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  • 5Fundamental science and engineering of the

    anaerobic digestion process for biogas

    production

    JERRY D. MURPHY and THANASIT THAMSIRIROJ ,University College Cork, Ireland

    DOI: 10.1533/9780857097415.1.104

    Abstract: The aim of this chapter is to explain, in a simplified manner,

    the complex microbiological process of anaerobic digestion and detail the

    relationship between feedstock, anaerobic digester and methane

    production. The chapter first reviews basic microbiology and explores the

    interplay between the different groups of bacteria, the conditions under

    which they prosper and potential sources of inhibition. Then, the chapter

    deals with engineering aspects categorisation of feedstocks, methane

    production per unit of feedstock, reactor configurations, mass and energy

    balance of digestion systems, up-scaling laboratory systems to

    commercial reality and modelling. In the analysis, an emphasis is placed

    on high solid content feedstocks. Two different feedstocks are highlighted

    the organic fraction of municipal solid waste and grass silage.

    Key words: anaerobic bacteria, methanogens, methane, digester

    configuration.

    5.1 Introduction

    5.1.1 Bacterial process and biogas

    Anaerobic processes are quite complex microbial processes that take place

    in the absence of oxygen. Bacteria are mainly involved in the process but

    higher trophic groups such as protoza and anaerobic fungi may be involved.

    The microbial population contains many diverse genera (types) of obligate

    anaerobic bacteria (strictly anaerobic) and facultative anaerobic bacteria

    (i.e. with the ability to function as aerobes in the presence of oxygen).

    104

    Woodhead Publishing Limited, 2013

  • The process brings about the conversion of organic raw materials to

    biogas. This biogas is dominated by methane (5070%) and carbon dioxide

    (3050%). Considering methane has a higher heating value (HHV) of

    ca. 37.8MJ/mn3 and carbon dioxide has no energy associated with it (as it is

    the product of complete combustion), biogas has an energy content of

    between 19 and 26MJ/mn3. Hydrogen sulphide is also present in the biogas;

    the proportion depends on the characteristics of the feedstock. Typically,

    biogas from animal slurry has a higher content of H2S than biogas from

    crops. The bulk of the energy content of the material undergoing

    biodegradation is conserved in the methane content, and only a minor

    fraction is made available for bacterial growth and reproduction.

    5.1.2 History

    The first documented account of an anaerobic reactor was in France in

    1891. The Mouras Automatic Scavanger was an airtight chamber in which

    organic material was liquefied. In 1895 in Exeter, England, an engineer by

    the name of Cameron invented the septic tank. It was utilised for the

    preliminary treatment of municipal wastewater downstream of the coarse

    and fine screens. The methane gas produced was used to heat and light the

    wastewater treatment plant.

    The septic tank evolved firstly to the Travis tank (1904) and then to the

    Imhoff tank (1905). The Imhoff tank was utilised as a primary sedimenta-

    tion step in wastewater treatment plants, with sludge storage within the

    tank. This sludge was stored for months in the chamber where it was

    biodegraded by anaerobic bacteria. After storage, it was inoffensive and

    easy to dispose of. In 1927, Ruhrverband installed the first sludge heating

    apparatus in a separate digestion tank. In the 1930s, a detailed description

    of anaerobic digestion was published in the USA (Buswell and Hatfield,

    1936). Much of the commercial applications of anaerobic digestion in the

    latter half of the 20th century were applied to either high-strength

    wastewaters (brewery and creamery wastewater with high organic loading

    rates) as a precursor to aerobic treatment or to sewage sludge and

    agricultural slurries. Anaerobic digestion is now viewed as a mature

    technology for the treatment of wastewater, for the treatment of slurries and

    sludges, for digestion of the organic fraction of municipal solid waste

    (OFMSW) and, more recently, for renewable energy production through

    digestion of crops (Murphy et al., 2011).

    The anaerobic digestion process for biogas production 105

    Woodhead Publishing Limited, 2013

  • 5.2 Microbiology

    Four different trophic groups are currently recognised in anaerobic

    processes (see Fig. 5.1). The coordinated activity of these trophic groups

    as a whole ensures stability.

