Combined biological and photocatalytic treatment of real coke oven wastewater

• Coke oven wastewater contains phenolics, aromatic hydrocarbons along with cyanide.
• Anaerobic–aerobic–anoxic bioreactors and photocatalysis was used for CWW treatment.
• Real CWW without pretreatment adversely affected the reactor performance.
• COD removal of 96.2% was achieved in the hybrid system with real CWW.
• The hybrid system was able to achieve discharge standards with real CWW.

Wastewater generated from coke oven industries contains highly toxic organic and inorganic compounds. In the present study, biological reactors were integrated in sequence of anaerobic–aerobic–anoxic and tested with real coke oven wastewater obtained from a Steel Plant located in India. Among several pretreatment methods, coagulation of raw CWW was found to be effective. COD removal efficiency of 78.5% was achieved by the integrated bioreactor system after coagulation of CWW with 1000 mg/L of alum. The effluent COD, NH4-N and TCN concentration from the integrated bioreactors was 420, 152 and 20 mg/L, respectively. It was observed that 420 mg/L of COD present in the effluent after integrated bio treatment was degraded in less than 4 h to minimum COD concentration of 94 mg/L using UV–TiO2 photocatalysis. In absence of photocatalyst (TiO2), there was no COD reduction from the bio treated effluent. Similarly, there was insignificant reduction in COD from the raw CWW using photocatalysis without biological treatment. Considerable COD reduction (78.5%) was observed only in the wastewater subjected to integrated biological system. The overall COD removal efficiency of the combined treatment system was found to be 96.2%, which resulted in an effluent COD concentration less than 94 mg/L.