This research and application project investigated the use of psychrophiles (cold loving microbes) for the purpose of improving efficiency in biogas digesters for generation of cooking and heating gas for Alaskan households.
Biogas technology is a mature technology with wide-spread implementation in India and China, with emerging efforts in Africa, California, and Europe. The technology is based on the biological production of methane by archeal microbes, namely methanogens, which naturally break down organic matter to produce methane gas. This project accessed deployment of cold-region digesters for potential use as a local, sustainable, alternative energy source for home cooking and heating.
Temperature range is a major restricting factor for most existing biogas digesters. Digester microbial communities consist primarily of mesophilic, or warm-loving (15 to >80 C), bacteria, which typically become inactive in colder winter months. This requires that the digester be stored indoors, heated, or retired in the cold season. Research in Alaska included the use of naturally-occurring, cold-loving (psychrophilic) methanogens ubiquitous in Alaskan lake bottom sediments.
The objectives of this project were to improve the efficiency of existing methane biogas digesters, produce a renewable and alternative fuel, reduce the release of harmful greenhouse gases, and implement dwelling-size applications to evaluate their acceptance and sustainability for widespread application in Alaska. The first phase compared the efficiencies of mesophites (conventional digester microbes) and psychrophiles on common household and rural Alaska feedstock at various temperatures. The second phase deployed digesters in practical household scale projects to operate appliances and an electrical generator in order to evaluate feasibility and sustainability in an applied setting for widespread use in Alaska.
With cooperation between the Cordova Electric Cooperative, the University of Alaska Fairbanks and the Cordova High School, a series of digesters were analyzed to determine biogas production rates of mesophiles and psychrophiles through summer and winter seasons. The project used different types of locally available and sustainable Alaskan materials for feedstock, construction material, and upkeep protocols. Phase 1 of the two-phase project experimented with six digester systems to determine which was the most efficient. Phase 2 utilized the most efficient system as determined in Phase 1 in local implementation, to power methane-fueled stoves and appliances, lights and an electrical generator.
In research Phase 1, three groups of variables were considered: microbial community (mesophiles vs. psychrophiles), feedstock (food scraps vs. vegetation/fishing/hunting mixtures), and temperature (indoor vs. outdoor). Microbes were obtained from Alaskan farms (mesophiles in ruminant manure) and thermokarst lake mud from Fairbanks (psychrophiles). We obtained feedstock from the Cordova High School cafeteria and the Alaska Commerical Value Center grocery store excess (food scraps), Cordova canneries and harbor (fisheries offal), and from yard clippings, trail maintenance and other locally and regularly produced leafy vegetation). Carbon to nitrogen ratios was maintained in feedstock at a C/N = 8 to 20 ratio, optimal for anaerobic methane production, by balancing ratios of C-rich vegetation and N-rich food and/or fish. Most food scraps fall naturally within this range and need no additives. C to N ratios of feedstock and digester effluent were measured periodically at UAF. To test temperature capacity, two 1000-L digester tanks were maintained indoors (15° to 27°C) and four insulated tanks above ground outdoors experiencing the seasonal range of temperature in Cordova, typically -5° to 20°C. For space and efficiency, Phase 1 deployed six household-scale (1000-L) experimental digester tanks. In two indoor tanks inoculated with proven warm-loving mesophilic microbes from Alaskan manure, we varied Alaskan substrate [(1) food scrap vs. (2) vegetation/fisheries offal]. Outdoors, four insulated tanks consisted of: (3) psychrophiles fed food scraps, (4) mesophiles fed food scraps, (5) psychrophiles fed vegetation/fisheries offal, and (6) temporally varying combinations of food scraps/fisheries offal/willow and spruce.
Outdoor digesters were covered with 2-inch foam insulation to keep them relatively within range of psychrophilic capacity. Should the temperatures drop below freezing, the naturally exothermic reaction of the methanotrophs coupled with the foam insulation kept the digester within a functional temperature range, without the need for an external heat source. All digesters were equipped with with hanging recycled fish nets to provide surface area to enhance microbial growth. An interior temperature data logger recorded temperature throughout the study. Additional data loggers collected ambient indoor and outdoor air temperature external to the digester tanks. Substrate quality (C/N), fermentor conditions (pH, temperature, dissolved oxygen, salinity, conductivity), and gas composition (percent CH4, C02, N2, 02) was monitored regularly (weekly to monthly depending on stage of experiment and parameter). Qualitative assessments of odor were made weekly for indoor and outdoor test sites. Units of effort (hours) to build and maintain the biogas digesters were recorded. Protocols optimized for biogas production efficiency based on sustainable organic Alaskan substrates and temperature regimes were delivered at the end of Phase 1.
