Guideline Number #2
INTRODUCTION
All waters contain traces of naturally occurring radionuclides, the concentrations depending on the origin of the water. The natural radionuclides of most relevance are Radon (Rn) and Uranium (U). Radon is volatile and as a result it can be released from water as a gas. This is of concern if the release occurs within a confined space with insufficient ventilation. Radon and Uranium are only found in significant concentrations in groundwater in certain parts of the UK, depending on the type of geology. Further advice is available from local authorities and the National Radiological Protection Board (NRPB). These substances are not significant for any surface water sources in the UK.
This guidance note is designed to assist in identifying suitable treatment regimes for improving the quality of raw and potable water with respect to Radon and Uranium content only; it is not designed to provide absolute advice, as each individual case must be independently evaluated with respect to cost, value and ‘best practice’, etc. In this respect Waterfall & O’Brien cannot be held responsible for individual decisions and judgements regarding the supply and installation of equipment and materials to safeguard or improve the quality of their private water supplies.
SCOPE
This guidance note is only applicable to raw and potable water that is intended for drinking, or for use as an agricultural or horticultural water source. These guidance notes are not intended to be used for the clean-up of effluent and contaminated waters.
GUIDELINES
The concentration of radioactive elements in water is expressed in terms of their activity, in Becquerels per litre (Bq/l). There is currently no official recommended level of Radon in drinking water in the UK. There is a draft European Commission recommendation on radon in drinking water supplies of 100 Bq/l for public supplies and an action level of 1000 Bq/l for non-commercial private supplies. The EU level of 1000 Bq/l is consistent with recent advice from the NRPB. Uranium limits were proposed to be in the range 20 to 100 μg/l. The Uranium guideline is currently 20 Bq/l (equivalent to a concentration of approximately 2 mg/l).
General Physico-Chemical Properties of Radon.
Treatment for Radon removal cannot use point of use systems fitted to the tap because, being volatile, it is released into the atmosphere whenever water is used. Under-sink treatment using an activated carbon filter is also inadvisable because the filter would become radioactive. Radon removal treatment therefore has to be installed before entry of water into a building. Aeration is the preferred treatment technique but other methods are feasible. Uranium removal is best achieved by point of use systems.
Treatment Methods
There are three main treatment methods for Radon removal – decay storage, GAC (Granular Activated Carbon) and aeration. Due to the large size of the Radon atom, it can also be removed by reverse osmosis and nano-filtration 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.
1. Decay Storage
As Radon (222Rn) has a half-life of 3.82 days (i.e. its radioactivity halves every 3.82 days), it is possible (provided that mixing and short-circuiting are avoided) to store it to achieve an adequate reduction in radioactivity. The amount of time required will depend on the level of activity. An eight-fold reduction would take two weeks to achieve. For household consumption, this would typically require two 10 m3 tanks, used alternately – impracticably large for most locations. With a lower activity level, requirements would be less and this option may be feasible.
2. Granular Activated Carbon
GAC was first used in the US in 1981 for Radon removal. The method has been found to be very effective and is generally quoted as achieving about 95% Radon removal. GAC is commonly 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 – the filters have to be handled with care.
With local authority permission, substances up to a radioactivity of 15 Bq/g can be disposed of 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.
3. Aeration
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. With aeration, Radon can be easily vented to the outside air. This prevents 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.
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