Most experts feel that if the water reaches a rolling boil it is safe. If one wishes to do this, a pressure cooker would allow the water to be kept at boiling with out loosing the heat to evaporation. African aid agencies figure it takes 1 kg of wood to boil 1 liter of water. Hardwoods and efficient stoves would improve on this. Water can also be treated at below boiling temperatures, if contact time is increased.
Heat exchangers recover most of the energy used to warm the water.
Solar pasteurizers have also been built that would heat three gallons of water to 65 deg C and hold the temperature for an hour. A higher temperature could be reached if the device was rotated east to west during the day to follow the sunlight. Regardless of the method heat treatment does not leave any form of residual to keep the water free of pathogens in storage. Reverse Osmosis Reverse osmosis forces water, under pressure, through a membrane that is impermeable to most contaminants. The most common use is aboard boats to produce fresh water from salt water.