What is a Combined Sewer Overflow

A Combined Sewer Overflow (CSO) is an engineered structure or discharge point within a combined sewer system, designed to protect public health, property, and critical infrastructure from hydraulic overload during storm conditions. When a sudden increase in flow—caused by heavy rainfall or snowmelt—threatens to exceed the capacity of the sewer network or treatment works, a CSO allows for the temporary discharge of excess, diluted sewage into an alternative pathway, such as another sewer, storage tank, or directly into a receiving watercourse.

While CSOs play an essential safety role in combined systems, they also pose considerable environmental risks due to the release of partially treated or untreated effluent into rivers, estuaries, and coastal waters. As such, they are subject to strict regulatory oversight and increasing public attention, especially in the context of climate change, water quality legislation, and sustainability goals.

Understanding the function, operation, and management of CSOs is critical for wastewater engineers, environmental regulators, urban planners, and infrastructure operators.

Background and Context

Combined sewer systems are common in older towns and cities throughout the United Kingdom and other parts of Europe. In these systems, both foul sewage (from domestic, commercial, and industrial sources) and surface water runoff (from roofs, roads, and pavements) are conveyed in a single pipe network. During dry weather, the system functions without issue, sending flows to a centralised wastewater treatment works (WwTW).

However, during heavy rainfall, the combined flow volume can surge dramatically. Without intervention, this can lead to surcharging—when sewage backs up through manholes, gullies, or into buildings—and overwhelm treatment plants, leading to operational failure.

The Combined Sewer Overflow was developed as a safety mechanism to avoid such outcomes. Located strategically throughout the sewer network, CSOs discharge excess flow—typically a mixture of rainwater and diluted sewage—once the system reaches a pre-defined threshold. The goal is not to bypass treatment unnecessarily, but to relieve pressure on the system and preserve overall functionality and public hygiene.

Structure and Function of a CSO

A CSO is typically located at a junction within the sewer network, often close to treatment works or near low-lying or flood-prone areas. It is a passive hydraulic feature, designed to activate automatically when certain flow conditions are met.

Key components of a CSO may include:

  • Weir wall or overflow channel: 
    • Directs normal flow toward the treatment works but allows higher flows to spill over once a certain level is reached. 
  • Overflow pipe or conduit: 
    • Transfers excess flow to a river, stream, or alternative sewer route. 
  • Screening system: 
    • Often includes coarse screens or mechanical bar screens to remove large solids and litter before discharge. 
  • Storage facilities (in advanced systems): 
    • May incorporate storm tanks or retention chambers to temporarily hold excess flow for later return to treatment. 
  • Flow measurement and monitoring equipment: 
    • Enables real-time tracking of discharge events and reporting to regulatory bodies. 

The activation of a CSO is based solely on hydraulic conditions. There are no pumps or manual controls involved; the system relies on gravity and pressure differentials.

Environmental Impact

While CSOs are essential from a system safety perspective, their environmental consequences have drawn increasing concern. Discharges from CSOs can introduce pollutants into natural water bodies, even when the majority of the flow is stormwater.

Common pollutants in CSO discharges include:

  • Suspended solids 
  • Faecal bacteria and pathogens 
  • Organic matter (measured as BOD and COD) 
  • Nutrients such as nitrogen and phosphorus 
  • Microplastics 
  • Litter and sanitary products 

These discharges can lead to a range of adverse environmental effects, including:

  • Eutrophication: Excess nutrients stimulate algal blooms, depleting oxygen and harming aquatic life. 
  • Public health risks: Contamination of bathing waters and shellfish beds. 
  • Visual pollution and odour: Litter and solids deposited in rivers and along banks. 
  • Degradation of water quality: Affecting ecosystems and breaching regulatory standards. 

Although the diluted nature of CSO discharges is often cited as mitigating their impact, frequent or high-volume overflows can cause cumulative harm to sensitive watercourses.

Regulatory Framework in the UK

In the United Kingdom, CSOs are legally permitted but highly regulated. They must operate within the terms of a Discharge Consent or Environmental Permit issued by the Environment Agency (EA).

