Basics

The actual methane yield that is obtained from a digester represents the recovered fraction of the BMP under practical process and operating conditions. Thus, the yield is dependent on numerous factors, including retention time, organic loading rate, degradation kinetics, mixing, inhibitory effects or nutrient (including trace element) deficiency. By definition, the actual methane yield cannot exceed the BMP of a substrate. A higher specific methane yield during continuous operation compared to the BMP is a clear indication of improper conditions in the BMP test (such as an unsuitable inoculum) or measurement error in continuous experiments.

BMP tests

The BMP is a key parameter for substrate characterisation and efficiency evaluation of anaerobic digestion plants. It is crucial for assessing quality and monetary value of different substrate types, and enables reliable process balancing.

BMP is typically measured in anaerobic batch tests in a laboratory setup (BMP test), but can also be estimated from principles of microbial stoichiometry, in conjunction with detailed substrate characterization and assumptions on substrate degradability and microbial growth. The fundamental principle of a BMP test is rather simple: A sample of the investigated substrate is digested with an active and well-functioning inoculum in bottles incubated at an appropriate temperature. Additionally, blank samples, only containing inoculum, are incubated as well. Assuming no synergistic/antagonistic effects, the resulting methane production of the substrate can be determined by subtracting methane production of the blank (inoculum) from the substrate sample (substrate + inoculum). The final value of cumulative methane production after test termination is defined as the experimental BMP of the substrate.

Power and limitations

As summarized in a recent publication: BMP tests are a powerful and useful tool in AD research and practice. They can be applied for the experimental determination of the BMP of pure or mixed substrates. Based on stoichiometric calculations, the anaerobic biodegradability can be estimated by dividing the obtained BMP by a theoretical value. Kinetic parameters achieved from BMP tests allow for a qualitative evaluation of process kinetics. However, kinetic parameters estimated in batch operation cannot be applied for detailed description of continuous processes. While acute toxicity of an inhibitor present in the substrate or mutually added can be detected in BMP tests, this is not possible for chronic toxicity. As in contrast to continuously operator systems, the substrate in a BMP test is only added once, and owing to the typically high share of inoculum present in the assay, synergistic or antagonistic occurring in co-digestion BMP tests can be different from those occurring in continuous processes. BMP tests cannot be utilized to assess long-term effects of nutrients or trace elements availability due to constant feeding. Similarly, the methane yield, process stability, rheology or kinetics in a continuously operated system cannot be elucidated by BMP tests.

Source:

Koch, K., Hafner, S.D., Weinrich, S., Astals, S. and Holliger, C. (2020): Power and limitations of biochemical methane potential (BMP) tests. In: Frontiers in Energy Research 8, 63. https://doi.org/10.3389/fenrg.2020.00063

Standard BMP Methods website

Many labs have long-term experience with BMP tests, and numerous methods are available. However, details about individual test methodology is often incomplete in papers and report, which hinders evaluation of experimental results. Inter-laboratory comparisons have shown low reproducibility among labs, which is related to a lack of standardization in both laboratory methods and calculations (processing of raw measurements). The Standard BMP Methods website was developed to help standardize measurements methods and calculations for more accurate and reproducible BMP determination. For detailed recommendations, select the Methods link. For background information, select Literature.