Introduction:
In the following two blogs I will write about the main parameters influencing biogas production, how much biogas to expect from 1 kg of substrate (both plant-based material and cattle manure), and how much biogas is used in various applications. In order to illustrate my points I will be using tables and data from the book: Biogas Plants: Designs and Details of Simple Biogas Plants by Ludwig Sasse © 1984. I intend to use Metric system for my calculations. Click here to open a new window that contains the necessary Imperial conversions from Metric.

Parameters of Biogas Production:
Biogas production depends on the amount and nature of the fermentation slurry, temperature, type of digester, and the retention time. Gas production is encouraged by high, uniform temperatures (e.g., 33°C), long retention times (e.g., 100 days), and through mixing of slurry. In contrast, gas production is adversely affected by low and fluctuating temperatures (15°C-25°C) (59°F-77°F), short retention times (e.g., 30 days) and poor mixing (Sasse, 18-19). To put this into perspective consider the following example:

--1 kg of cattle dung yields only 15 liters of biogas in a retention time of 30 days at a digester temperature of 20°C (68°F). If the retention time is increased to 100 days and the digester temperature to 33°C (91.5°F), 1 kg of cattle dung gives 54 liters of biogas.

As you can see, the amount of biogas created from 1 kg of feedstock depends on the temperature of the slurry and the retention time inside the digester. A lot of data on biogas production per unit of manure or material is derived in a laboratory setting. In the lab, the material is retained for only 30-35 days. This short retention time often considered the optimal time period to store the substrate before new substrate should be added to replace it. This is because up to 50% of the available methane is produced in the first four or five weeks. The remaining methane is available at a much slower rate as the materials are kept longer inside the digester.

This consideration matters when deciding whether to create a continuous or a batch system. When operating a continuous feed system the operator often wants to extract the greatest amount of biogas in the least amount of time. Furthermore, the operator needs to make more room in the system for daily arising of manure. When material is fed in a batch, all the material is loaded and the operator decides how to digest the material. Plant substrates are typically digested in batches because when introduced to the digester they do not mix well. In addition, plant matter has the potential to yield more gas when stored longer. Finally, one has to expend energy to collect plant materials and to process them into a form in which they can be digested. If we are going to go through the effort to create a substrate we need to try to get as much energy out of it as possible to make the process more feasible.

Substrate Information and Biogas Production:
Cattle manure has a typical methane content of 65%. Leaves (carbon) are 58% methane and fresh grass (nitrogen) is 70% (Sasse 12). The average of these two materials is 64%. It is important to note that successful anaerobic decomposition of plant-based substrates have been achieved, however there is considerably less data for these materials than with animal manure. Since the combination of two plant materials has fairly close methane content to cattle manure I feel that it is safe to use the following chart to set production expectations:

Therefore, 1 kg of cattle manure yields 40 liters of biogas at 26°C-28°C (79°F-82.5°F) when retained for 80 days (11 weeks, 3 days). If we start a 200 kg batch of material, keep a an average temperature of26°C-28°C (79°F-82.5°F) and retain it in the digester for 80 days then we can expect 8000 liters (8 cubic meters) of biogas.

This table can help us make general predictions related to biogas production from the cattle and plant-based feedstock described above. Keeping this information in mind I want to transition to the next blog where we can begin to scale a biogas plant to meet a specific gas use. Since we can make estimates related to how much gas can be expected from a given mass of feedstock, we can predict how much gas is produced from an already existing biogas plant. This will help us select the appropriate utilization of the gas, (i.e. cooking or engine fuel), based on daily biogas production.