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Mogden sewage treatment works

Mogden sewage treatment works is our third largest treatment works. It was built between 1931 and 1935 at a cost of £1.7m and covers an area of 55 hectares. Over half of the power used by the plant is renewable energy that's been generated on site as part of the sewage treatment process. The treatment works serves around 2.1 million people, and some of the wastewater has travelled over 20 miles by the time it reaches Mogden.

Investment at Mogden sewage treatment works

In more recent years, we've needed to significantly extend Mogden to reduce the number of times partially-treated sewage overflows into the River Thames when the works become overloaded after heavy rain. Our work to upgrade the site continues, with plans to implement the following:

  • £12 million to replace existing assets to improve efficiency, serviceability and performance
  • £10 million to install three combined heat and power engines, to increase serviceability and generation
  • £9 million to replace the air blower feed pipework and blowers to battery activated sludge treatment processes

Stay up to date with the latest changes with the reports available below.

Latest updates

Managing odour at Mogden treatment works

Historically, there was an issue with odour from the treatment works, and so we worked with the local community around Mogden to help control the smells. An independent odour survey carried out by OdourNet UK Ltd, in September 2008, showed that odour levels had significantly reduced.

We continue to monitor the smells at Mogden using two methods:

  • Measuring levels of hydrogen sulphide, which is one of the key compounds of odour at sewage treatment works. We carry out regular site odour inspections, using 13 odour monitors located in the middle and around the edge of the works.
  • Olfactometry, which involves taking samples of air from different locations around the works and studying them in a laboratory. These surveys are regularly undertaken by both Thames Water and the London Borough of Hounslow.

The reports below provide a breakdown of weekly odour monitor readings taken throughout the year.

Sludge gas monitor readings

There are eight monitors at Mogden, which monitor sludge gas readings taken throughout the year. The readings are available to access below.

London Borough of Hounslow site inspections

The local Environmental Health Officer is welcome to carry out a site visit whenever they wish. Following each visit, any observations are sent to the management team at Mogden for review and, when necessary, appropriate actions are taken.

Managing mosquitoes at Mogden

We also follow a strict programme of inspections and treatment aimed at controlling mosquitoes at Mogden. The processes we use are audited by local environmental health officers, and based on recommendations from leading independent experts. We also work with the London boroughs of Hounslow and Richmond upon Thames to raise public awareness of mosquito breeding sites in the wider local area and how to prevent further breeding to reduce the numbers.

Storm flows from Mogden

When it rains, the high volume of sewage, or wastewater, coming into Mogden can't all be treated at once. The excess is stored in storm tanks after first being screened and de-gritted. 

Once the flow reduces, the sewage is returned to the head of the works for processing. However, if the storm tanks fill up before this happens, we have no option but to send partially treated excess wastewater to the River Thames. This prevents it backing up into streets or homes.

You can view real-time data for our storm discharges on our EDM map

Mogden residents liaison meetings

We work closely with the local community surrounding Mogden Sewage Treatment Works and have held regular Mogden residents liaison meetings since 1992. The meetings invite local councillors, MPs and residents to discuss any issues they may be experiencing as a direct result of living near the treatment works. All previous residents' liaison meetings are available to download below: