the present disclosure is directed to a sewage disposal system having a central aeration chamber flanked on each side by a pair of biological filters each containing tiers of buoyant plastic hollow spheres through and over which the liquid from the aeration chamber flows upwardly and the filters are connected at the top by a crossover pipe positioned below the top level of sludge in the aeration chamber . the crossover pipe communicates with a vertical riser which functions with a vertical antisyphon vent . the riser communicates with a flexible hose which connects the primary central aeration chamber with the chlorine contact tank to receive the sludge from the biological filters for chlorinating the liquid and providing an adequate retention time for disinfection prior to discharge overboard .