Let's say that you are in place where you would like to produce your own drinking water. The water you are using might come from rain, or from a clean stream or lake. This is the type of system that can purify it for you:
The system uses a four-stage filter consisting of:
1) a sediment filter
2) two carbon filters
3) A reverse osmosis cartridge
Filtration is followed by a ultraviolet light system to kill any remaining bacteria.
Here are a lot more details on a typical multi-stage reverse osmosis filter system:
If you like the idea of ultraviolet sterilization, the Steripen is a portable way to use ultraviolet light on a small scale: