found higher losses (30–70%), but some of this could have been due to volatilization during pumping or mixing and agitation.

If EPA requires that water meet the MCL when it enters the distribution system, this method of treatment would not be acceptable. Furthermore, it is doubtful that it would consistently produce water with the same radon content, because retention times and therefore losses differ on the basis of the distance of travel and the demand. This method probably would be used only where the radon levels entering the distribution network are relatively low.

Field and others (1998; 1995) have shown that radon levels in a distribution system can actually increase when radon is released during decay of radium associated with iron-based pipe scale. A generation rate would have to be quantified if loss in the distribution system were considered as a treatment alternative in water supplies that have this type of scaling problem. It also has implications for where water samples are collected (at the origin of the distribution system or at the point-of-use) (Field and others 1995).

Vacuum Deaeration and Hollow-Fiber Membrane Systems

These are very new systems that have undergone only laboratory-or pilot-scale testing (Drago 1998; 1997). They have been developed to address the issue of off-gas emissions associated with aeration systems. In both technologies, the radon removed is trapped in a sidestream of water rather than being released to the air. Therefore, these systems have the potential to fill a niche where radon concentrations in the raw water are high (precluding use of GAC) and air emissions of radon are prohibited (precluding use of aeration systems alone). The sidestream water is passed through a GAC bed that sorbs the radon. The GAC is effective in these cases because the radon is dissolved in water, not in a vapor phase. Descriptions of vacuum deaeration and hollow-fiber membrane systems are found in appendix C.

The issues of gamma emissions and disposal of the spent GAC used in the vacuum deaeration and hollow-fiber membrane systems are similar to those for GAC when it is used directly to remove radon from water. Though disinfection might be required to prevent biofilm development on the GAC, microbial and disinfection-byproduct risks are not applicable, because water from the GAC unit used in vacuum deaeration and hollow-fiber membrane systems will not be released to the consumer. The issues of precipitate accumulation and backwashing are also minimized because the sidestream water can be fairly clean. The low transfer efficiency of radon from the sidestream water to the GAC dictates a long EBCT. This, along with the complexity of the systems, would increase the costs of these systems.

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