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topic 4405

Zinc waste treatment problems

A discussion started in 2000 but continuing through 2019


Q. I own and operate an electrocoating plant and have waste treatment problems.

Our process uses a zinc phosphate stage as part of the pretreatment for the e-coat. Our waste water permit allows us to discharge a monthly average of 1.0 mg/Liter and a peak discharge (daily limit) of 2.6 mg/Liter. Our normal stream concentration is about 20-30 mg/Liter. I need an economical way to remove the zinc. I have a filter press, settling tower and several 2000 gallon poly tanks to use for storage. I have tried to flock out the zinc using a flocking agent and raising then lowering the pH but when I send it through the filter press it clogs it up and also lets a lot of particles through the screens, and thus to drain.

Also, I am in need of an accurate and repeatable way to measure the zinc levels of our waste stream. Someone told me of some type of Atomic spectro something or other. Does anyone have any ideas on this? I have tried a colorimeter made by Hach Co. and it does not give accurate readings. (as compared to the water department monitoring our discharge)

Thank you

Rodger Smith
- washington, Michigan


A. Rodger, your supplier of the proprietary zinc phosphate should be available to help you waste treat the waste stream. The vendor should be a full service company with waste treatment chemicals and experienced sales/technical service, including a laboratory.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan


A. Dear Roger,

I assume you are already treating with hydrated lime [linked by editor to product info at Amazon] which facilitates wonderful settling. I am not too sure how sophisticated your laboratory is, but if you a need simple test for small quantities of zinc: we use the "Aquaquant" test from a company called Merck. Their catalog says they are represented in the USA through a company called EM Industries Inc.

Good luck


Trudy Hattingh
- Durban, South Africa


A. Zinc phosphate, by itself, gives a very slimy sludge. You might want to look at using some magnesium hydroxide to precipitate it. This stuff is itself rather insoluble so is irritating to work with. Try reacting it with the acid to get some in the system and then finish off with lime or other caustic.

James Watts
- Navarre, Florida


A. Hi Rodger,

I think you got enough replies for the first part of your problem. So let me throw some light upon the second part of your query.

I have a pretty good hand on an Atomic absorption unit or an (AA Unit).

This is a fine piece of accurate equipment which uses air and acetylene( or even nitrous oxide) and the sample of waste water is aspirated into this flame. The flame changes colour depending upon the concentration of the metal in it and this absorbency is decoded in the metal concentration present in the sample.

This is just a simple idea but the AA Unit is certainly a very reliable technique to detect any metal.

You use some hollow cathode lamps of the same metal (which is to be detected). Once you set up all calibration steps it takes you 5 seconds to detect a sample with 95% accuracy for sure. And you may proceed with any number of samples after calibration. So it is very fast too.

Ask your nearest Varian dealer. You may find a good deal in some used equipment ( say may be 5,000 USD).


Hemant Kumar
Hemant Kumar
- Florida, USA


A. Direct filtration through filter presses is possible, though problematic. It can be done if you have 2 (preferably 3) presses and can stage them, and if the volume of filter aid won't make the cost of waste treatment prohibitive. But most people prefer to use a clarifier, and then only have to dewater the precipitated sludge.

Magnesium hydroxide is indeed a great precipitant, but it is a slurry of particles not a solution, and therefore the reaction time can run 6 hours and even more as the particles dissolve ever so slowly into the acidic waste.

Hemant tells you true. While I like the economy and simplicity of Hach-type tests and colorimeters, your results are not atypical: When analyzing a dilute jobshop waste stream containing a mix of metals, sometimes the best you get is a good correlation factor rather than readings that are on the money. I.e., if you read 1.6 ppm in your lab when the sewer authority gets 2.6 using AA, it's a fair bet that if you next get 3.2, they'll get about 4.0 or more. The relationship may not be linear, but it is usually correlatable.

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


A. Perkin Elmer has a good model for Atomic Absorption..its the AAnalyst 100. Thats the one I personally run at work and I am within .1 ppm of what an independent lab runs our waste water at.

