Civil Engineering Reference
In-Depth Information
Design
Contact chambers are usually rectangular concrete tanks. Most often, the
system is operated with countercurrent flow, with water entering the top and exiting
at the bottom while exhausted air exits at the top.
Typical diffusers include perforated pipes, porous plates or tubes, and patented
impingement or sparger devices. The diffusers are located along the bottom of the
tank or are placed along one side of the tank to impart a spiral flow. A spiral flow
allows higher velocities to promote gas transfer. Porous tubes or plates are often located
mid-depth in the tank to reduce air compression head. Filtration of the air may be
necessary to avoid clogging.
Typical design ranges for diffused aeration systems are listed in Table 9-5. The
aim of the diffused aeration system is to distribute air uniformly through the water
cross section and to produce the desired air bubble size. The removal efficiency of
diffused aeration for stripping compounds can be improved by any of the following
actions:
Decreasing the bubble size. Finer bubbles are more efficient than coarse bubbles
because of their greater interfacial area; however, finer bubbles are more expensive
to produce and may coalesce into large bubbles, which would eliminate any ad-
vantage.
•
Increasing the water depth
•
Increasing detention time
•
Increasing the volumetric air-to-water ratio
•
Improving chamber hydraulics to prevent short-circuiting. Lateral baffles may be
used to prevent short-circuiting.
•
After aeration, the water is saturated with dissolved oxygen. The water is therefore
sent to a wetwell or inlet chamber configured to allow for the air bubbles to be released.
Performance
Table 9-6 presents removal efficiencies data for five VOCs and nine
synthetic organic chemicals (SOCs). Removal rates are based on diffused aeration units
with air-to-water ratios of 5:1 to 15:1 and contact times of 10 to 15 minutes. The
removal rates range from 11 to 95 percent for the 14 compounds.
TABLE 9-5. Typical Design Values for Diffused Aeration
Design Parameter
Value
Tank depth
9 to 15 ft (2.7 to 4.6 m)
Tank width-to-depth
ratio for effective
mixing
2
Tank length
10 to 30 ft (3.0 to 9.1 m)
Detention time
10 to 30 min
Air required
0.01 to 0.15 ft
3
per gal treated
(74.8 to 112.2 m
3
per ML treated)
Power requirements
0.5 to 2.0 kW (typically 1 kW) per mgd of plant capacity
(0.13 to 0.52 kW [typically 0.26 kW] per ML / d of plant capacity)
