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Understanding the Oily Water Separator: Construction, Working, and Maintenance

Writer's picture: Team MarineXTeam MarineX
Image Credit: AI

One of the most significant concerns for a seafarer or shipowner is the risk of oil pollution accidents. Onboard a merchant vessel, the daily operations generate a mixture of oil and water, which must be properly separated before discharging the treated water overboard. This process is critical in preventing oil pollution, and compliance with international regulations is mandatory.


To address this, the Oily Water Separator (OWS) is used on ships. This essential equipment is designed to separate oil from bilge water, ensuring that only clean water, with oil content below 15 parts per million (ppm), is discharged into the sea. The regulations governing this process are covered under MARPOL ANNEX I, and it is now a requirement for all ships to have an oil discharge monitoring and control system.


For a ship engineer, understanding the OWS is crucial, as it is one of the most inspected pieces of machinery by Port State Control (PSC) officers and surveyors. Throughout a career, engineers may encounter numerous makes and models of OWS, making it imperative to understand their working, maintenance, and regulatory compliance.


Function of the Oily Water Separator

As its name suggests, the Oily Water Separator is designed to separate oil from water in the bilges of a ship’s engine room, cargo holds, and other oil-contaminated areas. According to maritime regulations, the oil content in water processed by the OWS must not exceed 15 ppm before being discharged overboard.


MARPOL Regulations Governing Oily Water Discharge

Under Annex I of MARPOL, Regulation 4 outlines the conditions under which oil mixtures can be discharged:

  1. The ship must be en route – discharge is only allowed when the vessel is moving.

  2. Processing through the OWS – the oily mixture must pass through the OWS, which meets the regulatory standards set in Regulation 14 of Annex I.

  3. Oil content limit – the discharge must not exceed 15 ppm of oil content.

  4. Exclusion of cargo areas – oily mixtures originating from cargo pump-room bilges on oil tankers are prohibited from discharge.

  5. Segregation from cargo residues – in oil tankers, the oily water mixture must not be mixed with oil cargo residues.

Ships operating in Antarctic waters are strictly prohibited from discharging any oily mixtures.


Key Requirements for Oily Water Separator (OWS)

According to MEPC 107(49), the OWS must comply with several important standards:

  • The Oil Content Monitor (OCM) must be tamper-proof and certified by an authorized organization.

  • The OCM should sound an alarm and record data if the oil content exceeds 15 ppm or if freshwater is used for cleaning.

  • The OWS must achieve the regulatory discharge limit of 15 ppm.

  • Sensors and alarms must be placed in locations that cannot be easily accessed or tampered with.

  • Only trained officers, under the direct supervision of the Chief Engineer, are authorised to operate the OWS. When not in use, the system must be locked to prevent unauthorised access.

Sailing without a functional OWS is illegal, and ships must carry appropriate spares for maintenance.

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