Producing biofuel from wastewater seems like a natural equation: take a resource that is produced incessantly, and is an environmental pollutant, and turn it into energy—and, in turn, reduce fossil fuel consumption.

Doing this on a large scale is much easier said than done, so it isn't necessarily a widespread practice—but it might be getting there. Here's a look at a few projects or companies that are transforming human waste into usable energy.