Woody materials including bark, wood, and mixture of other residues from the forest contain cellulose, hemi — celluloses, lignin and small amount of other biomass

contents (Figure 1.4). Cellulose is the major component in plant biomass and it is made of anhydroglucopyra — nose or glucose residues, which can be converted to glucose and act as major source of hexoses in woody feedstock (Alvira et al., 2010). Due to the hydrogen bonds in it, cellulose is a highly crystalline structure and it is difficult to hydrolyze. Unlike cellulose, hemicel — luloses are heteropolymers composed of both five — carbon sugars and six-carbon sugars, including glucose, mannose, arabinose, xylose and others (Bochek, 2003). Due to its amorphous structure, hemicellulose is easily breakable by dilute acid or base. Lignin is the third ma­jor part in wood and comprises the glue that guards woody biomass from pathogens. Lignin mainly consists of phenolic units and with available technology we cannot use lignin as a source of bioethanol. Pretreatment
of these lignocelluloses separates the sugars and lignin and disrupts the crystalline portion of the biopolymers (Hu et al., 2008). Different pretreatment methods have been explored for achieving the optimistic situations with different biomass.

In general, pretreatment methods can be divided into biological pretreatment, physical pretreatment, and chemical pretreatment according to the pretreatment procedure. Some pretreatments combine two or more of broadly explored methodologies. Table 1.1 recaps some of the broadly explored pretreatment methods as per the classification (Sun and Cheng, 2002).

Biological Pretreatment

Most pretreatment technologies require selected and expensive amenities or equipment that have high power requirements, depending on the process. In particular, physical and chemical processes require rich energy for biomass conversion, whereas, biological treatment via microorganisms is a safe and environmentally friendly method and is increasingly being promoted as a process that does not require high energy, even for lignin removal from a lignocellulosic biomass (Okano et al., 2005; Potumarthi et al., 2013; Ravichandra et al., 2013).

Phanerochaete chrysosporium is one among the best microbial models to study the lignin degradation by white rot fungi. Fungi breaks down lignin anaerobi­cally through a family of extracellular enzymes collec­tively termed as lignases (Howard et al., 2003). Two families of lignolytic enzymes are generally consid­ered to play vital role in the enzymatic degradation: peroxidases (lignin peroxidase) and phenol oxidase

(Malherbe and Cloete, 2003). Other enzymes are not fully explored including glucose oxidase, methanol ox­idase, glyoxal oxidase, and oxidoreductase (Eriksson, 2000). Another best example was Trichoderma reesei, which is a mesophilic cellulolytic fungus isolated in the mid-1950s. By the mid-1970s, an impressive collec­tion of more than 14,000 cellulolytic fungi were isolated against cellulose and other insoluble fibers (Coyne et al., 2013). Trichoderma reesei, although a good producer of hemi and cellulolytic enzymes, is unable to degrade lignin (Mekala et al., 2008; Gupta et al., 2013).

Actinomycetes are also best tested for their task in lignin biodegradation. Lignolytic enzymes like peroxi­dases, ligninase and manganese peroxidase were discovered in P. chrysosporium (Saritha et al., 2012). Based on this, P. chrysosporium was taken up for biolog­ical delignification of wood and paddy straw in ethanol production. But, the extent of delignification was inade­quate to expose a significant portion of cellulose for enzymatic hydrolysis. Thus, from the reports available, it is evident that white rot fungi and actinomycete can be used jointly to remove lignin from lignocellulosic substrates, and further studies are required to shorten the incubation time and to optimize the delignification process.

Microalgae are unicellular microscopic organisms, like simple plants with no leaves and roots that grow through photosynthesis process. They capture carbon dioxide during photosynthesis and convert it into feed­stock that can be used as food, fertilizer, a source of med­icine and biodiesel (Chojnacka and Marquez-Rochaet,

2004) .

