The Biogas Handbook || Fundamental science and engineering of the anaerobic digestion process for biogas production
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<ul><li><p>5Fundamental science and engineering of the</p><p>anaerobic digestion process for biogas</p><p>production</p><p>JERRY D. MURPHY and THANASIT THAMSIRIROJ ,University College Cork, Ireland</p><p>DOI: 10.1533/9780857097415.1.104</p><p>Abstract: The aim of this chapter is to explain, in a simplified manner,</p><p>the complex microbiological process of anaerobic digestion and detail the</p><p>relationship between feedstock, anaerobic digester and methane</p><p>production. The chapter first reviews basic microbiology and explores the</p><p>interplay between the different groups of bacteria, the conditions under</p><p>which they prosper and potential sources of inhibition. Then, the chapter</p><p>deals with engineering aspects categorisation of feedstocks, methane</p><p>production per unit of feedstock, reactor configurations, mass and energy</p><p>balance of digestion systems, up-scaling laboratory systems to</p><p>commercial reality and modelling. In the analysis, an emphasis is placed</p><p>on high solid content feedstocks. Two different feedstocks are highlighted</p><p> the organic fraction of municipal solid waste and grass silage.</p><p>Key words: anaerobic bacteria, methanogens, methane, digester</p><p>configuration.</p><p>5.1 Introduction</p><p>5.1.1 Bacterial process and biogas</p><p>Anaerobic processes are quite complex microbial processes that take place</p><p>in the absence of oxygen. Bacteria are mainly involved in the process but</p><p>higher trophic groups such as protoza and anaerobic fungi may be involved.</p><p>The microbial population contains many diverse genera (types) of obligate</p><p>anaerobic bacteria (strictly anaerobic) and facultative anaerobic bacteria</p><p>(i.e. with the ability to function as aerobes in the presence of oxygen).</p><p>104</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>The process brings about the conversion of organic raw materials to</p><p>biogas. This biogas is dominated by methane (5070%) and carbon dioxide</p><p>(3050%). Considering methane has a higher heating value (HHV) of</p><p>ca. 37.8MJ/mn3 and carbon dioxide has no energy associated with it (as it is</p><p>the product of complete combustion), biogas has an energy content of</p><p>between 19 and 26MJ/mn3. Hydrogen sulphide is also present in the biogas;</p><p>the proportion depends on the characteristics of the feedstock. Typically,</p><p>biogas from animal slurry has a higher content of H2S than biogas from</p><p>crops. The bulk of the energy content of the material undergoing</p><p>biodegradation is conserved in the methane content, and only a minor</p><p>fraction is made available for bacterial growth and reproduction.</p><p>5.1.2 History</p><p>The first documented account of an anaerobic reactor was in France in</p><p>1891. The Mouras Automatic Scavanger was an airtight chamber in which</p><p>organic material was liquefied. In 1895 in Exeter, England, an engineer by</p><p>the name of Cameron invented the septic tank. It was utilised for the</p><p>preliminary treatment of municipal wastewater downstream of the coarse</p><p>and fine screens. The methane gas produced was used to heat and light the</p><p>wastewater treatment plant.</p><p>The septic tank evolved firstly to the Travis tank (1904) and then to the</p><p>Imhoff tank (1905). The Imhoff tank was utilised as a primary sedimenta-</p><p>tion step in wastewater treatment plants, with sludge storage within the</p><p>tank. This sludge was stored for months in the chamber where it was</p><p>biodegraded by anaerobic bacteria. After storage, it was inoffensive and</p><p>easy to dispose of. In 1927, Ruhrverband installed the first sludge heating</p><p>apparatus in a separate digestion tank. In the 1930s, a detailed description</p><p>of anaerobic digestion was published in the USA (Buswell and Hatfield,</p><p>1936). Much of the commercial applications of anaerobic digestion in the</p><p>latter half of the 20th century were applied to either high-strength</p><p>wastewaters (brewery and creamery wastewater with high organic loading</p><p>rates) as a precursor to aerobic treatment or to sewage sludge and</p><p>agricultural slurries. Anaerobic digestion is now viewed as a mature</p><p>technology for the treatment of wastewater, for the treatment of slurries and</p><p>sludges, for digestion of the organic fraction of municipal solid waste</p><p>(OFMSW) and, more recently, for renewable energy production through</p><p>digestion of crops (Murphy et al., 2011).