DISTRIBUTED MICROBIAL STATES IN NUTRIENT REMOVING ACTIVATED SLUDGE SYSTEMS: SIMULATION OF AN IMPORTANT BUT NEGLECTED PHENOMENON

Current state-of-the-art activated sludge simulation programs assume average, or “lumped,” system characteristics, rather than accounting for the distribution of individual bacterial states (such as internal concentrations of the microbial storage products polyphosphate, glycogen, and polyhydroxybutyrate) likely to occur in a given population. These individual states are hypothesized to vary because hydrodynamic variability produces a variety of possible histories in terms of upstream reactor residence times. This is particularly true in systems containing completely mixed reactors, where residence times vary across hydraulic elements. A MATLABbased bioreactor simulation program was built based on a standard biokinetic model (ASM2) to predict the potential effects of distributed states on simulation of the enhanced biological phosphorus removal (EBPR) process. A simple two reactor, anaerobic-aerobic EBPR system was simulated assuming either distributed or lumped characteristics. Lumped simulations consistently overestimated EBPR performance compared to distributed simulations, potentially leading to underestimation of required reactor sizes required for successful EBPR. The results demonstrated that improvements to activated sludge systems design and process control are possible by accounting for the distributed characteristics of their resident populations. Further research is needed for the development of the distributed simulation approach for its practical application.