Middlebury, Vt. — A 40-by-80-foot balloon, shaped like the risen dough for a homemade loaf of bread, fills one of the larger buildings on the Foster dairy farm here. It's a digester, the ``key element,'' as Bob Foster puts it, in a waste management system that converts farm manure into several useful products: electricity, sold to the local utility; odor-free fertilizer; and animal bedding that can also sell as an enriched potting medium in the garden trade. Or, because of its high protein content, it could be processed into feed for animals other than dairy cows.
Currently carbon dioxide is the only byproduct not put to use, but even that will be captured for greenhouse enrichment in the near future.
In 1981, the extended Foster family, descendants of the original Foster who moved to the Champlain Valley a century ago, had the system installed on their 1,800-acre farm. ``At the time,'' Bob Foster says, ``not all of us wanted it; today we're unanimous. If we were to start all over again, there wouldn't be one dissenting vote.''
With suburbia advancing and environmental concerns about runoff and water pollution growing, manure in such quantities could no longer simply be spread on the fields. ``We knew that whatever waste system we chose, it would cost us some. So we looked for something that would give us a return,'' says Foster.
What the Fosters got after linking up with Hadley & Bennett, an engineering/construction company in Bradford, N.H., is an anaerobic (without oxygen) digestion system that produces methane gas as the manure breaks down. The gas drives an electric generator about 14 hours a day, which feeds directly into the utility power lines. The remaining slurry is separated into a liquid fertilizer and a fine dry fraction.
Earl Gavett of the US Department of Agriculture's energy division estimates that there are fewer than 200 manure-to-electricity plants in the country. ``The procedure is simple enough,'' he says, ``but the economics aren't always there.''
A 1985 study by Dr. Billy Lessley, an agricultural economist with the University of Maryland, indicated that farmers would need to get 7 cents a kilowatt-hour to justify installing methane-to-electricity waste-management systems. In most rural areas farmers could not be guaranteed that figure by the utilities.
The Fosters get just under 7 cents at present and may receive less in future unless imported oil rises in price again. But they say there are other reasons this route is appropriate for them.
Currently electricity earns them some $70 a day; the processed fertilizer is valued at about $25,000 a year; and bedding derived from the remaining solids at something more than $20,000. In addition, energy and labor are saved by pumping the slurry to the fields rather than hauling it there by truck; the cows are healthier and more comfortable because the bedding is more absorbent than either the usual straw or sawdust; and neighbors are happier because of the reduced odor.
The system works this way: Twice a day when the cows are stall-fed, the manure is cleaned out and injected into the front end of the digester - a three-foot-deep concrete tank with a loose plastic cover that inflates as methane is produced. Each fresh charge of manure moves the mix slowly through the digester. It takes 22 days to complete the process.
The slurry is the consistency of thick cream, and Bob Foster doesn't hesitate to dip his hand into it. ``Here,'' he says, ``you'll barely smell a thing.'' I bend my nose to within an inch of Foster's hand and the absence of the expected odor is startling.
Odor control wasn't a top priority when the system was put in, but now Foster recognizes it as a valuable asset. ``Controlling odor problems is becoming increasingly important to farmers as housing moves out to the country,'' he says.
Foster recalls that a neighbor asked shortly after the system was installed what had been done to the manure. ``When you used to spread it, I could smell it for days afterward,'' she said. ``Then, yesterday, I saw you spread it again, but there was no odor and there's still no odor!'' She was pleased, because she had planned a family gathering for the very next day.
For odor control a methane system is ``justifiable right now,'' according to Agway's Stan Weeks. Otherwise, he says, ``It's economic only where electricity costs are high.''
Other positive things happen to the manure as it is processed: It improves as a fertilizer. The ammonia in the manure converts to a much more stable ammonium, says Spencer Bennett of the engineering firm. This means that far less nitrogen is lost into the air as a gas during storage. But the nutrients remain readily available to plants.
The bacteria population of the slurry explodes as it exits the digester, producing enzymes and other natural chemicals that boost soil fertility. And the manure's acid 5.4 on the pH scale becomes a slightly sweet 7.5, a distinct plus for acid-rain areas.
What sounds fine in theory also works out in practice. On the Foster farm, corn fertilized with the slurry produced cobs averaging four inches longer than corn fertilized with straight manure when the project first got under way. Today all manure on the farm is processed through the digester; unprocessed manure is no longer used on the farm.
With a herd of 600, the Foster farm produces enough manure to support a generating plant able to feed into a utility. Most farms in the region, however, are too small to justify the cost and effort involved. So Hadley & Bennett is planning a centralized facility that could process the manure from the region's 35,000 cows.
Such a facility will produce a megawatt of electricity an hour rather than the Fosters' one megawatt a day. On that scale and with the dry fraction sold as a potting medium, the ``economics are justifiable on under 7 cents a kilowatt-hour,'' says Bennett.
Dr. Weeks contends that the need during the present cheap-energy era ``is to keep the technology alive so that we are ready to run with it'' the moment energy costs increase.
Bennett expresses his views this way: ``Each cow in America is capable of producing 4,000 gallons of top-rate fertilizer and 10,000 hours of electrically powered light each year.''
Catalyst boosts methane-producing bacteria
In Switzerland, near Lake Geneva, the Roche treatment plant is incinerating all the sewage from Vevey and Montreaux in a very practical manner: It burns the sewage using the methane gas extracted from the waste as a fuel.
The process is highly cost effective, producing something in the region of 12 cubic feet for every pound of liquid sewage that goes in, or a 50 percent improvement over normal production.
Technical reports from Roche engineers give much of the credit to a Canadian discovery: a catalyst that boosts the methane-producing bacteria present in all liquid wastes.
Developed by biologists at Kamber Industries in Nanaimo, British Columbia, the catalyst was tested in the late 1970s in Nepal, where bio-gas production is commonplace. Then, ``just as we were ready to market the catalyst,'' says Kamber Industries' Alan McInnes, ``cheap oil flooded back onto world markets,'' and interest in enhanced gas production waned.
In recent years, however, several European plants have begun using the catalyst. Interest in North America has been nil, says Mr. McInnes, but that may change in the months ahead. Spencer Bennett, of Hadley & Bennett, a Bradford, N.H., company that builds methane digesters, plans to test the catalyst at the Foster farm in Vermont.
Bennett believes that if it proves as effective as Swiss reports indicate it should be, the economic picture for farm-produced methane could improve significantly.