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Vacuum Deaeration and Hollow-Fiber Membrane Technology
Vacuum deaeration (VD) exploits the high Henry's Law constant of radon by spraying the raw water into an enclosed tower that contains a packing material. A vacuum is applied to the top of the unit by an eductor or pump. The vapor is redissolved with an eductor into a continuously recirculating stream of water that passes through the GAC bed. Noncondensable gases (such as 02, N2, and CO2) are released from the sidestream via a constant-head tank and an oil trap. The efficiency of radon removal from the water is strongly linked to the strength of the vacuum. At high vacuums (under 0.1 atm absolute pressure), removals in the 70% range have been observed. Two difficulties with the process are the low efficiency of transfer of radon into the stream of recirculating water and the desorption of radon from the GAC (only 20–30% net radon removal observed). Implementation of this complex technology, which would be applicable only to medium and large flows, must await further testing.
The hollow-fiber membrane (HFM) technology is equally complex and differs from the VD process only in using a column of membranes, instead of a tower with packing, to remove radon from water initially. The raw water passes along one side of a series of microporous membranes. A stream of air induced by a vacuum passes along the other side. The radon and other dissolved gases are transferred to the air under vacuum. Again, the efficiency of transfer is a function of the strength of the vacuum applied. With a bench-scale system, a radon removal efficiency of 40–56% was obtained (Drago 1997). The problems with dissolving the radon in the sidestream and the removal efficiency of the GAC (40–80%) observed in evaluations of the HFM system are similar to those for the VD system. Applicability of HFM must be evaluated on pilot-and full-scales before it could be considered a best available technology for radon removal for medium and large communities, which need to remove radon from water to avoid discharging it to the atmosphere via the off-gas.