Handbook of Biofuels Production || Production of biogas via anaerobic digestion

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<ul><li><p> Woodhead Publishing Limited, 2011</p><p>12345678 9101234567892012345678930123456789401243X</p><p>266</p><p>12Production of biogas via anaerobic digestion</p><p>K. STAMATELATOU, G. ANTONOPOULOU and G. LYBERATOS, University of Patras, Greece</p><p>Abstract: Anaerobic digestion is a biological process that converts the organic matter present in various types of wastes (sewage sludge, agro-industrial wastes, OFMSW, energy crops) into: (1) biogas (rich in methane, suitable to be used for heat and/or electricity generation) (2) biosolids (microorganisms grown on the organic matter and unconverted particulate residues mostly fibres which can be used as soil conditioner), and (3) liquor (dissolved organic matter, recalcitrant to anaerobic degradation and nutrients, which may be used as liquid fertiliser). The vast improvement in various scientific fields (reactor engineering, modelling and control practices, molecular tools) helped to gain a better insight of the process. In addition, the policy to promote biogas utilisation contributed in boosting the application of the anaerobic digestion technology to achieve a two-fold goal: energy production and waste minimisation. All these aspects are discussed in what follows.</p><p>Key words: anaerobic digestion, biogas, control, modelling, utilisation.</p><p>12.1 Introduction: the anaerobic digestion process</p><p>Anaerobic digestion is a biochemical process conducted by the concerted action of a consortium consisting of several groups of microorganisms that degrade the organic matter into a gaseous mixture consisting of methane and carbon dioxide (biogas) in the absence of oxygen. It happens naturally in environments with lack of oxygen such as the bottom of lakes, swamps, the landfills or the intestine of animals. However, the term anaerobic digestion usually describes the technology of accelerating the naturally evolved bioprocess in an artificial environment of a closed vessel.</p><p>Anaerobic digestion was first applied in the tenth century BC for heating bath water in Assyria. In the seventeenth and eighteenth centuries, a flammable gas mixture produced was correlated with the decay of organic matter and, moreover, the correlation became quantitative; the more organic matter is decayed, the more flammable gas is produced. It was in 1808, when Sir Humphrey Davy discovered that methane was a constituent of the gas produced by cattle manure. The anaerobic technology was first demonstrated in Bombay, India, in 1859, by building an anaerobic digester (Meynell, 1976). The biogas recovered from a sewage treatment plant was used to fuel street lamps in Exeter, England, in 1895 (McCabe, 1957). The development of microbiology science in the 1930s brought up further improvement in the anaerobic technology through identification of the anaerobic bacteria, and the conditions favouring the process efficiency and the limitations </p></li><li><p> Production of biogas 267</p><p>123456789101234567892012345678930123456789401243X</p><p> Woodhead Publishing Limited, 2011</p><p>(Buswell and Hatfield, 1936). Since then, numerous anaerobic applications have been developed worldwide, mainly in the field of waste treatment, but also in manufacturing of chemicals, fibres, food, etc.</p><p>12.1.1 The process</p><p>Anaerobic digestion is a complex bioprocess consisting of successive, often interactive steps carried out by groups of microorganisms with different growth rates and sensitivity to environmental conditions (pH, partial pressure of hydrogen, etc.). The process can be outlined as consisting of the following steps (Fig. 12.1):</p><p> Disintegration: The complex particulate waste disintegrates to organic polymers such as carbohydrates, proteins and lipids. Disintegration lumps a number of steps such as lysis, non enzymatic decay, phase separation and physical breakdown (e.g. shearing; Batstone et al., 2002).</p><p> Hydrolysis: The organic polymers (carbohydrates, proteins and fats) are hydrolysed (depolymerised) by extracellular enzymes to their respective monomers (sugars, amino acids, lipids), which can be taken up by the microorganisms for further degradation. In the case of particulate complex organic matter consisting of lignocellulosic material (mainly of plant origin), </p><p>12.1 COD flux for a particulate waste consisting of 10% inerts and 30% each of the main organic polymers (in terms of COD) (Batstone et al., 2002).</p></li><li><p>268 Handbook of biofuels production</p><p>12345678 9101234567892012345678930123456789401243X</p><p> Woodhead Publishing Limited, 2011</p><p>pretreatment steps are necessary to enhance hydrolysis by rendering the substrate matrix more amenable to enzyme attack.