The Case for Variable Density: A New Perspective on Activated Sludge Settling
The effects of biomass density on activated sludge settling in full scale systems was evaluated. The driving force of sedimentation is the physical weight of the biological solids, but the role of biomass density in sedimentation has been largely ignored. This may be because density has been assumed to be more or less constant, and because convenient methods for its measurement have not been available. A new, simplified method for density measurement was developed and applied to full scale samples. Measurements demonstrated that density varied from system to system, and this variability was correlated with settleability. Non-soluble phosphorus content was a major contributor to density, and plants with enhanced biological phosphorus removal (EBPR) configurations generally had higher densities and better settleability than non-EBPR plants with similar filament contents. These results suggest that future work may benefit from consideration of density as a factor affecting activated sludge settling.