Growing algae in open pond system raise several con­cerns such as impossibility to control growth settings and contamination threats. Algal cells in open ponds are exposed to the environment, light deficiency, subject to risk of contamination, and heterogenous medium

depending upon the mixing mechanism, the shape of the ponds and the depth of the pond (Chojnacka and Marquez-Rochaet, 2004). On the other hand, closed ponds (photobioreactors) mitigate fluid culture contam­ination, and enhance full control over algal growth pa­rameters such as homogenous culture, pH, light penetration, and carbon dioxide input. They would use less space with high algal biomass yield. However, they are costly to build and maintain (Mulumba and Farag, 2012).

Design of tubular photobioreactor (TPBR) for algal cell growth was depicted in Figure 1.9. It has a main tank con­nected to two spiral tubes set in sequence. Both spiral parts were clear polyvinyl chloride (PVC) tubes of 1" external diameter and 3/4" internal diameter. The capac­ity of both spiral parts was 3.4 gallons. The main tank served as a feeding point of medium to the PVC tubes with a maximum capacity of 5 gallons (Chisti, 2007). Cul­ture medium was pumped into the tubings at a fixed flow rate. These tubes provided an area of 20 ft2 exposed to the fluorescence light. Air compressor supplies air to the
system for aeration and to serve as a source of carbon di­oxide (CO2). The air flow rate was set in the ranges of 190—210 gallons/h. In TPBR, selected algal strain was cultured using fresh medium with no modification. Algal growth and pH were measured over a period of time varying between 12 days and 14 days. A sample was taken every 2 days to quantify the turbidity using a spec­trophotometer at 682 nm and cell counts were performed using a microscope. The pH of culture was measured us­ing pH test strips.

The selected algal strain shows the typical growth curve of other microbes, which include lag, exponential or log, stationary and lytic phases. The length of each phase depends on light penetration, nutrients concen­tration, mixing mechanism, and the solubility of oxygen in medium. After reaching a stationary or lysis phase, algal culture was harvested by centrifugation followed by lyophilization to produce dry algal feedstock. Crude lipid from dried algal biomass was extracted using either modified Folch method (Cooksey et al., 1987) or Soxhlet extractor (Mulumba, 2010; Chojnacka and Marquez-Rochaet, 2004). In both methods, polar and nonpolar solvents such as methanol and chloroform/ hexane were used (Table 1.6). The combination of polar and nonpolar solvents enhances the extraction of both polar and nonpolar lipid.

BIOGAS

Biogas is obtained by anaerobic digestion (AD) of organic materials, which occurs inside the anaerobic bio­digester. Chemical composition of this biogas depends on several parameters, such as type of digester employed, the kind of organic material and the con­stancy of the feeding process of the biodigester. The most significant biogas components are methane (CH4), carbon dioxide (CO2) and sulfuric components (H2S). The composition of biogas is a crucial parameter,

because it allows identifying the suitable purification system, which aims to remove sulfuric gases and reduce the water volume, contributing to recover the combus­tion fuel conditions (Boe et al., 2007). Other important data collected from biogas analysis is referent to the low heat value, that combined to the efficiency and biogas consumption is important to estimate the electric generation potential. However, biogas production is much variable because it depends on several parame­ters, such as the kind of organic material (Liu et al.,

2004) . Biogas production involves three steps: fermenta­tion, which includes hydrolysis and acid genesis, acetone genesis and methane genesis. In the fermenta­tion process, during the hydrolysis the organic material is converted into smaller molecules and this material is transformed in soluble acids by acidogenese. Next step is acetanogenese process, transforming the products ob­tained in the first step into acetic acid, hydrogen and car­bon dioxide. The last step is referent to metanogenese process, producing methane gas through anaerobic bac­teria (Figure 1.10) (Seadi et al., 2008; Boe et al., 2007).

