Acidification of Waste Activated Sludge During Thermophilic Anaerobic Digestion

The effects of pH value, temperature, adding ratio of waste activated sludge (WAS) and hydraulic retention time (HRT) on WAS acidification in thermophilic anaerobic digestion system were investigated by batch experiments. The results indicated that the WAS acidification at pH 9.0 was the greatest (VFAs content was about 148 mg/L), followed by the pH uncontrolled (105 mg/L) and the neutral pH (87 mg/L). Compared to the conditions of pH uncontrolled, the VFAs accumulation was retarded at acidic (pH 3.0 and pH 5.0), neutral (pH 7.0) or strong alkaline pH values (pH 11.0). Optimal temperature of thermophilic anaerobic digestion on VFAs accumulation was shown to be 60°C (VFAs content was 148 mg/L), better than lower temperature 50°C (79 mg/L) or higher temperature 70°C (75 mg/L). Each adding ratio has its own optimal HRT to producing maximum VFAs. The VFAs accumulation was attained peak value at dosing 12.5% and HRT 8d (about 184 mg/L). In conclusion, in order to maximize the accumulation of VFAs for providing more carbon, the optimal operating conditions for WAS thermophilic anaerobic digestion observed in this study was pH 9.0, 60°C, adding ratio 12.5% and HRT 8d, revealing that the performance of WAS acidification was sensitive to operating conditions.