Unraveling membrane fouling in anoxic/oxic membrane bioreactors treating anaerobically digested piggery wastewater

Abstract

Anaerobically digested piggery wastewater (ADPW) draws increasing attention due to the high nitrogen and carbon level. In this study, four parallel lab-scale anoxic-oxic membrane bioreactors were applied to treat ADPW with the hydraulic retention time (HRT) of 75, 60, 45 and 30 h, respectively. Reactor performance and membrane fouling mechanisms also have been systematically evaluated. The results demonstrated a decline in chemical oxygen demand and total nitrogen biodegradation (from 65.8 ± 2.5% and 62.2 ± 5.5% to 38.6 ± 3.6% and 26.2 ± 5.7%, respectively) and a rise in membrane resistance (from 37.34 × 1012 m−1 to 51.69 × 1012 m−1) as the HRT decreased. The decline of HRT didn’t affect the functional group types of cake layer extracellular polymeric substances (EPS) and tightly bound EPS (TB-EPS) of activated sludge, but led to the increased concentrations of protein (PN) and polysaccharide (PS) in loosely bound EPS (LB-EPS) and soluble microbial products (SMP) of activated sludge and a decreased PN/PS ratio in SMP. These changes in SMP and EPS accelerated the formation of cake layer and aggravated the membrane fouling. Proteobacteria and Bacteroidetes were the predominant phyla on membrane surface. The abundant Proteobacteria in the reactor with longer HRTs was beneficial to mitigate membrane fouling while the relative abundance of Bacteroidetes remained stable in all reactors. The enrichment of Thauera was the primary reason for membrane fouling at genus level in the reactors. This study provides scientific insights on the practical application of membrane bioreactors in terms of membrane fouling control.