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${}_{ww}$). ... mehrX${}_B$ retention increased with larger X${}_B$ influent concentrations and lower vww, 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 XB 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.