What is a Combined Sewer

A combined sewer is a type of underground drainage system designed to collect and convey foul water (wastewater from domestic and commercial properties) and surface water runoff (rainwater from roads, roofs, and other impermeable surfaces) through a shared network of pipes. This system is a hallmark of traditional urban sewerage infrastructure and remains in use in many older towns and cities, particularly where it was installed before the development of separate sewer systems became common practice.

In a combined sewer, all flows are directed to a wastewater treatment facility, where the mixture undergoes processing before discharge into a watercourse. However, under certain conditions, particularly during periods of intense rainfall, the system may become overloaded, leading to the activation of Combined Sewer Overflows (CSOs)—controlled relief points that discharge excess flow directly into the environment to prevent flooding and protect infrastructure.

Understanding the operation, limitations, and future management of combined sewers is essential for engineers, utility managers, urban planners, and environmental regulators working to modernise and maintain sewerage systems in a changing climate and regulatory landscape.

How Combined Sewers Work

In a combined sewer system, wastewater from properties (including toilets, showers, sinks, washing machines, and commercial activities) is mixed with rainwater collected from roads, driveways, pavements, and other impervious surfaces. This combined flow enters the sewer network through a system of gully inlets, inspection chambers, branch connections, and street drains, and is transported under gravity—or via pumping stations—to a centralised wastewater treatment works.

During dry weather, the system carries mostly foul sewage at a relatively consistent flow rate. However, during rainfall events, the volume of water in the network can increase dramatically. The system must be able to accommodate these fluctuating loads without causing surcharging (flooding from manholes or gullies), pipe collapse, or uncontrolled discharge.

To manage this, combined sewers include:

• Storage capacity: Oversized pipes or purpose-built retention tanks provide temporary storage of storm flows. 
• Flow regulators: Devices such as weirs or penstocks to manage and prioritise treatment works inflows. 
• CSOs: Combined Sewer Overflows act as safety valves, discharging excess combined flow to receiving waters when design capacity is exceeded. 

The presence of surface water in the foul sewer network is what defines a sewer as combined, and it has significant implications for hydraulic design, treatment requirements, and environmental compliance.

Origins and Historical Context

Combined sewers originated in the mid-19th century, during a time of rapid urbanisation and growing public health awareness. At that time, sewage and rainwater were commonly allowed to flow through open channels, leading to severe sanitation problems and outbreaks of disease.

Engineers like Sir Joseph Bazalgette in London pioneered the construction of large-scale combined sewer systems, which conveyed waste away from populated areas using gravity-based pipe networks. These early systems were robust, easy to construct, and effective at transporting large volumes of water and waste from densely populated areas to river outfalls or later, treatment plants.

By the early 20th century, most major cities had adopted some form of combined sewer system. The approach was efficient at the time, especially given limited treatment technology and lack of understanding regarding the environmental impacts of discharging untreated wastewater.

From the 1950s onwards, however, increased environmental regulation and the development of advanced sewage treatment processes shifted design preference toward separate sewer systems, in which surface water and foul sewage are conveyed independently. Nevertheless, many areas still rely on combined sewers, and their operation continues to shape water management policy today.

Components of a Combined Sewer

A combined sewer system is composed of several key components that together ensure the collection and safe transport of wastewater and surface water.

Core Elements Include:

1. Main sewer conduits: 
   • Large-diameter pipes (often brick or concrete in older systems) that serve as the backbone of the network. 
2. Lateral and branch connections: 
   • Smaller pipes connecting individual properties or surface inlets to the main sewer. 
3. Gullies and surface water inlets: 
   • Collect rainwater from roads and footpaths. 
4. Manholes and access chambers: 
   • Allow maintenance and inspection. 
5. Pumping stations (if necessary): 
   • Used in areas where gravity flow cannot be maintained. 
6. Flow regulators and weirs: 
   • Manage flow direction and prioritise wastewater treatment inflows. 
7. Combined Sewer Overflows (CSOs): 
   • Discharge excess flow to rivers or seas during storm events to prevent upstream flooding. 
8. Stormwater storage tanks or oversized sewers: 
   • Provide temporary attenuation to reduce CSO activations. 

