Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
December, 2010

Chromium in Wastewater

Q. Mr. McGinness, I have 9000 gallons of chromic acid anodize in storage due to a ruptured tank. I experimented with sodium metabisulfite and reduced the hex to tri. (I had tested at lab) The wastewater regulations are for total chromium so is there a way to treat this under the limits so I can release to the water plant? I am the new quality manager and NDT Level III here and sure could use some help with this matter. Thanks

A. If you added enough sodium metabisulfite, all you need to do now is raise the pH back to about 9 (8-9), and let the trichrome precipitate and settle out of solution as an hydroxide sludge. Sodium hydroxide (liquid) is commonly used for this to raise the pH back up, but magnesium or calcium hydroxides (dry solid form) will work if you can handle and mix the dry material.

This will typically remove 99 to 99.9% of the chrome. The magnesium and calcium will reach lower chrome levels in solution, but the calcium makes more sludge. A little magnesium and the rest Sodium hydroxide will make less sludge. The Magnesium by itself is pH limiting on the high side, which is an advantage over sodium hydroxide and is far less dangerous safety wise than sodium hydroxide (which cause permanent eye damage on contact if it gets in the eyes). In a large batch operation, if I had the mixing ability for magnesium hydroxide (solid material, pellets, flake, powder), it would be my preferred choice. Note that sodium and calcium hydroxide are exothermic on contact with water, they get very hot, and if added too fast will boil and erupt in your face! So they must be added and mixed slowly!!!

If you have lab beakers and a scale, I suggest doing a bench scale test to see how much you need to add to reach the target pH, then just scale up the volumes. You may need to pre-dilute the additive about 100:1, to be able accurately measure the amount needed to raise the pH of say 500 ml in a beaker. In other words mix 1 ml of sodium hydroxide with 100 ml of DI water. Then add 1 ml at a time of the diluted NAOH to the 500 ml of trichrome waste water till the target pH is reached. That way you will be adding .001 ml of NaOH at a time to the 500 ml batch.

You did not say what plant or discharge limit you need to meet? If you mean discharge to your own in house waste water treatment for final metal finishing treatment the above may be good enough. If you mean direct discharge with out further treatment to a POTW, that may take more steps to meet a lower limit for total Chrome.

Feel free to give me a call, or email back if you have any further questions.

Sincerely,

Mike McGinness