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Key Words and Concepts

Aerator – A device used to bring water and air into close contact.

Volatile Organic Chemicals – A classification of volatile, manufactured, synthetic chemicals that are

primarily used as industrial solvents.

Process Description

How Aeration Works

The aeration of water is accomplished by bringing air and water into contact with each other which

enables the oxygen in air to transfer into the water. The more surface area contact between the air and

water, the more oxygen transfer takes place.

Other factors such as water temperature and salinity determine how much oxygen can be held in

solution. The aeration processes we will be discussing all work toward the same goals, which are to:

• Increase the surface area of contact between the water and air

• Provide a constant supply of air to provide maximum air to water transfer

• The interface surface area is a key component of aeration

Constituents Affected by Aeration

• Carbon dioxide

• Hydrogen sulfide

• Methane

• Volatile organic chemicals

• Radon

• Iron

• Manganese

• Tastes

• Odors

Carbon Dioxide

Water is able to hold large amounts of carbon dioxide in solution (1,700 mg/L at 68o

F). Water from

deep wells usually contains < 50 mg/L of CO2 however shallow wells may have CO2 concentrations as

high as 300 mg/L. When CO2 levels are above the range of 5-15 mg/L, it can create some problems.

Operating Problems:

• CO2 reduces pH which makes the water corrosive

• Lower pH keeps iron in solution making it difficult to remove

• CO2 reacts with lime during the softening process and requires more chemical usage to achieve

the desired hardness reduction

Carbon dioxide comes out of solution very easily and is removed by the stripping action of the aeration

process rather than oxidation. All forms of aeration are effective at reducing CO2 which can usually be

reduced to a concentration of 4.5 mg/L. The equilibrium between CO2 in water and air limits its

reduction to this level.

Hydrogen sulfide

Hydrogen sulfide is responsible for that rotten egg odor found in some groundwater supplies. It is both

corrosive and poisonous and must be removed from the water. It is very unstable and can be removed

by almost any form of aeration because it comes out of solution very easily.

Because it is heavier than air, it will collect in low lying areas around the treatment facility. Adequate

ventilation must be maintained at hydrogen sulfide reduction facilities to protect operations personnel

from the hazards associated with hydrogen sulfide.

Methane

Groundwater that is located near natural gas deposits often has elevated levels of methane. It is

colorless, odorless and tasteless in air but can cause water to have a garlic taste when it is present in

sufficient concentrations. It is not very soluble in water which means that almost any aeration method

will remove it.

Methane is extremely flammable and explosive at certain concentrations in air. It can also cause

asphyxiation by displacing the oxygen in air. For these reasons, it is very important that when methane

is removed from the water supply, it is adequately dissipated in the atmosphere.

Volatile Organic Chemicals

Many manufactured chemicals have found their way into the water supply. A group of these chemicals

are referred to as volatile organic chemicals (VOCs) and are regulated by the USEPA because they are

either, toxic, cause cancer, or are suspected of causing cancer.

Aeration is usually the most cost effective way to reduce VOC levels found in a groundwater supply.

Most VOCs are not as easily removed from water as CO2 or H2S. A special aeration method called

“packed tower aeration” is usually required to reduce VOCs to within legal limits.

Radon

Radon is a radioactive gas that is a decay product of uranium and may be found in certain ground waters

due to naturally occurring uranium deposits. The main method of exposure is through inhalation and it

is the second leading cause of lung cancer in the U.S. behind smoking. Radon can be removed from

water using various treatment technologies:

• Reverse osmosis

• Ion exchange

• Granular activated carbon

• Packed tower aeration

The most cost effective of these is packed tower aeration and is therefore most common.

Iron and Manganese

Many groundwater sources have elevated levels of iron and manganese which cause aesthetic problems

for water suppliers. These two inorganic contaminants are removed through a process of oxidation

which causes them to form a precipitate which can then be removed by settling or filtering.

The basic aeration process can effectively oxidize the iron while manganese usually requires the use of a

chemical oxidant or specialized aeration processes. For this reason, aeration is typically used in

applications where iron is the main concern due to low manganese concentrations.

