Dissolved Air Filtration Flotation

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Dissolved Air Filtration Flotation (DAFF) is a water treatment process used as an alternative to sedimentation. Due to certain inlet (raw) water quality characteristics (e.g. high colour and low turbidity), traditional sedimentation techniques are not suitable and thus, the DAFF process is preferable.

[edit] Basic Process

Raw water enters the plant where it is initially dosed with aluminium sulfate (incorrectly but commonly known in the industry as alum) to coagulate particles in the water together. The water will then pass into a rapid mixing tank where the alum is dispersed throughout the water. Either here or in the flocculation tanks the water is dosed with polyelectrolyte (poly) to aid coagulation. The water then will pass into several flocculation tanks. This is where the DAFF process differs from the sedimentation process.

In the typical sedimentation process the flocculant (floc particles) - the coagulated material - is allowed to fall to the bottom in large sedimentation tanks. In the DAFF process, high pressure product water (from the end of the plant) is saturated with air and then released up into the flocculation tanks. This causes a sudden pressure drop which releases small bubbles of air. These bubbles effectively 'lift' the flocculant to the surface of the flocculation tank. This flocculant then remains on the surface where it is either scraped off or overflows (by deliberate raising of the tank water level).

The water then travels down through a filter which is typically either sand or multimedia (anthracite-sand-gravel). This removes any floc particles which didn't properly rise to the surface.

The rest of the process is the same as the sedimentation process whereby the sludge (floc particles and water) is further dried and then this water (supernatant) is returned to the front of the plant. Also, the product water is then adjusted for pH and fluoride added. Note: these two steps may differ slightly between plants (e.g. sludge put down the sewer where it is treated).

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