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Documents, Tools and Resources
Anaerobic Digestion Gets New Life on Farms
by David Riggle
Twenty years ago, the concept of farm scale anaerobic digestion
was very much alive in the United States. Rising oil prices
in the 1970s triggered an interest in developing alternative
sources of energy, and biogas from livestock manure seemed
to have real potential. The prospect of farms meeting their
own energy needs - and perhaps generating enough power to sell
the surplus to local utilities - was very appealing. Methods
of trapping methane produced by manures in the absence of oxygen
were researched and, with encouragement from federal funding,
approximately 140 commercial farm scale biogas systems were
constructed. According to the U.S. Department of Energy (DOE),
71 were installed at commercial swine, dairy, and caged layer
farms, while another 70 digesters were constructed as university
research and demonstration projects in the years between 1972
and 1982. Technologies represented in these projects included
first generation covered lagoons and complete mix digesters,
as well as the new plug flow digesters developed by William
J. Jewell and other researchers at Cornell University in the
late 1970s (see below).
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ON-FARM BIOGAS TECHNOLOGY
Digesters optimize naturally occurring anaerobic bacteria
to treat manure and produce biogas. They usually are
covered with an airtight impermeable cover which traps
the biogas for on-farm energy use. The choice of which
digester to use is driven by climate and the existing
(or planned) manure handling system at the facility.
One of three basic options will generally be suitable
for most conditions, notes the AgStar Handbook:
Covered lagoons are used with liquid manure with less
than two percent solids. Generally, large lagoon volumes
are required, with depths preferably greater than 14
feet. Covered lagoons generally are more compatible with
flush manure systems.
Complete mix digesters are engineered tanks, above or
below ground, that treat manure with a solids concentration
in the range of about three to 10 percent. These structures
require less land than lagoons and are heated. Complete
mix digesters are compatible with combinations of scraped
and flushed manure.
Plug flow digesters are engineered, heated, rectangular
tanks that treat dairy manure with a range of about 11
to 13 percent total solids. The digester requires less
land area than a lagoon, and can be built above ground.
A plug flow digester is the most appropriate technology
available for relatively high solids manure that is scraped.
The system is only for ruminant manure and should not
be used for swine manure, due to its monogastric manure
characteristics.
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Progress was being made in the field, but then the ax fell.
The reduction of programs for sustainable energy projects during
the 1980s, and reduced energy prices under PURPA (a program
that required utilities to purchase power from independent
renewable energy sources), put the nascent on-farm anaerobic
digestion industry into a state of free fall.
"During the late 1970s and early 1980s, there was a lot of
government money available to do these things," says Kurt Roos,
program director for the U.S. EPA's AgSTAR program, which focuses
on farm scale anaerobic digestion and methane recovery. "There
just wasn't enough screening done of who got grant money and
that helped create a history of failed projects. Some were
just improperly designed for farm application using inappropriate
technologies. For others that were properly designed, there
was a very limited service structure in place to fix the systems
if they developed problems." Regardless of the reasons, the
failures contributed to the current poor technical perception
of anaerobic digestion held by the livestock industry and have
resulted in very limited biogas development since. In fact,
by 1994, there were only 25 commercial on- farm biogas systems
still operating in the U.S., according to a DOE study, although
there were at least 2,000 to 4,000 farms that could possibly
benefit from the technology.
A New Approach
Over the past three years, increasing attention has been given
again to on-farm anaerobic digestion systems by farmers, researchers
and government officials. Interestingly, the most prominent
reason has to do with a scientific reality that was barely
on the horizon 20 years ago - global climate change due to
methane and other human-related sources of greenhouse gases.
The Clinton administration's Climate Change Action Plan, released
in 1993, announced that there would be support for developing
voluntary pollution prevention programs to stabilize greenhouse
gas emissions to 1990 levels. One initiative to evolve from
this decision was creation of the AgSTAR program within EPA
and the U.S. Department of Agriculture (in cooperation with
the DOE) to provide information, tools and training designed
to help farmers make informed decisions about on-farm methane
recovery.
