Role of Changing Biomass Density in Process Disruptions Affecting Biomass Settling at a Full-Scale Domestic Wastewater Treatment Plant

Abstract

Biomass settleability, filament content, density, and factors known to affect density were monitored at a full-scale enhanced biological phosphorus (EBPR) plant to characterize system performance during start-up and several subsequent short-term process disruptions. Settleability fluctuated for two months after initial start-up, which corresponded to changes in both filamentous bacteria content and biomass density. Following start-up, a rain event was associated with greatly increased density (possibly because of washing of inorganic particulates to the system), and decreased filament content, and both of these factors may have contributed to improved settleability. Five months after start-up, a 36-hour repair-related process disruption resulted in decreased biomass density and stored polyphosphate simultaneously with degraded settleability, whereas filament content remained at low levels, and actually decreased after the disruption. During recovery, biomass density increased (simultaneously with increased storage of high-density polyphosphate) and settleability improved. A second, shorter aeration disruption also was linked to decreased density and degraded settleability, but not to changes in filament content. Although the observed changes in settleability were not severe, these results suggested that short-term process upsets can affect biomass density, and this in turn can affect the performance of biomass sedimentation. This research confirmed that well-performing EBPR can improve settleability by increasing biomass density, and it provided insights to how process disruptions can affect settleability. This may aid in the development of strategies for system recovery after disruptions; for example, avoiding planned disruptions during periods of lower density (e.g., winter months) may be useful to minimize negative effects on settleability, and prioritizing recovery of EBPR after a disruption may also help recover settleability.