    5.2.1 Acidogenic bacteria

    Known as anaerobic acidogenic bacteria, this group consists of fermentative

    and hydrolytic bacteria. The hydrolytic bacteria hydrolyse (i.e. break down

    polymers to monomers; make soluble material of particulate material) and

    5.1 Four trophic groups involved in anaerobic processes (adaptedfrom Colleran, 1991a).

    The biogas handbook106

    Woodhead Publishing Limited, 2013

  • the fermentative bacteria ferment the resultant monomers to a wide range of

    fermentation end products. End products of the acidogenic stage include

    acetic acid, hydrogen and carbon dioxide. However, the majority of the

    products are higher carbon number volatile fatty acids (VFAs) such as

    propionate, butyrate and alcohols.

    5.2.2 Acetogenic bacteria

    The obligate proton-reducing (OPR) acetogenic bacteria are unique as they

    are obligate syntrophs (i.e. they must act together with bacteria in a different

    trophic group to digest a substrate). They cannot be cultivated in a pure

    culture; their existence was not discovered until 1967, by Bryant and co-

    workers (McCarty, 1981). Their energy is derived from substrates if the

    hydrogen partial pressure is maintained at a very low level. Hydrogen is a

    product of their own metabolism and is toxic to them. These acetogenic

    bacteria require the syntropic action of a H2-utilising species. The role of the

    OPR acetogenic bacteria is crucial to the overall anaerobic process as they

    convert the fermentative intermediates (VFAs) to methanogenic substrates,

    H2, CO2, acetic acids and unicarbon compounds.

    5.2.3 Methanogenic bacteria

    This group consists of hydrogenotrophic methanogenic bacteria and

    aceticlastic methanogenic bacteria. The hydrogenotrophic bacteria utilise

    the H2 which the OPR acetogens produce. H2 uptake by the methanogens is

    very efficient, having an affinity of parts per million, which ensures very low

    hydrogen partial pressure. The relationship between OPR acetogens and

    hydrogenotrophic methanogenic bacteria is an excellent example of

    syntropic mutualism: bacteria in different trophic groups converting

    propionate, butyrate and long-chain fatty acids to methane and water.

    Species of only two genera Methanosarcina and Methanothrix can

    produce methane from acetic acid and be termed aceticlastic.

    Approximately 70% of methane produced comes from acetate and

    aceticlastic methanogens. It is usual in the literature to read that

    methanogens have slow doubling times (reproduction rates), but this is

    not entirely true: hydrogenotrophic methanogens are very efficient and have

    relatively fast doubling times (Pfeffer, 1979). Aceticlastic methanogens are,

    however, relatively inefficient in acetate uptake and as a result have slow

    doubling times.

    The anaerobic digestion process for biogas production 107

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  • 5.2.4 Homoacetogenic bacteria

    Hydrogen-consuming acetogens appear to be outcompeted by methanogens

    for hydrogen. The net result, however, is the maintenance of low hydrogen

    partial pressures and increased significance of acetate as an immediate

    methane precursor (Zeikus, 1979).

    5.2.5 Stoichiometry of acetogenic bacteria

    In 1967, Bryant and co-workers showed that the M. Omelianski culture

    contained two bacterial species (McCarty, 1981). One converted ethanol

    (CH3CH2OH) to acetate (CH3COOH) and hydrogen (H2); the other

    converted carbon dioxide (CO2) and hydrogen to methane (CH4). So, for

    the complete oxidation of a simple compound such as ethanol to CO2 and

    CH4 contributions from three separate species would be required, as

    follows.

    G (kJ/reaction)Species 2 CH3CH2OH H2O= CH3COO H 2H2 +5.95Species 4.1 2H2 O.5CO2 = O.5CH4 H2O 65.45Species 4.2 CH3COO

    H = CH4 CO2 28.35Net CH3CH2OH = 1.5CH4 0.5CO2 87.85

    . Species 2: OPR acetogenic bacteria. The standard free energy of thereaction (G) is positive. Thus, for this reaction to occur, the hydrogenpartial pressure has to be lowered. This is explained by Le Chateliers

    principle, which states that if some stress (in this case a reduction in the

    concentration of the products) is brought to bear on a system in

    equilibrium a reaction occurs which displaces the equilibrium in the

    direction which tends to undo the effect (towards the product).

    . Species 4.1: Hydrogenotrophic methanogenic bacteria form a syntropicassociation with the acetogenic bacteria. The standard free energy of the

    reaction is relatively high in the negative direction, indicating the affinity

    of the methanogen to hydrogen.

    . Species 4.2: Aceticlastic methanogenic bacteria. The standard freeenergy of the reaction is less negative than for its hydrogenotrophic

    relation, indicating the lower affinity of methanogens for acetate.

    In the theoretical degradation of ethanol two thirds of t

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