Full Retention Separators are used where there is a risk of regular contamination with oil, and foreseeable risk of significant spillages (such as in vehicle maintenance areas and retail fuel forecourts). You need to consider the flow rates of potential spillages delivered to the separator from the drainage system, and the oil storage volume of the separator needs to be sufficient to retain the entire spillage. Full retention separators treat the full flow that can be delivered by the drainage system. The ‘full flow’ is normally equivalent to the flow generated by a rainfall intensity of 65mm/hour.
Bypass Separators fully treat all flows, for the area served, generated by rainfall rates of up to 6.5mm/hour (this covers most rainfall events). Flows above this rate are allowed to bypass the separator. These separators are used when it is considered an acceptable risk not to provide full treatment for high flows, such as for sites where only small spillages can occur and the risk of spillage is small (short stay car parks etc.). In cases where a large spillage might occur, it is not acceptable to use a bypass separator.
Forecourt Separators are for all forms of liquid-fuel dispensing outlets, both retail and non-retail, including those where only diesel is dispensed. A forecourt separator must be a ‘full retention’ separator, large enough to serve the catchment area of the site and have a sufficient oil storage volume to retain any foreseeable spillages. It is important to install a forecourt separator of an appropriate size. On a forecourt where tanker deliveries are received, a separator must be sufficient in size to cater for a full fuel tank storage capacity, which is 7,600L.
Silt and Washdown Separators are for sites were detergents are present. The prime function of these separators is to retain and separator the silt with a small amount of oil separation; this is due to the presence of detergents which emulsifies the oil. Where a site is at greater risk due to the presence of an excessive amount of silt, then a Car Wash Silt Trap may be installed before the separator. The size of Washdown Separator depends on the type of vehicle and frequency of washing down taking place.