Microbial community and biomass characteristics associated severe membrane fouling during start-up of a hybrid anoxic-oxic membrane bioreactor

► A severe membrane fouling is observed during start-up of a hybrid anoxic-oxic membrane bioreactor (MBR). ► Microbial community shifts dynamically during start-up period, which leads to variations of several activated sludge properties. ► Small floc size, high fractal dimension and high EPS production are major contributors to the initial severe fouling. ► Microbial community shift is most likely the ultimate factor responsible for the severe membrane fouling of this period.