Reply #15: So "the heart of the corn belt" is all that needs consideration?

[JohnWxy]

I would say, without actually doing the numbers, that all the states where corn or soy beans or sugar cane, can be grown would equal more than a "very little of the U..S. land surface area.", whatever the significance of that is supposed to be. MOre to the point is the supply of animal waste. Generally speaking, wherever they are growing crops there is are also cattle and pigs being raised in some number.

In my view, anything that reduces fossil fuel use cost effectively (keep in mind a lot of money and no small amount of effort is being expended dealing with this waste anyway - and we're not getting anythng for it.) is worth pursuing and should not be minimized.

ONe point some have made, rather stridently, is the use of fossil fuel in the manufacture of ethanol. Just on this point alone, that fossil fuel can be greatly reduced or eliminated, makes this 'closed loop' technology a very significant development (especially to those who have raised the issue of fossil fuel use in ethanol production).

regarding what's left after anaerobic digestion:

http://en.wikipedia.org/wiki/Anaerobic_digestion

By-products of anaerobic digestion
There are three principal by-products of anaerobic digestion.

Biogas, a gaseous mixture comprising mostly of methane and carbon dioxide, but also containing a small amount hydrogen and occasionally trace levels of hydrogen sulfide. Biogas can be burned to produce electricity, usually with a reciprocating engine or microturbine. The gas is often used in a cogeneration arrangement, to generate electricity and use waste heat to warm the digesters or to heat buildings. Excess electricity can be sold to electricity suppliers. Electricity produced by anaerobic digesters is considered to be green energy and may attract subsidies such as Renewables Obligation Certificates.
Since the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle biogas does not contribute to increasing atmospheric carbon dioxide concentrations; because of this, it is considered to be an environmentally friendly energy source. The production of biogas is not a steady stream; it is highest during the middle of the reaction. In the early stages of the reaction, little gas is produced because the number of bacteria is still small in size. Toward the end of the reaction, only the hardest to digest materials remain, leading to a decrease in the amount of biogas produced.

Acidogenic anaerobic digestateThe second by-product (acidogenic digestate) is a stable organic material comprised largely of lignin and chitin, but also of a variety of mineral components in a matrix of dead bacterial cells, some plastic may be present. This resembles domestic compost and can be used as compost or to make low grade building products such as fibreboard.
The third by-product is a liquid (methanogenic digestate) that is rich in nutrients and can be an excellent fertilizer dependent on the quality of the material being digested. If the digested materials include low levels of toxic heavy metals or synthetic organic materials such as pesticides or PCBs, the effect of digestion is to significantly concentrate such materials in the digester liquor. In such cases further treatment will be required in order to dispose of this liquid properly. In extreme cases, the disposal costs and the environmental risks posed by such materials can offset any environmental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialised catchments.
Nearly all digestion plants have ancillary processes to treat and manage all of the by-products. The gas stream is dried and sometimes sweetened before storage and use. The sludge liquor mixture has to be separated by one of a variety of ways, the most common of which is filtration. Excess water is also sometimes treated in sequencing batch reactors (SBR) for discharge into sewers or for irrigation.