DISSOLVED AIR FLOTATION (DAF)

DISSOLVED AIR FLOTATION PRINCIPLES:

Dissolved Air Flotation / DAF is a clarification process for the separation of solids, grease, and oils. It works by producing a stream of micro-fine air bubbles that attach to solids and float them to the surface where they can be removed by a scraping machine.

Dissolved Air Flotation, or DAF, has been used as a separation technique commercially since the early 1960's. During the 1970's the process of dissolved air flotation / DAF was refined to make it more reliable. With the developments made in recent years the current DAF designs are far advanced on old DAF systems in terms of reliability and process. The most popular, efficient and process friendly dissolved air flotation system is a rectangular tank design. The flow dynamics are better and the shape allows for connected reaction tanks pre and post DAF with less plumbing and easier and more compact site layout.

The heart of the dissolved air flotation / DAF system is the production of the "dissolved air" stream of micro-fine bubbles. To be effective these bubbles need to be <0.1mm. The bubble stream is produced using a recycle of treated effluent (usually around 50% of inflow). This recycled effluent is pumped to a pressure of around 450kPa in a vessel where it is mixed with compressed air that will dissolve at the elevated pressure. At the entry to the dissolved air flotation / DAF system the pressure is released, allowing the dissolved air to precipitate as a micro-fine air bubbles that attach to and float the effluent solids.

Traditional dissolved air flotation / DAF systems have been designed with a co-current flow pattern. That is, the sludge is scraped in the direction of the flow and the recycle stream is added at the DAF entry. Some dissolved air flotation / DAF systems have incorporated full length sparging systems to try to alleviate the common problem of sludge blanket agitation resulting in sinking flocs around the beach area. At K2 we employ a counter current scraping system. This means that the sludge is scraped to the inlet end of the DAF tank. This configuration has traditionally been used for 'difficult to float' Waste Activated Sludge (WAS DAF's).

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