Biogas Production Systems

Biogas Production

Anaerobic digestion is the decomposition of wet and green biomass through bacterial action in the absence of oxygen to produce a mixed gas output of methane and carbon dioxide known as ‘biogas’, which can then be used as a substitute for fossil fuels. Both liquid and solid wastes or green crops can be digested to produce biogas. The resulting mixture of methane and carbon dioxide are both greenhouse gases.
 
The natural decomposition of organic wastes in the absence of oxygen (anaerobic decomposition) by bacteria also occurs on the bottom of lakes and wetlands indicated by gas bubbles rising. This is the reason why hydropower installations generate a major source of methane, one of the major greenhouse gases, when the surrounding land area is first flooded and the vegetation decomposes over fairly long periods of time.
 
The breakdown of the organic materials involves a number of biological steps, each involving a well defined class of bacteria that absorb energy from the gradually decomposing biomass which is finally converted to methane, carbon dioxide and water. The process can be encouraged by placing the organic material in large airtight tanks known as digesters, and the biogas produced captured for use. As a result, odours are removed and the pollution potential of the waste is reduced. Biogas can be burnt directly in thermal applications displacing natural gas in cooking and space heating, or used as fuel in internal combustion engines to generate electricity.

Biogas Production Installations in Australia

One example of an Australian biogas installation is Berrybank farm, located at Windermere, west of Ballarat. Berrybank Farm is home to 15,000 pigs, fed in an intensive feedlot farm, operated by Melville Charles. The large number of pigs at Berrybank farm produce the same quantity of effluent as a city of 40,000 people - almost two-thirds as much as the City of Ballarat. To address the disposal issue, Melville Charles has installed an anaerobic digester. Berrybank’s recycling system cost approximately $2 million to install, which was repaid in five years through sales of the products and efficiency savings (Victoria Museum, 1999). Products of the recycling system are; 7 tonnes of fertiliser, large amounts of mineralised and recycled water, and 1,700 cubic metres of biogas per day that is used in a cogeneration plant to generate 2900 kW of electricity daily (The University of Ballarat, 2004). The electricity is used on the farm and fed into the electricity grid, the mineralised water to irrigate crops, and the remaining solids is used as potting mix, fertiliser, compost and worm food (Victoria Museum, 1999).

Another Australian biogas project undertaken by Agricycle Pty Ltd in Western Australia was supported by a grant from the Waste Management and Recycling Fund to establish a facility for converting chicken manure into energy.

Digesters range in size from around 1m3 for a small household unit to as large as 2000m3 for a large commercial installation. In Western Australia, the Water Corporation operates wastewater digesters at its Woodman Point operation, which separates sludge from the water.

At Woodman Point the sludge is digested in one of two 38-metre tall anaerobic digesters. Biogas produced by the digester is used on-site to provide electricity, and excess power is sold to the electricity retailer, Western Power Corporation. What is left after the digestion is complete and the gas is extracted is a sludge. This biosolid is dried then sold as a soil conditioner and fertilizer to the agricultural and landscaping industries.

Benefits

A significant benefit of such systems is that it removes and converts wastes into useful and valuable products such as fertilisers and electricity. The removal of waste has several advantages, such as preventing eutrification of waterways, contamination of drinking water and stopping toxic chemicals from entering the wider environment. In addition the feedstock is derived from biological organisms themselves (or their waste) and therefore provides a renewable supply of biodigester feedstock. In addition this technology can be successfully used in developing countries to improve community sanitation and provide a sustainable supply of much needed energy for cooking and a myriad of other uses. There has been a large increase in small biodigester installations in many regions of the world due to their simplicity, effectiveness and practicality in solving many waste issues, as well as a general lack of sustainable domestic energy supply issues.

Constraints

Limitations of incorporating biodigesters tends to revolve around cost effectiveness and limited scales of operation. For example, the larger the amount of waste that is produced, the larger the amount of biogas that can be produced. Therefore for a relatively small farm with cows in a paddock, the impractical nature of collecting dung as feedstock for the biodigester becomes apparent. However with higher concentrations of animals in a small space the collection of wastes becomes technically easier. While the cost of a system may appear to be the most obvious barrier to implementation, this is not always the case. Connecting your system to the electricity grid involves technical and practical challenges as the system must meet the local standards and safety requirements. Operating, maintaining and servicing biodigesters is not a simple task and the costs and time involved may be another barrier to implementing such a technology.

Conclusion

There is a large potential for the application of biogas technologies to provide sustainable power supplies for distributed generation. The methane captured in these technologies is converted to energy, carbon dioxide and water. However as a greenhouse gas, methane is much more potent than carbon dioxide molecule for molecule. Therefore burning the methane for useful work before it is released into the atmosphere is beneficial to reducing anthropogenic climate change directly, while displacing some other fuel that may be used to perform the useful work. Unfortunately the energy supplies that humans currently use to provide energy services are mostly derived from unsustainable sources, such as coal, oil, gas, and radioactive isotopes. The development of biogas technology makes social, environmental and economic sense and it is being applied in many areas of the developing and industrialised nations to complement other renewable energy supplies.

Links to Suppliers and Further Information

RISE Information Portal - Information regarding renewable energy resources, technologies, applications, systems designs and case studies.

Waste to Energy – a Guide for Local Authorities

Environmental Solutions

Australian Biomass

Biofuture - Biodigesters

Management and Utilization of Biodigesters in Integrated Farming Systems - San Thy, 2003. (MSc Thesis, MEKARN-SLU)

Climate Solutions Special Report - Biogas (pdf)

A Small-Scale Biodigester Designed and Built in the Philippines by Gerry Baron

Environmental Health Perspectives - BIOGAS: A Bright Idea for Africa

The University of Adelaide - An Introduction to BIOGAS

Wikipedia - Biogas

United Nations University - Biogas Plant Technical Considerations

References

Victoria Museum 1999, “Pig Power”, (Online) http://www.museum.vic.gov.au/FutureHarvest/case1.html (Accessed 2 December 2008).