Aeration

Objective

In this lesson we will answer the following questions:

 How does aeration affect water?

 Which substances in water can be removed by aeration?
 What types of aerators are used in water treatment?

Reading Assignment

Read online lecture.

Lecture

The Process of Aeration

Introduction

We mentioned aeration briefly in the lesson on pretreatment. Aeration is used to treat

tastes and odors, to help remove minerals such as iron and manganese from water, and
to remove carbon dioxide from the water.

Aeration follows screening and (optional) pre-chlorination and precedes coagulation.

Like pre-chlorination, aeration is an optional procedure used when problematic
components exist in the raw water. In many cases, chemical alternatives exist to
aeration even when the raw water contains problematic components treatable by
aeration. A variety of factors must be taken into account when deciding whether
aeration is appropriate for a water treatment plant.

In general, aeration is more commonly used when treating groundwater than when
treating surface water. Surface water has typically run through creeks and rivers,
aerating the water before it reaches the treatment plant.

How Does Aeration Work?

Aeration is the intimate exposure of water and air. It is a way of thoroughly mixing
the air and water so that various reactions can occur between the components of the
air and the components of the water.
Aeration removes or modifies the constituents of water using two methods - scrubbing
action and oxidation. Scrubbing action is caused by turbulence which results when
the water and air mix together. The scrubbing action physically removes gases from
solution in the water, allowing them to escape into the surrounding air. In the picture
above, carbon dioxide and hydrogen sulfide are shown being removed by scrubbing
action. Scrubbing action will remove tastes and odors from water if the problem is
caused by relatively volatile gases and organic compounds.

Oxidation is the other process through which aeration purifies water. Oxidation is the
addition of oxygen, the removal of hydrogen, or the removal of electrons from an
element or compound. When air is mixed with water, some impurities in the water,
such as iron and manganese, become oxidized. Once oxidized, these chemicals fall
out of solution and become suspended in the water. The suspended material can then
be removed later in the treatment process through filtration.
Efficiency

The efficiency of the aeration process depends almost entirely on the amount of
surface contact between the air and water. This contact is controlled primarily by the
size of the water droplet or air bubble.

The goal of an aerator is to increase the surface area of water coming in contact with
air so that more air can react with the water. As air or water is broken up into smaller
drops/bubbles or into thin sheets, the same volume of either substance has a larger
surface area.

Problems with Aeration

Aeration typically raises the dissolved oxygen content of the raw water. In most
cases, this is beneficial since a greater concentration of dissolved oxygen in the water
can remove a flat taste. However, too much oxygen in the water can cause a variety
of problems resulting from the water becoming supersaturated. Supersaturated water
can cause corrosion (the gradual decomposition of metal surfaces) and sedimentation
problems. In addition, air binding occurs when excess oxygen comes out of solution
in the filter, resulting in air bubbles which harm both the filtration and backwash
process.

Aeration can also cause other problems unrelated to the supersaturated water.
Aeration can be a very energy-intensive treatment method which can result in overuse
of energy. In addition, aeration of water can promote algal growth in the water and
can clog filters.

Chemical Substances Affected by Aeration

Introduction

Aeration influences a great variety of chemical substances found in water. In general, aeration is
most effective in removing volatile substances from water and in removing substances which can
become oxidized. A volatile material is one that is capable of being evaporated or changed to
vapor at relatively low temperatures, meaning that it has a low boiling point.

Volatile substances removed from water through aeration include organic chemicals, such as
benzene (found in gasoline) and trichloroethylene, dichloroethylene, and perchloroethylene
(solvents used in dry-cleaning or industrial processes) and various chemicals causing taste and
odor. Carbon dioxide and hydrogen sulfide are also removed from water through scrubbing
action.
Methane can sometimes be removed from water through aeration, but it is not easily removed
because the gas is slightly soluble in water. Methane results in a gassy smell and in gas bubbles
escaping from the water. The gas is flammable and has a potential for explosions.

