| Mesophilic temperature regimes (30–40°C) have been adopted for anaerobic digestion, showing good operational performance for a long time. Recently, the thermophilic regimes (50–60°C) have also been adopted for anaerobic digestion showing several advantages, such as an increased destruction rate of organic solids and increased destruction of pathogenic organisms. However, the use of thermophilic anaerobic digestion has been limited because of some disadvantages like poor supernatant quality and poor process stability related to high propionate threshold concentrations, which caused the US EPA to exclude thermophilic anaerobic digestion as Processes to Further Reduce Pathogens (PFRP). The purpose of this research was to evaluate process stability and process efficiency of thermophilic anaerobic digestion by evaluating the key parameters of reactor configuration, microbial consortia proximity, and inorganic nutrient supplementation. For the sake of comparison, all experiments in this research were conducted at 55°C or 35°C. Stable pH, lower VFA, higher gas production, and stable VS removal of the non-mixed reactor configuration at both mesophilic and thermophilic temperatures showed well-balanced performance between acid producers and consumers. This implies the importance of microbial consortia proximity to enhance microbial scavenging of dissolved H2 concentrations to lower levels, improving the thermodynamic advantage of the process. Supplementing inorganic nutrients significantly reduced all VFA concentrations dramatically, except for propionate. In the case of the non-mixed reactor, propionate was also barely detected at both temperatures. Therefore, it turned out that microbial consortia proximity could solve the problem of thermophilic poor effluent quality. A proposed kinetic model for the effect of the distance between two syntrophic bacteria reasonably matched the real data in this study. An innovative process based upon an incorporation of rational concepts to enhance process stability and efficiency of anaerobic digestion, which is called Anaerobic Digestion Elutriated Phased Treatment (ADEPT), showed that this process could solve the problem of poor effluent quality in thermophilic anaerobic digestion while achieving greater stability. Additional experimental studies were conducted for the better understanding of anaerobic solubilization and start-up seed source of anaerobic sludge digestion. |
