SIMULATION OF BIO P REMOVAL IN CFD ENVIRONMENT Analysis of Macro- Environment Variations in Simultaneous Biological Nutrient Removal Systems

A self-consistent three-dimensional computational fluid dynamics (CFD) activated sludge model is being developed for a full-scale closed loop bioreactor. The objective of the study is to provide insights into simultaneous biological nutrient removal resulting from the reactor mixing pattern. This report investigates the effects on biological phosphorus removal. Based on ASM model No. 2, biochemistry rate expressions for heterotroph and phosphorus accumulating organisms (PAOs) have been introduced into the previously developed CFD model that characterizes the mixing pattern within a full-scale closed-loop bioreactor. The chemical species of interest are introduced into the CFD environment via source and sink scalar functions in a FORTRAN subroutine to satisfy the Navier-Stokes series governing equation for conservation of mass and momentum. Using acetate as the sole carbon and energy source, simulations for general heterotrophs and PAOs individually as well as the competition of the two organisms was conducted separately in ASM No. 2 and CFD models with varied oxygen input. A dynamic analytical solution for heterotrophs was also calculated as a comparison for the results of the CFD simulations.The results of the CFD simulations agreed well with those of the analytical solutions and ASM simulations for heterotrophs and PAOs individually. This suggests that the CFD mathematical model has correctly incorporated the relevant biochemical reactions. However, the competition of the two organisms in ASM and in CFD is not directly comparable, since the reactor setup in the ASM model (alternating anaerobic/aerobic reactors in series) for PAO and heterotroph competition is different from that of the CFD environment (single reactor with DO gradients). With the two populations together, the CFD simulations are more dynamic, and individual simulations take a much longer time to converge. However, the results of CFD simulations of competition between PAOs and heterotrophs can be used to test the hypothesis that heterotrophs and PAOs can coexist in a heterogeneous environment with oxygen gradients, rather than requiring separate anaerobic and aerobic zones – as has been observed in the full-scale plants studied - provided DO is sufficiently low. The simulation of the competition of the two organisms will be continued and reported on further.