Aeration is effective in removing iron and manganese from water through oxidation.

Below, we will consider in greater depth the effects of aeration on carbon dioxide, hydrogen
sulfide, and iron and manganese.

Carbon Dioxide

Carbon dioxide gas dissolves easily in water, resulting in carbonic acid:

H2O + CO2 <===> H2CO3

The carbonic acid increases the acidity of the water (lowers the pH) which can cause corrosion of
pipes in the distribution system. A large concentration of carbon dioxide in water can also keep
iron in solution, making it difficult to remove from the water.

Carbon dioxide in the water can be dealt with either through aeration of the water or through
addition of lime. Aeration is usually used in water with a high concentration of carbon dioxide.
At concentrations greater than 10 ppm, the carbon dioxide is loosely bound to the water and can
easily be stripped by aeration, raising the pH to a more normal level.

At lower concentrations, carbon dioxide is neutralized through the addition of an alkali, such as
lime or soda ash. Lime (Ca(OH)2) reacts with carbon dioxide, removing the carbon dioxide from
the water as shown below:

CO2 + Ca(OH)2 <===> CaCO3 + H2O

The most appropriate treatment for carbon dioxide may be aeration, addition of an
alkali, or a combination of the two. The best treatment will depend on the
concentration of carbon dioxide in the water, on the total alkalinity, and on other
substances in the water (such as hydrogen sulfide.)

Hydrogen Sulfide

Hydrogen sulfide is commonly found in well water, where it results in a distinctive

rotten egg odor. As the water passes through the ground, it comes in contact with
sulfates. If the water is highly mineralized or contains products of decomposition,
these minerals and other substances will react with the sulfates and change them to
hydrogen sulfide (H2S). Surface waters rarely have hydrogen sulfide problems since
the water is naturally aerated as it runs through streams.

Hydrogen sulfide gas turns into hydrosulfuric acid when it dissolves in water. The
acid is weak but highly corrosive, eating up electrical contacts, causing a slight odor,
and resulting in black water complaints. Water containing hydosulfuric acid will
become very dark after remaining in the water lines for a few hours. The black water
is most often noticed when flushing a fire hydrant.

The presence of larger quantities of hydrogen sulfide can be readily noted by odor.
The disagreeable rotten egg odor is very characteristic of this gas and unless it is
removed or reduced, the smell results in many complaints. As a result, even though
hydrogen sulfide gas in water is not injurious to people, it is usually removed when
present.

There are three methods used for the removal of hydrogen sulfide. If there is a heavy
concentration of the gas, the water should be aerated, allowing most of the gas to
escape into the air. Aeration of hydrogen sulfide requires that the pH of the water first
be lowered to 6 or less, and then the gas can be scrubbed away by aeration. The
remaining gas (or lower concentrations of the gas) can be oxidized by chlorine.
Alternatively, ozone can be used to convert hydrogen sulfide to sulfurous acid, but
ozone is also corrosive so it may cause as many problems as it solves.

Iron and Manganese

Both iron (Fe) and manganese (Mn) are minerals which can be found in water
supplies. The minerals cause stains on on porcelain plumbing fixtures and laundry
and cause coffee or tea to be cloudy and unpalatable. In addition, they can cause
diarrhea.

Water containing iron and manganese will be clear when first discharged from a well.
Upon exposure to air for several hours, the minerals oxidize and colored water
results. The presence of oxidized iron causes water to be red and results in stains of
the same color. Manganese is a dark brown mineral and the resulting stains are dark
brown or black.

The recommended limit on concentrations of these minerals in water is 0.3 PPM for
iron and 0.05 PPM for manganese. In addition, some industries cannot tolerate even
this quantity of either mineral. It is strongly recommended that whenever the
combined totals are more than 0.2 PPM, treatment facilities should be installed for
removal.
The usual treatment to remove iron and manganese from water is to oxidize the
minerals as rapidly as possible and then to remove the oxidized material through
filtration. Manganese oxidizes and discolors water at a slower rate than iron, which
affects the treatment method used for each mineral. In addition, pH affects the rate of
oxidation for both minerals, so it is often necessary to change the pH of the water
during treatment.

