Anaerobic digestion of crop residues: Technological developments and environmental impact in the Indian context

Author’s Accepted Manuscript Anaerobic Digestion of Crop Residues: Technological Developments and Environmental Impact in the Indian Context Muramreddy Jugal Venkateswara Rao

Sukhesh,

Polisetty www.elsevier.com/locate/bab

PII: DOI: Reference:

S1878-8181(18)30391-8 https://doi.org/10.1016/j.bcab.2018.08.007 BCAB840

To appear in: Biocatalysis and Agricultural Biotechnology Received date: 2 June 2018 Revised date: 12 August 2018 Accepted date: 13 August 2018 Cite this article as: Muramreddy Jugal Sukhesh and Polisetty Venkateswara Rao, Anaerobic Digestion of Crop Residues: Technological Developments and Environmental Impact in the Indian Context, Biocatalysis and Agricultural Biotechnology, https://doi.org/10.1016/j.bcab.2018.08.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Anaerobic Digestion of Crop Residues: Technological Developments and Environmental Impact in the Indian Context Muramreddy Jugal Sukhesh*, Polisetty Venkateswara Rao Environmental Engineering Laboratory, Water and Environment Division, Department of Civil Engineering, National Institute of Technology, Warangal, India-506004

Abstract India is an agrarian country generating surplus amounts of paddy, wheat, and maize crop residues at large scale, which are not being managed properly. The anaerobic digestion is an efficient way of managing these residues in an environmentally friendly manner. It results in high calorific methane gas and fertile rich digestate. The current study focused on reviewing the availability, methane potential of crop residues and technological developments to improve the methane production of crop residues. It is found that the methane potential from the anaerobic digestion of surplus paddy, wheat, and maize residues is estimated as 18,677 Mm3/year (632 ×109 MJ/year) in India. The resulted methane potential of crop residues could substitute 52 Mt/year of coal utilization that evades 46 Mt/year of CO2 emissions from releasing into the atmosphere. Keywords: Biogas; Methane; Anaerobic digestion; Crop residues; India;

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Abbreviations: AD, Anaerobic Digestion; C/N ratio, Carbon/Nitrogen ratio;

BMP, Biochemical Methane

Potential; GHG, Green House Gas; IESS, India Energy Security Scenarios; MT, Million Metric Ton; VFAs, Volatile Fatty Acids; L-AD, Liquid State Anaerobic Digestion; SS-AD, Solid State Anaerobic Digestion; NITI, National Institute for Transforming India

1. Introduction Energy plays a vital role in fostering the development, and shortage of energy jeopardizes the growth of the nation (Hiloidhari et al., 2014). In India, large fraction of energy demand was supported by fossil fuels imported which is affecting the country’s economy (NITI Aayog, 2015). The India Energy Security Scenarios (IESS) (NITI Aayog, 2015) estimated that the share of fossil fuel imports may raise from 32% (in the year-2012) to 59.3% (in the year- 2047). The Green House Gas (GHG)emissions may rise threefold from 1.7 tons per capita (in the year2012) to 5.8 tons per capita (in the year- 2047) with the current use of fossil fuels which may affect the environment adversely (NITI Aayog, 2015). In this context, it is necessary to look for self-sustainable, environmental friendly alternate source for energy demand for meeting the needs of the country. The consistent growth of the agricultural sector in India is causing the augmented generation of crop residues (Cardoen et al., 2015a) which are needed to be handled properly. The crop residues in India were estimated to contain an energy potential of 4.15 EJ (Hiloidhari et al., 2014) that could meet the partial energy demand if used properly (Balachandra, 2011). Among the crop residues produced, paddy (Oryza sativa), wheat (Tritium aestivum), and maize (Zea mays) occupy the majority of the crop area under cultivation (percentage of gross area under

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various crops is represented in Fig.1). The area of cultivation for the crops of paddy and wheat constitutes about 40% of the gross cropped area, whereas the cultivation of maize constitutes about 5% of the gross cropped area (Cardoen et al., 2015a). These crops generate 3.2 to 4.5 tons of residues per hectare of area under cultivation (Cardoen et al., 2015a) based on the crop type. It is estimated (Hiloidhari et al., 2014) that 686 Mt of crop residues are being generated annually and about 234 Mt (34% ) of generated residues can be considered as surplus quantity. In another study, it is estimated that 611Mt of crop residues are being generated annually and 158 Mt (crop 25%) of generated residues can be considered as surplus quantity (Cardoen et al., 2015a). Among the crops, paddy generates 154 Mt/year of residue, which was the highest among the generated, and resulting in 43.5 Mt/year as a surplus residue after its primary use as animal feeding. The wheat crop contributes the second largest generation of residue, about 131 Mt/year, resulting in 28.4 Mt/year as surplus residue, which needs to be managed. Whereas, maize contributes for the generation of about 35.8 Mt/year of residue, resulting in 9 Mt/year as surplus residue (Table 1) (Hiloidhari et al., 2014). All the three major crop residues are together generating surplus crop residue of 80.9 Mt/year, which is a significant quantity that needs to be handled properly. The common practices of utilization of crop residues include feeding the cattle, using for domestic fuel, roof thatching, fencing and packaging (Milhau and Fallot, 2013). In some areas of the country, residues are used directly for heating the water in boilers. The rice straw is widely used as cattle feed in major parts of the country, whereas in Punjab, Haryana and Uttar Pradesh people prefer wheat straw as cattle feed than rice straw (Hiloidhari et al., 2014). The highest surplus crop residues are generated in Uttar Pradesh (40 Mt) followed by Maharashtra (31 Mt) and Punjab (28 Mt) (Hiloidhari et al., 2014) as these states are having larger geographical area

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and plenty of water. The surplus residues generated in these states are either left uncollected or burnt openly in the field itself (Cardoen et al., 2015a). It is reported (Hiloidhari et al., 2014) that 22% of paddy straw and 10% of wheat straw is burnt openly in Uttar Pradesh. This kind of open burning leads to the emission of smoke, dust along with greenhouse gases. The open burning of one ton of paddy straw results in 1,460 kg of CO2, 1.20 kg of CH4, 34.7 kg of CO, 3 kg of NOX, 2 kg of SO2, 13 kg of particulate matter and fine particulate matter (PM

2.5)

of 12.95 kg into the

atmosphere (Gadde et al., 2009a). The resulted toxic gases and particulate matter causes respiratory ailments for the persons who are being exposed. It is also noted that the open burning will also lead to the death of microbial population, loss of plant nutrients such as nitrogen, phosphorous and potassium from soil beside the elevation of soil temperature. Keeping in the view of detrimental environmental effects, it is required to manage the crop residues in an environmental friendly manner (Cardoen et al., 2015b). The composition of crop residue decides the suitability for any alternative management option. The typical crop residue contains about 30-44% of cellulose, 30-50% of hemicellulose and 8-21% of lignin (Chandra et al., 2012a). The available methods to manage the crop residues are gasification, alcoholic fermentation (ethanol production) and anaerobic digestion (AD) (Chandra et al., 2012a; Singh and Gu, 2010). The AD of crop residues scores better when compared to alcoholic fermentation as the process derives high net energy (Chandra, 2015; Kaparaju et al., 2009; Wang et al., 2009). The AD process is efficient as there is a possibility to degrade the cellulose which is a major constituent in crop residues up to 80% (Ress et al., 1998). In addition, the AD process also controls direct emission of GHGs into the atmosphere (Liu et al., 2015; Senghor et al., 2017) and results in fertile rich digestate that could improve the fertility and holding capacities of the soil if applied to land ( Pathak et al. 2010) (Fig.2.). In

