Development of simplified flow models from CFD simulations

Reaction complexity in biological nitrogen removal (BNR) bioreactors has delayed the use of CFD simulations in this important field. The objective of this project has been to develop a method that can use the results of a CFD simulation of an activated sludge process to yield an equivalent model constituting a network of ideal reactors. The resultant simplified flow model will suitably account for the hydrodynamic behaviour of the process. This enables easy incorporation of complex physical phenomena such as activated sludge reaction kinetics, and can then be used for dynamic simulation of the whole plant and its incorporation into a control system, or the analysis of a badly performing process. A general methodology has been developed to determine the optimal flow model. The flow-field generated using a Fluent CFD simulation was used to predict the movement of neutrally buoyant particles. The particle tracks provide a characteristic transfer matrix between the different sections of an organised set of zones. The transfer matrix is then used to lump the zones into a network of ideal reactors with exchange flows between each of the adjoining reactors. The method is presented as a case study of a bioreactor presently operating in Brisbane, Australia, using simplified reaction kinetics.