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ship can be used to calculate important design variables, such as the empty-bed contact time (EBCT) and the volume of GAC needed to reach a desired effluent quality.
Ct = effluent radon concentration (kBq m-3),
Co = influent radon concentration (kBq m-3),
Kss = the adsorption/decay constant specific for the GAC and the water treated (h-1),
V = volume of GAC needed (m3), and
Q = flow of water treated (m3 h-1).
It has been suggested that saturation does not occur because the radon decays, allowing a new atom of the gas to be adsorbed to the same GAC site (Lowry and Brandow 1985). Others have suggested that very long run times occur before saturation because of the very low mass of radon being adsorbed (Kinner and others 1993; Cornwell and others 1999).
When used for small flows, the carbon is usually placed in a closed vessel (constructed of, for example, fiberglass or carbon steel), and the water is forced through the bed, using the pressure exerted by the well pump. Therefore, repumping is not required, because there is no break to atmospheric pressure. In large municipal facilities, operated at atmospheric pressure, the hydraulic head from the water above the GAC causes the water to flow past the GAC. In either system, head-loss problems resulting from accumulation of turbidity-causing substances or precipitates can be alleviated by backwashing. The effect of backwashing on radon removal is not clear; some studies have shown a decline in efficiency after backwashing (Lowry and Brandow 1985) (Lowry and others 1990), and others have not (Kinner and others 1990; Cornwell and others 1999). Lowry and others (1990) have observed desorption of radon during and immediately after backwashing, but the radon progeny remain sorbed.
When the efficiency of a GAC unit in removing radon from water begins to decline (Lowry and others 1991; Kinner and others 1989; Kinner and others 1990; Kinner and others 1993; Cornwell and others 1999), the GAC is usually not regenerated, although it is technically possible to remove accumulated 210Pb by using an acid pumped through the bed or by thermal desorption (Lowry and others 1990). Instead, it is usually easier to dispose of the old carbon before it accumulates a significant amount of radioactivity and add new GAC.