March 24, 2009 | General

Ontario Dairy Digester Grows With The Flow

BioCycle March 2009, Vol. 50, No. 3, p. 37
Multiple tanks enable experimentation with recipes combining manure and off-farm wastes to increase revenues and biogas production.
Peter Gorrie

STANTON Farm near the village of Ilerton, Ontario, Canada is a model of high-tech efficiency: Every cow wears a transponder; through the devices, computers monitor their movements, open and shut gates to direct each animal on its appointed route, and glean health information by measuring not only how much they produce during their three daily sessions in the scrupulously clean milking parlor, but also whether the rate of flow varies minute to minute.
The 2,000 dairy cows that mingle in the airy, indoor pens excrete huge quantities of manure – a daily total of 40,000 gallons. To manage the manure, the Stanton family has joined the small but growing ranks of North American farmers trying to recycle this disposal problem into renewable electricity, treating manure as a valuable resource, says Garry Fortune, the main consultant on the project for Laurie Stanton and his wife, Sandy.
Stanton Farm will, in full operation, generate a steady 1.3 megawatts of electricity. It will also create a low-pathogen liquid fertilizer, heat and a solid material similar to peat moss that serves as bedding for animals or a soil amendment for gardens. The project’s original catalyst was the stink of manure, which often sparks hostility between farmers and their neighbors. Two decades ago, the Stantons were farming on the outskirts of London, a small city about two hours west of Toronto, Ontario. When it became clear they’d soon be overrun by urban sprawl, they started to acquire land a few miles further north, near Ilerton. With their three adult sons and a daughter, the Stantons began to develop the new farm there in 2006.
“When we started, we wanted to have the least impact on the community, and hopefully be a positive influence,” Laurie said during a recent tour of the farm. The biogas system wasn’t required to site the dairy farm, he adds, but “we looked at this as an environmentally friendly way to deal with waste and odor issues.”
At the same time, with concern about climate change mounting, the Ontario government introduced North America’s first feed-in tariff, the Renewable Energy Standard Offer Program, to promote alternative electricity sources. Originally, in its simplest description, it paid about 34 cents (42 cents CAD) for each kilowatt-hour of solar-photovoltaic power that operators send to the grid. Wind, biomass, small hydro and all other renewables received the equivalent of nearly nine cents. The incentive was less generous than those offered elsewhere, particularly in Europe, where biogas production occurs at thousands of farms, but it helped the Stantons to commit to biogas production.
Stanton Farm cattle mingle in large indoor pens. Those areas, as well as walkways and the milking parlor, are equipped with flushing and scraping systems that collect the manure and move it to a small outbuilding where it’s sent to one of two mixing tanks. There, it’s combined with off-farm wastes – for now, mainly fats, oils and grease – trucked in from local food service and processing businesses and held in storage tanks until use. Current regulations require that animal-based ingredients be pasteurized, so the Stantons plan to construct units for that job. The initial food waste will be quite liquid: eventually, a chopper will be added to handle more solid matter.
The resulting slurry from the mixing tanks is piped into silo-like digesters, installed by an Ontario company, Dairy Lane Systems. The operation is one of only seven to date that employ vertical induced blanket reaction, a process developed at Utah State University and now designed and built by Andigen LC, of Logan, Utah. It features relatively small tanks instead of the usual one or two large vessels. The Stantons have eight tanks, each 32 feet tall, just over 13 feet in diameter, and with a capacity of 30,000 gallons. They are inside an insulated building that is heated, like the slurry, to 100°F.
The system operates in a continuous flow, from cattle to mixer to digester, to ensure the feedstock is as fresh as possible – for maximum energy potential – and digestion proceeds uninterrupted. In induction, the slurry moves up through a column of highly concentrated bacteria. The intense interaction cuts the breakdown time to between 5 and 7 days, compared with up to 30 in conventional digesters, and produces an enhanced volume of gas with high methane content. The process requires no mechanical agitation, explains Fortune. The bacteria attach themselves to the solid matter and rise with it as their digestion converts it into gas. At the top of the tank, they bump into baffles and drop down to begin feeding again. Paddles rotate at the top, but only to prevent crusting.
The multiple digesters enable gradual expansion of the system and eliminate the risk of having to shut down the process entirely if the bacteria fail, Fortune says. A crippled tank can be emptied and recharged from a healthy one for an almost instant restart. The Stantons’ system is also modified to let two digester tanks be taken offline. That feature, along with the two mixing tanks designed by Dairy Lane – one 76,000 gallons, the other half that size – allows for experimentation to produce a slurry the bacteria will thrive on and ensure consistent production of biogas. The mix is about 25 percent off-farm waste, but the actual composition can vary. Material from neighboring farms and food processors will be required to hit the 1.3-megawatt target, and each slurry mixture requires a different recipe.
