Michael was very concerned by Chobham Sewage Works’ reported discharges into the River Bourne, and he wrote to both the Environment Secretary and to the Chief Executive of the Environment Agency on 7 October 2022 to ask for their assistance.
13 October 2022 - Michael met representatives of Thames Water in Parliament to discuss the discharges from Chobham Sewage Works and Michael was reassured to hear that the matter is being taken seriously. He understood that work is due to take place in Chobham to resolve the situation, which should be completed in early 2023. Thames Water sent an email to Michael following that meeting:
Thank you for the opportunity to meet with you last week and explain the background to the storm sewage discharges that have been recorded at Chobham STW and our plans to address them.
- For many years the focus of both the water companies and our environmental regulator was on the quality of the treated sewage effluent discharges made to the environment rather than intermittent discharges made to the environment through storm overflows at the Sewage Treatment Works (STWs) or in the network. These discharges have not been recorded until the industry wide roll out of Event Duration Monitors (EDMs).
- The requirement to install Event Duration Monitors (EDMs) on permitted storm overflows in the planning period 2015-2020 gave all companies invaluable data to understand the performance of their sewerage system and STWs. The majority of Thames Water’s storm overflows are at STWs. As these monitors were installed and validated it has become clear that some sewage works are discharging storm flows more frequently than they should.
- The discharge of storm flows is an important part of the sewage inflow management process as it is not possible to stop the flow of sewage to a works. The overflows are designed to operate automatically when the sewer network and sewage works are about to be overwhelmed and release diluted wastewater into rivers rather than letting it back up into people’s homes. It is the way the system is designed to operate.
- Unmanaged excessive inflows can wash out a STW’s good bacteria (which break down sewerage). A wash out will take significant time to recover, which would leave sewage unsatisfactorily treated. Therefore, at the majority of STWs flow to treatment is controlled with excess initially spilt to a storm tank from which it would be returned when inflows to the works subside. Under prolonged high inflow conditions tank capacity can be exceeded and spills to the environment occur. The current regulatory process requires a works to be investigated if the EDM data hits predefined trigger points. For one year’s data this is more than 60 discharges in the year. For Chobham STW the trigger was hit in 2020 when 98 discharges were recorded.
- Chobham STW also has a number of other regulatory obligations that must be delivered in this period. These includes further monitors to record the flow being passed to full treatment and the flow overflowing from the treatment process to the storm tanks (the EDM monitors the discharges to the river from the storm tanks), a treatment upgrade to achieve an ammonia standard of 3mg/l (the current standard is 5 mg/l) as well as to provide sufficient capacity to meet projected growth in the area. The further monitors have already been installed and the required delivery date for the treatment upgrade is end March 2023.
- To meet these requirements a new inlet works is being delivered along with a balancing tank to enhance flow management across the site and a new tertiary treatment plant.
- It is envisaged that, taken together, these improvements will bring the storm discharges from Chobham STW back to within government targets.
The Government’s Storm Overflow Reduction Plan sets a 2050 objective of reducing all storm discharges to less than 10 events per year, with that target achieved by 2035 for environmentally sensitive sites and bathing waters. At Thames Water we have made our own commitment of a 50% reduction in the total annual duration of spills across London and the Thames Valley by 2030, and within that an 80% reduction in sensitive catchments, meaning that we are likely to outperform the government targets.
We regard any discharge of untreated sewage as unacceptable and I hope I have reassured you that we are working hard to make them unnecessary, with specific plans for Chobham STW. Please come back to me if you have any further questions.
18 October 2022 - Michael received the following email from Julia Simpson, the Environment Agency’s Director for the Thames Area:
‘Thank you for your email of 7 October 2022 to our Chief Executive, Sir James Bevan, regarding your concerns about Chobham Sewage Treatment Works. James has read your email and asked me to respond on his behalf. He will receive a copy of this response.
I can confirm that we are aware of the situation regarding storm sewage discharges at Chobham Sewage Treatment Works. In recent years, we have required water companies to install Event Duration Monitoring (EDM) to assess the frequency and duration of storm sewage discharges from their assets. The water companies are required to submit this data to us on an annual basis. This has increased the transparency of water company’s operations and environmental performance.
In 2020, EDM data for Chobham Sewage Treatment works confirmed a relatively high number of storm sewage spills, which required Thames Water to undertake an investigation under the Storm Overflow Assessment Framework to understand the reason for the high frequency of spills and plan improvements. Thames Water have completed an investigation and currently have plans in place for improvements at the site to reduce the number of storm sewage spills to an acceptable level. I note that you have already contacted Thames Water who are best placed to provide you with the latest progress on implementing their improvement plans.
In relation to the particular storm sewage incident you have referred to, this was reported by Thames Water on 28 August 2022. The information obtained shows that there was only a minor impact on the receiving watercourse.
Going forwards, we will continue to assess the frequency and duration of storm sewage spills at Chobham Sewage Treatment Works to confirm that the improvement schemes that Thames Water are implementing are effective. We also continue to progress our major national investigation into potential widespread non-compliance with flow conditions at sewage treatment works.
You will be aware that the Government has recently published the Storm Overflows Discharge Reduction Plan which sets strong new targets to protect people and the environment. We welcome the plan, water companies need to invest responsibly and go further and faster in tackling environmental damage caused by storm overflows. We will continue to work with government, the water industry, other regulators and NGO’s to deliver the plan and ensure that we have healthier and cleaner rivers.
Finally, I would be grateful if you would ask your constituents to report any future incidents they witness as soon as possible to our 24 hour free incident hotline number - 0800 807060.
I trust that the above information provides a useful response on the issues and concerns that you have raised.’
6 January 2023 - Michael received a response from Rebecca Pow MP, Minister for Environmental Quality and Resilience at the Department for Environment, Food & Rural Affairs, outlining the action the Government is taking. A copy of the letter is attached below.
11 February 2023 – Michael wrote to Thames Water to ask for an update on the progress of the upgrade at Chobham Sewage Works, and is awaiting a response.