Aeration

Aeration brings water and air in close contact in order to remove dissolved gases (such as carbon
dioxide) and oxidizes dissolved metals such as iron, hydrogen sulfide, and volatile organic
chemicals (VOCs). Aeration is often the first major process at the treatment plant. During
aeration, constituents are removed or modified before they can interfere with the treatment
processes.
Aeration brings water and air in close contact by exposing drops or thin sheets of water to the air
or by introducing small bubbles of air (the smaller the bubble, the better) and letting them rise
through the water. The scrubbing process caused by the turbulence of aeration physically
removes dissolved gases from solution and allows them to escape into the surrounding air.
Aeration also helps remove dissolved metals through oxidation, the chemical combination of
oxygen from the air with certain undesirable metals in the water. Once oxidized, these chemicals
fall out of solution and become particles in the water and can be removed by filtration or
flotation.
The efficiency of aeration depends on the amount of surface contact between air and water,
which is controlled primarily by the size of the water drop or air bubble.
Oxygen is added to water through aeration and can increase the palpability of water by removing
the flat taste. The amount of oxygen the water can hold depends primarily on the temperature of
the water. (The colder the water, the more oxygen the water can hold).
Water that contains excessive amounts of oxygen can become very corrosive. Excessive oxygen
can also cause problems in the treatment plant i.e. air binding of filters.
CHEMICALS REMOVED OR OXIDIZED BY AERATION
Constituents commonly affected by aeration are:

Volatile organic chemicals, such as benzene (found in gasoline), or trichloroethylene,

dichloroethylene, and perchloroethylene (used in dry-cleaning or industrial processes)

Ammonia

Chlorine

Carbon dioxide

Hydrogen sulfide

Methane

Iron and Manganese

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Carbon Dioxide
Surface waters have low carbon dioxide content, generally in the range of 0 to 2 mg/l. Water
from a deep lake or reservoir can have high carbon dioxide content due to the respiration of
microscopic animals and lack of abundant plant growth at the lake bottom.
Concentration of carbon dioxide varies widely in groundwater, but the levels are usually higher
than in surface water. Water from a deep well normally contains less than 50 mg/l, but a shallow
well can have a much higher level, up to 50 to 300 mg/l.
Excessive amounts of carbon dioxide (above 5-15 mg/L) in raw water can cause three operating
problems:

Increases the acidity of the water, making it corrosive. Carbon dioxide forms a weak
acid, H2C03 (carbonic acid).

Tends to keep iron in solution, thus making iron removal more difficult.

Reacts with lime added to soften water, causing an increase in the amount of lime needed
for the softening reaction.

Most aerators can remove carbon dioxide by the physical scrubbing or sweeping action caused
by turbulence. At normal water temperatures, aeration can reduce the carbon dioxide content of
the water to as little as 4.5 mg/l.
Hydrogen Sulfide
Hydrogen sulfide can present dangerous problems in water treatment. Brief exposures (less than
30 minutes) to hydrogen sulfide can be fatal if the gas is breathed in concentrations as low as
0.03 percent by volume in the air. The Immediate Dangerous to Life and Health level for
hydrogen sulfide is 300 ppm.
Hydrogen sulfide occurs mainly in groundwater supplies, and may be caused by the action of
iron or sulphur reducing bacteria in the well. The rotten-egg odor often noticed in well waters is
caused by hydrogen sulfide. Hydrogen sulfide in a water supply will disagreeably alter the taste
of coffee, tea, and ice. Occasional disinfection of the well can reduce the bacteria producing the
hydrogen sulfide.
Serious operational problems occur when the water contains even small amounts of hydrogen
sulfide:

Disinfection of the water can become less effective because of chlorine demand exerted
by the hydrogen sulfide.

Corrosion to piping systems and the water tanks, water heaters, and copper alloys.

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Aeration is the most common choice for removal of hydrogen sulfide from water. Hydrogen
sulfide has a low boiling point and vaporizes easily. The turbulence from the aerator will easily
displace the gas from the water; however the designer of the system needs to consider how the
gas is discharged from the aerator. If the gas accumulates directly above the water, the process
will be slowed and corrosive conditions can be created.
Methane
Methane is a colorless gas that is highly flammable and explosive and can be found in
groundwater. It may be formed by the decomposition of organic matter. It can be found in water
from aquifers that are near natural-gas deposits. Methane tends to make the water taste like
garlic. The gas is only slightly soluble in water, has a low boiling point, vaporizes easily, and
therefore is easily removed by the aeration of the water.
Iron and Manganese
Iron and manganese minerals are commonly found in soil and rock and can dissolve into
groundwater as it percolates through soil and rock.
Water containing more than 0.3 mg/l of iron will cause yellow to reddish-brown stains of
plumbing fixtures or almost anything that it contacts. If the concentration exceeds 1 mg/l, the
taste of the water will be metallic and the water may be turbid.
Manganese in water, even at levels as low as 0.1 mg/l, will cause blackish staining of fixtures
and anything else it contacts. Manganese concentration levels that can cause problems are 0.1
mg/l and above.
If the water contains both iron and manganese, staining could vary from dark brown to black.
Typical consumer complaints are that laundry is stained and that the water is red or dirty. Water
containing iron and manganese should not be aerated unless filtration is provided.
Oils and Algae By-Products
Many taste and odor problems in surface water could be caused by oils and by-products that
algae produce. Since oils are much less volatile than gases, aeration is only partially effective in
removing them.
TYPES OF AERATORS
Aerators fall into two categories. They either introduce air to water, or water to air. The water-inair method is designed to produce small drops of water that fall through the air. The air-in-water
method creates small bubbles of air that are injected into the water stream. All aerators are
designed to create a greater amount of contact between air and water to enhance the transfer of
gases and increase oxidation.