Phase 2 implemented the most productive, sustainable, cold-temperature improved biogas digester system determined from Phase 1; the most notable demonstration projects were the use of biogas as a cooking fuel with a cast iron single-burner stove, powering of a 4-cycle lawn mower engine, production of electricity using a converted gas-powered generator and use of digester effluent as liquid fertilizer in a student greenhouse project. These applications of conventional biogas digesters have been proven and are commonly used during warm seasons in households, industry, and community buildings in Germany, California, India, and China. We attempted to implement them as accepted and sustainable technologies in Alaska for public buildings (such as the Cordova High School) and local residences.
For the digester implementation, some primary materials were donated (six 1000 L tanks for the primary substrate containers were donated to the project). Many of the necessary construction materials were recycled from local waste. Some items were purchased, at minimal cost from local hardware stores or, in the case of some scientific equipment such as the data loggers, ordered by UAF. Mesophilic substrate material was freely procured in the form of manure from an Alaskan ruminant farm. Psychrophilic material was collected from thermokarst lake mud near Fairbanks. Feeding substrate was collected locally around Cordova: kitchen waste was collected by students from the high school cafeteria and grocery waste disposal, fisheries waste was collected for pick up at the canneries, and vegetative scrub was piled and shredded by local organizations for simple pick-up by students helping to maintain the digesters.
Biogas digester technology is proven and in widespread implementation throughout India and China, with emerging projects in Africa, California, and Europe. The technology is based on the biological production of methane by archeal microbes, namely methanogens, which naturally break down organic feedstock to produce methane. This process is commonly observed in landfills and in nature when bubbling methane seeps from lakes, peat bogs, and other organic-rich anaerobic environments.
The basic concept behind a biogas digester is the creation of an ideal environment for a methane-producing microbial community and then harvest the methane it produces. As the microbial needs are minimal, a relatively simple technology emerges: provided with a high-quality organic substrate under anaerobic (usually water-logged), neutral to high pH conditions, microbes produce methane which bubbles out of the substrate into a collection vessel plumbed to a natural gas fueled appliance or electrical generator.
Biogas digester systems are inexpensive to build and materials are widely available, one digester was built for about 300 dollars in Germany in 2008. Blended organic substrate is added to a liquid slurry of water and manure (or lake mud) containing methanogens in a large primary tank with an open top (the fermenter). A second large tank is inverted and telescoped into the first to contain the gas in the headspace and to serve as a piston moving up and down with gas production and release. A weight on top of the piston or a structured frame provides opposing force to the rising piston, pressurizing the gas to move through the gas outlet valve and hose for collection and/or use with an appliance.
Temperature range is a major restricting factor for most existing biogas digesters. Traditionally, ruminant manure is used as both a food substrate and source of methanogens for the microbial community since methane-producing microbes live in animal rumens at 40°C. However, these microbial communities consist primarily of mesophilic, or warm-loving (15 to >80°C), bacteria, which typically shut down (hibernate, in a manner) in colder winter months outside the animal's body. This requires that the digester be stored indoors, heated, or retired in the cold season.
If they are able to solve the cold temperature-limitation problem, biogas digesters could prove an excellent alternative energy source for Alaskans, who face particularly high seasonal fuel costs, and globally where communities experience cold seasons.
We proposed to mitigate the temperature limitation problem by improving existing biogas digesters with the addition of a naturally-occurring, cold-loving (psychrophilic), methane-producing microbial community that is ubiquitous in Alaska. Methanotrophic psychrophiles which thrive across Alaska in lake bottom sediments are known to produce strong methane seeps in thermokarst lakes year-round, even during extreme-cold (-40°C) winter temperatures when lake bottom sediment temperatures reach nearly freezing. Despite showing maximum methane production at 20-25°C in laboratory experiments, psychrophiles display significant biogas production down to 1°C, unlike the conventional mesophiles which become inactive when temperatures fall below 15°C. In mild climates, psychrophilic-based digesters could be kept outdoors above ground; in colder climates the digesters could be buried underground or kept in low-heated outer buildings to maintain the minimal temperature required for psychrophilic activity (0 to 40°C).
