Biofilm as a live and in-situ formed membrane for solids separation in bioreactors: Biofilm succession governs resistance variation demonstrated during the start-up period

Biofilms developed on support materials with large apertures can be used as live and in-situ formed membranes for solids separation in biofilm-based dynamic membrane bioreactors (bDMBRs). Unlike polymer membranes, biofilms are live, and their composition and structure vary, which further impacts the biofilm resistance. This study repeatedly observed that while starting up a bDMBR to treat biodegradable wastewater, the filtration resistance increased rapidly at the beginning and decreased to a low level corresponding with biofilm succession that was impelled by a variation in the food/microorganism (F/M) ratio. During the rapid rising stage, the F/M ratio was high. Moreover, Meganema, Rhodobacter, Bdellovibrio, Rhizobium, Micobacterium, and Bacillus were the dominant bacterial genera in the biofilms. This promoted biofilm attachment and growth by an excessive secretion of extracellular polymeric substances (EPSs). Consequently, the early biofilms were less permeable and exhibited a resistance of 15.6 × 1011 m−1 on the ninth day. As the start-up time increased and the F/M ratio decreased, the biofilm microbial community was altered significantly, with Legionella, Reyranella, Meganema, Hyphomicrobium, and a few uncultured_bacterium being dominant. Accordingly, the biofilm became more permeable and exhibited a resistance of 0.09 × 1011 m−1 on the 46th day. Then, a flux of 42 L/m2·h was achieved at a pressure of 0.1 kPa. This study indicated that the accumulated resistance in a bDMBR could decrease spontaneously without cleaning by the regulation of the biofilm succession on the support material.