Bacterial growth dynamics in activated sludge batch assays

Aerobic batch reactors are often used to estimate the maximum specific growth rates of bacteria in wastewater treatment environments. Results from these reactors vary, and they are sometimes confusing because they do not represent the bacterial populations in the original wastewater environment, i.e. in situ. For the first time in batch reactors, we present the in situ growth dynamics of these bacterial populations. We simultaneously measured bacterial numbers and growth rate, oxygen uptake rates (OUR), soluble COD and biomass COD (mg.l −1). With a substrate to biomass (S o/X o) ratio of 1.2, bacterial specific growth rates, measured using the thymidine growth assay, increased from 1 to 4.5 d −1. During the same period, the initial OUR remained relatively constant, with an OUR-based estimate of the maximum specific growth rate of 3.6 d −1. These observations challenge the fundamental theory of the batch assay that bacteria are growing at their maximum rate under substrate saturating conditions while the yield coefficient remains constant. Our results confirm that the batch reactor environment induces changes in the bacterial physiology and/or community structure compared to the original treatment environment, even at low S o/X o ratios. We discuss how, simultaneously using both OUR and thymidine assays, gives more information about how the cell allocates its energy resources between new cell synthesis and cell maintenance.