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Storm overflows: Understanding the challenges of our sewer system

Published on July 25, 2022

The storm overflow assessment framework was introduced in 2018 to confront the challenge of sewer overflows and protect our sewage and river networks. Mark Smith, RSK’s Strategic Business Development Director in the water sector, assesses the current situation and explains the concept of the storm overflow system.

There has been a lot of coverage in the media about the state of our UK rivers. Generally, this coverage has tended to be critical and the stories have focused on pollution incidents and resultant water quality. The finger of blame is usually pointed towards the water companies and their reliance on sewerage or storm overflow systems. The Storm Overflows Taskforce was created in 2020 to work on proposals to reduce the impact of sewage discharge from storm overflows. In addition, the government bought forward measures in the Environment Act 2021, essentially mandating a storm overflow discharge reduction plan and reduction targets to be in place by September 2022. In spring 2022, a consultation was held to seek wider stakeholder views and help shape this new storm overflow discharge reduction plan, which will be announced by Parliament on 1 September 2022.

Combined and separate sewerage systems

Interestingly, one cause of the problem is the success and longevity of our Victorian sewer network. In England and Wales, the sewerage system consists of about 350,000 kilometres of pipes collecting over 11 billion litres of wastewater. These pipes transfer the wastewater to approximately 9000 sewage treatment works, where it is treated and cleaned before being discharged to rivers, estuaries and the sea.

 Most of the sewer network today is what we call a combined sewerage system, which means that wastewater from our toilets, bathrooms and kitchens is conveyed into the same pipe as the rainwater. When it rains, the water that falls can very quickly exceed the capacity of the sewer network in any local situation. The water must go somewhere, so the sewer can very quickly become surcharged or overwhelmed. The result is sewage, albeit dilute sewage, either passing forward to the sewage works at a rate that the works cannot handle, leading to a potential major pollution incident, or alternatively, the sewage backs up in the pipe and is released through a manhole. In extreme cases, people’s homes and living spaces are flooded by this surcharge.

If you live in a modern house, these flows are separated at source and the rainwater from the gutters and pavements will flow to a different pipe containing the discharge from the toilets and kitchens in what is termed a separate sewer system.

What is a sewerage or storm overflow?

 To prevent any homes or living environments from being flooded with sewage during these surcharge events, our sewerage system has been designed to include safety valves, called storm overflows, to discharge excess sewage to the environment when rainfall exceeds capacity. In a combined system, they are termed combined sewer overflows (CSO) and in a separate system, they are called storm overflows (SO). In England and Wales, there are 21,462 licensed CSOs and pumping stations that have the purpose of protecting the network when it is overloaded. CSOs act as an emergency discharge valve and operate once the flowrate exceeds a certain value at a vulnerable point in the network.

It is important to point out that this process happens automatically and any discharge is not due to the manual intervention of a water company employee. According to the government consultation document, these overflows operated over 400,000 discharges, totalling over 3 million hours, in 2020. This is considered by many to be excessive and to have a negative impact on the environment and thus to be unacceptable. It must also be emphasised that overflows of diluted sewage during periods of heavy rainfall are not a sign that the system is broken – rather a sign that it is operating as it was designed to do.

Urbanisation and climate challenges

With increased population, urban growth and climate change driving extreme weather events, the sewerage system is being overwhelmed more often than historically experienced. This results in many more discharges occurring and those discharges lasting for much longer periods of time. Overflows are therefore not leaking but are merely overused to the point of it being considered a problem.

The Environment Agency (EA) is the regulatory body with responsibility for our rivers, working closely with the water companies to ensure that they are closely monitoring and reporting back on this increased discharge activity. This data are used to identify weak points of the system, with investigations and ultimately investment being put in place to target where solutions are most needed. In 2020, the EA identified over 700 CSOs in need of investigation and 40 in need of improvement before 2025.

Not enough action

Activists are not happy with this pace of improvement. The environmental campaign group Wild Justice is seeking a judicial review of the financial regulator Ofwat’s failure to monitor and take enforcement action against water companies that discharge raw sewage into the waterways. If that is not enough, the newly created Office for Environmental Protection (OEP) has said that it is going to investigate the regulation of CSOs following a recommendation by the Environmental Audit Committee.

What is obvious is that the CSOs’ days are numbered, with regulators and activists all wanting an end to uncontrolled discharges from our sewer system: what they discharge and what it would cost to stop those discharges are key questions for us all.

Mark Smith will continue to discuss this storm overflows topic in a forthcoming article, in which he will explain what can be done in this current climate to implement affordable solutions.

RSK has expertise across the water sector, with specialists in , sustainable drainage solutions and spill and pollution response.

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Mark Smith

Strategic Director

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Mark Smith is the Strategic Water Sector Director at RSK. He has over 30 years’ experience, both in the UK and overseas, most of which was at the Water Research Centre, and has had a career focus on the subject of environmental pollution reduction. He has worked on many projects involving discharges, legal and illegal, from the sewer network. He is a Chartered Chemical Engineer and sits as the current chairman of the Future Water Association.

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