Oily-water separator

Introduction: purpose and applications

Bilge water contains water, free oil, dispersed petroleum droplets, rust and cleaning chemicals from different sources. Large droplets separate more easily than a stable fine emulsion. Two batches that look similar can therefore require different treatment times and give different results on the same serviceable equipment.

A holding tank decouples treatment from the irregular arrival of waste. Settling and the specified removal of free oil reduce the load on subsequent processing. Excessive mixing, unsuitable cleaning agents and droplet breakup in a pump may make separation harder. The separator does not remove the cause of a continuing oil leak: control of contamination at its source and appropriate feed preparation directly affect treatment performance.

Construction and main components

• Feed pump and inlet strainer — deliver mixture within the operating range and retain coarse debris.
• Separation stage — removes oil using gravity, coalescence or centrifugal acceleration.
• Coalescing element, where fitted — helps small droplets combine into larger ones that separate more readily.
• Heater and additional stages — condition the stream and improve treatment where included in the equipment approval.
• Oil and sludge collection — routes separated residues to designated tanks for further disposal.
• Oil-content monitor and sample line — assess petroleum contamination in a representative part of the treated flow.
• Automatic stopping or diversion arrangement — sends unacceptable water to the prescribed safe route.
• Controls and records — connect equipment condition, alarms and evidence of completed operations.

Working principle, step by step

1. Bilge mixture arrives from the designated holding tank. Its composition and the avoidance of excessive mixing affect treatment difficulty.
2. Feed passes coarse filtration and the specified conditioning, then enters the separation stage.
3. Oil separates from water. Gravity permits it to rise, coalescence enlarges droplets, and centrifugal equipment accelerates separation with a rotating bowl.
4. Residues enter the receiving tank, while water passes any remaining stages and credible oil-content monitoring.
5. Discharge is possible only when every applicable condition is met. Unsuitable quality, failed monitoring or absent permission requires retention or return through the ship’s designated route.

An oil-content monitor measures properties of a representative treated-water sample to estimate petroleum content. In optical instruments, cell contamination, bubbles and other suspended particles can influence the reading. Sample piping, adequate flow, cell cleanliness and instrument condition are therefore integral to monitoring rather than secondary accessories.

An excessive reading must produce the designed automatic stopping or diversion action. Testing an audible alarm alone does not prove actuator movement or closure of the overboard route. Verification must cover the chain from credible sampling to actual action. Diluting the sample, supplying separate clean water to the monitor or bypassing safeguards does not demonstrate the quality of the treated stream and cannot be used to justify discharge.

MARPOL Annex I specifies conditions for oily-water discharge, rather than only a concentration limit. The applicable permission depends on the ship and water category, proceeding en route, approved equipment, absence of dilution and operating area. Oily discharge in the Antarctic area is prohibited. Polar Code Part II-A establishes the corresponding prohibition in Arctic waters; its clean or segregated ballast exception does not cover oily bilge water. Additional local restrictions and the ship's procedure may also prevent discharge despite a low monitor reading.

Resolution MEPC.107(49) superseded MEPC.60(33) for the relevant equipment standard; applicability to an individual installation depends on its date and approval conditions, including applicable amendments. An older unit cannot be declared compliant with a newer resolution from its appearance. Certificates, manuals, piping and required records provide the verifiable connection between treatment, monitoring and residue handling.

Key characteristics

Capacity describes treatment under approved conditions. It does not authorise overboard discharge, and oil content does not describe every substance in the water.

Maintenance: planning the checks

The calendar below is a planning framework, not mandatory overhaul intervals. The ship’s planned maintenance system (PMS), running hours, condition and maker instructions determine the actual scope and timing.

Troubleshooting: symptom, cause, action

Frequently asked questions

Conclusion

OWS equipment belongs to a chain of collection, treatment, monitoring and waste retention. Healthy separation and a low reading are necessary, but do not alone authorise discharge.

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