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Cyanide in Domestic Wastewater Treatment Effluent


The Key West Wastewater Treatment Plant is experiencing minor cyanide excursions in it's effuent. We find no trace of cyanide in the influent or in any sidestreams around the plant. We chlorinate with chlorine gas and dechlorinate with sulfer dioxide. Key West has no significant admitted contributors of cyanide and the amounts found at the plant are <14 micrograms, but still beyond the limit for our discharge point.

1. Can this be created in-plant?

2. Is there a viable treatment process for conversion or removal?

3. Any suggestions?

Greg Smith
- City of Key West - Key West Florida

This problem is quite common. There is an extensive discussion of it in East Bay M.U.D. water authorities annual report from 1997.

My own belief is that the cyanide is always there, but is masked in the raw influent by interferences. These are removed in the treatment, making it appear that cyanide has been "created". I do not know of any chemistry that can "create" cyanide under conditions like this. I suppose the only way to be sure would be to perform some kind of isotopically labelled spiking experiment.

At any rate, you're not alone in having this problem, and its cause is hotly debated among plant operators.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

There is a good article in the March/April 2004 issue of Water Environment Research, "The Occurance of Cyanide Formation in Six Full-Scale Publicly Owned Treatment Works" by Zeng, et al. The article discusses the formation of cyanide from thiocyanate during chlorination of wastewater. Although not routinely monitored for, thiocyanate is fairly common in municipal wastewater.

Tim Sheahan
- Ormond Beach, Florida

May 10, 2010

Your response was very interesting concerning the detection of cyanide after the raw water interferences have been stripped away but can you help explain why after running tests on my city's tap water we had cyanide hits near 20 ug?

Or why after adding chlorine bleach at a dosage rate of 500 ug to a Deionized Water sample waiting 5 minutes to remove the chlorine with Sodium Sulfite placing the treated then dechlorinated sample in the proper container and having it analyzed for cyanide while having and untreated Deionized Water ran simultaneously the sample spiked with chlorine then dechlorinated showed cyanide. While the other sample showed no detect.

I'm currently spiking Influent samples with 3 mg/L of chlorine and letting it set the estimated detention time that our plant has before removing the chlorine from the sample and having it analyzed along with an untreated sample.

Send me a response but stay tuned.

Sylvester Fox
- Monroe, North Carolina, USA

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