Utilities Wastewater Europe

Navigating the UK’s sewer overflow “trilemma”

Monday, 8 January 2024

The UK’s combined sewer overflow (CSO) challenge is highly complicated and political. A new whitepaper from environmental consultancy Jacobs aims to demystify the challenge – we take a look.


Operating beyond capacity

There are around 15,000 storm overflows being operated in England beyond their intended capability and they are significantly overwhelmed. If this is not addressed, this could lead to higher levels of sewer discharges which present two main types of harm: harm to public health and harm to the environment.

That’s according to a whitepaper from consulting firm, Jacobs, which looked into the UK water sector’s combined sewer overflow (CSO) challenge. 

As the country deals with the impacts of climate change, a growing population and infrastructure that's not up to the task, these storm overflows are being pushed beyond their intended limits.

When heavy rain combines with wastewater, it overwhelms the sewer systems, leading to discharges into nearby streams. These “old-fashioned safety valves”, designed by the Victorians, are struggling to cope and the consequences are affecting both the environment and local communities.

The Victorian solution

CSOs discharge excess sewage and rainwater to surface waters during strained sewer system scenarios. They prevent flooding and network backups during heavy storms, diluting combined sewage in water courses. However, with climate change, urbanisation and increased rainfall, CSOs operate regularly, exacerbated by sewer blockages. This results in discharges even during lighter rain periods, causing sewage to overflow into streets and homes.

Contributing to blockages are customer practices such as flushing fats, oils, grease, and wet wipes, which are further exacerbating the problem.


A collaborative effort

The CSO challenge embodies what Jacobs refers to as a trilemma: climate change intensifies weather extremes, aging infrastructure impacts water quality and rising costs pose financial challenges. To find a solution, the water industry must unite stakeholders, including regulators, consumers, shareholders, developers, NGOs, and innovators, Jacobs said. 

For example, the frequency and severity of weather events are increasing. More frequent and intense storms are exacerbating existing infrastructure deficiencies, impacting combined storm and sewer systems and catchment health.

Recent apologies from water companies highlight the need for urgent action against sewage spills, Jacobs highlights. Regulatory requirements demand large investments in CSO monitoring and improvements by 2025, with ambitious targets set for 2035 and 2050.

Government plans mandate a £60 billion investment by 2050 to address CSOs. Water companies must consider customer affordability, environmental costs and regional inequalities, seeking innovative funding approaches.

Investing for change


Water companies are investing billions in a National Overflows Plan, the most significant modernisation of sewers since the Victorian era. The investment addresses challenges posed by climate change, environmental impact and affordability. However, the burden is not evenly spread, with regional inequalities requiring careful consideration, the whitepaper points out.

Meeting the £60 billion investment mandate, for example, involves navigating credit ratings, environmental performance assessments and the cost of capital. Water companies must explore alternative funding approaches and maintain strong environmental performance ratings to secure investor confidence.

While customer preferences prioritise immediate impacts like internal sewer flooding, the broader significance of CSOs, including pollution, requires attention.


Five calls to action

There is broad awareness across the sector of the complexity of the issues described in Jacobs’ whitepaper. Conversations are happening at all levels, but there is a consensus that there are no quick fixes and a suite of solutions is required. 

To move forward, Jacobs recommends the following five calls to action to allow all stakeholders to come together and deliver this fundamental shift: 

1. Use a regulatory approach like the Regulators’ Alliance for Progressing Infrastructure Development, a partnership made up of three water regulators: Ofwat, the Environment Agency and the Drinking Water Inspectorate (DWI). This framework needs to be applied to the CSO challenge and include full customer representation, going further than what was recently proposed.

2. Incorporate adaptive planning approaches to allow more flexibility for water companies (and other third parties) to invest in the medium to long term. 

3. Remove unintended blockers to innovative investment by aligning incentives and risks to recognise the concept of ‘best value’ for all stakeholders. 

4. Explore alternative funding mechanisms that both complement the existing RCV (ranked-choice voting) approach and protect current customers from excessive, short term. 

5. Recognise that environmental protection and water quality management associated with CSOs are not solely the responsibility of water utilities and will require the adoption of integrated catchment management principles to achieve success. 


Nature-based solutions

The whitepaper focuses on two main types of solutions to address the sewer overflow dilemma, nature-based and data and technology enabled (D&T). 
Nature-based solutions (NBS) involve using nature to address water management issues, aiming to collaborate with natural processes. An example is planting woodland in the upper part of a river catchment to slow down water flow and reduce downstream flooding. While the main benefit is flood protection, creating woodlands also has additional advantages such as removing air pollutants, providing recreational spaces, enhancing aesthetics, and promoting biodiversity.

NBS, unlike traditional ‘grey solutions’ like flood defences, offer broader ecological benefits for society. They require collective catchment-wide support for success. 

With a growing population and urbanisation, there is a rising need to make space for nature. In the UK, 84 per cent of the population lives in urban areas, increasing by 0.5–1 per cent annually. Embracing NBS involves understanding and supporting these holistic solutions for effective water management.

D&T-enabled solutions

D&T-enabled solutions are vital for real-time monitoring of weather, river levels, sewer levels, and asset performance. Despite innovations, a significant amount of gathered data remains underused and unconnected to its full potential. For instance, data from telemetry and SCADA often ends up in traditional data centers, providing limited insights.

To harness the power of existing data, Jacobs highlights a need for a cultural shift towards insights-driven decision-making. Breaking down data silos, streamlining information and upgrading talent are crucial steps. 

Water company, United Utilities, has successfully employed D&T-enabled solutions to address sewer spills and flooding. Its use of Aqua DNA, a smart solution using sensors and AI analytics, resulted in significant improvements, reducing pollution events, sewer flooding, external flooding, and blockages.

To unlock untapped insights, D&T-enabled solutions can analyse existing data, like telemetry on wastewater pumping stations, providing valuable performance information. Exploiting regulatory requirements around CSO data collection and combining various monitoring efforts can offer a holistic view of catchment health.

A fundamental shift in water management, akin to nature-based solutions (NBS), involves regulators and water companies acting as "water management convenors" to collaborate with communities for comprehensive solutions.

Understanding inputs from other sectors, such as private septic tank discharges, agricultural run-off, and industrial discharges, is crucial, Jacobs points out. D&T-enabled solutions can integrate data from all sectors within a catchment to empower users in dealing with climate events and driving environmental improvements with potential cost savings. 

Nature-based solutions

Guidance from the House of Lords calls on the Government to publish a National Water Strategy, which, if appropriately devised and implemented, could tackle the above challenges and ideas holistically and facilitate a joined-up approach.

The aim of the Strategy would be to set clear expectations about the quality of the water environment and the resilience of water supplies, giving the water sector and regulators clear benchmarks to work towards. The sector would welcome this as a holistic way forward for all, Jacobs highlights.