Comparative performance and microbial diversity of hyperthermophilic and thermophilic co-digestion of kitchen garbage and excess sludge

The objective of this study was to evaluate the performance characteristics of a hyperthermophilic digester system that consists of an acidogenic reactor operated at hyperthermophilic (70 °C) conditions in series with a methane reactor operated at mesophilic (35 °C), thermophilic (55 °C), and hyperthermophilic (65 °C) conditions. Lab-scale reactors were operated continuously, and were fed with co-substrates composed of artificial kitchen garbage (TS 9.8%) and excess sludge (TS 0.5%) at the volumetric ratio of 20:80. In the acidification step, COD solubilization was in the range of 22–46% at 70 °C, while it was 21–29% at 55 °C. The average protein solubilization was 44% at 70 °C. The double bond fatty acid removal ratio at 70 °C was much higher than at 55 °C. These results suggested that the optimal operation conditions for the acidogenic fermenter were about 3.1 days of HRT and 4 days of SRT at 70 °C. Methane conversion efficiency and the VS removal percentage in the methanogenic step following acidification was around 65% and 64% on average at 55 °C, respectively. The optimal operational conditions for this system are acidogenesis performed at 70 °C and methanogenesis at 55 °C. The key microbes determined in the hyperthermophilic acidification step were Anaerobic thermophile IC-BH at 6.4 days of HRT and Thermoanaerobacter thermohydrosulfuricus DSM 567 at 2.4 days of HRT. These results indicated that the hyperthermophilic system provides considerable advantages in treating co-substrates containing high concentrations of proteins, lipids, and nonbiodegradable solid matter.