Data and Analysis
Results of Phase 1
We found that digesters containing psychrophiles were more robust to temperature and pH fluctuation. Among our experimental digesters, tanks containing psychrophile-rich lake mud produced more biogas (275 ± 82 L gas d-1, mean ± standard deviation) than tanks inoculated with
only mesophile-rich manure (173 ± 82 L gas d-1); however, digester temperature appeared to be the overarching control over biogas production among all tanks. Without knowing the temperature response from the microbial communities in our specific digesters, it is not possible to extrapolate these results with a high level of certainty; however, we can conclude that psychrophile-rich lake mud is a viable source of microbial inoculum for producing biogas at cold temperatures, albeit at only 28-56% of rates typical of warmer temperature regimes. Other benefits of the psychrophile-rich lake mud digesters included reduction of foul odor and a source of nutrient-rich, liquid organic fertilizer for growing plants.
Results of Phase 2
A Benefit-Cost Analysis and Sensitivity Analysis to assess the economic feasibility of the project showed that small scale biogas digesters are not cost-effective at the current prices of displaced fuels and electricity. Replication of the small, household-scale biogas digester technology is unlikely in Alaska due to the heat and energy requirements of maintaining digesters above freezing in winter, the time required for building and maintenance, and the relatively low energy yield. However, large-scale digester projects are becoming more widespread in the United States, Europe and elsewhere globally. Large-scale biogas operations may have potential in Alaska too in association with converting waste from fisheries into usable biogas and in landfill operations.
Continue to Links, Resources and Documents to learn more about the results of this project
Funding and Partnerships
This project is a Denali Commission EETG Program project. The funding goal of the EETG program is to develop emerging energy technology that has the potential of widespread deployment in Alaska and has the long-term goal of reducing energy costs for Alaskans.
The Alaska Center for Energy and Power (ACEP), an energy research group housed under the Institute of Northern Engineering at the University of Alaska, Fairbanks, is serving as the program manager of the EETG solicitation. As the projects deal with emerging energy technology and by nature are high risk, high reward, ACEP’s technical knowledge and objective academic management of the projects, specifically for data collection, analysis, and reporting, is a vital component to the intent of the solicitation, i.e., providing lessons learned and recommendations.
Originally the municipality of Cordova provided electric energy to the community. In 1978, the citizens of Cordova organized and voted to form a rural electric cooperative. These visionary citizens named the utility Cordova Electric Cooperative, Inc., and assumed member ownership and stewardship of the Cooperative. Cordova Electric Cooperative was energized on September 30, 1978. CEC currently serves 1,608 consumers, has 62 miles of line, one substation, and a generating capacity of 14.4 megawatts as follows: Orca Power Plant facility (diesel plant)-7.15 megawatts, Humpback Creek Hydroelectric facility-1.25 megawatts, and Power Creek Hydroelectric facility-6 megawatts.
University of Alaska Fairbanks
The University of Alaska is the technical contributor to this project and will provide research and development tasks including implementation, monitoring, evaluation, and publication of results. For the last two years, Katey Walter Anthony of UAF, Adam Low of Cordova Schools, and Clay Koplin of CEC have been collaborating on the identification and characterization of methane seeps in and around Cordova. The project is an extension of that effort, and played a primary role in selecting Cordova as the project site.
Thomas H. Culhane
Thomas H. Culhane is a critical technical contributor. He and Katey first developed the concept of marrying research in psychrophile methane production to existing, conventional digestors to improve and expand the efficiency and deployment of the digestors. Thomas is skilled in the construction, installation, and operation of methane digestors and will be training and assisting with those efforts for this project.
Cordova Schools is a primary contributor of project operations and maintenance. Cordova Schools collaborates closely with UAF and CEC on this project.
Native Village of Eyak
A supporting partner with no direct role in implementing the project, the Native Village of Eyak is collaborating with Cordova Schools and CEC on the implementation of a community wide renewable energy plan. It is likely that the Native Village of Eyak would contribute either vegetative feedstock collected from native lands, or some other supporting role.
Links, Resources, and Documents
- New Scientist: Cold climates no bar to biogas production
- Alaska Dispatch: Biogas could bring new energy to rural Alaska
- Biological Manipulation of Manure
- Biogas Digest Vol. 1: Biogas Basics
- Biogas Digest Vol. 2: Application and Product Development
- Biogas Digest Vol. 3: Cost and Benefits
- Biogas Digest Vol. 4: Country Reports
- ACEP Biogas Overview
EET Forum 2-14-11 Presentation 1/2
EET Forum 2-14-11 Presentation 2/2
ACEP Community Lecture 6-21-11 Presentation
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