Effects of granule disintegration and sludge re-aggregation on fouling behaviors and bio-cake formation in aerobic granular sludge membrane bioreactor (AGMBR)

The contribution of granules to membrane fouling alleviation in aerobic granular sludge-membrane bioreactor (AGMBR) has been increasingly investigated, yet the inoculated granules in MBR are more unstable than those in conventional AGS system and thus in-depth insight into the effects of the mutual transformation between granules and flocs on membrane fouling as well as corresponding bio-cake formation is necessary. Herein, we found that the disintegration of granules was accompanied by the release of soluble microbial products (SMP) and the increase of microbial diversity. The re-granulation phase occurred when granules gradually redominated the sludge system with the growth of biomass and the massive secretion of extracellular polymeric substances (EPS), corresponding to the declined microbial diversity and stable pollutants removal (COD: 93.45-98 %, TN: 21.06-36.86 %). Compared with the granule disintegration phase, less flocs deposition and fewer contents of SMP containing high molecular weight (>100 kDa) compounds in bio-cakes in the regranulation phase resulted in the longer filtration period, as evidenced by lower attractive interaction energies between sludge foulants and membranes according to extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. Microbial community analysis demonstrated higher relative abundance of filamentous bacteria (Thiothrix) and lower relative abundance of denitrifiers (e.g, Hydrogenophag, Trichococcus, Arenimonas, Acinetobacter) in bio-cakes in the re-granulation phase. Besides, the genera Thiothrix, norank_f__env.OPS_17 and Aeromonas may be responsible for the organic foulants accumulation on membrane surface. More importantly, stronger positive correlations among microbial populations between bio-cake and suspended sludge in the regranulation phase indicated the lower propensity of membrane fouling. The systematic insights into the effects of granules disintegration and sludge re-aggregation on fouling layer may have important implications for the future development of membrane fouling control in AGMBR system.