There are three main treatment methods for radon removal, including decay storage, GAC and aeration. Due to the large size of the radon atom, it can also be removed by reverse osmosis and nanofiltration techniques. With all the removal systems, location is a key issue, both for hydraulic reasons and radiation exposure. Some systems may require an additional pump to be installed, or a bypass system. This increases the complexity of the system. Where a system results in the build-up of radioactive substances, it may not be appropriate to keep it under the sink. On the other hand, if the radiation risk means that there is a need to build an extra outbuilding to house a unit, the cost may be prohibitively expensive. For radon removal, point-of-use systems fitted to the drinking water tap (as opposed to point-of-entry systems, which treat the entire supply) are not acceptable as radon is released wherever water is used in the house and it can then be inhaled. Table 2 summarises the features of the principal methods for radon removal.
As radon (222Rn) has a half-life of 3.82 days (i.e. its radioactivity halves every 3.82 days), it is possible to store it to achieve an adequate reduction in radioactivity. The amount of time required will depend on the level of activity. Storage will have to be well mixed with no short circuits. An eightfold reduction would take two weeks to achieve. For household consumption, this would typically require two 10m3 tanks, used alternately, which is impracticably large for most locations. With a lower activity level, requirements would be less and this option may be feasible.
Granular activated carbon
GAC is very effective and is generally quoted as achieving about 95% radon removal. GAC is used for removing taste, colour, odour and synthetic organic chemicals. It works by adsorption and the extremely high internal surface area within the porous structure is responsible for its effectiveness.
The main drawback of GAC is that as the radon is trapped in the filter, the radioactivity of the filter increases. Although radon decays rapidly, there is a continuously increasing radioactivity due to other radionuclides being trapped and the build-up of longer lived radionuclides further down the radon decay chain (notably 210Pb). As such, it is important to either shield the filters or place them in a separate shed outdoors or in an unused basement. There are also disposal problems with these levels and the filters have to be handled with care. With local authority permission, substances up to a radioactivity of 15Bq/g can be disposed of 63 to landfill with other household waste. The time before the filter reaches this level will depend on activity levels of radon in the raw water and on the retention time in the GAC. Once a GAC filter is taken out of service its radioactivity will fall as the adsorbed radon and other radionuclides decay. It has been shown that after three to four weeks out of service the activity of a GAC unit can be close to background levels.
In order to avoid clogging of the filter and to extend its life, it may well be necessary to pre-treat the water. Typically, this would involve a sediment filter and possibly an ion- exchange unit to remove other radionuclides but the treatment required would depend on the levels of other contaminants in the water.
Aeration is the preferred treatment for radon removal. In the natural environment this process ensures that most waters coming from springs in radon emitting rocks quickly lose their radon to the atmosphere. The main reason why problems occur with radon in many private supplies is because the water is either abstracted from the rock directly or very soon after. Aeration allows radon to be easily vented to the outside air thus preventing build-up of radiation levels and means there are no disposal issues. As such, the system will typically require less maintenance. Depending on the system, there may be a need for a pressure tank or an additional pump.
Unlike radon, uranium does not transfer from water to air once inside houses and thus treatment at point-of-use seems more appropriate than treatment at point-of-entry. Point-of-use treatment has the potential advantage that much smaller volumes require treatment. Many methods are available for removing heavy metals from water and as such there is no shortage of possible solutions to a problem with uranium but ion exchange and reverse osmosis are the only suitable options for private water supplies, the former is normally the preferred method. For ion exchange resins are available that can provide effective treatment systems. Reverse osmosis is also effective, it has the advantage that packaged point-of-use systems are available that can be used without any modifications. Uranium removal is rarely practised so advice should be sought from professional water treatment equipment suppliers or consultants. The Table below summarises available systems.