Evaluation of the effects of varying the water velocity at the entrance of the contact zone on the performance of a dissolved air flotation (DAF) unit using the image analysis method

For a more efficient design of dissolved air flotation (DAF) units, it is important to deepen our knowledge of the role that each constituent part plays with regard to overall performance. The velocity control of the flocculated water at the entrance (Ve) of the contact zone of DAF units is important because the degree of agitation in this region may affect the size of the flocs and the efficiency of the flotation process. This article shows the results of an experimental study conducted to verify the influence of Ve (and its associated average velocity gradient, Ge) on the performance of a DAF pilot plant (DAFPP) fed with synthetic water. Two different coagulation conditions of the same raw synthetic water were tested, and 11 assays with different Ve values for each condition were performed. The floc size distribution curves regarding two limit situations, the lowest and the highest Ve values, were obtained by using an appropriate image analysis method. The best results were obtained for Ve values ranging from 125 to180 m·h⁻¹, and for Ve values outside this range, the decrease in DAFPP performance was not as substantial as expected.