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Germany, more than 50% of the biogas is produced from energy crops using 700 anaerobic digestion plants (Li et al., 2011a). From the experience of Germany, the anaerobic digestion of crop residues could become a sustainable option for energy generation in India. The anaerobic digestion can be defined as a biochemical process that converts the organic waste/residues to high calorific methane gas and fertile digestate in the absence of oxygen (Fig.3.). However, the process suffers from the limitations such as slow biodegradability due to lignin content and imbalanced nutritional composition in crop residues for widespread application. The limitations can be overcome with appropriate pre-treatment methods to improve the degradation of lignin coat and co-digesting substrates with the nitrogen rich animal manures to improve the nutritional balance (Fig.4.). Several pre-treatment methods and co-digestion substrates have been reported widely in the scientific literature to improve the efficiency of the AD system (Abudi et al., 2016b; Wang et al., 2012). However, an overview of a specific focus on the AD of crop residues and recent technical developments in the scientific literature is missing. The present paper is aimed to summarize the various influencing factors in the AD of paddy, wheat, and maize residues, to review the technological developments in improving the methane production and to estimate the environmental impact of the AD of crop residues.

2. Influencing factors in the AD of crop residues The AD is a complex biochemical process that decomposes organic matter by a variety of microorganisms under anaerobic environment in four phases (Mussoline et al., 2012b). The first phase is the hydrolysis during which polymeric substrates (carbohydrates, proteins, and lipids) present in organic matter gets converted into water-soluble monomers (sugars, amino acids, and long chain fatty acids). The second phase is an acidogenic phase that converts formed monomers in the hydrolysis phase into acids, alcohols, CO2 and H2. The third phase is the acetogenesis that 5

converts products of acidogenesis into acetic acid, H2, and CO2. The fourth phase is methanogenesis that takes up the generated products in earlier phase, i.e., acetic acid, H2, CO2 and converts into energy rich methane gas. The AD process efficiency depends on the syntrophic interrelation of these four phases. Further, the AD process is also influenced by factors such as the composition of the substrate, total solids (TS%) content, temperature of digestion , inoculum content, and nutritional balance. The detailed discussion of the influence of the factors on the AD system is discussed with a special focus on paddy, wheat, and maize crop residues (pictorially represented in the Fig. 5.). 2.1 Influence of composition in crop residues The composition of crop residues had a strong influence on its biodegradability, affecting the efficiency of AD process (Amon et al., 2007). Typically the crop residues consist of cellulose, hemi cellulose and lignin as the major proportion and little amounts of proteins (3-4%) and fats(1-2%) (Chandra, 2015). The cellulose is a linear polymer of cellulobiose units and hemicellulose is a branched network of pentose and hexose units whereas, lignin is the threedimensional network of phenyl propanoid units in the lignocellulosic biomass (Martínez et al., 2005). The three components cellulose, hemicellulose, and lignin, are intermeshed with each other making as the complex substrate to degrade biologically. Cellulose is linked physically with hemicellulose whereas linked physically and chemically with lignin. Lignin is linked chemically with hemicelluloses with ester or ether bonds. Table 2 shows the principal composition of these three components for paddy, wheat, and maize residues. It shows that the cellulose, hemicellulose, and lignin components of the three residues lie in the range of 30% 45%, 16%-44 %, and 1.9% - 34% respectively. The time of harvest, harvesting pattern (mechanical/manual) and silaging of crop residues

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affect their composition and methane production (Amon et al., 2007) (Fig.3). It is reported that silaged maize residue produced 25% higher methane yield than non-silaged maize residue due to the pre-decomposition of the crude fiber during silaging (Amon et al., 2007). The maize residues harvested at the stage of milk ripeness produced 16-27% higher methane yield than maize residue harvested at the stage of full ripeness due to changes in its composition over harvesting period (Amon et al., 2007). In contrary to this, methane yield per hectare of cropland is high for harvested maize residue at the stage of full ripens due to the high quantity of residue generation per hectare (Amon et al., 2007). Furthermore, the type of harvesting (mechanical/manual) also influences the structure of residues. The manual harvesting preserves the original structure of the harvested crop whereas mechanical harvesting shreds the crop residue to small pieces, which is favorable for the better AD. Moreover, the climatic conditions vary with the geographical position

that affects the composition, and the methane production (Amon et al., 2007).

Therefore, the composition and structure plays a significant role in the anaerobic digestion of crop residues. The composition of crop residues affects the AD process and several researchers mathematically correlated it with the methane production (Table 3). A positive correlation of methane production in maize was observed with crude protein, crude fat and hemicellulose and negative correlation to the acid detergent lignin (ADL) (Amon et al., 2007; Dandikas et al., 2014; Rath et al., 2013). In another study, a negative correlation of hemicellulose with methane production along with the lignin content is reported (Bekiaris et al., 2015). A slight variation of methane production with respect to composition was observed in different works which may be due to a wide variety of plant biomasses considered (Bekiaris et al., 2015). A strong negative relationship of methane production to the lignin content is reported in

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several works (Li et al., 2013; Liu et al., 2015; Triolo et al., 2011). The very low first-order rate constants (0.05-0.06 1/d) for methane production were reported for residues containing high lignin content (Li et al., 2013). It indicates that lignin content in the crop residues results in slow degradation rates. The slow degradation is due to the protective action of the lignin content to the cellulose and hemicellulose. In order to depolymerize the lignin, extracellular enzymes require oxygen, which is not available in the AD system (Triolo et al., 2011). However, the depolymerization of lignin can be achieved with an appropriate pretreatment method (Reilly et al., 2015). Several physical (Chandra et al., 2012b; Ferreira et al., 2014, 2013), chemical(Khatri et al., 2015; Reilly et al., 2015; Song and Zhang, 2015; Yuan et al., 2015) and biological pre-treatments (Mustafa et al., 2016; Zhao et al., 2014b; Zhou et al., 2017) have been proven to be effective to degrade the lignin content in crop residues. The high-energy requirements and high costs associated with the pre-treatment methods preventing its application (Abudi et al., 2016a, 2016b). Therefore, a low cost and energy efficient pre-treatment method is required to improve the methane production in crop residues. 2.2 Influence of total solids (TS) content The TS content represents dry matter excluding the moisture. Typically, the AD system can be categorized into two systems based on TS content. The AD which is carried out at TS <15%, termed as liquid state AD (L-AD), and >15%, termed as solid state AD (SS-AD) (Xu et al., 2014). The TS content of a substrate significantly influences the AD system (AbbassiGuendouz et al., 2012). Several researchers attributed the influence of TS content to the rate of hydrolysis and gas-liquid transfers between the digester content and gas phase. (Xu et al., 2014) reported that the rate of methane production was increased with an increase in

