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Lowering chrome results in my effluent

Do the levels of chrome before you treat have anything to do with the levels after treatment? We find it hard to get below .5ppm when the batch is initially low. Also, final results seem to vary greatly, any reasons why? Currently, we drop pH to 2.2 add Metabisulfite, raise pH to 9.5 and floc etc.

David Nunno
Aluminum Painting - Rutherford, New Jersey
2007

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It is true that it can be difficult to get a chromium-bearing floc to agglomerate and settle if the concentration is too low (or any metal-bearing floc for that matter). I think the first thing to do is to filter a sample to determine whether the problem is failure to precipitate out of solution or failure to settle out. But adding ferric chloride [on eBay or Amazon] or aluminum sulphate will probably help.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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The only thing I could add to what Ted already said is that you might be able to treat with ferrous sulphate ferrous sulphate [affil links] alone, at about pH 6. This is a good idea only if the hexchrome is pretty low.

Ferrous is a great chrome reductant, but if there's a lot of Cr6+ in the waste, you have to use a lot. That means you get a lot of extra sludge.

Dave Wichern
Consultant - The Bronx, New York
2007