OPTIMIZATION OF THE ANAEROBIC DIGESTION OF BIOMASS: A REVIEW

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1 OPTIMIZATION OF THE ANAEROBIC DIGESTION OF BIOMASS: A REVIEW Nittaya Boontian School of Environmental Engineering, Suranaree University of Technology Muang Nakhon Ratchasima District, Nakhon Ratchasima, 30000, Thailand n.boontian@sut.ac.th) Abstract - Anaerobic digestion is considered an efficient, cost effective and competitive means of producing renewable energy. Biological conversion of biomass to methane has received increasing attention in recent years. Grasses have been explored for their anaerobic digestion potential to methane. In this review, extensive literature data have been tabulated and classified. The influences of several parameters on the potential of these feedstocks to produce methane are presented. Almost all the land and water grown species examined to date either have good digestion characteristics or can be pre-treated to promote digestion. Lignocellulosic biomass represents a mostly unused source for biogas and ethanol production. Many factors, including lignin content, crystallinity of cellulose, and particle size, limit the digestibility of the hemicellulose and cellulose present in the lignocellulosic biomass. Pretreatments have are used to improve the digestibility of the lignocellulosic biomass. Each pretreatment has its own effects on cellulose, hemicellulose and lignin, the three main components of lignocellulosic biomass. Focus is placed on substrate pre-treatment in anaerobic digestion (AD) as a means of increasing biogas yields using today s diversified substrate sources. Current pre-treatment method to improve AD are being examined with regard to their effects on different substrate types, highlighting approaches and associated challenges in evaluating substrate pre-treatment in AD systems and its influence on the overall system of evaluation. WWTP residues represent the substrate type that is most frequently assessed in pre-treatment studies, followed by energy crops/harvesting residues, organic fraction of municipal solid waste, organic waste from food processing and manure. Overall, substrates containing lignin or bacterial cells appear to be the most amendable to pre-treatment for enhancing AD.. Anaerobic digestion, lignocellulosic biomass, Methane production, Optimization, Pre- Keywords: treatment. 1. Introduction Anaerobic digestion (AD) is a method engineered to decompose organic matter by a variety of anaerobic microorganisms under oxygen-free conditions. The end product of AD includes biogas (60 70% methane) and an organic residue rich in nitrogen. For a long time research is being done to enhance the digestibility of lignocellulosic biomass for mainly the efficient conversion of (hemi-) cellulose to ethanol, methane and, in the last years, also to hydrogen. It is however not clear which characteristics of the lignocellulosic biomass are important, to determine a successful pretreatment. Further more additional problems, like production of recalcitrant or inhibitory products, are to be solved. A lot of literature is written about different pre-treatment methods to enhance the digestibility of lignocellulosic material. Methane production through biomethanation technology has been evaluated as one of the most energy-efficient and environmentally benign way of producing vehicle biofuels and can provide multiple benefits to the users. The important processes in anaerobic digestion are hydrolysis, acidogenesis, acetogenesis, and methanogensis, where hydrolysis step is an extra cellular process where the hydrolytic and acidogenic bacteria excrete enzyme to catalyze hydrolysis of complex organic materials into smaller units. The hydrolyzed substrates are then utilized by acidogenic bacteria. Product such as acetate, hydrogen and carbon dioxide can directly be used by methanogenic bacteria producing methane and carbon dioxide, while other more products such as alcohol and volatile fatty acids are further oxidized by acetogenic bacteria in syntrophic with the methanogens [1]. The whole process is carried out with the help of microorganisms and the growth of microorganisms depends on various parameters like ph, temperature, C/N ratio, organic loading rate, reactor designing, inoculums and HRT. Therefore, to harness fully the anaerobic digestion potential, these parameters should be maintained in the optimized range. Pre-treatment, additives and reactor designing according to feedstock can solve the major limitations like low gas production from agricultural residues, large hydraulic retention time and low gas production in winters. This review paper explores the methane potential of various substrates along with their essential properties. There is specific focus on anaerobic degradation of lignified 48

