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 148mg/L), followed by the pH uncontrolled (105mg/L) and the neutral pH (87mg/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 degrees C (VFAs content was 148mg/L), better than lower temperature 50 degrees C (79mg/L) or higher temperature 70 degrees C (75mg/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 184mg/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 degrees C, adding ratio 12.5% and HRT 8d, revealing that the performance of WAS acidification was sensitive to operating conditions. (C) 2012 Selection and/or peer-review under responsibility of Basel Convention Coordinating Centre for Asia and the Pacific and National Center of Solid Waste Management, Ministry of Environmental Protection of China.