Anaerobic digestion can be an integral part of the solution to two of the most pressing environmental concerns in urban centers: waste management and renewable energy.
In the anaerobic digestion process, specialized bacteria decompose organic matter (leaves, grass, tree clippings, food waste, pet waste, paper that's been in contact with food; items high in moisture) in an oxygen-depleted environment to produce biogas and a stable solid. Each of these products can be used for beneficial purposes to close the loop in organic waste management. Anaerobic digestion has been successful in reducing the volume of waste going to landfill, decreasing emissions of greenhouse gases and creating organic fertilizer. (from www.earthinstitute.columbia.edu/nyc/projects). The photo at right is a concept drawing for a UK digester.
It’s doable - Anaerobic digestion is widely used in Europe, and many US cities already recycle organics, including Toronto and Seattle who started with pilots that then went city wide. Closer to home, Hutchinson, MN collects organic matter, selling the processed product as composting material. The photo at right is a proposed biomass plant on the River Tees. Click here for more info.
Hennepin County has done a few small pilot studies of collecting organics- e.g. Wayzata collecting kitchen waste, St Louis Park High School, Minnetonka and Hopkins High schools. Now over 60 schools are collecting organics in Hennepin County, including 23 in Minneapolis.
In Manhattan, some high rises are considering small scale anaerobic digesters; plus they're being considered for areas with high concentrations of restaurants -e.g. Hells Kitchen, Chinatown, etc.
Currently Minneapolis residents recycle around 35% of their garbage. Collecting organics could increase that to about 75%, leading to a massive reduction in landfill space.
At present, 75% of Minnesota’s electricity comes from burning coal. If the Linden Hills digester is successful, the technology could be replicated in neighborhoods across the city, state and beyond.
Print a copy of our vision here -feel free to distribute!
The photo at right shows an anaerobic digester in Germany.
When organic matter (food, paper, plant matter) degrades in landfills it creates methane - a green house gas with 70 times more warming capacity than carbon dioxide (23% times worse over a 100 year span, 72 times worse in a more realistic 20 year time frame) The impact of methane:
Robert Watkins, the co-chair of the Intergovernmental Panel on Climate Change’s Third Assessment, recently wrote:
“T(he) Copenhagen talks focused on the leading climate change culprit: CO2. But reversing global temperature increases by reducing carbon emissions will take many decades, if not centuries. Even if the largest cuts in CO2 contemplated in Copenhagen are implemented, it simply will not reverse the melting of ice already occurring ...The most obvious strategy is to make an all-out effort to reduce emissions of methane. Methane’s short life makes it especially interesting in the short run, given the pace of climate change. If we need to suppress temperature quickly in order to preserve glaciers, reducing methane can make an immediate impact. Compared to the massive requirements necessary to reduce CO2, cutting methane requires only modest investment. Where we stop methane emissions, cooling follows within a decade, not centuries. That could make the difference for many fragile systems on the brink.”
Robert Watson and Mahamed El-Ashry, “A Fast, Cheap Way to Cool the Planet,” The Wall Street Journal (December 29, 2009).
Portable Digester
LHP&L's Jim Postiglione built a portable digester, that can be moved by trailer to demonstrate the technology to the community. Sponsored by the MN Pollution Control Agency, the digester uses food scraps to produce methane, just like a full sized AD would.
Energy from Waste
Energy from waste can be used to describe a variety of processes that use mixed or pure organic wastes to generate energy or energy-producing products. Well-known energy-from-waste processes include:combustion-based incinerators, landfill gas collection and aerobic composting. A lesser known option is anaerobic digestion.
Anaerobic Digestion : How does it work?
Controlled biological processes such as AD involve treating organic waste using bacteria in an oxygen-free environment. The bacteria eat away at the waste and produce a methane rich biogas. This gas can be used to generate electricity and/or hot water/heating and as a transport fuel similar to compressed natural gas (CNG). Other solid and liquid byproducts also result from the process (see diagram).
San Francisco creates green energy from food scraps with help of the EPA:
This 4 minute YouTube video shows a digester with different technology at the University of California, Davis.
Feasibility Study
LHP&L completed a feasibility study funded by the MN Piollution Control Agency and the MN Department of Commerce's Office of Energy Security. Browse that document here. Are there examples or extant anaerobic digesters on a neighborhood scale?
Although there are various digesters in Europe; we do not know of any in the US. Many farms and organic waste producing facilities in North America use digesters. They use a variety of technologies that involve digestion in some form. The Old Dutch Potato Chip Factory in Roseville, MN, apparently uses a digester to power their plant using potato skins, however that is a much smaller scale than our proposed project. There are also many examples on large farms that use animal waste to produce methane and reduce waste. Some large scale digesters in Minnesota are used to process waste produce methane and other products. One example is WLSSD in Duluth, MN.
Our example is a tough engineering question because of the variety of material input to the digester seasonally and the challange of siting in an urban area.
What uses are the gases created in the digester?
There are many possibilities for use of gas created from this process. (see article on landfill gases)
Raw bio-gasses (unrefined) contain water, CO2, methane, and hydrogen-sulfur gas traces and can be used directly to make heat energy in a boiler for heating of connected buildings through steam pipes. With specialized equipment (link1) or (link2), it can be made directly to electricity too.
Refined gasses (methane) can be used to create electricity on-site with a standard turbine or piston engine generator and sold to electric utility at market rate.
Refined gasses (methane) can be injected directly into gas lines and sold to gas utility at market rate.
Refined gases (methane) can be compressed (CNG) and used as a transportation fuel.
Refined gases (methane) can be liquified (LNG) and sold as a transportation fuel but is tricky to store.
Any use of a refined gas to create even a small amount of electricity can be utilized in the plant and create the heat necessary for the thermophillic digestion process we would like to aim for. Refining and compressing will consume some percentage of the gas produced in the refining process. Efficient refining can produce the highest rates of return in the for of a high quality energy source but will require larger and larger amounts of product to pay for the infrastructure.
Where will the digester be sited?
We are examining various sites as informed by our draft feasibility study. Ideally it would be close to or in Linden Hills to save on transportation, but it may end up being elsewhere due to space and zoning requirements.
How small a community can do this. Would it work with a town house association of say 80 homes? An association of 80 homes could definitely have a thermophillic anaerobic digester to produce methane to power their homes. The smallest commercial size for this technology is approximately 5-10 tons per day and this size will produce enough pipeline quality gas to provide all of the heating needs for 30-50 homes. The economics are a question that Linden Hills Power & Light is wrestling with. We have just completed a feasibility study that compares the economics of a 5 ton per day facility with larger sizes that might provide close to all of the heating needs of our 4,000 home community. You can view the feasibility here. Here is generally what we know now--
1. Operating costs without including revenues from energy and bi-products will cost around $90 per ton.
2. With current natural gas prices included into the revenues, the operating cost may come down to $45-$65 per ton.
3. Revenues for carbon credits, compost, and CO2 might bring the operating cost below the estimates in #2.
4. The cost of garbage collection exceeds $100 per ton. If any cost savings and/or tipping fees for Source Separated Organics such as yard waste and food wastes can be received by the operating entity and these revenues cover operating costs, then you may have a viable project.
massive reduction in landfill space.