MECHANICAL COMMINUTION

The objective is to cut the particle size and crystal­linity of lignocellulosic biomass in order to increase the surface area and reduce the degree of polymeriza­tion. Methods like chipping, grinding and milling can be used to improve the further enzymatic hydrolysis. However, this process is not economically feasible due to the high energy requirement (Tassinari et al., 1980). During comminution, vibratory ball milling is found to be more efficient in breaking down the cellu­lose molecules of spruce and aspen chips and improving the digestibility of the biomass than ordi­nary ball milling (Sun and Cheng, 2002). The power requirement of mechanical comminution of agricultural materials depends on the final particle size and the waste biomass characteristics.

STEAM EXPLOSION

It is a hydrothermal pretreatment in which the ligno — cellulose is subjected to pressurized water vapors for few seconds to several minutes, and then suddenly dep­ressurized. In this process, combination with the partial hydrolysis of hemicelluloses and solubilization, the lignin is redistributed and removed up to certain level from the material (Pan et al., 2005). Although this tech­nique is cost-effective, it generates toxic by-products and the hemicelluloses degradation is partial (Saritha et al., 2012).

ULTRASONIC PRETREATMENT

This technique is extensively used for the treatment of sludge from wastewater treatment plants. An experi­ment on carboxyl methyl cellulose with irradiation increased the rate of enzymatic hydrolysis up to 200% approximately (Imai et al., 2004). The mechanism of ac­tion in ultrasonic treatment remains unknown. One guess is that, the hydrogen bonds in the crystalline cel­lulose structures were broken due to irradiation energy, whose energy is higher than the hydrogen bond energy (Bochek, 2003).

EXTRUSION

This process disrupts the crystal structure of lignocel — lulose and increases the accessibility of carbohydrates to enzyme. In this case, materials are subjected to heating, mixing and shearing resulting in physical and chemical modifications in biomass structure (Karunanithy et al.,

2008) . However, the process is novel and not widely applied.

A wide range of biomass types canbe used as substrates for the production of biogas by AD. The most common biomass categories used in biogas production are listed below and in Table 1.7. Animal manure and slurry, agricul­tural residues and by-products, digestible organic wastes from food and agro industries, organic fraction of munic­ipal waste and from catering, and sewage sludge, etc. are best study sources for biogas production.

Recently, various novel feed stocks has been tested and introduced for biogas synthesis in many countries, the dedicated energy crops (DECs), crops grown specif­ically for energy and biogas production. DECs can be herbaceous (grass, maize, and raps) and also woody crops (willow, poplar, and oak), although the woody crops need particular delignification treatment before

AD.

In AD, substrates can be classified according to the following criteria: methane yield, origin, dry matter (DM) content, etc. Table 1.7 gives a summary on the
characteristics of some digestible feedstocks. Substrates with DM content less than 20% are used for what is called wet digestion (wet fermentation), which includes animal slurries and manure as well as various wet organic wastes from food industries. When the DM con­tent is as high as 35%, it is called dry digestion (dry fermentation), and it is typical for energy crops and si­lages. The choice of types and amounts of feedstock for the AD substrate mixture depends on their DM con­tent as well as the content of sugars, lipids and proteins.

ACID HYDROLYSIS

During acid hydrolysis, concentrated acids like HCl and H2SO4 have been used to pretreat lignocellulosic biomass. Although acids are influential agents for cellu­lose hydrolysis, intense acids are poisonous, corrosive, and require chemical reactors that are resistant to corro­sion. In addition, concentrated acid must be removed af­ter hydrolysis of celluloses into simple sugars, which simultaneously enter into alcoholic fermentation (Potumarthi et al., 2013; Ravichandra et al., 2013). Hydro­lysis using dilute acid has been industriously developed for pretreatment of lignocellulosic biomass (O’Donovan et al., 2013). The dilute sulfuric acid pretreatment can attain high reaction rates and drastically improve cellu­lose hydrolysis. Dilute acids at lower temperatures, saccharification suffered from low yields because of sugar decomposition (Chen et al., 2009). High tempera­tures, with dilute acids are favorable for cellulose hydro­lysis. In recent times, dilute acid hydrolysis processes use less harsh environment and achieve high xylan to xylose conversion rates. Achieving high xylan to xylose conver­sion yields is required to achieve favorable economics, because xylan is the third most promising carbohydrate in many lignocellulosic feedstocks (Sun and Cheng,

