Where the
world gathers for metal finishing
Q&As since 1989


-----

Dissolved Ni Removal

April 2, 2008

I am dealing with a greatly reduced discharge limit on Ni at my WWTP. Our discharge limit is now at 1ppm. We are normally at the .7-.9 range, occassionaly flirting at 1-1.1 .

Can I get the dissolved Ni to percipitate out to any degree by elevation of pH to 10.5 without the use of a polymer? If I can see a minimal reduction this would avoid a costly alternative for removal

Ray Krause
Manufacturing - Gurnee, Illinois, USA

If you are looking at getting the Ni to below 1ppm on a consistent basis you will probably require some type of polymer to aid with the coagaulation/flocculation. Standard hydroxide precipitation can get the Ni down to below 1ppm (provided there are no chelators present), but usually good water/solids separation is required to eliminate any solids in the final effluent. Increasing particle size (achieved through coagulation or flocculation) allows the precipitated particle to drop out of solution quickly, maximizing the effeciency of the hydroxide reaction, by reducing the possibility of having small floaters in the final effluent.

Gordon Djani
- Greenville, South Carolina
April 8, 2008

---

Odd that you're not using a polymer now - what's the objection? Surely it can't be the expense.

If you mean "precipitant", you might get somewhat better results, without one, by dosing in some ferrous sulphate ferrous sulphate [affil links] before the elevation of the pH; or, by using calcium hydroxide to elevate the pH instead of NaOH. Both would likely increase the sludge volume, possibly A LOT.

My approach to the problem would be to add a wee bit of ferrous, bring the pH to 8.5-9 with NaOH, then add sodium sulfide/polymer. I would routinely get Ni down to 0.2 ppm, with some waste that was pretty tough to treat.

Dave Wichern
Consultant - The Bronx, New York
April 8, 2008