In order to investigate the feasibility of biological treatment of hypersaline wastewater produced from toilet flushing with seawater at low temperature, pilot-scale studies were established with plug-flow activated sludge process at low temperature (5-9℃) based on bench-scale experiments. The critical salinity concentration of 30 g/L, which resulted from the cooperation results of the non-halophilic bacteria and the halophilic bacteria, was drawn in bench-scale experiment. Pilot-scale studies showed that high COD removal efficiency, higher than 85 %, was obtained at low temperature when 30 percent seawater [ seawater/( seawater + sewage) ] was introduced. The salinity improved the settleability of activated sludge, and average SV dropped down from 38%to 22. 5% after adding seawater. Sludge bulking could be forborne effectively because filamentous bacteria couldn t subsist under high salinity concentration.