2002) . Primarily two types of dilute acid pretreatment processes are well studied: high-temperature

(T > 160 °C), continuous flow process for low solids loading (5—15% (weight of biomass/weight of reaction mixture)) (Converse et al., 1989), and low-temperature (T < 160 °C), batch process for high solids loading (10—40%) (Esteghlalian et al., 1997). Although dilute acid hydrolysis can significantly improve the cellulose
breakdown, overall cost is typically higher when compared with few other physicochemical pretreatment processes such as steam explosion.

ALKALINE HYDROLYSIS

Usually alkaline hydrolysis was carried out at low temperature and pressure and it may be completed even at ambient conditions. The only drawback of this process is time; it might be hours or even days to com­plete the hydrolysis. During lime pretreatment, some cal­cium is tainted into nonrecoverable salts or included in the biomass (Chang et al., 2001). Other alkaline pretreat­ment methods include calcium, potassium, sodium and ammonium hydroxides as reactants based on biomass category. Among these reactants, sodium hydroxide re­ceives the most attention followed by lime, due to its advantage of being low cost and secure to use, as well as it is easily recoverable from water as insoluble CaCO3 by reaction with CO2. Further delignification of feedstocks can be enhanced by supplying surplus air/ oxygen (Hu et al., 2008). We can compare alkaline pre­treatment of feedstocks to Kraft pulping, where lignin was removed efficiently, thus improving the reactivity of polysaccharides. Alkaline hydrolysis also effectively removes acetyl groups and uronic substitutions from hemicellulose; thus, the surface of hemicellulose becomes more accessible to the hydrolytic enzymes.

AMMONIA HYDROLYSIS

Ammonia has abundant desirable characteristics as a pretreatment reagent. It is a valuable swelling reagent for lignocellulosic biomass, and ammonia has high selectivity for reactions with lignin over those with car­bohydrates (Kim et al., 2003). It is one of the most exten­sively used commodity chemicals with about one-fourth the cost of sulfuric acid on molar basis. Its high volatility makes it easy to recover and recycle. It is a nonpolluting and noncorrosive chemical. One of the known reactions of aqueous ammonia with lignin is cleavage of ether (CeOeC) bonds in lignin as well as ester bonds in the
ligninecarbohydrate complex (Lewin and Roldan, 1971). This above reaction indicates that ammonia pre­treatment selectively cuts the lignin content in biomass. Lignin is believed to be a major hindrance in enzymatic hydrolysis and there are several advantages by removing lignin early in the conversion process before it faces the biological treatment.

OZONOLYSIS

Ozone is a leading oxidant that demonstrates high delignification efficiency. This ozonolysis is done at room temperature and at normal pressure. In this case we do not locate any inhibitory by-products, which affect the simultaneous fermentation steps (Saritha et al., 2012). An important drawback is ozone require­ment in large quantities, which can make the process economically unapproachable (Sun and Cheng, 2002).

Bioethanol Fermentation

Once the lignocelluloses were hydrolyzed into simple sugars, they have to be fermented to ethanol. The hydro — lyzate now contains various hexoses and pentoses, mainly glucose and xylose, depending on the substrate and the pretreatment method applied. Currently, fermentation of simple sugars is mostly done using yeast cultures (Saccharomyces cerevisiae), because of its well-known char­acteristics, toughness and high ethanol yield. However, S. cerevisiae can only ferment hexoses and not the pentoses. The pentose sugars can be fermented in an additional step by another microorganism or by S. cerevisiae itself through genetic engineering approaches, so that it is able to ferment pentoses as well (Van Zyl et al., 2007). List of most popular yeast strains used for ethanol fermen­tation are mentioned in Table 1.2. Besides a high yield, an important aspect with fermentation is alcohol tolerance in the fermenting organisms. A strategy to defeat this crisis is to have a system where the ethanol is recovered at reg­ular intervals to keep the alcohol concentrations under control. Another problem is inhibitory compounds that

are produced during the pretreatment. As mentioned above they can be reduced by an additional detoxification step, but this is an expensive operation (Van Maris et al.,

2006) .

