FAQ: BMP testing
49 questions with answers. Click a question to expand the answer; each has a permanent link.
How does BMP differ from the theoretical methane potential?
The theoretical methane potential is calculated from the elemental or biochemical composition of the substrate, while BMP is measured experimentally, so it also reflects what the microorganisms were actually able to degrade.
Theoretical calculations (e.g. using the Buswell equation) assume full conversion. Substrates rich in lignocellulose have a BMP much lower than the theoretical one, because lignin is not degraded anaerobically. The ratio of BMP to the theoretical value shows biodegradability. Note: for fats the theoretical potential is in the order of 1000 NmL/g VS, for proteins about 500, and for carbohydrates about 415, which shows how strongly composition affects the result.
Can a BMP result be transferred directly to a biogas plant?
No. BMP determines the potential of a sample under ideal laboratory conditions. In a plant, the yield is determined by retention time, loading, mixing, the composition of the recipe and process stability.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Science & Technology; IEA Bioenergy Task 37 — Energy from Biogas; VDI — wytyczne VDI 4630 dotyczące fermentacji materiałów organicznych
Discussed in the article: What is a BMP test and why is it key for a biogas plant?
Can an installation have a higher yield than BMP?
The actual yield from a substrate rarely exceeds BMP. If the balance shows more, data errors are usually to blame: understated masses or VS, overstated gas measurement, unreported additional substrates.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Sci. Technol. 74(11), 2515–2522; Holliger C., Fruteau de Laclos H., Hack G. (2017), Methane Production of Full-Scale Anaerobic Digestion Plants Calculated from Substrate's Biomethane Potentials Compares Well with the One Measured On-Site, Front. Energy Res. 5, 12; Weiland P. (2010), Biogas production: current state and perspectives; IEA Bioenergy Task 37
Discussed in the article: BMP and biogas plant yield: why laboratory and full-scale results differ
Can BMP per gram of VS be compared with BMP per tonne of fresh matter?
Not directly — a result per VS describes the quality of the organic matter, while a result per tonne of fresh matter also depends on the water and ash content of the substrate.
To convert BMP from VS units to fresh matter, you have to multiply it by the share of TS in fresh matter and the share of VS in TS. This is why silage with 30% TS yields much more methane from a tonne of fresh matter than slurry with 8% TS, although their BMP per VS may differ less. When comparing, always check that the units and the basis of conversion are the same.
Can every BMP test be ended after 30 days?
No, a fixed number of days is not a universal criterion; the test should be ended only when the daily methane production over 3 consecutive days falls below 1.0% of the cumulative methane volume (the BMP1% criterion according to Holliger et al.).
For some materials 30 days is enough, while for others (straws, sludges, fats) production continues much longer. A test that is too short underestimates the result and distorts comparisons with publications with longer incubation. For this reason it is worth giving on charts not only the final value but also the measurement time. A "30-day" result is not identical to BMP at the plateau.
Can missing days be filled in on a BMP curve?
They can be estimated by interpolation between neighbouring measurements, but they must be marked as estimated values. It is not permitted to add points outside the measurement range or to extend the curve so that all series have the same length.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Sci. Technol. 74(11), 2515–2522; Hafner S.D. et al. (2020), Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP), Water 12(6), 1752; Holliger C. et al. (2021), Towards a standardization of biomethane potential tests: a commentary, Water Sci. Technol. 83(1), 247–250
Discussed in the article: Missing points on a BMP curve: when interpolation is honest and when it is not
Can the theoretical potential be lower than the BMP result?
It should not be. If the test result exceeds the theoretical ceiling calculated from a reliable elemental composition, this is a sign of error: in gas measurement, in the VS determination, in the elemental composition, or an overstated inoculum contribution.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Sci. Technol. 74(11), 2515–2522; Angelidaki I. et al. (2009), Defining the biomethane potential (BMP) of solid organic wastes and energy crops, Water Sci. Technol. 59(5), 927–934
Discussed in the article: Theoretical methane potential: the Buswell and Boyle formula with calculation examples
Can inoculum from a wastewater treatment plant be used to test agricultural substrates?
