Oily Water Separator
An Oily Water Separator (OWS) is equipment in the engine room that separates oil from the bilge water that collects in the machinery spaces, so the water can be discharged overboard legally. It reduces the oil content to below the MARPOL limit of 15 parts per million, monitored by an oil-content meter that stops the discharge if the limit is exceeded.
Discharge of oily bilge water is one of the most heavily regulated and enforced areas under MARPOL Annex I. Every overboard discharge is recorded in the Oil Record Book, and tampering with the OWS or its monitor — the so-called magic pipe — is a serious offence that has led to large fines and criminal prosecutions.
Illegal discharge of oily water is one of the most prosecuted pollution offences at sea, with heavy fines and jail terms for falsified records. A properly working, properly used oily water separator is what keeps the ship legal and protects the operator from catastrophic regulatory and reputational risk.
What is the MARPOL limit for oily water discharge?
15 parts per million (ppm) of oil in the discharged water, monitored automatically; above that the oily water separator must stop the overboard discharge.