Modeling pH Dynamics in the Nutrient Removal Activated Sludge Process

A comprehensive biological model (MANTIS2) including a pH model was developed on GPS-X simulation platform. Each biological, physical and chemical reaction equation in the model was modified to include the stoichiometric changes in the concentration of inorganic state variables contributing to charge balance. The model was applied to study the pH dynamics in experimental batch reactors performing the biological reactions of nitrification, denitrification and phosphorous release/uptake. The model was able to predict the change in the various parameters of soluble COD, PO4-P and NH3-N during the anaerobic and aerobic conditions of the batch test. The simulated pH profile compared reasonably well with the experimentally measured profile. When the batch reactor conditions were switched from anaerobic to aerobic, nitrification was observed with an increase in the pH. Further analysis of this observation highlighted the importance of the dynamics of CO2 accumulation/stripping and CO2 mass transfer rate in determining the pH in the anaerobic/anoxic/aerobic nutrient removal systems.