"One of the first things we did when we started AgSTAR and
began developing this next era of anaerobic digestion projects
was to look at the history of all the past failures," says
Roos. "For instance, the U.S. biogas experience taught us the
technology is not suitable for all farms, but in certain situations,
it can be a cost-effective method for treating manure and liquid
waste." In addition, technical standards and specifications
are under development. One standard for a covered lagoon has
just been finished and two more - for complete mix and plug
flow digesters - are in process, all based on successful commercial
systems with track records. "What this really boils down to
is some level of consumer protection," Roos notes. "It creates
a common framework for everyone who wants to work in this industry
and offers flexibility in expansion."
AgSTAR helps interested participants determine if anaerobic
digestion makes sense for their operation. A computer software
package called "FarmWare," for instance, provides a means of
surveying potential sites, assessing energy options and applications,
and selecting the most profitable installation. The AgSTAR
Handbook, to be finalized in 1997, will complement the software
by providing a comprehensive method to developing biogas systems
at commercial farms. It will address issues such as technical
design, odor control, vendor evaluation and financial performance.
Farmer interest in this information is picking up. "Right
now, we're seeing the most activity in the biogas area since
the early 1980s," Roos says. "In the first two years of the
program, we had about 20 farms sign up each year. Between this
year and last year, another 70 new farms have made a commitment
to evaluate the options through AgSTAR." Calls to a program
hotline increased from 70 in 1995 to over 400 inquiries through
October, 1996. Four AgSTAR installations presently are under
construction and expected to be completed by the end of the
year. In 1997, Roos expects at least six or seven others to
begin construction. "And that's not including what other people
in the industry are doing," he says.
Cooperative Projects
One of the lessons learned from analyzing past projects is
that biogas production is best suited for farms that handle
large amounts of manure as a liquid, slurry or semisolid with
little or no bedding added. While experience has shown that
anaerobic digestion can make economic and operational sense
on these kinds of individual farms (see below), another interesting
way that the technology is being applied is through cooperative
projects in which a number of farms take their manure to a
central facility.
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HOW THE SYSTEM WORKS AT LANGERWERF DAIRY
Leo and Linda Langerwerf installed a plug flow digester
at their dairy with 350 milkers in 1982. "We've been
operating it for 14 years, for better or for worse," says
Linda. "It's kind of like a marriage. The system needs
its first significant maintenance right now." The project
was initiated to take advantage of the California Department
of Food and Agriculture's low interest loan program that
was available to dairies for the installation of anaerobic
digesters as an alternative waste management tool. A
plug flow digester was selected because the manure solids
could be more easily removed and land application of
the liquid manure can be incorporated as part of the
irrigation system. The digester itself is installed in
a greenhouse that protects it from the elements.
The Langerwerf plug flow digester measures 14 feet
deep, 25 feet wide and 125 feet long. Black plastic covers
the digester pit. Biogas is then collected under the
plastic cover and suctioned through an underground pipeline
to an engine that generates an average of 40 KW of electricity
24 hours/day, supplying all the farm needs before selling
excess to the utility. As much as 30,000 cubic feet/day
of biogas is recovered. The dairy scrapes manure into
a mixing pit where water is added as needed to create
the proper consistency before it is pumped into the digester.
A series of pipes pump water through the digester to
heat the manure to around 100 F, activating the methane
producing bacteria.
In addition to heating the digester, enough hot water
is produced for cleaning the milking parlor twice a day,
and to heat the Langerwerf's home. The final stage effluent
is pumped into a solids separator and applied to the
land for fertilizer. The separated solids are used for
stall and calf barn bedding, and are sold for mulch to
local gardeners. The entire methane recovery system adds
about $68,000 per year to the dairy's revenues.
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The MEAD project in Tillamook, Oregon is a cooperative public/private
venture involving the Tillamook People's Utility District (TPUD),
the Soil and Water Conservation District (SWCD) and the Tillamook
County Creamery Association. MEAD is planning to build a treatment
facility that will process an estimated 145,000 gallons (approximately
560 tons) of manure per day from 40 or more farms with more
than 10,000 head of dairy cattle. Liquid manure is expected
to be taken by tanker trucks from participating farms to the
treatment facility. Methane from the resulting biogas will
fuel a generator, producing electricity for TPUD customers.
