Seasonal variability of biomass density and activated sludge settleability in full-scale wastewater treatment systems

Biosolids sedimentation is a critical component of the activated sludge wastewater treatment process. Seasonal variability of biomass settleability has been previously reported and linked to variable filament content in some studies, but others have reported seasonal variations without changes in filament content. Biomass density (mass per microbial floc volume, not including pore spaces) has recently been shown to vary substantially and to affect settleability in full-scale systems, but its potential role in seasonal variations has not been previously evaluated. Four full-scale activated sludge systems were monitored for density, filament content, and settleability for a year. Biomass density values were significantly higher in warm weather than in cold weather in all plants. Settleability was significantly worse in cold weather in three of the four systems, and the inverse of the buoyant density was correlated with settleability in these three systems. Filament content, on the other hand, exhibited seasonal variability and was correlated with settleability in only one of the four plants. Non-volatile suspended solids content was correlated with buoyant density and exhibited seasonal variability in all four systems. Biomass phosphorus content measurements suggested that seasonally variable enhanced biological phosphorus removal activity affected density in one of the systems. These results suggest that variable density plays a role in seasonally variable settleability in some full-scale wastewater treatment systems, they help to clarify previously unexplained reports of seasonally variable settleability that were independent of changes in filament content, and they provide the basis for development of strategies for improved performance.