To investigate the underlying causes of instability, reactor overload was induced by implementing step-type disturbances in the organic load rate (OLR), followed by a comprehensive characterization of microbial com-munities (genes), process parameters, and kinetics parameters. The results revealed significant reductions in kacetate and kpropionate, with decreases of 52.87 +/- 0.14 % and 73.23 +/- 0.94 %, respectively, compared to their maximum values (1.74 +/- 0.09 gVS/(gVSS center dot d) and 0.71 +/- 0.05 gVS/(gVSS center dot d)), when the OLR was set at 3 kgVS/ (m3 center dot d) and the methane yield (MY) was stabilized at 0.52 +/- 0.02 L/gVS. Similarly, at an OLR of 3.5 kgVS/ (m3 center dot d), the significant deviations of kacetate and kpropionate from their normal range (kacetate: 0.71 +/- 0.02-1.74 +/- 0.09 gVS/(gVSS center dot d); kpropionate: 0.39 +/- 0.02-0.71 +/- 0.05 gVS/(gVSS center dot d)) were observed and a substantial decline in MY subsequently was observed. Notably, the continuous and significant decline in the relative abundance of SPOB, AMA, and their related metabolic genes (prpE, ACADM, paaF, PCCA, MCEE, sucD, cdhC, and metF) in response to the high OLR stress was observed, providing valuable insights into the reduction in kacetate and kpropionate, ultimately leading to their accumulation. Moreover, intricate interactions between acetate and pro-pionate metabolism under overload conditions were revealed, with propionate metabolism displaying the highest sensitivity.

A modified Anaerobic Digestion Model No. 1 (ADM1) with optimized kinetic parameters was presented to model methane production in the anaerobic digestion of food waste. Experimental data from batch and semi-continuous fermentations were used to calibrate and verify the model. Modified ADM1 simulation was carried out using AQUASIM 2.0 software. Sensitivity analysis was used to identify and evaluate the most sensitive kinetic parameters during biogas production. The decay constant of microorganisms, the disintegration constant, the hydrolysis constant of carbohydrates, the Monod maximum specific substrate uptake rate, and the half-saturation constants affected biogas production significantly. The optimized values of these parameters were 0.001, 0.16, 3, 1 and 0.23, respectively. Optimization results were validated using batch and semi-continuous experiments. The modified ADM1 well-predicted methane production, with R-2 values for the validation experiments all above 90%. These results can be used as basic data to simulate methane production in full-scale reactors.

A long-term high solids anaerobic digestion of food waste was conducted to identify microbial mechanisms of ammonia inhibition during digestion and to clarify correlations between ammonia accumulation, microbial community dynamics (diversity, composition, and interactions), and process stability. Results show that the effects of ammonia on process performance and microbial community were indirectly caused by volatile fatty acid accumulation. Excess free ammonia blocked acetate metabolism, leading to process instability. Accumulated acetate caused feedback inhibition at the acetogenesis stage, which resulted in considerable accumulation of propionate, valerate, and other long-chain fatty acids. This high concentration of volatile fatty acids reduced the abundance of syntrophic acetogenic bacteria and allowed hydrolytic fermentative bacteria to dominate. The normally interactive and orderly metabolic network was broken, which further exacerbated the process instability. These results improve the understanding of microbial mechanisms which contribute to process instability and provide guidance for the microbial management of anaerobic digesters.

Organic loading rate (OLR) disturbances were introduced into a mesophilic anaerobic digester treating food waste (FW) to induce stable and deteriorative phases. The microbial community of each phase was investigated using 454-pyrosequencing. Results show that the relative abundance of acid-producing bacteria and syntrophic volatile fatty acid (VFA) oxidizers increased dramatically at deteriorative phase, while the dominant methanogens did not shift from acetoclastic to hydrogenotrophic groups. The mismatching between bacteria and methanogens may partially be responsible for the process deterioration. Moreover, the succession of predominant hydrogenotrophic methanogens reduced the consumption efficiency of hydrogen; meanwhile, the dominant Methanosaeta with low acetate degradation rate, and the increase of inhibitors concentrations further decreased its activity, which may be the other causes for the process failure. These results improve the understanding of the microbial mechanisms of process instability, and provide theoretical basis for the efficient and stable operation of anaerobic digester treating FW.