In some cases the oxidation is accomplished entirely by the addition of chemicals. In

other cases the water is first aerated, then an alkali is added to complete oxidation.
The alkali optimizes the pH and uses the oxygen in the air to oxidize the iron and
manganese. At the same time, the alkali reduces the carbon dioxide concentration in
the water.

Methods of Aeration

Introduction

There are several different methods used to aerate water, but all either involve passing
water through air or air through water. Water can be exposed to air by spraying or by
distributing it in such a way that small particles or thin sheets of water come in contact
with the air. Water can also by aerated by pumping large volumes of air through the
water.

The method of aeration to be used depends on which materials on the water are to be
removed. The chemical characteristics of the water to be treated also influence which
treatment method is used. Finally, each method has a different efficiency. In general,
pumping water through air is much more energy efficient than pumping air through
water. Different types of aeration and other methods of treatment should all be
compared to determine the most efficient and practical method of treatment in each
case.

Types of Aerators
Air diffusion is a type of aerator in which air is blown through a trough of water. As
water runs through the trough, compressed air is blown upward through porous plates
on the bottom. This method is not very efficient due to limited air transfer.

Most of the other aeration methods work by passing raw water through air in small
streams rather than by passing air through water. A few, such as spray nozzle
aerators, pump water through nozzles breaking the water into a fine spray.

Cone tray aerators and cascade aerators both work by forming little waterfalls.
The cone tray aerator, shown below, consists of several cones in which water flows
through the cone and over the rim of the cone. Cone aerators are primarily used to
oxidize iron.
Cascade aerators allow water to flow in a thin layer down steps. In both the case of
the cone tray aerator and the cascade aerator, the waterfalls allow the water to come in
contact with air.

Coke tray aerators also pass water through air in small streams. A coke tray aerator
is comprised of a series of activated carbon trays, one above another, with a
distributing pan above the top tray and a collecting pan below the bottom tray. The
distributing pan breaks the water up into small streams or drops. The holes in the
trays should be designed to develop some head loss to provide for equal distribution to
the lower tray.

As the water moves through the coke tray aerator, small streams of water flow
through the air from tray to tray. A great amount of water surface area is also exposed
to air as the water passes over the coke beds. The water is collected in the bottom pan
and given further treatment if necessary.

In addition to aerating water, the activated carbon trays in a coke tray aerator filter
organic contaminants out of the water. A similar method was once used to treat
people who had swallowed poison. Bread was toasted in the oven until it blackened,
turning into activated carbon. Then the patient ate the burnt toast. The carbon drew
the poison into the carbon and out of the patient's system. Coke tray aerators work in a
similar manner, drawing contaminants out of the water.

The last type of aerator which we will discuss here, the forced draft aerator,
combines both methods: it blows air through water which has been broken into fine
streams. The forced draft aerator consists of a series of trays over which raw water
runs. As the water comes to the end of each tray, it cascades off and falls down to the
collecting tray (also known as a drip pan). At the same time, a fan at the top of the
aerator pulls air up through the water. So, as small streams of water fall from the
trays, they comes in intimate contact with the strong updraft of air. This type of
aerator is most effective in the reduction of hydrogen sulfide and carbon dioxide.
Review

Aeration is an optional part of the water treatment process which uses scrubbing
action and oxidation to remove or modify constituents of the water. Substances
affected by aeration include volatile organic chemicals, carbon dioxide, hydrogen
sulfide, methane, iron, and manganese.

Aerators work by increasing the amount of surface area of air coming in contact with
water. This may be achieved by passing air through water, as in an air diffusion
aerator. In contrast, many aerators pass water through air, as in spray nozzle, cone
tray, cascade, and coke tray aerators. Finally, forced draft aerators both pass air
through water and water through air.

