The mechanical scouring of bio-carriers improves phosphorus removal and mediates functional microbiomes in membrane bioreactors

Abstract

Integrated fixed-film activated sludge combined with a membrane bioreactor (IFAS-MBR) is a promising alternative to improve biological nutrient removal (BNR). The synergistic benefits of the attached-growth biofilm on reactor performance have been extensively reported, yet the effects of moving carriers on driving changes in the bulk sludge bacterial community are poorly documented. In this work, two parallel MBRs (i.e., with and without carriers) were operated to determine the roles of moving carriers in shifting the bacterial community composition, particularly the active microbiota, and their impact on BNR. The results showed that the presence of moving carriers significantly improved phosphorus removal (p < 0.01) during stable operation, which was in good agreement with the higher specific phosphorus release/uptake rates of the bulk sludge from the carrier-dosed MBR compared to that from the control MBR determined by a series of batch tests. LIVE/DEAD BacLight viability staining and the use of propidium monoazide (PMA) together with 16S rRNA amplicon sequencing indicated that the bulk sludge in the carrier-dosed MBR had a higher viability and biodiversity within the active bacterial community compared with that in the control MBR, attributable to the mechanically induced decrease in floc size and an increase in mass transfer. Notably, a higher abundance of phosphate-accumulating organisms (PAOs) and lower GAOs/PAO ratio (GAOs: glycogen-accumulating organisms) were observed in the active microbiota of the carrier-dosed MBR than in the control MBR. By contrast, the abundance of ammonia oxidizing bacteria, nitrite oxidizing bacteria and denitrifiers was comparable in both MBRs. Intriguingly, these functional bacteria had highly divergent abundances between the total and active bacterial communities, likely attributable to their different susceptibilities to oxygen and substrates. Overall, this study substantially extends our understanding of the roles of fluidized particles in mediating microbiota changes in an IFAS-MBR system.