Effect of HRT and recycle ratio on removal of cyanide, phenol, thiocyanate and ammonia in an anaerobic–anoxic–aerobic continuous system

A combined system consisting of three sequentially arranged suspended growth reactors under anaerobic, anoxic and aerobic environments were used to treat a synthetic wastewater containing phenol, cyanide, thiocyanate and ammonia. The total hydraulic retention time (HRT) of the three-stage system, internal recycle ratio to anoxic reactor and feed cyanide concentration were used as variable parameters for the study. During HRT study, it was observed that at total HRT of 6 days and recycle ratio of 1, 90% COD, 96% cyanide, 50% total nitrogen and 100% phenol and thiocyanate (influent: 2500 mg/L COD, 27 mg/L cyanide, 712 mg/L total nitrogen, 1000 mg/L phenol and 400 mg/L thiocyanate) removals were achieved. Total nitrogen removal efficiency increased with increase in recycle ratio. The anaerobic reactor had removed more than 75% feed cyanide from a feed concentration of 68 mg/L. The efficiency of three-stage system in terms of COD and total nitrogen removal remained unaffected when feed cyanide concentration was increased from 27 to 68 mg/L.