8

TS (%) upto a threshold limit of 20% , thereafter decreased. The decreased methane production at higher TS(>20% TS) was attributed to a single hypothesis, i.e., mass diffusion limitation that leads to accumulation of hydrolysed products (i.e. sugars) causing product type inhibition of hydrolysis. It is

hypothesised that different inhibition

mechanisms play roles at different ranges of TS% (Abbassi-Guendouz et al., 2012). For the case of 10% ≤ TS ≤ 25%, the low rate of hydrolysis is the reason for low methane production. For the case of TS ≥30%, the limitation of liquid-gas transfer of CH4, CO2, and H2 gases resulted in low methane production. The TS content affects the stability of the AD system. The AD of palm oil residues (analogous to crop residues) has resulted in higher methane production at a TS of 16% compared to TS of 25% and 35% (Suksong et al., 2017). The low methane production at higher TS may be due to low mass transfer coefficient (Abbassi-Guendouz et al., 2012), the formation of dead zones in the reactor (Sawatdeenarunat et al., 2014) or low microbial activity due to low water content (Suksong et al., 2017). Moreover, the TS content had a significant interactive relationship with operational temperature also. For instance, with an increase in TS content from 22% to 27% at thermophilic temperatures (55ºC), the methane production was decreased by 29.8% (Li et al., 2011b). But, for the same increase in TS content at mesophilic (35 ºC) conditions decrease in methane production was not observed indicating a possible interactive relationship between TS content and temperature. Currently, most of the AD plants in India are operated at liquid state. However, the SSAD is common in Europe where crop silages are generally used as a feedstock (Kalamaras and Kotsopoulos, 2014). More than 60% of recently installed anaerobic digesters in Europe were based on SS-AD (Karthikeyan and Visvanathan, 2013; Y. Li et al., 2011) because of certain 9

advantages. The major advantages include smaller reactor volume for the same loading of volatile solids, low investment costs for the same production of methane and do not require mixing in the digester (Brown et al., 2012; Xu et al., 2014). The SS-AD of corn stover, switch grass and wheat straw yields 2 to 7 folds of the higher volumetric methane production than the liquid state-AD (Brown et al., 2012). SS-AD avoids the requirement of extra energy (Lianhua et al., 2010) as there is no need of mixing. The amount of water required is less in SS-AD compared to L-AD which requires more water to maintain the TS content. Moreover, the problems such as floating and stratification in digester cannot be found in SS-AD and handling of sludge is easier with its low water content (Li et al., 2011a). Even though SS-AD is advantageous, certain complications associated with high TS loading need to be addressed. The complications such as low hydrolysis, limited substrate availability to microbes and low gas-liquid transfer need to be addressed for efficient use of SSAD in crop residues. Further, cost to benefit analysis of crop residues under SS-AD needs to be evaluated for practical implementation. 2.3. Influence of temperature Temperature significantly affects the microbial communities present in the AD system leading to variations in methane production (Chae et al., 2008). The microbial communities in the AD system can be categorised into psychrophilic (<20ºC), mesophilic (20-45ºC), thermophilic (45-60ºC) and hyperthermophilic (>60ºC) based on temperature. Among them, mesophilic and thermophilic temperature conditions are adopted commonly for AD process (Liu et al., 2017). The AD at mesophilic temperature conditions is more stable and less sensitive to the variations in temperature, whereas AD at thermophilic temperatures is highly unstable and

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sensitive to the variations due to high metabolic growth rates at thermophilic systems (ElMashad et al., 2004). The effect of temperature on methane production is uncertain due to changes in the growth of microbial communities as different organisms have different comfortable temperatures. For instance, the AD of composted rice straw obtained maximum methane production at 35 ºC (in the temperature range of 30-45ºC) (Yan et al., 2015) whereas corn stover obtained maximum methane production at 44 ºC (in the temperature range of 35-44ºC) (Liu et al., 2017). On the other hand, SS-AD of rice straw at mesophilic conditions (35ºC) resulted in higher methane production (123.5 mL of CH4/g VS) compared to thermophilic conditions (55 ºC) (76.3 mL of CH4/g VS) (Lianhua et al., 2010). The observed low methane production at thermophilic conditions (55 ºC) is due to the accumulation of VFAs. The accumulated VFAs resulted in increased acidity that inhibited the methanogens. In contradiction to this, corn stover (at TS 22%) at thermophilic conditions(55ºC) resulted in improved biogas production of 36% compared to mesophilic conditions (37 ºC) (Li et al., 2011b). However, from an economic point of view, mesophilic conditions are preferable due to the consumption of low energy input compared to high energy input to maintain the thermophilic temperature in the reactor (Yan et al., 2015). The AD at mesophilic conditions results in higher methane production than ambient conditions. Methane production of rice straw was increased by 53% when operating temperature increased from ambient (25 ºC) to mesophilic (35 ºC) (Acharya, 1935). Whereas, AD of rice straw at mesophilic (35 ºC) temperature resulted in an increase in methane production by 3133% than ambient conditions (25 ºC) in both liquid and solid state digestions (Lianhua et al., 2010). In addition to this, quicker energy recovery is possible at mesophilic temperature due to 11

the high rate of methane production. The quicker energy recovery facilitates the efficient use of digester volume compared to ambient conditions. However, the improved methane yield obtained for enhancing the temperature from ambient to mesophilic conditions needs to be investigated to guarantee the economic viability of the temperature change. 2.4. Influence of inoculum (microbial population) Inoculum represents the seed with high active microbial population and low biodegradable matter. It facilitates the anaerobic digestion with quick start-up and reduces the overall digestion time. The appropriate quantity of inoculum is essential for the stable and efficient performance of the AD system (Li et al., 2011b). The low inoculum content ( high S/I ratios) results in the accumulation of VFAs (acidification) subsequently inhibiting the methanogenic population (Xu et al., 2016), (Zhou et al., 2017). In the AD of corn stover rapid acidification caused the accumulation of VFA at low inoculum content (high S/I ratio) (Li et al., 2011b).Inoculum content also affects the mass transfer of the substrate to microbial mass. In the AD of rice straw, a low inoculum proportion caused poor mass transfer with low production of methane (Zhou et al., 2017). Hence, the optimum inoculum content is required for the stable and optimal production of methane in the AD system. The requirement of optimal inoculum is different at mesophilic and thermophilic conditions (Li et al., 2011b). At mesophilic conditions, higher inoculum favored the methane production, whereas in thermophilic conditions higher inoculum proportion retarded the methane production during the AD of corn stover (Li et al., 2011b). The specific reason attributed for this effect is the low tolerance limit of free ammonia (4 N g /L) for thermophilic bacteria with the supply of high inoculum. Because the high ammonium nitrogen carried with the high quantity of inoculum supplementation into the digester inhibited thermophilic 12

