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Domestic Biogas Plant
Bio-gas plant diagram

Domestic Biogas Plants produce renewable fuel from organic biomass and are primarily used in developing countries and rural areas as an alternative to using fossil fuels, whose combustion contributes to global warming. Biogas is a carbon neutral fuel that is produced when bacteria degrade biological material in the absence of oxygen, a process known as anaerobic digestion, or fermentation of biodegradable materials such as biomass, manure, sewage, or municipal waste1 . The fuel consists primarily of methane (CH4 or natural gas) and carbon dioxide (CH4) mixed with trace gases and can be used to generate electricity for cooking and heating when it is burned.

The ability to capture methane and use it as a fuel source is important since methane is a significant contributor to global warming. Methane is 20 times more effective than CO2 at trapping heat in the atmosphere than CO2. Animal wastes are inherently anaerobic and release methane into the atmosphere during the process of decomposition unless they are managed2 .  Through the use of domestic biogas plants, the natural generation of methane can be captured and used as a clean, non toxic fuel source. Furthermore, the use of methane as a fuel source will offset the need for fossil fuels. Switching from fossil fuels to biogas will result in CO2 emission reductions as half of the green house gas emissions being released into the atmosphere are from CO2 from fossil fuel combustion3 .
Biogas history

Humans have been using anaerobic digestion has been used for centuries. Over 3,000 years ago, biogas was used to heat bath water in Assyria. In the late 19th century, biogas recovered from a sewage treatment facility fueled street lamps in Exeter, Devon in the United Kingdom. Biogas from sewage sludge was converted and used for fuel in automobiles in Germany in the 1950s4 . Today, biogas is being used worldwide, particularly in developing countries such as India, China, and Costa Rica.
Technical aspects

Domestic biogas plants (biodigesters) in developing nations convert livestock manure and night soil (human excreta) into biogas and fermented manure. The size of the plants varies, but most systems are feasible for households or small communities and use 110 lbs/50 kg manure per day, the amount produced by about 6 pigs or 3 cows. Manure must be collected, mixed with water and fed into the biodigester.  The optimal biodigester temperature of 97°F/36°C which promotes rapid anaerobic activity and allows for a smaller volume tank, makes biogas plants ideal for tropical climate locations5 .  As the manure ferments, the methane bubbles to the top of the container where it can be collected and piped to be used as fuel for a stove or lamp.

There are two types of conventional anaerobic digestion, a liquid and a solid process. In the “liquid” process, waste is mixed with water to facilitate digestion and in the “solid” process occurs on landfill sites and produces landfill gas (LFG)6 .  The liquid process is used for domestic biogas plants, however many biogas system designs and design variations exist.  

Types of biogas systems used in developing countries include:

    Home Biomass Systems (HBS) which are common in developing countries in tropical environments.
    China Fixed Dome (CFD) which are used in temperate and cold climates and are built underground
    India Floating Cover (IFC) uses cow manure as the main biomass input, the floating cover rises and falls with methane production and use7 .

Rural Costa Rican communities have built HBS of various designs . "Media bolsa” is a design that includes the basic construction elements of a domestic biogas plant and is used here as an example of a domestic biogas plant  The main component is a large underground tank which is covered by a large inflatable plastic sheet. This system includes an entrance tube, a mixing tub an exit tube, and a collection tub. In most biogas plants, water is necessary to turn biomass fluids into solution and immerse solids to create an anaerobic environment.  The media bolsa system requires a water-to-manure ratio of 2:1 for grazing livestock and a 1:1 ratio for grain-fed livestock.   Manure and water is put into the entrance tube which flows into the mixing tub. As the microbes digest the manure, the biogas bubbles up from the surface of the water/manure mixture and causes the plastic bag to inflate.  The methane can then be piped through a PVC tube connecting the tank to the kitchen stove for cooking or used for lighting purposes.
Domestic Biogas Plant Diagram

Diagram of a 50 m3 Biogas Digester
Source: http://www.smartinnova.com/index.php?p=re-biogas. Author: CITT, KIST. Permission: Smart Innovations.

Daily, manure and water are added to the system.  If the mixture in the fermenting tank is at the same level as the exit tube, the manure/water mixture introduced through the entrance tube will force the same volume of liquid out the exit tube and into the holding tank.  This fermented liquid can be used as a crop fertilizer and is rich in Nitrogen, Phosphorus, and Potassium. 8 .
Biogas plant construction and monitoring

In tropical environments, the biogas mixing tank may be constructed out of cement blocks, and is placed underground while the plastic bag is above ground.  In this type of design, building a roof over the digester is suggested to protect the plastic bag from the elements.

