Fouling and cleaning of thin film composite forward osmosis membrane treating municipal wastewater for resource recovery

Abstract

Concentrating municipal wastewater by forward osmosis (FO) membrane to a high level of water recovery rate to facilitate downstream resource recovery might cause more serious membrane fouling. This study investigated the concentration of synthetic and real municipal wastewater to 90% water recovery rate by hollow fiber and flat-sheet thin film composite (TFC) FO membranes and their associated membrane fouling and cleaning. Results show that the FO membrane had high rejection rates of COD, phosphate, Ca2+ and Mg2+ with concentration factors at around 8 when achieving a 90% water recovery rate, which facilitated downstream phosphate recovery by precipitation and energy recovery by anaerobic digestion. Ca2+ concentration in municipal wastewater at 61 mg/L was found to be the main factor to cause inorganic scaling, and the fouling caused by calcium precipitates was harder to be cleaned by physical cleaning compared with suspended solids (SS) such as cellulose particles. In addition, the TFC FO membrane for treating real sewage with SS is not applicable for the hollow fiber configuration used in this study due to lumen clogging, while the TFC flat sheet configuration was able to achieve a 90% water recovery rate. The use of a spacer in the flat sheet configuration improved the efficiency of the following physical cleaning by around 15% although it did not alleviate membrane fouling during the membrane filtration process. This study highlighted the importance of the chemistry of FS and DS and FO membrane configuration on membrane fouling particularly at high water recovery rates and the necessity of pre-treatment of municipal wastewater by removing suspended solids.