How does dissolved air flotation (DAF) thickening work, and what types of sludge are best suited for this process?

Dissolved air flotation (DAF) thickening works by injecting fine air bubbles into the sludge, which attach to the suspended solids, causing them to float to the surface where they can be removed. Sludge thickening is a process used to increase the solids concentration of sludge, reducing its volume and making it easier to handle and treat. In DAF, pressurized water is saturated with air and then released into the sludge at atmospheric pressure. This rapid pressure drop causes the dissolved air to come out of solution in the form of very small bubbles, typically 20-100 micrometers in diameter. These microbubbles attach to the suspended solids in the sludge, reducing their overall density. The bubble-solids aggregates then become buoyant and float to the surface, forming a thickened sludge layer that can be skimmed off. The clarified water is removed from the bottom of the DAF tank. DAF thickening is particularly well-suited for sludges that are difficult to settle, such as activated sludge from wastewater treatment plants. Activated sludge often has a low density and a high water content, making it challenging to thicken using gravity settling alone. DAF is also effective for thickening algae-laden water or sludges containing oily or greasy materials. Chemical addition, such as polymers, is often used to enhance the DAF process by improving the attachment of air bubbles to the solids. For example, a DAF system might be used to thicken waste activated sludge from a secondary treatment process, reducing its volume before it is sent to anaerobic digestion.