This is a very simple and effective solution to clean drinking water: Contaminated water is filled into transparent plastic bottles and exposed to full sunlight for six hours.
Sunlight is treating the contaminated water through two synergetic mechanisms: Radiation in the spectrum of UV-A (wavelength 320-400nm) and increased water temperature. If the water temperatures raises above 50°C, the disinfection process is three times faster.
In a survival situation, you are bound to come across discarded clear plastic bottles. This tip could come in handy. You should also carry other means to purify water for drinking.