What is a Main sewer

The main sewer is a critical component of the public wastewater management system. It refers to the primary, large-diameter pipes forming the central sewer network owned and maintained by the local water authority. Unlike private drains or lateral connections serving individual properties, the main sewer is part of a shared and publicly funded infrastructure designed to transport large volumes of wastewater or surface water from numerous sources to treatment facilities or receiving water bodies.

As the foundation of urban sanitation, the main sewer system supports the health, hygiene, and environmental integrity of entire communities. Understanding its function, design, regulation, and maintenance is essential for engineers, planners, and stakeholders involved in water and wastewater management.

Function and Purpose of the Main Sewer

At its core, the main sewer serves the purpose of collecting and conveying sewage from multiple upstream pipelines and service connections. These pipelines include:

  • Private drains from homes, businesses, and industrial sites

  • Lateral sewers that connect property-level pipes to the public network

  • Secondary and trunk sewers, which progressively combine flow as they feed into the main sewer line

The main sewer system ultimately transports this wastewater to one of two destinations:

  1. Wastewater Treatment Works (WWTW) – where sewage is treated before being safely discharged into rivers or the sea

  2. Combined Sewer Overflows (CSOs) – used only in extreme cases to relieve pressure during storm events by temporarily diverting flow

In some networks, main sewers also carry stormwater, especially in older combined sewer systems, adding to their capacity and design complexity.

Main Sewer vs Private Drainage Systems

One of the key distinctions in urban drainage systems lies in ownership and responsibility. The main sewer, managed by the water authority or sewerage undertaker, is part of a broader public service. This means:

  • The local utility is responsible for inspection, maintenance, repair, and upgrading.

  • Access is regulated, and unauthorised alterations or discharges are prohibited.

  • Capital improvements are funded through public investment or regulated charges.

In contrast, private drains and sewers fall under the responsibility of property owners until they connect to the main network.

A clear understanding of this distinction is vital during construction, renovation, or when investigating blockages and failures. For instance, issues within a private drain up to the connection point (often at the boundary of the property) are the owner’s responsibility, while blockages or damage within the main sewer require water authority intervention.

Types of Main Sewers

Depending on the layout of the sewer system and the development history of an area, main sewers can vary considerably in their design and function. Broadly, they fall into the following categories:

1. Combined Sewers

These carry both wastewater and stormwater in a single pipe. Found predominantly in older cities, they were built before the separation of foul and surface drainage became standard practice. Combined sewers require careful management to avoid flooding and pollution during heavy rainfall.

2. Foul Sewers

Designed exclusively for domestic, commercial, and industrial wastewater, foul sewers direct sewage to treatment works. They are the preferred system in modern developments.

3. Surface Water Sewers

These are dedicated to rainwater and surface runoff, discharging directly into watercourses. They must remain free of foul discharge to prevent pollution.

4. Trunk Sewers

These are major conduits within the sewer hierarchy, conveying large flows over long distances. They form the core routes within the main sewer system and often run beneath main roads, green corridors, or designated utility easements.

Design and Construction Considerations

Designing a main sewer involves a host of technical, environmental, and regulatory considerations. These include:

Hydraulic Design

Engineers must model the expected flow rates based on population density, peak usage, infiltration, and rainfall. Safety factors are applied to account for future growth and unexpected surges.

Materials and Structure

Main sewers are typically constructed from:

  • Reinforced concrete – ideal for large-diameter and high-load areas

  • Clay or vitrified clay – corrosion-resistant and common in older networks

  • Ductile iron or plastic (HDPE, uPVC) – used in pressurised or chemical-resistant applications

Large trunk sewers may be tunnelled using microtunnelling or pipe-jacking methods, particularly in congested or sensitive urban areas.

Gradient and Velocity

Proper gradient ensures self-cleansing velocities are maintained to prevent sediment buildup and blockages. Too steep a gradient, however, can lead to erosion or excessive turbulence.

Access and Inspection

Main sewers are accessed through manholes, chambers, or inspection shafts strategically placed at junctions, changes in direction, and intervals suitable for maintenance operations.

Operation, Maintenance and Challenges

Routine operation and maintenance of the main sewer network are essential for public health and environmental protection. Key activities include:

  • CCTV inspections to detect defects, roots, infiltration, or structural collapse

  • Jetting and flushing to clear sediment and fat accumulations

  • Structural rehabilitation using methods like CIPP lining or pipe bursting

  • Flow monitoring to ensure capacity limits are not exceeded

Despite these efforts, several challenges threaten the integrity of main sewers:

  • Fatbergs – large congealed masses of fat, oil, and non-biodegradable waste

  • Infiltration and Inflow (I&I) – where ground or stormwater enters the sewer, increasing volume

  • Ageing Infrastructure – much of the UK’s network is over 100 years old, requiring constant upgrades

  • Climate Change – increasing rainfall intensity leads to more frequent overflows and surcharge events

  • Urbanisation – more paved surfaces mean higher surface runoff and greater loading on systems

Regulation and Governance

In the UK, the main sewer network is regulated under the Water Industry Act 1991, which outlines the responsibilities of sewerage undertakers and the rights of property owners. The Environment Agency also plays a role in monitoring discharges and environmental compliance.

Since 2011, under new rules, most formerly private sewers have been adopted by water authorities, greatly expanding the main sewer network. This move was aimed at improving long-term asset management and simplifying ownership structures.

Discharges into the public sewer require permission (called a Section 106 agreement) from the water authority. Developers must design their drainage systems to avoid overloading the main sewer and may be required to use sustainable drainage systems (SuDS) to reduce demand.

Integration with Modern Infrastructure

Main sewers must now coexist with a wide range of modern infrastructure needs, including:

  • Smart monitoring systems for real-time data on flow, blockages, or pollution risks

  • Sustainable drainage schemes (SuDS) designed to limit runoff before it enters the main sewer

  • Green-blue infrastructure that combines landscape and water management

  • Resilience planning against flood risk and asset failure

Where possible, new urban developments aim to reduce reliance on traditional main sewers by promoting decentralised solutions, such as permeable paving, retention ponds, or green roofs.

Future Outlook

The role of the main sewer is evolving in the face of growing urban populations, environmental legislation, and climate pressures. Innovations in trenchless technology, sensor-based diagnostics, and real-time control are helping utilities maintain and enhance ageing networks.

Water authorities are increasingly expected to balance affordability with service reliability and environmental protection. As such, investment in sewer infrastructure is rising, often under government scrutiny or Ofwat regulatory cycles.

Public awareness also plays a role. Campaigns such as “Bin it, don’t block it” aim to reduce misuse of the sewer system and prolong its operational life.

Conclusion

The main sewer is far more than just an underground pipe — it is the essential backbone of public sanitation and urban resilience. By transporting wastewater safely from homes and businesses to treatment facilities, the main sewer protects public health, the environment, and the quality of urban life.

Properly managing and investing in this network is one of the most important responsibilities of modern infrastructure planning. As cities grow and climates change, the main sewer will remain a central focus in the effort to build sustainable, liveable, and water-smart communities.