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

Abstract

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 re-dominated 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 re-granulation 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 re-granulation 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.