These components work together to transport all combined flow toward a treatment facility, while managing the risks associated with hydraulic overload.

Benefits of Combined Sewer Systems

Although modern engineering generally favours separate drainage systems, combined sewers still offer several advantages in appropriate contexts:

1. Infrastructure efficiency: 
   • Using a single pipe system reduces the need for dual infrastructure and simplifies construction. 
2. Cost-effective in historic developments: 
   • Retrofitting separate systems into established urban areas can be prohibitively expensive and disruptive. 
3. Compatible with densely built environments: 
   • Combined systems reduce spatial requirements where streets and footways are narrow or congested. 
4. Simplified flow routing: 
   • All flows directed to a single treatment works streamlines process management. 

In many towns and cities, the continued use of combined sewers remains the only practical option short of full reconstruction, making their intelligent management a key engineering challenge.

Limitations and Challenges

Combined sewers are not without significant drawbacks, especially in modern urban environments where impervious surfaces are widespread, and rainfall events are becoming more intense due to climate change.

Key Limitations Include:

• Combined Sewer Overflows (CSOs): 
  • When system capacity is exceeded, diluted but untreated sewage may be discharged directly into rivers or coastal waters. 
  • This presents a risk to human health, aquatic life, and water quality. 
• Variable hydraulic loading: 
  • Treatment works must accommodate large fluctuations in flow rate, making process control and energy optimisation more difficult. 
• Pollutant mixing: 
  • Stormwater that might otherwise not require treatment becomes contaminated by contact with foul sewage. 
• Maintenance and inspection complexity: 
  • Sediment and debris from surface water runoff can accelerate blockage formation and structural deterioration. 
• Regulatory pressure: 
  • Water utilities are under increasing scrutiny to reduce CSO events and meet tightening environmental standards. 

These issues have prompted significant investment in monitoring systems, green infrastructure, and engineering modifications to mitigate the negative effects of combined sewers.

Environmental and Regulatory Considerations

Combined sewers must be managed in compliance with national and regional environmental laws. In the UK, this includes:

• Environment Agency discharge permits for CSOs, which limit frequency and volume. 
• Urban Waste Water Treatment Regulations 1994, which govern treatment standards and environmental protection. 
• Water Framework Directive objectives, which aim to improve the ecological status of water bodies. 

In response to regulatory pressure and public concern, many water companies have initiated large-scale programmes to reduce CSO discharges, often through a combination of infrastructure upgrades, data-driven control systems, and green engineering.

Mitigation Strategies and Future Management

Several strategies have been developed to improve the performance and sustainability of combined sewer systems:

1. Attenuation storage: 
   • Installing storm tanks, oversized pipes, or underground basins to store peak flows and return them to treatment after the storm subsides. 
2. Real-time control (RTC): 
   • Using sensors and automated gates to optimise flow distribution and maximise use of available storage. 
3. Surface water separation: 
   • Gradually disconnecting impermeable area drainage from combined sewers and redirecting it to soakaways or dedicated surface water systems. 
4. Green infrastructure integration: 
   • Implementing Sustainable Drainage Systems (SuDS) such as swales, green roofs, permeable paving, and detention ponds to reduce runoff at source. 
5. CSO monitoring and transparency: 
   • Increasing the use of telemetry and live public reporting to build accountability and support planning. 

These approaches allow authorities to manage legacy combined sewers more sustainably while planning long-term transitions where appropriate.

Conclusion

A combined sewer is a fundamental, albeit ageing, element of urban wastewater infrastructure. Designed to transport foul water and stormwater in a single network, combined sewers reflect a time when sanitation priorities focused on practicality and rapid urban expansion.

Today, these systems face growing scrutiny due to their vulnerability to overflow, pollution, and climate-related stress. While wholesale replacement is rarely feasible, effective management—combining smart monitoring, targeted infrastructure investment, and sustainable drainage practices—can mitigate their drawbacks and prolong their utility.

For professionals in the plumbing, civil engineering, and environmental sectors, knowledge of combined sewer systems is essential. It remains a key challenge to balance the operational realities of these historic systems with modern expectations for clean, resilient, and sustainable water environments.