It can, but this must be stated. Inoculum from sewage sludge has a different microbial community and often lower activity towards lignocellulosic materials; for difficult substrates the result and kinetics may differ from a test with agricultural digestate.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Sci. Technol. 74(11), 2515–2522; Angelidaki I. et al. (2009), Defining the biomethane potential (BMP) of solid organic wastes and energy crops
Discussed in the article: Inoculum in the BMP test: where to get it, how to prepare it and how to assess activity
Do different laboratories obtain the same BMP results?
Not always; interlaboratory studies have shown that without a unified procedure, results for the same material can differ significantly.
The causes are differences in inoculum, ISR, sample preparation, measurement method and calculations. For this reason standardisation recommendations have been developed, and many laboratories use a cellulose control and common protocols. When comparing between laboratories, it is best to be guided by their compliance with a uniform methodology and the results of the positive control, not by the number alone.
Sources: Towards a standardization of biomethane potential tests: a commentary — Water Sci. Technol.
Do drying and grinding of the sample change the BMP result?
Yes, both drying and grinding can change the BMP result, because they affect losses of volatile compounds and the availability of the material to microorganisms.
Drying at a high temperature evaporates volatile fatty acids and alcohols present in silages, which underestimates BMP. Grinding too fine can speed up hydrolysis and acidification, and too coarse can slow the process. It is recommended to prepare the sample in a way as close as possible to that in which it enters the digester, and to describe the method. For TS and VS analyses of silages, a correction for losses of volatile compounds is recommended.
Does a higher k mean a higher BMP?
No. B₀ describes the quantity, k the rate. A substrate may have a high k and a low B₀ or the reverse (lignocelluloses).
Sources: Batstone D.J. et al. (2002), The IWA Anaerobic Digestion Model No. 1 (ADM1); Holliger C. et al. (2016), Towards a standardization of biomethane potential tests
Discussed in the article: First-order kinetics in the BMP test: the rate constant k, B₀ and the t90 time
Does a small spread of BMP results prove that they are correct?
No, a small spread means good repeatability, but it does not guarantee that the result is true — a systematic error can give consistent but wrong values.
Repeatability is assessed by the standard deviation or coefficient of variation between bottles, but accuracy is checked with a positive control and interlaboratory comparisons. If all bottles have the same error (e.g. a wrong volume conversion factor or a leak), the spread will be small and the result falsified. For this reason, assessing reliability requires the full set of information about the test.
Does BMP determine the output of a full-scale biogas plant?
No, BMP is the maximum potential of a substrate under ideal laboratory conditions, and the actual yield in a biogas plant is usually lower, because retention time, mixing and loading are limited.
In a plant the substrate spends a finite time in the digester and part of the matter leaves with the digestate. Added to this are storage losses, feedstock variability and imperfect mixing. For this reason the BMP result is used for comparison and planning, not as a production guarantee. It is sensible to take into account the degree of degradation realistic for a given retention time and to correct it on the basis of plant operating data.
Sources: Towards a standardization of biomethane potential tests — Holliger et al., Water Sci. Technol. (2016); Towards a standardization of biomethane potential tests: a commentary — Water Sci. Technol.
Does BMP mean the amount of biogas or of methane?
BMP means the amount of methane, not of the whole biogas; biogas production is described separately as biogas potential (BP).
Biogas potential also includes CO₂, so it is always greater than BMP (with a typical methane share of 55–60%). From an energy perspective methane is what matters, which is why NmL CH₄/g VS is the standard in the literature. Always check that the stated value concerns methane — confusing units is sometimes the source of large errors in profitability calculations.
Does pretreatment always increase BMP?
No, pretreatment (thermal, mechanical, chemical, enzymatic) increases BMP only when the limiting factor is the availability of the matter and its energy cost does not exceed the gain obtained.
