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"Beef plant effluent sedimentation"

October 11, 2016

Q. Hello,

I would like to introduce chemicals to a sedimentation tank in order to significantly reduce (>90% reduction) Total Suspended Solids, BOD & Turbidity in beef plant wastewater. The plant consist of:
Rotary Screeners ==> Fat Seperation ==> Sedimentation Tank ==> Bioreactor ==> Fat Traps.
I have done bench jar tests and found FeCl3 to coagulate nicely around 600-650 ppm. The challenge now is in settling the flocs formed. I have found that the best settling takes place around pH of 6.5-7.5 with the aid of 2 ppm anionic flocculant. To get this pH, I had to add up to 200 ppm NaOH since the alkalinity in the water is all consumed by the FeCl3. However, the jar tests results are not consistent over different days when samples are taken. I can't seem to reproduce the good settling rates I get on some days. Any ideas how I can overcome this? Any pointers as to better/more consistent coagulant & flocculant aids?

I'm thinking of taking a chance with the plant trial even though the jar tests are not entirely convincing.

Oswell Kapotsa
Computer Programmer - Namibia

October 12, 2016

A. You might try using either magnesium or calcium hydroxide instead on NaOH for your pH adjustment Both have good coagulant properties.

You might also try a different flocculant.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

October 2016

A. Hi Oswell. Dave's advice is definitely sound. Calcium hydroxide (lime water) is simply better than NaOH at reducing everything in your effluent. Magnesium hydroxide performs similarly, and has the advantage that you can't overdo the neutralization and go excessively alkaline; but remember that MgOH2 is a slurry, not a solution (solubility <0.001 g/l), so you need very long retention time for it to be used effectively.


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

October 13, 2016

A. My recommendation would be to use lime, with the following precaution. lime is heavy and will help in sedimentation if it doesn't completely dissolve. Lime sludges also de-water well. But, lime dissolves slowly so you need to allow 30-45 minutes of reaction time.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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