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 60ć (VFAs
content was 148mg/L), better than lower temperature 50ć (79mg/L) or higher temperature 70ć (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, 60ć, adding ratio 12.5% and HRT 8d, revealing that the performance of WAS acidification
was sensitive to operating conditions.