Treatment of coal gasification wastewater by a two-phase anaerobic digestion

Coal gasification wastewater (CGW) is a refractory wastewater, whose anaerobic treatment has been a severe problem due to its toxicity and poor biodegradability. Using a two-stage anaerobic digestion as a control, the two-phase anaerobic digestion of real CGW was investigated. After 210 d of operation, the maximum removal efficiencies of COD and total phenols (TP) reached 50-60% and 55-60%, respectively, in the two-phase anaerobic digestion at total hydraulic retention time (HRT) of 48 h; the corresponding efficiencies were at low levels of 40-45% and 42-50%, respectively, in the two-stage anaerobic digestion. COD and TP removal efficiencies increased to 69.7 and 65.9%, when 50% of the effluent was recirculated to the hydrolytic acidogenic reactor. Phenol utilization rate and specific methanogenic activity (SMA) were decreased to 57.6 mg phenol/(gVSS d) and 394.3 mg COD-CH4/(gVSS d), respectively, as the HRT in the hydrolytic acidogenic reactor was reduced to 18 h. After the two-phase anaerobic digestion, the wastewater concentrations of the aerobic effluent COD could reach below 150 mg/l when compared to 237.2 mg/l if two-stage anaerobic digestion was done and 328.5 mg/l if sole aerobic pretreatment was done. The results suggested that the two-phase anaerobic digestion improved significantly both anaerobic and aerobic biodegradation of real CGW.