What is a Combined Network

A combined network, also known as a combined sewer system, is a type of sewer infrastructure designed to collect and transport both surface water (rainfall runoff) and foul water (wastewater from domestic, commercial, and industrial sources) in a single pipe system. It is a traditional model of sewerage still found in many older urban areas throughout the United Kingdom and other parts of the world, particularly those with historical infrastructure dating back to the Victorian or Edwardian eras.

Combined networks remain in widespread use today, especially in established city centres and densely developed zones. While they offer certain economic and logistical advantages in terms of reduced pipework and simplicity of design, they also pose distinct environmental and operational challenges—particularly in managing the risks of flooding and pollution during periods of heavy rainfall.

Understanding the structure, benefits, limitations, and management requirements of combined networks is essential for professionals involved in drainage planning, wastewater engineering, urban development, and environmental protection.

How a Combined Network Works

In a combined sewer network, both foul sewage and surface runoff are collected by the same underground pipe system. The flow typically consists of:

  • Domestic wastewater: Discharges from toilets, sinks, baths, showers, washing machines, and dishwashers. 
  • Industrial effluent: From commercial or manufacturing facilities, where permitted by regulation. 
  • Surface water: Rainfall or snowmelt collected from roofs, roads, pavements, car parks, and other impermeable surfaces. 

All these flows are directed through the same infrastructure to a wastewater treatment works, where the mixed influent undergoes treatment before being safely discharged into receiving water bodies.

In dry weather, the system mainly conveys foul sewage at relatively low volumes. However, during wet weather, rainfall significantly increases the flow volume—sometimes exceeding the hydraulic capacity of the system or the treatment plant.

To manage such events, combined sewer systems are often equipped with Combined Sewer Overflows (CSOs)—relief points that allow excess flow to be discharged directly into rivers or coastal waters to prevent upstream flooding. While CSOs are regulated and designed to operate only during extreme conditions, their operation has become a matter of growing environmental concern.

Historical Context and Evolution

Combined sewer networks were first widely implemented in the 19th century as part of large-scale urban sanitation projects. At the time, the public health imperative was to remove waste and stormwater from densely populated areas as efficiently as possible.

The use of a single system for both types of flow was considered logical due to:

  • Simpler design and construction 
  • Lower installation costs 
  • Faster implementation during urban expansion 

As a result, most towns and cities constructed before the mid-20th century relied on combined sewers. Separate systems—where foul and surface water flows are collected independently—only became standard practice in new developments during the latter half of the 20th century, driven by increased environmental regulation, urban planning reforms, and better understanding of pollution impacts.

Components of a Combined Sewer Network

While combined and separate networks share many structural features, there are specific components that define and support the functionality of a combined network:

  1. Combined sewer pipes: 
    • Typically larger in diameter to accommodate variable flow volumes. 
    • Constructed from vitrified clay, brickwork, concrete, or modern plastic materials in refurbishment projects. 
  2. Manholes and access chambers: 
    • Provide access for inspection, maintenance, and cleaning. 
    • Located at pipe junctions, bends, and regular intervals. 
  3. Gullies and surface inlets: 
    • Collect surface runoff from roads and footpaths. 
    • May include silt traps to reduce sediment entering the system. 
  4. Interceptor chambers: 
    • Historically used to prevent odours or solids from entering main sewers. 
  5. Pumping stations: 
    • Installed where gravity flow is insufficient due to topographical constraints. 
  6. Combined Sewer Overflows (CSOs): 
    • Act as pressure relief valves for the system. 
    • Divert excess flow into rivers or watercourses when capacity is exceeded, typically during storms. 
  7. Storage tanks or attenuation facilities: 
    • Provide temporary storage of excess flow to reduce frequency and volume of CSO discharges. 
  8. Flow regulators and penstocks: 
    • Used to control and divert flow depending on system conditions. 

These components are carefully calibrated and monitored to balance everyday operational flow with emergency capacity needs.