2 biomass with the help of pre-treatment which ensures the complete harnessing of the energy and fertilizer aspect of biomass. Fundamental requirement of the optimization of operating parameters is discussed with major constraints. Moreover, conclusions with respect to promising pretreatment techniques. II. The composition of lignocellulosic material Lignocellulosic material consists of mainly three different types of polymers, namely cellulose, hemicellulose and lignin. 1. Cellulose Cellulose exists of D-glucose subunits, linked by b- 1,4 glycosidic bonds. The cellulose in a plant consists of parts with a crystalline (organized) structure, and parts with a, not well-organized, amorphous structure. The cellulose strains are bundled together and form so called cellulose fibrils or cellulose bundles. These cellulose fibrils are mostly independent and weakly bound through hydrogen bonding [2]. 2. Hemicellulose Hemicellulose is a complex carbohydrate structure that consists of different polymers like pentoses (like xylose and arabinose), hexoses (like mannose, glucose and galactose), and sugar acids. The dominant component of hemicellulose from hardwood and agricultural plants, like grasses and straw, is xylan, while this is glucomannan for softwood [3]. Hemicellulose has a lower molecular weight than cellulose, and branches with short lateral chains that consist of different sugars, which are easy hydrolyzable polymers. Hemicellulose serves as a connection between the lignin and the cellulose fibers and gives the whole cellulose hemicellulose lignin network more rigidity [2]. 3. Lignin Lignin is one of the most abundant polymers in nature and is present in the cellular wall. It is an amorphous heteropolymer consisting of three different phenylpropane units (p-coumaryl, coniferyl and sinapyl alcohol) that are held together by different kind of linkages. The main purpose of lignin is to give the plant structural support, impermeability, and resistance against microbial attack and oxidative stress. The amorphous heteropolymer is also non-water soluble and optically inactive; all this makes the degradation of lignin very tough. Lignin, just like hemicellulose, normally starts to dissolve into water around 180 _C under neutral conditions. The solubility of the lignin in acid, neutral or alkaline environments depends however on the precursor [4]. III. The biomethanation process The biomethanation is a complex biological process, which can be divided in four phases of biomass degradation and conversion, namely hydrolysis, acidogenesis, acetogenesis, and methanation. 1. Hydrolysis phase Undissolved compounds like cellulose (a form of carbohydrates), proteins, and fats are cracked down into monomers (water-soluble fragments) by exoenzymes (hydrolase) of facultative and obligatorily anaerobic bacteria. The hydrolysis of carbohydrates takes place within a few hours, while hydrolysis of proteins and lipids may take a few days. The degradation of lignocellulose and lignin is slow and incomplete. The facultative anaerobic micro-organisms take the oxygen dissolved in the water and thus cause the low redox potential necessary for obligatorily anaerobic microorganisms. The conversion of carbohydrates into simple sugars, lipids (fats) into fatty acids and proteins into amino acids take place in the hydrolysis phase [5]. 2. Acidogenic phase The monomers formed in the hydrolytic phase are taken up by different facultative and obligatorily anaerobic bacteria and are degraded into short-chain organic acids, C1 C5 molecules (e.g. butyric acid, propionic acid, acetate, and acetic acid), alcohols, hydrogen, and carbon dioxide. The concentration of the intermediately formed hydrogen ions affects the kind of the products of the fermentation. The higher the partial pressure of hydrogen, the fewer reduced compounds (like acetate) are formed. 3. Acetogenic phase The products from the acidogenic phase serve as substrate for other bacteria, in the third phase. In the acetogenic phase, homoacetogenic micro-organisms constantly reduce exergonic H2 and CO2 to acetic acid. Acetogenic bacteria grow in a symbiotic relationship with methane-forming bacteria. During this phase, organic acids and alcohols are converted into acetate. For example, when ethanol (CH3CH2OH) is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced. If the hydrogen accumulates and significant hydrogen pressure occurs, then the termination of activity of acetate-forming bacteria comes into play and the lost of acetate production occurs. 4. Methanogenic phase In this phase, the methane formation takes place under strict anaerobic condition. This reaction is categorically exergonic. As follows from the description of the methanogenic micro-organisms, not all methanogenic species degrade all substrates. One can divide substrates acceptable for methanogenesis into the following three groups: (I) Acetoclastic methanogenesis Acetate CH 4 + CO 2 (II) Hydogenotrophic methanogenesis H 2 + CO 2 CH 4 (III) Methyltrophic methanogenesis Methanol CH 4 + H 2 O 49

