Direct concentration of municipal sewage by forward osmosis and membrane fouling behavior

•FO membrane had an excellent rejecting effect for real sewage.•Concentrating multiple of pollutants did not reach concentrating volume multiple.•FO membrane flux was affected mainly by DS concentration.•Cross-flow velocity had little influence on membrane flux change.•Fouled FO membrane could be remitted by physical and chemical cleaning. Forward osmosis (FO) draws attention due to its advantages compares to traditional pressure-driven membrane processes. In this study, a FO membrane concentrating system was built for sewage concentration to investigate membrane rejection, concentrating effect, membrane fouling behavior. Sewage could be concentrated to 1/10 original volume by FO membrane, while pollutants concentrating multiple could not reach 10. The FO membrane had excellent rejecting effect, with effluent COD, ammonia nitrogen, total nitrogen, total phosphorus concentration of 18, 2.5, 2.8, 0.4mg/L, respectively. The FO membrane flux was mainly associated with the draw solution (DS) concentration, which increased with DS concentration but more severe membrane fouling engendered in the meantime. Scanning electronic microscope and fourier transform infrared spectroscopy analysis indicated the formation and constitution of the fouling layer, which included humic acid, protein, and polysaccharide. After concentration, fouled FO membrane was remitted by physical and chemical cleaning, with recovery of 90% and 96%.