Effect of HRT on the biological pre-denitrification process for the simultaneous removal of toxic pollutants from cokes wastewater

A lab-scale serial anoxic–aerobic reactor for the pre-denitrification process was continuously operated to efficiently and economically treat actual cokes wastewater containing various pollutants, such as phenol, ammonia, thiocyanate and cyanide compounds. The biodegradation efficiencies of the pollutants were examined by changing hydraulic retention time (HRT) as a main operating variable. The long-term operation of the pre-denitrification process reactor showed that 100% phenol, 100% free cyanide, 100% SCN−, 97% ammonia, 85% COD, 84% TOC (total organic carbon) and 83% TN (total nitrogen) were removed at HRT above 11.9 h. Removal efficiency of total cyanides significantly decreased with a decrease in the HRT. Free cyanide and some of total cyanides were removed in anoxic reactor, whereas thiocyanate was removed in aerobic reactor. Phenol was completely removed under successive anoxic and aerobic conditions. Although actual cokes wastewater contained high concentrations of various toxic pollutants, the pre-denitrification process showed stable and successful performances in both nitrification and denitrification reactions.