It is likely that reusing water is as old as sanitation, since almost every community that pipes its liquid waste sees merit in using that stream. There is general support for deploying purified sewage as a resource. Two principles have been established to guide re-use practice, but there is an inherent tension between them. The first principle is that of 'fit for purpose' - only use water of a quality just good enough to meet each need, as opposed to producing drinking-quality water and applying it for the full spectrum of uses. The second is that water should only be reused when there is a net economic benefit accruing to society, so the total cost of purifying it and delivering it (including externalities) is less than that of alternative sources.
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