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Why is mechanical agitation better than air for hexavalent chromium reduction process?

It has been my observation that mechanical mixers tend to react differently to air mixing when trying to reduce hexavalent chrome . where the ORP reading remains stable when using the mechanical mixer it is significantly increased when mixing by air. I don't understand why this should be so. Is the air that is being introduced for mixing causing this problem?

Junior Williams
water treatment consultant. - Brooklyn, NY
June 22, 2011

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The only thing I can think of is that dissolved oxygen is throwing it off.

I much prefer mechanical agitation for any kind of chemical waste treatment, because some of the air tends to dissolve, is released later, and floats up some of the solids.

A nice trick I used to use: lower a sump pump, with no hose attached, into the treatment tank and turn it on. This gives you good mixing without the need to install a permanent prop mixer.

Dave Wichern
Consultant - The Bronx, New York
June 28, 2011

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July 1, 2011

Hi,

Air agitation is not recommended for treatment of hex-chrome due of the possibility of re-oxidation of trivalent, however, aeration is recommended as a support to lower COD Levels of the treated water prior the sewer.

Kind regards,

Dominik Michalek
- Melbourne, Australia

The suggestions that you both made were very helpful in my research. I am now using a sump pump for mixing and will post my observations at the conclusion of my study.

Junior Williams
Water treatment consultant - Brooklyn, New York, USA
March 22, 2012