Abstract
Particulate substrate (XB) is the major organic substrate fraction in most municipal wastewaters. However, the impact of XB on aerobic granular sludge (AGS) systems is not fully understood. This study evaluated the physical retention of XB 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 XB retention in AGS SBR were evaluated. XB mass-balancing and magnetic resonance imaging (MRI) were applied. During the anaerobic plug-flow feeding, most XB was retained in the first few cm of the settled sludge bed within the interstitial voids, where XB settled and accumulated ultimately resulting in the formation of a filter-cake. Sedimentation and surface filtration were thus the dominant XB retention mechanisms during plug-flow conditions, indicating that contact and attachment of XB to the biomass was limited. XB retention was variable and influenced by the XB influent concentration, sludge bed composition and upflow feeding velocity (vww). XB retention increased with larger XB influent concentrations and lower vww, which demonstrated the importance of sedimentation on XB retention during plug-flow conditions. Hence, large fractions of influent XB 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 XB concentrations increased XB retention. Elevated XB retention was observed after short mixing times < 60 min when flocs were present, and the contribution of flocs towards XB 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 XB during aerobic fully-mixed conditions of AGS SBR. Therefore, a complete wash-out of flocs is not desirable in AGS systems treating municipal wastewater.
Highlights
Our understanding of the effect of particulate organic substrate (XB) on the formation, operation and overall process performance of aerobic granular sludge (AGS) remains limited, despite many full-scale installations (Derlon et al, 2016; Ali et al, 2019)
Our first main result is that XB accumulated predominantly within the voids at the bottom of the settled sludge bed, indicating that XB retention was governed by sedimentation and surface filtration during plug-flow feeding (Figs. 3e6)
If XB retention was governed by sedimentation and surface filtration, it is likely that only a minor fraction of XB is in contact with the granules during anaerobic plug-flow feeding (Figs. 5e6; Ranzinger et al, 2020)
Summary
Our understanding of the effect of particulate organic substrate (XB) on the formation, operation and overall process performance of aerobic granular sludge (AGS) remains limited, despite many full-scale installations (Derlon et al, 2016; Ali et al, 2019). Finger-type granules are rarely observed in AGS systems treating municipal WW (Pronk et al, 2015b; Derlon et al, 2016). A noticeable growth of flocs is observed, so that flocs represent a substantial fraction of 10e20% of the AGS formed during treatment of municipal WW (Wagner et al, 2015a; Derlon et al, 2016; Layer et al, 2019; Pronk et al, 2015b). Understanding the connection between influent XB and the presence and role of flocs is required
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