3.8 How to measure or estimate the biogas or biomethane potential?

To assess faecal sludge suitability for anaerobic digestion, it is recommended to conduct a BMP test. This test takes approximately 30 days to complete in a lab setting. For more details on how to set up a BMP test, please consult the following resources.

The test can be conducted at different levels of sophistication, accuracy, and price, as the picture below illustrates. On the left is a simple, low-cost solution, and on the right is an automated and sophisticated analytic tool.

Left: Low-Cost BMP Field Test in Bangladesh (UPM), Right: Automated & High-Tech BMP Analytic Tool (bpcinstruments, 2023)

The methods for conducting a BMP assessment following literature can be consulted.

For the documented experience in the field, please review “Sampling for faecal sludge and other liquid wastes in emergency settings” – page 46 (Link).

For a “quick and dirty” estimation, for digesters connected to the toilet, one can use the figures 20 to 30L of biogas produced per person and day. However, it should be noted that this estimate may be lower if the toilet is only used for a portion of the day, such as in schools or other scenarios with limited usage.

Another “quick and dirty” estimation can be conducted if COD, i.e. for faecal sludge, is known. The estimation is based on the theoretical value that 1kg of COD removed can produce 350L of methane. However, this value assumes that all COD is converted into methane, which is not realistic in practice. Therefore, if 50 – 80% of removed COD is converted to methane, it can be used as a rationale for a more realistic estimation, resulting in 175 – 280L of methane per 1kg of COD removed.

There are alternative ways to calculate the biological methane potential, such as using the Bushwell equation. You can visit the link provided for more information in the materials tab if you are interested.

Further Reading:

  • Value of Batch Tests for Biogas Potential Analysis (Link)
  • Theoretical Biogas and Methane Production Estimation Tool (Link)