Electroplating Wastewater Treatment Q&A's, Problems & Solutions

"Operation & Maintenance of Surface Finishing Wastewater Treatment Systems"
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A. Chelate breaking usually involves reducing the pH to about 4.0, maybe lower, and then supplying another ion like aluminum (from aluminum sulphate) or iron (from ferric chloride ⇦this on eBay or Amazon [affil links] ) or perhaps magnesium from epsom salt [affil links] to replace the problem metal, then raising the pH quite high, Clay. Try it in the lab to see how high you can keep the pH when you lower it, how low you can keep it when you raise it, and which co-precipitant works best. People hate the operational mess of lime, but it combines the co-precipitant/chelate breaker with the pH raiser and may be best. Good luck.

A. Are you doing acid zinc plating with a bath that contains ammonium ions?

If so, that might be the cause of your problem. At pH 10, there's enough free ammonia to cause complexation problems. If that's your situation, try precipitating at a lower pH, perhaps 9 or so, with a product like DTC or sodium sulfide.

Funny, eh, that the enforcement authorities should not be a little more helpful when you're doing a good thing and conserving water? Once, I got dinged for Zn being about 20% high in a batch of water that I had discharged. This water had been recycled twice and had a TDS of about 15,000 ppm.

Funny, the priorities people have. It's a perverse incentive to leave well enough alone and just leave the water turned up.

A. As metals enter the treatment process, they are in a stable, dissolved aqueous form and are unable to form solids. The goal of metals treatment by hydroxide precipitation is then to adjust the pH (hydroxide ion concentration) of the water so that the metals will form insoluble precipitates. Once the metals precipitate and form solids, they can then easily be removed, and the water, now with low metal concentrations, can be discharged.
Metal precipitation is primarily dependent upon two factors: the concentration of the metal, and the pH of the water. Heavy metals are usually present in wastewaters in dilute quantities (1 - 100 mg/L) and at neutral or acidic pH values (< 7.0). Both of these factors are disadvantageous with regard to metals removal. However, when one adds caustic to water which contains dissolved metals, the metals react with hydroxide ions to form metal hydroxide solids:

During this process I would keep the pH 8.0 to 8.5 SU.

A. Chris,
You are correct in general with your statements concerning metal hydroxides and pH. If there are chelates present the whole world can change. I would suggest that Clay investigate possible sources of chelates, burnishing compounds, etc.

The whole regulatory issue is could be a never ending discussion. we have an RO unit to produce water that we can recycle. The nature of this beast is it concentrates our discharge. When we get close to the limits we turn off the unit and stop recycling. The real dicotomy here is that we are sending the same total pounds of metal to drain. The POTW would never know the difference either way because we are such a small part of their total treatment.

I understand that there are discussion by those in the compliance field about possibly changing the regs from concentration to total pounds or some combination of the 2. Lets hope clearer heads prevail.

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A. Hi Wan. The first step is jar testing while adhering to the basics. If you can't get proper levels in jar tests with the basic approaches, then you would need to either see how to modify the process which is generating the nickel problem or start applying more complex reductants like TMT-15 or sulphides or DTC.

What is the source of the nickel, what other processes mix with it before WWTP, and what pH do you precipitate at? What is the allowable nickel ppm in the discharge, and what is your current dissolved and filterable nickel in ppm? Thanks.

Regards,

A. Hello Mr. Muhammad,
Check the source chemical for addition of a chelating agent, which will make your treatment method more difficult. For example, the ubiquitous Reliant Nickel acetate seal has a chelator added. To perform a 'destruct' rendering it possible to separate as solids and clean dischargeable wastewater, it must be brought to pH greater than 11 to break the chelation effect. We don't have to use any polymers etc to do this, just solid sodium hydroxide added while mixing (watch your temperature!). If a smut reducing additive has been put in the bath, which contains even more chelator, an additional first step may be necessary, which is to first drop the pH to about 3.5 using ferric chloride (about 1 kilo per 1000L) and then Nitric acid to further reduce the pH. Then proceed bringing the pH way back up again. Not knowing what exactly you are starting with, this is just a simple example of one way to deal with a heavily chelated plating chemical.
As Ted says, you'll want to jar test before trying to full-scale the treatment process. Good luck!

A. Hi All

Excellent advice on the treatment of waste water but I would suggest that there is another approach.

If expensive chemicals do not enter the waste stream they do not have to be removed by even more expensive treatment.

Assuming we are discussing plating wastes, I would look first at the plant and its operation. Much can be achieved by …

Jigging with minimal solution traps (not so easy on a barrel line).
Counter-flow rinses
Adequate draining time over the tank (without drying out); Operators rarely understand the importance of this.
Fine demin spray rinse over the tank; With a little practice you can balance evaporation/dragout losses, you save chemicals ($s) and never have to top up again!
Recirculated rinses with deioniser.

It is possible in some cases to completely eliminate the need for any effluent treatment.

A. Hi Carolina. We added your inquiry to a long thread on the subject, which should offer food for thought. You might also look at thread 40394, "Cyanide plating waste water treatment", for additional insights. But just for four quick thoughts:

1. You must figure out where the 1400 ppm of COD is coming from, not only to control it, but because it might be interfering with the nickel removal.
2. Filter a sample of the wastewater before testing so you are sure whether the excess nickel is in dissolved or precipitated form so you know whether the chemistry or the solids-liquid separation step must be improved.
3. Just a reminder that you do jar testing to figure out what will work, you don't make adjustments to your treatment regiment based on untested theory.
4. ferric chloride is a chelate breaker/co-precipitant and there may be better ones; but what the word 'co-precipitant' implies is that you add it before things are precipitated, not after. The idea is that there is a limit to the total metal that can stay dissolved at a given pH and if you add iron or another metal it reduces how much nickel can stay dissolved. If you're sure that cyanide is absent, lower the pH in your beaker [beakers on eBay or Amazon [affil links] to 3.5 with acid, then add the co-precipitant, then raise the pH and see if there is improvement.

Luck & Regards,

A. You may have chelators or ammonia that are interfering with metals precipitation. I'd recommend that you jar test with a stronger chelate breaker.