Why does the oily water separator 15 ppm bilge alarm keep activating?

The oil content meter fitted downstream of an oily water separator continuously measures the oil concentration in the treated water about to be discharged overboard, and under MARPOL Annex I it must automatically stop the discharge, usually by closing a three way valve back to the bilge holding tank, the instant that reading reaches or exceeds 15 parts of oil per million parts of water. A persistent or recurring alarm means one of two things: either the separator genuinely is not cleaning the water to that standard, or the monitor itself is giving a false reading, and both possibilities need to be worked through rather than assumed.

On the separation side, the most common cause is an overloaded or poorly conditioned feed: bilge water carrying detergents, degreasers or other emulsifying chemicals used in engine room cleaning breaks the normal separation of oil and water into a stable emulsion that the unit cannot physically separate, no matter how well it is maintained, so what goes into the bilges upstream matters as much as the separator itself. Beyond that, a dirty or saturated coalescer element that has not been cleaned or renewed on schedule, accumulated sludge and free oil in the separator that has not been drained down, or simply running the unit at a throughput higher than its rated capacity will all push oil content in the effluent above 15 ppm even with a mechanically sound unit.

On the monitoring side, false high alarms are also common and worth ruling out: a dirty measuring cell or sample chamber that has scaled or fouled over time, air bubbles trapped in the sample line disturbing the optical or infrared reading the meter relies on, a fouled sample pump or restricted sample flow, or a genuinely faulty sensor can all produce a reading that does not reflect the actual discharge quality.

The correct response, always, is to stop the discharge, work through the equipment methodically, back flush and clean the separator internals, renew or clean the coalescer element, drain accumulated sludge and free oil to the sludge tank, and clean the monitor's sample cell, sample lines and sample pump, then retest before resuming discharge. What must never happen under any circumstance is bypassing, tampering with, or falsifying the monitor or its recording, whether by physical bypass piping or by manipulating the readings, since this is illegal under MARPOL, has led to serious criminal prosecutions and heavy fines against companies and individual engineers in numerous well publicized cases, and is exactly the kind of falsification Port State Control and flag state inspectors specifically look for when they audit the oil record book against the equipment's own data logs. Keeping the feed genuinely separable, by keeping detergents and emulsifiers out of the bilges, and keeping the separator and monitor on a real maintenance schedule rather than running until it alarms, is what keeps this from becoming a recurring problem or a legal one.