A wastewater treatment system for use on marine vessels or land-based applications treats wastewater by solids separation and oxidation. The system includes a solids separation tank, an oxidation tank having a plurality of chambers and a fluid circuit for re-circulating treated wastewater into the solids separation tank and oxidation tank for further treatment. An ozone generator produces ozone which is dissolved in the treated wastewater in the re-circulating fluid conduit and is used for the solids separation and oxidation processes. The treated wastewater is periodically discharged into the environment.