Civil Engineering Reference
In-Depth Information
The quantity of air required for good DAF performance depends on the raw water
quality and the treatment steps upstream of the flotation tank.
19,23
Generally, DAF
plants recycle between 5 and 15 percent of the plant flow through the air saturation
system. For a saturator pressure of 80 psig (550 kPa [gauge]), saturator efficiency of
70 percent, and water temperature of 50
F (10
C), recycle ratios of 5 and 15 percent
correspond to bubble volume concentrations of 3,000 and 8,100 ppm, respectively.
Advantages and Disadvantages of DAF
DAF has several advantages over conventional sedimentation. These advantages can
mean better performance of the unit process and overall plant, as well as lower capital
and operations costs.
•
More compact design.
The higher loading rates used with DAF allow for much
smaller tanks than those used in sedimentation. Although the saturation system in a
DAF plant requires space that is not needed in sedimentation, the overall footprint of
the DAF process is still substantially smaller.
•
Shorter start-up time.
The smaller tanks used with DAF result in good effluent
quality in less time. Typical start-up times for DAF are about 45 min, whereas a
sedimentation basin may require hours to produce a good clarified water.
•
Better performance.
For many water quality types, DAF produces a superior ef-
fluent than sedimentation. This is particularly true for waters containing low-density
particles.
•
Lower chemical dose.
In many cases, DAF requires less coagulant than sedimen-
tation for optimal performance. Unlike sedimentation, DAF does not rely on the for-
mation of large, dense floc for effective particle removal.
•
Shorter flocculation time.
DAF typically performs well with pinpoint-size floc.
Flocculation times for DAF are therefore one-half to one-fifth those used with sedi-
mentation.
•
Thicker sludge.
The floated sludge from a DAF tank normally has a much higher
solids concentration than sludge produced from sedimentation.
Although DAF offers several advantages over conventional sedimentation, the con-
ditions of each individual application must be considered to determine the best process.
Sedimentation is more appropriate for some types of water quality, particularly waters
with high turbidity and those requiring softening. In addition, where land is available,
the size of sedimentation basins may not be an issue. The mechanical equipment used
in DAF requires a higher level of operator skill and more maintenance than sedimen-
tation. These aspects may be undesirable to a utility.
The relative costs of DAF and sedimentation depend on several factors, including
the plant capacity, location, and water quality. When comparing the two processes, a
utility needs to consider both capital costs and operation and maintenance (O&M)
costs. The capital costs are lower for the smaller DAF tanks, but equipment and equip-
ment housing costs are higher. For a new plant, the smaller flocculation basins needed
for DAF may make for lower overall capital costs. Energy costs are higher with DAF
because of the recycle / saturation system, but lower chemical doses often offset the
energy costs.
