A ship produces oil and water mixture on a daily basis which needs to be separated from each other, before discharging the dirty water out of the ship using equipment such as an oily water separator.
MARPOL has a regulation under ANNEX I which limits the oil content in the bilge water that a vessel can legitimately discharge into the sea. It is now a requirement for all vessels to have an oil discharge monitoring and control system along with oil filtering equipment known as the Oily Water Separator (OWS).
As the name indicates, the function of an oily water separator is to separate the maximum amount of oil particles from the water to be discharged overboard from the engine room or cargo hold bilges, oil tanks, and oil-contaminated spaces. As per maritime regulations, the oil content in the water processed from the OWS must be less than 15 parts per million of oil.