Combining CFD, FLOC Dynamics, and Biological Reaction Kinetics to Model Carbon and Nitrogen Removal in an Activated Sludge System

A number of studies have been performed in the simulation of activated sludge systems with many utilizing ideal reactor concepts to reduce the overall complexity of the model framework. Observance of non-ideal behaviour in actual reactors, however, led others to recognize the importance of hydrodynamics in modelling wastewater treatment processes. More recently, new experimental studies have demonstrated the influence of floc size on the reaction environment inside a floc. However, no reported attempts have been found in the literature in developing a mathematical model of activated sludge system that incorporates effects of spatial variation in floc size on the biological processes. This study presents an approach in modeling activated sludge systems that combines Computational Fluid Dynamics (CFD), floc dynamics, and biological reaction kinetics in simulating the carbon and nitrogen removal process at both the reactor scale and internal floc scale. The use of CFD allowed the simulation of the flow field in a baffled flow-through reactor. The introduction of floc aggregation and breakup dynamics provided spatial estimates of the steady-state sizes of flocs inside the reactor allowing for a more comprehensive description of the transport and reactive processes occurring both within the reactor and inside the floc. The results showed that significant differences in reactor performance can be predicted when spatial variation in floc size is incorporated in the model framework.