steel pipework, the galvanized carbon steel pipework supports and the galvanized carbon steel support frames. The pipework, valves and the inline equipment was assembled on to the frames and then transported from the manufacturing facility and subsequently lifted into position. This made things easier and quicker with regards to the biogas pipework being fully welded and site run with the routes being determined between Franklyn Yates Engineering, Costain, Atkins, Black & Veatch and Thames Water working collaboratively, thus providing the best solution for the whole process.

With storage areas and work faces at a premium it was extremely important that there was a high level of collaborative planning adopted for the project. This required all stakeholders involved to embrace the collaborative working approach.

The work detail was reviewed on a weekly basis by site managers for various trades and disciplines including, civil, mechanical, electrical, Thames Water, design, equipment suppliers and commissioning. Planning sessions were also carried out each day Monday to Thursday at 4pm between the site supervisors and Eight20 management that enabled them to review work carried out that day and allowed closer planning of the work the following day. This helped to identify potential problems and any positive or negative affect to the program. Each Friday there was a more intense meeting with the various front line supervisors and Eight20 for review of the projected work through the next four weeks. The whole approach to the collaborative planning regime by everyone involved with this scheme made sure that the whole project was a complete success.

The Eight2O team have created a short video to explain the project and showcase their great work, which you can watch here.

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