Process for treating waste water on ships in which the waste water is pretreated by surface filtration, the pretreated waste water being collected in a first tank acting as a mixing tank and optionally enriched with oxygen. The waste water from the first tank is continuously fed to a three stage bioreactor, in which aerobic waste water treatment takes place in a second tank, waste water is fed in batches from a second tank to a third sealed tank, in which denitrification and sedimentation of the solids takes place. Water from the third tank is fed into a fourth tank acting as a settling tank with the largest part of the sludge being recirculated in batches from the third tank into the first tank and, after disinfection, the water from the fourth tank is subjected to fine filtration.