ABRASION OF SUSPENDED BIOFILM PELLETS IN AIRLIFT REACTORS - IMPORTANCE OF SHAPE, STRUCTURE, AND PARTICLE CONCENTRATIONS

The detachment of biomass from suspended biofilm pellets in three-phas e internal loop airlift reactors was investigated under nongrowth cond itions and in the presence of bare carrier particles. In different set s of experiments, the concentrations of biofilm pellets and bare carri er particles were varied independently. Gas hold-up, bubble size, and general flow pattern were strongly influenced by changes in volume fra ctions of biofilm pellets and bare carrier particles. In spite of this , the rate of biomass detachment was found to be linear with both the concentration of biofilm pellets and the bare carrier concentration up to a solids hold-up of 30%. This implies that the detachment rate was dominated by collisions between biofilm pellets and bare carrier part icles. These collisions caused an on-going abrasion of the biofilm pel lets, leading to a reduction in pellet volume. Breakage of the biofilm pellets was negligible. The biofilm pellets were essentially ellipsoi dal, which made three-dimensional size determination necessary. Calcul ating particle volumes from two-dimensional image analysis measurement s and assuming a spherical shape led to serious errors. The abrasion r ate was not equal on all sides of the biofilm pellets, resulting in an increasing flattening of the pellets. This flattening was oriented wi th the basalt carrier inside the biofilm and independent of the absolu te abrasion rate. These observations suggest that the collisions causi ng abrasion are somehow oriented. The internal structure of the biofil ms showed two layers, a cell-dense outer layer and an interior with a low biomass density. Taking this density gradient into account, the wa shout of detached biomass matched observed changes in volume of the bi ofilm pellets. No gradient in biofilm strength with biofilm depth was indicated. (C) 1997 John Wiley & Sons, Inc.