Combined sewer overflows - it's time we cut the crap

Flooding, Management & Regulation, Processed Water

Combined sewer overflows - it's time we cut the crap

A private-member’s bill aims to punish water companies that pollute our rivers with discharges from combined sewer overflows. Is it misguided to single out the water companies, asks Alastair Chisholm

When The Guardian – and CIWEM's The Environment magazine – published articles earlier this summer about the number and impacts of discharges from combined sewer overflows (CSOs), these presaged fresh criticism about the lack of monitoring and transparency regarding these incidents, the UK’s total lack of designated bathing waters in rivers and water-company profits.

The Guardian had to file a freedom of information request to press the Environment Agency to release last year’s data. This revealed 200,000 discharges into English rivers in 2019.

Now, Conservative MP Philip Dunne, who chairs the House of Commons’ influential Environmental Audit Select Committee, has secured a slot on the legislative timetable late this autumn for his private-member’s Sewage (Inland Waters) Bill.

Detail of this bill is still being assembled but the headline is that it seeks to make it a legal duty for water companies not to pollute the water environment with raw sewage. Dunne has pledged to introduce the water companies “to the concept of ‘the polluter pays’.”

We could argue that the water companies are already perfectly well acquainted with the concept, via the permitting system – although perhaps not to the satisfaction of the Rt Hon member for Ludlow. However, the narrative is rather one-sided; the problem is far from straightforward to fix and water companies are not the only polluters here.

It’s all too easy and tempting to point the finger at water companies, who own and manage the sewer networks. And the criticisms are valid, concerning transparency of disclosure about discharges and the historic levels of dividend payments to water-company shareholders.


But CSOs are a legacy of our Victorian sewer system, designed as a public-health intervention to tackle water-borne diseases such as cholera. These were designed as combined systems, taking foul sewage and rainwater to treatment works, then to the point of discharge.

As the sewers were developed and extended – and as urbanisation and population growth expanded their catchments – they have had to tackle ever more foul sewage and surface-water runoff from rooftops, yards, increasing pressure on capacity at their lower points. That has increased the risk of overloading, causing the system to back up and flood homes with sewage.

CSOs release pressure on the system during heavy rain, when the amount of surface water entering it raises the risk of sewer flooding. CSOs are designed to discharge into swollen watercourses or into the sea – the added flow diluting pollution load to mitigate its negative effects on the environment.

The Guardian’s evidence shows discharges happening at lower flows, in lighter rain.

Years back the frequency of discharges and pollution incidents was unacceptably high and water companies were told to invest in upgraded sewer systems, largely adding storage where the problem was particularly bad. Since then, the problem has been gradually improving.

The reasons why watercourses fail to meet their European Union Water Framework Directive targets have also changed. Before, the main reasons related to water-industry practices. Now, agricultural runoff is the more common problem.

Nevertheless, our rivers’ compliance with WFD targets has been pretty dire, despite trumpeted improvements to river-water quality over recent decades – which owes much to the decline, over the years, of heavy industry in the UK – returning species such as salmon and otters to some water bodies.

There is every reason for people to be concerned that in the 2020s – well over a century since the UK developed effective wastewater-treatment processes – we still discharge raw sewage at a high frequency into our rivers. How do we stop it? And are we prepared to make stopping it a priority?

In so many of our towns and cities, our Victorian heritage is all too evident in building stock and infrastructure. The scale of the sewer assets is huge; the UK has well over half a million kilometres of sewers.

As our towns and cities have grown this century, all that extra housing and associated infrastructure, from roads to shops, has considerably increased the hard surfaces off which rainwater runs, traditionally entering the drains that connect to these sewers. Another aspect of urban creep is the inexorable paving over gardens to create driveways to house our ever-expanding fleet of cars.

Meanwhile, we also consume more water, which means more foul sewage. And yet new developments have an automatic right to connect to existing sewer networks. It all adds up to the perfect recipe for increasing pressure on an ageing sewer and drainage network.


One river that has frequently fallen foul of regulatory breaches on sewage discharges and water quality under the Urban Wastewater Treatment Directive is the Thames. The solution – building the Thames Tideway tunnel super sewer – has split opinion, but the project’s £5 billion cost highlights just how much it costs to build solutions to CSO discharges.

Take how much we will spend to fix London’s sewage problems, extrapolate it to the rest of the UK, and the costs will fall into many tens of billions. Add this to the additional water challenges that require our attention over coming decades and it will clearly be impossible to pay for everything by bearing down on investor dividends.

If this is a priority issue for customers, the customer is almost certainly going to have to pay higher bills to fund all this.

Perhaps the answer is not to see this as just a water-company issue. Sure, the water companies have a major role to play, as guardians of the sewer network, but surface water is a major factor. And government has recently published a review of how to better determine ownership and responsibility for surface water drainage assets. We must all find ways to put less of it down our drains.

Recent years have seen a shift towards managing surface water better, above ground, using sustainable drainage systems (SuDS). Following the summer 2007 floods the Pitt Review called for a concerted effort to take SuDS forward.

I’ve banged on enough about what happened next, but England has failed to deliver this properly, its watered-down, half-baked so-called “planning-led” approach underpinned by virtually non-existent, non-statutory standards.

In contrast, Wales embraced Pitt’s recommendations and passed its own Flood and Water Management Act. After a steep learning curve, Wales is starting to see a light at the end of the tunnel.

Multifunctional SuDS schemes can contribute to a green recovery thanks to their magical powers to deliver the threefold benefits of better water quality, new amenity space and better habitats for nature recovery.

Incorporating SuDS into new developments is essential. But so is retrofitting this kind of drainage into our existing concrete and tarmac jungles. Spongifying our cities means less rainwater runoff into combined sewers and more space in them for foul sewage, and so fewer CSO spills.

Returning briefly to the role of water companies, the new Environment Bill proposes to make drainage and sewage-management plans statutory for water companies to manage their sewer and drainage network investment strategically, for the long-term. At the moment these are embryonic and non-statutory – but they will become a major vehicle for tackling CSO discharges.

What else needs to change?

Coming back to Philip Dunne’s private member’s bill, perhaps its needs to cast its scrutiny beyond the water companies. While obliging the water companies in law not to pollute inland waters, it might also set out two additional areas to tackle:

  • Amend new developments’ automatic right to connect to the public sewer, making this conditional on meeting the requirements set out by the Design Construction Guide for the Codes for Adoption
  • Oblige local authorities and other highways agencies to seek opportunities, in maintaining, upgrading or building new infrastructure, to prevent untreated runoff from roads and other urban surfaces being discharged into watercourses. The US uses this principle US under the Clean Water Act, to drive more sustainable management of surface water. We could employ this here too.

Sorting CSO discharges is part of the jigsaw, if we are to tackle our climate and nature emergencies. To achieve this, we need a nuanced, multifaceted approach that goes beyond clobbering the water companies.

Alastair Chisholm image

Statement by Alastair Chisholm, Director of Policy - CIWEM

contact Alastair at

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