Bio-digester

Needs Address

Sustainable agricultural practices

Adaptation effects

• Provides a sustainable source of organic fertiliser for increased crop yields and heightened food security – the fertiliser is rich in nutrients including nitrogen, phosphorus, and potassium
• Promotes sanitation
• Improved health impacts due to limited use of chemical fertiliser
• Enhances economic resilience by reducing costs of fuel and fertiliser and enabling enhanced income due to the high commercial value of the fertiliser produced and the related job opportunities available
• Reduces deforestation, soil erosion and loss of cultivable land by providing an alternative fuel source
• Reduces cooking times, thereby allowing more time for alternative livelihood activities

Overview and Features

Utilises livestock waste to produce bio-gas that can be used for cooking and other energy needs and organic fertiliser for increased agricultural production. Models include fixed dome models, developed in China, and floating drum models, developed in India. Fixed dome digesters are filled through an inlet pipe, which is positioned so that slurry falls into the bottom layer while biogas accumulates in the top part of the dome chamber. Gas pressure is created due to differentiated levels of accumulation between the slurry level and the expansion chamber. This pressure causes the slurry to move into the expansion chamber and once the gas is released via an outlet pipe and tap, the slurry flows back into the digester. Floating drum digesters involve a movable inverted drum, which acts as a storage tank and is placed on a well-shaped digester that is able to move up and down depending on the volume of gas accumulated. The weight of the drum pressurises the gas, allowing it to move through the outlet pipe and be released via a tap. The remaining slurry can be used as a fertiliser.

Figure 1: Fixed Dome Digester (Source: Rajendran, Aslanzadeh, and Taherza, 2012)

Figure 2: Floating Drum Digester (Source: Rajendran, Aslanzadeh, and Taherza, 2012)

Cost

Costs vary from USD 200-400 according to place and size

Energy source

Human resources

Ease of maintenance

• Maintenance fairly straightforward but repairs necessary to reduce leakage
• Most bio-digesters last for 25-30 years

Technology performance

• Provides a reliable and continued source of clean energy
• Can be used in urban and rural environments – in urban environments it reduces animal waste providing an efficient means of disposal
• Technology being scaled up in China, Nepal, Bangladesh, Vietnam and India
• In comparison to no fertiliser, application of bio-digester fertiliser has been found to increase the yields from potato cultivation by 27.5 per cent and forage by 1.5 per cent
• The fertiliser nurtures earthworm culture and vermicomposting

Considerations

• Construction and design must be adapted according to context of application, including consideration of geographical location, availability of material and local climate
• Requires training in use and maintenance to ensure appropriate and sustainable application
• Promotion of benefits of fertiliser production in addition to bio-gas production is necessary
• Leakages release methane and greenhouses gases into the air and must therefore be avoided

Co-benefit, suitability for developing countries

• Can be constructed using locally available materials
• Bio-digesters produce geo-thermal energy therefore it is advised that they are located underground in tropical countries
• Use is limited by cold temperatures therefore adoption in cold countries is less likely
• The process can be time consuming therefore appropriate marketing strategies are required to encourage use
• Financial support and government subsides can enable adoption in poorer communities

Information Resources

Rajendran, K. Aslanzadeh, S. and Taherza, M.J. 2012. Household Biogas Digesters—A Review. Energies 5: 2911-2942.