Management of cyanide in municipal wastewaters.
2006
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.
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