Here's a surprising fuel source: dog doo.
Many European farms have been turning animal waste into biofuel for 20 years now.
The city of San Franciscans is launching a program to recycle the waste from its 120,000 plus pooches and other pets. It's estimated that about 3.8% of their residential garbage collected is composed of pet litter. They want to reduce that number to zero. Not only are they going to recycle pet waste from residential garbage, but they're going to pick it up from the city's dog parks.
Since we feed our dogs and cats such rich diets, their feces have high energy potential. The feces contain bacteria called methanogens that, under the right conditions, convert the organic matter in feces to methane gas. San Francisco will collect its pet feces in biodegradable bags and then run them through a tank, where the natural process of methanogenesis will be exploited to produce methane gas. The gas can then be piped directly into homes.