What is a Separate system

A separate system is a type of drainage and sewer infrastructure in which two distinct pipelines are provided to convey two different types of wastewater. One pipeline is designated for foul water (also known as wastewater or sewage), which comes from domestic and commercial sources such as toilets, sinks, baths, washing machines and dishwashers. The second pipeline is used for surface water, which includes rainwater runoff from roofs, driveways, roads and other impermeable surfaces.

This design contrasts with a combined system, where both foul and surface water are conveyed in a single pipe. The key aim of a separate system is to reduce the volume of water requiring treatment at wastewater treatment works and to prevent the overloading of sewers during storm events.

Separate systems are now standard in new developments throughout the UK and many other countries, following environmental, engineering and regulatory best practices.

Historical Context and Development

Historically, most urban areas developed combined sewer systems, particularly in the 19th and early 20th centuries. These systems carried both sewage and stormwater to treatment works or directly into watercourses, often with minimal or no treatment. While simple to install, combined systems have several drawbacks:

• They are prone to overloading during heavy rainfall, which can lead to combined sewer overflows (CSOs), discharging untreated wastewater into rivers.

• They require larger capacity pumping and treatment facilities, increasing capital and operational costs.

• They reduce the effectiveness of wastewater treatment, as rainwater dilutes sewage but adds pollutants from urban surfaces.

To address these issues, regulations and design codes began to favour the separation of foul and surface water at source. The Building Regulations (Part H) and Sewers for Adoption guidelines have long supported the installation of separate systems in new developments. Retrofitting older systems remains a challenge but is sometimes undertaken during major infrastructure upgrades.

How a Separate System Works

In a separate drainage system, two independent pipe networks are installed:

• Foul water drainage: All wastewater from appliances, kitchens, bathrooms and industrial processes is collected and conveyed to a sewage treatment works. This pipe typically connects to public foul sewers managed by the local water authority.

• Surface water drainage: Rainwater from roof gutters, paved areas, car parks and highways is collected and discharged, either to a watercourse (river, stream, or ditch), a soakaway, a sustainable drainage system (SuDS), or sometimes a dedicated surface water sewer.

The two networks are designed to operate entirely independently, with no interconnection points. Any cross-connection between them (intentional or accidental) is considered a defect and must be corrected, as it undermines the function and legality of the system.

Key Components of a Separate System

A properly functioning separate system includes a variety of drainage components and devices:

• Foul drainage elements:

  • Soil stacks and waste pipes

  • Inspection chambers and access fittings

  • Foul drainage gullies and backflow preventers

  • Gravity or pumped connections to foul sewer

• Surface water drainage elements:

  • Rainwater downpipes and hoppers

  • Yard gullies and channel drains

  • Silt traps and catch pits

  • Surface water sewers or infiltration features (e.g. soakaways, swales, attenuation tanks)

Each element must be clearly identified during design and installation to prevent confusion and cross-connection.

Benefits of a Separate System

A separate system offers numerous operational and environmental advantages:

1. Improved treatment efficiency:

   • Wastewater reaches treatment plants without being diluted by rainwater, improving biological processing and reducing energy use.

   • Treatment plants are not overwhelmed during storms, maintaining compliance and effluent quality.

2. Reduced risk of pollution:

   • Surface water is discharged directly to the environment or via SuDS, avoiding the risk of CSO discharges during heavy rainfall.

   • Isolating pollutants allows more targeted control and protection of watercourses.

3. Lower capital and maintenance costs:

   • Smaller pipe diameters for foul sewage reduce excavation and material costs.

   • Less volume to pump and treat reduces electricity and chemical consumption.

4. Adaptability and resilience:

   • Each network can be adapted or expanded independently.

   • System failures or blockages in one pipe do not necessarily affect the other.

These benefits have led to separate systems becoming the preferred approach in most urban and rural development schemes.

Challenges and Limitations

Despite the advantages, separate systems also face some limitations and design challenges:

• Misconnections: One of the most common problems in practice. For example, washing machines connected to rainwater downpipes or rain gutters draining into foul sewers. These faults are difficult to detect but cause significant environmental harm.

• Maintenance duplication: Two separate systems require two sets of inspections, cleaning, and repair work.

• Public confusion: Homeowners and contractors may not fully understand the difference between foul and surface water drainage, leading to errors during alterations.

• Limited effectiveness if water quality is not managed: Surface water runoff can be contaminated with oil, heavy metals, or litter. Without treatment, even a separate system can pollute receiving watercourses.

• Legacy infrastructure: In older neighbourhoods with existing combined systems, full separation is often unfeasible without major reconstruction.

These issues are typically addressed through education, enforcement, planning controls and SuDS integration.

Design Considerations

The design of a separate system must follow national and local guidance, including:

• Minimum pipe gradients to maintain self-cleansing flow

• Correct pipe diameters based on anticipated flow volumes

• Separate access points for inspection and maintenance of each network

• Flow control devices for surface water systems to prevent downstream surges

• Storage or infiltration features to manage runoff at source

In many cases, surface water components are designed in accordance with Sustainable Drainage Systems (SuDS) principles, which aim to mimic natural hydrological processes.

In residential settings, this may include:

• Permeable paving

• Rainwater harvesting

• Soakaways

• Green roofs

• Swales and detention basins

These reduce the volume of surface water requiring discharge and enhance water quality before it enters rivers or streams.

Regulatory Context in the UK

The installation and operation of separate systems are governed by several key documents:

• The Building Regulations (Approved Document H): Specifies requirements for separation of foul and surface water drainage in new buildings.

• Sewers for Adoption (now superseded by the Design and Construction Guidance 2020): Outlines standards for adoptable drainage systems, including separate sewer networks.

• Environment Agency and local water authority requirements: May require flow attenuation, water quality controls, and SuDS elements on all surface water discharges.

Planning authorities and developers are expected to demonstrate compliance through drainage strategies submitted as part of development approvals.

Separate System vs Combined System

The choice between a separate and a combined system depends largely on the age of the infrastructure and development constraints.

For new developments, a separate system is almost always required by regulation. Combined systems remain in place in older cities and are subject to phased upgrades where feasible.

Conclusion

A separate system provides a more efficient, environmentally responsible approach to managing foul and surface water in modern developments. By using two independent pipelines, it prevents stormwater from overloading treatment works and ensures that only wastewater requiring treatment reaches the sewage plant. While it demands greater design care and installation accuracy, particularly to prevent misconnections, the long-term benefits in terms of performance, resilience, and watercourse protection make it the system of choice for urban planners, engineers, and regulators alike.