Immobilization of hydrolytic/fermentative bacteria to achieve ultra-low fouling in anaerobic membrane bioreactor

Abstract

Membrane fouling remains the key challenge that limits the application of anaerobic membrane bioreactors (AnMBRs). Free-living bacteria, particularly hydrolytic/fermentative bacteria (HFB) in sludge supernatant, have recently been identified as the predominant foulants in AnMBRs. Here, a pretreatment tank filled with porous polyurethane sponges as carriers for biomass attachment and/or entrapment was developed for influent organic hydrolysis/fermentation, named HFB-immobilization tank. Then, two parallel AnMBRs (i.e., with and without an HFB-immobilization tank) treating synthetic domestic wastewater were operated to determine the roles of HFB capture in fouling mitigation as well as reactor performance. Higher methane yields were observed in the experimental than in the control AnMBR. Interestingly, the HFB-immobilization helped to achieve a non-fouling state of the experimental AnMBR for more than 100 days, while the average fouling cycle of the control AnMBR was 20 days. Dead-end filtration tests corroborated better filterability of sludge supernatant in the experimental than the control AnMBR. The 16S rRNA gene sequencing revealed substantially lower abundances of HFB (e.g., Aminicenantales, SJA-15) but higher abundances of methanogens (e.g., Methanosaeta, Methanolinea, Methanospirillum) in the mixed liquor of the experimental than the control AnMBR, indicating efficient niche separation by the HFB immobilization. Particularly, the sequencing data, together with fluorescent staining, showed that the amounts and abundances of HFB in sludge supernatant were largely lower in the experimental than the control AnMBR. This study provides a promising alternative for fouling control in AnMBRs.