Key legislative and policy frameworks include:

  • Urban Waste Water Treatment Regulations 1994 
  • Environment Act 2021 
  • Water Framework Directive (until Brexit; principles continue in UK law) 
  • Bathing Water Regulations 2013 

The EA and Ofwat, the economic regulator of water companies, both have roles in overseeing CSO performance, particularly in terms of frequency, duration, and environmental impact. Permits typically specify:

  • The conditions under which discharge is allowed (e.g., during rainfall events only) 
  • Location and design standards for overflow points 
  • Requirements for flow measurement and data reporting 
  • Maximum allowable frequency of discharge 

Failure to comply with permit conditions can lead to enforcement action, fines, or prosecutions, particularly if pollution causes environmental damage or affects designated waters.

Mitigation and Modernisation Strategies

Water companies and sewerage undertakers across the UK are engaged in a wide range of strategies aimed at reducing the frequency, volume, and impact of CSO discharges. These interventions fall into both engineering-based solutions and nature-based strategies.

Common engineering measures include:

  • Storage tanks and tunnels: 
    • Construction of large underground chambers to hold excess flow until it can be treated. 
  • Real-time control (RTC): 
    • Use of sensors, valves, and software to manage flow dynamically and maximise available capacity. 
  • Sewer rehabilitation and upsizing: 
    • Increasing pipe diameters or upgrading aging infrastructure to improve conveyance. 
  • Screening upgrades: 
    • Installing finer or automated screens to reduce solids in CSO discharges. 

Complementary green infrastructure includes:

  • Sustainable Drainage Systems (SuDS): 
    • Features such as swales, rain gardens, and permeable paving to reduce surface runoff at source. 
  • Roof gardens and tree pits: 
    • Reduce volume and delay peak flow into the combined sewer. 
  • Surface water disconnection programmes: 
    • Re-routing roof or yard drainage to soakaways or waterbutts instead of the combined system. 

These solutions are often delivered through long-term investment programmes as part of water companies’ Asset Management Plans (AMPs), particularly in AMP8 (2025–2030) and beyond.

Monitoring and Transparency

In recent years, there has been growing demand for greater transparency around CSO activity. In response, most water companies have implemented Event Duration Monitors (EDMs) at active overflow sites. These devices record:

  • The start and end time of each overflow event 
  • The duration and estimated volume of each discharge 
  • The number of events within a reporting period 

This data is shared with the Environment Agency and, increasingly, made available to the public through live dashboards and online platforms.

The aim is to:

  • Build public trust 
  • Improve accountability 
  • Enable better policy and investment decisions 
  • Inform recreation and bathing water users of pollution risks 

The Future of CSOs in the UK

The future role of CSOs is the subject of significant national debate and legislative review. Key developments include:

  • Storm Overflows Discharge Reduction Plan (2022): 
    • A government strategy outlining targets to reduce CSO discharges and investment pathways. 
  • Environment Act 2021: 
    • Introduced new legal duties on water companies and regulators to reduce discharges and report performance. 
  • Nature-based regulation: 
    • Exploring more holistic approaches to stormwater management that reduce reliance on hard infrastructure alone. 
  • Public and political pressure: 
    • Campaigns by NGOs, communities, and environmental organisations have brought the issue to the forefront of water policy. 

The long-term goal is not necessarily to eliminate CSOs entirely—given the practical challenges and cost—but to significantly reduce their frequency, improve their transparency, and mitigate their environmental impact through an integrated, system-wide approach.

Conclusion

A Combined Sewer Overflow is a critical component of older sewer systems, offering protection against flooding and hydraulic failure by redirecting excess flows during storm conditions. While necessary from an operational standpoint, CSOs can cause significant environmental harm when used frequently or without adequate controls.

Modern wastewater management strategies aim to reduce reliance on CSOs through a mix of engineering upgrades, green infrastructure, real-time monitoring, and public engagement. As environmental standards tighten and climate pressures increase, the responsible management of CSOs will remain central to the challenge of delivering resilient, sustainable, and publicly accountable sewerage systems in the 21st century.