Distributed microbial state effects on competition in enhanced biological phosphorus removal systems A.J. Schuler Department of Civil and Environmental Engineering, Duke University, Box 90287, Durham, NC 27708, USA Abstract Computer simulation of activated sludge population dynamics is a useful tool in process design, operation, and troubleshooting, but currently available programs rely on the assumption of “lumped,” or average, system characteristics in each reactor, such as microbial storage product contents. In reality, the states of individual bacteria are likely to vary due to variable residence times in reactors with completely mixed hydraulics. Earlier work by the present author introduced the MATLAB-based distributed state simulation program, Dissimulator 1.0, and demonstrated that distributed states may be particularly important in enhanced biological phosphorus removal (EBPR) systems, which rely on the cycling of bacteria through anaerobic and aerobic reactors to select for a population accumulating multiple microbial storage products. This paper explores the relationships between distributed state profiles, variable anaerobic and aerobic SRTs, and the process rates predicted by lumped and distributed approaches. Consistent with previous results, the lumped approach consistently predicted better EBPR performance than did the distributed approach. The primary reason for this was the presence of large fractions of polyphosphate accumulating organisms (PAOs) with depleted microbial storage product contents, which led to overestimation of process rates by the lumped approach. Distributed and lumped predictions were therefore most similar when microbial storage product depletion was minimal. The effects of variable anaerobic and aerobic SRTs on distributed profile characteristics and process rates are presented. This work demonstrated that lumped assumptions may overestimate EBPR performance, and the degree of this error is a function of the distributed state profile characteristics such as the degree to which fractions of the biomass contain depleted microbial storage product contents. Keywords Activated sludge; enhanced biological phosphorus removal; modeling; population dynamics; simulation Introduction Computer simulation of competition in activated sludge microbial populations is import- ant for wastewater treatment system design, operation, and troubleshooting, and so improvements to simulation accuracy have great potential to improve system performance and to reduce treatment costs. Microbial storage products (MSPs), such as polyphosphate, glycogen, and polyhydroxyalkanoates (PHAs), are critical to the enhanced biological phosphorus removal (EBPR) process, where microorganisms are induced to accumulate these MSPs by cycling them through anaerobic and aerobic environments, and they may also be of broader importance in activated sludge systems. Simulation programs that con- sider MSP accumulating organisms generally operate on the fundamental assumption that MSP concentrations are uniform throughout a given population in each of a system’s reactors. This has been termed the “lumped” or “macroscopic” approach (Gujer, 2002). Gujer (2002) presented a distributed-type program that simulated individual bacteria accumulating a single (unspecified) MSP as they moved through a multiple-reactor, suspended growth system. Schuler (2005) presented a program (DisSimulator 1.0) utiliz- ing the distributed approach to simulate an EBPR system including multiple MSPs, anaerobic and aerobic reactors, competition between PAOs and a population of non-MSP-accumulating heterotrophs, wasting of bacteria from the system, and splitting Water Science & Technology Vol 54 No 1 pp 199–207 Q IWA Publishing 2006 199 doi: 10.2166/wst.2006.388 Downloaded from https://iwaponline.com/wst/article-pdf/54/1/199/432947/199.pdf by guest on 12 June 2020