Treatment of coking wastewater using oxic-anoxic-oxic process followed by coagulation and ozonation

[Display omitted] •Long time treatment of coking wastewater by O-A-O process, coagulation and ozonation.•High removal of NH4+-N in O-A-O process as preoxidation of inhibitors to nitrifiers.•Effluent COD from O-A-O process cannot be reduced below 260 mg/L even with high HRT.•Organics outflowing from the first aerobic tank cannot be utilized by denitrifiers.•Coagulation and ozonation effectively remove COD, PAHs and change MWD in the effluent. An oxic-anoxic-oxic (O-A-O) system followed by coagulation and ozonation processes was used to study the treatment of coking wastewater. In the O-A-O process, the removals of NH4+-N, total nitrogen and COD were 91.5–93.3%, 91.3–92.6% and 89.1–93.8%, respectively when employing hydraulic residence times of 60 h for the biochemical system. High removal of NH4+-N was obtained due to the placement of an aerobic tank in front of A-O system which can mitigate the inhibitory effect of toxic compounds in coking wastewater on nitrifying bacteria. Addition of methanol into the anoxic reactor greatly increased the removal of total nitrogen, indicating that denitrifiers can hardly use organic compounds in coking wastewater as carbon source for denitrification. COD values of the effluent from the O-A-O system were still higher than 260 mg/L even with a prolonged time of 160 h mainly due to the high refractory properties of residual compounds in the effluent. The subsequent coagulation and ozonation processes resulted in the COD removal of 91.5%–93.3% and reduced the relative abundance of large molecular weight (MW) organics (>1 kDa) from 55.8% to 20.93% with the ozone, PAC and PAM dosages of 100, 150 and 4 mg/L respectively. Under these conditions, the COD value and concentration of polycyclic aromatic hydrocarbons in the final effluent were less than 80 and 0.05 mg/L, respectively, which meet the requirement of the Chinese emission standard. These results indicate that the combined technology of O-A-O process, coagulation and ozonation is a reliable way for the treatment of coking wastewater.