Turning Yucky Stuff into Energy – It’s a Gas

Two things we try to avoid stepping in are garbage and manure. Yet, disgusting as they may be, these two members of the biomass clan are sources of renewable energy. Just not in their usual forms.

Take garbage. Day after day it is trucked out to huge landfills where it gets buried by more garbage. As the trash piles up, the lower layers become starved of oxygen and the conditions near the bottom of the heap become anaerobic, allowing anaerobic bacteria and other microorganisms to feast on the garbage, creating landfill gas, a mixture of methane and carbon dioxide.

Once a landfill is full, it is usually capped by thick layers of dirt and often a sealing membrane, and left to sit, while more landfill gas accumulates. Finally, collection wells are drilled and cased to the base of the landfill. The section of the casing penetrating the waste layers is perforated so the landfill gas can enter the pipe. Unlike natural gas wells, landfill gas must be pumped out of its reservoir.

Agricultural wastes such as manure, crop residue, and silage are collected in a digester, a large, domed tank, often built underground. Again, as the waste accumulates, the lower section becomes oxygen-starved and anaerobic microbes acting on the waste produce methane and carbon dioxide. Because the material in the digester is a thick liquid slurry, the biogas rises to the top of the digester where it can be siphoned off. Once the slurry has been digested, the residue can be used as fertilizer.

With both processes, the carbon dioxide must be removed before the biogas can be used as fuel. Biogas can be used as a substitute for natural gas in fuelling electricity generation, space heating, and natural gas powered cars and buses.