Factors Influencing the Selection of Biogas Plant Size

In this article, the first part provides an overview of biogas plant size selection before dwelling on factors influencing the selection of the biogas plant size. These factors include feedstock supply, capital availability, environmental analysis and consciousness, and local government policy, among others.

An Overview of Biogas Plant Size Selection

Several factors that influence biogas output, adoption, and plant size selection have been established. Feedstock availability, environmental awareness, income, and local political governance are among them (Yang, Liu, Thrän, Bezama, & Wang, 2021). Biogas facilities come in a variety of sizes and levels of sophistication. Balloon plants, fixed-dome plants, floating-drum plants, and portable biogas plants are some examples of biogas plants. However, technical constraints such as scaling the digester and/or the gasholder size have an impact on the scale of the biogas plant under consideration.

To calculate the scale of a biogas plant, certain parameters are used (Sasse, 1988). The following are characteristics/concepts associated with biogas plant size:

  1. Daily fermentation slurry: Slurry is a by-product of biogas production retrieved from the anaerobic digestion of animal waste and crop residues.
  2. Specific gas production per day: This depends on the retention time and the feedstock incorporated into the biogas plant on a daily basis.
  3. Dry matter (DM): The water content of natural feed ingredients varies. As a result, the solids or dry matter content of the feed material is analyzed.
  4. Organic dry matter (ODM): For digestion, only the organic or volatile elements of the feed material are required. As a result, only the organic portion of the dry matter composition is taken into account.
  5. Digester Loading: The digester loading determines the amount of organic material that must be fed to or digested each day. The organic dry matter loading in the digester is measured in kilos per cubic metre of digestive volume (Kg/m3).
  6. Retention time (RT): This is the amount of time the feed material spends in the digester. The retention time is significantly less than the time required for complete digestion of the feed material.

Digester-gasholder size ratio: The size ratio between the digester and the gasholder determines the shape of a biogas plant. The ideal shape for the digester in floating-drum systems with a low digester/gasholder ratio (1:1 to 3:1) is a cylinder. Shell and vault constructions are worthwhile if the ratio is higher.

Depending on its scale, a biogas plant could take anywhere from two months to two years to build. Energy planners typically use a Geographical Information System (GIS) to analyze spatial aspects of the supply, distribution, and demand sides of energy management. It is particularly beneficial for assessing renewable energy potential and choosing locations for difficult geographical problems like biomass usage.

Factors affecting the size of a biogas plant

The following are the factors affecting the size of a biogas plant:

1) Hydraulic retention time (HRT): This is defined as the average time interval over which the substrate/slurry is kept inside the digester. It determines the size of the biodigester and, as a result, the plant’s cost. High HRT means the slurry will be retained for a long time. There is the need for building a big biogas plant in these situations. On the other hand, low retention time means that the slurry takes a short time in the biodigester. Hence, hydraulic retention time is directly proportional to a big-sized plant in biogas production.

2) Feedstock supply: This is the volume of feed material to be supplied to the biodigester in a day. High amounts of feedstock encourage the construction of big biogas plants. The opposite is also true. Low amounts of feedstock supplied to the biogas plant encourage the construction of standard-sized biogas plants.

3) Biodigester size: The volume of the biodigester in cubic meters, is determined by size of the digester multiplied by the hydraulic retention time (HRT) – SD X HRT. The biodigester is where the anaerobic fermentation occurs. The size of the biodigester cylinder impacts upon the size of the gasholder, an ultimately affects the biogas plant size.

4) Gasholder size: The gasholder provides the space for the storage of biogas. It is directly dependent on the cylinder, which is also called the biodigester. The gasholder size chosen should be able to receive gas produced at any time, as well as gas produced during or outside of consumption periods, as well as adequate space to accommodate daily changes. A large biogas digester together with a large gasholder would ultimately mean that the biogas plant being produced will be large in size compared to a combination of a small biodigester and gasholder size (Bensah, Antwi, Ahiekpor, 2021).

