ABSTRACT
The flotation thickening process feeds air into the sludge to be thickened under pressure (40 to 80 psi), so that a large amount of air can be dissolved into the liquid.
Air is dissolved into the wastewater, under pressure, in a retention tank by means of an air compressor. This air/water mixture is injected into and mixed with the sludge.
The air saturated sludge then flows into an open tank where, at atmospheric pressure, much of the air comes out of solution as tiny bubbles that attach to sludge particles and float them to the surface.
As additional solids rise, compaction occurs, resulting in a layer of thickened sludge or “blanket” at the surface of the tank.
This thickened sludge is removed by skimmers that continually skim off the upper surface of the DAF tank sludge blanket.
Part of the DAF effluent is pumped through the retention tank. Air is fed into the pump discharge or into the retention tank.
The flow through this recycle system is controlled by a valve. Effluent recycle can range from 50 to 300 percent of the influent flow. This recycle flow and sludge feed are mixed in a chamber at the entrance to the DAF unit.
Polymer, due to its ability to bridge individual sludge particles, is often used to improve DAF performance and solids capture. If a polymer flotation aid is used, it is usually fed to the mixing chamber.
Clarified effluent flows under and over a weir, and bottom sludge collectors are used to remove any settled sludge or grit.
