What is a Combined Sewer Overflow (CSO)

A Combined Sewer Overflow (CSO) is an engineered relief mechanism in a combined sewer system that prevents sewers or sewage treatment works from being overwhelmed during periods of intense rainfall or snowmelt. When the capacity of the sewer or treatment facility is exceeded, a CSO allows excess, diluted sewage to discharge into an alternative outlet—commonly a watercourse, stormwater drain, or dedicated overflow structure—thus safeguarding the integrity of the sewer network and preventing upstream flooding.

While CSOs are an essential part of many historic and urban drainage systems, they are also a focus of considerable environmental concern due to their potential to discharge untreated or partially treated wastewater into natural water bodies. Balancing the operational necessity of CSOs with the environmental imperative to minimise pollution is a major challenge facing water companies, regulatory authorities, and infrastructure planners.

Background: Combined Sewer Systems

In a combined sewer system, both foul sewage (from homes, businesses, and industrial processes) and surface water runoff (from roofs, roads, and paved areas) are transported through a single network of pipes. This design was historically common, particularly in the UK’s Victorian-era cities, where it was seen as a practical solution to improving public health and hygiene.

Under dry weather conditions, these systems function efficiently, directing sewage to wastewater treatment works. However, during periods of heavy rainfall, the additional inflow from surface water can dramatically increase the volume of flow in the system, risking:

  • Surcharging of the sewer network (backflow through manholes, gullies, or into buildings) 
  • Overloading of treatment plants (causing process failure or bypass) 
  • Structural damage to pipes and pumping stations 

To avoid these outcomes, CSOs are installed at strategic points within the network. These overflow structures automatically divert excess flow once hydraulic thresholds are reached, allowing the system to continue operating and protecting public health and infrastructure.

How a CSO Works

A typical CSO consists of a diversion point where the main sewer pipe includes an overflow weir or bypass conduit. During normal flow conditions, sewage passes through the system to the treatment facility. During storm conditions, once the flow exceeds a predetermined level, the CSO activates.

Key components of a CSO include:

  • Weir wall or overflow chamber: Allows excess flow to spill over into the CSO outlet once the upstream level rises. 
  • Overflow pipe or conduit: Carries the excess flow away from the main sewer to its alternative destination. 
  • Screening system: Most modern CSOs include static or mechanical screens to capture large solids, sanitary waste, and litter before discharge. 
  • Flow control devices: Flow regulators or throttled outlets help modulate the volume and rate of discharge. 
  • Telemetry and sensors: Increasingly common in modern systems to track activation times, duration, and volume of discharges for reporting and analysis. 
  • Storage provision (optional): Some systems include stormwater tanks to hold excess flow temporarily and return it to the sewer once levels recede. 

CSOs are passive hydraulic structures—they rely on gravity and water pressure rather than mechanical operation. Their effectiveness is based on careful siting, accurate hydraulic modelling, and routine maintenance.

Environmental Considerations

Despite their functional importance, CSOs can have significant environmental impacts, especially if they discharge too frequently or are poorly managed. Even diluted sewage may contain:

  • Faecal bacteria and pathogens 
  • Suspended solids 
  • Organic matter (high BOD/COD) 
  • Ammonia and nutrients (nitrogen, phosphorus) 
  • Detergents and hydrocarbons 
  • Plastic debris and sanitary products 

Discharging this mixture into rivers, lakes, or coastal waters can result in:

  • Deterioration of water quality 
  • Algal blooms and eutrophication 
  • Oxygen depletion and harm to aquatic life 
  • Health risks for recreational users 
  • Pollution of shellfish beds and bathing waters 

In response to these risks, CSOs are now subject to increasingly strict regulation and public scrutiny. UK water companies are under legal obligation to monitor and reduce the environmental impact of these overflows.

Legal and Regulatory Framework

In the United Kingdom, CSOs operate under the regulatory oversight of the Environment Agency (EA), with additional scrutiny from Ofwat, DEFRA, and regional authorities in Scotland, Wales, and Northern Ireland.

