15, 1997). Although those studies suggest that corrosivity decreases with aeration, it might still be necessary in some systems to add corrosion inhibitors (such as lime and sodium silicate) to reduce the potential for increased release of lead and copper from the plumbing and distribution system.

Precipitate Formations

One problem observed in some aeration systems is formation of precipitates (scale) which can cause operational problems (such as fouling of equipment) and aesthetic concerns (such as release of precipitates to consumers). Depending on the chemical characteristics of the raw water, the most common precipitates are oxide, hydroxide and carbonate species of iron, manganese and calcium. Typical A:W ratios for radon removal systems might be 15:1, and this could result in precipitate formation at iron concentrations as low as 0.3 mg L-1 (Kinner and others 1993).

Common methods of eliminating precipitate-formation problems involve periodic addition of weak acid solutions to clean the equipment or addition of sequestering agents that bind the cations (Dyksen and others 1995; AWWA 1991). Another approach is to install cation-exchange filters before the aeration system. These filters are very effective at trapping iron, manganese, or calcium, but they also concentrate other naturally occurring cations, some of which can be radionuclides (such as Ra2+). The brine used to regenerate the ion-exchange filters can also become contaminated with long-lived radionuclides. In some small-scale applications, aeration equipment is followed by sand or cartridge filters that trap the precipitates (Drago 1998; Malley and others 1993). That is a simple method of removal especially useful in plants that do not have full-time professional operators. After sufficient precipitate has collected in the filter, substantially decreasing water flow, it must be backwashed. Both the brine from the ion-exchange unit and the backwashed material from the sand or cartridge filter are usually discharged to the nearest sewage-treatment system, which in many rural areas is a subsurface leach field. In its cost estimates for radon treatment with aeration, EPA did not adequately consider the cost of precipitate treatment, nor did it address adequately the problems of precipitate formation.

In some cases, it has been shown that iron precipitates have enriched concentrations of long-lived radionuclides, such as radium, lead, or uranium (Cornwell and others 1999; Kinner and others 1990). Depending on the concentrations of these radionuclides, the brine or backwash residuals might require special disposal, as discussed in EPA's (1994c) Suggested Guidelines for Disposal of Drinking Water Treatment Wastes Containing Radioactivity. Special treatment of the residuals can increase the cost of operation and the risk to workers who must oversee the disposal process.

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