Process hydraulics, distributed bacterial states, and biological phosphorus removal from wastewater

Abstract

Hydraulic characteristics of biological wastewater treatment systems were shown to affect bacterial state distributions and system performance through mathematical simulations. The term "state" is used here to mean the microbial storage product and biomass content of a bacterium. The traditional approach to simulating biological treatment processes assumes "lumped" (average) states, rather than accounting for variable states across bacterial populations. Distributed states were previously suggested as critical to enhanced biological phosphorus removal (EBPR), but the factors that cause distributed states were not evaluated. A primary driver for distributed state development is variable hydraulic experiences of bacteria as they cycle through completely mixed reactors, and so process characteristics that affect hydraulics were hypothesized to affect state distributions. Two design characteristics affecting system hydraulics were evaluated using a new distributed state simulation program (DisSimulator 1.0): total hydraulic residence time (HRT) and numbers of reactors in series. Distributed predictions consistently predicted worse EBPR performance than did the lumped approach. Increasing HRTs (with constant solids retention times) tended to increase state distributions, to increase the differences between lumped and distributed simulation predictions, and to decrease predicted EBPR performance. As the numbers of reactors in series increased, distributed predictions tended to converge with lumped simulation predictions. Distributed simulations tended to predict a greater benefit to using reactors in series than did lumped simulations. This work provides guidance for new strategies to improve EBPR by minimizing state distributions. The targeted hydraulic characteristics may be more important to EBPR than previously recognized due to their effects on distributed states.