methanogens that have a low tolerance for ammonia. It is observed that the diluted inoculum facilitated the higher substrate loadings with improved biogas production compared to concentrated inoculum(Zhou et al., 2017). It is also observed that the supplementation of high inoculum (low S/I ratio of 2) resulted in higher methane production during AD of corn stover and wheat straw (Liew et al., 2012). In this case, the corn stover and wheat straw resulted in the methane production of 81.2 mL CH4/kg VS and 66.9 mL CH4/kg VS respectively (at S/I ratio of 2). During solid state AD, the addition of inoculum fetches additional moisture content and benefits quick mass transfer and microbial growth. For instance, the additional moisture content improved the mass transfer of VFAs to methanogens in the AD of rice straw that led to improved methane production (Zhou et al., 2017). Hence, maintaining an optimal S/I ratio plays a significant role in the AD of crop residues. In addition to the initial inoculum dosage, the richness and diversity of microbial population over the time plays a significant role in the stability and efficiency of the AD system (Song and Zhang, 2015). The microbial population in the AD system mainly constitutes with two anaerobic microbial groups: bacteria and archae (nearly 100%) (Griffin et al., 1998). However, nutritional balance (Briones et al., 2014), pre-treatment and environmental conditions such as pH (Zhou et al., 2016) and temperature (Pap et al., 2015) affect the richness and diversity of these microbial groups. In anaerobic co-digestion of rice straw and pig manure, hydrogenotrophic methanogenic genus Methanothermobacter was predominant with more rice straw addition due to increase in C/N ratio(Zhou et al., 2016). On the other hand, with an increase of temperature (from 35 ºC to 44 ºC) in L-AD of corn stover,

hydrogenotrophic methanogens

are increased

compared

to acetotrophic

methanogens,(Liu et al., 2017).

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High acetotrophic methanogens (methanosarcinales) were found at mesophilic conditions,

and

high

hydrogenotrophic

methanogens

(methanomicrobiales

and

methanobacteriales) were found at thermophilic conditions during co-digestion of rice straw, clay residues and pig manure (Jiménez et al., 2014). In SS-AD of composted rice straw, acetoclastic methanogens (Methanosarcina) were dominated at initial and growth stages and hydrogenotrophic methanogens (Methanoculleus) were dominated at stabilization period (Yan et al., 2015). It indicates that methane production at initial and growing

stages

was

mainly

contributed

by

acetoclastic

methanogenesis,

and

hydrogenotrophic methanogenesis at stabilization stage. Moreover, in SS-AD of corn stover, higher cellulolytic and xylanolytic bacteria (10-50 times) were found to be flourishing under thermophilic conditions due to increased hydrolysis compared to mesophilic conditions (Shi et al., 2013. The increased hydrolytic bacteria caused higher degradation of cellulose and hemicellulose (Shi et al., 2013). Hence, the presence of an appropriate microbial population is required for stable and efficient AD system which can be sourced by the selection of appropriate inoculum or maintaining favorable environmental conditions in the system. 2.5. Influence of nutritional balance Nutritional balance in the AD system is represented with carbon to nitrogen (C/N ratio) although other factors such as phosphorous and other trace elements influence the methane production. Generally, the manures are having the C/N ratio of 4-34, vegetable waste of 8-36, kitchen waste 26-30 and crop residues of 40-151 (Siddique and Wahid, 2018) (Table 2). However, the optimal C/N ratio for efficient performance of AD system is 20-30 (Suksong et al., 2017; Wang et al., 2012; Yan et al., 2015; Yen and Brune, 2007). The high C/N ratio in AD

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system causes the accumulation of volatile fatty acids (VFA) whereas low C/N ratio causes the release of high total ammonia nitrogen (TAN) in the digester causing low methane production (Wang et al., 2012). The optimal C/N ratio can be achieved if substrates containing low and high C/N ratios are mixed (Wang et al., 2012) at appropriate proportions. Several researchers optimised the C/N ratio in improving the methane production of crop residues. Higher methane production with C/N ratio of 29.6 for the AD of composted rice straw with the addition of urea was obtained (Yan et al., 2015). In another study, the improved methane production was obtained with an optimal C/N ratio of

20 to 25 in the anaerobic co-

digestion of waste paper mixed with algal sludge (Yen and Brune, 2007). The presence of other nutritional elements such as phosphorous also plays an important role in the AD of crop residues. The addition of phosphorous in AD of rice straw accelerated the digestion process that caused 7-10 days of earlier appearance of the methane peaks (Lei et al., 2010). The presence of other trace elements such as Fe, Ni, Co, Zn, W, and Se also plays a significant role in maintaining the overall stability and efficiency in the AD of crop residues (Demirel and Scherer, 2011). The depletion of trace elements will affect methane production phase and may lead to souring of the AD system in mono-digestion of crop residues (Demirel and Scherer, 2011). The depletion of iron (Fe) and nickel (Ni) caused the accumulation of VFAs during AD of wheat stillage (Schmidt et al., 2014). In this case, the depletion of Fe affected the methanogenic population and propionate oxidizing bacteria (Schmidt et al., 2014). The daily addition of Co, Ni, Mo, Se in the AD of Napier grass, caused 40% improvement in methane production (Wilkie et al., 1986). The improved methane production was attributed to higher conversion of VFA to methane with the addition of the micronutrients. Similarly, the addition of trace elements of iron, nickel, and cobalt caused 35% of improvement in biogas

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production in the AD of corn residues (maize) (Hinken et al., 2008). However, the quantity of addition is also important as it retards and the methane production and may inhibit the process if excessively added. For instance, a higher concentration of Ni (greater than 1 g /m3) inhibited methanogens during anaerobic digestion of sewage sludge (Ashley et al., 1982). The trace elements can also be supplemented with the co-digestion of wastewater sludge or animal manures that contains trace elements naturally (Demirel and Scherer, 2011). The nutritional balance in the AD system plays a key role that affects the methane production. However, the research work to draw the concise conclusions about the impact of trace elements lacks in the literature, which needs to be extended in future research work.

3. Improving the methane production from crop residues The AD of crop residues is having certain barriers for their efficient degradation leading to the production of methane. The barriers include recalcitrant lignin coat, nutritional imbalance, and the requirement of the microbial population in large quantity. The lignin coat is a protective hydrophobic layer, prevents the accessibility of carbohydrates (cellulose and hemicellulose) for the microbial population. It is reported that that presence of 1% of lignin in crop residues reduces the methane production by 3% (Chandler et al., 1980). An appropriate pre-treatment method that degrades the lignin facilitates the microbial action on carbohydrates. However, careful selection of the pre-treatment method among the available physical, chemical and biological methods is required in economic perspective. Though the physical pre-treatment methods (such as irradiation) are effective in lignin degradation, it requires high energy input making them more expensive. The chemical pre-treatment methods (such as acids, alkali or ammonia pre-treatments) causes secondary pollution to the environment, corrosion of the equipment, releasing toxic furfural and phenolic compounds during pre16

