DAF

DAF is a purely physical process which operates based on a reasonably simple design philosophy. Incoming effluent may require pre-treatment as necessary, e.g. the addition of chemical coagulant(s) and/or flocculant (s) may be required with associated mixing and coagulation/flocculation stages. Adjustment of pH may also be a consideration to ensure optimum conditions for coagulation and flocculation.

DAF system may be designed for pressurization and air dissolution of the total flow or, more commonly, the incoming effluent enters the flotation vessel where it comes into contact with a portion of recycled, treated effluent (sometimes termed whitewater). The percentage of the total effluent flow into which air is dissolved under pressure and subsequently recycled will be determined by several factors. Increasing the pressure within the vessel where the air is being dissolved ensures that a higher concentration of air dissolves into the liquid phase than is possible at atmospheric pressure. Once this portion of saturated effluent enters the flotation tank, the pressure is released back to atmospheric pressure. This immediately results in the recycled flow becoming supersaturated, resulting in the generation of micro bubbles as the dissolved air comes back out of solution. These bubbles attach to, and form within, the solids or chemical flocs entering the vessel, causing them to float to the surface where they are retained and subsequently removed by a mechanical skimmer.

In the case of rectangular flotation tanks, the skimmer mechanism consists of a series of paddles or 'flights' which run on a belt or chain and skim just below the surface of the tank removing the 'float' into a trough for further treatment or, in some instances, recovery of materials. The alternative of circular DAF tanks may incorporate rotating skimmer blades feeding a 'float' trough or involve use of a circulating, revolving scoop. In cases where some gross solids may be present and there is risk of gradual accumulation of sludge build-up on the flotation tank floor, the design may also incorporate a floor scraper.

There are limits to what can efficiently be removed by applying flotation technology. It would seem a logical step therefore to apply DAF systems to effluents where the solids present are of approximately neutral or perhaps even positive buoyancy so that the bubbles produced are working with gravity rather than against it. Under these circumstances DAF systems would, on first approximation, appear to be a process worthwhile of consideration should standard sedimentation systems not provide the required removal of contaminants.