Determination of the critical draw solution concentration for anaerobic microfiltration osmotic membrane bioreactors

Anaerobic microfiltration osmotic membrane bioreactors (AnMF-OMBRs) have the advantages of resource recovery such as energy, water, and nutrients, but they have some drawbacks of low flux and severe membrane fouling. The concentration of draw solution (DS) is effective on the flux and fouling of FO membrane, and microbial inhibition. The aim of this study is to determine the critical DS concentration in AnMF-OMBR and up-flow anaerobic sludge blanket microfiltration osmotic membrane bioreactor (UASB MF-OMBR) systems operated with DS concentrations of 0.47, 1, and 1.5 M. The analytical hierarchical process (AHP) was applied to determine the critical DS concentration considering the criteria of forward osmosis (FO) performance, anaerobic performance, water recovery, microbial inhibition and operation time. The results demonstrated that the DS concentration of 0.47 M showed a better system performance, by causing less membrane fouling and microbial inhibition than that of the 1 and 1.5 M, and higher methane yield was obtained with 0.47 M in both bioreactors. The membrane autopsy analyses verified that the membrane fouling was severe with increasing DS concentration. Microbial community structure was affected by salinity build-up, high free ammonia (FA), and low organic loading rate (OLR). The relative abundance of Synergistota, a syntrophic acetate oxidizer, showed a parallel trend with Methanobacterium, a hydrogenotrophic methanogen in the conditions of high FA and salinity build-up due to the FO operation. Consequently, the critical DS concentration was determined as 0.47 M MgCl2.