treatment that harms the microbial communities (Jönsson and Martín, 2016). Biological pretreatment methods (fungal treatment and aeration) require mild operating conditions and are environmental friendly. Nevertheless, the AD of crop residues has operational difficulties such as clogging of pumping tubes, floatation of biomass, stratification and scum formation due to bulky nature of crop residues (Li et al., 2011a). The difficulties in the AD system can be overcome with the recirculation of leachate back into the AD system. The following section discusses the effectiveness of biological pre-treatment methods such as pre-aeration, fungal pretreatment, and recirculation of the leachate in the AD of crop residues. 3.1. Aeration Aeration is the supply of oxygen for certain time period either before AD process which can be referred as composting (Zhou et al., 2017) or during AD process (Jagadabhi et al., 2010). As mentioned earlier, the lignin coat protects the degradation of cellulose and hemicellulose. The aeration of crop residues mineralises the lignin content which will facilitate the hydrolysis of cellulose and hemicellulose (Fu et al., 2014). The key mechanism in the process is that the aerobic microorganisms release hydrolytic enzymes (cellulases, proteases) that decompose the lignocellulose structure (Charles et al., 2009; Zhou et al., 2017). The aeration pretreatment of rice straw decomposed the cellulose, hemicellulose, and lignin by 64.5%, 7.5% and 13.6% respectively (Yan et al., 2015). The major additional benefits are avoiding the formation of toxic substances (such as lactic acid and ethanol) (Zeng and Deckwer, 1996) and producing lipids required for synthesis of the cell membrane of anaerobes (Ghaly and El-Taweel, 1995). The aeration would even generate heat which is sufficient to maintain thermophilic temperature during start-up without the requirement of the external heat source (Charles et al., 2009).

17

It is reported (Jagadabhi et al., 2010) that aeration had improved the production of VFAs in the AD of grass silage and facilitate the accessibility of VFAs to methanogens (Nguyen et al., 2007) leading to improved methane production. The aeration of 5 mL O2/g.VS of corn straw under thermophilic (55º C) conditions lead to a 16% improvement of methane production (Fu et al., 2014). Pre-aeration of rice straw for two days (35º C) had also improved the volumetric methane production (at an S/I ratio of 4 and TS content of 16%) (Zhou et al., 2017). It also needs to be considered the period of aeration, which plays a significant role in the degradation of lignocellulose biomass. Pre-aeration for too longer periods consumes substrates (Oxidised to carbon dioxide) which would otherwise be utilised for methane production (Charles et al., 2009; Zhou et al., 2017). Comprehensively, it is observed that, even though pre-aeration is found to be improving the methane production, it requires an energy balance to be conducted to verify that the extra methane obtained with the energy required for pre-aeration. 3.2. Fungal pre-treatment The growth of fungi on lignocellulose mass causes effective degradation of the lignin content and also resulting partial degradation of carbohydrates (cellulose &hemicellulose) (Zhou et al., 2015). It is observed that the best fungal pre-treatment has a higher selectivity for lignin degradation when compared to carbohydrates (cellulose and hemicellulose) for methane production (Zhou et al., 2015). The probable reason can be the growth of fungal hyphae on the surface of lignocellulose mass preventing the accessibility of these carbohydrates to some extent (Zhao et al., 2014a; Zhou et al., 2015). However, careful selection of fungal type is important for effective lignin degradation as it varies intragenerically and intraspecifically among different categories of fungi (Zhou et al., 2015). Among the fungal groups, white rot fungi (basidiomycetes) are most efficient fungal 18

groups for AD (Rouches et al., 2017) which had high selectivity for lignin degradation and consumes little cellulose and hemicellulose whereas brown rot fungi comparatively consumes high cellulose and hemicellulose apart from lignin (Zhao et al., 2014a). The white rot fungi (basidiomycetes) releases lignolytic enzymes such as lignin peroxidase and manganese peroxidase that breaks the lignin network (Zhao et al., 2014a). Ceriporiopsis subvermispora white rot fungi degrade 40% of lignin in 42 days with low cellulose degradation(5% ) (Zhao et al., 2014a). The secretion of lignocellulosic enzymes depends on the type of fungal strain, nature of the lignocellulosic structure, and on culture conditions (Wymelenberg et al., 2010), (Grigoriev et al., 2011),(Mustafa et al., 2016). The fungal strains suitable for one substrate may not be suitable for other substrates for effective degradation (Zhou et al., 2015).The fungal pre-treatment of wheat straw with polyporus brumalis improved the methane production by 20% (Rouches et al., 2015) whereas the fungal pre-treatment of yard trimmings had resulted in 154% (Zhao et al., 2014b). Similarly, fungal pre-treatment of rice straw caused lignin degradation by 33% leading to improvement in methane production of 120% (Mustafa et al., 2016). Rice straw when fungal pre-treated with Pleurotus ostreatus and Trichoderma reesei caused an increase in methane production by 120% and 78.3% respectively compared to untreated rice straw (Mustafa et al., 2016). The fungal pre-treatment can be performed under both solid and submerged states. However, the solid state fermentation secrets concentrated enzymes along mycelia and are close to their substrate causing effective degradation of lignocellulose. Moreover, diffusion of oxygen is more in solid state fermentation that favors the oxidative degradation of lignocellulose. Hence, solid-state fermentation is a better choice compared to submerged 19

state fermentation (Zhou et al., 2015). Solid-state fermentation requires low aeration, low heating, and minimal water consumption. It also allows higher feedstock loads and facilitates attachment of enzymes to the substrates (Rouches et al., 2015) resulting in maximum degradation. The improved degradation of lignocellulose mass causes high methane yield. However, the fungal pre-treatment requires longer degradation times. The longer duration times can be avoided by conducting fungal pre-treatment during the storage of crop residues. Therefore, the careful attention is required for the selection of strain, duration of pretreatment time and appropriate culture condition need to be addressed for efficient use of the method. 3.3. Recirculation of liquid fraction of digestate The stability and efficiency of the AD system mainly depend upon the syntrophy in interlinked hydrolysis, acidogenesis, acetogenesis and methanogenesis phases. Sometimes, syntrophy is disturbed due to either slow hydrolysis or fast hydrolysis that causes either shortage of VFAs or accumulation of VFAs respectively, both affecting the activity of methanogens (Schievano et al., 2010; Zhou et al., 2017). It has been reported that the supply of microbial population plays a key role than the supply of buffering capacity in enhancing the methane production (Charles et al., 2009). It is also to be noted that the growth of methanogenic microbial population is slower compared to acidogenic microbial population leading to imbalanced microbial population more specifically at higher substrate loadings. The recirculation of methanogen rich digestate solves the problem of poor methanogenic population in the AD system leading to balanced phases. Moreover, it can improve the nutritional balance, moisture content and reduces the lag phase time required for initial startup time for methane production.

