More than half of the 15 trillion gallons of sewage Americans flush annually is processed into sludge that gets spread on farmland, lawns, and home vegetable gardens. In theory, recycling poop is the perfect solution to the one truly unavoidable byproduct of human civilization. But sludge-based as fertilizer can contain anything that goes down the drain—from Prozac flushed down toilets to motor oil hosed from factory floors. That's why an increasing number of cities have begun to explore an alternative way to dispose of sludge: advanced poop-to-power plants. By one estimate, a single American's daily sludge output can generate enough electricity to light a 60-watt bulb for more than nine hours. Here are the six most innovative ways that human waste is being converted to watts:

Poop-Eating Bacteria

Turd Cell Smashers

Geological Toilets

Feces Ponds

Gassifiers

Poop Pyrotechnics

patented SlurryCarb process converts sludge

into char pellets that replace coal at a nearby cement kiln