</p><p>The anaerobic digestion process for biogas production 105</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>5.2 Microbiology</p><p>Four different trophic groups are currently recognised in anaerobic</p><p>processes (see Fig. 5.1). The coordinated activity of these trophic groups</p><p>as a whole ensures stability.</p><p>5.2.1 Acidogenic bacteria</p><p>Known as anaerobic acidogenic bacteria, this group consists of fermentative</p><p>and hydrolytic bacteria. The hydrolytic bacteria hydrolyse (i.e. break down</p><p>polymers to monomers; make soluble material of particulate material) and</p><p>5.1 Four trophic groups involved in anaerobic processes (adaptedfrom Colleran, 1991a).</p><p>The biogas handbook106</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>the fermentative bacteria ferment the resultant monomers to a wide range of</p><p>fermentation end products. End products of the acidogenic stage include</p><p>acetic acid, hydrogen and carbon dioxide. However, the majority of the</p><p>products are higher carbon number volatile fatty acids (VFAs) such as</p><p>propionate, butyrate and alcohols.</p><p>5.2.2 Acetogenic bacteria</p><p>The obligate proton-reducing (OPR) acetogenic bacteria are unique as they</p><p>are obligate syntrophs (i.e. they must act together with bacteria in a different</p><p>trophic group to digest a substrate). They cannot be cultivated in a pure</p><p>culture; their existence was not discovered until 1967, by Bryant and co-</p><p>workers (McCarty, 1981). Their energy is derived from substrates if the</p><p>hydrogen partial pressure is maintained at a very low level. Hydrogen is a</p><p>product of their own metabolism and is toxic to them. These acetogenic</p><p>bacteria require the syntropic action of a H2-utilising species. The role of the</p><p>OPR acetogenic bacteria is crucial to the overall anaerobic process as they</p><p>convert the fermentative intermediates (VFAs) to methanogenic substrates,</p><p>H2, CO2, acetic acids and unicarbon compounds.</p><p>5.2.3 Methanogenic bacteria</p><p>This group consists of hydrogenotrophic methanogenic bacteria and</p><p>aceticlastic methanogenic bacteria. The hydrogenotrophic bacteria utilise</p><p>the H2 which the OPR acetogens produce. H2 uptake by the methanogens is</p><p>very efficient, having an affinity of parts per million, which ensures very low</p><p>hydrogen partial pressure. The relationship between OPR acetogens and</p><p>hydrogenotrophic methanogenic bacteria is an excellent example of</p><p>syntropic mutualism: bacteria in different trophic groups converting</p><p>propionate, butyrate and long-chain fatty acids to methane and water.</p><p>Species of only two genera Methanosarcina and Methanothrix can</p><p>produce methane from acetic acid and be termed aceticlastic.</p><p>Approximately 70% of methane produced comes from acetate and</p><p>aceticlastic methanogens. It is usual in the literature to read that</p><p>methanogens have slow doubling times (reproduction rates), but this is</p><p>not entirely true: hydrogenotrophic methanogens are very efficient and have</p><p>relatively fast doubling times (Pfeffer, 1979). Aceticlastic methanogens are,</p><p>however, relatively inefficient in acetate uptake and as a result have slow</p><p>doubling times.</p><p>The anaerobic digestion process for biogas production 107</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>5.2.4 Homoacetogenic bacteria</p><p>Hydrogen-consuming acetogens appear to be outcompeted by methanogens</p><p>for hydrogen. The net result, however, is the maintenance of low hydrogen</p><p>partial pressures and increased significance of acetate as an immediate</p><p>methane precursor (Zeikus, 1979).