What is a Sewer

A sewer is an underground conduit or pipeline that conveys drainage water or wastewater from multiple properties to a designated treatment facility or discharge point. Unlike a drain, which typically serves a single property, a sewer forms part of a broader public or private network designed to collect waste and stormwater from entire streets, neighbourhoods or industrial areas. The term applies both to foul sewers, which carry domestic and trade wastewater, and to surface water sewers, which collect rainwater from roads, roofs and paved areas.

Sewers are a critical component of modern sanitation infrastructure, ensuring that wastewater is safely removed from populated areas to protect public health and prevent environmental pollution. Their design, maintenance and regulation are governed by civil engineering principles, environmental standards and legal requirements in each jurisdiction.

Types of Sewers

There are several types of sewers, each with a specific function depending on the kind of water they transport and the layout of the surrounding infrastructure. The primary categories include:

1. Foul Sewers

Foul sewers transport wastewater from toilets, sinks, baths, kitchens and commercial processes. This type of sewage contains organic matter, pathogens, nutrients and pollutants that require full treatment before discharge. Foul sewers connect homes and businesses to sewage treatment plants.

2. Surface Water Sewers

These sewers carry clean rainwater collected from gutters, gullies, driveways and public spaces. Surface water typically flows directly into local watercourses, such as rivers or lakes, often without treatment. However, in urban areas, surface water can be contaminated with oil, sediment or litter.

3. Combined Sewers

Combined sewers carry both foul and surface water in the same pipe. This system is common in older towns and cities, particularly in the UK, where sewer infrastructure was developed in the 19th and early 20th centuries. While cost-effective at the time, combined sewers can become overwhelmed during heavy rainfall, leading to combined sewer overflows (CSOs).

4. Private and Public Sewers

  • Public sewers are maintained by local water companies and serve multiple properties, usually from the boundary of private land onwards.

  • Private sewers or lateral drains may serve several buildings but are not adopted by the water company. Responsibility for these often lies with the property owners.

The transition between private and public responsibility is a common source of confusion, particularly during property transactions or when dealing with blockages.

How Sewers Work

Sewers function primarily on gravity flow, meaning that pipes are laid on a continuous downward slope to allow waste to travel unaided towards treatment plants. Where gravity flow is not possible due to geography or urban layout, pumping stations are installed to move sewage uphill or over long distances.

A typical sewer system includes:

  • Lateral connections from individual properties

  • Inspection chambers (manholes) for access and maintenance

  • Trunk sewers that carry large volumes over longer distances

  • Overflow structures to manage excess flow during storms

  • Ventilation points to release gases and prevent odours

The size, material and layout of the sewer depend on the expected volume of flow, population served, soil conditions and regulatory standards.

Materials Used in Sewer Construction

Historically, sewers were constructed from brick, vitrified clay or cast iron. Modern systems use materials chosen for their durability, corrosion resistance and hydraulic performance. Common materials include:

  • PVC (Polyvinyl Chloride): Lightweight, easy to install, used mainly for small-diameter sewers.

  • HDPE (High-Density Polyethylene): Flexible, resistant to chemical corrosion and ideal for trenchless installations.

  • Concrete: Used for large trunk sewers and stormwater systems.

  • Ductile iron: Strong and durable, often used in high-pressure systems.

Proper material selection helps reduce the risk of collapse, leakage or root intrusion, all of which can compromise the system.

Sewer Maintenance and Inspection

Regular inspection and maintenance are vital to keep sewers functioning efficiently. Common issues include:

  • Blockages: Caused by fat, oils, grease, wet wipes and foreign objects. These restrict flow and can lead to overflows or backups.

  • Structural damage: Cracks, collapses or root ingress may compromise pipe integrity.

  • Sediment build-up: Reduces flow capacity and encourages anaerobic conditions.

  • Vermin infestation: Rodents may enter through open or broken pipes and access buildings.

Maintenance techniques include:

  • High-pressure water jetting

  • Mechanical rodding

  • CCTV surveys

  • Root cutting

  • Manhole refurbishment

Many water companies use digital monitoring tools and predictive modelling to detect and prevent problems before they cause disruption.

Legal and Regulatory Context

In the UK, the ownership, operation and maintenance of sewers are regulated under several legislative frameworks, including:

  • The Water Industry Act 1991: Defines responsibilities for sewerage undertakers and property owners.

  • The Building Regulations (Part H): Sets standards for the design and connection of drainage systems.

  • The Environmental Permitting Regulations: Govern discharges from sewers to the environment.

Water companies are responsible for public sewers, while property owners are responsible for private sewers and internal drainage systems. Changes to the law in 2011 transferred many private sewers to public ownership, but exceptions still exist.

Before connecting a new development to a public sewer, developers must seek approval from the local sewerage undertaker and ensure that their proposals comply with design and capacity requirements.

Environmental Considerations

Sewers play a crucial role in protecting water quality and public health, but they can also present environmental risks if not managed properly. Key concerns include:

  • Combined Sewer Overflows (CSOs): During heavy rain, combined systems may discharge untreated sewage into rivers or the sea to prevent flooding. While permitted under certain conditions, these overflows are increasingly scrutinised for their impact on ecosystems.

  • Pollution from misconnections: Where appliances (e.g. washing machines) are wrongly connected to surface water sewers, pollutants can reach watercourses untreated.

  • Leaks and infiltration: Groundwater or rainwater entering foul sewers increases treatment loads, while exfiltration may contaminate soil and groundwater.

Improving the performance of sewers involves not only technical upgrades but also public education and cross-sector collaboration.

Modern Developments and Innovations

Modern sewerage planning incorporates new technologies and sustainable approaches to reduce environmental impact and improve resilience. Examples include:

  • Sustainable Drainage Systems (SuDS): Green infrastructure that mimics natural water processes, reducing the burden on conventional sewers.

  • Smart sewers: Systems fitted with sensors that detect blockages, flow rates and gas build-up, enabling predictive maintenance.

  • Trenchless technology: Methods like pipe bursting or cured-in-place pipe (CIPP) lining reduce the need for excavation during sewer repairs.

  • Separate drainage strategies: Promoting the use of rainwater harvesting and greywater recycling to reduce overall sewer loads.

These innovations support long-term infrastructure resilience and align with environmental and climate targets.

Responsibilities of Property Owners

While public sewers are managed by utility companies, property owners have important responsibilities for the drainage systems within their boundaries. This includes:

  • Ensuring proper connection to the correct type of sewer

  • Avoiding disposal of fats, oils, wipes or hazardous chemicals

  • Maintaining any private sewers not adopted by the local authority

  • Reporting signs of sewer problems such as slow drains, bad odours or localised flooding

Ignorance of these responsibilities can lead to costly repairs, enforcement notices or legal disputes with neighbours or local authorities.

Conclusion

A sewer is more than just a pipe in the ground. It is an essential component of urban infrastructure that protects health, supports development and preserves environmental quality. Whether foul, surface water or combined, sewers must be properly designed, maintained and integrated into wider planning frameworks to function effectively. As urban populations grow and weather patterns shift, investment in sewer systems and smarter management practices will become increasingly important for building resilient and sustainable communities.