Osmotic membrane bioreactor (OMBR) is an emerging technology with great potentials for wastewater treatment and reclamation. However, the contradiction between the alleviation of salinity build-up and the achievement of a high biomass concentration remains a main obstacle. Here, we attempt to simultaneously achieve a low salinity environment and a high biomass concentration in OMBRs through integrating a micro-filtration (MF) membrane system in the OMBR and operating it at a longer sludge retention time (SRT) for controlling the salt accumulation and extending the retention of activated sludge, respectively. The results indicated that the salinity in terms of conductivity in the OMBR assisted with the MF membrane (MF-OMBR) was successfully kept at a low range of 2.5-4.0 mS/cm. In the meanwhile, a high concentration of mixed liquor suspended solid (MLSS) in the range of 8.09 +/- 1.02 g/L was achieved in the MF-OMBR through operating at a longer SRT of 30 d. A high MLSS concentration enhanced the removals of ammonia nitrogen (NH4+-N) and total organic carbon (TOC) in the mixed liquors, and consequently increased the water quality of MF and forward osmosis (FO) permeates. In addition, the high MLSS concentration strengthened the removal of some trace organic chemicals (TrOCs) such as Trimethoprim by the activated sludge in the MF-OMBR system. Moreover, a high MLSS concentration did not induce severe FO membrane fouling compared with a low MLSS concentration in OMBRs, which might be due to a low fouling tendency of FO membrane operating at a low water flux.