For readily degradable substrates, pretreatment often gives no benefit and can be harmful, e.g. when compounds that are hard to degrade form at high temperature (Maillard reactions). For lignocellulosic materials the effect can be noticeable, but it depends on the conditions. When assessing profitability, the additional methane is compared with the energy and cost of the treatment. In the service's database, a record with pretreatment is marked separately.
How long does a BMP test take?
A typical BMP test takes from a dozen or so to several dozen days (often 3–6 weeks), but the right duration depends on the substrate and the termination criterion, not on a number of days fixed in advance.
According to current guidelines (Hafner et al. 2020; Holliger et al. 2021), the test ends when the daily methane production from a bottle over 3 consecutive days is lower than 1.0% of the cumulative net methane increase from the substrate (after subtracting the mean of the blanks). Easily degradable substrates (sugars, fats) may reach a plateau faster, and lignocellulosic materials, straws or sludges more slowly. Ending the test too early underestimates the result, and ending it too late increases costs. Always state the test duration together with the result.
Sources: Towards a standardization of biomethane potential tests — Holliger et al., Water Sci. Technol. (2016); Towards a standardization of biomethane potential tests: a commentary — Water Sci. Technol.; Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP) — Hafner et al., Water 12(6), 1752 (2020)
How many replicates are needed in a BMP test?
A BMP test is usually carried out in at least three replicates for each substrate, blank and cellulose control, because a single bottle does not allow repeatability to be assessed.
Replicates are independent bottles containing separate weighed portions. Three readings from the same bottle are not three independent tests — they measure only the reading uncertainty. The spread between bottles is described by the standard deviation or the coefficient of variation. A large spread signals sample heterogeneity, a problem with the inoculum or dosing errors.
How much does a BMP test cost?
The price of a BMP test depends on the laboratory, the number of samples, replicates, incubation time and the scope of the report, so there is no fixed rate — the price is given by the contractor in the quotation.
The cost is affected by the length of the test (several weeks), sample preparation, controls (blank and cellulose) and TS/VS analyses. Cheaper offers may limit the number of replicates or controls, which affects reliability. When ordering, it is worth asking about the method used, temperature, ISR, termination criterion and the way uncertainty is reported.
How should a sample for BMP testing be collected correctly?
A BMP sample should be representative of the whole batch of substrate, taken from many points, sent quickly to the laboratory in a sealed container and, if necessary, frozen.
For silage, material is taken from several places in the clamp, removing the lower-quality surface layer. For slurry, after mixing the tank. Care must be taken that transport does not cause fermentation or loss of volatile compounds. Information is attached to the sample: name, origin, date of collection, storage method. A badly collected sample gives a useless result even with an excellent laboratory.
How is methane volume converted to normal conditions?
Methane volume is converted to normal conditions by correcting it for temperature, pressure and water vapour content, according to the ideal gas law.
The general formula: V₀ = V · (p − p_H₂O) · T₀ / (p₀ · T), where p is the gas pressure, p_H₂O the water vapour pressure at the measurement temperature, T₀ = 273.15 K, p₀ = 1013.25 hPa. Omitting the water vapour correction overestimates the result by several per cent at mesophilic temperatures. In the test documentation it is worth stating which reference conditions were adopted.
How can you recognise a reliable BMP laboratory?
A reliable laboratory describes its method (temperature, ISR, termination criterion), runs replicates as well as blank and cellulose controls, and states uncertainty and conversion conditions in the report.
It is worth asking about the standards or guidelines used (e.g. VDI 4630, ISO 11734 and the methodological recommendations published in the literature), experience, participation in interlaboratory studies and the way samples are stored. The report should contain TS, VS, the results of the positive control, cumulative curves and an indication of whether the results were converted to normal conditions. The absence of such information is a warning sign.
Sources: Towards a standardization of biomethane potential tests: a commentary — Water Sci. Technol.
How can BMP be used to design a feed recipe?
BMP is used for a preliminary estimate of methane production from different components, which allows recipes to be compared and the digester loading to be planned, but it requires verification during operation.
