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?
Read online lecture.
The Process of Aeration
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.
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
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 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 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
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.
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.
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?
Please take Quiz 5. When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and submit your grade directly into the database for grading purposes.