distributed over the trays or slats. Natural, forced, or induced draft causes air to flow past the thin film of water formed.
Cascade aeration, a very simple technology, involves construction of a series of steps over which water tumbles as in a waterfall. The system requires a hydraulic head to operate, but little else except a method of ventilation to remove the radon-contaminated air from the unit.
Use of jet-aeration systems is favored in Europe, where they can be retrofitted into existing small storage-tank systems. The water is pumped through a venturi-like device (such as a jet, eductor, or ejector) that aspirates air into the water. The radon-contaminated air is released, and the treated water falls into an atmospheric storage tank. The water must be recirculated through the system several times before high removal efficiencies (>75%) are obtained. A venturi system tested on two US water supplies achieved radon-removal efficiencies of 78–95%.
In pressure aeration, air is injected into a pressurized chamber [for example, tank (hydrophor), pipe]. The gas is released when the water is allowed to come to atmospheric pressure. Although this technology uses lower A:W ratios (1:1) compared to other aeration methods (10:1 to > 100:1), it might be applicable only to special situations because the energy required to inject the air can be very high.
Granular activated carbon (GAC) is made by subjecting materials such as bone, wood, or coconut shells to high heat and pressure. These processes increase the surface area of the material and activate it, improving its ability to adsorb substances, including organic chemicals, and dissolved gases. GAC has a finite number of sites where it can adsorb a specific substance. Hence, it normally becomes saturated with the contaminant that it is removing from water over the course of days to months and must be replaced or regenerated (for example, by steam-cleaning when used to remove volatile organic compounds) to sustain an adequate level of treatment.
Adsorption of radon from water does not follow the typical saturation model observed for many contaminants, but instead can be modeled with a steady-state first-order equation first proposed by Lowry and Brandow (1985). The relation-