Tastes and Odors

The organic materials that cause bad tastes and odors in water supplies can generally be easily oxidized

by aeration. Some taste and odor causing compounds such as oils produced by algae cannot be

removed sufficiently by aeration and therefore require the use of chemical oxidants or GAC adsorption.

Dissolved Oxygen (DO)

Water that contains 2-4 mg/L of DO tastes better than water containing less than 1 mg/L of DO. Too

much of a good thing can be a bad thing. Too much DO will cause the water to be corrosive because

excess oxygen accelerates the rate of corrosion.

The amount of DO that water can keep in solution depends of the temperature and salinity. As the

temperature goes up, solubility goes down. Cold water can hold more DO than warm water. The lower

the salinity, the more DO the water can hold.

Types of Aerators

General Categories

There are two main methods that that are used to increase the oxygen levels in water: water-into-air

type and air-into-water type.

Water-into-air Type

• Cascade

• Cone

• Slat and coke tray

• Draft

• Spray

• Packed towers

Air-into-water Type

• Diffuser

• Draft tube

• Mechanical

• Pressure

Water-into Air Aerators

Cascade Aerators

• Oxidize iron

• Partially reduce dissolved gases

Cone Aerators

• Oxidize iron

• Partially reduce dissolved gases

Slat-and-coke-tray Aerators

• Oxidize iron

• Partially reduce dissolved gases

Draft Aerators

• Oxidize iron and manganese

• Reduce dissolved gases

Spray Aerators

• Oxidize iron and manganese

• Increase DO

Packed Tower Aerators

• Removal of VOCs

Air-into Water Aerators

Diffuser Aerators

• Increase DO to reduce poor tastes and odors

Draft Tube Aerators

• Increase DO in existing basin or tank

Mechanical Aerators

• Increase DO levels

Pressure Aerators

• Oxidize iron and manganese

• Increase DO levels

Operating Problems

Common Problems

• Corrosion

• Floating floc

• Filter air binding

• Hydrogen sulfide removal

• Algae

• Clogged diffusers

• Energy consumption

Corrosion

When water and oxygen come into contact with metal surfaces, corrosion can occur. Therefore,

excessive levels of DO in a water system can cause corrosion problems.

Solution:

• Don’t over-aerate (2-4 mg/L)

• Apply protective coating to metallic surfaces

Floating Floc

If the water in a clarifier has been over-aerated, the dissolved gases may come out of solution and small

bubbles will rise to the surface. These bubbles may be attached to the floc particles and cause them to

float to the surface as well.

Solution:

• Don’t over-aerate

Air Binding

When gases come out of solution in the media of a gravity filter, it can cause air binding which will

prevent flow through the filter. The corrective action if this occurs is to backwash the filter to release

the air from the media.

Solution:

• Don’t over-aerate

Hydrogen Sulfide Removal

Hydrogen sulfide removal happens best when the water pH is < 6. Since normal groundwater pH is

higher than 6, reducing the pH to 6 using CO2 is a good solution to this problem. When the H2S is

removed during the aeration process, the excess CO2 will also be removed which will return the pH to an

acceptable level.

Algae

Slat-and coke-try aerators and positive draft aerators are often exposed to the air and sunlight which

can promote algae or slime growth. This algae and slime growth can lead to other operational problems

and increase organic loading.

Solution:

• Shade the aerators to minimize direct sunlight

Clogged Diffusers

Air diffusers have very small openings that may become clogged with:

• Dust

• Oil

• Debris

• Chemical deposits

Solution:

• Maintain clean air filters

• Don’t over-lubricate blowers

• Prevent backflow into diffusers

Energy Consumption

All methods of aeration will require some form of energy to operate. Even cascade aeration requires

the water to be pumped at the beginning or end (or both) of the process. Other aeration methods

require the use of motors, compressors or blowers in addition to pumps. There are a few things an

operator can do to minimize the consumption of power:

• Don’t over-aerate

• Properly maintain pumps, motors and blowers

• Keep air diffusers clean and free of debris