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AERATION AND AIR STRIPPING 5.7

Draft-Tube Aerators. A draft-tube aerator consists of a submersible pump that rests on
the bottom of a basin and is equipped with an air intake pipe extending to above the wa-
ter surface. The partial vacuum created by the pump pulls air through the tube and mixes
it with water at the pump intake. Aerated water is then directed outward along the floor
of the basin. This type of aeration is an inexpensive and relatively effective means of
adding aeration to an existing basin.
In-Well Aeration. A variation of diffused aeration to remove VOCs from groundwater
supplies is in-well aeration. This technique has been investigated by the North Penn Wa-
ter Authority, a public water utility located in Lansdale, Pennsylvania. The Authority re-
ceived a grant from the AWWA Research Foundation to conduct detailed tests on the fea-
sibility of in-well aeration.
Aside from obvious advantages of this type of treatment system, several disadvantages
exist. One disadvantage is the dissolution of large quantities of air into water, causing wa-
ter to appear milky. For practical use of this type of aeration system, water needs some
atmospheric contact time to allow the milky appearance to disappear before it is pumped
into the distribution system. The study found this treatment method to have relatively low
efficiency.

Mechanical Aeration
Mechanical aerators employ motor-driven impellers alone or in combination with air in-
jection devices.

Surface Aerators. Mechanical surface aerators are used extensively in wastewater ap-
plications for supplying oxygen to water. To a lesser extent, they are used to control taste
and odor problems in water treatment and are commonly installed at a reservoir rather
than at the treatment plant. They generally consist of an electric motor suspended on a
float, with a driveshaft operating a propeller located a short distance below the water sur-
face. The water is drawn up by the blade and thrown into the air in tiny droplets so that
the water can pick up oxygen.
A variation in design is the surface aerator equipped with a draft tube extending be-
low the propeller. With this design, water is drawn up from near the bottom of deeper
basins.

Submerged Aerators. Submerged aerators operate in the reverse of surface aerators. The
submerged blade draws water downward and, in the process, draws in air, which is dif-
fused into the water. This type of aerator results in relatively calm water at the surface
compared with surface aerators. Submerged aerators are best used for increasing dissolved
oxygen levels.

Pressure Aeration
There are two basic types of pressure aerators, and the object of both is to aerate water
that is under pressure. In one type, water is sprayed into the top of a closed tank while
the tank is continuously supplied with compressed air. Aerated water leaves at the bot-
tom of the tank. In the second type of pressure aerator, compressed air is injected directly
into a pressurized pipeline and adds fine air bubbles to the flowing water.
With both systems, the higher the pressure used, the more oxygen dissolves into the
water. Pressure aerators are primarily used for oxidizing iron and manganese for subse-
quent removal by settling, filtration, or both.