Quantifying the In Situ Growth Rate of a Filament and a Floc-former in Activated Sludge

The goal of this project was to develop a species-specific method to quantify the in-situ growth rate of one filament and one floc-former in activated sludge. Chemostats were used to study pure cultures of Sphaerotilus natans, as the representative filament, and Arthrobacter globiformis, as the representative floc-former, to determine the relationship between growth rate and RNA:DNA ratio for each species. Real-time qPCR and reverse transcription real-time qPCR were used to measure DNA and RNA levels respectively. The relationship between RNA:DNA ratio and growth rate was found to be positive and linear, with R2 of 0.58 and 0.98 for S. natans and A. globiformis respectively. This relationship was used to determine in-situ growth rate of S. natans in activated sludge from the aeration basin of a full-scale WWTP and in samples collected from a previous study on the effect of different substrates on bulking. A. globiformis could not be detected in any of the WWTP samples. This study represents the first time qPCR and RT-qPCR have been used to quantify in-situ microbial growth rates in activated sludge.