By multiplying the mass of each component by its TS, VS and BMP, the expected methane production can be calculated. Then the organic loading rate (OLR), retention time, nitrogen content and risk of inhibition are checked. Recipes are introduced gradually, monitoring FOS/TAC, pH and gas composition. With high feedstock variability, the analyses are repeated periodically.
Is 413.5 mL/g VS the BMP of rice straw?
No. It is the biogas yield (not methane) from lime-treated straw under the conditions of a specific experiment. In that study methane made up about 46% of the biogas, and its yield in the lime variant was 198.0 mL/g VS. Results depend on the inoculum, temperature and sample preparation.
Sources: Du J., Qian Y., Xi Y. et al. (2019) The feasibility of shortening the pretreatment time for improvement of the biogas production rate from rice straw with three chemical agents. BioResources 14(2), 3808–3822; Zhong B. et al. (2020/2021) Anaerobic co-digestion of rice straw and pig manure pretreated with a cellulolytic microflora. Front. Bioeng. Biotechnol. 8:579405
Discussed in the article: Chinese research on rice straw: chemical pretreatment raises biogas yield by 36–45%
Is a result in line with VDI 4630 comparable with a result according to ISO 11734?
Not directly. The documents have different purposes, and the procedures differ, among other things, in the type of inoculum, the loading and the way of interpretation. Results from the same procedure and with explicitly described parameters can be compared.
Sources: VDI — Verein Deutscher Ingenieure (wytyczne VDI); ISO — International Organization for Standardization; Holliger C. et al. (2016), Towards a standardization of biomethane potential tests
Discussed in the article: VDI 4630 and ISO 11734: what they regulate and how they differ from BMP protocols
Is the BMP of a mixture the average of the BMP values of its components?
Often it is, approximately, but not always: in a mixture, synergistic or antagonistic effects may occur that change the result relative to the weighted average of the components.
Adding slurry to silage improves the buffer and provides trace elements, which can speed up the process. On the other hand, a substrate with a high protein content can increase the ammonia concentration and inhibit the whole. For this reason, for new recipes it is recommended to carry out a BMP test of the mixture and observe behaviour at pilot scale. Calculations based on individual BMP values serve as a first estimate.
Is the substrate with the highest BMP always the best for a biogas plant?
No, the highest BMP does not mean the best substrate, because price, availability, dry matter content, kinetics, impact on the process and the cost of processing the digestate also count.
A substrate with a high BMP but a low dry matter content gives little energy per tonne. Another may have a high BMP but inhibit the process. A practical comparison is based on energy yield per tonne of fresh matter, unit cost and impact on stability. BMP is an important, but only one, criterion in the analysis.
What does the "N" in NmL CH₄/g VS mean?
The letter N denotes gas volume converted to normal conditions (0°C and 1013.25 hPa), so that results from different laboratories can be compared regardless of the temperature and pressure of measurement.
The volume of methane in the bottle measured at the test temperature and at atmospheric pressure would differ from that under normal conditions. For comparisons, a correction for water vapour content is also applied. Some authors define "standard" conditions differently (e.g. 15°C or 20°C), so when comparing publications the definition must be checked. The unit NmL CH₄/g VS is the basic result format on Substraty.pl.
Discussed in the article: What is a BMP test and why is it key for a biogas plant?
What does the BMP kinetics curve show?
The BMP kinetics curve shows how quickly methane production builds up over time, which tells us about how easily the substrate degrades, not only about its final potential.
Readily degradable substrates (sugars, fruit residues) reach most of their production within a few to a dozen or so days, while difficult ones (straws, sludges, lignocelluloses) need many times longer. The lag phase is lengthened by inhibitors or an unsuitable inoculum. The curve can be described by a first-order model B(t) = B₀·(1 − e^(−kt)), where k is the rate constant and B₀ the final potential. The parameters k and B₀ are important when selecting the retention time in a plant.
What does the first-order model mean in BMP?
