Abstract: Biomass and microbial activity were investigated in backwashing processes of a biofilter for tertiary treatment.The microbial groups revealed a new distribution along the biofilter depth after low flow rate backwashing for a short time.Then the start-up process was accelerated by backwashing.The biomass profile and microbial activity profile,both varying with depth,can be mathematically described by quadratic equations.Using the profiles,the demand of oxidation can be calculated to determine the air flow rate in a backwashing.Combined with the difference between biofilter and rapid gravity filters,analysis of biomass and microbial activity can determine the required air flow rate in a backwashing more accurately.