Combined sewer overflows (CSOs) made headlines earlier this month when The Guardian named and shamed companies and the Environment Agency (EA) for allowing untreated sewage to flow into waterways for thousands of hours every year. Utility Week explores why there has been an increase in the use of CSOs and what can be done to mitigate issues related to them.
The UK’s sewer network is still largely based on the Victorian-built system, carrying both foul sewage and surface water as well as a small amount of groundwater infiltration. During periods of heavy rainfall, the surface water flows within the network can increase significantly, leading to the network reaching capacity. At that point CSOs can spill the flows from the system to consented discharge points. The released water will have been so diluted by the rainwater, which caused the system to overflow, that it will not pose a risk.
Without CSOs, sewer flooding would occur in properties and businesses, and sewage treatment plants would be inundated with flow but as The Guardian reported, the instances of these being used has increased exponentially.
“CSOs are operating more than they should be,” says David Elliott, a consultant at Indepen and a former director at Wessex Water, who points to a combination of human and environmental pressures.
The effects of climate change mean there are more intense and frequent storms, which pose a challenge for companies: the design criteria is already being exceeded quicker than it is being invested in.
Another issue is uncontrolled urbanisation, for example, gardens being paved over and resulting in run-off increases. Although newer developments have separated sewer systems or some kind of sustainable drainage system (SuDS), all too often those re-join the combined sewer.
Elliott says limiting the impact of urbanisation will require a change in attitude towards planning and management of surface water and an acceptance by the public that “doing things that may be harmful to the environment is anti-social behaviour”.
This includes landing the message that sewers are not designed as waste bins and that wet wipes, plastics and other undesirables flushed down the toilet are causing chaos.
“Overflows are designed to prevent flooding of property, so it’s a trade off in terms of public health – do you flood the property or discharge to the environment?” Elliott does not believe the idea of raw sewage going into the environment will ever be broadly accepted by the public as a solution but there are ways of finding acceptable trade-offs.
“Many (people) are starting to articulate that CSOs don’t have a place in modern society but the chance of them being eliminated anytime soon is unlikely because we are talking about billions of pounds and significant disruption to eliminate them entirely.”
As well as behavioural change to protect the sewers, natural solutions such as storm overflows with reedbeds around them offer some treatment and filtration as well as boosting biodiversity, and SuDS can offer improvements but often re-join combined systems.
Ben Aynsley, market group manager for water and property at engineering firm GHD, says in an ideal world we would have a system where surface water drainage is fully separate from foul water flows.
“For decades, planning law has dictated that new developments have had to be constructed in just this way. However, the issue remains that a large proportion of these systems are then connected into the existing combined sewer network and that benefit of separating surface water flows is often lost. There have been some significant improvements with the introduction of SuDs, which look to manage stormwater flows locally on new developments by mimicking natural drainage and infiltration to groundwater. To date, the impact of these measures has been to prevent further burden on the system and is not significant enough to remove the need for CSOs in the network alone.”
Why is this in the news now?
“This is a scab that is being picked at because companies are doing more to monitor and report on CSOs. That has raised awareness of the issue, but given the challenges that companies face around CSOs there is no acceptable solution to this other than eliminating them, which many companies are getting to. But the reality is getting there is going to take time, money and a lot of disruption”, Elliott says.
Some discharges are licenced and legitimate – which is why overflows exist – to operate as a safety valve. This is why the EA as regulator sets permittable overflow levels, but does it need to be doing more?
Elliott says the EA is stuck between a rock and a hard place. “They recognise why overflows exist and they recognise the challenge of constraining their utilisation. When there is negligence they take action but the only action they can take in terms of controlling urban creep, climate change, and changing behaviours are not in the companies’ remit. It will require a joint effort by companies, policy makers and regulators to address these challenges and make them socially unacceptable.”
The EA claims it now has a more robust and consistent approach for how water and sewerage companies monitor CSO spills, which identified over 700 overflows to be investigated and 40 overflows to be improved within the next five years.
It has developed the Drainage and Wastewater Management Planning (DWMP) framework, to better understand, communicate and act on any increased risk to the environment before pollution occurs. The first round of DWMPs will be reported by companies in 2022.
Aynsley also points to moves to increase online storage to reduce the number and duration of spillages; improving the screening of overflows to minimise the discharge of debris and repair maintenance regimes to ensure the hydraulic capacity of sewers is not reduced by blockages
“The implementation of upper catchment solutions to reduce the volume of surface water entering the network has seen more novel solutions coming forward,” Aynsley explains. “This involves taking an integrated catchment solution approach and initiatives such as land management, tree planting and identifying misconnections.
As well as catchment management, advances to network modelling can help with asset management. Aynsley says his company has developed an artificial intelligence tool that uses machine learning to accurately predict wet weather impacts faster than traditional hydraulic models. It does this by understanding the link between past rainfall imagery and overflow occurrences.
Incentive and ambition
The question of what to do about this is going to require action by companies, regulators, legislators, policy makers as well a combined action on sentiment.
Elliott points out that the price of water is so low that offering discounts for not putting surface water into drains may not be enough for most customers to their change behaviour. But developing community values around purpose could have a significant impact.
Households pay for water companies to take the problems away, so companies have internalised the issues. A major argument for privatising the industry was to make large investments to improve public health and the environment, but rather than costly asset infrastructure, the community engagement argument could prove effective.
Aynsley believes that incentive as well as the ambition to reach carbon net-zero by 2030 means innovation plays a greater role. “I feel this will drive innovation in the sector to find smarter, lighter impact solutions that create efficiencies in existing assets. For example, the arrival of internet of things (IoT) technology in network sensors, coupled with AI, will improve the accuracy of modelling to achieve this. Major projects will no doubt have their place in the future but there will be a more diverse set of alternative solutions to consider.”