Surface water drains typically discharge into a watercourse or indirectly into underground waters with the help of a soakaway. The surface water can get contaminated with chemicals, oil, or suspended solids that can cause these discharges to severely impact the receiving water. In places where run-off is actually draining from an area with relatively low risk like non-operational areas and car parks, a source control approach like a permeable surface or infiltration trenches can offer a suitable means of treatment that can remove the requirement of a separator. On surface water drainage systems, oil separators are installed for protecting the receiving water from getting polluted by oil that can be present because of minor leaks from plants and vehicles or from accidental spillage. The effluent from industrial processes and vehicle washing must normally be discharged to the foul sewer for additional treatment at a municipal treatment plant. These oil separators can completely treat all the flows generated by rainfall. Higher rated flow is allowed for bypassing the separator. When it is considered as an acceptable risk for not providing full treatment for high flows, these devices are used. For example, when the risk of a large spillage and heavy rainfall occurring at the same time is small.