Assignments

Answer the following question. When you have completed the assignment, either,
email or mail your work to the instructor.

1. Why do we use aeration in the treatment of water?

2. What is the addition of oxygen, the removal of hydrogen, or the

removal of electrons from an element or compound called?

3. What is meant by the term volatile?

4. What is the chemical formula for carbonic acid?

5. Hydrogen sulfide gases turn into what when dissolved in water?

6. For iron and manganese, how is oxidation accomplished?

7. What is air diffusion?

8. What is a coke tray?

9. What does aeration remove from water?

10. What chemical removes carbon dioxide from water?

 AERATION

Aeration is the process of bringing water and air into close contact in order to remove dissolved
gases, such as carbon dioxide, and to oxidize dissolved metals such as iron. It can also be used to
remove volatile organic chemicals (VOC) in the water. 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.

HOW AERATION REMOVES OR MODIFIES CONSTITUENTS

In water treatment the aeration process brings water and air into close contact by exposing drops
or thin sheets of water to the air or by introducing small bubbles of air and letting them rise
through the water. For both procedures the processes by which the aeration accomplishes the
desired results are the same:
 Sweeping or scrubbing action caused by the turbulence of water and air mixing together
 Oxidizing certain metals and gases

Undesirable gases (e.g. hydrogen sulfide) enter the water either from the air above the water or
as a by-product of some chemical or biological reaction in the water. The scrubbing process
caused by the turbulence of aeration physically removes these gases from solution and allows
them to escape into the surrounding air.

Aeration can help remove certain dissolved gases and minerals 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 suspended material in the water. The suspended
material can then be removed by filtration.

The efficiency of the aeration process depends almost entirely on the amount of surface contact
between the air and water. This contact is controlled primarily by the size of the water drop or air
bubble.
DISCUSSION OF CHEMICAL SUBSTANCES AFFECTED BY AERATION

Aeration of water removes gases or oxidizes impurities, such as iron and manganese, so that they
can be removed later in the treatment process. The constituents that are commonly affected by
aeration are:

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

dichloroethylene, and perchloroethylene, examples of
solvents are used in dry-cleaning or industrial processes.

 Carbon dioxide
o Hydrogen sulfide (rotten-egg odor)
o Methane (flammable)
o Iron (will stain clothes and fixtures)
 o Manganese (black stains)
Various chemicals causing taste and odor

CARBON DIOXIDE

Carbon dioxide is a common gas produced by animal respiration. Apart from being naturally
present in the air, it is produced by the combustion of fossil fuels. It is used by plants in the
photosynthesis process.

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 a range of 5 to 15 mg/l in raw water can cause three
operating problems:

• It increases the acidity of the water, making it corrosive. Carbon dioxide forms a
“weak” acid, H2C03 (carbonic acid).
• It tends to keep iron in solution, thus making iron removal more difficult.
• It 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 carbon dioxide per liter.

HYDROGEN SULFIDE
A poisonous gas, 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 (IDLH) level for hydrogen sulfide is 300ppm.

Hydrogen sulfide occurs mainly in groundwater supplies. It 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. Hydrogen sulfide gas is corrosive to piping, tanks, water heaters, and
copper alloys that it contacts. 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 the chlorine demand exerted by
the hydrogen sulfide.
• There could be corrosion of the piping systems and the water tanks.

Aeration is the process of choice for the removal of hydrogen sulfide from the water. The
turbulence from the aerator will easily displace the gas from the water. 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 gas 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 is a
colorless gas that is highly flammable and explosive.

When mixed with water, methane will make the water taste like garlic. The gas is only slightly
soluble in water 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. Iron and manganese
compounds can dissolve into groundwater as it percolates through the soil and rock.

Iron in the ferrous form and manganese in the manganous form are objectionable for several
reasons. 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.
TASTE AND ODOR
Aeration is effective in removing only those tastes and odors that are caused by volatile
materials, those that have a low boiling point and will vaporize very easily. Methane and
hydrogen sulfide are examples of this type of material.

