Assessment of whey anaerobic digestion stability based on organic loading rate fluctuation: In terms of syntrophic volatile fatty acid degradation and microbial signals of recovery.

Abstract

Anaerobic digestion (AD) is a promising method for treating organic waste, but it requires thorough management. While it is known that organic loading fluctuations affect anaerobic digestion, their specific impact has not been verified. Therefore, this study hypothesized that the response to overload differs depending on the organic loading rate (OLR) strategy to which anaerobic digesters are adapted. To test this hypothesis, sequencing batch reactors were operated using whey as the substrate. Five operational conditions were compared under three fixed OLRs of 0.25, 1.0, and 2.0 g COD/L∙day (i.e., low-L, medium-M, and high-H, respectively) and two dynamic OLR regimes of stepwise oscillation (SO; M-L-M-H conditions) and drastic oscillation (DO; L-H conditions). After the initial operation period, all reactors were subjected to the same overload, recovery, and treatment (C-control, B-bioaugmentation). The LB and SOB reactors showed marked pH decreases and caproic acid accumulation, whereas the HB and DOB reactors showed delayed propanoic acid degradation. In contrast, the MB reactor exhibited superior performance in terms of volatile fatty acid (VFA) removal and pH recovery on D159. The microbial community changed dynamically according to the OLR history. PICRUSt2 analysis suggested that syntrophic VFA degradation and amino acid metabolism were affected by the OLR histories. Network analysis revealed putative associations between minor syntrophic acetate-oxidizing bacteria and Methanosarcina during recovery. Additionally, the connection of protein-related consortia (Aminobacterium-Methanomassiliicoccus) was inferred by network analysis. Overall, the results of this study indicate that prior OLR history shapes overload resilience by altering VFA degradation trajectories and microbial recovery patterns.