What are Oil Water Separators and where are they used
Oil Water Separators, or Interceptors, form part of a surface water drainage system to prevent pollution from oils and other hydrocarbons and solids. Any oil that drains into the interceptor is separated from the water and is retained in the tank. Heavier silt also settles out and is retained in the chambers of the tank.
There are many applications for separators, but essentially they are used most places where oil leaks, oil contamination and spillages may occur in surface water. Examples are car parks which are over 800 sqm (which equates to 50 or more parking spaces), roads, vehicle maintenance facilities, petrol stations and other refuelling sites, or industrial sites that have oil storage.
How do they work?
Interceptors or Separators were designed to act as storage vessels to allow hydrocarbons and solids to separate from water and to allow the water to discharge to the approved outfall under gravity.
Originally, interceptors were traditionally three chambers and are easily recognisable by the three manhole covers over the turrets. The interconnecting pipework between each chamber allows flow to gravitate through each chamber resulting in a better separation of hydrocarbons and solids.
Newer interceptors tend be one, or two chambered in design in most cases utilising coalescing filters to remove the oil from the effluent.
The separator retains the oil within the body of the tank.
There are two standards or classes of interceptor.
Class 1
These separators are designed to achieve a discharge concentration of 5mg/litre of oil (5 parts per million) and are used where discharge to surface water drains has been permitted. The 5 parts per million will have been achieved under standardised test conditions and may vary in actual use.
Class 2
These separators are designed to achieve a discharge concentration of 100mg/litre of oil (100 parts per million) and are used where a lower quality of discharge is permitted. The 100 parts per million will have been achieved under standardised test conditions and may vary in actual use.
Both classes of separator can be produced and supplied as ‘full retention’, ‘bypass’ or ‘forecourt’ depending upon the use and application.
Full Retention Separators
Full Retention Separators treat the ‘full flow’ that can be delivered by the drainage system which is normally equivalent to the flow generated by rainfall of 65mm/hour. They are used where there is a regular risk of oil contamination and a risk of significant spillages. This may be vehicle maintenance yards and washdown facilities.
Bypass Separators
Bypass Separators are used when it is considered an acceptable risk not to give full treatment for full flows, where there is NO risk of any spillages or erratic flows. An example of this is for a short stay car park. The probability is there will be a small risk of spillage and any spillages are small. These separators treat rainfall up to 6.5mm/hour, flows above this are allowed to bypass the separator.
Forecourt Separators
A forecourt separator must be a ‘full retention’ type. It must be large enough to serve the site area catchment and should be sized to have an oil storage volume sufficient to retain spillages. Where tanker deliveries are made to the site, the oil storage capacity of the separator should be at least 7600 litres to comply with BS EN 858-2. 7600 litres is the normal capacity of a tanker compartment. If a tanker with larger compartments is used to deliver fuel to the site, then the separator must be sized to suit. A ‘forecourt’ is a site where liquid fuel is dispensed and may be retail or non-retail.
