I'm using a relatively new, fairly inexpensive technology that effectively purifies even heavily contaminated water without any power requirement at all.
It's a ceramic dome .2 micron filter, packed with activated charcoal, silver, and other materials, that removes bacteria, heavy metals, etc. It produces safe drinking water from even heavily polluted ditch water. It first came to my attention when used in disaster relief after the big tsunami in India a few years ago, and then again in Haiti after the hurricane. Now it is being used by overseas troops in distant postings. The smallest dome filter, which retails for about $22, can be used with 2 buckets or barrels to form a very simple gravity drip filter system which will process about 15 gallons a day, or by pressurizing the feed tank with a small bicycle pump, it can process 70 gallons a day.
With a small amount of simple maintenance... cleaning the dome surface every two weeks with a brush or soft cloth... the filter is good for 6 months.
You can buy the filter alone and DIY, or buy a complete gravity drip system (filter, 2 5-gallon buckets, spigot) for about $50, or get the pressurized one for about $60. For a more "First World" installation, you can even get an under-sink housing for conventional plumbing for about $100, or step up to a larger size "candle" style filter for whole-house type applications. The results of lab tests of these filters seem almost too good to be true, but they've been verified multiple times by various independent organizations. (Results below) The combination of the .2 micron pore size and activated packing material creates an effective filter water an order of magnitude finer than the conventional water filters sold in hardware stores, literally finer than bacteria. You can read more about them or purchase them from Monolithic Domes or from Just Water.
Removal capabilities as follows:
>99% Arsenic 5 and 99% Arsenic 3 (special order)
>99% Hydrogen Sulfide (H2S)
>95% Chlorine and Chloramines
>95% Poly Aromatic Hydrocarbons
>99.999% of particles larger than 0.5 micron (Staffordshire University Labs) (includes Anthrax)
>99.7% of particles larger than 0.3 micron (Staffordshire University Labs)
>98% of particles larger than 0.2 micron (Staffordshire University Labs)
>100% Giardia Lamblia
>100% removal of live Cryptosporidium (WRc Standard)
>100% removal of Cryptosporidium (NSF Standard 53 – A.C. fine dust – 4 log challenge)
>100% removal of E. Coli, Vibrio Cholerae (Johns Hopkins University)
>99.999% removal of Salmonella Typhil, Shigella Dysenteria, Kiebsiella Terrigena (Hyder Labs)