Sewage—refuse liquids or waste matter usually carried off by sewers—is at the front lines of a global movement for clean energy. Innovative U.S. cities are digging into their dirtiest depths to create new sources of power that optimize economic benefits, generate clean energy, and control pollution. This wastewater-to-power movement is just beginning to catch on in China. But with some of the largest and most densely populated cities in the world, the country could be poised to lead a sludge-to-energy revolution.

Ten years ago, amateur tilapia farmers in China were able to dig a pond, fill it with fish, add antibiotics and chemicals, and a few months later sell the fish to numerous unregulated processors. In those early days, fish farming created a great economic boom for first-time aquaculture farmers.

“Made in China” products surround us, yet few consumers have anything more than a foggy idea of where in China their phones, computers, and other goods come from. Hunan Province in South Central China is not only the home of spicy food, but the world’s largest mines for non-ferrous metals used in many electronic devices. Nearly all the glass panels for Apple and Samsung smartphones are manufactured in Hunan as well. While this multibillion-dollar phone industry has been a boon for Hunan’s economy, it has also produced seriously polluted rivers and soil.