Aeration is the process by which the area of contact between water and air is increased either by natural methods or by mechanical devices. In other words, it is the method of increasing the oxygen saturation of the water. Aeration is usually effective against several pollutants like carbon dioxide, some taste and odour producing compounds like methane, hydrogen sulphide, volatile organic compounds like industrial solvents etc. Principle of treatment underlines on the fact that volatile gases in water escape into atmosphere from the air-water interface and atmospheric oxygen takes their place in water, provided the water body can expose itself over a vast surface to the atmosphere. This process continues until an equilibrium is reached depending on the partial pressure of each specific gas in the atmosphere.
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 sulphide 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 to 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.
Oxygen is added to water through aeration and can increase the palpability of water by removing the flat taste. The amount of oxygen in which the water can hold depends primarily on the temperature of the water. (The colder the water, 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.
Efficiency
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. 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.
Aeration Process
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. Scrubbing action will remove tastes and odours 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.
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 over use of energy. In addition, aeration of water can promote algae growth in the water and can clog filters.
Types of Aerators
Aerators fall into two categories. They either introduce air to water or water to air. The water-in-air 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. Pumping water through air is much more energy efficient than pumping air through water.
I) Water-Into-Air Aerators
1) Cascade Aerators
A cascade aerator consists of a series of steps that the water flows over similar to a flowing stream. Cascade aerators allow water to flow in a thin layer down steps. 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.
Fig. 1 Cascade Aerator
2) Cone Aerators/Cone Tray AeratorCone 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. The cone tray aerator consists of several cones in which water flows through the cone and over the rim of the cone.
Fig. 2 Cone Tray Aerator
3) 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 fist-sized 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. 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.
Fig. 3 Coke Tray Aerator
4) Draft AeratorsDraft 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. This type of aerator is most effective in the reduction of hydrogen sulphide and carbon dioxide.
Fig. 4 Forced Draft Aerator
5) 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.
Fig. 5 Spray Aerator
II) Air-Into-Water Aerators
1) 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.
2) Centrifugal Aerators
Centrifugal aerators create enhanced conditions for dissolving gas into liquid phase, including bubble size, bubble size distribution and duration of interaction with liquid. Centrifugal aerators combine several elements like high turbulence swirling flow of liquid, orthogonal flow of liquid and gas, constant pressure inside the vessel, optimum flow velocity generating centrifugal forces thereby extending diffusion rate within the vessel and very small pores, through which gas permeates into the liquid and is sheared off into liquid phase, thereby forming small bubbles.
3) Air Diffusion Aerator
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.
Fig. 6 Air Diffusion Aerator