Vicki Harwood
- Wyoming, Michigan, USA


A. We also use an AA machine to detect the cadmium levels in the waste water test. I would suggest filter the solution before running it. Let the flame burn for five minutes after the test to clear the orifice. Soda ash will cut the metals Floc will help the metal settle in solution. You will still need another chemical as a negative charge.

Dale May
mining - Bartlesville, Oklahoma


A. Dear sir,
I read about the problem you are having with your Zinc lines. I have read the other responses and I must say, some of those things I read made no sense to me. Atomic Absorption or called Atomic Spectrophotometry is definitely in advantage.

But for a simple process, it's overkill not to mention cost. A colorimeter or reagent test kit would suffice. I understand that you have tried a hatch test kit but were unable to achieve accurate results, I was curious as to why it wasn't working. I have used AA, Ultra-violet spectro, gas spectro and colorimeters; all of which were very accurate.

I could easily suggest some remedies that might solve your problem but would be wrong in doing so just for the simple fact, that to my ears there is to much missing information which can be vital in making any decision whatsoever.

Anyone that has been involved in Industrial chemistry should know this. I would be glad to help you in any way I can.

But first I need to know the following:

1. What kinds of chemicals do use in pre-process?
2. What type of processor operation are you supporting?
3. What is your rate of production?
4. How is your plating line setup? concentration levels,   rinse types etc...
5. How many and what type of cleaners are you using?
6. How often do you dump and remake tanks?
7. What is your W.T. setup consist of?
7. What is your rate of discharge in waste treatment?
8. What are you using to control pH and what are the optimum pH levels you are using.
9. How knowledgable are your technicians?
10. What is the climate like where you are at?

As I mentioned earlier, I would be happy to help. But all these questions I ask can mean everything, why? Because that's chemistry for ya!

Currently I oversee a zinc phosphate line and waste treatment and to be honest with you we haven't had any problems at all.

I hope by now you have found a solution to your problem.

Eric Fahling
- Sanford, North Carolina

December 2, 2018

Q. How to treat effluent from acid zinc plating. Which chemicals to use and how to use them?

Sam Jay
- Mzn India

Digital version

(No longer published, but Elsevier hasn't yet de-commissioned the online version of the Guidebook)
Download it before it disappears.
December 2018

A. Hi Sam. That's a better question for a week-long training course than for a forum response :-)
-- even the simple step of pH adjustment can be done with caustic soda, or lime, or magnesium hydroxide, some new proprietary precipitants, or some other chemicals :-)

But the on-line version of the Metal Finishing Guidebook has several good chapters on the subject and will get you started. I'd suggest the "Wastewater Treatment" chapter by Thomas J. Weber as step one. Best of luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

December 3, 2018

Q. Sir
I have been running an acid zinc plating plant for around 6 months. It's a very secluded area so for treating; the advice I get here is just dump the waste, who cares. But I am hell bound on discharging the waste water in an eco-friendly manner. The book was definitely helpful.

Currently I am running my ETP in the following manner. Please let me know if it is correct or how can I improve it.

1. I collect all my waste water in a common pit (approx. 3000 litre capacity)
2. Once the pit is full, I add sodium metabisulphite (SMBS) in the pit itself. The pit has an air blower to mix the solution.
3. Then I add lime to increase pH to around 10 to 10.5. Sometimes I use caustic instead of lime. I really don't know which is to be used when.
4. I feed the solution to reaction tank from the pit.
5. Add alum to reduce ph to around 6.5 to 8.5
6. Add polyelectrolyte so the solution flocculates.


