Q
Dear professor, I had a potential customer’s water tested and found uranium in the well water. I am looking for an affordable whole-house filtering system to get it out. Do you know of anywhere I can look?
A
Uranium sources: Uranium is a naturally occurring radionuclide found in both surface and groundwater. It comes from leaching uranium-containing minerals, especially in alkaline water. It shows a great affinity for oxygen and tends to form an oxycation in aerated water. Since bicarbonate is usually also present in most waters, the uranium oxycation readily combines with the bicarbonate to become an anionic-uranium complex. Ninety-nine percent of natural uranium is uranium 238.
EPA standard: The current standard is 30 ppb (30 µg/L, or 30 pCi/L).
Health Risks: Uranium has been shown to be a kidney toxin, possibly leading to cancer. The U.S. Environmental Protection Agency (EPA) says uranium is also believed to cause bone cancer and other cancers at high exposure levels.
Removal: Uranium exists as anionic uranium complexes in water. Although cation exchangers can remove some of it, that technology is made less efficient due to the competition with calcium and magnesium. Anion exchange will remove uranium to a much greater degree. Both Type 1 and Type 2 anion exchange resins are known to be extremely efficient in removing uranium from potable water. These are regenerated with sodium or potassium chloride using a solution of about 10 percent at 40 gallons per cubic foot of resin.
Activated alumina and reverse osmosis (RO) are also effective. With the recently established maximum contaminant level (MCL) of 30 micrograms per liter for uranium, water treatment dealers may already be seeing growing demand for removal of uranium from central and private well water supplies.
Practitioners wishing to add this capability to their product line can benefit from a deeper understanding of the RO and ion-exchange processes involved. Other contaminants, such as nitrate, sulfate, chloride, arsenic or tannins, will have an effect on the uranium capacity. Therefore, theoretical capacity (if using anion resin) is generally very high but actual operating capacity for uranium will depend on the concentration of the aforementioned competing ions. There are existing guidelines for estimating this capacity from ion exchange resin manufacturers. In designing a brine regenerated system, two options can be considered, one for removing only uranium and the other for simultaneous removal of other contaminants such as the above competing anions.
You ask what the capacity is. With both ion exchange and RO the capacity is dependent on the competing ions. The capacity for uranium (using anion resin) is at least an order of magnitude higher than typical capacities for either nitrate or arsenic.
Capacity for uranium for typical waters is generally above 50,000 bed volumes, while capacities for nitrate and arsenic is generally about 300 to 800 bed volumes for typical waters. Therefore, regenerating the resin bed based on the first contaminant to break through, such as arsenic or nitrate, will automatically result in control of the uranium plus the arsenic and nitrate.
Because uranium is so tightly bound by strong base anion resin, there is no fear that the remaining uranium will leak from the resin during a subsequent service cycle. After a number of cycles (typically no more than 20 cycles) the amount of uranium taken up by the resin during the previous service run is equal to the uranium removed during the subsequent brining cycle. The recommended dosage for regeneration is a minimum of 10 pounds of salt per cubic foot of resin, at a 10 percent concentration assuming at least one regeneration per month. The next question to answer is what salt dosage should be used to ensure good leakage control over all contaminants. For this, a dosage of 15 lbs/ft3 would be adequate to regenerate off about 50 percent of the uranium picked up by the resin.
In designing for single-use disposable resin, designers must know the predicted capacity as well as the maximum radionuclide loading permissible and design for the lower of the two values.
For disposable use, Type I SBA resins offer the best use cost, although Type 2 resins can also be used. Using proprietary software, the capacity can be predicted once the anionic composition of the water is known, including sulfate, nitrate, bicarbonate, chloride and uranium, as a minimum.
In designing a brine regenerated system, two options can be considered: One for removing only uranium and the other for simultaneous removal of other contaminants such as nitrate, arsenic or tannins.
Combining removal of uranium, nitrate and arsenic
As part of our design, we need to answer a number of questions:
- What is the capacity of each type of resin for each contaminant?
- Which resin is most suitable given its relative capacity for each contaminant?
- Which contaminant will break through first and must, therefore, be used for controlling when the unit is regenerated?
- What dosage of brine (pounds of salt per cubic foot of resin) is adequate to handle all the contaminants? Again, even though computer projections of capacity for each contaminant are not expected to be 100 percent accurate, such estimates when made by an experienced user, can be accurate enough to answer the questions above.
Anion resins, RO, activated alumina and distillation are viable options for uranium removable. For whole-house systems they should be compared as to cost and efficiency. In addition, you must consider which part of the house really needs uranium removal. All the treatments listed above are generally understood to be effective if properly applied. Like ion exchange, RO has an “order of priority” of removal (in this case it is more like an “order of rejection.”) Contact a membrane manufacturer or a knowledgeable RO equipment manufacturer in order to compare the uranium with the ions present in the feedwater. Then, contact a similar ion-exchange resin manufacturer, with the same complete water analysis, to find out where uranium is on the list of “order of affinity” of anions. Then, choose which water in your house you want to make “uranium-free” and compare the above mentioned methodologies.
