An Overview of Microbiological Processes of Anaerobic Digestion
There are four main steps that occur during the process of anaerobic digestion. They are:
There are four main steps that occur during the process of anaerobic digestion. They are:
- Hydrolysis- insoluble carbohydrates, protein, and fats are broken down into soluble sugars, fatty acids, and amino acids
- Acidogenesis- Acid forming bacteria (acidogens) convert soluble products of hydrolysis into volatile fatty acids, carbon dioxide, and hydrogen gas
- Acetogenesis- volatile fatty acids are converted by acetogens to acetate, carbon dioxide, and hydrogen gas
- Methanogenesis- methanogens convert acetate into methane
Source: Bendfeldt, E., Collins, E.R. Jr., Ignosh, J., Ogejo A.J., Wen, Z. (2009). Biomethane Technology. Virginia Cooperative Extension, 3, 442-881.
- Where are the digesters?
- Vermont has the third most digesters in the country, after Wisconsin and New York, respectively
- In May of 2013, there were approximately 200 digesters in the country.
- Concentrated heavily in the northeast and midwest.
Beer waste-to-energy: Anaerobic Digestion in the media
Burlington Free Press article: Magic Hat digester turns brewery waste into energy
Magic Hat Brewing Company, whose facilities are in South Burlington, VT, has been using anaerobic digestion for several years now to turn brewery waste into energy. The "waste" is barley, hops, and yeast. I am interested what ratio of these three feedstocks was used. During my internship with Avatar Energy, we picked up the same brewery waste feedstocks from The Alchemist Brewery in Waterbury, VT, known for its world renowned, super hoppy Heady Topper. Every time we tried to do small batch start up digestions (performed in oversized Mason jars), the digest failed because the slurry became too acidic and the methanogens couldn't eat up the acid quick enough. Aside from my scientific wondering, I am quite pleased to see anaerobic digestion taking place at this scale and not on a farm. It shows the public that this waste-to-energy system can use waste sources other than manure, and it is accessible to more mainstream, nonagricultural industries. Hats off to Magic Hat for representing Vermont in the field of anaerobic digestion and setting an example of corporate environmental responsibility and ingenuity. The New Belgium Brewing Company based in Fort Collins, CO is another example of waste-to-energy implementation in the beer industry.
Source: John, C. (2011, February 14). Magic Hat digester turns brewery waste into energy. The Burlington Free Press. Retrieved from: http://www.burlingtonfreepress.com/viewart/20110214/NEWS02/110213026/Magic-Hat-digester-turns-brewery-waste-into-energy
My experience: Summer Internship at Avatar Energy in South Burlington
I spent twelve long weeks interning for Avatar Energy in South Burlington, VT this summer. Much of my time was spent in the garage behind the office sorting screws, nails, and washers. Some days I helped with feedstock testing, such as pH levels, volatile organic acid assays, and total suspended solids. My favorite days of all were spent on the farm in Stowe, VT fixing the separator pump, or scraping poop splatters off the separator box. Here are a few paragraph excerpts from my reflection at the end of the summer:
Avatar has anaerobic digester
systems at two farms, one in Stowe, VT and the other in British Columbia,
Canada. The digester at Keewaydin Farm
in Stowe, VT was built for a medium sized farm, with about 75 cows, and has a
life expectancy of 25-30 years. The
system is above ground, fully automated and can be remotely monitored. It is made up of four main parts. First, poop enters a preheat tank, where it
is warmed to about 105 degrees Fahrenheit.
Next, it is pumped into a large hull that receives 1,500 gallons per day
of fresh material. In total, the
digester holds about 22,000 gallons. The temperature in the hull is in the
mesophilic temperature range (95-105°F).
The exterior of the hull is made of fiber glass, and the inner layer is
made with corrosion and gas resistant vinyl ester resin. Next, the poop is pumped into a holding tank,
and then to a separator. Here, the
solids are removed and the liquid goes to a different storage tank underground,
where it is then pumped to a spreading truck to be spread on the fields. The
separator can process between 5 and 10 gallons of effluent per minute. The methane gas is pulled through a pipe that
takes it to an on-site generator that, when fully optimized, can produce 20 kW
of electricity. It is a cogeneration
system, meaning heat is reused in the pre-heat tank and hull, and power is sold
to Stowe Electric.
The digester in Stowe has a
home in what appears from the outside to be a barn. The hull is covered in a foam insulation that
was painted gray to protect against UV damage.
Avatar uses a plug-flow digester system.
Material enters the digester in batches with equal amounts exiting at
the opposite end. This system ensures
that pathogens are being killed and are not being mixed with the clean poop. Therefore, the solids, which are slightly
heavier than sawdust, are almost completely pathogen and odor free and are used
as bedding for the cows. Other types of
digesters include complete mix, which is a mechanically mixed unit at a
constant volume. The hydraulic retention
time is about the same, 15-20 days, for both plug flow and complete mix
systems.
While funding is a challenge, the future of the digester is promising. Its biggest success factors are investment, location, and concern for energy security. As investors engage in power purchase agreements with farmers, and digesters are sold as a part of that contract, investors will make money from the electricity generated through green energy incentive credits, and farmers will receive the value added products such as treated fertilizer and bedding for their cows. Location is important because facilities with on-site waste, such as farms, school cafeterias, restaurants, and hospitals, will be more geared towards fueling an anaerobic digester since transportation will not be a burden. As waste disposal fees at landfills increase, large commercial and industrial waste producers will look for cheaper ways to get rid of their wastes. This could mean distributing to anaerobic digestion facilities for a tipping fee, which is a fee paid by the waste producer to the digestion facility at a cheaper cost than having the waste delivered to a landfill. There is speculation that in Europe, fats, oils, and greases from restaurants and other industries that had previously been given away to lower the cost of waste collection, but as once these wastes were viewed as a commodity, industries began charging for them.
