We were recently involved in an industrial waste water project for the removal of Molybdenum. We conducted jar testing with pH adjustment, addition of coagulant and a flocculent along with a body feed of bentonite clay. We were successful at clarifying the waste water and removing the molybdenum in the jar testing. The jar testing went so easily, that we were surprised when it failed on a pilot scale.
The pilot plant was a continuous flow treatment system with three 36 gallon CSTRs (continuously stirred tank reactors) followed by simple filtration. Chemicals were added to the first tank, a rapid mix tank, followed by the next two tanks at much lower mixing speed. When we did not achieve the removal of Molybdenum we began to look at the chemistry. Why does it work in the jar testing but not in the pilot testing? Because the jar testing was batch operation and the pilot testing was done as a continuous flow operation we suspected the flow rate was the issue.
We altered the flow rate to change retention time and we experimented with different chemical dosages, different mixer speeds all with no success. Why was this happening? When we looked into the top of the reactor tanks, the waste water appeared to be mixing well and when we increased the mixer speed we sheared the floc. We then decided to change the impellers from 3.5 inches to 5.0 inches to give more mixing energy at the same rpm. This simple change made the difference. The larger impeller increased the mixing energy without shearing the floc. Problem solved.
Mixing energy is critical for a successful waste water treatment system. Mixing must be robust enough to get the particulate collisions to form a floc but not so much as to shear the floc. This slight adjustment was all it took.