Keeping Your Water Safe

Radionuclides RULE!

No I don't mean in that old high school connotation where  it is implied that they "awesomely" preside over other non-ruling subjects. Today I am talking about the Radionuclides Final Rule published by the EPA in 2000 that replaced old radionuclide regulation from 1977.

This ruling set revised standards for the levels that radionuclides should be found at in our water. They devised a MCL or Maximum Concentration Level for the radionuclides of Uranium, Combined Ra226/Ra228, beta/photon emitters, and the GAA mentioned in the previous post:

Beta/photon emitters 4 mrem/yr
Gross alpha particle 15 pCi/L
Combined radium- 226/228 5 pCi/L
Uranium 30 µg/L

These standards were set for the aforementioned reasons of keeping down public dose. It also should be noted that Uranium in the water can also have harmful chemical effects on the human kidneys apart from its radiological effects.

Removal of Nuclides

So now we know how nuclides are tested and what the standards for these tests are, but how do we remove them? Reverse Osmosis has been selected by the EPA as what could be the best possible method for removal. How does it work? Osmosis is the movement of water down its concentration gradient from low solute concentration to high concentration. When solute is dissolved in a lot of water it will have low solute concentration because concentration is equal to solute amount divided by solvent amount. A high concentration would thus take place in a relatively low amount of water. Thus during osmosis water moves down its gradient by going from a low solute concentration (high amount of water) to high solute concentration (low amount of water). This is what is thermodynamically favorable and occurs spontaneously. Thus reverse osmosis makes sense to be water moving up it gradient. It moves from a high solute concentration to a low one. This is an unfavorable process and requires the input of energy to sustain it. The water with high solute level (radionuclides) is pressurized and passed against a filter with such small pores that the nuclides cannot pass through. The other side of the filter is the resulting low solute concentration (radionuclide-free) water. This process is not a 100% efficient as the all the contaminated water cannot be passed through the filter. Thus there is waste water. Also, the filter must be strong as to not burst under the pressure. If this happens the process will fail. The filters must also be cycled as the pores will get clogged reducing the ability for the water to pass through.  The result of this process is waste water that can be sent to the sewer or a further processing plant and finished water.

Cited:
(1) http://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=30006644.txt