Sewage Treatment Plant
A Sewage Treatment Plant (STP) treats the sewage — black water from toilets and medical spaces — generated on board so that it can be discharged within the limits of MARPOL Annex IV. Biological plants use bacteria to break down the waste, followed by disinfection; other systems collect sewage in holding tanks for discharge at sea beyond set distances or to shore reception facilities.
Where and how treated or untreated sewage may be discharged depends on distance from land, the speed of the ship and whether the plant is type-approved. Passenger ships and special areas have stricter rules, and records are kept of the systems and discharges.
Sewage discharge is regulated to protect coastal waters and public health, and non-compliance brings fines and detention. A working, type-approved sewage treatment plant is what lets a ship manage black water legally, especially near coasts and in the stricter special areas.
What is black water on a ship?
Sewage from toilets and medical spaces, treated by the sewage treatment plant. It is distinguished from grey water — the waste water from sinks, showers and galleys — which is regulated differently.