Combined storm overflows aren’t the problem – delivery models are

What the public debate on combined sewer overflows is missing

Sewer discharges into watercourses are both a public and political issue. Solving the challenge efficiently and cost effectively calls for catchment and systemic approaches, according to Mott MacDonald’s UK storm overflows director Tony Smithson.

The storm overflow challenge is no longer simply about identifying where spills occur or selecting the right engineering intervention. It is about whether the water market can make the right decisions quickly enough to deliver credible improvement in AMP8, without committing investment that limits what can be achieved in AMP9, AMP10 and beyond.

Closing that gap requires delivery models that connect strategy, assessment, design, consenting and construction much earlier. Bringing these disciplines together from the outset can help programmes move faster while ensuring that immediate interventions support, rather than constrain, the improvements needed in AMP9 and AMP10.

Combined sewer overflows (CSOs) were designed as invisible safety mechanisms to protect homes, treatment works and communities during heavy rainfall by releasing excess flow into waterways. Today, increased monitoring, public data, media scrutiny and environmental expectations have made them a highly visible test of trust in the water market. That visibility has created pressure on water companies to deliver rapid action. But speed alone is not progress.

Addressing a single overflow in isolation without understanding its role in the wider catchment can simply transfer the problem elsewhere. A local intervention may reduce spills at one asset while increasing flood risk, surcharge levels or spill frequency downstream. Such interventions can also struggle to remain effective as climate change drives more frequent and intense rainfall events, potentially requiring further investment in the future. Without a catchment-wide understanding of the drivers behind spills, there is a risk of investing significant sums in solutions that deliver limited long-term environmental benefit and lack the resilience needed to adapt to future climate pressures.

Monitoring of England’s 14,000 CSOs is now automated, making discharges far more visible. But records only show when and for how long a spill occurs, not its contents or environmental impact. Therefore, very different incidents can appear the same in the data. This creates a fundamental delivery challenge: how can the market target investment quickly when the measures driving action do not provide a complete picture of risk or harm?

The industry needs to move beyond a CSO-by-CSO mindset

The delivery gap

The scale and timescale of the challenge make sequencing critical. With government setting distinct expectations for 2035, 2045 and 2050, water and sewage companies need to deliver visible improvements now while building programmes that remain effective across successive investment periods.

Expectations from the government on CSO discharge reduction are clear:

• By 2035, water companies must improve CSOs discharging into or near designated bathing waters
• By 2045, CSOs discharging into or near high-priority nature sites must not cause local adverse ecological impact
• By 2050, no CSO should discharge more than an average of 10 times a year over a 10-year period or cause adverse ecological harm.

Unfortunately, there is no silver bullet to address this. Storage, separation, larger pipes, screens, nature-based solutions and blue-green infrastructure all have a role to play, but none will solve the challenge on their own. Each may deliver some of the improvements required but won’t fix the problem in the long-term.

Having a minimum viable outcome mindset is essential. This doesn’t mean aiming for zero discharges everywhere immediately, which is unlikely to be feasible or affordable. It means defining the most credible route through AMP9 and AMP10 to reducing harm while also showing visible progress towards the market’s longer-term environmental obligations.

The industry needs to move beyond a CSO-by-CSO mindset. If a single CSO is causing concern, the obvious response might be to look immediately upstream and see where a tank, swale, permeable car park or other intervention could reduce pressure on that asset. Those interventions may work but opportunities for wider benefits could be missed. Treating each CSO as a separate project risks solving problems at a local level while ignoring catchment-wide issues.

Rapid solutions or false progress?

The danger of moving too quickly without a wider view is that future progress may stall. The public rightly want to see meaningful environmental improvements and evidence that action is being taken to reduce spills. However, while a project may improve the performance of a single asset or meet an immediate compliance requirement, it could inadvertently constrain future options for wider catchment improvements.

Improvements to the highest-risk locations, including those affecting public health or sensitive ecological sites, should come first. Other associated projects, for example catchment strategies, blue-green infrastructure, public engagement and longer-term engineering plans need to be designed, consented or safeguarded now so they can be implemented efficiently in later periods. The important point is that these decisions are made as part of a sequenced plan so that early action does not close off future solutions.

This requires asset management, environmental assessment, feasibility, design, consenting, construction and stakeholder communication to operate as an integrated programme rather than a series of handovers. This is the approach taken by Mott MacDonald and Mott MacDonald Bentley, combining strategic planning, environmental and engineering expertise and delivery capability within a single integrated model. It can also help identify repeatable approaches, reduce duplication and make better decisions about what should happen first, what should happen in parallel and what can wait.

What next?

The market has already made a major step change in transparency, investment and ambition, but AMP8 provides an opportunity to show the public and government that change is happening. However, success depends on more than delivering quickly. The market must act in the right places and in the right order to deliver benefits now without making AMP9 and AMP10 harder to deliver.