Causes of variable biomass density and its effects on settleability in full-scale biological wastewater treatment systems

Both variable biomass density and floc structure were determined to affect the settleability of microbial biomass produced during biological wastewater treatment (activated sludge). Average biomass density varied from 1.022 to 1.056 g/mL in a survey of 17 full-scale biological wastewater treatment systems with a variety of configurations. Biomass settleability was correlated with density in samples with higher filament contents and/or more open floc structures, but settleability was independent of density in biomass with lower filament contents and more rounded floc structures. Biomass density increased with polyphosphate content, and enhanced biological phosphorus removal (EBPR) plants had higher density and better settleability than non-EBPR plants, including two systems that converted to EBPR during the course of this study. Density also increased with increasing nonvolatile suspended solids content, which was linked both to polyphosphate and to increasing solids residence times. Both density and floc structure should be considered in future analyses of activated sludge settleability, and it may be possible to improve system performance by adopting a new set of operational and design strategies to increase density.