Advantages of Combined Networks

Despite the drawbacks discussed later, combined networks offer certain practical advantages, particularly in specific contexts:

  1. Simplified construction: 
    • A single pipe system reduces trenching, materials, and installation time. 
  2. Lower initial capital cost: 
    • Fewer components are required compared to dual systems. 
  3. Compatibility with dense urban environments: 
    • Useful where space is limited and retrofitting new pipework is unfeasible. 
  4. Streamlined flow management: 
    • All wastewater reaches the treatment works together, assuming system capacity is adequate. 
  5. Resilience through built-in overflow mechanisms: 
    • CSOs provide a controlled alternative to uncontrolled flooding in emergencies. 

As such, while combined networks are not typically used in new developments, they remain a manageable and often necessary part of existing infrastructure in older cities.

Disadvantages and Environmental Concerns

Combined sewer networks present several operational and environmental challenges, many of which are the focus of active mitigation programmes across the UK and Europe.

Key Disadvantages:

  • Risk of pollution through CSOs: 
    • During intense rainfall, untreated sewage mixed with stormwater may be released into rivers or coastal areas. 
    • This poses risks to water quality, aquatic ecosystems, and public health. 
  • Variable hydraulic load: 
    • Wastewater treatment plants must be designed to cope with highly fluctuating flows, increasing energy and processing costs. 
  • Sediment and silt accumulation: 
    • Surface runoff carries debris, grit, and hydrocarbons that can settle in sewers and reduce capacity. 
  • Difficulties in upgrading: 
    • Retrofitting combined networks to separate systems is technically complex, disruptive, and costly. 
  • Regulatory scrutiny: 
    • Water authorities are under increasing pressure to reduce CSO frequency in line with environmental directives. 
  • Climate vulnerability: 
    • Heavier, more frequent rainfall due to climate change may exceed system design limits more regularly, increasing overflow risk. 

Management and Mitigation Strategies

Water companies and municipal authorities implement a variety of measures to improve the performance of combined networks and reduce environmental impact.

Key Strategies Include:

  1. Monitoring and real-time control: 
    • Use of sensors and SCADA systems to track flows and optimise sewer usage dynamically. 
  2. Storage tanks and attenuation solutions: 
    • Installation of below-ground tanks or oversized sewers to temporarily store peak flows. 
  3. Sewer cleaning and maintenance: 
    • Regular desilting, CCTV inspection, and repair reduce blockages and maximise capacity. 
  4. Surface water separation: 
    • Diverting surface runoff into SuDS or separate drainage systems where feasible. 
  5. Public education and misconnection prevention: 
    • Identifying properties wrongly connected to combined systems and correcting them to prevent cross-contamination. 
  6. Green infrastructure integration: 
    • Promoting permeable surfaces, swales, green roofs, and rain gardens to reduce runoff at source. 

These interventions contribute to better compliance with the Urban Waste Water Treatment Directive, Environment Agency permits, and wider environmental goals.

The Future of Combined Sewer Systems

While combined networks are unlikely to be replaced wholesale in the near future, their management is undergoing significant transformation. As urban areas densify and climate patterns shift, utilities are embracing smart sewers, nature-based solutions, and integrated water management strategies to enhance system resilience.

Innovations include:

  • Digital twin modelling of sewer networks for predictive maintenance and planning. 
  • AI-driven control systems that automatically adjust flows based on weather forecasts. 
  • Decentralised treatment units to reduce loads on centralised systems. 
  • SuDS-led planning policies that reduce reliance on hard infrastructure. 

The goal is to adapt existing combined networks to modern demands without complete replacement—striking a balance between heritage infrastructure and contemporary performance expectations.

Conclusion

A combined network is a legacy sewerage system that transports both foul water and stormwater in the same set of pipes. While simple and cost-effective in design, it presents substantial challenges in terms of pollution risk, climate resilience, and capacity management.

For professionals working in urban drainage, environmental protection, or infrastructure renewal, understanding the operation, limitations, and opportunities for improvement within combined networks is vital. As regulatory frameworks tighten and environmental expectations grow, the future of combined sewerage lies not in elimination but in smarter, cleaner, and more adaptable management practices that respect both historical engineering and contemporary sustainability goals.