3 IV. Optimization of operational parameters for biomethanation There are some basic requirements for effective play of anaerobic bacteria (hydrolytic, acidogenetic, acetogentic and methanogenetic) those degrade the particular biomass in terms of feed compositions and environmental conditions inside the reactor. These basic requirements for efficient operation of biomethanation system are as follows. 1. Retention time Generally higher retention time yields higher cumulative biogas yield and results higher total volatile solid mass reduction. Rate of gas generation is initially high and then gradually declines as the digestion approaches towards completion [6]. High retention time values help to permit biological acclimation to toxic compounds [5]. The design of the retention time is a function of the final disposition of the digested sludge. The retention time may be relatively high or low, if the digested sludge is to be land applied or incinerated, respectively. However, increase in retention time >12 days do not contribute significantly to increase the destruction of volatile solids. 2. Process temperature The variations in operating temperature of digester even a few degrees affect almost all the biological activity including the inhibition of some anaerobic bacteria, especially methane-forming bacteria. Most methane-forming bacteria are active in two temperature ranges. These ranges are the mesophilic range C and the thermophilic range C. At temperatures between 40 and 50 C, methane-forming bacteria are inhibited. Methane-forming bacteria are active and grow in several temperature ranges. Anaerobic digestion in the psychrophilic temperature range (10 20 C) is usually confined to small-scale treatment units such as Imhoff tanks, septic tanks, and sludge lagoons. The psychrophilic, mesophilic and thermophilic methanogens convert the organic substrates into methane. The psychrophilic produces methane when the process temperature is up to 20 C and converts a lesser quantity of biodegradable volatile solids, thus producing a very low amount of biogas [7]. Mesophilic come into play in the temperature range of C and the biogas production reaches the maximum when the process temperature is maintained around 35 C [8]. Thermophilic operation provides benefits of short degradation time, good reduction of pathogens, high gas production and good sludge separation. 3. ph In biomethanation process, ph significantly affects its performance and is an important parameter affecting the growth of a variety of micro-organisms involved in various stages during operation of the process [7]. The ph of the digester can be kept within a desired range by feeding an optimal organic loading rate. A ph outside the range of starts showing toxic effect on methanogens population. The ph of the system depends on the rate at which intermediates are formed during fermentation. The drop in ph below 6.6 adversely affects the activities of the methanogens while a ph of 6.2 becomes toxic. The acceptable enzymatic activity of acid-forming bacteria occurs above ph 5.0, but acceptable enzymatic activity of methane forming bacteria does not occur below ph 6.2. Most anaerobic bacteria, including methane-forming bacteria perform well within a ph range of Solid concentration The degradable part of feed material in a unit volume of slurry is defined as solid concentration. The total solids (TS) concentration of the waste influences the ph, temperature and effectiveness of the microorganisms in the decomposition process. The solid concentration is optimized according to the reactor design. Normally 7 9% solid concentration is best suited for floating dome reactors [9]. The CSTR was simulated over a range of % TS concentration of 4 10, at a maximum fractional conversion of 0.8 to cater for system inefficiencies [10]. High organic loading rate (OLR) reduce the HRT and capital cost generated by size of digesters. 5. Organic loading rate The organic loading rate (OLR) is defined as the amount of volatile solids (VS) or chemical oxygen demand (COD) components fed per day per unit digester volume. Higher organic loading rates can reduce both the digester s size and consequently, the capital cost. However, enough time (retention time) should be permitted for the micro-organisms to break down the organic material and convert it into gas [11]. Methanogens vary considerably with regard to specific carbon requirements and growth response to organic additions. For optimum gas yield through biomethanation process, normally an % total solid content in the feed is desirable [12]. High total solids anaerobic digestion (25 30% effluent total solids) showed the need to address ammonia toxicity and trace nutrient limitations. 6. C/N ratio The C/N (carbon to nitrogen) ratio in the feedstock is very important because high level of nitrogen (>80 mg/l) as undissociate ammonia (at low C/N ratio) can cause toxicity, while low level of nitrogen (at high C/N ratio) can inhibit the rate of digestion. It is necessary to maintain proper C/N ratio of substrate in desired range. It has been established that during biomethanation process microorganisms utilize carbon times more than nitrogen. C/N ratio also can be optimized according to the type of reactor, the two-stage reactor has been reported as reliable process with C/ N ratios less than 20 [13]. The Co-digestion of different compatible substrate can also be used to maintain the C/N ratios. 50

