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7.38 CHAPTER SEVEN

Recycle System
About 5% to 10% of clarified water is recycled into the air saturation system by recycle
pumps. Recycle water is pumped through an eductor that introduces high-pressure air in
the line from the air cushion of the saturation tanks. The air-water mixture is then dis-
charged into the bottom of the saturation tank. The eductor increases the efficiency of air-
to-water transfer by increasing contact time between the two media.
Air is supplied from an air compressor controlled by the saturation tank water level.
Operating efficiency of this type system is in the 80% to 85% range and may be improved
by using packed bed saturation tanks to increase the air-water interface. Packed bed sat-
urators are more expensive, but their greater efficiency may make them more economi-
cal in large plants. Saturated water flows from the saturation tank through a system of
headers to the proprietary recycle dispersion system, where the pressure is dropped to
atmospheric and microbubbles are created.

Design Criteria
The size of DAF tanks is based on the surface loading rate. Standard practice has been
to design in the range of 4 to 5 gpm/ft 2 (10 to 12 m/h), although higher rates may be pos-
sible. Edzwald et al. have reported that rates as high as 12 to 16 gprn/ft 2 (28 to 40 m/h)
may be possible depending on water temperature (Edzwald et al. 1999). The use of pilot
plant studies is recommended for rates higher than standard (Grubb, Arnold, and Harvey,
1994). Ondeo Degremont markets the AquaDAF system as being capable of handling
loading rates of up to 20 gpm/ft 2 (50 m/h) or more. The efficiency of AquaDAF is said
to be based on a proprietary effluent collection system.
Design should allow for a recycle rate of 5% to 10% of plant flow at a dissolved air
pressure of 60 to 90 psi (414 to 620 kPa). Recycle rate and air pressure should be ad-
justable to allow for process optimization.
The recycle dispersion system must be selected to evenly distribute recycle flow across
the width of the unit into the flocculated water flow.
One unique use of DAF is "stacked DAF," where the flotation and filtration take place
in the same basin. In this case the water depth above the filter media is such as to allow
flotation to take place with solids being removed at the surface. Flotation effluent goes
directly to the filter. DAF loading rates in this case are limited to the rates the filter can
handle.

Basin Dimensions
Basin dimensions may be flexible; the length-to-width ratio is not important. However, a
maximum length of about 40 ft (12 m) is recommended, because in this distance, all the
air bubbles would have typically risen to the surface. Tank depths are usually around l0
ft (3 m). If tanks are too shallow, forward velocities could carry air bubbles into the ef-
fluent. Tanks could be deeper, but there is no added benefit for increasing depth.

Influent Design
As noted above, flocculated water must be introduced uniformly along the width of the
tank near the bottom. This can be done by means of a continuous slot or through uni-