Microbial conversion pathways of particulate organic substrate conversion in aerobic granular sludge systems: limited anaerobic conversion and the essential role of flocs

Abstract

Particulate organic substrate (X_B) represents the major fraction of organic substrate in low-strength municipal wastewater (MWW) but its hydrolysis, conversion and utilisation in aerobic granular sludge (AGS) are not well understood. We systematically analysed through modelling the microbial conversion pathways of X_B in AGS systems operated as sequencing batch reactors and treating municipal wastewater. Based on our current understanding of AGS systems, we developed an AGS model in Sumo® to better understand (1) during which phases of the SBR cycle (anaerobic plug-flow feeding or aerobic fully-mixed conditions), (2) by what biomass fraction (granules or flocs) and (3) via which microbial processes X_B is converted into soluble organic substrate (S_B) and volatile fatty acids (S_VFA) and further utilised. The validity of all our results was evaluated by assessing the sensitivity of findings on the biomass distribution (floc fraction) (SCENARIO #1), SBR cycle (SCENARIO #2), and biofilm properties (SCENARIO #3). Our results indicate that only 20% of influent X_B was hydrolysed and 11% of influent X_B was then used for internal carbon-storage during 90 min anaerobic plug-flow feeding. The major fraction of influent X_B (80%) leaked into the aerobic SBR phase, where X_B was then hydrolysed and oxidised aerobically by ordinary heterotrophic organisms (OHOs). Under aerobic fully-mixed conditions, the flocs successfully competed for X_B with the granules. Aerobic X_B hydrolysis and its further utilisation by OHOs occurred mainly in flocs despite their minor biomass fraction (13% of total suspended solids), preventing excessive OHO growth on granules and demonstrating that flocs are beneficial for AGS systems treating MWW. Also, the model allowed us to better understand that the SBR sequence can be optimised to make better use of X_B under anaerobic conditions and to reduce the leakage of X_B to aerobic conditions. The introduction of a 60 min anaerobic idle phase after a 60 min plug-flow feeding phase almost doubled the anaerobic X_B hydrolysis and internal carbon-storage by phosphorus and glycogen accumulating organisms (PAOs and GAOs), respectively, compared to 90 min plug-flow feeding. Nonetheless, most influent X_B was still utilised by OHOs during the aerobic SBR phase.