This is part of IEEE Spectrum's special report: Critical Challenges 2002: Technology Takes On
"Stick it right up to your nose," farmer Robert Aman told me. "You'll be pleased."
I scooped up a handful of fluffy black compost and sniffed. It smelled slightly sweet, like rich potting soil. I never would have guessed it was recycled cow manure if I hadn't seen AA Dairy's barn and its tenants: 550 Holsteins whose digestive tracts are just the starting point of Aman's system for managing his animal waste.
In the United States, animal waste, agricultural chemicals, and eroded sediment from irrigation foul over 275 000 km of waterways and account for 70 percent of the nation's water pollution. The situation is not unique to the States. "In virtually every country where agricultural fertilizers and pesticides are used, they have contaminated groundwater aquifers and surface waters," according to "Solutions for a Water-Short World," a report by the Population Information Program at the Johns Hopkins University Bloomberg School of Public Health, in Baltimore, Md.
The choices farmers make about how they farm have a direct impact on the quality of our food and the purity of our water. Sustainable farming systems minimize the use of chemical fertilizers and rely instead on the processing and judicious application of animal manures, alongside minimal tillage and crop rotation. Farms could even one day produce enough power to sustain themselves as well as their rural neighbors. But can such approaches feed a growing world population while preserving the integrity of the water supply?
In 1997, prompted by complaints from his neighbors about a strong odor of manure, Aman visited Mason Dixon Farms Inc., in Gettysburg, Pa., looking for ideas to solve the problem. Twenty years earlier, Mason Dixon had become the first commercial farm in the United States to process its manure through an anaerobic plug-flow digester. The digester is basically a long trough with an airtight cover, which expands as anaerobic bacteria break down waste material and release mostly methane gas. The gas powers a generator and has made Mason Dixon Farms independent of other power sources.
For years, anaerobic digesters have been used to process manure in countries like Germany, Denmark, Sweden, China, India, and Zaire. The idea is finally catching on in the States, and may get a push as U.S. farmers grapple with the consequences of consolidation in the livestock industry. The last 30 years have seen more animals and more animal waste being produced within smaller geographic areas. In response, thousands of farms have constructed huge waste-storage lagoons, some of which hold millions of liters of liquefied waste. If handled improperly, the waste may spill into surface waters or leach into groundwater. Neighbors often complain about noxious odors emanating from large open-air lagoons, a stench strong enough to sting the eyes and shorten the breath.
Inspired by his visit to Mason Dixon, Aman decided to install a digester on his own large dairy farm in Candor, N.Y., located about 400 km northwest of New York City. The design engineering work was partly funded by AgSTAR, a program sponsored by the U.S. Environmental Protection Agency (EPA), in Washington, D.C. To date, AgSTAR has provided technical assistance to 31 digester installations on swine and dairy farms throughout the country.
The AA Dairy setup is fairly straightforward [see diagram, above]. In their barn, the Holsteins eat from a trough and eliminate on the floor, between trips to the milking parlor. Manure and urine are scraped into a receptacle and flow down into a holding tank. Periodically, a pump feeds a batch of the waste into the digester, a receptacle about 9 meters wide by 41.5 meters long and 4.25 meters deep, with an airtight cover that swells as gas is produced. Each new plug of waste pushes material further through the digester, where anaerobic bacteria feast on the sludge, releasing on average 1600 m3 of biogas per day, about 55-60 percent of which is methane. Other gases produced during the digestive process include carbon dioxide (approximately 40 percent) and a small amount of hydrogen sulfide.
The biogas passes through a series of pipes to the generator station, where a diesel engine converted to run on methane drives a 130-kW generator. The generator powers the farm by producing about 80 kW of three-phase 208-V electricity around the clock. About a year after his visit to Mason Dixon, Aman's farm began producing electricity. Now he sells 10-20 percent of what he produces back to the grid in the summer, when fans used to cool the cows push power usage on the farm to its peak. In the winter, anywhere from 35 to 40 percent of the electricity goes to the grid.