Performance Of An Integrated Microfiltration–Osmotic Membrane Bioreactor For Wastewater Treatment At High Sludge Concentration

Osmotic membrane bioreactor?OMBR?has drawn increasing interests in the field of wastewater treatment owing to its excellent water quality and low foyling tendency.However,high salinity due to salt accumulation during the operation of OMBR has become a main drawback of OMBR.In order to alleviate the salinity build-up,OMBR always applied a low value of sludge retention time?SRT?for enhancing the discharge of activated sludge containing soluble salt.However,a low SRT resulted in a low sludge solid,which reduced the biomass in the OMBR and thus leading to a decrease of organic matters,ammonia nitrogen and trace organic matters?TrOCs?removals.Besides that,draw solute recovery and its impcats on FO performance in OMBRs are still unclear.Thus,the microfiltration?MF?membrane was integrated in the OMBR?MFO-MBR?for controlling salt accumulation through discharging the soluble solute.In this case,the SRT can be enlarged to achieve a high sludge solid due to no need for controlling salt accumulation by SRT.The removal of TOC and NH₄⁺-N,removal route of TrOCs,salt accumulation and FO membrane fouling in the MFO-MBR operating at high sludge solid concentration was investigated.Moreover,reverse osmosis?RO?was integrated into OMBR for recovering the darw solute,and the optimal conditions for draw solue recovery and impcats on FO performance were analyzed.The main research contents and results are listed as follows:?1?The submerged MFO-MBR system was constructed.Owing to the MF membrane,the SRT realized the separation of waste sludge discharge and salt accumulation control.When the MFO-MBR was operated at SRT of 30 d,a high MLSS concentration of 8.09±1.02 g/L and a low salinity environment of 2.5-4.0mS/cm were simultanerouly achieved.It demonstrated that the addition of MF membrane can simultanerouly achieve high MLSS concentration and low salinity environment in OMBRs.High sludge concentration increased the biomass and thus enhanced the removal of TOC and NH₄⁺-N by the activated sludge,which resulted in a decrease of TOC and NH₄⁺-N concentrations in the sludge supernatant.Comparing with the OMBR operating at a low MLSS concentration,high MLSS concentration did not cause a serious forward osmosis?FO?membrane fouling owing to a reduction of extracellular polymer substances?EPS?including proteins and polysaccarides.The FO membrane fouling at high MLSS concetration included inorganic fouling and the biofouling mainly induced by polysaccarides and microorganisms was the dominant fouling.?2?The high sludge concentration increased the amount of microorganisms in the reactor and enhanced the low biodegradability and the removal rate of low molecular weight TrOCs.With the help of high rejection of FO membrane and the metabolism of microorganisms,the removal of the TrOCs by the MFO-MBR was above 94%.Furthermore,TrOCs had no impacts on the TOC and NH₄⁺-N removals and microbial activity in the MFO-MBR.?3?Application of RO can effectively recover the draw solute of NaCl in the MFO-MBR.Owing to the lower rejection of FO membrane for nitrate and phosphorus than RO membrane,the accumulation of total nitrogen and phosphate occurred in the draw solution.The final total nitrogen and phosphate were stable at 67.97±0.95 mg/L and 2.9±0.07 mg/L,respectively after operating 45 d.The contaminants accumulation in the draw solution induced the fouling of support layer of FO membrane.The analysis of the foulants on the surface of the membrane indicated that the fouling of the FO active layermainly included biological and inorganic foulants,and the?-D-glucopyranose polysaccharides and the total cells played an important role in the fouling of the active layer.However,in the support layer of FO membrane,inorganic and total cells were the main foulants.