It is difficult to accept one particular type of digester for household biogas production. The design of the digesters is diversified based on the availability of sub­strate, geographical location, and climatic conditions. For example, a digester designed in mountainous re­gions has less gas volume in order to avoid gas loss. For tropical countries, it is recommended to have di­gesters underground due to the geothermal energy (Bin, 1989). Of all the different digesters developed, the fixed dome model developed in China and the floating drum model developed in India sustained to perform well until today (Rajendran et al., 2012). Recently, plug flow digesters are gaining attention due to its portability and easy operation.

Fixed Dome Digesters

The fixed dome digesters (Figure 1.11) is also called "hydraulic" or "Chinese" digesters and it is the most frequent model developed and used in China for biogas production (Rajendran et al., 2012). In this case, digester is filled through the inlet pipe until the level reaches the base level of the expansion chamber. Biogas that is pro­duced is accumulated at the upper part of the digester called storage part. The difference in the levels between the slurry inside the digester and the expansion chamber develops pressure inside due to accumulation of biogas. This accumulated biogas requires space and presses the substrate apart and enters into the expansion chamber. The slurry flows back into the digester straight away after

gas is released (Adeoti et al., 2000). Fixed dome digesters are usually built underground and the size of the digester depends on the place, number of households, and the amount of substrate available every day. Generally size of these digesters normally varies between 5 m3 and 150 m3 in various parts of Asia (Tomar, 1994). Instead of having a digester for each home, a large-volume digester is used to produce biogas for 10 to 20 homes, and is called community-type biogas digesters. In countries where houses are clustered as in Africa, these types of biogas digesters are more viable (Adeoti et al., 2000).

In the last few years technologies breakthrough has compelled us for an alternative feedstock due to consid­erable shortage in agricultural land. In this sense, ad­vances in metabolic pathway engineering/genetic engineering have led to the development of microbes skilled enough to convert biomass into ethanol (Das Neves et al., 2007). Generally, such development de­pends on expansion of the substrate range and inclusion of other biomass sources like arabinose or xylose in strains that cannot ferment sugars other than glucose. Examples of such microorganisms include genetically modified Escherichia coli, Saccharomyces sp., and Zymomo — nas mobilis, etc. (Davis et al., 2006).

In cellulosic ethanol industry, aside from Pichia stipitis, natural xylose fermenting yeast, more efforts are being taken in obtaining recombinant bacterial and yeast strains that are able to ferment pentose sugars, such as arabinose and xylose. Figure 1.5 is one among the best examples depicting recombination process in microbes, where the tail end in E. coli and Klebsiella oxytoca or the front end of

S. cerevisiae and Z. mobilis can be recombined for improved production of ethanol (Hagerdal et al., 2006).

Moreover, genetic engineering of plants is another promising area, which most likely plays a key role in bio­fuel industry. The latest hybrid varieties have helped us considerably in improving starch yield from energy crops. For example, 25 kg of corn contains about 15 kg of starch. In the near future, that same 25 kg may contain as much as 17 kg of starch through hybrid corn. This would result in a gain of nearly $2 million in annual in­come by processing the same amount of corn in a 120 million liter per year ethanol production (DOE, 2007).

