How does a ship's sewage treatment plant work?

A biological marine sewage treatment plant is essentially a compact version of an activated sludge works, fitted into a machinery space and built to keep working reliably despite motion, temperature swings, and an intermittent, unpredictable load that shore plants rarely see.

Raw sewage, black water from toilets and, on some systems, grey water from galleys and washrooms, enters the aeration chamber, where a blower supplies a continuous stream of fine air bubbles. This does two things: it keeps the mixture agitated and in suspension, and it supplies the oxygen that aerobic bacteria need to metabolise the organic matter in the sewage, breaking it down biologically in much the same way as a shore treatment works, just at a smaller scale and with a shorter retention time. The bacterial population, the activated sludge, has to be maintained and given time to establish itself, which is why a newly commissioned or recently pumped-out plant often performs poorly for a period until the biomass rebuilds, something engineers need to plan for rather than be caught out by.

From the aeration chamber the mixture flows to a settling, or clarification, chamber, where flow is slowed enough for the biological solids to settle out as sludge, which is either returned to the aeration chamber to maintain the bacterial population or periodically wasted off, while the clarified liquid on top moves forward for final treatment. Before discharge, the effluent is disinfected, historically by dosing with chlorine or a hypochlorite generator producing chlorine from seawater, more recently by ultraviolet disinfection in some installations, to reduce the bacterial and pathogen content to the standard required before the treated effluent can leave the ship.

MARPOL Annex IV governs where this discharge may happen: effluent from an approved treatment plant meeting the required standard can generally be discharged at any distance from land in ordinary sea areas, though comminuted and disinfected untreated sewage requires greater distance, and special areas designated under Annex IV impose stricter distance and, in some cases, treatment standards. Ships without an adequate treatment plant, or wanting flexibility, use a holding tank instead, retaining sewage for controlled discharge at sea beyond the permitted distance or for pump-out to a shore reception facility in port.

Keeping the plant working properly is a routine but easily neglected job: overloading it with more flow than it is sized for, dosing it with strong disinfectants or cleaning chemicals poured down toilets that kill off the bacterial culture, or letting the air blower or disinfection dosing pump fail unnoticed, all degrade effluent quality, which is why a badly run plant discharging under-treated effluent defeats the entire purpose of Annex IV.