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Water-Into-Air Aerators
Cascade Aerators
A cascade aerator (one of the oldest and most common aeratrors)
consists of a series of steps that the water flows over (similar to a
flowing stream). In all cascade aerators, aeration is accomplished
in the splash zones. Splash zones are created by placing blocks
across
the incline. (They are the oldest and most common type of
aerators.) Cascade aerators can be used to oxidize iron and to
partially reduce dissolved gases.
Cone Aerators
Cone aerators are used primarily to oxidize iron and manganese
from the ferrous state to the ferric state prior to filtration. The
design of the aerator is similar to the cascade type, with the water
being pumped to the top of the cones and then being allowed to
cascade down through the aerator.

Slat and Coke Aerators

Slat and coke trays are similar to the cascade and cone aerators.
They usually consist of three-to-five stacked trays, which have
spaced wooden slats in them. The trays are then filled with fistsized pieces of coke, rock, ceramic balls, limestone, or other
materials. The primary purpose of the materials is providing
additional surface contact area between the air and water.

Draft Aerators
Draft aerators are similar to other water-into-air aerators, except
that the air is induced by a blower. There are two basic type of draft
aerators. One has external blowers mounted at the bottom of the
tower to induce air from the bottom of the tower. Water is pumped
to the top and allowed to cascade down through the rising air. The
other, an induced-draft aerator, has a top-mounted blower forcing
air from bottom vents up through the unit to the top. Both types are
effective in oxidizing iron and manganese before filtration.

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Spray Aerators
Spray aerators have one or more spray nozzles connected to a pipe
manifold. Water moves through the pipe under pressure, and leaves
each nozzle in a fine spray and falls through the surrounding air,
creating a fountain affect. Spray aeration is successful in oxidizing
iron and manganese and increases the dissolved oxygen in the
water.
Air-Into-Water Aerators
Pressure Aerators
There are two basic types of pressure
aerators. One uses a pressure vessel;
where water to be treated is sprayed
into high-pressure air, allowing the
water to quickly pick up dissolved
oxygen.
The other is a pressure aerator
commonly used in pressure filtration
Air is injected into the raw water
piping and allowed to stream into the
water as a fine bubble, causing the iron
to be readily oxidized.
The higher the pressure, the more
readily the transfer of the oxygen to the
water. The more oxygen that is
available, the more readily the
oxidation of the iron or manganese.

Centrifugal Aerators
Centrifugal aerators create enhanced conditions for dissolving gas into liquid phase, including
bubble size, and bubble size distribution and duration of interaction with liquid. Centrifugal
aerators combine several elements: 1. High turbulence swirling flow of liquid; 2. Orthogonal
flow of liquid and gas; 3. Constant pressure inside the vessel; 4. Optimum flow velocity
generating centrifugal forces thereby extending diffusion rate within the vessel; and 5. Very
small pores, through which gas permeates into the liquid and is sheered off into liquid phase,
thereby forming small bubbles.

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AIR STRIPPING
Air stripping can be quite effective in removing volatile organic chemicals (VOCs) from water.
Air stripping has been shown to be capable of removing up to 90 percent of the most highly
volatile VOCs. Water flows over cascade aerators, or in specially designed air-stripping towers.
In the air stripping towers, water flows down over a support medium or packing, while air is
being pumped into the bottom of the tower.
OPERATING
CONSIDERATIONS
Aeration raises the dissolved oxygen
content of the water. If too much
oxygen is injected into the water, the
water becomes supersaturated, which
may cause corrosion or air binding in
filters. Other problems with aeration
may include slow removal of the
hydrogen sulfide from the towers,
algae production, clogged filters, and
overuse of energy in some aerators.
Corrosion
A certain amount of dissolved oxygen
is present in raw and treated waters.
However, dissolved oxygen can cause
corrosion. Corrosion can occur
whenever water and oxygen come
into contact with metallic surfaces.
Generally, the higher the dissolved
oxygen concentration, the more rapid
the corrosion. The solution to this
problem is to not over-aerate. This
may be difficult because no definite
rule exists as to what constitutes
over-aeration. The amount of aeration
needed will vary from plant to plant
and will also vary with the season.

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False Clogging of Filters (Air Binding)

Filters in water containing a high amount of dissolved oxygen will have a tendency to release the
oxygen in the filter as it passes through. The process can continue until the spaces between the
filter media particles begin to fill with bubbles. Called air binding, this causes the filter to behave
as though it is plugged and in need of backwashing.
Slow Removal of Hydrogen Sulfide
Hydrogen sulfide is most efficiently removed, not by oxidation, but by the physical scrubbing
action of aeration. This removal is dependent on the pH of the water. At a pH of 6 or less, the
hydrogen sulfide is easily removed. If the water has a high pH, the hydrogen sulfide will ionize,
precluding removal by aeration.
Three basic control tests are required for aeration:

Dissolved oxygen - The concentration of dissolved oxygen can be used to determine if

the water is over or under-aerated. The pH test will give an indication of the amount of
carbon dioxide removed.

pH - pH increases as carbon dioxide is removed. pH can also be used to monitor the

effective range for hydrogen sulfide, iron, and manganese removal.

Temperature - The saturation point of oxygen increases as the temperature decreases. As

water temperature drops, the operator must adjust the aeration process to maintain the
correct dissolved oxygen level.

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Notes:

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