Chelants in Industrial wastewater - From the Water and Wastewater.com Help Forum

Water and Wastewater.com Help Forum - Chelants in Industrial wastewater:

Question by Catman

What can be done to minimize the effect of chelating agents in Industrial Wastewater where we are trying to precipitate metal hydroxides. We do zinc plating and the problem occurs when we get an influx of the cleaning baths into the WWT system. We supposedly don't have any "chelants" in our cleaning baths (according to the suppliers)but there are constituents that act as chelants (Pyrophosphates,etc.)

answer

I would try coagulation with iron salts. If chelating agents are present the iron will partly displace the zinc bound .
The heavy metals will adsorb to the precipitated iron hydroxides.
The phosphates should not interfere.
Addition of minor amounts of flocculants might improve the coagulation/precipitation process.

reply by Catman

Our pH is controlled closely we raise to 9.8 before adding a cationic polymer and gravity flow to a Lamella clarifier where an anionic polymer is added in the flocculation tank. The system is a continuous flow system which runs about 40 gpm. We raise the pH with sodium hydroxide. The cleaners are usually recovered for reuse but on occasion tanks are overflowed or inadvertently dumped to WWT. On these occasions it takes us several hours (sometimes days) to recover from the cleaner influx. We have about 30,000 gallons of capacity in our equalization tanks and the cleaner still causes us problems. Our polymer supplier suggested a calcium magnesium blend product to help with the problem but we see little effect when this is used. They have also mentioned carbamate but we are hesitant to use this due to it's toxicity.

answer by Dedalus

A pH of 9.8 is a little too high for Zn. When I used to treat a lot of cyanide zinc rinses I used 9.3 as my control level.
Sobisch offers good advice - dose with iron salts. I would add: bring the pH down to 3 - 4 before doing so. This tends to protonate things like organic acid salts and EDTA, disrupting metal complexes. If when the pH is raised again, there is a large excess of ferrous or ferric ions, the chelants will latch onto those, leaving the target ions free to be precipitated as hydroxides.

question by Catman

What effect will the iron salts have on the amount of sludge generated? Our filter cake is classified as non-haz and costs little to dispose of, but our filter press is about at capacity. Just wondering if we would have to add a bigger press. At present we aren't lowering the pH of the incoming stream. It generally is around the 5 - 5.5 range. At one time we were having a problem with small spikes of cadmium but have since eliminated the source. We were running the pH at 9.8 - 9.9 to try and catch the zinc and the cadmium. Since the cadmium is no longer an issue should we lower the pH regardless of whether we add the iron salts?

answer by Dedalus

A modest dose of ferrous sulfate at the incoming pH ought to help you. I would avoid the use of ferric salts, which will precipitate immediately at the pH you indicated, giving little benefit from coprecipitation.

You will see some increase in sludge volumes from this addition. The main thing is to be moderate and use only as much ferrous sulfate as is needed to produce the desired effect.

Can you batch treat those cleaners? They are notorious for causing system upsets. I always excluded them from continuous flow systems I was responsible for running.

And, don't forget to change the final precipitation pH. You will do much better with Zn at 9 - 9.3.


answer by Superglide

I have a little experience with Chelants giving me problems. The stuff is in everything. Some worse than others. I use a chemical called Clearmet. It's really just organic sulfur, but it is not pH dependent. I have found that just relying on solubility curves with pH, will not solve your problem. That only seems to work in the lab. Try as much product substitution upstream as possible.

comment by Dedalus

Re: Solubility products, solubility curves, etc. Superglide is entirely right. They will not reliably predict what will work. If I had a nickel for every time some...person from the front office came to visit me in the field with a table of theoretical data that was supposed to solve my problems, I could retire. My pH 9.3 value was derived from actual batch treatments, not textbook data.

What that theoretical stuff is good for is not telling you what will work, but what absolutely will not work. This can cut down the number of bench tests and sour batches considerably.