The remaining slurry will be dried and the solids converted
to commercial fertilizer and soil amendments. Extracted liquid
will be distributed to participating farmers for land application.
According to Gregory Booth, MEAD project manager, bids have
been received and a decision on a specific developer will be
made in the near future. Construction is expected to begin
soon, with start-up projected for the Fall of 1998. "This really
got started in 1989, but we didn't get funding to start on
any scale until 1993, with most of the development money coming
from EPA's Region 10 office," says Booth. The Oregon Department
of Energy also provided financial assistance and Oregon State
University horticultural researchers have been involved in
developing fertilizer products using a 300 gallon digester.
As part of the same overall project, the Craven farm - an
800 cow dairy on the far end of TPUD's electric service territory,
and a partner in the AgSTAR program - has just completed construction
of a plug flow digester. "Most of the other dairies here are
within a short distance of the central plant," says Booth. "If
we're going to have a digester at a single dairy, they are
the very largest one and it made sense to do it there." The
Craven farm will serve as a pilot facility for further work
with products beyond the energy that will be sold to TPUD. "The
economics here really depend on creating a high quality potting
soil business," Booth adds. "Having more material available
from the Craven farm before the central digester comes on line
will be a big help for future market development."
Nutrient Management in New York
Two of the biggest reasons farmers are becoming interested
in anaerobic digestion are odor control and water quality.
To comply with the federal Clean Water Act, farmers are being
required to develop nutrient management plans that specify
what steps they are taking to control nonpoint source pollution
of surface and ground water. In New York State, the Agricultural
Management Practices Catalogue describes methods (such as composting)
farmers can use to address these concerns in a variety of different
circumstances. Anaerobic digestion was formally approved as
an acceptable nutrient management practice for the first time
in April, 1996. This will enable a farmer interested in biogas
to apply for some of the $2.1 million in state funds available
for implementing the designated agricultural practices. "We
haven't funded any anaerobic systems yet, but they are approved," says
Rich Lewis of the New York State Soil and Water Conservation
Committee. "The money is made available through a competitive
grant process, so if a farmer wants a digester as part of his
manure management system, then that would compete against other
projects and technologies. Cost-effectiveness would be a big
factor."
Also in New York, Jewell and Peter E. Wright at Cornell are
completing a study of a potential centralized system south
of Rochester in an area where there are approximately 30,000
cows within a 10-mile radius. "We've not been focusing on anaerobic
digestion for a number of years," Jewell says, "but research
has recently been diverting back into the area in a comprehensive
and intensive way." This project, financed through AgSTAR,
is examining the economic and technical feasibility of collecting
manure from dairies and deodorizing it with anaerobic digestion,
producing energy and returning the digested manure to participating
farmers. "At this stage, the study is 90 percent complete,
and under certain scenarios, results look positive," notes
Jewell. A report is expected to be completed by March, 1997
that will be available through the U.S. Natural Resource Conservation
Service.
Having seen the shortcomings of the industry in its earlier
days, one of Jewell's main concerns with the present renewed
interest in anaerobic digestion is that there are relatively
few commercial providers of digester systems. "There were maybe
six or seven major companies and 40 or 50 consultants who made
a living in the field, and that's just not there now," he notes.
In an ideal scenario, Jewell says he can envision a delivery
system in which a supplier sets up a digestion system and the
farmer would then pay for the service and the maintenance,
but not be required to get involved in operations. The commercial
provider could service a number of farms in a given area, with
everyone aiming for 90 to 100 percent waste reduction and utilization
of as many by-products as possible, including electricity,
digested solids and effluent.
"The whole idea is to commercialize projects and allow a broader
biogas and coproduct industry to develop," says Roos. To help
this move along, AgSTAR is holding two workshops in Raleigh,
North Carolina in February and March, 1997. "We hope that a
lot of private sector people come and look at anaerobic digestion
as a possible way of expanding their businesses, or expanding
the equipment and services they sell," he says. For more information,
contact AgSTAR, US EPA Atmospheric Pollution Prevention Division,
401 M Street SW (6202J), Washington, DC 20460. |
