While much of what goes into biodiesel could be considered biodegradable waste, the concept deserves its own section because of its enormous potential. As we said before, biomass happens. When the quantities were relatively low, few people considered waste management a problem people used to burn trash in their back yards, or bury, or put it out for the trash truck without a thought. Industrialization, population growth, and dozens of other factors caused the amount of waste produced annually to skyrocket, and for a long time everything got dumped into old mines, quarries and other big holes: landfills.

Soon enough it became apparent that in solving the problem of what to do with the waste, other problems had cropped up. Before landfill developers thought to line the pits, contaminated water leeched into the aquifers. They attracted rats and other scavengers. "Landfill gas" (methane and CO2) filled the air, killing surface vegetation and contributing in a major way to greenhouse gases (1.2 tons of CO2 per ton of municipal solid waste, or MSW). And, quite simply, they stank.

Still, economics caused most MSW to end up in landfills. Then huge incineration plants called waste-to-energy facilities (WTEs) were built to take care of the problem. They do solve some issues: they reduce the volume of the MSW by 90 percent, and with 40 million Americans each producing an average of more than 1,600 pounds a year, that's important. WTEs can convert a ton of garbage into 525 kilowatt-hours of electricity (and 300 to 600 pounds of ash). Today, 14 percent of the MSW in the U.S. get incinerated.

But as with landfills. WTEs were not without their own problems. What remains after incineration is ash, and much of it hazardous. High concentrations of the metals (e.g., lead, cadmium) in the MSW remain in the ash, from dyes, inks, batteries, ceramic materials and more. Pollutants overall have decreased substantially-roughly 85 percent overall-since EPA introduced the Maximum Achievable Control Technology (MACT) standards in 1995 as part of the Clean Air Act. But U.S. facilities combine their fly ash (airborne) with their cleaner bottom ash, which brings the overall percentages of toxic materials into compliance standards for expanded reuse.

The newest development is converting the useful biomass energy stored in MSW to feedstock and ethanol in biorefineries. A number of processes are in the R&D and pilot stages, including one in Texas, that uses a biological/chemical process that anaerobically digests the biomass into liquid mixed-alcohol fuels.

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