Volatile Fatty Acids and Biomethane Recovery from Thickened Waste Activated Sludge: Hydrothermal Pretreatment's Retention Time Impact

The main objective of this study was to evaluate the hydrothermal pretreatment's retention time influence on the volatile fatty acids and biomethane production from thickened waste activated sludge under mesophilic conditions. Six different retention times of 10, 20, 30, 40, 50, and 60 min were investigated while the hydrothermal pretreatment temperature was kept at 170 degrees C. The results showed that the chemical oxygen demand (COD) solubilization increased by increasing the hydrothermal pretreatment retention time up to 30 min and stabilized afterwards. The highest COD solubilization of 48% was observed for the sample pretreated at 170 degrees C for 30 min. Similarly, the sample pretreated at 170 degrees C for 30 min demonstrated the highest volatile fatty acids yield of 14.5 g COD/L-substrate added and a methane yield of 225 mL CH4/g TCODadded compared to 4.3 g COD/L-substrate added and 163 mL CH4/g TCODadded for the raw sample, respectively. The outcome of this study revealed that the optimum conditions for solubilization are not necessarily associated with the best fermentation and/or digestion performance.