4 7. Fatty acids VFA s (acetic acid, propionic acid and butyric acid) are key intermediate in the biomethanation process which are capable of inhibiting methanogensis at high concentration [7]. Sudden increase in organic loading rate is expected to cause an accumulation of high VFA s, since acetogens grow at a slower rate and subsequently a significant drop in ph occurs. High VFA inhibits growth of acid producing bacteria thus reducing rate of acidogensis. Fermentation of sugar is inhibited by total VFA concentrations above 4 g/l [14]. Formation of volatile fatty acids from fats/lipids and ammonia from proteins beyond a particular range inhibit the methane production [15]. 8. Inoculation During the degradation of waste within an anaerobic digester, facultative anaerobic bacteria like Enterobacter spp., produce a variety of acids and alcohols, carbon dioxide and hydrogen from carbohydrates, lipids and proteins. Anaerobes are active in absence of oxygen and some anaerobes are strong acid producer, such as, Streptococcus spp. In anaerobic digestion, strict anaerobes, methanogens are used to convert the acetate, alcohol, carbon dioxide and hydrogen into methane by methane forming bacteria like Methanobacterium, Methanococcus, etc. For efficient degradation of waste in biomethanation specific group of micro organisms are necessary [16]. 9. Reactor designing Various digester configurations are employed using different approaches such as one-stage or two-stage digesters, wet or dry/ semi-dry digesters, batch or continuous digesters [17], attached digesters with combination of different approaches [18]. The gas production varies considerably with time, and several units must be operated simultaneously to maintain a constant gas supply [10]. The fermentation works out normally with solid content (6 10% TS) known as wet fermentation and at high concentration (more than 20%) known as dry fermentation. High total solid substrates are mainly treated in continuous flow stirred tank reactors (CSTRs), while soluble organic waste are treated using high rate biofilm systems such as up flow anaerobic sludge blanket (UASB) reactors. 10. Mixing Mixing is a physical operation which creates uniformities in fluids and eliminates any concentration and temperature gradients. The main aim of stirring the digester contents is to provide an intimate contact between micro organisms and substrate for enhancing the biomethanation process. Mixing doesn t always take place continuously because excessive mixing can reduce biogas production. It is reported that slow mixing allow the digester to better absorb the disturbance of shock loading than high mixing of the reactor contents [13]. Excessive mixing can disrupt the granules (microbial biomass) structure; reduce the rate of oxidation of fatty acids which can lead to digester instability [19]. A survey conducted by the German Federal Agricultural Research Centre (2006) observed 60% of anaerobic digesters installed operate submersible mixers. 40% paddle, long shaft, central mixers or a combination of them. A study carried out by the University of Natural Resources and Applied Life Sciences, Vienna revealed average mixing times of 3 4 h per day rpm is suitable for high solid contents. V. Pretreatment 1. Mechanical pretreatment Milling (cutting the lignocellulosic biomass into smaller pieces) is a mechanical pretreatment of the lignocellulosic biomass. The reduction in particle size leads to an increase of available specific surface and a reduction of the degree of polymerization (DP). The milling causes also shearing of the biomass. The increase in specific surface area, reduction of DP, and the shearing, are all factors that increase the total hydrolysis yield of the lignocellulose in most cases by 5 25% (depends on kind of biomass, kind of milling, and duration of the milling), but also reduces the technical digestion time by 23 59% (thus an increase in hydrolysis rate) (Hartmann et al., 1999). 