When sewage is treated at a wastewater treatment facility, biosolids are the byproduct. After being separated from the water, biosolids are usually sent to a landfill or incinerated. That doesn't mean that they're without value, however. Vitruvian Energy has created a process to make a usable fuel out of this human waste product, and while the source is pretty gross, it is undeniably abundant, and the results are much cleaner.
Here's the recipe: separate the water out of some sewage, leaving the biosolids behind. Then pipe those solids into airless tanks and let some microbes go to town. These bugs release a gas (roughly 60 percent methane, 40 percent CO2) which you can burn to power the plant where you're doing all this work but remember to send some of the methane to a tri-generation machine that leaves you with electricity, heat and hydrogen.