Sam jay
- Mzn India

December 2018

A. Hi Sam.

2. Sodium metabisulfite is a reducing agent which is used to reduce hexavalent chromium to trivalent chromium. If you are not using hexavalent chromates you do not need it; but if you are, you do. It will accomplish that reduction at a pH between about 2 and 5. It's faster acting at low pH; but if the pH is too low, toxic sulfur dioxide gas is generated. It's slow at pH 5 and above about 5 it won't work, but there is much less sulfur smell at pH 4.5 - 5 than at low pH.
3. Lime is a more effective precipitant than caustic soda as well as cheaper and safer; it does generate more sludge and can clog things up, but if lime is not causing serious problems, that's what to use.
5. I was not aware that alum efficiently reduced pH. I'm not sure why you ever need to get below pH 8.5; I don't think I would.
6. Polyelectrolyte is fine, but the point of flocculation is so the solids will settle out separately from the liquid, and you haven't mentioned yet what you do after the solution flocculates.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

simultaneous December 4, 2018

A. Hi Sam,

I would adjust pH around 9 for polyelectrolyte to work, because zinc precipitates better around 8.5-9 and not so good below 7.5.

I don't know when or where you remove the sludge, after or before the polyelectrolyte. If you can, make a flow chart so we can understand better!

Best regards,

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

December 4, 2018

Q. Sir,

I use hexavalent chromium for passivation. Thus I use sodium metabisulphite. But what is the right way to determine how much quantity to add. As of now I arbitrarily add 15 kgs.

Thank you for advising me on the use of lime. Caustic was indeed expensive. Can I add lime powder directly to my collection pit as there is an air blower to mix it. Or should I make a solution of lime and water and then add it to the collection pit?

Alum is cheap and effective. Would you suggest any thing else. I would like to give it a shot if it's better.

After flocculation the water goes to another tank where the impurities settles down which later go to a sludge tank. In the sludge tank water trickles back to collection pit and the sludge is collected and disposed.
The clean water on the other hand, goes to another tank from which it passes through two filters after which it is fed for rain water harvesting.

If there is any other good practice i should follow I would be happy to hear it out.

Thanks a ton for all the valuable info you have shared. It has helped me a lot.

Warm regards

Sam jay [returning]
- Mzn India

December 2018

A. Hi again. Assuming your waste is translucent/transparent rather than opaque, the cheapest and easiest way to know how much sodium metabisulphite to add is from the color. Hexavalent chromium tints the water amber colored even at concentrations of a couple of ppm. When the hexavalent chromium has all been reduced to trivalent, the water will be a very light green color (some say blueish-green). Try some samples in a beaker to get a good feel for this obvious color change. You can test for hexavalent chromium concentration and/or control the addition of metabisulphite with an ORP controller if you wish, but I don't think it's necessary or the best use of always scarce resources. Ideally you should do the chrome reduction step before mixing in the other wastes if it's possible.

As for how to add the lime, whatever works works. If you can add it as a powder directly to the pit without making a mess or getting it airborne, then no need to complicate things.

Alum is a fine thing to add as it is both a coagulant and a co-precipitant (when you put aluminum into the water you drive a little of the zinc and other metals out). I was just surprised to hear that alum had substantial power to lower the pH.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

December 20, 2018

Q. Hello.
After the treatment effluent there is a tinge of yellow in the water. What could be the reason for it?


Sam jay [returning]
- Mzn India

December 2018

A. Hi again, Sam. Did you get a chance to do what I suggested -- repeatedly treating beakers of solution with sodium metabisulfite at controlled pH until you are very comfortable with how the color changes when hexavalent chromate has been reduced to trivalent? Please do your development of treatment strategy and your practice a beaker at a time rather than 3000 liters at a time. Only after you are comfortable with this operation will you be able to make an educated guess as to whether the yellow color is related to hexavalent chromium or some other thing. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

January 2, 2019

A yellow tinge in the wastewater is probably - but not certainly - hexavalent chromium that has not been reduced. You can check this easily with the 1,5 - diphenylcarbohydrazide test, which will give a strong colorimetric reaction with hexavalent chromium. One description of this test is given in the US EPA's test 7196A which is available at
While the test describes a quantitative method, the same procedure can be used qualitatively. A reddish purple result is indicative of hexavalent chromium. This is a pretty robust test.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
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