20

The recirculation of the digestate which is rich in microbial population and buffering capacity promotes the mass transportation and redistribution of the enzymes in the AD system (Lü et al., 2008). However, the direct application of the whole digestate occupies the volume of the reactor earmarked for the feedstock. The liquid portion of digestate that can be obtained with liquid-solid separation of digestate can be mixed with the fresh feedstock before feeding into the AD system (Fig.6a). The recirculation of leachate from AD system improved the pH, alkalinity, methanogenic population and nutrients (Nordberg et al., 2007; Peng et al., 2016; Pezzolla et al., 2017). The recirculation also allowed to increase the organic loading rate that leads to improved methane production (Nordberg et al., 2007). The leachate effluent from the AD of wheat straw improved the methane production by 21% due to the enrichment of nutrients and microbial population (Peng et al., 2016). The leachate recirculation also improved the buffering capacity in

SS-AD of fungal pre-treated rice straw (Lianhua et al., 2010)

and digestate quality

(Pezzolla et al., 2017). Sometimes the full recirculation of digestate results in the accumulation of VFAs leading to a drop of pH causing inhibition to methanogens (Nordberg et al., 2007). The problem could be solved by replacing the proportion of leachate to be circulated with water instead of full leachate recirculation which will dilute the accumulated VFAs facilitating the stable AD process (Nordberg et al., 2007). The solid-state digestion which is suitable for crop residues also requires a significant quantity of inoculum for quick start-up and control (Xu et al., 2016). A novel process has also been developed (Yebo LiJiying ZhuCaixia Wan, 2014) with the integrated design of L- AD and SS-AD system, which are built together in stages (Fig.6b.). Leachate from L-AD is used as inoculum for SS-AD system resulting in improved stability and optimal performance. Therefore, the recirculation of leachate fraction with good buffering capacity causes better stability and

21

improved methane production if the appropriate quantity of leachate is applied in the AD of crop residues.

4. Methane production of crop residues The methane production of crop residues in various scientific studies and their critical observations are discussed in the following sections. 4.1. Paddy Paddy is the largely cultivated food crop in India generating highest quantity of crop residue (Mussoline et al., 2012b). The yield of one kg of rice grain from paddy generates 1.7 kg of straw and husk (Hiloidhari et al., 2014). The paddy is largely being cultivated in West Bengal, Andhra Pradesh and Punjab (Gadde et al., 2009b). In northern parts of India, the paddy is being cultivated on a rotation basis with the wheat crop. The actual utilisation of the rice residue is different throughout the country. In some parts of India, wheat straw is preferred as animal fodder when compared to rice straw. It is also a common practice that the farmers leave the harvested crop in the field itself which is burnt subsequently (Gadde et al., 2009b). The open burning of paddy straw results in the emission of GHGs such as carbon dioxide, carbon monoxide, methane, and nitrous oxide and is affecting the environment (Gadde et al., 2009a). A wide range in experimental methane yields (193-535 mL of CH4 /g VS) has been observed for AD of paddy residues (Table 4). The wide range in methane production is due to varied operating conditions, pre-treatment methods and co-substrates applied in the studies. The co-digestion with nitrogen rich manures and pre-treatment methods improved the methane production. The co-digestion of rice straw with nitrogen rich manure ( dairy manure, chicken manure, and swine manure) improved the methane production by about 33% to 43% (Wang et 22

al., 2013). The improved methane production is because of the synergistic effect of nitrogen rich manures with rice straw. Rice straw when pre-treated with fungi increased the methane production about 31% to 46% (Ghosh and Bhattacharyya, 1999). The acid pre-treatment (HCl) of rice straw is better compared to alkali pre-treatment (NaOH) that caused 43% of higher methane production than alkali pre-treatment (Wang et al., 2015). Whereas, the alkali pretreatment (NaOH) of rice straw is better compared to hydrogen peroxide pre-treatment (H2O2) that caused 20% of higher methane production (Abudi et al., 2016b). The improved biogas production may not suffice to cover the extra expenses incurred during pre-treatment (Abudi et al., 2016a). However, the combined co-digestion and pre-treatment of substrates may sometimes make the process economically feasible (Abudi et al., 2016a). In addition to this, the other residue from paddy, rice husk is shown improved biogas production when co-digested with poultry droppings which acted as a nitrogen source (Okeh et al., 2014). It is estimated that 100ha of paddy field could generate 1 Mm3 of CH4 per year that could yield 328 MWh of electrical energy (Mussoline et al., 2012a). The various pre-treatment methods and co-digesting substrates for improving the methane production from paddy residues is comprehensively listed in Table 4. 4.2. Wheat Wheat is a second most cultivated food source in India that occupies 16% of the total gross cropped area (27, 505 K Ha) (Cardoen et al., 2015a). The yield of one kg of wheat grains from wheat crop generates about 1.8 kg of residues (Hiloidhari et al., 2014). In terms of area of cultivation, it generates about 4.5 tons of residues per hectare. After its primary utilisation such as cattle feed and domestic use, the wheat crop was found to be generating 28 Mt of residues annually (Hiloidhari et al., 2014). The major wheat cultivating states are Uttar Pradesh, Punjab, Haryana, Madhya Pradesh, Bihar, and Rajasthan (Cardoen et al., 2015a).

23

The conversion of wheat straw to methane is energetically most efficient process (Kaparaju et al., 2009). The methane production of wheat straw was found to be improved with various pre-treatment conditions, co-digesting substrates which are in the range of 67- 380 mL of CH4/g VS (Table 5). The wide range of methane production is due to various pre-treatments/codigesting substrates under varied experimental conditions. The co-digestion of untreated wheat straw with cattle manure at 40:60 ratio resulted in improved methane production of 191mL CH4 /g of VS (Krishania et al., 2013). The additional alkali and combinational calcium hydroxide and sodium carbonate pre-treatments decomposed lignin that leads to improved methane production by 94%-99% (370-380 mL CH4 /g of VS) (Krishania et al., 2013). Pre-treatment with H2O2 caused the degradation of hemicellulose by 12.5%-45.2%, cellulose by 9.3-30.2% and lignin by 5.4-21.9% and improved the soluble fraction by 30.5-77.3%. Among the tested concentrations of H2O2 (1%, 2%, 3% and 4%), wheat straw pre-treated with 3% concentration of H2O2 resulted in highest methane production (Krishania et al., 2013). The methane production was increased from 84.3 mL of CH4/g VS (untreated) to 128.4 mL of CH4/g VS with 3% concentration of H2O2 pretreatment. Wheat straw co-digested with cattle manure at 40: 60 ratio improved the methane production to 320.8 ml of CH4 /kg of VS. Pre-treatment of steam explosion improved the methane production from 233 mL of CH4/g VS to 296 mL of CH4/g VS (increased by 27% ) (Ferreira et al., 2014). It is also crucial to consider net energy balance for practical implementation for chosen pre-treatment (Ferreira et al., 2014). The microwave pre-treatment of wheat straw improved the methane production with structural modifications of wheat straw (Jackowiak et al., 2011). However, the improved methane had not compensated the energy consumed for the microwave pre-treatment (Jackowiak et al., 2011). The operating conditions of the AD process also

