Page 180 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.16 CHAPTER FOUR

TECHNOLOGY AND COMPONENT
DESCRIPTION

AERATION

Aeration is a gas transfer process in which water is brought into contact with air for the
purpose of transferring volatile substances to or from the raw water. It is used most often
to remove undesirable gases such as carbon dioxide, hydrogen sulfide, and methane.
Aeration, by introducing oxygen, is also used to remove iron and manganese and to lower
the amount of volatile organic compounds (VOCs) from ground water. The following are
criteria for its use:
1. Reduction of carbon dioxide by 90 percent is obtained by near saturation with oxygen.
Carbon dioxide dissolved in ground water will consume lime in the lime-soda soften-
ing process without any accompanying softening. Generally accepted practice indicates
that aeration is not economical for carbon dioxide concentrations in water of less than
10 mg/L. The cost of the lime saved should be compared to the cost of purchasing and
operating the proposed aerator.
2. Aeration will partially remove VOCs from raw water by oxidation, making them insoluble.
They would then be coagulated and removed from the water.
3. Aeration alone could be used for the removal of hydrogen sulfide in concentrations
of 2 mg/L or less. Above this level, it could be used in conjunction with chlorination,
which oxidizes hydrogen sulfide.
4. Iron and manganese can be removed by aeration if this cannot be done by other methods.
These metals are oxidized to form insoluble hydroxides which precipitate out at proper
pH levels. They can then be removed by settlement or filtered out of the water stream.
Each ppm of dissolved oxygen will oxidize about 7 ppm of iron or manganese.
Several types of aerators are commonly used: waterfall, diffusion (or bubble), and
mechanical. The most commonly used system in utility water treatment is the waterfall.
Waterfall aerators are made in several types: cascade, spray nozzle, and multiple tray.
They operate by having the raw water enter the unit from the top and fall by gravity to the
bottom, using various methods to evenly distribute and disperse the water throughout the
unit. The most common waterfall aerator is the naturally ventilated, multiple tray type. This
unit consists of a series of trays, one above the other, with perforated, slot, or mesh bot-
toms. The trays are filled with 2 to 6 in (40 to 140 mm) of a medium such as coke, stone, or
ceramic balls to improve water distribution and gas transfer as well as providing catalytic
oxidation in the medium. The vertical openings between trays range from 12 to 30 in,
depending on the number of trays required. Water loading on the trays is usually in the
2
range of 10 to 20 gpm/ft . Efficiency can be improved by the use of enclosures and forced
air blowers to provide counterflow ventilation.
The simplest type of aerator is the diffusion type which bubbles compressed air up
through the water tank. The large volume of air required limits the diffusion aerator to
smaller flows of water. Air requirements vary from 0.1 to 0.2 SCFM per gallon of water
aerated. A detention time is necessary, ranging from 10 to 30 mm. Advantages of this type
of aerator are the freedom from cold weather operating problems, very low head loss, and
the possibility of using this process for chemical mixing.
The mechanical aerator consists of an open impeller operating on the water surface of a
tank. It is not as efficient as either of the two systems previously described, and so, longer

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