Key regulatory instruments include:

  • Environmental Permitting Regulations (EPR) 
  • Urban Waste Water Treatment Regulations 1994 
  • Bathing Water Regulations 2013 
  • Water Framework Directive (legacy of EU legislation) 
  • Environment Act 2021 

CSOs must be permitted and operate under a set of defined conditions, such as:

  • Only discharging during storm events 
  • Not causing deterioration in the ecological or chemical status of receiving waters 
  • Being equipped with appropriate screening and monitoring 
  • Reporting frequency and volume of discharges 

Failure to meet permit conditions may result in enforcement action, financial penalties, or legal proceedings.

Monitoring and Public Reporting

The monitoring of CSOs has advanced significantly in recent years. Most active CSOs in the UK are now fitted with Event Duration Monitors (EDMs). These devices collect data on:

  • Start and end time of each overflow event 
  • Duration of discharge 
  • Estimated volume 
  • Frequency over a defined period 

Some water companies publish this data in near-real time via public dashboards, increasing transparency and enabling citizens and stakeholders to hold providers accountable.

Public interest in CSO activity has grown, fuelled by environmental campaigns, media coverage, and community action groups. This scrutiny is helping to drive investment in infrastructure upgrades and operational improvements.

Strategies for Mitigation and Improvement

Addressing the impact of CSOs involves a combination of engineering interventions, system optimisation, and green infrastructure solutions. The aim is to reduce both the frequency and volume of overflows while maintaining system integrity.

Key mitigation approaches include:

  1. Storage solutions: 
    • Construction of storm tanks, retention basins, or oversized sewers to hold excess flow and release it later. 
  2. Real-time control (RTC): 
    • Intelligent systems that manage flow distribution across the network dynamically based on rainfall and sewer levels. 
  3. Flow separation and disconnection: 
    • Redirecting surface water (e.g. from roofs and paved areas) away from combined sewers into Sustainable Drainage Systems (SuDS). 
  4. Green infrastructure: 
    • Use of permeable paving, swales, green roofs, and detention ponds to reduce and delay runoff entering the sewer system. 
  5. Upgrading treatment capacity: 
    • Expanding or enhancing wastewater treatment works to accept greater flow during storm conditions. 
  6. CSO refurbishment and screening: 
    • Retrofitting older CSOs with improved screens and hydraulic design features to reduce solids and litter discharge. 
  7. Catchment-wide planning: 
    • Coordinating urban development, water management, and infrastructure investment to align with long-term resilience goals. 

In many regions, these strategies are being delivered through water companies’ Asset Management Plans (AMPs) and supported by government targets to reduce environmental harm from overflows.

The Future of CSO Management

The role of CSOs in modern drainage systems is under intense review. While their elimination is technically and financially unfeasible in most areas, their impact must be reduced significantly to meet legal obligations and public expectations.

Key trends shaping the future of CSOs include:

  • Digital transformation: Greater use of data analytics, AI, and modelling to predict and manage overflow events. 
  • Legislative reform: Tougher environmental performance standards and enforcement. 
  • Investment and innovation: Funding for infrastructure upgrades and nature-based solutions. 
  • Community engagement: Collaboration with stakeholders to build support for change and encourage water-sensitive behaviours. 

The UK’s Storm Overflows Discharge Reduction Plan and the Environment Act 2021 both signal a shift toward stronger accountability and a renewed commitment to protecting rivers, coasts, and biodiversity from wastewater pollution.

Conclusion

A Combined Sewer Overflow (CSO) is a vital safeguard in combined sewer systems, designed to prevent overloading by allowing excess, diluted sewage to discharge when capacity is exceeded during storm events. While CSOs help protect infrastructure and public health, their discharges pose real environmental risks, particularly to rivers, bathing waters, and marine habitats.

Effective management of CSOs requires a multi-faceted approach: robust infrastructure, real-time monitoring, targeted investment, and strong regulatory oversight. As the climate changes and urban areas expand, reducing CSO frequency and improving their environmental performance will remain a key challenge—and opportunity—for the water industry, regulators, and communities alike.