Biofilm development is determined by the balance between growth and detachment. The detachment of biomass from suspended biofilm pellets was investigated in three-phase internal loop airlift reactors under non-growth conditions, and in the presence of bare carrier particles. Particle collisions were found to dominate the detachment. These collisions caused an on-going abrasion of the biofilm pellets. The abrasion rate was linear with both the concentration of biofilm pellets and the bare carrier particles up to a solids hold-up of 30%. An increase in particle size drastically increased the abrasion rate. Carrier roughness also strongly influenced the detachment rate. Experimental results were interpreted in terms of collision frequency and collision impact, and could partly be described by conventional collision theory. Particle responses to flow fluctuations, the effect of carrier roughness and the effect of growth rate on biofilm strength and elasticity need to be included. Some consequences for reactor operation and start-up are discussed.

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