09/11/2022

At the end of September we met with senior managers from Thames Water at their Hogsmill Sewage Treatment Works in Berrylands, to discuss the pollution discharges in the Hogsmill and Green Lanes Streams.

We were given a tour of the sewage treatment works by Thames Water managers, who told us about the challenges they face in keeping sewage out of the streams, particularly during heavy rainfall.

The Hogsmill is deemed to be a sensitive catchment as it is a chalk stream that should be a haven of wildlife and is rare in the natural world.

The root causes of sewage pollution:

Rainwater and surface water going into the foul sewers and into the sewage system is the most common problem and occurs across large areas of the boroughs.

The sewage and rainwater drainage systems in an area should be separate as the sewage needs to go to the sewage works while the rainwater should go into the water table via a soakaway or into local streams via pipes. Rainwater would naturally end up in the water table or local streams if it wasn’t for human urbanisation (our concreting and building on the natural world) when lots of rainwater and surface water in the sewers fills them up and mean they overflow. In Epsom, Ewell, Kingston and Surbiton the systems are often mixed. In some areas there is a clean separation however in large areas of the boroughs they are not. The mixing can go either way with differing consequences.

This causes major problems when it rains meaning the sewage system regularly overflows at the storm tanks in Epsom and Ewell depositing large amounts of sewage into the streams.

Housing Missconnections:

The foul sewers from homes and businesses going into the rainwater / surface water drainage system, are commonly known as misconnections. Most homes’ and businesses’ foul sewers are connected correctly however in the Hogsmill catchment there are around 200,000 properties and 400,000 people and some aren’t (the number is estimated to be in the hundreds) and that means that hundreds of homes’ and businesses’ sewage is going into the rainwater system everyday and then into the rivers. Where these mistakes have been made it is up to the property owner to correct them. If property owners can’t afford to do this, then sometimes Thames Water can help.

Fixing the issues:

The problems could be fixed by fitting larger storm tanks, separating the systems fully or by installing intelligent sewers that can hold back flows in the system at times of heavy rainfall, using the whole system as a massive storm tank (currently Thames Water’s preferred solution).

Thames Waters 25 year plan:

Thames Water are currently creating a 25 year plan for the area and its draft is available here: https://www.thameswater.co.uk/about-us/regulation/drainage-and-wastewater-management

The final plan is due to be published in March 2023. The draft plan for the area does not appear to provide a feasible way forward to meet the objectives of reducing pollution into the streams by 80%.

We want to see a credible plan from Thames Water of significant investment in the Hogsmill catchment to fix the pollution issues.

This plan needs to show projects that deliver in the next 1-3 years not in 25. Be it smart sewers, investment in fixing misconnections, or storm tanks. But, as Thames Water are driven by profit, not clean rivers, we shouldn’t be under any illusion that getting them to do this is going to be easy.

Please keep sharing the petition. Every signature ups the pressure on Thames Water! https://www.change.org/HogsmillSewage