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topic 33822

Treatment of Chrome Plating Wastewater


Q. I am starting up a hard chrome plating plant in China over the next couple months and was wondering if there are any new options for treating chrome contaminated waste water. Back in Canada we are using E-cells (steel plates with applied voltage) to reduce the hex chrome to tri. Our sister plant uses sodium metabisulfite and then raises pH, filter press, etc...

I plan to recycle all the chrome from the rinses, pre-controls, and scrubber though I am a bit hesitant to recycle and purify the clean up water (simple as mop water) as there will be unknown contaminants. I will be dealing with very small volumes, approx. 1000L/week at concentrations from 50-500mg/L.

I am hesitant to install ion exchangers since we will not have the experience or resources to maintain such a system. My problem with the sodium bisulfite or ferrous sulfate methods are the added chemicals needed for pH adjustment and reduction.

Is there a simpler, reliable solution for such a small flow? Or, should I consider recycling/purifying the mop water? The government has suggested a zero discharge process though we will have no problem meeting legal requirements.


Chris Maciejko
Hard Chrome Plating - Guelph, Ontario, Canada


A. Having experienced electrolytic reduction why would you consider anything else? If the electrolytic process does not also precipitate the chromium then you may need to tune it some. By definition, an electrolytic process will not give you re-useable water, as some conductivity (i.e. dissolved solids) are required simply to conduct the electricity. You can follow with another technology to remove them.

paul morkovsky
Paul Morkovsky
- Shiner, Texas, USA

June 24, 2008

A. We're pondering also different options for water treatment. One of those was the use of e-cells, but we were uncertain on it. Is there a way to find out more about the e-cells?

Thanks Chris!

Francisco Cruz
- Edmonton, AB, Canada

To minimize your searching efforts and to offer multiple viewpoints, we've combined some threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.


Q. What is the procedure to convert hexavalent chrome to trivalent chrome ?

How to make colourless water from Treated Effluent?

Imran Bagwan
Electroplating Related - Ichalkaranji, Maharashtra, India


A. Hi Imran. I assume you are speaking about wastewater treatment rather than about how to do trivalent chrome plating.

At low pH, sodium bisulfite will convert hex chrome to trivalent chromium. This process evolves toxic sulfides in proportion to how low the pH is, so keep it as high as practical without stopping the process. Probably about 3.5-4.0. The yellow color will be gone and the water will be greenish.

The chrome will then precipitate out at normal wastewater treatment pH. If you are treating only chrome, the pH should be about 8.5, but you can go higher to remove other dissolved metals without stopping the precipitation of chrome.

Wastewater treatment, however, involves dangerous chemicals and the potential to evolve toxic gases, and operators should receive hands-on training, not proceed based on brief internet messages. Good luck!

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


A. Electrocoagulation using steel plates and DC current works well without any chemical additions and will remove the chromium as a precipitate at any pH over about 6.

paul morkovsky
Paul Morkovsky
- Shiner, Texas, USA


A. You can use also the Catalytic Precipitation, ECP, without any chemicals, without sludge or precipitate, without sacrificial electrodes. No dependence on pH or temperatures, or concentration. The results are oxides and minerals filtered like solids directly from water at the time of treatment process. You will have good water in a few minutes (hours), depending on flow rate and wastewater analysis.


Damian Dinu
- Constanta, Romania


Q. Hello all

Usually I work with hexavalent chrome wastewater. The wastewater is provided by a de-tanning process and the final slurry is a mix among chrome and little bit of organic fibers.
I use sodium sulfite to reduce hexavalent chromium to trivalent chromium at pH=2. After that the chromium is flocculated and precipitated by using NaOH until pH 8,5.
The problem is: Sometimes after addition of sulfite and increasing the pH, the chromium won't precipitate. I'm wondering if there is any relation about amount of sulfite and ability to precipitate or not?
If I put sulfite in excess, the chromium precipitation would be screwed up? I can't figure out what is happening. Does anybody know why I can't precipitate chromium after sulfite addition?

Thanks to all!

Rodrigo Lousada
- Brazil

A. Hi Rodrigo. In my experience, reduction of hexavalent chromium to trivalent is one of the easier processes because you can so easily track it visually. If there is any yellow/amber/orange tone whatsoever, the chromium has not been reduced to trivalent and will not precipitate. If the color has turned to light green-blue, it will.

If you have sufficient treatment time, I would not run the pH down to 2, but would try to keep it closer to 4 -- the reason being that a lot of noxious sulfur gases are released at low pH, and that drives you towards wanting to go light on the necessary bisulfite; whereas at the higher pH you can add sufficient bisulfite with less gassing.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Q. I would like to know; how to reduce TDS in Chrome plating Waste water. I am getting TDS very high after regular treatment.

- Nashik, Maharastra state, India

A. Hi Ashok. You must analyse the wastewater to see what the TDS is comprised of before you can make any plans.

For example, you cannot practically precipitate chlorides, so any attempt to reduce the TDS if it's high in chlorides might only increase it. Different metals precipitate best at different pHs, which is a possible optimization. Low pH pretreatment with chelate breakers may be needed. Please give us an analysis and some details. Thanks.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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