When designing drainage systems for large car parks, incorporating oil separators is crucial to prevent environmental pollution. Oil separators, also known as oil interceptors, play a vital role in ensuring that oil and other hydrocarbons do not contaminate watercourses, groundwater, or sewage treatment systems.
This article explores the necessity of oil separators, the regulations governing their use in the UK, and the importance of their placement upstream of any attenuation storage.
Why Oil Separators Are Essential
Car parks, especially large ones, are prone to oil contamination due to vehicle leaks and accidental spillages. Without proper treatment, these contaminants can enter the drainage system and eventually pollute natural water bodies.
Oil separators are designed to capture and retain oil, allowing only clean water to proceed through the drainage system. This not only protects the environment but also helps businesses avoid legal consequences and potential fines for pollution incidents.
UK Regulations Governing Oil Separators
In the UK, the use and design of oil separators are governed by several regulations and guidelines. The primary document is the Guidance for Pollution Prevention (GPP) 3: Use and Design of Oil Separators in Surface Water Drainage Systems.

This guidance is produced by the Environment Agency for England and Wales, the Scottish Environment Protection Agency, and the Environment and Heritage Service for Northern Ireland.
Key Aspects of GPP 3
Site Requirements:Â
Car Parks: Oil separators are required for car parks larger than 800m² or those with more than 50 parking spaces. Smaller car parks discharging to sensitive environments also need oil separators.
Other Locations: Areas where goods vehicles are parked, vehicle maintenance areas, busier road sections, and industrial sites where oil is stored or used must also have oil separators.Â
Types of Oil Separators:
Full Retention Separators: These treat all the water that enters the drainage system and are used in high-risk areas where significant oil contamination is likely.
Bypass Separators: These treat lower flow rates during moderate rainfall and are suitable for low-risk areas.
Forecourt Separators: Specifically designed for areas like petrol stations where there is a high risk of oil contamination.
Design and Installation:
Sizing: The size of the oil separator must be appropriate for the area it serves. This involves calculating the flow rate and potential oil load.
Location: Oil separators should be located upstream of any attenuation storage to ensure contaminated water is treated before it reaches storage areas.
Maintenance:
Regular Inspections: Oil separators require regular inspections to ensure they are functioning correctly. This includes checking for blockages and ensuring the oil storage capacity has not been exceeded.
Cleaning: Periodic cleaning is necessary to remove accumulated oil and debris. This helps maintain the efficiency of the separator and prevents environmental pollution.
Environmental Compliance:
Discharge Consents: Businesses must obtain discharge consents from the relevant environmental agency to ensure compliance with local regulations.
Record Keeping: Detailed records of maintenance and inspections must be kept to demonstrate compliance with environmental regulations.
Importance of Upstream Placement
Placing the oil separator upstream of any attenuation storage is critical for several reasons:
Preventing Contamination: Attenuation storage is designed to temporarily hold excess stormwater to prevent flooding. If oil-contaminated water enters this storage, it can lead to widespread pollution when the water is eventually released.
Efficient Treatment: By positioning the oil separator upstream, contaminated water is treated before it reaches the attenuation storage. This ensures that only clean water is stored and later discharged, maintaining the integrity of the drainage system.
Regulatory Compliance: Ensuring that oil separators are correctly sited and maintained helps businesses comply with environmental regulations and avoid penalties.
In conclusion, oil separators are a vital component of drainage design for large car parks. They protect the environment from oil pollution, help businesses comply with regulations, and ensure the efficient operation of the drainage system.Â
Proper placement and maintenance of these separators are essential to maximise their effectiveness and safeguard natural water resources.
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