In Britain, small mom-and-pop shops are thriving by selling filtered used cooking oil as biodiesel for cars. They can sell the fuel at a discount to regular diesel because they get the grease free from fish-and-chips shops and other establishments. Besides cutting CO2 emissions, the business performs an important recycling service. Restaurant owners are legally banned from discarding their used oil in landfills; it also causes terrible clogging problems in drains, and is no longer allowed to be used in animal feed because of concerns about mad cow disease.
Although large-scale biodiesel refinery operations begun on both sides of the pond have flopped with the price of petroleum, smaller operations seem to have hit a sweet spot. (A US example is TriState Biodiesel in the New York City area.) Sometimes finding the right sustainable business model is a question of scale.