Water that is unsafe for human consumption is considered unfit to drink. This unsuitability stems from the presence of contaminants like pathogens, toxins, or excessive levels of minerals. An example includes greywater from showers; while usable for irrigation, it should never be ingested.
The segregation of water sources based on usability is vital for public health and resource management. Utilizing water unsuitable for drinking for applications like toilet flushing, landscape irrigation, and industrial cooling conserves valuable resources. Historically, communities have often relied on diverse sources of water, adapting practices to use the most appropriate water quality for different needs.