BMP Testing 101:
Everything You Need to Know

What it is, and how can it help you with digester operation?
by Dan McKeaton, Senior Researcher and Anaerobic Specialist

Bio-methane potential (BMP) testing is a standard anaerobic lab test used to assess the methane production potential of different feedstocks in wastewater treatment systems. This testing can be applied pretty much universally for anaerobic sites, including mixed systems, lagoons (though sampling here can be a major hurdle), and UASBs. While BMP testing procedures can vary by site, the key points are always the same.

What is a BMP test?

BMP testing involves combining sludge with feedstock in a sealed bottle connected to some form of methane collection and measuring system.  This mixture is contained for a set amount of time to see how much methane is produced from the substrate. To ensure accurate results, it is ideal to use starved seed sludge—sludge that has been deprived of food sources before the test—to prevent it from contributing additional methane. In some cases, the chemical oxygen demand (COD) of the inoculum (seed sludge) can also be used to approximate the impacts of undegraded feed in the seed sludge.

Additionally, purging the digester headspace with an inert gas like nitrogen can be helpful in providing a more consistent startup to the bottles and limit stress on the anaerobic microbes.  While it is ideal to mix the reactors in most cases, sometimes, especially in the case of landfill or lagoon samples, be more representative of the site.

Best uses for BMP Testing

BMP testing is primarily used to evaluate the potential methane production of a specific feedstock in anaerobic environments. This means, this test is intended to use a healthy seed sludge to ensure seed sludge is not limiting methane production.  Many BMP testing sites collect inoculum from healthy sites or grow healthy inoculum in house to ensure a healthy starting bacteria and archaea population BMP testing can be used for:

• Testing feedstock nutritional supplements: To help assess how additives like micronutrients and macronutrients or co-digestion (combining multiple feedstocks) impact methane yield.
• Evaluating toxicity: Identifying the effects of chemicals, such as disinfectants, on biogas production. We have used BMP tests to study the impacts of Quaternary ammonium cation disinfectants on biogas production.  However, there is no guarantee that a seed sludge from a different site will respond the same way to a toxin, so in these cases, it is important to collect seed sludge from the same site the feedstock is coming from. Additionally, this testing can be misleading if using a standard seed sludge as bacteria and archaea often acclimate over time to toxins.

These more niche applications for BMP testing often break away somewhat from standard testing methane but can provide quite useful results. BMP can be used for some applications other than the ones listed above but the further you diverge from a standard BMP test; the more difficult experimental methodology becomes (and the less reliable a test becomes).

Limitations of BMP Testing

While BMP testing is a valuable tool, it has limitations. The main problems that you can run into with BMP testing are, not having a representative loading rate on the bacterial population as it requires batch loading and the inability, in most cases, to feed any new substrates for the duration of the test.  This stresses the methanogen population in many cases and makes it very challenging to assess the impacts of SRT on the degradability of a substrate.  This can skew one’s perspective somewhat on the degradability of a feed substrate.  In addition, while having a standard inoculum is good in most cases for BMP testing, different bacterial populations are better or worse suited for degrading specific substrates (for example, one digester site might be much better suited for the degradation of fats than another). 

BMP testing is also not very good for determining the health of a methanogen population.  This is because any health determination will require a standard inoculum from a healthy site as a comparison but since you also have a variable feed substrate this direct comparison can often be misleading. 

Finally, they are not effective when studying things like reduction of hydrogen sulfide or general biogas quality as these things are determined heavily by loading parameters.  BMP studies are generally quite poor for site optimization in general as well as they cannot mimic operation parameters.  BMP testing has also been found to be usable for determining the impacts of small doses of additives like enzymes, but the batch loading aspect of this testing can sometimes hide if enzyme additions are helpful or not in full scale applications.

Alternative Testing Methods

No matter what testing method you use, it is important to be aware of the potential shortfalls of that methodology.  In practice here, I do a lot of rougher screening tests for feed substrates and additives as a BMP test as this allows for efficient testing of feed variables.  I like to follow these up with a 2-L fed digester study to make sure all findings are realistic for full scale operation.  Once this is complete, it is then safe and helpful to do a full-scale case study to confirm that any operational, feed, or additive changes produce the desired impact on a full-scale site. But generally, lab studies should produce results consistently to a full-scale case study.

Feel free to contact us at Aquafix with any questions about anaerobic testing recommendations to understand your specific site as well as possible!

Anaerobic Food Supplement

• Provides COD and soluble micronutrients specific for anaerobic microbes, allowing better methane generation
• For use in anaerobic processes with variable loadings and of all types
• Provides more consistency in operations and methane generation
• Improves the digester microbiota, stabilizing the pH

About the Author

Dan McKeaton is a senior researcher and anaerobic digestion specialist at Aquafix Inc.  He has performed wastewater research for over 10 years and has presented his work at WEFTEC’s technical symposium on the impacts of quaternary ammonium cation disinfectants on biological wastewater treatment.  Currently, Dan is responsible for designing and executing our studies on anaerobic digestion and developing anaerobic products with a current focus on controlling hydrogen sulfide production in anaerobic digesters.