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topic 45127

Best pH for precipitation of zinc

A discussion started in 2000 but continuing through 2018


Q. Our company is a manufacturer of latex paint, and we are looking to install a filter press to reduce our wastewater disposal costs. We also have an issue with zinc contaminants, as we have trouble flocculating it to below 3 ppm. The filter press will likely require a wastewater pretreatment stage to improve performance, probably some sort of surfactant. I am wondering if there is a surfactant that will also precipitate the zinc. If anyone has experience with this, I appreciate your help.

Mike Rompala
- Garland, Texas


A. I maybe can help. I have experience treating zinc to 0.5 ppm. If I could get more information?

danny tynes
Danny Tynes
used electroplating equipment
- Birmingham, Alabama


A. In your case I would advise a filter press with a post treatment to remove the last few ppm of zinc. Concentrate from the post treatment can be recycled to the pH-adjustment before the filter press. This always ensures low zinc level in the effluent and results in a lower moist content in the filter cake, compared to the use of flocculants, surfactants and other additives.

Ruud Gerritsen
- Simpelveld, Netherlands

May 17, 2011

A. Yes, filter press will responsible for the receiving water only, that is, the wastewater need to be treated before filtering.

pH adjusting is also a method for thickening the solids and shifting those tiny ones to bigger ones; is it possible to add flocculant agent?

Or just discharge the wastewater to tanks to check its thickening conditions?

The smallest ones are the key point "role" during filtering, as it will stay on the filter cake growing side or filter cloth side for cake building.

Qi Yangqi
- Hangzhou, Zhejiang, China

Why High Zinc Levels in Wastewater


Q. I am searching for possible causes of High Zinc Level readings in our waste water. I have had the incoming well water tested with normal readings. Our effluent has the DEQ concerned. We have covered all of the chemicals used in house with the manufacturers and MSDS's. I have walked the plant and looked at all possibilities. The only thing odd I have noticed is we have a few of our indoor Air Conditioner units condensate drain into the sinks. Could this be a possibility?

Thank you for any help,

Paul Dofermire
- Front Royal, Virginia, USA


A. What is it that you do at this plant? I would not think that you would get high zinc from AC condensate, as most heat exchangers in AC units are aluminum, not zinc treated metals.

Perhaps a facility walkthrough with a water treatment engineer familiar with metals and treatment might be of help?

tom baker
Tom Baker
wastewater treatment specialist
Warminster, Pennsylvania


Q. At what pH value we have the best precipitation of Zinc in a wastewater?

Sergio Zeni
- Montevideo, Uruguay

ph vs. dissolved metal

A. It's not quite as exacting as that, Sergio. According to the famous EPA chart that people have been both relying upon and laughing at for 3 decades, it's about 10.2. I think in practice it's been more like 10.5 for typical zinc-bearing electroplating wastes. The thing is, the "common ion" effect shifts it quite a bit based upon what other metals are in the waste solution.

So please remember to design the system only after jar tests with the specific waste. Good luck.

Ted Mooney, amadeya42.com Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

O&M of Surface amadeya42 Wastewater Treatment Systems
Clarence Roy


A. Optimum pH for precipitation of zinc as zinc hydroxide is 9.45 from my experience. Above this pH zinc effectively comes 'back' into solution as a zincate Zn(OH)4 - - ...thus causing a rise in zinc concentration in your effluent outfall.
Hope this helps!

Nigel Gill
- Glasgow

January 24, 2018

Q. Hi, I am investigating a problem with too much zinc in effluent.

The site produces metal ceilings; they are consistently over the 2 mg/litre on their consent. The zinc contaminates the water when washing the perforated metal prior to painting (powder coating).

They have an effluent treatment system in place which is:

Caustic (to bring pH up to between 9.8 - 10.5)
34% Ferric chloride (as the coagulant) made up to a 1000 L solution.
They then have one 18 m3 tank with weirs to help clarify.

But this does not work. Zinc is still over. Any ideas?

Craig Jenkins
- UK

January 2018

A. Hi Craig. Step 1 is to simply take a sample and tell us the Zinc concentration before and after filtering it. If there is a lot of dissolved zinc, we will have to talk about the source and the chemical treatment. If the problem is poor clarification, that's something else. Get back to us.


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

February 3, 2018

A. Try 9.2 to 9.5.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

February 14, 2018

A. Hello Craig,

As Dave said, pH values around 9 are better for zinc precipitation. Ferric chloride alone is not the best way for metal precipitation, I would use an anionic polymer after you add the ferric chloride and adjust pH.

And most important:

1 - First add ferric chloride
2 - Adjust pH
3 - Add polymer

Ferric chloride is always acid, so if you adjust pH before adding it, pH values will be not consistent and you will have problems as you are experiencing.

First of all, see if the step order is correct. If it is, try adding a polymer.

Best f luck!

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

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