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AERATION AND AIR STRIPPING 5,11

ing literature can be applied to water treatment for trace organics removal. Based on prin-
ciples described in the last section, the design of an air stripping installation is primarily
dependent on the following factors:
• Temperature of the water and the surrounding air
• Physical and chemical characteristics of the contaminant to be removed
• The ratio of air to water being provided in the process
• Contact time between the air and water
• The water surface area available for mass transfer
The first two factors are fixed by source water quality and location of the installation.
The other factors can be varied with the type of aeration equipment used. For all aeration
and air stripping equipment, the water exiting the treatment equipment must be disinfected
before the water is pumped to the distribution system.

Design of Diffused Air Equipment
When diffused aeration is employed, air stripping is accomplished by injecting bubbles
of air into the water. Ideally, diffused aeration is conducted counter to the flow of water.
Untreated water should be entering at the top and treated water exiting at the bottom while
fresh air enters at the bottom and exhausted air exits at the water surface. Gas transfer
can be improved by increasing the basin depth, producing smaller bubbles, improving the
contact basin geometry, and incorporating a turbine to produce smaller bubbles and in-
crease bubble holdup.
Diffused air aerators usually provide a longer aeration time than waterfall aerators,
generally an advantage, but other factors influencing performance are the turbulence pro-
vided, air-volume ratio, and gas-transfer resistance. Because of these factors, comparison
between the two types of equipment cannot be made solely on the basis of aeration con-
tact time.
Basin Design. Tanks used for the diffused air process are usually made of concrete and
are commonly 9 to 15 ft (3 to 5 m) deep and 10 to 30 ft (3 to 9 m) wide. The ratio of
width to depth should not exceed 2, to achieve optimum mixing. Tank length is governed
by the desired detention time, which usually varies from 10 to 30 min. Air diffusers are
generally mounted along one side of the tank to impart a spiral flow to the water. A spi-
ral flow pattern produces higher water surface velocities, which in turn promotes better
gas transfer. In addition, with a spiral flow, a substantial number of bubbles do not es-
cape immediately, but are carried across the basin where they are held in a more or less
fixed position by the descending water.

Diffusers. Common types of diffusers are perforated pipes, porous plates or tubes, and
various patented impingement or sparger devices. Compressed air is generally furnished
by a rotary compressor sized to produce the correct volume and pressure. Diffusers pro-
duce small bubbles that rise through the water and cause turbulence and the opportunity
for the exchange of volatile materials.
Diffusers are generally located near middepth in the tank, usually about the optimum
efficiency point. Deeper location of the diffusers requires greater pressure head, which in-
creases compressor power costs. The amount of air required ranges from 0.01 to 0.15
ft3/gal (0.0008 to 0.012 m3/L) of water treated. Sufficient diffuser capacity must be pro-
vided to supply air at the required rate without excessive pressure loss. Some installations
include lateral baffles to prevent short-circuiting.