Particulate substrate retention in plug-flow and fully-mixed conditions during operation of aerobic granular sludge systems

Particulate substrate (X(B)) is the major organic substrate fraction in most municipal wastewaters. However, the impact of X(B) on aerobic granular sludge (AGS) systems is not fully understood. This study evaluated the physical retention of X(B) in AGS sequencing batch reactor (SBR) during anaerobic plug-flow and then aerobic fully-mixed conditions. The influence of different sludge types and operational variables on the extent and mechanisms of X(B) retention in AGS SBR were evaluated. X(B) mass-balancing and magnetic resonance imaging (MRI) were applied. During the anaerobic plug-flow feeding, most X(B) was retained in the first few cm of the settled sludge bed within the interstitial voids, where X(B) settled and accumulated ultimately resulting in the formation of a filter-cake. Sedimentation and surface filtration were thus the dominant X(B) retention mechanisms during plug-flow conditions, indicating that contact and attachment of X(B) to the biomass was limited. X(B) retention was variable and influenced by the X(B) influent concentration, sludge bed composition and upflow feeding velocity (v(ww)). X(B) retention increased with larger X(B) influent concentrations and lower v(ww), which demonstrated the importance of sedimentation on X(B) retention during plug-flow conditions. Hence, large fractions of influent X(B) likely re-suspended during aerobic fully-mixed conditions, where X(B) then preferentially and rapidly attached to the flocs. During fully-mixed conditions, increasing floc fractions, longer mixing times and larger X(B) concentrations increased X(B) retention. Elevated X(B) retention was observed after short mixing times < 60 min when flocs were present, and the contribution of flocs towards X(B) retention was even more pronounced for short mixing times < 5 min. Overall, our results suggest that flocs occupy an environmental niche that results from the availability of X(B) during aerobic fully-mixed conditions of AGS SBR. Therefore, a complete wash-out of flocs is not desirable in AGS systems treating municipal wastewater.