Low-cost, durable water filtration system

We will build a rugged, durable, self-contained water filtration unit that will supply sanitary water to meet the household water needs of a family in the third world, 40 l/day. It will kill or remove bacteria in the water. The filtration unit will have a target annual cost of $20 or less. The unit will not require external fuel or power for operation but may require occasional, modest human power for cleaning