Polymer Dosage Control in Dissolved Air Flotation

Abstract

Using a three‐cylinder batch unit with an incorporated dissolved oxygen probe, as well as a full‐scale continuous flow unit, for DAF tests, the relationship between polymer residual in the effluent and general performance was studied. A simple titrimetric procedure, sensitive to 1 mg/L, was used for residual polymer measurements, and the Zeta potential for the effluent solids was also obtained. It was found that below a polymer dose of about 4 mg/g of dry solids for the batch unit, there was no improvement in the thickening performance (or float solids concentration), that no residual polymer could be detected in the effluent, and that there was no effect on the negative Zeta potential. Above this dose, there was a step‐wise improvement in the thickening behavior, no further reduction in effluent SS, a detectable residual polymer, and a change to a positive value of the Zeta potential. A similar pattern was observed in full‐scale tests. Thus, it is suggested that polymer residual or Zeta potential measurements could be used for polymer dosage control. Both methods are quicker than the SS measurements, can differentiate between lack of polymer or another cause of performance deterioration, and show how much polymer overdosing had occurred.