Management of cyanide in municipal wastewaters.

Cyanide present in municipal sewage is derived from industrial sources. Cyanide can also be input to or generated within municipal wastewater treatment processes, such as by the introduction of biosolids incineration scrubber water or in the oxidizing conditions of the disinfection process. Cyanide compounds are commonly used in a number of industries, and are present in associated process wastewaters. Industries that generate large quantities of cyanide-bearing wastewaters include electroplating and metal finishing, chemical production, steel production, and petroleum refining (see Chapters 4 and 26). As cyanide can be toxic to the microorganisms used in biological treatment processes if present at sufficiently high concentrations, and because of concern about potential for HCN gas releases in sewers, many industrial generators of cyanide-bearing wastewater are required to pretreat their wastewater prior to discharge to publicly-owned treatment works (POTWs), for example, iron and steel manufacturers [1]. Thus, concentrations of cyanide in influent to POTWs are generally low, that is, nondetectable to 500 µg/l total cyanide [2-4]. The fate of cyanide in a municipal wastewater treatment plant depends on the form and amount of cyanide. Cyanide in raw wastewater is predominantly (e.g., 60 to 70%) in the complexed form. Free cyanide species, HCN and CN−, are readily biodegradable if not present at concentrations inhibitory to aerobic bacteria. They can also be removed from solution by adsorption onto organic particles (see Chapter 5) in primary treatment and biosolids in secondary treatment. HCN, which is relatively volatile, can be removed (“stripped”) by aeration processes within wastewater treatment plants. The understandingCYWS “L1666_C025” — 2005/10/12 — 17:39 — page 502 — #2less well developed. Weakly associated metal-cyanide complexes are subject to decomposition and hence ultimately the same fate processes relevant to free cyanide. On the other hand, strong metal-cyanide complexes are essentially nonbiodegradable at the typical residence time of a municipal wastewater treatment plant, and they will often pass through biological treatment processes. Strong metal-cyanide complexes may be removed through adsorption on mineral particles (Chapter 5) in primary treatment.