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How do I size a separator for my site?

Nominal size separators are tested in accordance with the standard test procedure in the European Standard. Each separator is allocated a nominal size (NS) on the basis of the test results. Therefore, Full Retention and Bypass Separators are referred to as NS and NSB respectively.

The nominal size of a Full Retention Separator that is required for a catchment area (A) is obtained using the following formula:

NS = 0.018 x A (in m2)

For a Bypass Separator, the formula is:

NSB = 0.0018 x A (in m2)

In addition, capacity for silt storage (C) must be provided for all separators – either as an integral part of the separator or as a separate upstream unit – according to the following:

C (in litres) = NS x 100 or C (in litres) = NSB x 100

Silt capacity for a Bypass Separator must be provided either upstream of the separator or in the bypass weir chamber, and not in the main oil separating chamber. Separators must be designed such that when the silt chamber or silt area of the separator is full of silt, this will not affect the operation of the separator, the skim pipe or the bypass device.

Oil storage capacity

The oil storage capacity is defined as the volume of separated oil that can be stored in the separator without any of the stored oil entering the inlet or outlet of the separator. The oil storage volume (V) is given by the following:

V (in litres) = NS x 10 or V (in litres) = NSB x 15

On sites where significant oil spillages are foreseeable, make sure the oil storage capacity is sufficient to retain any such spillage and a separator larger than that identified in Section 4a may be required.

Minimum size

The minimum working capacity (which excludes any provision for silt deposition) of a separator should be 1,000 litres; though for forecourts, it is likely that risk assessment will indicate the need for a larger separator. For Bypass Separators, the minimum capacity is defined as the working capacity of the oil separating chamber only.