Memorably called the “Reinvent the Toilet Challenge,” the competition solicited designs for toilets that do not require a sewer connection or electricity and cost less than five cents per user per day. The winning entry was a toilet system from Caltech that uses photovoltaic cells to power an electrochemical reactor that treats human waste, producing clean water for flushing or irrigation and hydrogen that can be stored in fuel cells. The system is entirely self-contained; it has no need for an electrical grid, a sewer line, or a treatment facility. The only input the toilet requires, beyond
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