FORMATION AND DETACHMENT OF BIOFILMS AND GRANULES IN A NITRIFYING BIOFILM AIRLIFT SUSPENSION REACTOR

The influence of the bare basalt carrier concentration on nitrifying b iofilm formation in biofilm airlift suspension (BAS) reactors was stud ied. The time needed to develop fully covered biofilm particles strong ly increased with an increasing initial carrier concentration. This wa s caused by the increased shear and subsequent higher detachment. Duri ng startup the diameter of the biofilm particles increased, leading to a higher biomass concentration, larger biofilm surface area, and lowe r surface-specific substrate load. Eventually a ''steady state'' thick ness is obtained, where surface-specific growth and detachment are equ al. The biofilm density decreased with st decreasing bare basalt conce ntration. The steady state biomass concentration was not very much inf luenced by the initial carrier concentration. Up to 75% of the produce d biomass was retained in the reactors. Maximum biomass retention was found when the initial bare basalt concentration was in between 10 and 30 g . L(-1). The continuous detachment of biofilm fragments leads, u pon growth of these small fragments, to the formation of granules (bio films without a carrier). The granules have average density and size e qual to those of the biofilm particles. The granules are formed, if th e detached biofilm fragments are large enough to be retained by the th ree-phase separator on top of the BAS reactor. Since the superficial l iquid velocity in the three-phase separator is scale dependent, granul e formation will be influenced by reactor scale, hydraulic load, and d imensions of the three-phase separator.