Dissolved Air Flotation

May 31, 1995
Rev. April 1999

Dissolved Air Flotation (DAF) is one of the common wastewater treatment systems. It is used in a variety of industries to remove fine suspended solids. This is achieved by pumping the wastewater into a large rectangular tank where three elements are combined.

The three elements are the suspended solids, air bubbles, and a flocculent. The flocculent is usually a polymer in the form of a long chain molecule. The suspended solids in the wastewater become attached to these molecules. At the same time, air pumped into the bottom of the tank forms air bubbles which rise to the water surface. Agglomeration occurs between the three elements.

As the air bubbles rise to the surface of the tank they form a layer of sludge from one to three inches thick. Traveling bars are dragged across the top of the tank at the water surface level. This drags the sludge to and over the far edge of the tank.

Sludge from DAF systems is always very high in moisture content, typically 95% or more. This sludge is so wet that free water runs from it, making it messy to handle.

Most sludge is sent to landfill. In the case of paper recyclers, the sludge is usually blended into lower grade paper products, like egg containers and paper trays. Some industries have wastewater that can be treated with food grade flocculents, so the sludge can be made into animal feed. Failing this alternative, most sludges end up in landfill.

Dewatering sludge in a screw press can easily reduce the physical volume by two thirds or more (if it works). Also, the press cake formed in the press does not drip, making the solids suitable for handling, transporting and storage. Because of this there is a great demand for dewatering DAF sludge with a screw press.

The small Model CP presses are suited for certain DAF sludges. We have had success dewatering DAF sludge in paper mills where the biological content is minimal. Plastic recyclers, because of the fiber content in their waste stream, are a good application. This contrasts with zero success on DAF sludge from slaughterhouse wastewater treatment facilities. Only with testing can we tell if it will work.

Issue 27