The first-order model describes methane production with the equation B(t) = B₀·(1 − e^(−kt)), in which B₀ is the final potential and k is the degradation rate constant.
It assumes that the rate of production is proportional to the amount of matter not yet degraded. It fits homogeneous substrates well, but mixtures of readily and slowly degradable matter less well, for which two-stage models are used. The model is a tool for description, not "the truth": parameters fitted to data from an incomplete run can be inaccurate. On multi-BMP charts the modelled curve is clearly marked.
What gas measurement methods are used in BMP tests?
The most common are measuring the pressure in the bottles (manometry), measuring gas volume by liquid displacement, or automated systems with gas meters, and the gas composition is determined by chromatography.
Each method has limitations: displacement in a liquid can cause dissolution of CO₂ and part of the methane, manometry requires correction for solubility and temperature, and automatic meters require calibration. It is important that the methane content is measured, not assumed. A BMP result always concerns methane, not the whole biogas, so a gas volume without analysis of composition is not enough.
What is BMP (biomethane potential)?
BMP (biochemical methane potential) is the maximum amount of methane that can be obtained from a unit of substrate in an anaerobic laboratory test, most often expressed in NmL CH₄ per gram of volatile solids (VS).
The test consists of incubating a sample with sludge containing microorganisms (inoculum) at a controlled temperature and measuring methane production until the increase ceases. The result describes the potential of the material, not the performance of a specific plant. It is used to compare substrates, plan recipes, assess susceptibility to degradation and check the effects of pretreatment.
What is the specific methanogenic activity (SMA) of the inoculum?
Specific methanogenic activity (SMA) tells us how much methane the inoculum produces from a reference substrate per unit of its biomass over time, and describes its "fitness".
It is measured similarly to BMP, but with a readily degradable substrate (e.g. acetate or volatile fatty acids), observing the maximum production rate. In a BMP test it is worth knowing the activity of the inoculum, because it affects the lag phase and kinetics. Low activity means a risk of underestimating the result. For this reason it is recommended to use fresh, degassed inoculum from an active plant.
What result should cellulose give in a BMP test?
In a correctly performed BMP test, microcrystalline cellulose should give a mean result in the range of 340–395 NmL CH₄/g VS, i.e. approx. 82–95% of the theoretical value (approx. 415 NmL CH₄/g VS).
This is one of the two validation criteria of the test according to Hafner et al. (2020) and the updated guidelines of Holliger et al. (2021); the other is a relative standard deviation of the cellulose result of no more than 6%. A result outside the range means that the series is not validated and requires verification: the inoculum may have been inactive or inhibited, there may have been a leak or a mistake in the volume conversion. The laboratory should document the result of the positive control in the report, and the recipient of the report should ask about it. Older publications and standards may give different ranges.
Sources: Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP) — Hafner et al., Water (2020); Towards a standardization of biomethane potential tests: a commentary — Holliger et al., Water Sci. Technol. (2021)
What spread is "normal"?
For agricultural substrates, differences in the order of several to a dozen or so per cent between well-run laboratories are not uncommon; for difficult or heterogeneous materials they can be larger. It is better to assess agreement than to demand identical numbers.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests; Hafner S.D., Fruteau de Laclos H., Koch K., Holliger C. (2020), Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP), Water 12(6), 1752
Discussed in the article: Scatter of BMP results between laboratories: where it comes from and how to read it
At what temperature is a BMP test run?
Most often the BMP test is run under mesophilic conditions, at a temperature of around 35–38°C, less often under thermophilic conditions (around 50–55°C).
The choice should correspond to the target plant: mesophilic results do not always translate to thermophilic plants. A higher temperature usually speeds up kinetics but increases sensitivity to ammonia and may change the final potential. The temperature is given together with the result — records from different temperatures are compared on multi-BMP charts while retaining information about the regime.
What to do when cellulose gives a result below the criterion?