</p><p> Acidogenesis: A versatile group of microorganisms are able to convert the simple monomers to a mixture of volatile fatty acids, alcohols and other simpler organic compounds. This step is also often called fermentation. During acidogenesis, large amounts of carbon dioxide are produced as well as hydrogen. Especially in the case of sugars fermentation, the amount of hydrogen produced can be high and may be harvested for energy recovery. The growth rate of acidogens is quite high (doubling time of the order of one hour or even less) and low pH resistant (56) giving them the advantage of prevailing in the anaerobic consortium at adverse conditions. As a result of the rapid acid formation, there is a danger of acid accumulation (and concomitant pH drop) if the acids are not degraded in time in the steps that follow.</p><p> Acetogenesis: The higher volatile fatty acids (propionate, butyrate, valerate, etc.) as well as the other organic molecules produced in the acidogenesis step are transformed to acetic acid, carbon dioxide and hydrogen by the acetogenic bacteria. This step is thermodynamically inhibited by hydrogen, meaning that, unless hydrogen is depleted by the hydrogen-consuming bacteria in other steps, there is an accumulation of mainly propionic and butyric acid. The acetogenic bacteria are slow growing microorganisms doubling time of the order of days.</p><p> Methanogenesis: There are two distinct groups of microorganisms that produce methane and carbon dioxide: (1) the acetoclastic methanogens that grow on acetic acid and produce approximately 70% of methane in the biogas, and (2) the hydrogen utilising methanogens that consume hydrogen and carbon dioxide. The methane content of biogas depends on the oxidation state of the organic carbon in the initial substrate (ranging from 4 for methane to +4 for carbon dioxide); the more reduced the initial substrate is, the more methane will be produced, but on average the biogas contains 60% of methane. Acetoclastic methanogens are slow growing microorganisms (doubling time of the order of days) and are particularly sensitive to a number of factors such as pH, lack of nutrients, certain compounds, etc.</p><p>12.2 Factors affecting the anaerobic digestion process</p><p>The anaerobic consortium consists of several microorganism groups with different physiology that coexist syntrophically or antagonistically, resulting in a different response to environmental changes. As a consequence, when the activity of one of the microorganism groups is inhibited, the growth rates of other microorganisms are affected, changing the population balance, often causing a decrease in process efficiency or even failure. It has been recognised that the most important factors affecting the anaerobic digestion process are the pH, the temperature, the nature of the feedstock (composition, nutrients), the presence of toxic or inhibitory substances and the organic loading rate.</p></li><li><p> Production of biogas 269</p><p>123456789101234567892012345678930123456789401243X</p><p> Woodhead Publishing Limited, 2011</p><p>12.2.1 The pH</p><p>The pH affects the dissociation of weak acids and bases, and therefore, the formation of undissociated acids and bases which can easily penetrate the cellular membrane changing the internal pH of the cells. The pH also influences the function of the extracellular enzymes and has an impact on the hydrolysis rate. In most cases, the anaerobic transformation of organic matter is achieved most efficiently at a neutral pH. Many species though can grow at lower or higher pH values.</p><p>Low values of pH and the concomitant intermediate acid accumulation are more inhibitory to the methanogens than the acidogenic bacteria. Acidogens can grow and continue to produce acids at low pH values (56), intensifying the inhibitory conditions to the methanogens. However, it is known that methanogenesis can occur in extreme environments where very low or high pH values prevail such as swamps, hot springs, etc.</p><p>It is common to the acidogens that produce a mixture of metabolic products to switch their metabolism towards the formation alcohols to avert any further pH decrease (Huang et al., 1986; Gottschal and Morris, 1981; Lowe and Zeikus, 1991).</p><p>12.2.2 Temperature</p><p>As temperature increases, the biochemical reactions take place at a higher rate, up to a point where the structure of the cellular components (proteins, nucleic acids, etc.) change, rendering the cell inactive. Generally the microorganisms are distinguished into three groups according to the temperature range in which they grow: thermophilic (optimum above 50C), mesophilic (optimum 3040C) and psychrophilic (optimum below 20C). The enzymes developed in a microorganism, after proper adjustment, can be tolerant to temperature changes. As a result there are bacteria that can grow in more than one temperature range. In the majority of anaerobic systems, the acetoclastic methanogens, being the most sensitive microorganisms group in anaerobic digestion, are influenced dramatically by small changes in temperature. Temperature in combination with other factors influences the number of bacteria that coexist in heterogeneous populations, and as a result, it has a significant part in the selection of the microorganisms group that will prevail in an anaerobic digestion system.