Speculative thinking over on the AltEnergy tribe (by Roy, but it sounds cool to me so I'm copying it here) :

I've thought up a better system, at least for the humid tropics. Make a large artificial marsh to purify ag runoff water. This needs swamp type plants to do this job. So we plant taro, an arum that rapidly produces huge starchy tubers. There's the starch to make sugars to ferment. The distilling fuel is methane gas but we'll get to that in a moment. The spent mash from this ethanol producing operation is part of the feed for a hog operation. This operation produces two valuable products, pork (and hides) and pig manure. The manure goes into a large biodigester, along with the taro leaves and other wastes, which, of course, produces the methane for the distillation process. Also the biodigester produces solid fertilizer for other operations and waste water to be purified in the artificial marsh. This waste water is rich in fertility for the growth the taro. A closed cycle with little expensive input other than labor. Comments?

Tribe Discussion: Solar Power and Alternative Energy - Tribe.net