An aerobic sewage treatment system is constructed of a plurality of series-connected turbulent, aerated stages which progressively purify the waste to essentially organic-free water without building up sludge. The system is especially suitable for a compact toilet, such as a toilet on a pleasure boat or recreational vehicle. Discharge of effluent can be avoided by evaporation of water from the system by venting the stages and if necessary by heating the effluent from the last stage.