Letter 0016

Waste Water Treatment

----- Our company paints and Zinc plates automotive parts. We usually segregate the waste water from both the departments. But by accident, all the water got mixed up, and now we are having problems with the precipitation of Zinc. By the way, we use hydroxide precipitation to remove metals. We have been able to get down the Zinc concentration to 4 ppm using various chemicals, but not any lower than that. Our discharge limits are 2.61 (daily) and 1.48 (monthly).   Various chemicals we tried :   Caustic soda, Lime, Magnesium oxide, Ferrous sulfate, Calcium chloride, Sodium borohydride, Calcium polysulfide, two proprietary organometallic polymers.   Any suggestions will be highly appreciated.   Thanks in advance. prasad s [last name deleted for privacy due to age of posting] Traveller Information Services ----- We assume these values are after filtration? If not, your problem is easily solved. If so, you have your hands full. My personal experience is that sulphide chemistry is often the most powerful precipitating method for zinc. Even though you have already tried calcium polysulphide, you may obtain success with DTC (sodium diethyl dithiocarbamate) or DeGusa's proprietary TMT15. Ted Mooney, P.E.  finishing.com Brick, NJ

Dear Mr. Mooney :  

Thank you for your reply. I forgot to mention, but we tried the carbamate too. Yes, we are filtering the effluent.   Since my last letter, we had successfully precipitated out zinc using Magnesium Oxide and a proprietary organometallic polymer 326-C. But now we are having problem removing Nickel. This is not caused by 326-c. We had the problem with Nickel to start with.   One more chemical we tried in the last couple of days is Ferric Chloride at around pH 8.0. Please suggest any chemicals to remove Nickel and at what pH, they work best.   Thanks again for your reply.   -Prasad.        

prasad surapaneni
Traveller Information Services

It is difficult and unwise to try to prescribe a solution to a problem when I haven't seen it; and by now you have quite a witch's brew to deal with! So I can't. All I can say is that: 1). In general, nickel precipitates best at high pH, about 10.5 2). I have read that magnesium hydroxide has to be 'topped off' with caustic or another stronger alkali when used for the treatment of nickel because it doesn't get the pH high enough. Good luck! Ted Mooney, P.E.  finishing.com Brick, NJ

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