The series is invalid. Check the activity and degassing of the inoculum, tightness, the calibration of gas measurement and the I/S ratio, and then repeat the test. Do not correct substrate results with a factor.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Sci. Technol. 74(11), 2515–2522; Hafner S.D., Fruteau de Laclos H., Koch K., Holliger C. (2020), Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP), Water 12(6), 1752; Holliger C. et al. (2021), Towards a standardization of biomethane potential tests: a commentary, Water Sci. Technol. 83(1), 247–250
Discussed in the article: Cellulose control and blank in the BMP test: why they matter and how to interpret them
What unit is the BMP result expressed in?
Most often NmL CH₄ per gram of volatile solids (VS), which is equivalent to litres of methane per kilogram of VS. Publications also give values per tonne of fresh matter, which must not be mixed with values per VS.
Sources: Chala B. et al. (2018), Biogas Potential of Coffee Processing Waste in Ethiopia, Sustainability 10(8), 2678; Keche et al. (2022), RSC Advances 12(14) — hiacynt wodny z jeziora Abaya, obróbka wstępna mocznikiem; Oduor et al. (2022), Heliyon 8(9) — współfermentacja odpadów żywności z hiacyntem wodnym (Kenia); Awiszus S., Latif S., Müller J. (2017), Tropentag — skórki manioku i miazga z Tanzanii (abstrakt)
Discussed in the article: BMP in Ethiopia: coffee husks, pulp and mucilage and water hyacinth as biogas substrates
What units is BMP given in?
Most often in NmL CH₄ per gram of volatile solids (VS), less often per gram of dry matter or per tonne of fresh mass. The volume is converted to normal conditions (0 °C, 1013 hPa), dry gas.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests, Water Sci. Technol. 74(11), 2515–2522; Angelidaki I. et al. (2009), Defining the biomethane potential (BMP) of solid organic wastes and energy crops, Water Sci. Technol. 59(5), 927–934
Discussed in the article: What is BMP? Biochemical methane potential: definition, uses and limitations
When can a BMP result be misleading?
A BMP result can be misleading when the sample is not representative, the substrate contains inhibitors or the test was ended too early, or when results obtained under different conditions are compared.
Inhibitors (ammonia, disinfectants, antibiotics, large amounts of salt) can underestimate the result, and the effect of synergy or antagonism in a mixture is not visible in a single-substrate test. A short-term test will not reveal problems of long-term operation. For this reason, in assessing a recipe, tests in continuous or semi-continuous reactors are sometimes needed alongside BMP.
When should you choose Gompertz instead of a first-order model?
When the curve has a clear lag phase and an inflection (an S shape). When production starts immediately and declines exponentially, a first-order model is sufficient.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests; Batstone D.J. et al. (2002), The IWA Anaerobic Digestion Model No. 1 (ADM1)
Discussed in the article: The modified Gompertz model in BMP tests: lag, rate and potential
Which is more accurate: pressure or volume measurement?
Either method can be reliable if it is properly calibrated and the conversions are taken into account. What matters is avoiding leaks, controlling the temperature and correctly converting to dry gas at normal conditions.
Sources: Hafner S.D., Fruteau de Laclos H., Koch K., Holliger C. (2020), Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP), Water 12(6), 1752; Holliger C. et al. (2016), Towards a standardization of biomethane potential tests
Discussed in the article: Gas measurement in the BMP test: manometric, volumetric and GC methods
Why is BMP reported per VS rather than per fresh matter?
Fresh matter contains a variable amount of water and ash. VS describes the matter that can actually be broken down, which allows substrates with different moisture content to be compared.
Sources: Holliger C. et al. (2016), Towards a standardization of biomethane potential tests; FNR — Fachagentur Nachwachsende Rohstoffe
Discussed in the article: TS and VS in BMP testing: how to determine dry matter and measurement pitfalls
Why do BMP results for the same substrate differ between publications?
Results differ because BMP is affected by the type and variety of the substrate, harvest date, particle size, inoculum, temperature, ISR, test duration and measurement method.
