Effect of driving force on the performance of anaerobic osmotic membrane bioreactors: New insight into enhancing water flux of FO membrane via controlling driving force in a two-stage pattern
Anaerobic osmotic membrane bioreactor (AnOMBR) has shown great promise for wastewater treatment and energy recovery. One of the major obstacles limiting the performance of the AnOMBR is the low water flux of forward osmosis (FO) membrane. Here, impacts of draw solution (DS) concentration on the performance of the AnOMBR were investigated for enhancing the water flux of FO membrane via controlling the driving force. The results indicated that the flux variations of FO membrane in the AnOMBR included two stages of a rapid and a mild flux declines at all DS concentration conditions. In addition, the FO membrane fouling correlated well with the flux variations for all DS concentration levels, i.e., a severe fouling in Stage 1 and a slight fouling in Stage 2. When the driving force or DS concentration exceeded a critical value, a worse FO membrane performance especially a severer flux decline in Stage 1 was observed. It could be attributed to an aggravated membrane fouling in Stage 1 owing to an increased initial flux with the rise of the driving force, implying that a critical initial flux exists in the AnOMBR except for a critical driving force. Inspired by the critical initial flux and the significant role of the membrane fouling in Stage 1, this study provides a novel method to enhance flux performance of FO membrane via controlling the driving force in a two-stage pattern. It utilizes a low driving force in Sage 1 to form a thin and loose fouling layer on the FO membrane surface and then a high driving force in Sage 2 to alleviate the flux drop. Compared with the stable driving force, the two-stage pattern obtained a better flux performance owing to a mitigation of membrane fouling with thinner layer thickness and less deposited foulants.