Dynamic modeling of biological water treatment processes

Biological processes using biomass in suspension, such as activated sludge, are widely used to treat urban or industrial wastewater.

Generally speaking, these processes are dimensioned on the basis of material balance equations established on the assumption of a steady state.

Yet these systems are subject to continuous disturbance. In particular, they are subject to significant variations in pollutant load (flow and concentration) on different temporal scales: daily, weekly, annual, reflecting fluctuations in human activity.

What's more, these purification processes are managed by operations that are sometimes discontinuous (sequenced aeration, sludge purging, batch feeding, etc....). Consequently, even if pseudo-steady-state regimes can be achieved, dynamic analysis of biological purification processes is essential to understand the actual operation of units and to optimize sizing parameters: tank volume and layout, aeration management, sludge extraction, clarifier buffering role.

This is all the more important as the configurations and geometries of biological ponds have become more diverse and complex, in order to integrate nutrient removal.

The aim of this article is, on the one hand, to present the scientific basis of these tools and, on the other, to propose methodologies for their use by engineers.