MadSci Network: Environment & Ecology

Re: does ultraviolet light cleanse drinking water, is equipment available?

Date: Wed Dec 30 14:11:49 1998
Posted By: Bob Peeples, MadSci Admin
Area of science: Environment & Ecology
ID: 911239571.En

I am sorry that it took so long to get back to you. This question has been passed through five scientists without an answer. I guess, as I am moderator for this group, it is my duty to try and answer it for you as best I can.

Ultraviolet (UV) water treatment systems use light wavelengths near 250 nm to alter the genetic material of microbes in such a manner that they are no longer useful for essential cellular processes. Mold, algae, bacteria, viruses, etc. are all sensitive to this treatment in sufficient doses.

The dose (D) of UV delivered within a UV chamber is the product of the average intensity (we will use IAVG because "I with a bar over it" is too hard to code) of UV light multiplied by the retention time (t) of the water within the chamber:

D = IAVG × t

The dose-response curves are logarithmic, so lethal doses are usually presented in terms of units per log10-1 lethal dose. Dose units are in mWs/cm2 or µWs/cm2. For instance, coliform bacteria require about 3˝ mWs/cm2 for each log10-1 of microbial reduction (reduction to 10% of original population). Twice that dose would reduce the population to 1% , three times the dose reduces the population to 0.1%. Similarly, if the goal was to reduce a million colony forming units (CFUs) of bacteria to one CFU (99.9999% reduction), it would take 6 times the dose that was required to reduce by 90%. The cost of UV treatment is usually based on contact time. If the flow rate through the system is slow, it is easy to get 10 seconds of residence time in a small UV light chamber. Increase the flow rate and the cost goes up quickly.

There has been some research done with ozone treatment that suggests a synergistic relationship between ozone and UV treatments. UV light seems to really get the old ozone fired up (that’s about as scientific as I get with that particular explanation).

I will also caution you that UV light, although effective on microbes, does not do much to reduce chemicals in the water. It can reduce some toxic organic chemicals to less toxic chemicals. If there is a lot of biomass removed in the final filter, the settling mass can and often does reduce suspended solids by simple flocculation—the particles are caught up in a stampede of dead bodies settling out of solution. But that is about it. You really need another system to remove things like dissolved lead. In fact, removing as many particles as possible prior to the UV treatment stage helps to maximize transmittance (T) of UV light in the UV chamber. Transmittance is the percentage of UV light that passes through 1 cm of water. The dirtier the water, the lower the transmittance. Average intensity is directly related to transmittance as the dose is directly related to average intensity.

For equipment, try searching something like yahoo! or your local yellow pages.

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