Bioreactors in Ethanol Production

A major commitment in cost-effective lignocellulosic bioethanol production is to employ reactor systems yielding the maximal cellulose conversion with the min­imal enzyme. As a result, one of the most vital parame­ters for the fabrication and operation of bioreactors for lignocellulosic conversion is the efficient use of the en­zymes to gain high specific rates of cellulose conversion (yield of glucose attained/amount of enzymes). The maximization of the product concentration, i. e. the amount of glucose obtained per liquid volume, is also a significant parameter as well as the optimization of the volumetric productivity.

When hydrolysis is carried out with biomass comprised of high cellulose levels, the product concentration will drive up. For this reason, few researchers are attempting the enzymatic biomass conversion with high biomass loads (Jorgensen et al., 2007). The most imperative

difficulty in high biomass loads is related to the viscosity of reaction mixture, which also influences the rheology of the mixture. In particular, mixing and mass transfer limita­tions and presumably increased inhibition by intermedi­ates come into play. A variety of fed-batch strategies have been adopted with the scope of supplying the substrate without reaching excessive viscosities and unproductive enzyme binding to the substrate (Rudolf et al., 2005).

General criteria in bioreactor design and in the choice of the operating conditions could be use of bioreactors or reaction regimes that allow a rapid decrease in the glucose concentration; running of the reactions at low to medium substrate concentrations in order to maintain higher conversion rates and thus obtain higher volu­metric output of the reactor (Andric et al., 2010).

The combination of the bioreactor with a separation unit has obtained prospective results with product inhibited or equilibrium limited enzyme-mediated con­versions, because it potentially removes the products as they are accumulated (Gan et al., 2002). In this regard, membrane bioreactors could be a feasible process configuration. Unlike the Solid State Fermentation (SSF) approach in which the glucose consumption is car­ried out by the microbes simultaneously accessible in the hydrolyzate, the use of membrane bioreactors would finish the same function without any compromise in the reaction parameters. A membrane bioreactor (Figure 1.6) is a multitasking reactor that combines the reaction with a separation, namely, in this case the product was taken away by membrane separation, as one integrated unit (in situ removal) or alternatively in two or more separate units. The membrane bioreactors used for this separa­tion processes are mainly ultra — and nanofiltration types (Pinelo et al., 2009). However, the use of this technology is restricted by the accumulation of unreacted lignocel — lulosics in large level and/or continuous processing (Andric et al., 2010). Already in the past, few scientists enhanced the efficiency of the continuous stirred tank

bioreactor by incorporating membrane separation tech­nologies during the reactor design.

Recently, an advanced reactor system was intended that removes the reducing sugars during the enzymatic hydrolysis of cellulose through a system consisting in a tubular reactor, in which the substrate was retained with a porous filter at the bottom and buffer entered at the top through a distributor (Yang et al., 2006). This hol­low fiber ultrafiltration module with polysulfone mem­brane enabled the permeation and the separation of the sugars. To keep the volume constant in the tubular reactor, the entire buffer was recycled back from the ultrafiltration membrane and the makeup buffer was continuously sup­plied from the reservoir. In some applications an addi­tional microfiltration unit has exceptionally been used to retain the unconverted lignin-rich solid fraction due to the presence of firmly bound enzymes or has been employed to remove the unconverted substrate from the reactor. These setups result in slightly complex process layouts for the hydrolysis (Knutsen and Davis, 2004).

It is obvious that the optimization of the reactor designs will allow overcoming both the rheological and inhibition limit of the bioconversion and maximizing the enzymatic conversion. Therefore, the reactor design becomes more relevant for large-scale processing of cellulosic biomass.

The floating drum digester was first time constructed by Khadi and Village Industries Commission and this model was developed in 1962 (Figure 1.12). Although the model is old, it is one of the most extensively used designs for household purposes in India. This design includes a movable inverted drum placed on a well­shaped digester. The inverted steel drum acts as a stor­age tank, which can move up and down depending on the quantity of accumulated biogas at the top of the digester. The weight of this inverted drum applies the

pressure needed for biogas flow through the pipeline (Singh and Sooch, 2004).