2. Thermal pretreatment During this pretreatment the lignocellulosic biomass is heated. If the temperature increases above C, parts of the lignocellulosic biomass, firstly the hemicelluloses and shortly after that lignin, will start to solubalize [20]. The composition of the hemicellulose backbone and the branching groups determine the thermal, acid and alkali stability of the hemicellulose. From the two dominant components of hemicelluloses (xylan and glucomannan), the xylans are thermally the least stable, but the difference with the glucomannans is only small. Above 180 C an exothermal reaction (probably solibilization) of the hemicellulose starts. This temperature of 180 C is probably just an indication of the temperature at which an exothermal reaction of the hemicellulose starts, because the thermal reactivity of lignocellulosic biomass depends largely on its composition. During thermal processes a part of the hemicellulose is hydrolyzed and forms acids. 3. Acid pretreatment Pretreatment of lignocellulose with acids at ambient temperature are done to enhance the anaerobic digestibility. The objective is to solubilize the hemicellulose, and by this, making the cellulose better accessible. The pretreatment can be done with dilute or strong acids. The main reaction that occurs during acid pretreatment is the hydrolysis of hemicellulose, especially xylan as glucomannan is relatively acid stable. Solubilized hemicelluloses (oligomers) can be subjected to hydrolytic reactions producing monomers, furfural, HMF and other (volatile) products in acidic environments [21]. During acid pretreatment solubilized lignin will quickly condensate and precipitate in acidic environments [22]. The solubilization of hemicellulose 51

5 and precipitation of solubilized lignin are more pronounced during strong acid pretreatment compared to dilute acid pretreatment. The advantage of acid pretreatment is the solubilization of hemicellulose and by this, making the cellulose more easily accessible for the enzymes. 4. Alkaline pretreatment Alkaline pretreatment causes hemicellulose and parts of lignin to solubalize. The removal of hemicellulose has a positive effect on the degradability of cellulose. There is however often a loss of hemicellulose to degradation products and the solubilized lignin components often have an inhibitory effect. This is probably caused by the products formed from the lignin during the alkaline heat pretreatment. The loss of fermentable sugars and production of inhibitory compounds makes the alkaline pretreatment less attractive for the ethanol production. The production of inhibitors is less severe for methanogens as compared to yeasts for ethanol production. Methanogens are (often) capable of adapting to such compounds. 5. Oxidative pretreatment An oxidative pretreatment consists of the addition of an oxidizing compound, like hydrogen peroxide or peracetic acid, to the biomass, which is suspended in water. The objective is to remove the hemicellulose and lignin to increase the accessibility of the cellulose. During oxidative pretreatment several reactions can take place, like electrophilic substitution, displacement of side chains, cleavage of alkyl aryl ether linkages or the oxidative cleavage of aromatic nuclei [23]. In many cases the used oxidant is not selective and therefore losses of hemicellulose and cellulose can occur. A high risk on the formation of inhibitors exists, as lignin is oxidized and soluble aromatic compounds are formed. Teixeira et al. (1999) [24] have investigated the use of peracetic acid at ambient temperatures as a pretreatment method for hybrid poplar and sugar cane bagasse. Peracetic acid is very lignin selective and no significant carbohydrate losses occurred. The enzymatic hydrolysis of the cellulose increased from 6.8% (untreated) to a maximum of about 98% (pretreated) at a 21% peracetic acid pretreatment. 6. Combinations, ammonia and carbon dioxide pretreatment 6.1. Thermal pretreatment in combination with acid pretreatment A way to improve the effect of thermal steam or LHW pretreatment is to add an external acid. This addition of an external acid catalyzes the solubilization of the hemicellulose, lowers the optimal pretreatment temperature and gives a better enzymatic hydrolysable substrate. The lignocellulose is often impregnated (soaked) with SO 2 or H 2 SO 4. During steam pretreatment the SO 2 is converted to H 2 SO 4 in the first 20 seconds of the process; after that, the catalytic hydrolysation of the hemicellulose starts. Another important point is that gradual removal of hemicellulose and lignin can trigger reorientation of cellulose to a more crystalline form. Latter is true for every pretreatment that gradually removes hemicellulose and lignin. The effect of the added acid is however still not clear. Tengborg et al. (1998) [25] showed a severe inhibition in the ethanol production step at a severity factor of 3 and higher with the addition of an external acid Thermal pretreatment in combination with alkaline pretretment Another way to improve the thermal pretreatment is to add an external alkali instead of an acid to the process. A very common alkaline thermal pretreatment is lime pretreatment. This pretreatment is usually carried out at temperatures of C with lime addition of approximately 0.1 g Ca(OH) 2 g substrate _1 [26]. Chang and Holtzapple (2000) [27] attribute the effectiveness