Joules Before Swine

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Family pig farms used to be as much a part of the old South as homemade sausage and red-eye gravy. What’s left of swine farming in the Southeast today, however, has gone corporate — generating larger profit margins, but also a flood of new wastewater.

Recently, all in the name of bioenergy, a portion of that effluent has been used to fertilize and irrigate an experimental stand of Southeastern coastal Bermudagrass (Cynodon dactylon L.).

Normally used as livestock forage, these particular grass swaths were cut and dried for analysis as bioenergy-rich hay. The details are in a paper to appear in the journal Bioresource Technology.

But from 2003 to 2005, lead author Keri Cantrell, an agricultural engineer at the USDA’s Agricultural Research Service in Florence, S.C., and her colleagues used a form of subsurface wastewater drip irrigation over a half-hectare (1.2 acre) of Bermudagrass in Duplin County, N.C.

By using treated swine wastewater for irrigation from an adjacent 4,400-animal swine facility, Cantrell said their process also automatically freed up fresh groundwater for other use.

The group’s experiment registered impressive potential Bermudagrass energy yields — ranging from 127.4 to 251.4 gigajoules per hectare (2.47 acres).

A single gigajoule represents a significant amount of energy — some 278 kilowatt hours, or roughly a third of a two-bedroom household’s average monthly electric consumption. But the problem of just how to best convert this potential Bermudagrass bioenergy remains open for debate.

Ken Stone, an agricultural engineer at the Florence facility and one of the paper’s co-authors, says one possibility might be some type of fermentation to produce ethanol.

While the Midwest has used its principal corn crop to produce as much ethanol as possible, Cantrell says that the Southeast has an advantage with its plethora of potential bioenergy foodstocks.

And unlike ethanol produced from corn, which in the Midwest is normally harvested once a year, Bermudagrass grows so quickly in the Southeastern summer, Cantrell says it can be harvested three or four times a year.

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