Purification of water
Providing clean drinking and washing water is one of the basic acts of public health.
Water obtained from near the top of a watercourse may be acceptably pure, but most supplies need filtering to remove particulate matter, followed by exposure to a disinfectant agent to eliminate viral and bacterial infectious agents. Some water supplies may need removal of chemical substances - sodium and chloride are the most striking ones, but ground water in large areas is contaminated with arsenic.
Avoiding adding sewage to the water supply is a desirable component of the plan.
Individual water purification
Filtration through fine fabric, such as a Milbank bag, or under pressure through a ceramic filter followed by careful dosing with iodine or a chlorine bleach are common methods. Silver ions may also be used. Exposure of water to sunlight will reduce the bacterial count. Boiling for sufficiently long also works.
District water purification
More complex filtration systems, followed by automatic dosing systems are common, but a large scale version of the individual routine, involving water trucks rather than bottles may be used at times.
Chlorine is the mainstay, ozone and ultraviolet may be used as alternatives. Water desalinated by heat from a nuclear reactor or other large-scale heat source will be presumed sterilised at some point, reverse osmosis may also achieve this. While an individual probably cannot monitor their water supply, a district supply should be regularly tested for pathogens and chemical contaminants.