</p><p>5.2.5 Stoichiometry of acetogenic bacteria</p><p>In 1967, Bryant and co-workers showed that the M. Omelianski culture</p><p>contained two bacterial species (McCarty, 1981). One converted ethanol</p><p>(CH3CH2OH) to acetate (CH3COOH) and hydrogen (H2); the other</p><p>converted carbon dioxide (CO2) and hydrogen to methane (CH4). So, for</p><p>the complete oxidation of a simple compound such as ethanol to CO2 and</p><p>CH4 contributions from three separate species would be required, as</p><p>follows.</p><p>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</p><p> H = CH4 CO2 28.35Net CH3CH2OH = 1.5CH4 0.5CO2 87.85</p><p>. 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</p><p>principle, which states that if some stress (in this case a reduction in the</p><p>concentration of the products) is brought to bear on a system in</p><p>equilibrium a reaction occurs which displaces the equilibrium in the</p><p>direction which tends to undo the effect (towards the product).</p><p>. Species 4.1: Hydrogenotrophic methanogenic bacteria form a syntropicassociation with the acetogenic bacteria. The standard free energy of the</p><p>reaction is relatively high in the negative direction, indicating the affinity</p><p>of the methanogen to hydrogen.</p><p>. Species 4.2: Aceticlastic methanogenic bacteria. The standard freeenergy of the reaction is less negative than for its hydrogenotrophic</p><p>relation, indicating the lower affinity of methanogens for acetate.</p><p>In the theoretical degradation of ethanol two thirds of the methane comes</p><p>from acetate and one third comes from hydrogen.</p><p>The biogas handbook108</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>5.2.6 Sulphate-reducing bacteria</p><p>Sulphate-reducing bacteria (SRB) can utilise multicarbon compounds and</p><p>methanogenic substrates: carbon dioxide, hydrogen and acetate. With high</p><p>additions of sulphate, the SRB metabolise unicarbon compounds and</p><p>hydrogen, more effectively to the detriment of the methanogens. Due to the</p><p>syntrophic nature of anaerobic processes, this affects all trophic groups</p><p>(Zeikus, 1979).</p><p>5.3 Microbial environment</p><p>5.3.1 Reducing effects</p><p>By its very definition, anaerobic digestion takes place in the absence of</p><p>molecular oxygen and the environment changes from an oxidising state to a</p><p>reducing one. This may be explained in chemical terms by noting that</p><p>alternate electron acceptors must be found to replace molecular oxygen.</p><p>Usually, carbon atoms associated with organic compounds will become</p><p>electron acceptors and will be reduced while other organic compounds will</p><p>be oxidised to carbon dioxide and volatile acids. The end product of this</p><p>reaction still contains large amounts of energy (potential to accept electrons)</p><p>in the form of methane.</p><p>For the bacterial cell to yield enough energy for cell growth and</p><p>maintenance, a large quantity of substrate needs to be processed</p><p>(Eckenfelder et al., 1988). Thus, bacterial production is much less than</p><p>would occur in aerobic conditions with oxygen as the electron acceptor</p><p>(Colleran, 1991b). The sludge/bacteria produced from the aerobic conver-</p><p>sion of glucose is 450 kg of dry microbial biomass per tonne of chemical</p><p>oxygen demand (COD) converted. This may be compared with 45 kg of dry</p><p>microbial biomass per tonne of COD converted in the anaerobic reaction</p><p>(Colleran, 1991b) as follows:</p><p>G (kJ/reaction)Aerobic: Glucose 6O2 = 6CO2 6H2O 2826Anaerobic: Glucose = 3CO2 3CH4 403</p><p>For industrial wastewater treatment this offers significant advantage to</p><p>anaerobic processes over aerobic processes. Dissolved oxygen as low as</p><p>0.1mg/l can completely inhibit methanogenic growth (Casey, 1981). Not</p><p>only oxygen but other highly oxidised materials, which are electron</p><p>acceptors, may inhibit methanogenesis (Pfeffer, 1979). Nitrites, nitrates</p><p>and sulphates are examples of such oxidised materials.