Pressure Retarded Osmosis Membrane Bioreactors:Construction And Performance

Osmotic membrane bioreactor(OMBR)has gained increasing interests in wastewater treatment and reclamation owing to its high water quality and low membrane fouling tendency.It is necessary for OMBR to further treat the draw solution in order to achieve wastewater reclamation and draw solute recovery.Currently,the commonly used technologies for draw solute recovery includes nanofiltration(NF),reverse osmosis(RO)and membrane distillation(MD).However,these technologies require a large amount of energy and thus significantly hindering the application of the OMBR technology in wastewater treatment.Pressure retarded osmosis(PRO)can effectively harvest osmotic energy existing in both sides of the forward osmosis(FO)membrane during the operation of OMBR.Given that,we proposed pressure retarded osmosis membrane bioreactor(PROMBR)using the PRO process instead of the FO process for balancing the energy consumption during the draw solute recovery via recovering the osmotic energy.To the best of our knowledge,there is no literature on PROMBR.Based on the construction of PROMBR,impacts of various operating parameters on the performance of PROMBR were investigated,and then the feasibility of PROMBR was evaluated through comparing the performance between PROMBR and OMBR under the selected operating parameter.In addition,FO membrane fouling in the PROMBR was analyzed and then the fouling mitiagtions were investigated based on the obtained fouling mechanisms.The main research contents and results are listed as follows:(1)Based on the construction of PROMBR,impacts of various operating parameters on the performance of PROMBR were investigated.Membrane orientation,draw solution concentration and activated sludge concentration had no significant impacts on the effluent water quality.However,they dramatically affected the FO membrane flux and power density.Compared with the active layer facing feed solution(AL-FS)mode,a higher operating water flux and a higher power density were achieved in the active layer facing draw solution(AL-DS)mode.A higher draw solution concentration resulted in an increase of water flux and power density.However,when the draw solution concentration exceeded the critical value,the water flux would significantly decrease.In addition,the internal concentration polarization of FO membrane would be enhanced as an increasd of activated sludge concentration,and thus resulting in a significant reduction of water flux and power density.Overall,the optimal membrane orientation,draw solution concentration and activated sludge concentration for PROMBR operation were AL-DS mode,2 M NaCl and 3 g / L,respectively.(2)The feasibility of PROMBR for wastewater treatment was evaluated by comparing its performance with OMBR under the optimal conditions.Due to the high rejection of FO membrane and the microbial metabolism in the bioreactor,the effluent quality of the PROMBR and OMBR was similar,i.e.,concentrations of total organic carbon(TOC),ammonia nitrogen,total nitrogen and total phosphorus in the FO permeate were less than 3 mg/L,less than 0.4 mg/L,less than 1.5 mg/L and 0 mg/L.In addition,the FO membrane fouling was similar in both PROMBR and OMBR,and the recovery rate of water flux through physical cleaning were exceeding 80% for both reactors.Although the water flux of PROMBR was slightly lower than that of OMBR due to a lower net driving force of the FO membrane induced by the added pressure in the draw solution,PROMBR could effectively recover the osmotic energy and thus had a lower operating cost.(3)The fouling mechanism of FO membrane was investigated based on the fact that a serious membrane fouling significantly affected the performance of PROMBR.During the operation of PROMBR,the FO membrane support layer fouling included internal fouling and external fouling,which were occurred simultaneously.Although the external fouling was identified to be the major type of fouling,the internal fouling dominated the overall flux decline of FO membrane.The development of support layer fouling correlated well with the variation in water flux of FO membrane and could be divided into three stages,i.e.,the initial stage,the rapid decline srage and relatively stable stage.At the initial stage,there were fewer foulants and they exsited on the surface and in the pores of the support layer at the same time.At the rapid decline srage,the amount of foulants significantly increased in the support layer including the surface and the pores.At relatively stable stage,the presence of microorganisms could be observed in the pores of the support layer,and the organic fouling continuing increasing on the surface and in the pores of the support layer but the rising rate became very slow.(4)Based on the membrane fouling mechanism,the feasibility of adding chemical cleaning reagents in the draw solution to alleviate membrane fouling was investigated.The results indicated that it was feasible to control FO membrane fouling via directly adding suitable chemical cleaning reagents in the draw solution.Adding HCl into draw solution could effectively enhance the water flux of FO membrane while adding NaClO into draw solution had no impacts on FO membrane flux.Although adding HCl into draw solution could not mitigate biofouling and organic fouling on the surface and in the pores of the support layer,it could effectively alleviate inorganic fouling of support layer.With regard to adding NaClO into draw solution,it could alleviate organic fouling and biofouling on the surface and in the pores of the support layer,but had no impacts on inorganic fouling of support layer.Comparing with the organic fouling and biofouling,inorganic fouling had a more contribution to the flux decline of FO membrane in the PROMBR.