Table 8.1 Efficiencies, Flow, and Construction Costs for Mitigation Systems Being Used in the United States to Remove Radon from Drinking Water

Treatment Method

Removal Efficiency, %

Flow Range, m3 d-1

Unit Construction Cost, $m-3 d-1

No. of Systems Evaluated

I. Aeration Methods

1. Packed tower (PTA)

79 to >99%

49 to 102,740

18 to 481

11

2. Diffused bubble

a. Single-stage

93

431

312

1

b. Multi-stage

71 to >99

65 to 6,540

11 to 433

8

3. Spray Aeration

~88a (estimated)

1,025

5.3

1

4. Slat tray

86 to 94

1,989 to 2,453

5.3 to 124

6

5. Cascade aeration

~88a (estimated)

5,450

7.9

1

6. Surface aeration

83 to 92a

54,504

42

1

II. Granular Activated Carbon

20 to >99

11 to 981

77 to 365

5

a Estimated.

Source: Drago (1998), Pontius (1998).

Aeration

In July 1991, when EPA proposed regulations for radon in drinking water, it specified aeration as the best available technology to meet the proposed maximum contaminant level (MCL) of 11,000 Bq m-3 . The agency's choice was based on the large removal efficiencies attainable (over 99.9%), the compatibility of aeration with other water-treatment processes, and the availability of aeration technologies in public water supplies. The documentation used to support the decision was published in 1987 (EPA 1987b) and updated in 1988 (EPA 1988a). In the 1991 proposed rule, EPA did not specify a particular type of aeration, but did cite packed-tower aeration (PTA) and diffused-bubble and spray-tower technologies. Mention was also made of less technology-intense aeration methods suitable for small water systems. Evaluations of aeration methods removing radon from drinking water are presented in Lowry and Brandow (1985), Cummins (1987) and Kinner and others (1989).

Aeration methods all exploit the principle that radon is a highly volatile gas and will readily move from water into air. The rate of removal from drinking water is governed by the ratio of the volume of air supplied per unit volume of water treated (A:W), the contact time, the available area for mass transfer, the temperature of the water and air, and the physical chemistry of radon (EPA 1987b). The dimensionless Henry's constant for radon at 20 °C and 1 atm pressure is 4.08 which is higher than values for CO2 or trichloroethylene that are usually removed from water by aeration methods (Drago 1998).



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