Biogas generation. Here is a 52 °C thermophile anaerobic digestion system converting putrescible residual organic matter (i.e. brown trays), organic residues from slaugtherhouse activities and biosludge from wastewater treatment plants to energy. Image: http://bit.ly/biogas-power.
By Lonnie Coplen, LEEDAP, CPHC
Biogas is a combustible gas composed largely of methane, carbon dioxide, and hydrogen sulfide that is produced by anaerobic decomposition of organic plant and animal matter. Small-scale biodigesters are typically custom-designed and built. The process, which generates biogas and a rich fertilizer byproduct, typically requires warmth – much as the mammalian body requires warmth to support metabolic processes.
Utility company methane has an energy density of about 1,000 BTU per cubic foot due to additional hydrocarbon constituents such as ethane, propane and butane. Backyard biogas contains about 500 BTU per cubic foot with NO other hydrocarbons. (Note that biogas production, unlike the extraction and use of natural gas, is a natural process that does NOT produce greenhouse gases that wouldn’t naturally occur. Organic matter is bound to decompose and create volumes of greenhouse gases – on the forest floor, at the bottoms of lakes, and in landfills across America.)
BIOGAS PRODUCTION – source: smallstarter.com
The Assyrians used biogas to heat baths in 3,000 BCE. Today in rural China, some 50 million households use small, home and village-scale biogas plants. India boasts similar and growing biogas system penetration. Large scale U.S. systems in Yaphank Long Island and San Jose California are pioneering municipal applications. Farm-based systems are gaining popularity. Residential and community-based systems have not attracted much interest.
Yes, biogas benefits abound. Distributed waste management could reduce publicly funded infrastructure. Biogas replacement of extracted hydrocarbons eliminates a wide range of negative impacts. On-site distributed energy production offers lower distribution losses, better energy outcome per unit fuel, and fosters competitive market dynamics.
Small-scale biogas isn’t a standard part of our renewable energy mix due to a lack of knowledge and confidence in system design and construction, unproven and finicky gas yields, and a paucity of appliances that convert the gas into useful outcomes.
Opportunities to support design, construction and use of home and community scale biogas systems abound. Where winters get cold, demand will grow for insulated concrete tanks with inputs, outlets and baffles that foster favorable tank circulation, and ports for gas and fertilizer outputs. Equipment that appeals to off-grid communities has an immediate market in the rural northeast U.S. where power outages are not uncommon, and where grid-independence is valued. As home-scale biogas takes off, so will the market for a variety of devices, from standard system components and system integration devices to end-use appliances. Integration of RE-based process heat should be standardized.
Cooking is the most obvious use of biogas, although gas yields needed to meet meal preparation demand for a family of four typically can’t be met by that family’s food scraps alone. Passive House projects may present the most significant opportunity for future biogas applications. Some calculations show that 75% of the heating demand of a small (1,000 SF) house could be met by a home-scale biodigester operating on the continuous sanitary and food waste of a four-person family in a rural setting that includes a summer vegetable garden feedstock stockpiled for winter production.
ANAEROBIC DIGESTION (SMALL-SCALE) source: www.sswm.info
To effectively challenge building codes and engender adoption of home-scale biogas requires focus but is within reach. In most northeastern jurisdictions where independent sewage treatment systems (septic tanks) are common, a system that complies with the intent of sanitary storage or treatment and fuel storage or distribution requirements can be designed by a professional engineer and installed with the blessing (as applicable) of the county health department and the municipality’s code-enforcement official. Tanks and gas storage systems shouldn’t need to be any more robust than a standard septic tank, although they will have to be insulated to foster microbial activity. Gas distribution and use should be based on existing, prescriptive systems, adjusted to minimize process energy (production heating, gas compression and circulation). Ultimately, the most economically feasible systems will leverage combined heat and power, and be sited indoors – such as within the insulated building foundation, isolated for sanitary and safety.
If home-scale biogas has piqued your interest, follow up online for a wealth of DIY information. Check out the Solar CITIES Biogas Innoventors and Practitioners Facebook page, which offers useful tips that can be harnessed with some focus and commitment. Learn more at https://www.facebook.com/groups/methanogens/.
If you catch the bug, there’s every reason to begin experimenting today, and plenty of ways to connect to likeminded folks with a burning desire to literally harness the power of waste.
Lonnie Coplen, LEEDAP, CPHC is founder and President, ARC Alternative and Renewable Construction, LLC.
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