Comparing bioreactors with activated sludge and moving bed
Part 1. Kinetic approach to the estimation of the efficiency (for discussion)

Summary

The objective of this work is to analyze the conditions that provide for moving-bed bioreactors actually surpassing the traditional activated sludge systems in efficiency. The test subject is kinetic and diffusion processes that determine the rate of substrate consumption in both types of reactors. The study uses Monod models supplemented with analytical equations describing substrate consumption in the biofilm with account of the diffusion limitations. The modes that provide for the kinetics of substrate consumption in a biofilm passing from zero order to 0.5 order and first order are considered, depending on the substrate concentration, biofilm thickness and diffusion coefficient. An analytical expression is proposed for determining the maximum substrate concentration, upon reaching which the oxidative capacity of a moving-bed bioreactor exceeds the similar indicator of an activated sludge bioreactor. Numerical examples and engineering formulas for practical application are given. The obtained results provide for estimating the real potential of moving-bed bioreactors and identifying the cases where their use is truly justified from the point of view of kinetics and diffusion, rather than perceived as a universal solution.

Key words

wastewater , biological treatment , activated sludge , biofilm , moving bed , Monod kinetics , diffusion limitations , permeability criterion , maximum concentration