Humans in the U.S. generate about 220 million tons of municipal solid waste every year. About 70% of it is organic waste, including paper, cardboard, wood, food, grass, leaves, and brush. Humans also create a similar amount of sewage sludge, grease trap waste, and industrial waste. Agricultural animals generate an estimated 170 times more organic wastes than humans. Using conventional techniques to dispose of these wastes can be very expensive, both in terms of dollars and to the environment.