After dozens of posts, readers may be growing impatient with my focus on dairy cow manure. So, for a change of pace, I turn to pondering the organic waste that comes out of own homes--the majority of it food waste but also excrement. This potential energy leaves by two main routes--the sewer system (from both toilets and sink disposals) and the garbage system. In rough terms, each American dumps and flushes a pound of "fuel" every day, to be dispersed in sewage-treatment plants and landfills. I say dispersed because although both wastewater-treatment plants and landfills can be designed to capture some of the energy in garbage and sewage, their main goal is to make waste go away. And the material that arrives at these plants is already heavily diluted by water or inert solids, so energy production remains inefficient.
But what if we diverted all this organic waste--perhaps sending it in special pipes to small, energy-producing local digesters? One pound per person per day, converted into biogas, can be burned to produce about one kilowatt-hour of electricity--with enough extra heat (left over after warming the digester) to bring five gallons of water to a comfortable temperature for showering. Unfortunately, the typical American needs about twelve kilowatt-hours for household use, along with similarly excessive amounts of heat.
So there, in short, is why I focus on dairy manure: it produces far more energy than the farms could ever use and involves a lower "yuck factor" than what we dump and flush!