What is a Foul system

A foul system is a type of drainage or sewerage network that is specifically designed to collect and convey foul sewage—that is, wastewater generated from domestic, commercial, or industrial sources—without any contribution from surface water or rainwater. The system exclusively handles wastewater from toilets, sinks, baths, kitchens, and other internal plumbing fixtures, directing it to a treatment facility or private treatment unit for safe processing and discharge.

Foul systems are a fundamental part of separate drainage systems, which are now the standard for modern developments, as opposed to combined systems that transport both foul and surface water in the same pipework.

Purpose and Function

The main purpose of a foul system is to safely transport contaminated water away from buildings and infrastructure to avoid risks to human health and environmental pollution. Because foul sewage contains organic waste, chemicals, pathogens, and nutrients, it must be treated at a wastewater treatment works or via an approved on-site treatment system (e.g. septic tank or small package plant) before being discharged to the environment.

By separating foul flows from surface water, foul systems:

  • Prevent unnecessary treatment of clean rainwater, reducing operational costs and energy use at treatment facilities
  • Minimise the risk of sewer overflows during heavy rainfall, as surface runoff is not mixed with foul sewage
  • Improve hydraulic efficiency by managing constant, predictable flows instead of peak storm inflows

Key Components of a Foul System

A foul drainage system typically includes the following engineered components:

  • Foul drains and lateral pipes: Underground pipework conveying wastewater from internal plumbing fixtures to the public sewer or private treatment plant.
  • Inspection chambers: Provide access for maintenance and inspection of underground foul pipes.
  • Foul sewer mains: Larger-diameter pipes that collect flows from multiple properties and convey them to treatment works.
  • Pumping stations: Used where gravity flow is not possible, especially in low-lying or flat terrain.
  • Backflow prevention devices: To prevent reverse flow during surcharges or blockages.

Foul systems are generally constructed from durable materials such as uPVC, vitrified clay, or concrete, and are installed with appropriate gradients to ensure self-cleansing flow velocities.

Design and Layout Principles

When designing a foul system, engineers must ensure that:

  • Foul and surface water systems remain entirely separate to prevent cross-contamination
  • Pipe gradients, diameters, and junctions support reliable, efficient conveyance of wastewater
  • Access points and manholes are provided at key locations to enable cleaning and inspection
  • The system complies with Building Regulations Part H, Sewerage Sector Guidance, and water authority standards

Proper planning also considers population equivalent (PE) and expected daily wastewater volumes to size pipes and pumping infrastructure correctly.

Regulatory and Legal Considerations

In the UK, connection to a foul sewer is governed by the Water Industry Act 1991 and regulated by local water companies. Key legal aspects include:

  • Mandatory connection: In areas where a public foul sewer is available within 30 metres of a property, new developments must typically connect to it.
  • Misconnection compliance: Discharging foul sewage into a surface water drain is illegal and may result in enforcement action and environmental damage.
  • Permitted discharges: Where no public foul sewer is available, discharges from septic tanks or treatment plants must meet conditions under the General Binding Rules or require an environmental permit from the Environment Agency.

Regulators and planning authorities may require developers to submit Drainage Strategy Reports and obtain formal approval before connecting to the public foul network.

Common Misconnections and Risks

Incorrect plumbing or legacy systems may result in foul sewage being misconnected to surface water drains. This can cause:

  • Pollution of watercourses
  • Public health risks
  • Breach of legal regulations
  • Fines and costly rectification works

Typical misconnections occur when toilets, washing machines, or kitchen sinks are inadvertently or illegally plumbed into rainwater pipes, particularly in older buildings.

Maintenance and Monitoring

Foul systems, while generally reliable, require routine maintenance to prevent blockages, leaks, or failures. Maintenance activities include:

  • CCTV drain surveys to inspect internal pipe condition
  • Jetting and rodding to clear blockages
  • Monitoring flows for early detection of infiltration, illegal connections, or excessive discharge

Public foul sewers are maintained by water companies, while private drainage (e.g. from a property boundary to the internal system) is typically the responsibility of the property owner.

Conclusion

A foul system is a critical component of modern drainage infrastructure, designed to handle wastewater from internal sources and transport it for treatment. By keeping foul and surface water systems separate, foul systems help reduce the environmental impact of sewage, improve efficiency, and ensure compliance with public health standards. For developers, engineers, and property owners alike, understanding the function and requirements of foul systems is essential to building and maintaining safe, effective, and legally compliant drainage networks.