If you are looking for a gravity oil water separator, here is a rule of thumb to determine the size. Multiply your estimated flow rate times 10 and that will be the minimum tank size. For example, if your flow rate is around 30 gallons per minute (gpm) you will want an oil water separator with a separation tank that is at least 300 gallons. This will give the oily water a 10 minute retention time in the separation tank. 10 minutes is usually enough time for gravity separation. If you want to be sure, collect a sample of your oily water in a clear container and let it set for ten minutes. What you see separated, is what will you get. Waiting long than ten minutes usually does not yield better results.