The high protein and lipid content of fish waste makes mono-digestion a difficult bioprocess for an anaerobic digestion (AD) system. On the other hand, the massive increase in fish and seafood consumption worldwide has led to an inevitable fish waste mono-AD. Therefore, this study was conducted to investigate the effects of food-to-microorganisms (F/M) ratios and temperatures during the start-up period of fish waste mono-digestion. F/M ratios of 0.5, 1, 2, and 3 on a g-COD/g-VSS basis were operated at 35°C and 45°C, representing mesophilic and hyper-mesophilic conditions, respectively. The increase in F/M ratio improved the maximum methane (CH4) production rate at both temperatures. However, F/M ratio of 0.5 generated the highest CH4 yield in mesophilic and hyper-mesophilic conditions (0.23±0.00 L-CH4/g-CODinput). Further increase in F/M ratio decreased CH4 yield up to 21.74% and 39.13% when the reactors were operated at 35°C and 45°C, respectively. When reactors were supplied with FM ratios of 0.5, 1, and 2, hyper-mesophilic temperature improved methanogenesis by up to 2.61% and shortened the lag phase by 22.88%. Meanwhile, F/M ratio 3 at 45°C decreased cumulative CH4 production by up to 26.57% and prolonged the lag phase by 10.19%. The result of this study is beneficial to managing the input substrate of a batch-AD system that treats fish waste as a sole substrate.

The design of a full-scale bioprocess is typically based on parameters derived from smaller-scale experiments from a previous study. However, disagreements often occur at up-scaling of waste-to-energy processes due to the fluctuations of the substrate characteristics, etc. Therefore, once a commercial-scale waste digester has been built and operated, it is essential to test if the performance of the process agrees with its design value; during this process, fluctuations might occur in digesters operated at marginal organic loading rates. In this study, triplicate full-scale anaerobic digesters treating food waste were monitored for five months. The digesters, operated at the design feeding ratio, showed increasing volatile fatty acid (VFA) trends (per total alkalinity) due to a 30% higher chemical oxygen demand of the influent, than the design. The organic loading rate was adjusted on a daily basis until a stable performance was observed. Significant shifts of methanogen populations from Methanobacteriales to Methanomicrobiales and Methanosarcinales were observed during the stable operation period.

The influence of introducing fluctuations in acetate feeding concentrations on the process stability of a lab-scale horizontal anaerobic reactor (HAR) was investigated to ascertain its effects on acetoclastic methanogenesis. Acetate concentrations were randomized at 85 g COD/L ± 20% and discontinuously fed in the duplicate HARs for five days per week (giving the system time to rest and equilibrate for two days). The reactors were monitored daily with respect to performance indicators (physicochemical variables). The residual acetate concentration was observed to fluctuate at the initial stages, followed by a converging trend (decrease in variance) until the end of operation. Furthermore, letting the system self-neutralize and equilibrate during rest days resulted in improved process performance. The daily acetate degradation efficiency at ~90% and methane concentration at ~60% were attained after operating the reactors for 80 days. The results divulged that introducing fluctuations in acetate feeding concentrations does not affect the stability of biogas production and methane concentration. The acclimatization of the methanogenic population (predominantly Methanosaeta, then shifting to Methanosarcina) was also observed.

Food waste (FW) sourced from treatment facilities is predominantly in solid form, with low water content and high variations in organic content. High organic content in FW is ideal in anaerobic digestion for bioenergy applications, but proper monitoring during start-up operations should be employed to avoid imbalance in the acidogenic/methanogenic population due to volatile fatty acid (VFA) accumulation in the system. The seed inoculum (5 L) in each horizontal anaerobic reactor (HAR) was fed with food waste without effluent flow (filling-up phase) until it reached the final working volume of 10 L (continuous phase). The pH, alkalinity, chemical oxygen demand (COD), VFA, biogas production, methane concentration, and microbial community dynamics were set as stability indicators during reactor operation. The results revealed that introducing fluctuations in FW concentrations does not negatively affect the biogas production (1.7 ± 0.2 L/L_R/d) and methane concentration (59.0 ± 2.5%). Acclimatization of the methanogenic and bacterial population was also observed. This study aimed to evaluate the influence of fluctuating FW concentrations on the process performance of horizontal anaerobic reactors, focusing on process stability, microbial dynamics, and biogas output during filling-up and continuous phases.