“The whole key is to mix it in the right recipe,” Fortune says. “This is a trial-and-error process. Unique to our facility is the ability to undertake full-scale research in this regard. It’s one thing to do testing in a lab; it’s quite another to do it in a full-scale environment.”
Stanton Farm installed two 150-kWh Cummins diesel generators, modified to burn methane. The farm opted for two small generator units to avoid total failure, facilitate expansion and servicing, and increase flexibility. There’s no biogas storage: It flows straight to the generators, so the system must be able to quickly adapt to changes in output from the digesters. Heat from the generators is captured to warm the slurry and digester building and supply in-floor heating throughout the farm. The generator operates at about 40 percent efficiency, Fortune says, although cogeneration lets the system exploit more than 90 percent of the biogas energy.
Digestate from the system is about 4 percent fine lignin fibers suspended in liquid. Three-quarters of the solids are recovered in a process designed by Dairy Lane, to create bedding and a peat moss substitute. Some liquid is removed as the digestate is carried along a horizontal corkscrew device. Then, five roll presses reduce the moisture content to 30 percent.
Overall, the system “is a lot to put together,” Fortune says. “Although the fundamentals are relatively simple, the operation and controls can be complex.” Along with getting the slurry recipe correct, the design must prevent manure and slurry from plugging equipment, crusting or foaming. Sophisticated computer, electrical and mechanical systems are required to control temperatures, the flow of heat and materials, removal of hydrogen sulfide from the biogas, and the connection to the electricity grid. And everything must mesh.
“It’s a state of the art facility,” Laurie Stanton says. “If there’s a better way to do it, we want to find out. We didn’t compromise. We’ve tried to get things fixed as we went along; we got things right so others could follow in our footsteps. We’re the template.” To date, the system has cost $4.1 million. Tipping fees for the off-farm waste are an important part of the financing, but the payback time – in fact, the economic viability of the project – depends on whether the feed-in tariff is increased, notes Fortune. Because of the initial expense, substantial operating costs and health and environmental benefits, he’d like to see it doubled to at least 16 cents/kWh (20 cents CAD). The province of Alberta pays roughly 13 cents under a different form of subsidy. Germany’s feed-in rate translates to roughly 25 cents.
Fortune also wants the Ontario government to, as in Europe, dramatically reduce grid connection costs. That expense at the Stanton Farm was $400,000. The change could happen. In February, the provincial government introduced a Green Energy Act that, if passed by the Ontario legislature, would empower it to revise the feed-in tariff rates and reduce red tape. Details haven’t been decided, but, Fortune says, “It’s a great start.”
Biogas is a good source of renewable energy, although the only uncertainty is its economics, says Mahendran Navaratnasamy, a renewable energy expert with the Alberta government. The operator’s cost of purchasing the system and construction/installation for biogas power is about $6,500/kWh, he says. Industry sources say the equivalent for wind power is $1,500 to $3,000/kWh; for solar photovoltaic, $6,000 to $10,000/kWh. But unlike those two, biogas systems can generate electricity nonstop.
Stanton Farm “is one step ahead of others,” Navaratnasamy says, and it fits his view of how biogas will develop. That is, it won’t likely become an industrial-scale source of electricity; it’s too expensive to transport organic wastes any distance to a centralized plant. “This is microgeneration: If you want to produce a considerable amount of energy you need a lot of small digesters,” he explains.
Navaratnasamy adds that organic wastes are not created equally for biogas production. The manure from dairy cows is an ideal seven percent solids. Because beef cattle drink less water, their excrement is much drier; difficult to handle and slow to decompose. So, too, is typical household organic waste. On the other hand, manure from hogs and poultry, and the human sewage that floods into municipal treatment plants, are too watery. Feedstocks could be combined to create a suitable mixture, but, then, transportation costs come into play, he notes. Diluting dry manure with water also adds to the expense.
The Stantons will have to contend with such issues as they expand from their just-right slurry to off-farm sources. They’ll also continue to seek additional uses of the digester’s outputs. The solid digestate might be incorporated into plastics. If the technology ever makes commercial sense, it might eventually be processed to create cellulosic ethanol, making use of cogeneration heat. That same source might also warm greenhouses, where plants would absorb carbon dioxide emitted by the generator engine or stripped from the biogas – if the Stantons decide to sell purified methane into the pipeline distribution system.
Researchers at the nearby University of Western Ontario are investigating whether liquid digestate and algae combined in clear tubes strung through greenhouses would produce a worthwhile quantity of biodiesel. If it did, dead algae from the process would create a protein supplement for cattle. “This is exciting,” Fortune says. “There have been challenges, but when you’re pioneers like the Stantons, the road is always much more bumpy.”
Peter Gorrie is a freelance writer based in Toronto, Ontario, specializing in energy and environment issues.

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