What is a Petrol interceptor

A petrol interceptor, also known as a fuel interceptor or oil separator, is a drainage system component designed to capture and remove hydrocarbons, oils, fuels and suspended solids from surface water runoff before it enters the public sewer or natural watercourse. These devices are commonly found in areas where vehicles are refuelled, parked or serviced, such as car parks, garages, forecourts, and vehicle wash-down areas.

The main purpose of a petrol interceptor is to prevent pollution of surface and groundwater by intercepting contaminants that are washed off paved surfaces during rainfall or routine cleaning.

Where petrol interceptors are used

Petrol interceptors are required in locations where there is a risk of petroleum-based substances contaminating stormwater. These include:

  • Petrol stations and fuel forecourts

  • Car parks and transport depots

  • Garages and vehicle repair workshops

  • Industrial sites with heavy vehicle movement

  • Roadside rest areas and service stations

  • Airport aprons and maintenance areas

  • Car wash facilities

In these environments, spills and leaks from vehicles, fuel tanks or machinery are common, and rainfall can carry pollutants into drainage systems unless intercepted.

How a petrol interceptor works

A petrol interceptor is typically installed as part of the surface water drainage system. It consists of a series of chambers that use gravity and differences in fluid density to separate hydrocarbons from water.

The basic operation includes:

  1. Inflow of surface water
    Contaminated surface runoff enters the interceptor through an inlet pipe.

  2. Primary separation
    In the first chamber, heavy solids such as grit and debris settle at the bottom, while oil and petrol float to the surface due to their lower density.

  3. Secondary treatment
    The partially treated water passes into a second chamber, allowing further separation of lighter hydrocarbons.

  4. Outflow of clean water
    The cleaned water exits through a submerged outlet pipe that prevents floating oil from escaping.

  5. Oil storage
    Hydrocarbons remain trapped at the surface until they are removed through routine maintenance.

Some systems include a coalescing filter or additional media to enhance separation efficiency, especially in environments with fine oil droplets or emulsified fuel residues.

Types of petrol interceptors

There are two main types of petrol interceptors, defined by their performance and application:

  • Class I interceptors
    Designed to achieve a discharge concentration of less than 5 mg/l of oil. These units often include coalescing filters and are suitable for environmentally sensitive areas or where water is discharged to a natural watercourse.

  • Class II interceptors
    Allow a discharge concentration of up to 100 mg/l of oil. Suitable for less sensitive locations where water discharges to the public sewer rather than directly into the environment.

Additionally, petrol interceptors can be:

  • Full retention
    Designed to treat all flow from a site, including during high rainfall.

  • Bypass
    Allow normal rainfall to pass through the interceptor but divert high flows around it. These are used in larger catchments where only initial runoff needs treatment.

  • Forecourt-specific
    Tailored for petrol station use, typically full-retention Class I interceptors.

Regulatory requirements

In the UK and many other jurisdictions, petrol interceptors are regulated to prevent pollution under environmental protection laws. Key regulations include:

  • The Water Resources Act 1991 and Environmental Permitting Regulations
    Make it illegal to discharge polluting substances such as oil or fuel into controlled waters without authorisation.

  • Environment Agency Pollution Prevention Guidelines (PPGs)
    Although now withdrawn, they remain a useful design reference. PPG 3 (Use and design of oil separators) sets standards for interceptor design and maintenance.

  • BS EN 858-1 and 858-2
    European standards that define performance criteria, sizing methods, installation and maintenance for oil separators.

Local planning authorities and water companies may impose additional requirements depending on the sensitivity of the receiving environment.

Maintenance and inspection

Petrol interceptors require regular maintenance to function effectively. Poorly maintained units can become overwhelmed or allow contaminated water to escape. Key maintenance tasks include:

  • Checking oil and sludge levels with dip rods or sensors

  • Removing trapped oils and solids at prescribed intervals

  • Inspecting inlet and outlet structures for blockages

  • Replacing coalescing filters or other internal components

  • Keeping accurate service records for regulatory compliance

Some modern interceptors include automatic alarm systems that alert operators when the unit needs servicing. Failure to maintain interceptors properly can lead to pollution incidents and enforcement action.

Environmental benefits

By removing hydrocarbons from runoff, petrol interceptors help to:

  • Protect rivers, streams and groundwater from contamination

  • Preserve aquatic ecosystems and biodiversity

  • Prevent the spread of pollutants to drinking water supplies

  • Reduce the environmental impact of vehicle-based activities

  • Maintain compliance with legal discharge limits

Their use is a practical example of source control in sustainable drainage design, where pollution is managed close to its origin.

Integration with other systems

Petrol interceptors are often integrated with other stormwater or site management features, such as:

  • Silt traps or catch pits upstream to reduce sediment load

  • Attenuation tanks to manage peak flows

  • Oil storage facilities with bunding to contain accidental spills

  • Surface water quality monitoring systems

  • Sustainable Drainage Systems (SuDS) where the interceptor protects downstream features like swales or ponds

The overall effectiveness of a site drainage plan depends on how well these components work together.

Conclusion

A petrol interceptor is a vital pollution control device used in drainage systems to trap oils, fuels and suspended solids from surface water runoff in vehicle-dense environments. By separating and storing contaminants before they reach watercourses or public sewers, these units protect the environment, meet legal requirements and support sustainable development.

When properly designed, installed and maintained, petrol interceptors offer a reliable and effective means of preventing hydrocarbon pollution and ensuring that commercial and industrial sites operate responsibly.