CSOs: Solutions for complex problems

Awareness and concern about storm overflows in inland waters and seas has gained considerable momentum, with everyone from activists and politicians to pop stars campaigning for uncontrolled discharges to cease. RSK Group’s strategic business development director, Mark Smith, examines the impact of storm overflows and asks, is it feasible to stop all discharges? If so, what will it cost and who should pay? 

The UK sewerage system consists of hundreds of thousands of kilometres of pipes that vary significantly in both size and material laid in different soils at varying depths.

The vastly complex system is a legacy of Roman infrastructure, with significant and lasting contributions from the Victorians. It has, of course, undergone changes and upgrades, however, some pipes have been in continuous service for more than 200 years.

It conveys almost 11 billion litres of wastewater to more than 9,000 treatment plants for processing and cleaning before being discharged it back into rivers, estuaries, and seas.

Given the age of our sewerage system, pipes may no longer be up to the job when impacted by poor maintenance, misuse, increased population, and extreme weather events.

In its 21st Century Rivers report published in 2021, Water UK stated that removing all CSOs would cost hundreds of billions of pounds and divert funding from issues that cause greater ecological harm to waterways.

In an ideal world we would not have storm overflows at all. So, how do we minimise their impact and ultimately operate the network to prevent uncontrolled discharges?

Redirecting surface water

Sewers are designed to handle wastewater from kitchens and bathrooms but should not contain groundwater or rainwater. Due to age however, the network is prone to groundwater infiltration and cross-connections with the surface water system are endemic. When it rains the network becomes inundated with flows it cannot handle.

As a first step to stop excess water entering the network, we need to take a different approach to managing surface water. Today, we try to direct it off the streets as quickly as possible and into a sewer, which simply transfers the problem elsewhere.

Instead, we need to increase the use of sustainable urban drainage systems (SuDS) in our communities and employ nature-based solutions throughout the system. SuDS are already used in conjunction with strategic drainage design and, increasingly, required to comply with planning, building regulations and lead local flood authority policies.

Nature-based solutions include modifications using swales, wetlands, and balancing ponds to help capture and divert surface water from the sewer. There are huge advantages to bringing these green assets into our communities in terms of increasing biodiversity. However, they need to be managed and maintained into the future, unlike a sewer which can be fitted and forgotten. But there is an opportunity here for mobilising local community involvement, a new and potentially game-changing experience for all. Rather than taking a problem away and solving it in isolation from the community, the water industry could work alongside the community to manage water together.

Secondly, in areas of high groundwater, or after heavy periods of rain, many of our sewers are subjected to serious infiltration because the wall structure of the pipe is compromised. This can be simply due to age or a result of structural factors like ground movements or tree root intrusion. For many years the wastewater network has been the Cinderella asset of the industry and in contrast to the potable network, where leakage has had significant amounts of capex allocated to improving it, Ofwat has regularly rejected plans to improve the wastewater network.

The consequence of this lack of attention and investment is that in a lot of places the wastewater network is degrading. We need a programme to map our sewers, their condition, and to repair and reline those that are most critical with respect to infiltration. Again, local communities can be mobilised to assist in this national mapping exercise.

Currently the public view CSOs as a water industry problem, but the reality is that the issue will not be addressed without people taking responsibility for their own surface water management. Education is key to achieving this and we need to move on from the “out of sight, out of mind” approach that exists. Why not have a water management module mandated in the school curriculums?

Intelligent network management

The sewer network is a passive asset in that it is unpressurised and powered by gravity, with intermittent pump stations at low points. It is unlike the potable water network in that there are very few interventions an operator can make.

We need to change this and make our sewers more intelligent. Thankfully, water companies are now starting on that journey: millions of level sensors need to be deployed across the national network to give early warning alerts of increased water levels. Identifying local blockages means they can then be cleared before flooding occurs and keep the network operating at full capacity.

New technology that accesses the condition of the network needs to be deployed to understand the condition and its operational status in real time. This data together with weather forecasts can be used to operate sewers in readiness for heavy rains. In other words, operators will be able to “empty” the sewer and storage tanks in advance of a storm to maximise the capacity available.

Artificial intelligence (AI), in combination with this real time data from multiple sensor points has the potential to revolutionise how we go about managing our wastewater networks.

Capturing and treating the pollution

The struggle to stop all uncontrolled discharges will take massive investment over many decades and we will have to deal with discharges from storm overflows for many years. Until they are no longer operational, we need to manage the network: not just to protect both ends of the pipe but also with one eye on the environment which can be adversely affected when the CSOs spill, which they will continue to do so for a while.

Encouragingly, the Environment Bill imposed duties on government to publish a plan to reduce sewage discharges from storm overflows and report to Parliament on progress on implementation. Most importantly, the Bill stipulates that all water companies will have a duty to publish data on storm overflows. This means all overflows will be mapped and monitored, producing a better picture of where the spillages and pollution incidents occur.

When conditions require CSOs to discharge, we need to manage the flows better. That means the industry must implement new designs to prevent litter from entering the environment. Solutions that ensure that the extremely polluted first flush is retained in the sewer and passed forward for treatment and screening with the later more dilute sewage being discharged.

The more conventional way to prevent pollution from entering the environment is to build storage tanks which retain the flows until the storm has subsided, at which time the storage tank contents are passed forward for treatment. However, this solution does not come cheap. As an example, a 12,000 cubic metre storage tank built by United Utilities at Anchorsholme Park on the Fylde Coast in April 2016, which has a capacity of five Olympic-sized swimming pools, cost £80 million to construct over five years – this is not practical to implement at every CSO in the country.

The solution

The role we all play in cleaning up our rivers and preventing discharges from CSOs is a vital part of the solution to the challenge we face. Everyone needs to do their part from homeowners and the public through the water companies to the regulators and, ultimately, the Government.

It has taken 200 years of development to get to where we are now, and it will take decades to fix.

Earlier this year, a consultation was held to attract wider stakeholder views to help develop a new storm overflow discharge reduction plan, which was announced by Parliament on 26 August. Let us hope this puts us on the right path.