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.

DISSOLVED OXYGEN
Oxygen is injected into water through aeration. This is, in most cases, beneficial. It increases the
palpability of the water by removing the flat taste. The amount of oxygen that the water can hold
is dependent on the temperature of the water. The colder the water, the more oxygen the water
can hold.

However, water that contains excessive amounts of oxygen can become very corrosive.
Excessive oxygen can cause additional problems in the treatment plant by, for example, causing
air binding of filters.
TYPES OF AERATORS

Aerators fall into two general categories. They either introduce air into the water or water into
the air. The water-to-air method is designed to produce small drops of water that fall through the
air. The air-to-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 the air and water to
enhance the transfer of the gases.

WATER INTO AIR

Cascade Aerators

A cascade aerator consists of a series of steps that the water flows over. In all cascade aerators,
aeration is accomplished in the splash zones. The aeration action is similar to a flowing stream.
Splash areas are created by placing blocks across the incline. Cascade aerators can be used to
oxidize iron and to partially reduce dissolved gases. They are the oldest and most common type
of aerators.
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

The slat and coke trays are similar to the cascade and cone types. They usually consist of three-
to-five stacked trays, which have spaced wooden slats in them. The trays are filled with fist-sized
pieces of coke, rock, ceramic balls, limestone, or other materials. The primary purpose of the
materials is to provide additional surface contact area between the air and water.
Draft Aerators

A draft aerator is similar to the others 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.

Spray Aerators

This type of aerator has one or more spray nozzles connected to a pipe manifold. Moving
through the pipe under pressure, the water leaves each nozzle in a fine spray and falls through the
surrounding air, creating a fountain affect. In general, spray aeration is successful in oxidizing
iron and manganese and is successful in increasing the dissolved oxygen of the water.
AIR INTO WATER
These are not common types used in water treatment. The air is injected into the water through a
series of nozzles submerged in the water. It is more commonly used in wastewater treatment for
the aeration of activated sludge.
Pressure Aerators
There are two basic types of pressure aerators. One uses a pressure vessel. The water to be
treated is sprayed into the high-pressure air, allowing the water to quickly pick up dissolved
oxygen.

A pressure aerator commonly used in pressure filtration is a porous stone installed in a pipeline
before filtration. The air is injected into the stone 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.

AIR STRIPPING
If operated properly, a process called air stripping can be quite effective in removing volatile
organic chemicals (VOCs) from water. The presence of VOCs, many of which are man-made or
formed during industrial processes, is increasingly becoming a problem for public water
suppliers. US EPA has set Maximum Contaminant Levels for many VOCs (see the Public Water
Supply Regulation chapter). A major concern is that VOCs may be carcinogens. Example of
VOCs are benzene from gasoline and trichloroethylene from dry cleaning establishments.

Air stripping has been shown to be capable of removing up to 90 percent of the most highly
volatile VOCs. It can be accomplished by letting the water flow over cascade aerators or in
specially designed air-stripping towers. In these, water is allowed to flow down over a support
medium or packing contained in the tower, while air is being pumped into the bottom of the
tower.

COMMON OPERATING PROBLEMS

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 are slow removal of the hydrogen sulfide from the towers, algae
production, clogged filters, and overuse of energy.

CORROSION
A certain amount of dissolved oxygen is present in raw and treated waters. However, dissolved
oxygen may 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.

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.

HYDROGEN SULFIDE REMOVAL

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.

OPERATIONAL TESTING
Three basic control tests are involved in the operation of the aeration process:
• Dissolved oxygen
• pH
• Temperature

The concentration of dissolved oxygen can be used to estimate whether the process is over or
under aerated. The pH test will give an indication of the amount of carbon dioxide removal. pH
increases as the carbon dioxide is removed. pH can also be used to monitor the effective range
for hydrogen sulfide, iron, and manganese removal. The temperature is important as 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 DO level.