Dear Fine People:
The precipitant we use in our industrial wastewater treatment plant to remove chelated metals from our rinse waters is dithiocarbamate, or DTC. DTC has the unfortunate ability to form the undesirable (read: strictly regulated) byproduct/decomposition product CS2 (carbon disulfide), especially under acidic conditions. Can anyone offer help in any of the following areas regarding this dilemma?
1. DTC management to minimize/eliminate CS2 formation? Preventing DTC bearing waters from experiencing acidic conditions seems to be one of the more obvious interventions.
2. DTC variants which cannot form CS2 (if they exist)?
3. Alternatives to DTC? We have tried several, none seems to match DTC's ability to precipitate a highly chelated, high lead bearing wastewater stream to under 0.2 ppm lead consistently.
4. Easy test kit/method for CS2?
Thanks for your help
Keith Perrin,
Compliance Specialist
Printed Circuit Corp.
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Your problem may be where you add the DTC and what it is reacting with. DTC is a reducing agent and if you have a metal oxide in solution, it reduces to the metal and oxygen. You may have a lot of oxygen in solution that is causing the problem. If this is an old installation and a new problem, you may want to look upstream to see what processes have changed recently.
DTC should be added late in the clarification stage, as a secondary step to polish the waste stream. I you add it early, you loose the advantage of hydroxide precipitation. DTC should be added last as a polishing step to take out any residual metal. It's a very expensive way to precipitate metal.
I think our newsletter number 4 or 5 talked about DTC use.
Bob Mesick
Remco Engineering
Water and Wastewater Treatment Systems
http://remco.com/home.htm
Remcobob@remco.com
Keith,
We used to use DTC in three of our facilities, and understand the CS2 issue, as well as the other issues you describe. We affectionately refer to DTC as Damn Toxic Chemical. We even had one POTW propose in a permit revision that we must have at least 1 ppm copper in our discharge so that we wouldn't overfeed DTC! For almost four years now, we have converted four of our facilities to Romar chemistry, which is based on ferrous dithionite. It is essentially an iron substitution chemistry for chelated metals, much like ferrous sulfate, but without all the sludge, since copper is precipitated in an acidic solution. In fact, our experience is that we cut the sludge generation in HALF from both DTC and ferrous. So, switching from DTC to Romar is not only P2, but waste minimization. One of the benefits of Romar is its powerful nature in treating chelated metals. We have not had any problems removing lead from chelated solutions. In fact, several facilties report non-detectable lead levels (using AA analysis). One has even treated two spent baths of 1000 gals each and achieved non-detectable levels. We'd be glad to show you our treatment system since we're only a half hour away.
Lee Wilmot HADCO Corp
603/896-2424
HYPERLINK mailto:lwilmot@hadco.com lwilmot@hadco.com