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Aeration and Air Stripping

Overview
 
Figure 1: Typical packed tower aerator configuration.
Figure 1: Typical packed tower aerator configuration.
Figure 2: Typical diffused aeration configuration.
Figure 2: Typical diffused aeration configuration.

Aeration processes are, generally, used in two types of water applications: air stripping - the process in which gas is removed from water, and aeration - the process in which air or oxygen is transferred to water. Major applications for aeration technologies include: (1) oxidation of substances such as iron and manganese by air or oxygen addition, and (2) removal of carbon dioxide, taste and odor causing substances, VOCs and volatile SOCs, ammonia, trihalomethanes, pesticides, herbicides, and gases such as methane, hydrogen sulfide, and radon.

Henry's law describes the tendency of a constituent to transfer from the liquid to the gas phase at equilibrium. The Henry's Law constant is the ratio of the equilibrium concentration of a particular contaminant in air to its concentration in water. Thus, a higher Henry's Law constant indicates a greater tendency of species to volatilize. High temperature and turbulence promotes gas transfer by reducing thickness of film at air-water interface. The efficiency of aeration depends almost entirely on the amount of surface contact between the air and water.

Method of aeration can be classified into four general categories.

(1) Waterfall aeration uses the desorption principle and accomplishes gas transfer by causing water to break into drops of thin films, increasing contact surface area between the air and water. There are five common types of waterfall aerators.

(a) Spray Aerators have been used in water treatment for many years for iron oxidation and stripping VOCs and dissolved gases. Spray aerator installation generally consists of fixed nozzles or a pipe grid located over an open-top tank. The nozzles spray fine water droplets into the surrounding air, creating the air-water interface necessary for contaminant transfer. The configuration of pipe grid on top of open-top tank is preferable for stripping applications. Spray aerators requires a large installation area and pose operating problems during freezing weather, furthermore, for effective application they require supplemental air exchange.

(b) Cascade aerators consist of a series of steps that allows water to fall in thin layers from one level to another, where aeration is accomplished in the splash zones. The exposure time of air to water is increased by increasing number of steps, and the area-volume ratio is improved by adding baffles to produce turbulence. The major operating problems include corrosion and slime and algae buildup.

(c) A multi-tray or slat tray aerator consists of series of trays equipped with slats or perforated or wire-meshed bottoms. Water is distributed over the trays and is allowed to fall from each tray onto the collection basin at the base. Air is forced or induced to flow perpendicular to water path. Often coke, stone, or ceramic balls of size 2 to 6 in. are used to improve efficiency of gas exchange. There are typically 3-9 trays spaced 12-30 inches apart in multi-tray aerators. Multi-tray aerators are analogous to cooling towers and require adequate ventilation. They provide excellent oxygen absorption and carbon dioxide, ammonia, and hydrogen sulfide removal.

(d) Cone aerators are primarily used to oxidize manganese and iron as a pretreatment measure. The design of the aerator is similar to cascade aerators, with stacked pans arranged such that water fills top pan and cascades down to each succeeding pan.

(e) Packed tower or air strippers are capable of removing from water fuel, solvents and volatile SOCs VOCs, ammonia, hydrogen sulfide, and carbon dioxide. A packed column consists of a cylindrical tower, a packing material (usually plastic, steel or ceramic) and a centrifugal blower. The contaminated feed water is pumped into the top of the tower and blowers are used to introduce air countercurrent through the bottom of the tower. The large surface area provided by the packing material allows more liquid-gas transfer compared with other air stripping methods. The number of stages is determined by Henry┐s Law constant and the tower loading rate. A typical configuration for packed tower aerator is shown in Figure 1.

Frequently, the off-gas from the column has to be treated. Typically, the gas is heated to reduce humidity and is passed through carbon adsorption chamber to trap the volatile compounds. Other process of treating off-gas includes thermal oxidation which is expensive, and photo-oxidation which is less feasible for continuous, full scale operations. A most common problem with packed tower aerators is fouling of packing material with solids, resulting in loss of plant capacity, efficiency, and increased pressure drop.

(2) Bubble aerators consist of a rectangular concrete tank in which perforated pipes, porous diffuser tubes or plates are installed. Compressed air is forced through these pipes to produce air bubbles that rise through water, producing turbulence resulting in effective water-air mixing. Bubble aerators are effective in removing carbon dioxide, VOCs, gasoline components, hydrogen sulfide, methane, and radon from contaminated groundwater. They are mostly used by small water systems with low flow rates. A typical bubble or diffused aerator configuration with perforated pipe installation is shown in Figure 2.

Air sparging is similar to bubble aeration except it does not involve the use of a diffusion mechanism. In air sparging, pressurized air is injected directly into the reservoir, well, or aquifer. The removal efficiency depends on the amount and pressure of air injected, water column depth, and air venting conditions.

(3) Mechanical aerators employ motor driven impellers alone or in combination with air injection devices. They are also installed at water reservoirs to control taste and odor. The mechanical aerators can be installed as surface aerators (on the float) or submerged (below the water level) aerators. They are normally used for increasing dissolved oxygen levels, but they can also remove certain contaminants.

(4) Pressure aerators are typically used for oxidizing iron and manganese. There are two basic types of pressure aerators in which either compressed air is injected directly in pressurized pipeline or water is sprayed into the top of a closed tank while the tank is continuously supplied with compressed air.

The effectiveness of aeration depends upon the aeration method selected, Henry's law constant of contaminant, design factors such as air to water ratio, flow and loading rate, available area of mass transfer, temperature, pH and algae production. As Henry's law constant increases, design factors such as air to water ratio becomes less critical for effective removal.





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