According to Lucina Melesio Friedman in an article on scidev.net, the nopal (or prickly pear) cactus is a cheap and effective means of purifying water and is being advocated for use in rural areas of Mexico.
Knowing about properties of things and how they can be put to use can provide interesting story possibilities for cinema and fiction, whether for a character taking advantage of a property that is not so well known or writing about a hypothetical future in which a property is more commonly known and utilised.
Here is the abstract of a study on the water purification property of the nopal cactus.
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