Methanogenic Potential

The methanogenic power or potential (MP) of organic matter refers to its capacity to produce methane (CH₄) when treated by biomethanization.

Biomethanization is the transformation of organic matter (table scraps, sludge from municipal water treatment plants, residuals from the agri-food industry, agro-inputs, etc.) by anaerobic bacteria into a biogas that is mainly composed of methane and carbon dioxide, but also into a solid fraction known as digestate.

Methanogenic potential, generally expressed in normal cubic metres of methane (Nm3 CH4) per ton of volatile matter (VM), is a key indicator for sizing digesters and assessing the energy potential of inputs.

The BMP (Biochemical Methane Potential) test, performed in the laboratory, quantifies the theoretical methanogenic potential of different types of inputs under controlled conditions. It involves mixing a representative sample of the input with an inoculum (microbial flora) and incubating it under mesophilic (35-40°C) or thermophilic (55-60°C) anaerobic conditions to reproduce the conditions of a methane digester. The gas emitted by the sample is analyzed for an average of 60 days until the maximum potential is determined.
When a plateau is observed in gas production, this indicates that the bacteria have completely degraded the assimilable carbon contained in the tested input. However, since laboratory conditions are optimal for bacterial growth, the methanogenic potential measured in this way tends to be overestimated compared to the results obtained under actual conditions in the digester.

An anaerobic digester can receive various types of inputs, such as:

• Industrial waste (e.g. paper mills, agri-food industry)
• Agricultural waste (e.g. animal waste such as slurry and manure, crop residues)
• Sludge from water treatment plants (primary or secondary treatment)
• Organic waste sorted at the source (table scraps)

The table below provides a range of values for the methanogenic potential of different substrates. These values may vary from one region to another, as operating conditions may differ.

Although inputs such as manure and slurry have relatively low methanogenic potential, other significant advantages should be considered in any biomethanization project when compared to the conventional spreading of these slurries/manures on fields :

• Significant reduction in odours compared to spreading raw slurry, as biomethanization breaks down the volatile compounds responsible for odours.
• Reduction in atmospheric emissions of NH3, a harmful gas.
• Easier assimilation by plants. As a matter of fact, digestate, which is rich in mineral fertilizers (N, K, P) like those found in manure, serves as both a soil amendment and a suitable fertilizer.
• Significant reduction in bacteria, viruses and pathogens in digestate compared to slurry. This reduction is even greater in thermophilic conditions (55-60 °C) when compared to mesophilic conditions (35-40°C).

Whey is an excellent substrate for methanization if we consider only its methanogenic potential. It has a high organic matter content and therefore a high chemical oxygen demand (COD). It is therefore an excellent source of food for bacteria. However, whey should preferably be co-digested with other inputs, such as slurry, as digesting it alone is unstable. Since whey has a low buffering capacity, if digested alone, the pH could drop dramatically, inhibiting the activity of methanogenic bacteria.

Among the inputs with high methanogenic potential are grease, pasta and bread, and oils. Bakery waster has excellent methanogenic potential because it is rich in organic matter, and more specifically in fats, which are particularly suitable for conversion into methane during methanization.

When it comes to sludge from water treatment plants, sludge from primary treatment contains more carbon than sludge from secondary treatment. This is because, during biological treatment, some of the carbon is already broken down by bacteria, therefore reducing its presence in secondary sludge.

In summary, controlled supply and a good balance between the various inputs will determine the performance of the digester. It is essential to have an effective input management system, including a receiving tank for mixing and homogenizing the substrate before it is introduced into the digester.

Moreover, it is essential to know precisely the nature of the inputs to establish the best input recipe to ensure that the biological balances necessary for anaerobic digestion are respected.
In order to properly characterize the inputs, the following analyses are generally performed on the raw material received:

• Percentage of dry matter (% DM)
• Percentage of total volatile matter (% TVM)
• Kjeldahl nitrogen concentration
• COD (chemical oxygen demand) concentration
• And, of course, the methanogenic potential of each input

The organic matter biomethanization sector for biogas production is still in its infancy in Quebec. However, with Énergir’s goal of achieving a 10% RNG (Renewable Natural Gas) injection target in its network by 2030, there is currently strong enthusiasm for new biomethanization projects in Quebec. This is therefore an area of expertise that is still developing, both in terms of the design and construction of biomethanization plants and their operation.

At AQUASAN, we understand and know the challenges of this new industry and are here to assist you, whether it be in choosing the right polymer for the digestate dehydration stage, managing foam, or determining the micronutrient requirements needed to boost the biomass responsible for anaerobic digestion.

Sources:

• https://biogasworld.com/biogas-calculations/
• “Biogas et Compost 101” training given by Enviro-Compétences, winter 2024
• April 2024 RNG Forum presentations
• https://solagro.org/images/imagesCK/files/publications/2023/2022_Rapport_Record.pdf (in French only)
• https://www.iea.org/reports/outlook-for-biogas-and-biomethane/assessing-the-sustainable-potential-and-cost-of-feedstocks-for-biogas-and-biomethane
• https://www.arvalis.fr/infos-techniques/quel-est-le-potentiel-methanogene-des-couverts-vegetaux (in French only)
• https://www.mapaq.gouv.qc.ca/sitecollectiondocuments/agroenvironnement/1457_rapport.pdf  (Page 64, figure 16)
• https://ville.montreal.qc.ca/pls/portal/docs/PAGE/RES_AGGLO_GMR_FR/MEDIA/DOCUMENTS/ETUDE_POTENTIEL_MATIRES_ORGANIQUES.PDF (in French only)
• https://methasynergie.fr/wp-content/uploads/2020/05/Guide_m%C3%A9thanisation_industrie-agroalimentaire.ORDIF_Avril-2016-1.pdf (in French only)