Constructing a domestic biogas plant requires basic building materials and can be done by hand.   Common materials include: a plastic bag, cement blocks, rebar, materials for building a roof over the digester, shovels for digging, PVC piping,  and a safety valve.  The average construction time using manual labor is about one week.
Biogas as a fuel
"Media Bolsa" Domestic Biogas Plant in Rural Costa Rica

"Media Bolsa" Domestic Biogas Plant in Rural Costa Rica
Source: http://www.ruralcostarica.com/biodigester.html. Author: Rural Costa Rica. Permission: Rural Costa Rica.

Since biogas is physically identical with natural gas, it can be used as fuel for vehicles. However, according to the University of Adelaide biogas website, biogas should be used directly for cooking, heating, lighting, or absorption refrigeration since electricity generation and compression of gas (for storage or vehicle use) uses large amounts of energy for a small output of useful energy. The biogas system works best as a stationary, not mobile fuel with a variable volume storage: such as a flexible bag or floating drum 9 .

 Developed countries also use biogas, but with modern technology.  Digesters are often stainless steel and the mixing system consists of two stainless steel propellers, powered by an electric motor operated by an inverter through the control panel. The digesters are equipped with temperature and pH probes to allow for easy system monitoring.  In European countries, anaerobic digestion is becoming increasingly widespread due to legislative tools aimed to increase biogas production and also to increasing energy prices10 .

Biogas methane content ranges from 50 – 80% 60%, 35% CO2, small amounts of water vapor, hydrogen sulphide H2S, carbon monoxide (CO) and N2 (Biogas China).
Benefits and advantages

Biogas plants have numerous advantages for household and community users.  First, biogas doubles as both a cheap fuel source and a sanitation device for waste11 .  Also, the decentralized biogas systems allow waste to be treated near the production source and sludge to be reused locally, which minimizes transport and initial capital cost compared to a centralized system.  In addition, biogas can replace the use of wood for fuel of indoor stoves.   Since biogas burns without smoke and is nontoxic, when it replaces wood as a fuel source, it can prevent respiratory distress and disease.  Futhermore, the relatively cheap cost of domestic biogas plants has lead to the widespread adapation in developing countries which allows rural people to have access to renewable fuel sources.  For example, in 2005, China had 17 M digesters and 50 M rural people were benefitting from biogas plant production12 .
Barriers and disadvantages

The main disadvantage of domestic biogas plants is the safety issue that the volatile nature of methane presents.  Biogas mixtures containing more than 50% methane are combustible.  Thus, naked flames should not be used near the digester and electrical equipment must be explosion proof.  In addition, the digester area must be well ventilated to minimize the risk of fires and explosions. 

Another risk is the potential for diseases to spread due to handling animal waste.  People should avoid contact with the digest contents and wash after working near the digester13 .

Since maintenance costs are low, the primary costs of domestic biogas plants are capital construction and labor costs.  These costs vary based on location and system design and size.  For instance, in Costa Rica due to the tropical location, costs range from $120 - 300 USD whereas in Asian countries, costs typically range between $300 - 600 USD due to the temperate climate14 .
"Media Bolsa" Domestic Biogas Plant in Rural Costa Rica
"Media Bolsa" Domestic Biogas Plant in Rural Costa Rica
Source: http://www.ruralcostarica.com/biodigester.html.

1. Harris, Paul. Beginner’s Guide to Biogas. University of Adelaide, Australia. February 2010.

2. U.S. Environmental Protection Agency (EPA). In Brief: The U.S. Greenhouse Gas Inventory. May 2002.

3. Speth, James Gustave. Red Sky at Morning: America and the Crisis of the Global Environment.  New Haven: Yale University Press 2004.

4. U.S. EPA. Region 9: Waste Programs. Organics: Anaerobic Digestion. November 2009.

5. New Zealand Ministry of Agriculture and Forestry. Methane from Animal Waste Management Systems. October 2008.

6. University of Adelaide, Australia. Biogas: An Introduction. February 2010.

7. Biofuelswork.com. Biofuels. 2010.

8. Rural Costa Rica. Small-Scale Biogas Use with Biogidesters in Rural Costa RicaBiodigester Design & Construction. 2007.

9. University of Adelaide, Australia. Biogas: An Introduction. February 2010.

10. Comino, Elena, et al. Increasing Organic Loading Rate in the Co-Digestion of Energy Crops and Cow Manure Mix. Science Direct. September 2009.

11. Comino, Elena, et al. Increasing Organic Loading Rate in the Co-Digestion of Energy Crops and Cow Manure Mix. Science Direct. September 2009.

12. Institute of Science in Society. Biogas China. 2006.

13. University of Adelaide, Australia. Beginner's Guide to Biogas: Safety Page. 2010.

14. Institute of Science in Society. Biogas China. 2006.

 Deublein, Dieter and Steinhauser, Angelika.  Biogas from Waste and Renewable Resources: An Introduction. Weinheim, Germany: Wiley-VCH, 2008.



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