Mechanism of biofouling enhancement in a membrane bioreactor under constant trans-membrane pressure operation

In a membrane bioreactor, flux and organic loading rate conditions essentially affect membrane fouling. In constant TMP mode, flux and organic loading rate dynamically change, while they are rather static in constant flux mode. This study investigated fouling mechanism in MBR under these different filtration modes. Fouling development was more significant in constant TMP mode than in constant flux mode when MBR was operated under an equal flux condition in average. In constant TMP mode, microbial community in cake sludge was distinct from bulk sludge and had higher abundance of biofilm-forming bacterial group. The cake sludge contained more SMP and LB-EPS with high polysaccharide contents in constant TMP mode. These results suggested that, in constant TMP mode, (i) initially high flux in a filtration cycle enhanced initial deposition of bacterial cells and rapid development of fouling layer, which resulted in distinct microbial community in cake sludge; (ii) the high abundance of the biofilm-forming bacteria in cake sludge and fluctuation of organic loading rate stimulated polysaccharide production, (iii) higher polysaccharide contents in cake layer caused faster development of fouling layer and increase of hydraulic resistance by compression.