http://www.survivalblog.com/2009/02/solar_water_disinfection_and_p.html
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This article describes so me simple and practical methods for providing drinkable water in disaster situations. They fit with my motto: "Keep calm, and carry on!"

Cardboard and aluminum foil are unlikely tools for disinfection water until you factor in sunshine. Solar water pasteurization uses the heat of the sun to raise the temperature of water to a point where microbial pathogens are destroyed. Disease-causing organisms in water are killed by exposure to heat in a process called pasteurization. Water that has been heated to 65ºC (149ºF) for a short period of time is free from microbes including Escherichia coli, Rotaviruses, Giardia and the Hepatitis-A virus.

Although traditional fuels can be used to pasteurize water, on sunny days solar energy is the better choice. A major problem with boiling water for disinfection is its energy consumption in relation to cost and availability of the fuel supply. If you do not have electricity you do not want to spend all of you fuel supply sanitizing your water. Boiling water for five to ten minutes takes a lot of fuel.

With full sunshine, it takes about two hours to pasteurize two liters of water. In order to determine when water has reached pasteurization you will need to invest in a simple device called a Water Pasteurization Indicator (WAPI). The WAPI is a simple reusable device containing a special soy wax. The wax melts at the same temperature as the water is pasteurized. The tube hangs on a string inside the container with the wax end up, and once the water around it becomes hot enough to kill the bacteria the wax melts, running from the top part of the tube to the lower end. Although it is designed for solar pasteurization, the WAPI can be used for pasteurizing over most fuel sources including gas, wood, and charcoal. WAPIs generally cost between $5 and $10.

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