The influence of particle size on microbial hydrolysis of protein particles in activated sludge

The influence of particle size on hydrolysis rates was evaluated in batch respirometers using protein particles derived from hard-boiled egg whites and activated sludge. It was found that initial hydrolysis rates for large particles were low but increased over time, contradicting commonly used mathematical modeling approaches to describe hydrolysis. A modified mathematical model was proposed based on particle breakup that described the observed hydrolysis kinetics for small and large protein particles. In the particle breakup model, hydrolysis results both in the release of readily biodegradable substrate and, at the same time, breakup of larger aggregates resulting in an increase of the specific surface area available for hydrolysis. Using this model, initial specific hydrolysis rates were calculated for the small and large particles, and ranged from 0.038 to 0.24 d(-1) and 0.19 to 0.98 d(-1) for large and small particles, respectively. The specific hydrolysis rate at the point of maximum overall hydrolysis rate ranged from approximately 2 to 3 d(-1) for all particles.