</p><p>The anaerobic digestion process for biogas production 109</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>5.3.2 Volatile fatty acids and bicarbonates</p><p>The acid base system that controls pH in anaerobisis is the carbonate acid</p><p>base system. If the total volatile acid of the system is low the bicarbonate</p><p>alkalinity is a measure of the total alkalinity of the system. Should the</p><p>volatile acid concentration increase, bicarbonate alkalinity will neutralise</p><p>the acids.</p><p>Un-ionised volatile acids are toxic to methanogenesis if the pH is less than</p><p>6. Un-ionised aqueous ammonia is toxic to methane-forming microorgan-</p><p>isms at pH levels greater than 8. Thus, high concentrations of volatile acids</p><p>can be tolerated if sufficient buffering capacity exists to prevent the</p><p>formation of free acids and a shift in pH (Noone, 1990).</p><p>Most problems in anaerobic digestion can be attributed to an accumula-</p><p>tion of acids and a fall in pH (Archer and Kirsop, 1990). The adverse effect</p><p>of VFAs on methanogenic bacteria is particularly problematic as VFAs are</p><p>intermediates of the process. The effect of a decrease in pH is described by</p><p>Casey (1981) as follows. The growth of methanogens is inhibited below a pH</p><p>of 6.6. However, the acidogenic bacteria continue to function until the pH</p><p>drops to 4.55. The result is a rapid accumulation of VFAs. A useful</p><p>indicator of impending process failure is the accumulation of acetate</p><p>followed by propionate (Pfeffer, 1979). The following rules of thumb may be</p><p>applied to the management of a carbonate acid base system (Water</p><p>Pollution Control Federation (WPCF), 1987).</p><p>. Volatile acids/alkalinity > 0.30.4 indicates that the process is unstableand that corrective action is required.</p><p>. Volatile acids/alkalinity > 0.8 indicates that the process is failing due topH depression and inhibition of methanogenesis.</p><p>Microorganisms may degrade proteins, and produce ammonia. Ammonia</p><p>reacts with water to produce hydroxide ions in the following reaction.</p><p>NH3 H2O = NH4 OH</p><p>Carbon dioxide, formed by anaerobisis, partly escapes to gas phase,</p><p>however it is relatively soluble in water. In solution it reacts with hydroxide</p><p>ions in the system to form bicarbonate ions, HCO3.</p><p>CO2 H2O= H2CO3 = H HCO3H2CO3 OH= HCO3 H2O</p><p>Thus, the higher the protein content of the substrate, the more alkaline</p><p>radicals are available to form bicarbonate ions with aqueous CO2. At lower</p><p>temperatures, according to Henrys law, more carbon dioxide goes into</p><p>solution and more bicarbonate ions may be formed, leading to a higher</p><p>buffering capacity. Thus, substrates that which are treated anaerobically at</p><p>The biogas handbook110</p><p> Woodhead Publishing Limited, 2013</p></li><li><p>mesophilic temperatures will have higher buffering capacities if hydroxide</p><p>ions are available than if treated at thermophilic temperatures.</p><p>Ammonia nitrogen concentrations in the range of 15003000mg/l at pH</p><p>levels above 7.4 are said to be toxic, while levels above 3000mg/l are</p><p>supposed to be toxic at all pH levels (Van Velsen and Lettinga, 1979). It has</p><p>been found, however, that anaerobic processes have operated efficiently</p><p>with higher levels of ammonia nitrogen (Sugrue et al., 1992: Van Velsen and</p><p>Lettinga, 1979); this is a reflection of the ability of the methanogenic</p><p>population to acclimatise to toxins. Nizami et al. (2011b) found ammonia</p><p>nitrogen levels of 2400mg/l in a wet digestion process treating grass silage.</p><p>Marin-Perez et al. (2009) state that the inhibition level for grass digestion is</p><p>4700mgN/l. A slaughterhouse waste digester in Austria (International</p><p>Energy Agency (IEA), undated) found ammonia nitrogen levels varied</p><p>between 4500 and 750...</p></li></ul>
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