Ammonia and propionate are known inhibitors of anaerobic methanogenesis at higher concentrations, and are likely to coexist in digesters treating high-strength wastewater. Therefore, this study was conducted to assess the effects of ammonia and propionate on acetate-utilizing methanogenesis when they coexist. Response surface analysis with face-centered central composite design was used to explore the total ammonia nitrogen (TAN) level of 2–5 g/L and the propionate level of 2–8 g/L in acetate-fed batch incubation. Two models were successfully derived to estimate the lag period and the methane yield in response to the concentrations of the two chemicals. The lag period was affected by both inhibitors, with clues obtained of a synergistic effect at a higher concentration rage ([TAN] > 3.5 g/L and [propionate] > 5 g/L). The methane yield was also affected by the two inhibitors; between the two, it was more significantly dependent on the TAN concentration than on the propionate concentration. Real-time PCR showed that Methanosarcinaceae was the major methanogen group in this system. The results of this study improve our understanding of the inhibition of biogas reactors.

A demo-scale (600 m³ working volume) anaerobic digester treating food waste leachate was monitored during its startup period. The operation strategy was adjusted twice (i.e., three distinct phases) during the operation to recover the process from instability. During the first phase, the organic loading rate (OLR) > 2.7 kg chemical oxygen demand (COD)/m³∙day corresponded to volatile fatty acid (VFA) accumulation along with a decreasing pH, resulting in the drop in biogas yield to 0.43 ± 0.9 m³/kg COD_in. During phase 2, fast recovery of this process was aimed at using a sequencing batch operation. One batch cycle (5 to 2 days) consisted of the combined drawing and feeding step (5 h), the reacting step (91 to 17 h), and the settling step (24 h). The duration of the reacting step was determined for each cycle such that (1) the biogas production ceased before the cycle end and (2) the residual VFA concentration was < 1 g/L. In total, 11 cycles were operated with a gradual increase in biogas yield to 0.55 m³/kg COD_in with the absence of any sign of system disturbance. After phase 2, the digester was fed at the designed OLR of 4.1 ± 0.3 kg COD/m³∙day. The biogas yield was elevated to 0.58 ± 0.2 m³/kg COD_in during phase 3 with the residual VFA concentration maintained at 2.2 ± 0.6 g/L. Methanogen populations, as determined by real-time PCR, did not change significantly throughout the period. These results imply that the adaptation of this process to the OLR of ca. 4 kg COD/m³∙day was not due to the increase in methanogen population but due to the elevation of its activity. Overall, this study suggests that the sequencing batch operation with adjustable cycle duration can be one successful recovery strategy for biogas plants under system instability.

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.