24

influence the AD performance. The co-digestion of wheat straw (9% on fresh matter basis) with cattle manure (91% on fresh matter basis) under thermophilic (50º C), liquid state conditions (TS:14.8%) resulted in methane production of 351 mL of CH4/g VS (Xavier et al., 2015). Whereas, the co-digestion of wheat straw with cow feces under psychrophilic, solid state (TS-27%) conditions (20º C), resulted in 187 mL of CH4/g VS which is comparatively lower which may be due to higher substrate loadings and low operating temperature (Saady and Massé, 2015). Hence, proper pre-treatment and selection of co-digestion substrate, operating conditions play a significant influence on the improvement of methane production. 4.3 Maize (Corn) Maize is the third most cultivated cereal crop in India accounting about 5% of the gross cropped area (8,103 K Ha) (Cardoen et al., 2015a). One kg yield harvested corn produces about 2.3 kg of maize residue indicating highest quanity when compared to paddy and wheat (Hiloidhari et al., 2014). The cultivation of maize crop generates 35 Mt of residue /year, and 9 Mt surplus after its primary use as animal feeding in India (Hiloidhari et al., 2014). In India, it is largely being cultivated in Andhra Pradesh, Karnataka, Bihar, Gujarat, Himachal Pradesh, Jharkhand, Rajasthan Madhya Pradesh, and Uttar Pradesh (Cardoen et al., 2015a). A wide range in experimental methane production yields (81- 383 mL of CH4/g VS) have been observed for AD of maize residues (Table 6). To improve the accessibility of substrate, pre-treatment, and co-digestion strategies were widely reported for maize residues in the literature (Zhou et al., 2014). Corn straw when pre-treated under thermophilic (55ºC), microaerobic conditions (5 mL of oxygen load/ g VS) have shown enhanced methane production by 16.24% (325.7 mL of CH4/g VS) due to improved hydrolysis rate (Fu et al., 2014). It is also reported that the corn stover using ammonia pre-treatment improved the biogas production by

25

26.70% (Yuan et al., 2015). In addition to the improved biogas production, the ammonia pretreatment reduced the digestion time from 52 days to 37 days while producing 90% of biogas. The combined effect of pre-treatment and co-digestion was also reported in the literature. The pre-treatment along with co-digestion of corn stover with food waste improved the methane production by 12.2% at C/N ratio of 20 compared to mono-digestion of corn stover (Zhou et al., 2014). The co-digestion of cornstalk (60%) with vermicompost (40%) caused improvement in methane production due to a reduction in crystallinity of the cornstalk (Chen et al., 2010). Anaerobic co-digestion of corn straw with blue algae (at optimal C/N ratio of 20) shown improved methane production by 46% compared to mono-digestion of blue algae (Zhong et al., 2013). Co-digestion of corn stover with chicken manure also improved methane production (resulted in the energy of 8.0 MJ/Kg VSadded) due to their synergistic effect (Y. Li et al., 2014). However, the co-digestion of maize residues with swine manure had not shown any synergistic effect in semi-continuous experiments (Cuetos et al., 2011). Hence, it is required to select an appropriate pre-treatment method and co-digestion substrate to enhance the methane production from the maize residue.

5. Energy potential and carbon dioxide emission reduction in the AD of crop residues The present section deals with the estimation of the cumulative energy potential from surplus crop residues and corresponding CO2 emissions reduction with the utilisation of from paddy, wheat, and maize crops via anaerobic digestion. The CO2 emissions reduction was calculated based on the assumption that produced energy from the AD of crop residues substitutes the coal utilisation.

26

The methane potential from the anaerobic digestion of surplus paddy, wheat, and maize residues is estimated to be 18,677 Mm3/year (Table 7). The corresponding energy potential is estimated to be 670×109 MJ. Further, the net energy potential is estimated as 632 ×109 MJ after the deduction of energy required for shredding (207 MJ/ ton) of crop residue (Adl et al., 2012) which is required before feeding into the AD system. Other energy requirements such as mixing, feeding the feedstock and withdrawal of digested material were neglected due to their low energy requirement. The coal substitution is estimated based on the assumption that the generated methane in the AD of crop residues was utilized for thermal heating (instead of coal) according to the method prescribed (Eq. 1) in (Yanli et al., 2010).

Coal substitution (t)=

( B  P  Qb  Eb ) Qc  E c

(1)

Where, B is surplus crop residue (t), P is methane potential (m3/t), Qm is calorific value of methane (35.9 MJ/m3),Em is efficiency of methane for thermal heating (0.9), Qc is calorific value of coal (20,900 MJ/ton), Ec is efficiency of coal for thermal heating (0.6). The coal substitution is found to be 52 Mt. The corresponding CO2 emissions with the coal substitution is estimated based on the assumption that combustion was taken in an environment of excess air (Eq. 2) (Yanli et al., 2010). CO2 emissions = P (Cp-Cs)

Co

(2)

Where, P is quantity of coal(t), Cp is percentage of carbon in coal (60%), Cs is percentage of unburned carbon (10%);Co is carbon oxidation rate (80%). The amount of CO2 emissions that could be avoided releasing into environment with coal substitution is found to be 76 Mt. The net CO2 emissions reduction is estimated after the deduction of CO2 emissions of vehicle during 27

transportation of residues to the AD plant. The vehicle for transportation of crop residue was assumed to carry 2 ton of crop residues over a haul distance of 5 km with a mileage of 35 Km/L of diesel. It was also assumed that one liter of diesel utilisation by the transporting vehicle emits 2.6 kg of CO2 into the atmosphere while estimating net CO2 emission (Canada, 2016). After subtracting the CO2 emissions during transportation of residues, the net CO2 emission is found to be 46 Mt. Based on these estimations, it can be observed that the AD of crop residues can significantly avoid the CO2 emissions while contributing for the energy generation.

6. Conclusions The current study reviewed the technological developments to improve the low degradability,and poor nutritional balance in anaerobic digestion of paddy, wheat, and maize residues. The selection of appropriate substrate for co-digestion of crop residue, application of appropriate pre-treatment techniques such as fungal pre-treatment, aeration, modifications to conventional process such as solid-state anaerobic digestion, and recirculation of liquid leachate improves the efficiency of anaerobic digestion. The effective usage of surplus crop residues of paddy, wheat, and maize in India for anaerobic digestion had net energy potential of 632×109 MJ/year. However, the collection and transportation of these residues for anaerobic digestion process remains a challenge and may be practically feasible if the governing states adopt an appropriate policy for their effective use. It is observed that the generated methane from anaerobic digestion could substitute 52 Mt/year of coal utilization that could avoid 46 Mt/year of net CO2 emissions from releasing into the atmosphere.

28

Acknowledgments The authors wish to thank Department of Biotechnology-Government of India for funding this work (Sanction order No: BT/PR6328/GBD/27/387/2012) and Department of Civil Engineering, National Institute of Technology, Warangal for providing support to carry out present work.

Table 1: Gross and surplus residue potential of major crops In India

S. No

Crop

Gross potential(Mt)

Surplus potential(Mt)

1

Rice

154.0

43.5

2

Wheat

131.1

28.4

3

Maize

35.8

9.0

Source:(Hiloidhari et al., 2014)

29

Table 2: Composition of rice straw, wheat straw, and maize straw Crop residue Rice straw

Cellulose (%)

Hemicellulose (%)

Lignin

C/N

(%)

ratio

Ref.

