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Analysis for Anerobic Digestion
anaerobic digestion

Background to Anaerobic Digestion

Anaerobic digestion (AD) is a process whereby microorganisms break down organic matter under conditions where oxygen is absent. The degradation involves a number of stages, with the production of a biogas with a high biomethane content usually being the target output. These stages are outlined below:

  • Hydrolysis:

      In this stage bacteria break apart the biomass polymers into their constituent monomeric units. For example, the polysaccharides of biomass (cellulose and hemicellulose) will be broken apart into their individual sugar units (e.g glucose, xylose, mannose etc.) whilst proteins will be hydrolysed to amino acids and lipids to fatty acids. However, lignin is highly resistant to degradation by anaerobes meaning that feedstocks with high lignin contents (e.g. wood) may be unsuitable for anaerobic digestion unless pre-treatments are undertaken.

  • Acidogenesis:

      This involves the products from hydrolysis being further broken down by acidogenic bacteria. The major products are acetate, carbon dioxide, hydrogen sulphide and ammonia.

  • Acetogenesis:

      During this stage, the products of acidogenesis are digested by acetogens to primarily produce acetic acid, along with carbon dioxide and hydrogen.

  • Methanogenesis:

      In this final stage of anaerobic digestion methanogens utilise the products from the previous stages of digestion to produce methane, carbon dioxide and water. Two types of reaction occur in this stage: one involves the production of methane and carbon dioxide via the splitting of acetic acid; whilst the other involves hydrogen and carbon dioxide combining to produce methane and water. The remaining material consists of indigestible biomass and any dead microbes and is referred to as digestate.

  • Anaerobic digesters can consist of a single reactor or more than one. In the case of a single reactor, all of the stages of digestion occur in that reactor. In a two-stage reactor system hydrolysis, acetogenesis, and acidogenesis occur in the first reactor whilst methanogenesis occurs in the second reactor. Single stage systems are a lot cheaper to build but suffer with regards to performance as methanogenesis can be limited by pH, which is reduced as a result of the actions of acidogenic bacteria.

    Anaerobic digestion is often used to treat biodegradable wastes and sewage/wastewater sludges. The process has also been increasingly used as a means by which to generate renewable heat and/or electricity. Energy production involves the biogas being used in a gas engine or the biogas can be upgraded to biomethane than can be of a comparable quality to natural gas and suitable, in some cases, for direct injection into the gas grid. Upgraded biogas has also been used as a fuel in vehicles, particularly in captive fleets (e.g. local buses).

    Important Analytes for Anaerobic Digestion

    The AD Feedstock

    Maxmium Methane Potential via the Buswell Equation

    The maxmium theoretical methane potential possible from a given feedstock would be attained under conditions where all of the organic material of the sample was converted to biogas, with no residual digestate produced. This can be calculated using the Buswell Equation which uses the stoichiometric ratio of the products methane, carbon dioxide, ammmonia and hydrogen sulphide under the assumption that these are the only products from the complete breakdown of biomass of chemical composition CHONS, as described in the equation below:

      CcHhOoN nSs + 1/4(4c - h - 2o +3n + 2s)H2O
      = 1/8(4c + h - 2o -3n - 2s)CH4 + 1/8(4c - h + 2o +3n + 2s)CO2 + nNH3 + sH2S

      Where c, h, o, n, and s represent the molar proportions of mass fractions of elements C, H, O, N, and S in the organic fraction of biomass.

    We report the theoretical maximum methane potential when an analysis package that determines the elemental composition and ash content of the sample has been used. Such analysis packages include: "P33 - Ultimate (Elemental) Analysis"; "P34 - Calorific Value and Elements"; and "P40 - Combustion Package".

    Of course, the actual biomethane outputs of anaerobic digestion will always be less than the theoretical maximum as some of the biomass will be converted into microbial matter and also because it is unlikely that all of the organic matter (e.g. the lignin) will be fully converted.

    Biochemical Methane Potential (BMP)

    The biochemical methane potential (BMP, also referred to as the biomethane potential) is a more direct method of determining the amount of biomethane that can be produced from a sample. It involves mixing the organic substrate with an anaerobic inoculum in a closed reactor that is incubated at a set temperature, with the contents mixed, for a set period of time. During this period the sample is digested and biogas is produced. The volume of biogas is monitored allowing for a cumulative plot of biogas production over time to be derived. This biogas can then be analysed for its composition, in particular the methane content, to allow the BMP to be determined.

    The BMP test also requires that control experiments, involving only the inoculum (in water, with no substrate), be undertaken. The methane production from these blanks is then subtracted from the amount of methane produced in the experiments involving substrates in order to provide the volume of methane associated with the digestion of the substrate.

    The BMP can be considered to be the experimental theoretical maximum amount of methane produced and is expressed in terms of methane per gram of volatile solids. Volatile solids are defined as the non-ash component of dry biomass.

    At Celignis we determine the biochemical methane potential potential of samples using our BMP-RBP equipment. This is an automated device that contains 15 one-litre reactors held in a water bath and subjected to consistent levels of mixing. We analyse each sample in triplicate and run a concurrent blank/control analysis in triplicate. The digestion experiments can last either 14, 28, or 40 days, depending on the requirements of the sample and its expected rate of digestion. We produce a chart showing the daily cumulative level of biogas production and, upon completion of the experiment, we undertake an analysis of the gas for methane content. Click here for a list of packages determining the BMP.

    Chemical Composition

    Volatile solids covers all organic matter in biomass. However, not all organic matter is suitable for anaerobic digestion. For example, since lignin is typically not digested in most AD processes, it can be useful to determine the lignin content of the feedstock in order to see what proportion of volatile solids is suitable for AD. Celignis has a number of analysis packages that can be used to determine lignin content. For example, package "P8 - Lignin Content" undertakes direct acid hydrolysis of the sample. However, we have found that for many samples it is necessary to first remove the extractives before undertaking this hydrolysis as, if they are not removed, they can condense during the acid treatment leading to the formation of pseudo-lignins that can give artificially high values for the Klason lignin and acid soluble lignin contents. This phenomenon is described in more detailhere. We would recommend that analysis package "P6 - Full Extractives" is undertaken, in addition to P8, in order to get the most accurate lignin results. Ideally, analysis package "P10 - Sugars, Lignin, Extractives, and Ash" would be undertaken as this will also report the amounts of structural carbohydrates present in the sample, which would give a more detailed picture regarding the chemical composition of the sample and how it may be degraded in the AD process.

    The Digestate