Aerobic digestibility of waste aerobic granular sludge (AGS) assessed by respirometry, physical-chemical analyses, modeling and 16S rRNA gene sequencing.

Research has evolved on aerobic granular sludge (AGS) process, but still there are very few studies on the treatment of excess AGS sludge, with almost none considering its aerobic digestion. Here therefore, the aerobic digestibility of typical AGS sludge was assessed. Granules were produced from acetate-based synthetic wastewater (WW) and were subjected to aerobic digestion for 64 d. The stabilization process was monitored over time through physical-chemical parameters, oxygen uptake rates (OUR) and 16S rRNA gene sequencing. The microbial analyses revealed that the cultivated granules were dominated by slow-growing bacteria, mainly ordinary heterotrophic organisms with potential for polyhydroxyalkanoates (PHA) aerobic storage (PHA-OHOs), polyphosphate and glycogen accumulating organisms (PAOs and GAOs), fermentative anaerobes and nitrifiers (AOB and NOB). Differential abundance analysis of the bacterial data (before versus after digestion) discriminated between the most vulnerable microbiome genera and those most resistant to aerobic digestion. Furthermore, modeling of the stabilization process determined that the endogenous decay rate constant (bH) for the heterotrophs present in the granules was notably low; bH = 0.05 d-1 (average), four times less than for common activated sludge (AS), which is rated at 0.2 d-1. For first time, the research reveals another important feature of AGS sludge, i.e. the slow-decaying character of its bacteria (along with their known slow-growing character). This results in slower stabilization, need of bigger digesters and reconsideration of the specific OUR limits in biosolids regulations (SOUR limit of 1.5 mg/gTSS.h), for waste AGS compared to conventional waste AS. The study suggests that aerobic digestion of waste AGS (fully-granulated) could differ from that of conventional AS. Future work is needed on aerobic digestibility of real AGS sludges from municipal and industrial WWs, compared to synthetic WWs.

Mechanisms of persistence and impact of ordinary heterotrophic organisms in aerobic granular sludge.

The stability of granules, contaminant removal and microbial structure of an aerobic granular sludge (AGS) process were investigated with a focus on ordinary heterotrophic organisms (OHOs). Long-term stable granules and high removals of COD (97 %), NH4+ (98 %), P (85 %) and total N (77 %) were achieved. Sequencing analyses identified 6.6 % of phosphorus-accumulating organisms in the sludge, concordant with the observed bio-P removal capacity. However, OHOs were the most abundant bacteria in the sludge (70-93 %) without resulting in unstable aggregates. Under current dogmas of microbial competition in activated sludge, it seemed contradictory that OHOs could persist in the long term in the AGS where COD was depleted beginning in the anaerobic phase. Microbial analyses showed that OHOs could survive in granules by micropredation, proteolysis, fermentation and EPS consumption. Heterotrophic-nitrification/ aerobic-denitrification was an active pathway in the AGS. These findings contribute to a better understanding of microbial competition in AGS and its stability.