5) Ratio of biodigester to gasholder volume: It is the volume of the biodigester divided by the volume of the gasholder. A ratio of 6 or greater favors dome-shape biodigesters. Cylindrical biodigesters are cost-effective for smaller ratios (<3).

6) Organic loading rate (OLR): This is calculated as the amount of volatile solids fed to the biodigester per day divided by the biodigester volume.
The equation is (Kg Manure X TS X VS)/ Biodigester Volume.

OLR SI units are kg VS/day/ m3
TS are the total solids of the feed material.
VS are the Volatile solids of the biodigester.

If the organic loading rate is expected to be high, then a sizeable biogas plant will be constructed. Conversely, if the organic loading rate is expected to be low, then a much more reduced biogas plant will be constructed (Bensah, et al., 2021).

7) Volume of specific gas production target by time (Daily, weekly, monthly): The biogas being produced is key to determining the volume of the biogas digester and the dome. These conditions will determine the size of the biogas plant to be installed. Volume can be measured in m3. If the target is 24m3 of biogas per day, then the volume and consequently the size of the biogas plant will have to be modified to fit the conditions. A 50m3 gas volume will be much bigger than a 24m3 biogas cylinder. This affects the size of the biogas plant.

8) Environmental analysis (terrain and land analysis): The biogas plant needs to be supported by a conducive environment. Therefore, careful analysis of the environment has to be done in order to carry out the biogas construction. Flat and steady areas are mostly preferred, but if not available, tractors and excavators can level the land enough for it to be flat, and also dig troughs for the pipelines to pass through. It is possible to level land but the spending of resources will be considerable depending on the amount of land. It is advisable to use Geographic Information Systems (GIS) to study the feasibility of the space that will be used in the production of the biogas.

9) Use of the biogas plant: How the biogas plant will be utilized is key in determining the size. The biogas plant could serve a school, household, a company or even the general community. All these institutions vary in magnitude. Therefore, investigations should be done as to who the biogas plant will service before planning for it.

10) Availability of capital: The capital or amount of money in hand determines the size of the biogas plant. Money is a very important resource in any project being undertaken. Other resources such as the masons being paid, the supervisor of the project, the architect and equipment used in biogas construction can be incremented or decremented, all because of money. The increase in biogas plant size is directly proportional to the amount of money available in production.

11) Government policies: The government creates policies which govern the way we do things. Consequently, government policies can encourage or limit the biogas plant construction based on size alone. In some areas such as urban areas, the government can regulate the size of the biogas plants set up. In rural areas or near schools, the government may have less restrictions based on the setup of the area.

12) Owner’s preference: The owner may determine the biogas plant size based on his/ her judgement. As a result, the size of the plant is defined by the operator’s position and requirements, such as feedstock transportation costs, production costs, and available working time (Yang, et al., 2021).

As observed from the article, many factors influence size selection. There exists special technology known as Geographical Information System (GIS) which help in the analysis of land or space to be used in determining the size of biogas plant layout. Feedstock supply, capital availability, environmental analysis and consciousness, and local government policy are factors that determine the size of a biogas plant. In conclusion, one has to plan precisely and do due diligence about the size of the biogas plant.

Edem Bensah, Edward Antwi, Julius Ahiekpor, (May 2021). Guide For The Design And Construction Of Fixed-Dome Biodigester. Retrieved from https://www.researchgate.net/publication/351818542

Sasse, L. (1988). Biogas plants. Wiesbaden: Vieweg & Sohn.

Yang, X., Liu, Y., Thrän, D., Bezama, A., & Wang, M. (2021). Effects of the German Renewable Energy Sources Act and environmental, social and economic factors on biogas plant adoption and agricultural land use change. Energy, Sustainability and Society, 11(1), 1-22.


This article is an excerpt from the book “The Blue Flame: Biogas Training Blueprint”

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

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