You have the filter/purifier part rightshell need both in those areas. And that isnt a knock on Third World water standards. Its increasingly the case that a purifier is needed everywhere, the United States included.
I think the best way to approach this is through a two-stage approach. First, filter to remove grit, chemicals, and tiny creatures that can wreak havoc in your gut. Then, purify to kill viruses and other baddies the filter doesnt catch.
Shell be in these areas for at least a couple of years, correct? So Id get a really reliable filter. MSRs WaterWorks EX ($150; msrcorp.com) is sturdy, effective, easy to use, and easy to maintain. Id pack a spare cartridge ($40), membrane filter ($60), and maintenance kit ($20). Katadyns Pocket Filter ($220; katadyn.com) is another excellent model. Its a little more rugged than the WaterWorks and equally effective at first-stage cleaning of water. Both units will remove chemical tastes, and catch nearly all measurable quantities of things such as giardia and cryptospiridia.
After the cleaning comes the purifying. The simplest way to achieve this is with MSRs SweetWater ($10 for a 2 oz. bottle, which is enough to treat 80 gallons). You add five drops of this stuff to a liter of water, mix it up, let it stand for five minutes, and drink. Potable Aqua ($8.50 for 50 tablets; potableaqua.com) works well also, but it can end up costing more than the SweetWater solution.
A good filter and water purifier is a combo that should keep your sister safe. And thats a wonderful thing shes doing.
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