Guidelines
Guidelines on the sizing of oil separators
The size of the separator will depend on the design, rainfall intensity and the catchment area draining to the separator.
The maximum rainwater flow rate Qr in L/s shall be calculated using equation 4 in accordance with EN 752 – 4.
(4) Qr = Y · i · A
Where:
i is the rainfall intensity, in L/s · ha A is the area receiving rainfall, measured horizontally in ha; Y is a dimensionless runoff coefficient (in most cases taken as 1) 
However in South Africa it is more correct to work in M2. The rainfall intensity analysis for a given station can be obtained from the SA WEATHER BUREAU in Pretoria. We have a good contact: info1@weathersa.co.za 

For a retention time of 6 minutes as promulgated by the standard a easy calculation of L\Sec X 360 = the required separator volume. e.g. 16L\Sec X 60 = 960L\Min X 6Min = 5760 Litres Volume. 
However the primary function of the forecourt separator is to contain one compartment of a fuel delivery vehicle and we have assumed a worst case scenario of 4500 litres storage capacity requirement. 
Guidelines on petrol/oil separators
Separators must conform to the National Building Regulations.
We refer you to SANS 10400 P (Sewage and Drainage) Edition 3 Normative reference SANS 50858 1 & 2
SANS 50858 1 & 2 DEFINES PETROL\OIL SEPARATORS
(A) All separators must be fitted with an automatic closure device.
(B) Separators must confirm to the water quality tests.
(C) Test procedure 8.3.3.1.3 defines water quality to be 120mg/l (120ppm) for Class 2 gravity separators and 10mg\l (10ppm) for Class 1 coalescing separators under laboratory conditions.
(D) Rated flows and volumes are calculated on a retention time of 6 minutes ,therefore the flow rate in litres per second multiplied by 360 is the required volume of the separator.