Influence of growth history on sloughing and erosion from biofilms

The development of biofilms is determined by the balance of growth and detachment. But while the growth of biofilms is well studied, the influence of growth history and detachment on biofilm development is not. Here we report on laboratory scale experiments where heterotrophic biofilms were grown in a tubular reactor. Biofilm detachment was categorized based on particle size as erosion or sloughing. Erosion results in small particles and was approximated by the effluent suspended solids while sloughing was determined from the larger pieces of biomass that settled in a mixing tank. It was found that for all experiments, overall detachment was a combination of erosion and sloughing where erosion had a slightly larger contribution to the overall solids removal. However, sloughing had a significant influence on the biofilm morphology. Once the smooth biofilm surface was disturbed by a sloughing event (e.g., initiated through increasing liquid shear in the reactor), the biofilm became unstable resulting in spontaneous sloughing during subsequent operation. We propose that experimental investigations should consider sloughing events as an integral part of biofilm development rather than a disturbance.