Abstract
Operational disturbances in aerobic granular sludge (AGS) systems can result in aerobic availability of readily biodegradable COD (rbCOD). Different from activated sludge, morphological consequences on the short and long term are not well described in literature. This study investigated the effect of incomplete anaerobic uptake of acetate on the morphological and process stability of AGS using a lab-scale reactor. A fraction of the total acetate load was dosed aerobically, which was increased stepwise while monitoring granular morphology. A good granular morphology and an SVI of 40 ml/g were obtained during initial enrichment and maintained for ≤20% aerobic acetate load dosed at 4 mg COD/g VSS/h. Biological phosphorus removal efficiency was initially unaffected, but the aerobic acetate dosage rate did decrease the aerobic phosphate uptake rate. This led to loss of phosphorus removal for >20% aerobic acetate load dosed at 8 mg COD/g VSS/h over the course of 12 days. Subsequently, significant outgrowth formed on the granular surfaces and developed over time into finger-like structures. Under these high aerobic acetate loads the SVI increased to 80 ml/g and resulted in significant biomass washout due to deteriorating settling properties of the sludge. The sludge settleability and biological phosphorus removal recovered 10 days after aerobic feeding of acetate was stopped. Aerobic presence of rbCOD can be tolerated if mostly anaerobic acetate uptake is maintained, thereby ensuring stable granular morphology and good settleability. The high enrichment of phosphate accumulating organisms in the granular sludge through bottom-feeding and selective wasting of flocs makes aerobic granular sludge resilient to morphological deterioration in aerobic presence of rbCOD.
Highlights
Aerobic granular sludge (AGS) is becoming a well-established technology for wastewater treatment due to the compactness, energy savings and good effluent quality (Bengtsson et al, 2018; Derlon et al, 2016; Pronk et al, 2017a)
A good granular morphology and an sludge volume index (SVI) of 40 ml/g were obtained during initial enrichment and maintained for ≤20% aerobic acetate load dosed at 4 mg COD/g VSS/h
We investigated the effect of an increasing fraction of the total acetate load on the morphology of aerobic granular sludge (AGS) enriched for anaerobic PHA formation through enhanced biological phosphorus removal (EBPR)
Summary
Aerobic granular sludge (AGS) is becoming a well-established technology for wastewater treatment due to the compactness, energy savings and good effluent quality (Bengtsson et al, 2018; Derlon et al, 2016; Pronk et al, 2017a). The storage polymers are oxidised in the subsequent aeration phase where they are used for microbial growth and nutrient removal. This process design selects for well-settling, granular sludge, which is further enhanced by subsequent selective discharge of the worst-settling, flocculent sludge fraction (Beun et al, 2002; De Kreuk and Van Loosdrecht, 2004; Liu et al, 2005). Mirghorayshi and M.C.M. van Loosdrecht et al / Water Research 187 (2020) 116402 over, this potentially hampers implementation and development of alternative technologies for acquiring granular sludge in full-scale municipal wastewater treatment
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