Even within a single substrate, e.g. maize silage, the spread can be considerable: plant maturity, starch and lignin content and the way the sample is prepared change the result. Differences in the methodology of measuring gas volume (manometry, displacement), determining its composition (chromatography) and converting to VS also matter. For this reason the service shows all records, their conditions and a weighted mean, rather than a single number.
Sources: Towards a standardization of biomethane potential tests — Holliger et al., Water Sci. Technol. (2016); Towards a standardization of biomethane potential tests: a commentary — Water Sci. Technol.
Why do fats have a high BMP yet are difficult to digest?
Fats have a very high methane potential (in the order of 1000 NmL/g VS), but in excess they disturb the process, because long-chain fatty acids inhibit methanogens and promote foaming.
The high BMP results from the high chemical energy content of fatty acids. The problem, however, is kinetics: the conversion of fats is slow, and the intermediate products are toxic to archaea. For this reason fats are added gradually, in small portions, and the parameters (FOS/TAC, pH, acids) are monitored. The BMP result shows the potential, not a safe loading.
Why does a lag phase appear on the BMP curve?
The lag phase is an initial period without a clear increase in methane, resulting from the adaptation of microorganisms, the presence of inhibitors or the need to hydrolyse complex polymers.
It can last from several hours to several days. A long lag suggests an unsuitable inoculum, too high a substrate dose or inhibition by volatile fatty acids. In kinetic models it is accounted for by a separate parameter. A short lag is typical for readily degradable substrates, and a longer one for difficult substrates or those containing inhibitors.
Why does straw have a low BMP compared with sugars?
Straw contains a lot of lignin and a compact cellulose structure, which are poorly accessible to enzymes under anaerobic conditions, so a significant part of the organic matter is not converted.
Lignin is practically non-degradable in methane fermentation and shields cellulose and hemicellulose. Pretreatment (grinding, steaming, chemical or enzymatic hydrolysis) can increase accessibility, but it usually involves costs. The BMP of straw depends strongly on the species, moisture, harvest date and degree of grinding, so the range of results in the literature is wide.
Why does the inoculum dose affect the BMP result?
The inoculum-to-substrate ratio (ISR, usually calculated on VS) affects the result because too small an amount of microorganisms can cause overload, acidification and incomplete conversion.
Methodological recommendations often propose an ISR of at least 2:1 on a VS basis, and a higher one for readily acidifying substrates. Too large a share of inoculum increases the background (the blank) and reduces accuracy, because the material tested gives a small signal. The appropriate dose depends on the substrate: it is better to justify and describe it than to apply a single value to everything.
Why is a blank run in a BMP test?
The blank contains only the inoculum without substrate and makes it possible to subtract the methane originating from the organic matter contained in the inoculum, so that the result concerns only the material tested.
The inoculum, e.g. digestate, itself produces biogas from residual organics. Production from the blank bottles is subtracted from production from the bottles with substrate after conversion to VS. If the inoculum is poorly degassed, the blank gives a large signal and the result has greater uncertainty. For this reason a pre-incubated (degassed) inoculum is used and blanks are run in replicates.
Why is a cellulose control used in a BMP test?
The cellulose control is a parallel run with microcrystalline cellulose of known potential, which confirms that the inoculum is active and that the measuring system works correctly.
The theoretical potential of cellulose is about 415 NmL CH₄/g VS, and according to current validation criteria (Hafner et al. 2020; Holliger et al. 2021) the mean cellulose result must fall within the range of 340–395 NmL CH₄/g VS, with an RSD of no more than 6% — this is a condition for the validity of the series. A result that is too low suggests a weak or inhibited inoculum, leaks or a measurement error, and one that is too high suggests errors in volume conversion or dosing. Results from a series without a correct positive control should be treated with caution.
Sources: Towards a standardization of biomethane potential tests — Holliger et al., Water Sci. Technol. (2016); Towards a standardization of biomethane potential tests: a commentary — Water Sci. Technol.; Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP) — Hafner et al., Water 12(6), 1752 (2020)