The mechanism of dissolved air flotation for potable water: basic analysis and a proposal

The principal mechanism of flotation of suspended solids in natural waters is by collision of free micro-bubbles with flocs and attachment at hydrophobic sites. Such sites are formed by adsorption of naturally-occurring organic compounds on to the precipitated alumina (or ferric “hydroxide”) derived from the coagulant added.Although effective, the current flotation process uses supersaturated water inefficiently, mainly because the average bubble size obtained is above the optimum. Unfortunately, no method is known for generating a dense cloud of micro-bubbles (< 50 μm) without adding surfactants.A more economical procedure might be based on mixing bubble-free super-saturated water with the chemically treated water, followed by conditioning in a flocculator under moderate shear. Under these conditions bubbles grow from nuclei within the flocs, which can subsequently be floated in a quiescent tank. Bench tests indicated that the method is feasible, the growth time of the bubbles being comparable with that needed for good flocculation.