What is a Public sewer

A public sewer is a section of the underground sewerage network that is owned, operated and maintained by a statutory water authority or utility company. It is part of a large-scale, centralised system that collects and transports wastewater and, in some cases, surface water runoff from residential, commercial and industrial properties to a wastewater treatment facility or an approved discharge point.

Public sewers form the core of modern wastewater management, playing a critical role in maintaining public health, protecting the environment and supporting urban development. Their design, operation and maintenance are governed by strict standards and legislation to ensure they remain reliable, safe and capable of meeting the demands of growing populations.

Characteristics of a public sewer

Public sewers are distinguished from private drains and sewers by several key features:

  • They are legally adopted and maintained by the local water authority

  • They serve multiple properties or developments

  • They lie in public land or have legal access rights via easements or adoption agreements

  • They are accessible for maintenance, inspection and repair by the sewerage undertaker

Once a sewer is formally adopted, the responsibility for its upkeep and performance shifts entirely to the authority, relieving individual property owners of maintenance obligations beyond their boundary.

Types of public sewers

Public sewers can be classified by the type of waste they carry:

Foul sewers

These transport wastewater from toilets, sinks, baths and other domestic or industrial waste sources. The sewage is conveyed to treatment plants where solids are removed and water is processed to meet discharge quality standards.

Surface water sewers

Also known as stormwater sewers, these convey rainwater runoff from roofs, driveways, roads and other impermeable surfaces. They usually discharge into rivers, ditches or soakaways, often without treatment.

Combined sewers

Found mainly in older parts of cities, combined sewers carry both foul and surface water in the same pipe. During heavy rainfall, the volume of water may exceed capacity, leading to the use of combined sewer overflows (CSOs) to prevent flooding by releasing diluted sewage into the environment.

Each type requires specific design considerations, flow calculations and maintenance regimes to operate effectively.

Responsibilities of the sewerage undertaker

The local water company or sewerage undertaker is responsible for:

  • Maintaining the condition and structural integrity of the sewer

  • Clearing blockages and repairing defects

  • Ensuring capacity for current and future demand

  • Responding to flooding incidents and pollution events

  • Inspecting and mapping the network

  • Authorising new sewer connections under Section 106 of the Water Industry Act 1991

They are also responsible for meeting environmental targets and complying with the conditions of discharge consents set by regulators such as the Environment Agency.

Adoption of sewers

Under UK legislation, newly constructed sewers can be adopted by the local authority through a Section 104 agreement, which ensures they are built to an agreed standard. Once adopted, they become part of the public sewer network.

Additionally, under the 2011 sewer transfer regulations in England and Wales, many formerly private sewers and lateral drains connected to the public network were automatically transferred to water company ownership. This transfer greatly expanded the reach of the public network and clarified maintenance responsibilities.

Design standards and construction

Public sewers are designed and constructed to meet the standards outlined in guidance such as Sewers for Adoption and Design and Construction Guidance (DCG). Key design principles include:

  • Sufficient capacity to handle peak flows and storm events

  • Access points such as manholes for inspection and maintenance

  • Durable materials such as vitrified clay, concrete or plastic

  • Gradients that ensure self-cleansing flow velocities

  • Hydraulic modelling to assess load from proposed developments

Properly designed public sewers help prevent flooding, structural failure and pollution incidents.

Access and maintenance

Access to public sewers is critical for effective maintenance. Water companies use tools such as:

  • CCTV camera inspections to identify defects or blockages

  • High-pressure water jetting for clearing obstructions

  • Man-entry inspections for large diameter pipes

  • Trenchless repair techniques such as lining or patching

Regular maintenance ensures the network continues to function efficiently, particularly in high-risk areas or ageing infrastructure.

Protection and regulation

Public sewers are protected by legal regulations, which include:

  • Restrictions on building over or near sewers without approval

  • Prohibition of unauthorised connections or discharges

  • Requirement for water companies to keep sewer maps and respond to public sewer enquiries

  • Regulatory oversight by Ofwat and the Environment Agency

Developers and property owners must seek permission before connecting to or building near a public sewer. Failure to do so can result in enforcement action or fines.

Public awareness and interaction

While the public sewer system operates mostly out of sight, its importance becomes evident during heavy rainfall, flooding or pollution incidents. Public campaigns often remind users:

  • Not to flush inappropriate items such as wipes, nappies or sanitary products

  • To avoid pouring oils, fats or chemicals down the drain

  • To report signs of sewer collapse or flooding

Public cooperation is essential in maintaining the long-term health and efficiency of the network.

Conclusion

A public sewer is a vital component of the urban drainage system, owned and maintained by the local water authority to manage and convey wastewater safely and efficiently. Its role in protecting health, property and the environment cannot be overstated.

With growing demands from climate change, urban expansion and ageing infrastructure, continued investment in inspection, renewal and innovation is necessary to ensure public sewers remain fit for purpose. As part of a larger system that includes treatment works, pumping stations and outfalls, public sewers represent the foundation of civil sanitation and water resource management in modern society.