Interceptors and Separators

Extract from the Pollution Prevention Guidelines: PPG3

Pollution Prevention Puidelines
Use and design of oil separators in surface water drainage systems: PPG 3

Introduction

How do oil separators work?

Oil separators can be fitted to surface water drainage systems to protect the environment from pollution by oils. They separate the oil from the water, and then retain the oil safely until it is removed. They are installed to contain oil leaks from vehicles and plant and accidental spillages. To be effective, oil separators need to be correctly designed, installed and maintained.

Where are separators needed?

Surface water may be contaminated by oil at a number of different sites. These sites need to have measures in place to prevent this oil from polluting the environment. These sites include:

• car parks typically larger than 800m² in area or for 50 or more car parking spaces
• smaller car parks discharging to a sensitive environment
• areas where goods vehicles are parked or manoeuvred
• vehicle maintenance areas
• roads
• industrial sites where oil is stored or used
• refuelling facilities
• any other site with a risk of oil contamination.

Trapped gully pots can provide adequate protection for car parks that are too small to justify the installation of a separator, but they must be properly maintained.

You might not need an oil separator if you use ‘sustainable drainage systems’ (SUDS). The SUDS approach should be used on all sites to minimise the impact of the development on the environment. In Scotland, the use of the SUDS approach is a legal requirement. Techniques that control pollution close to the source, such as permeable surfaces or infiltration trenches, can offer a suitable means of treatment for run-off from low risk areas such as roofs, car parks, and non-operational areas. In higher risk areas you might need other SUDS facilities such as constructed ponds, wetlands or swales. Where there is a high risk of oil contamination, it may be appropriate to use an oil separator as part of the SUDS scheme.

If you do need an oil separator, you will need to consider where it will discharge. It is important to speak to us as early as possible if you plan to discharge to surface water drains, to a watercourse or to the ground, as you might require our consent. In Northern Ireland any discharge from an oil separator will require consent. We do not issue these consents automatically and, if we allow a discharge, we might impose strict controls on the level of polluting substances in it such as oils. If you install a separator discharging to surface water you will need a Class 1 separator.

If your separator will discharge to a foul sewer, you must contact your local sewer provider before doing so. For discharges to foul sewer you will need a Class 1 or Class 2 separator. If your separator will discharge to a surface water sewer that is owned by the sewer provider, you must also contact them before you connect to that sewer.

Drainage from areas such as scrapyards, storage and handling areas for chemicals (solvents, acids etc), and washing bays are likely to be contaminated with substances other than oil, and should normally drain to the foul sewer with the approval of the sewer provider. The local sewer provider might require the discharge to have a separator and you must consult them. Discharge from such areas is not suitable for drainage to surface water drains, a watercourse or to the ground.

Drainage containing detergents should not pass to a separator that discharges to surface water because the detergents prevent the separator from working properly.

Separator Standards and Types

The UK has adopted a two-part European Standard (BS EN 858-1:2002 and BS EN 858-2:2003; Reference 5) for the design, use, selection, installation, operation and maintenance of prefabricated oil separators. The Construction Products Regulations require that new prefabricated separators (made off site and then installed) must satisfy certain essential requirements. Demonstration of fulfilment of these requirements can be provided by compliance with the mandated clauses of BS EN 858-1.

Separator classes

BS EN 858 refers to two 'classes' of separator, based on performance under standard test conditions.

Class 1 separators are designed to achieve a discharge concentration of less than 5 mg/litre of oil under standard test conditions. These separators are required for discharges to surface water drains and the water environment. Many Class 1 separators contain coalescing devices, which draw the oil droplets together and facilitate the separation.

Class 2 separators are designed to achieve a discharge concentration of less than 100 mg/litre of oil under standard test conditions. They are suitable for dealing with discharges where a lower quality requirement applies such as discharges to the foul sewer (but check first with your sewer provider).

Both classes can be produced as 'full retention', 'bypass' or 'forecourt' separators.

The oil concentration limits of 5 mg/litre and 100 mg/litre only apply under standard test conditions. You should not expect separators to always perform within these limits under field conditions. In addition, these levels of oil might be too high in some environmentally sensitive areas to allow the discharge to pass into the water environment without additional treatment.