In diffused aeration systems, water is contained in basins that feature diffusers at their base. Compressed air is forced into this system through the diffusers. This air bubbles up through the water, mixing water and air and transferring pollutants from the former to the latter or, more often, introducing oxygen into the water.
Diffused aeration systems are relatively inexpensive to build but expensive to operate. They are easily installed as retrofits to improve existing water treatment basins—a common use for this technology. They can run continuously and automatically, requiring only periodic maintenance and regular monitoring.
Diffused air systems are prone to problems of particulate buildup, rust-producing bacteria, and other contaminants, which can clog a system and shut it down. The air must be put under considerable pressure to get it into the bottom of the diffusion basin, hence diffusion systems are limited to relatively low air-to-water ratios. As a result diffused aeration systems can only cost-effectively remove highly volatile contaminants, like radon, and are more often used for the purpose of introducing oxygen or ozone into the water.
Mechanical aeration systems are fairly simple, but they are not among the most common purification techniques. These aerators work by vigorously agitating source water with mechanical mixers. As the waters churn, they become infused with purifying air.
These systems can be easily retrofitted to existing storage facilities, where they can add basic treatment capacity where none exists. Mechanical aeration systems are able to remove most volatile contaminants, but they are limited to removals of 50 to 80 percent, depending on conditions. If mechanical aerators are installed in the top of an existing covered reservoir, ventilation will be required.
Tray aeration systems arrange simple packing materials like crushed coke (a carbon-rich material produced from coal) or rock in a vertical sequence. Water is delivered to the top of this stack, spread out to increase the surface contact area, and dripped through small openings in the bottom of each tray.
As water falls from tray to tray and splashes on the packing, it encounters air. Air flow in these systems can be aggressively driven by an electric air compressor or, more commonly, by simply employing natural draft.
Mechanically-forced air systems are more effective in removing less volatile constituents, like solvents, but natural draft is a viable option when more volatile constituents like hydrogen sulfide, radon, or vinyl chloride are the targets.
When the series of trays has been passed through, treated water is simply collected at the base of the system.
Tray aeration systems are very susceptible to algae and slime growth, which can make treatment less effective. This growth is sometimes checked by the addition of chemicals like chlorine or copper sulfate, though these additives represent an additional expense and may remain present even in the system’s finished treated water.
Because they can achieve high air to water ratios and removals approaching 99 percent, packed tower systems are a popular option for removing volatile solvents from ground water media. They employ a five to 12 meter tower equipped with a distributor at the top. The distributor introduces water evenly across the top of a tower packed with plastic, ceramic or metal objects engineered to maximize air-water contact. Air is pushed or drawn upward through the tower against the direction of water flow. A pump at the base collects and removes treated water.
Tower systems are often permanent installations, but they may be constructed on a portable trailer and moved from place to place.
While simple in principle, packed tower systems, like other air stripping systems, are prone to clogging because of particulate buildup, rust-producing bacteria, and the precipitation of calcium carbonate. Treatment costs increase significantly if water must be pre-treated or if system air must be purified before it is released into the atmosphere.
The base cost associated with air stripping systems is in electric power, used to run the pumps and air blowers that drive this purification technology.