91Dissolved Air Flotation (DAF)
Dissolved air flotation (DAF) is another clarification process in which air is diffused as fine bubbles
and suspended particles are floated to the surface and removed by skimming.
Generally, DAF is most effective for small, fine, low-density particles like algae and may not be
effective is all instances. Like conventional sedimentation, solids not removed by DAF are removed
during filtration.
Clarification Process
Two parameters frequently used to describe the clarification process are the overflow rate and the
detention time. The overflow rate is the process loading rate and is usually expressed in gpm/sf or
gpd/sf. Overflow rates for conventional sedimentation generally range from 0.3 to 1 gpm/sf (500 to
1500 gpd/sf).
Overflow rates for other processes can vary significantly. There are proprietary sand-ballasted
clarification systems that have been demonstrated to operate effectively at overflow rates as high
as 20 gpm/sf. Typical detention times range from 1 to 2 hours, although many states require up to
4 hours for full-scale surface water treatment.