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Current IssueAugust 28, 2015
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Ask The Expert: Distilled vs. reverse osmosis
Question: Q: My name is Wanda Richardson, and I am a subscriber. I have been combing through various WC&P publications trying to determine whether there are any significant advantages or disadvantages to using water for drinking purposes that has been purified via distillation vs a system that combines reverse osmosis, UV sterilization and ozonation. Based on what I have read so far, the only difference that jumps out is taste. Help! Thanks.
Wanda Richardson
Accra, Ghana

Answer: A: As evident by your question, choosing a home water treatment system is not an easy task. Although each treatment can produce a different taste in the finished water, there are other health-related points to consider. The following is a brief summary of each of the processes that you mentioned and some (not anywhere near all) of the good and bad points to consider.

Distillation -- The process of separating contaminants by boiling the water to steam and condensing or cooling the purified water into a separate reservoir.

Advantage: Kills bacteria, viruses, and other microbes effectively. Water boils to steam before many heavy organics, inorganics and metals. Softens water. Disadvantage: Produces a flat, tasteless, and sometimes bitter product. May not boil extensively. Does not remove volatiles and other chemicals with boiling points lower than or near that of water. May be corrosive to plumbing.

Solution: Vent volatiles. Add granular activated carbon (GAC) filter to pick up the slack from any "carryover" contaminants.

Reverse osmosis -- Forces water through semi-permeable or selective membrane that separates contaminants from the water.

Advantage: Removes 98-100% of pesticides, 90-99% of dissolved pollutants and many bacteria.

Disadvantage: Cannot remove lighter molecules, i.e., trihalomethanes (THMs) and nitrates. Requires frequent membrane replacement. Slow, uses lots of excess water.

Solution: Combine with GAC.

UV and ozone -- Both are disinfectants aimed at reduction of microbiological pathogens. Each are effective provided that the water is low in turbidity, which may require pretreatment to remove total dissolved solids (TDS) as well as other more technical considerations for both operation and maintenance. Protozoa are easily killed by ozone, but relatively resistant to UV—although more recent studies show it to be much more effective than was previously thought at lower doses of ultraviolet irradiation.

Based on this information, a combined RO, UV and/or ozone system would offer more health based protection compared to a distillation unit alone. But whether this combined system is necessary for you as an individual depends on the original quality of your source water. UV and ozone aren't usually required for either process unless the water is to be stored for extended periods, or comes from a microbiologically contaminated or suspect source.

Hope this helps.

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