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Slake lime consumption calculation

I'm working in a waste water treatment plant of a copper smelter plant. The major heavy metal contents in the waste water are As (about 1000 mg/L), Cu (500), Fe (150), and Cd (100). Others elements, like Zn, Ni, Se, are below 100 mg/L. Daily volume was about 700 m3. Same as other common industrial waste water treatment process, we utilize slake lime solution (made from 88% Ca(OH)2 mix with water) to increase the pH and catch the heavy metals into the sludge. The pH increment is from 2.20 to 11.40. We plan to evaluate the reasonability of the actual slake lime powder consumption compared to the theoretical consumption calculation. Our actual consumption was about 750 T of 88%Ca(OH) powder each month. What is the best & simple formula to find out how many Ca(OH)2 powder is needed to increase the pH? Were there any important matter that should be considered in calculating Ca(OH)2 powder needed to react with those heavy metals, besides the concentration and the waste water ! flow?

Tommy Nugroho
- Indonesia

pH is a measure of how much ionization is taking place, but it is not a measure of total acidity or alkalinity. The way to complete your assignment is not to do theoretical calculations based on pH, but to do bench testing of a tiny slipstream of the actual effluent.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey


Thank you Mr. Mooney for your sincere response! Prior to seek the consumption formula, my team had conducted small test by adding gradually the slake lime powder into 500 ml actual waste water. At that time, pH increment was from 2.10 to 11.20. The test consumed about 4 grams of powder, so, it was about 8 kg slake lime/T waste water. Comparing to the plant condition from our monthly inventory, we had used about 10 kg slake lime/T waste water. So, the test result and the actual had a small discrepancy.

Then, more detail question was aroused from that test, i.e. could this consumption be described mathematically (to judge its reasonability)? Unfortunately, as I already mentioned on my previous letter, I could not find a good formula to calculate it.

Tommy Nugroho
- Indonesia

I don't think you will find such a formula. What you did in your best test is called "titration" and is widely used in all fields that employ chemistry, and I don't think there is any practical theoretical substitute for it.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

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