of lime pretreatment to the opening of the acetyl valve and partly opening the lignin valve, making the substrate more accessible to hydrolysis. According to Kaar and Holtzapple (2000) [28] lime pretreatment (with heating) is sufficient to increase the digestibility of low-lignin containing biomass, but not for high lignin containing biomass. Chang et al. (2001) [26] mention that lime pretreatment of switchgrass and corn stover did not inhibit the enzymatic saccharification and fermentation steps. Pretreated softwood however was washed before the enzymatic saccharification and fermentation step to prevent possible inhibiting by (the large amount of) solubilized lignin. A positive effect of lime is that it is relatively cheap and safe and that the calcium can be regained as insoluble calcium carbonate by the reaction with carbon dioxide Thermal pretreatment in combination with oxidative pretreatment Ando et al. (1988) [29] mentions that the saccharification of cedar, soaked in peracetic acid and steam treated at 231 C for 10 min, was directly proportional to the amount of peracetic acid adsorbed in the chips. Wet-oxidation is another oxidative pretreatment method, which uses oxygen as oxidator. The soluble sugars produced during wet-oxidation pretreatment of wheat straw are mainly polymers opposite to the monomers produced during steaming or acid hydrolysis as pretreatment. Phenolic monomers are no end products during wet-oxidation but are further degraded to carboxylic acids Thermal pretreatment in combination with alkaline oxidative pretreatment According to Chang et al. (2001) [26] thermal lime pretreatment is not capable of removing enough lignin of high-lignin biomass to enhance the enzymatic digestibility and therefore oxygen as oxidant must be included during the pretreatment. A low sugar degradation was observed, probably as a result of the relative low temperature of 150 degrees, applied during the pretreatment. The enzymatic digestibility of the treated biomass was 13 times higher than for the 52

6 untreated biomass. The pretreated biomass was however washed to remove the probably produced inhibiting soluble lignin compounds[26]. After the oxidative lime pretreatment about 21% of the added lime could be recovered by carbon dioxide carbonation Ammonia and carbon dioxide pretreatment Other applied pretreatments are ammonia and carbon dioxide pretreatment. The objective of the ammonia pretreatment (also called AFEX pretreatment). The ammonia pretreatment is conducted with ammonia loadings around 1:1 (kg ammonia/kg dw biomass) at temperatures ranging from ambient temperature with a duration of days, to temperatures of up to 120 C with a duration of several minutes [30]. Alizadeh et al. (2005) [30] reported a six-fold increased enzymatic hydrolysis yield and a 2.5-fold ethanol yield after pretreatment. Kim and Lee (2005) [31] mention swelling of the cellulose and delignification as the responsible factors for the increased yield. Carbon dioxide pretreatment is conducted with high-pressure carbon dioxide at high temperatures of up to 200 C with a duration of several minutes VI. Conclusion The biodegradability of lignocellulosic biomass is limited by several factors like crystallinity of cellulose, available surface area, and lignin content. Pretreatments have an effect on one or more of these aspects. Several factors are mentioned to have a positive effect on the overall economy of the process. It is for example favourable to avoid the production of inhibitors, because detoxification of the liquid fraction showed to be costly and/or ineffective; to leave the lignin with the substrate and remove it after the hydrolysis of the (hemi) cellulose to minimize the overall costs of the process for ethanol production; low water, energy and alkali/acid use and a pretreatment which can be performed continuously, which is very attractive for industrial appliance. It can be concluded that pretreatments like concentrated acids, wet oxidation, solvents and metal complexes are effective, but too expensive compared to the value of glucose. Steam pretreatment, lime pretreatment, LHW systems and ammonia based pretreatments are the ones that, according to the factors determining the economic effectiveness mentioned above, and the effects of the pretreatments, have a high potential. An economical evaluation of five different pretreatment technologies (dilute acid, hot water, ammonia fiber explosion (AFEX), ammonia recycle percolation (ARP), and lime). Not much research on the carbon dioxide explosion based pretreatments has been done, so it is difficult to judge if this pretreatment is a potential one or not. The effect of the pretreatments is however very dependent on the biomass composition and operating conditions. All these pretreatments have their advantages and disadvantages and future research is needed for optimization. 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