Some problems are amazingly simple and inexpensive to solve.
For instance, village wells in northern Nepal are contaminated with up to 700ppb of arsenic. One solution is a filter made of gravel, sand, iron nails and shards of brick, requiring no energy and costing roughly twenty dollars to manufacture.
The filtering device, based on a design created by the Massachusetts Institute of Technology, relies on nothing more complex than iron nails, sand and gravel layered in a large plastic bucket....Under the direction of trained workers, Nepalese villagers are taught to take sand from riverbanks and sieve it through a wire mesh. Gravel and sand are layered in a clean plastic can topped by a diffuser containing iron nails and shards of brick. When water is poured through the filter, the arsenic bonds to the iron in the nails.This simple device removes 96 percent of the arsenic from the water. Meanwhile, chelating agents and vitamins can help to decontaminate the villagers themselves.
A more general form of water purification is provided by the Musaffa sachet currently being used in Pakistan:
Each Musaffa sachet weighs 250gm and when added to water in a bucket it provides effective water purification for one month at a cost of 60 rupees (less than USD $1).Going back to the subject of Nepal's water, the region is a hotbed of microhydropower, utilizing inexpensive turbines that can generate hydropower from a relatively small change in elevation. ("Drop," of course, is an abundant commodity in the Himalayas). The Nepalese have long used streams to power prayer wheels, so this seems like a logical development. (These days, they also have solar-powered prayer wheels!)
The sachet contains sand and a silver base to disinfect clear water. Silver is a well-known bactericide; it has been used since Roman times to add to liquids to keep them fresher for longer, and is often used aboard space shuttles to sterilize recycled water. Milk kept in silver jugs, or with silver coins in it, is known to delay spoilage.
Vietnam, however, leads the world in microhydropower. A 300-watt turbine costs about $20; it's adequate for a rural family's needs, and is considerably cheaper than getting energy from the power grid.