Bedding for the cows made from solid organic materials after liquid separation
Vasu Sojitra (a fellow intern) scraping poop splatters on the inside of the separator box.
Interview with Perry Scofield from Avatar Energy (edited)
CHARLOTTE: What is the biggest reason why these systems aren't more
widely used across the U.S.? Is it more of a financial set-back or a problem
with cultural attitudes?
PERRY: Digesters are expensive and subsidies from the government
are small compared to wind, solar,ethanol etc.
There are not many companies that build them that are based in the US,
most are from Europe (primarily Germany and Denmark) so most of the technology
is shipped over. This elevates the price substaintailly.
As far as the public knowing about digesters, I think it's
more difficult for them to grasp that those funny looking tanks and buildings
are making power 24/7/365 as compared to wind and solar which rarely ever
produce all the time. It's easy to see the PV and wind arrays and know that
when it is sunny and there is wind they are putting juice on the grid. Unless
you work in the business or are doing research on the subject, recognizing a
digester and its attributes (heat, power, pathogen kill, GHG reduction, nutrient
management/solid waste reduction) is a bit more difficult.
CHARLOTTE: Where do you see the digester in 10 years? Is it
reasonable to think that we might start seeing food digester systems in homes?
PERRY: I see the number of working digester in the US growing, with
the combination of foreign and domestic technologies boosting efficiencies of
digestion (combo psychro/ meso/thermophilic, multi stage systems, feedstock
analysis etc..etc.) and competition driving down the price.
I just finished the home digester prototype for food waste
and started running it a few weeks ago.
It took a year to build and will take another six months of testing to
engineer it to a point where it can fit under a sink or in a basement. A small
unit (20 gallons) would be able to produce enough gas for cooking needs. Larger
units would be able to supply a multi family home with enough gas for CHP
(combined heat and power).
CHARLOTTE: Are there incentive programs for AD in homes or
institutions (universities, hospitals, etc.)?
PERRY: VTC (Vermont Technical College) is building a digester on its campus in Randolph. They got funding from many sources (grants,
educational bonds.....) for their digester.
Rutland Herald article: VTC readies anaerobic digester for use
With ACT
148* being instated in 2014 (restricting organics from
landfills) there is going to be a ton of food waste being generated in the
state that will have nowhere to go. The state still does not know what to do
with this waste stream. It would be a great chance for institutions like UVM to
further greenwash themselves, install a food waste digester and make power and
heat for a campus building.
VTC opinion: Will Act 148 help cow power expand?
I am very hopeful about the future of anaerobic digestion because of Act 148. Having digesters on farms can surely close the loops on energy needs in the agricultural industry, but I think our society has more pressing issues about food waste and landfill expansion. The Coventry, VT landfill recently received permission from the state to expand the amount of garbage it can accept per year, from around 350,000 to 600,000. Landfills cannot grow forever; at some point, either new facilities must open or trash must decrease. With more trash comes more methane, which must be flared off. I see anaerobic digestion as an excellent opportunity for food waste to be diverted from landfills. Act 148 not only serves to decrease the amount of trash in landfills, but by necessitating the separation of food waste from trash, it also makes people more aware of how much food they are wasting. However, since waste is inevitable, anaerobic digestion could be the perfect answer for diverting this waste from landfills, thereby keeping our water, air, and soil healthier. I'm proud to see this legislation take place in Vermont, and it compliments Act 250, Vermont's environmentally conscious Land Use and Development Act. It is my hope that in 10 years, we will have entered a new realm of thinking and acting when it comes to waste, energy, and land use.
*Source: Vermont Agency of Natural Resources Department of Environmental Conservation. Universal Recycling Law (Act 148) Summary Sheet. Retrieved from: http://www.anr.state.vt.us/dec/wastediv/solid/Act148.htm
I am very hopeful about the future of anaerobic digestion because of Act 148. Having digesters on farms can surely close the loops on energy needs in the agricultural industry, but I think our society has more pressing issues about food waste and landfill expansion. The Coventry, VT landfill recently received permission from the state to expand the amount of garbage it can accept per year, from around 350,000 to 600,000. Landfills cannot grow forever; at some point, either new facilities must open or trash must decrease. With more trash comes more methane, which must be flared off. I see anaerobic digestion as an excellent opportunity for food waste to be diverted from landfills. Act 148 not only serves to decrease the amount of trash in landfills, but by necessitating the separation of food waste from trash, it also makes people more aware of how much food they are wasting. However, since waste is inevitable, anaerobic digestion could be the perfect answer for diverting this waste from landfills, thereby keeping our water, air, and soil healthier. I'm proud to see this legislation take place in Vermont, and it compliments Act 250, Vermont's environmentally conscious Land Use and Development Act. It is my hope that in 10 years, we will have entered a new realm of thinking and acting when it comes to waste, energy, and land use.
*Source: Vermont Agency of Natural Resources Department of Environmental Conservation. Universal Recycling Law (Act 148) Summary Sheet. Retrieved from: http://www.anr.state.vt.us/dec/wastediv/solid/Act148.htm
Anaerobic Digestion Funding
Avatar Energy’s funding came from:
- Lake Champlain Basin Program
- Biomass Research Development Initiative (USDA)
- Clearn Energy Developmet Fund
- Environmental Quality Assessment Program through the USDA Natural Resource Conservation Service
- Vermont Methane Pilot Program
~CHARLOTTE ADAMS
