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Mogden STW Air Main pipework manufacture, supply and installation

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.


Franklyn Yates Engineering contracted with Eight20 to design, manufacture and install the new air main system for batteries A and B. The whole project was challenging in several aspects. The area chosen where the new blowers were to be sited was an elevated sloping green field area of the site for a start. The logistics for the new air main pipework involved connection into the existing ASP lane structures, that were 80 years old, as well as having to cross over access roads with new pipe bridges and modifying existing pipework sections within the ASP lane pipe trenches. We were also tasked with keeping the process operating and keeping equipment accessible at all times for Thames Water maintenance operators to work on, as and when necessary.


Due to the complex interfacing issues with the existing process systems and structures, plans needed to be put into place to allow installation of the new pipework and equipment to progress without compromising the running operation of the plant. As a result, new butterfly valves were installed in the existing pipework system within the ASP lane trenches. These valves were ultimately sacrificial and provided no long-term benefit but formed a critical part of the installation process and allowed the new pipework to be connected to the existing system with minimum delay. The introduction of the butterfly valves meant that there would be no need to shut down any of the 24 No ASP lanes for longer than 4-hours, thus saving time and money and helped protect the whole air distribution process and program for the works.

Franklyn Yates Engineering developed the overall design and logistics for the pipework system providing detailed 2D CAD drawings and a 3D model showing the interface from the new air blowers, the pipework fabrication and inline equipment through to the existing pipework connections as well as providing calculations for the pipe bridges, support frames, access structures, walkways and stairs.

Franklyn Yates Engineering fabricated all the pipework and steelwork structures, including the pipework supports and access platforms at our manufacturing facility in North Wales. Wherever possible the pipework and valves were assembled at the manufacturing facility and mounted on to galvanised carbon steel frames and delivered to site as “modules” ready to be lifted into position on pre-cast concrete plinths. The interconnecting pipework was then coupled to the inlet and outlet flanges on each of the 24 No modules. 

Franklyn Yates Engineering were also contracted to install 4 No new air main blowers. This involved working hand in hand with the blower supplier and Eight20. This allowed the design to be developed for the interface of the blowers and acoustic enclosures with the new pipework system and the necessary walkways and stairs to provide access to the associated inline equipment. Each blower weighed more than 28-ton and the whole process for unloading, lifting, positioning, lining, levelling and securing the blowers was very complex and critical to the project’s success.       

All the above works were completed on time and to the original contract program. 

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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