Floating drum digesters manufacture biogas at a sta­ble pressure with variable volume. In floating drum reactor, by position of the drum, the amount of biogas accumulated under the drum is easily noticeable. How­ever, the floating drum needs to be coated with paint at regular intervals to avoid rusting. Additionally, fibrous materials in biomass will block the movement of the digester. Hence, their accumulation must be avoided if possible (Adeoti et al., 2000). In Thailand, the floating dome has been customized with two cement jars on each side of the floating drum. The average size of these digesters is around 1.2 m3 (Gosling, 1982). For small and medium-size farms the size varies from around 5 to 15 m3. Singh and Singh (1991) compared 14 different biogas plants with a floating drum model and optimized the various parameters for maximum biogas production.

For bioreactor application, immobilization of cells is a technique that has proved augmented ethanol productiv­ity, operation stability and easier downstream process­ing, compared to processes using suspended cells (Das Neves et al., 2007). However, the specific advantages of immobilized cells depend on the nature of cells, reactor design and nature of the process. Entrapment of cells in natural polymers by ionic gelation (alginate) or by ther­mal precipitation (carrageenan and agar) is a method commonly used for cell immobilization (Ogbonna et al.,

1991) . Immobilization by passive adhesion to surfaces has great potential for industrial application since the immobilization method is relatively simple. The use of cheap carriers ensures that this method can be exploited with minimal increase in the overall production cost. Thus, one limiting factor of this technology is that it can only be adapted for practical industrial production if the expected increase in bioethanol productivity can overcome the increase in the production costs (cost of the carrier and immobilization) (Ogbonna et al., 1996).

BIODIESEL

pollutants than petroleum-based diesel. Biodiesel can be used in its pure form (B100) or blended with petroleum diesel. Common blends include B2 (2% biodiesel), B5, and B20.

Biodiesel is an ideal biofuel contender that eventu­ally could replace petroleum based diesel. Currently, biodiesel production is still too costly to be commercialized. Due to the static cost associated with oil extraction and biodiesel processing and the variability in biomass production, future cost-saving efforts for biodiesel production should focus on the production of oil-rich feedstocks like microalgae, nonedible oils, etc.

As discussed above, biodiesel is costlier than conven­tional diesel fuel, although it is rarely quoted as being competitive, as it will be if existing fluctuations in feed — stocks/product prices are favorable. Using the distribu­tion of these prices over the last 20 years, less than 5% of cost—benefit analyses based on fixed prices over the project life will show a positive result in producing bio­diesel. If the feedstocks/product prices are varied each year, as will be the case in reality, biodiesel production will always be more expensive than conventional diesel (Duncan, 2003).

Change in the global climate is a major threat that the world is facing today. The nonrenewable energy con­sumption in the past has led to global warming that needs to be addressed (Bilen et al., 2008). The household digesters could reduce the pressure on the environment by dropping deforestation and GHG emissions followed by loss of cultivable land, and soil erosion (Gautam et al.,

2009) . Biogas production in rural areas can partly reduce global warming (Pei-dong et al., 2007). By using biogas in rural households economical, environmental, and so­cial benefits were achieved (Yang et al., 2011). Even though both carbon dioxide and methane are major con­tributors to the greenhouse effect, the global warming
effect of methane is 21 times greater than that of carbon dioxide (Dhingra et al., 2011). However, houses equip­ped with biogas systems exhibit leakage of gases in the biogas systems. Fortunately, the households with biogas plants have 48% less emissions compared to households without biogas systems (Pathak et al., 2009). It is worth talking about 10% of households, which had methane leakage (Yang et al., 2011). Research has already shown that by replacing firewood and coal with biogas, the emission of CO2 and SO2 would be reduced by 4193 thousand tons, and 62.0 thousand tons, respectively (Pei-dong et al., 2007).

CONCLUSION