</p><p>Mesophilic and thermophilic anaerobic digestion are preferred against psychrophilic due to the higher rate of the process at these temperature ranges. However, psychrophilic temperatures are often imposed by local climatic conditions and it is important to improve the process in these conditions. Most research has focused on mesophilic bacteria acclimated to low temperatures and not on real psychrophilic bacteria isolated from naturally cold habitats (Kashyap et al., 2003).</p></li><li><p>270 Handbook of biofuels production</p><p>12345678 9101234567892012345678930123456789401243X</p><p> Woodhead Publishing Limited, 2011</p><p>12.2.3 Feedstock composition</p><p>Anaerobic bacteria can degrade a variety of organic compounds (carbohydrates, proteins, lipids, etc.). The methane content of the biogas mixture depends on the oxidative state of carbon in the compounds present in the feedstock; the more reduced the carbon is, the higher the content of biogas in methane is (Gujer and Zehnder, 1983). The feedstock should also be balanced with respect to the ratio of carbon and nitrogen (C:N = 20:30), since the microorganisms use carbon and nitrogen at this ratio range. Quite often, organic feedstocks contain these nutrients at lower or higher ratios. In such cases, the codigestion of selected feedstocks can adjust the required balance (diet) and enhance biogas production, e.g. codigestion of sewage sludge with agricultural wastes or municipal solid wastes (Alatriste-Mondragon et al., 2006) as well as cattle manure with municipal solid wastes (Hartmann and Ahring, 2005). Apart from C and N, other elements present at trace concentrations are also crucial to the growth of anaerobic microorganisms. For example, Ni (involved in the synthesis of coenzyme F430), Fe (constituent of electron carriers), Mg (stabilising the cellular membranes), Ca (stabilising the cellular wall and contributing to the thermal stability of the endospores), Co (component of the vitamin B12), Zn (constituent of several enzymes). etc. If these trace elements are not contained in the feedstock, they should be supplied since their absence is correlated with decrease in efficiency (Zandvoort et al., 2006).</p><p>12.2.4 Toxic compounds and inhibitors</p><p>Oxygen: The tolerance of the microorganisms in relation with oxygen classifies them as aerobic (when growth requires oxygen), facultative anaerobes (when growth may occur on oxygen when available but does not require it) and anaerobes, classified further to strictly anaerobes (when oxygen is toxic) and aerotolerant anaerobes (when growth may occur in the presence of oxygen but without utilising it). Strict anaerobes include Clostridia, methanogens, sulphate reducers and homoacetogens. The sensitivity to oxygen varies widely among the strict anaerobes. All bacteria contain enzymes to react with oxygen and produce toxic free radicals that destroy vital cellular components. However, it is the presence of other enzymes that remove the toxic oxygen radicals that determine the degree of tolerance to oxygen. In anaerobic environments the traces of oxygen are rapidly consumed by the facultative anaerobes of the consortium, decreasing the redox potential to acceptable levels (400 mV). For this reason, the facultative anaerobes are usually found in external layers in systems where spatial distribution of the various populations is possible (e.g. lagoons, heterogeneous or hybrid bioreactors).</p><p>Ammonia: It is the degradation product of nitrogenous compounds such as proteins and amino acids. Anaerobic digestion of feedstocks such as manure results in the production of high amounts of ammonia. Non-ionised ammonia is quite inhibitory to methanogens. Since the concentration of non-ionised ammonia </p></li><li><p> Production of biogas 271</p><p>123456789101234567892012345678930123456789401243X</p><p> Woodhead Publishing Limited, 2011</p><p>is a function of pH, the inhibition is less at neutral pH. There are contradictory reports on the levels of tolerance to ammonia, due to differences in substrates, inocula, environmental conditions and acclimation periods. The inhibitory total ammonia concentration causing a 50% decrease in the methane production ranges from 1.7 to 14 g/L (Chen et al., 2008). Acclimation plays a significant role in making the anaerobic consortium tolerant to high levels of ammonia (Bhattacharya and Parkin, 1989; Angelidaki and Ahring, 1993).</p><p>Long chain fatty acids (LCFAs) and other organic compounds: LCFAs tend to be adsorbed on surfaces and interfere with the molecule transfer mechanisms or the protection functions of the cell wall or membrane. Moreover, flotation of biomass can occur as a result of the adsorption of LCFA (Rinzema et al., 1989). The inhibition of LCFA in thermophilic anaerobes is more severe because of the different composition of their cell membranes (Hwu and Lettinga, 1997). Biodegradation of LCFAs, although difficult, has been observed in mesophilic and thermophilic conditions.</p><p>Other organic compounds which have been found to be toxic to anaerobic digestion are: alkyl benzenes, halogenated benzenes, nitrobenzenes, phenol and alkyl phenols, halogenated phenols, nitrophenols, alkanes, halogenated aliphatics, a...</p></li></ul>