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Human urine could replace a quarter of the commercial fertiliser currently
used for food crop production, estimates A+ Science of Gothenburg, Sweden. But
health authorities worry about hygiene. The company has tested a system which
purifies human urine by removing all traces of solid excrement. The urine is
cooled until half is frozen. The remaining liquid, which contains most of the
nutrients, is then mixed with magnesium oxide to precipitate crystals of
magnesium ammonium phosphate hexahydrate—known as struvite—which can
be used as a crop fertiliser.