Fixed Dome Biogas Digester Installation in Kenya

By Anthony M. Wanjohi1 and Alex Ngaruiya Mumbi2
1,2 Kenya Projects Organization
P.0 Box 15509-00503, Mbagathi, Nairobi

Corresponding Author Email: [email protected]

The continued use of fossil fuels, as well as the environmental impact of greenhouse gases (GHGs), has prompted research into the production of alternative fuels from natural sources. (Achinas, Achinas, & Euverink, 2017).  Due to the increased cost of living and inflation in today’s society, there is a necessity for the development of alternative sources of energy. Cooking accounts for 90 percent of energy consumption in developing-country households. Furthermore, access to electricity is limited in rural areas. As such, biogas is an energy-efficient alternative to firewood and cattle dung that can meet the needs of the rural population. Biogas is a renewable energy source that can be used in place of natural gas or liquefied petroleum gas (Rajendran, Aslanzadeh, & Taherzadeh, 2012). Biogas is a source of energy, one which is economically friendly if conserved and utilized. As such, there is a need for the construction of systems that are compatible with biogas consumption. One such mechanism is the fixed dome biogas digester.

This article provides an overview of a fixed dome biogas digester including production, construction and materials used, and size of the system. The article further outlines a summary of the comparison between fixed dome and floating drum biogas digesters.

An Overview of Fixed Dome Biogas Digester

The fixed dome digester was designed to be low-cost, long-lasting, and low-maintenance. It is made up of an underground reactor or digester with a fixed cover where the gas and input slurry are stored, as well as an effluent displacement tank or expansion chamber with an outlet (Uche, Emmanuel, Paul, Olawale, Frank, Rita, & Martin,2020).

Biogas production

Many factors, including pH, temperature, and microbial population are found to have an impact on biogas production (Eze, & Agbo, 2010). The most common designs are fixed dome, floating drum, and plug flow. Biogas produced by anaerobic digestion can be used for cooking, lighting, and electricity generation (Rajendran, et al., 2012).

Fixed dome biogas digester construction 

Fixed dome digesters are typically constructed underground. The size of the digester is determined by its location, the number of households, and the amount of waste available on a daily basis (Rajendran, et al., 2012).

The digester is filled through the inlet pipe (1) until the level reaches the expansion chamber’s bottom level (7). The biogas produced is accumulated in the upper part of the digester known as the storage part. (4). Gas pressure is created by the difference in level between the slurry inside the digester and the expansion chamber (6). The gathered gas takes up space and presses a portion of the substrate into an expansion chamber. The biogas is then emitted to serve the connected sources (5). The process repeats itself all over. The figure below illustrates a fixed dome system.

As shown in the figure, there is an underground reactor/ digester (7). This is where water and animal waste reacts, releasing biogas in the process.

There is also an expansion chamber (4). This is where the biogas accumulates after being dispensed from the digester.

Slurry, on the other hand, is a watery mixture of insoluble matter coming from the digester.

Materials and Size of Fixed dome digesters

Materials used in the construction of household digesters are determined by geological, hydrological, and local conditions, as well as locally available materials. Stones, bricks, PVC, and polythene materials may be used in the construction of a biogas plant (Rajendran, et al., 2012).

The size of the biogas plant depends on several factors: The amount and type of organic waste to be disposed of in the digester. The nature of the excreta, be it animal refuse or kitchen waste; The demand for natural gas, and the consumption pattern. This depends on the number of people the biogas plant is servicing; The level of groundwater and the site of the soil. Some areas are preferred due to the nature and odor of the biogas. For better integration, it should be put in a considerable distance far from people; Air temperature in the region, and wind direction. This determines the speed of the biogas decomposition and the interaction of the community with the biogas plant and the objective treatment of organic waste. How the waste will be processed (Samer, 2012).

Comparison between Fixed Dome and Floating Drum Biogas Digesters

Fixed dome and floating drum biogas digesters are two types of biogas digesters that have been in use in Kenya for some time. Table 1 shows a comparison summary of the two types of biogas digesters..

Table 1 shows a summary of the comparison between the fixed dome and floating drum biogas digesters.

Pros and Cons of Fixed Dome Biogas System

Pros of Fixed Dome Biogas System include: Solidly built and easy to construct with low-cost materials; well insulated – the underground design, combined with typically porous building materials, allows the biodigester to remain productive for longer periods of time; provides an alternative source of energy to fuel and electricity, and is relatively cheaper than fuel or electricity.

Cons of Fixed Dome Biogas System include: Anaerobic digestion is a slow process that necessitates a lengthy period (more than 30 days). This increases the volume and cost of the digester; Low loading rates and slow recovery after a failure; Difference in the year-round temperature variation. The year has varying climatic periods, with each resulting to a fluctuation in temperature; Leakage from biogas digesters increases methane and carbon dioxide emissions into the environment. When methane leaks from the digester, another disadvantage is the fire explosions in homes connected to biogas supplies (Rajendran, et al., 2012).

Conclusion

A fixed dome biogas system is one of the ways of incorporating a biogas infrastructure. There is a need to adopt biogas technology as a sustainable renewable energy solution at household and institutional levels despite the issues of cost implications and policy gaps.


References

Achinas, S., Achinas, V., & Euverink, G. J. W. (2017). A technological overview of biogas production from biowaste. Engineering, 3(3), 299-307. Retrieved from https://www.sciencedirect.com/science/article/pii/S2095809917304228

Eze, J. I., & Agbo, K. E. (2010). Studies on the microbial spectrum in anaerobic biomethannization of cow dung in 10 m3 fixed dome biogas digester. International Journal of Physical Sciences, 5(9), 1331-1337. Retrieved from https://academicjournals.org/journal/IJPS/article-abstract/9883FC930204

Rajendran, K., Aslanzadeh, S., & Taherzadeh, M. J. (2012). Household biogas digesters—A review. Energies). Retrieved from  https://www.mdpi.com/38682

Ranisha Basnet & Romas Radtke, Fixed dome biogas plants, (2016). Retrieved from https://energypedia.info/wiki/Fixed-dome_Biogas_Plants

Samer, M. (2012). Biogas plant constructions. Biogas, S. Kumar (ed.), 343-368. Retrieved from https://books.google.com/books?hl=en&lr=&id=76yZDwAAQBAJ&oi=fnd&pg=PA343&dq=construction+of+a+fixed+dome+biogas+plant&ots=WZ2ZPRT5ax&sig=LzzmlRnoVQry_3is_-9FjppxcJ0

Uche, A. M., Emmanuel, O. T., Paul, O. U., Olawale, A., Frank, K. B., Rita, O. O., & Martin, O. S. (2020). Design and construction of fixed dome digester for biogas production using cow dung and water hyacinth. African Journal of Environmental Science and Technology, 14(1), 15-25. Retrieved from https://academicjournals.org/journal/AJEST/article-abstract/DE9738662550

Suggested Citation in APA

Wanjohi, A.M. (2022). The Blue Flame: Biogas Training Blueprint. Nairobi: Kenya Projects Organization


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Further Reading

Cost Implication of Installing a Fixed Dome Biogas Plant in Kenya


For inquiries about Biogas installation for Domestic and Commercial use at household and institutional levels, kindly contact KENPRO Support Team or call us at +254725788400

KENPRO works in partnership with Som Biogas and Energy Solutions among other partners in providing technical support in portable and fixed biogas digester installations.