EFFECTS OF POTW TREATMENT PROCESSES ON TRANSFORMATION AND PRODUCTION OF CYANIDE SPECIES

This research involved laboratory studies to investigate causes of aqueous phase cyanide formation which has been observed at some Publicly Owned Treatment Works (POTWs). Four reaction groups / processes identified as potential contributors to cyanide formation in POTWs were investigated in a bench-scale study, including (1) chlorination; (2) chloramination (from hypochlorite reaction with residual ammonia); (3) nitrosation (reaction with nitrite); and (4) UV irradiation. Experiments were conducted with POTW wastewater (unchlorinated secondary effluent) and with synthetic solutions. For reaction group 1, results indicated that total cyanide was detected in thiocyanate solutions in which the chlorine dose was not sufficient to destroy SCN completely. For reaction group 2, it was revealed that cyanogen chloride could be detected in the chloramination of POTW secondary effluents, which implies that precursors of CNCl can exist in the POTW secondary effluent. It was also found that detectable cyanide, around 10 μg/L, was observed in chloramination of synthetic solutions of selected organic compounds and in chloramination of POTW effluents followed by dechlorination. For reaction group 3, free cyanide was identified as a product of nitrosation of selected organics under ambient conditions (T=25°C, neutral pH). Experiments performed with some POTW effluents revealed that nitrite-reactive compounds likely occur in municipal wastewaters, and cyanide may be formed as a result of in-plant nitrosation of these organics. For reaction group 4, results indicated that thiocyanate can be broken down by UV irradiation to yield cyanide.