34.9

16.7

23.3

47.0

(Ye et al., 2013)

30.2

17.6

31.9

60.5

(Yan et al., 2015)

Wheat straw 45.6

33.4

6.4

-

(Xavier et al., 2015)

38.6

25.1

7.3

32.8

44.1

1.9

59.1

(Cuetos et al., 2013)

38.8

29.5

7.1

34.9

(Yuan et al., 2015)

Maize straw

(Saady and Massé, 2015)

30

Table 3: Effect of composition of crop residue on methane yield Ym – Yield of methane; CCellulose; HC – Hemicellulose; L-Lignin; ADL-Acid detergent lignin; CP-Crude protein CFCrude fat; R2 –Correlation coefficient; C/L ratio- cellulose to lignin ratio Methane yield( Ym )

R2

Reference

41

Ym =371+0.13×HC -2 ADL

0.80

(Dandikas et al., 2014)

12

Ym=19.05×CP +27.73 CF+1.8 ×C

-

(Amon et al., 2007)

Substrate No of samples Energy crops Maize

+1.7 ×HC Energy

10

Ym= -2.58× L+460.6

0.76

(Triolo et al., 2011)

14

Ym=113.14×(C/L ratio)-26.62

0.78

(Liu et al., 2015)

crops Crop residues

Table 4: Summary of experimental conditions and methane production of paddy residues

Subst

Co-

Pre-

Mo

Operating

Methane Remarks

rate

substra

treatment

de

conditions

producti

te

Reference

on

Rice

Food

Size

Batc Mesophilic,

535 mL

High

(Chen et al.,

straw

waste

reduction

h

of CH4/g

methane

2015)

food waste

31

0.5-1 cm

to rice

and alkaline

straw ratio -

obtained

pre-

3.88 and S/I

with butyric

treatment of

ratio -0.5

acid

rice straw

based on

fermentation

VS,

.

Rice

Kitchen

Size

straw

waste

reduction-

and pig

<1 mm

Mesophilic,

of VS

production

383 mL

VFAs

(Ye et al.,

Batc ratio of

of CH4/g

accumulatio

2013)

h

kitchen

VS of

n was

waste, pig

methane

observed at

manure

manure,

high kitchen

and rice

waste

straw is

loading

0.4:1.6:1

(>26%).

(C/N ratio21.7) Rice

Chicken

Size

Batc Mesophilic,

378 mL

Co-digestion (Zhang et al.,

straw

manure

reduction 2- h

TS-8%, rice of CH4/g

of substrates

3cm

straw to

of

improved

chicken

VSremoved

the stability

2014)

manure ratio -50:50

32

Rice

Food

Size

Batc Mesophilic,

307 mL

Co-digestion (Haider et al.,

Husk

waste

reduction

h

C/N ratio

of CH4/g

avoided

20,

VS*

VFAs

<10 mm

S/I ratio

2015)

inhibition

0.25. Rice

-

straw

Size

Mesophilic,

287 mL

Acid pre-

reduction-

Batc alkali-

of CH4/ g treatment

<2mm,

h

NaOH

COD(H

resulted in

acid-HCl

Cl)

higher

pre-

193 mL

methane

treatments

CH4/g

yield

COD

compared to

(NaOH)

alkali pre-

alkali, acid

(Wang et al., 2015)

treatment Rice

Sewage

Size

Batc Thermophil

266 mL

Two stage

(Kim et al.,

straw

sludge

reduction -

h

ic,

of CH4/g

system

2013)

two stage

of VS

resulted in

2mm

system,

higher

sewage

methane

sludge-150

yield

mL and rice

compared to

straw- 27g,

one stage

TS-17%.

system

33

Rice

-

straw

Size

Batc Mesophilic,

263 mL

Fungal pre-

(Mustafa et

reduction,

h

solid state

of CH4/g

treatment

al., 2016)

fungal pre-

conditions,

VS

enhanced

treatment

moisture

the methane

content

yield by

75%, 20

120%.

days Rice

-

straw

Size

Batc Mesophilic,

234 mL

Pre-aeration

(Zhou et al.,

reduction

h

TS-16%,

of CH4/g

and

2017)

and pre-

I/S ratio of

VS

inoculum

aeration for

2

dilution

2 days at

improved

35º C

the hydrolysis.

Rice straw

-

Size

Batc Mesophilic,

227 mL

Extrusion

(Chen et al.,

reduction,

h

OLR is 50

of CH4/g

pre-

2014)

extrusion

kg/m3 and

VS

treatment of

pre-

I/S ratio of

rice straw

treatment

2.5

reduced the digestion time.

34

Rice

Food

Size

Plug Mesophilic,

245 mL

Inhibition

(Jabeen et al.,

Husk

waste

reduction

flow C/N ratio

of CH4/g

VFAs was

2015)

VS*

observed at

<10 mm

28, OLR of 5 kg VS/ m3

high OLR

/day Rice

Pig

Size

Batc Mesophilic,

straw

manure

reduction- 1 h, mm

cont

220-247

Stable

(Li et al.,

rice straw :

mL of

biogas

2015)

pig manure

CH4/g

production

VS*

was found at

inuo 1:1 on VS us

Rice

-

straw

basis

an OLR of

6-8 kg

6-8 kg

VS/m3/day

VS/m3/day

Size

Batc Mesophilic,

reduction,

h

composting

194 mL

C/N ratio of of CH4/g 30

VS*

Composting

(Yan et al.,

enhanced

2015)

the biodegradati on.

Rice

Cow

Size

Batc Mesophilic,

straw

manure

reduction- 1 h, mm

cont

193 mL

Stable

(Li et al.,

rice straw :

of CH4/g

biogas

2015)

Cow

VS

production

inuo manure 1:1

was found at

us

on VS basis

an OLR of

6kg

3-6 kg

35

VS/m3/day

VS/m3/day

Rice

Pig

Size

Batc Thermophil

1.38 g

Presence of

(Jiménez et

straw

manure,

reduction

h

ic - 20.1 g

CH4-

high amount

al., 2014)

clay

VSS/L of

COD/ g

of clay

residues

manure+

VSS/day

residue

10.18 g

reduced the

VSS/L of

methane

straw +

production

3.05 g VSS/L of clay residue Rice

Pig

Size

Batc Mesophilic,

1.31 g

Clay

(Jiménez et

straw

manure,

reduction

h

manure(28.

CH4-

residues had

al., 2014)

clay

35 g VSS/

COD/ g

higher

residues

L) +

VSS/day

influence on

straw(17.6

methane

g VSS /L) +

production

clay residue

compared to

(8.3 g

rice straw

VSS/L)

36

Rice

Goat

Size

Batc Mesophilic,

8,584

Co-digestion (Zhang et al.,

straw

manure

reduction-

h

GM: rice

mL of

of substrates

(GM)

2-3 cm

straw ratios

CH4 * in

improved

of 30: 70

55 days

the biogas

and 50:50,

(30:70)

production

TS-8%, 700 8633

due to

mL

mL of

improved

working

CH4 * in

nutrient

volume

55 days

balance.

2013)

(50:50) Rice

Dairy

Size

Sem Mesophilic,

straw

manure

reduction-

i-

2-3 cm

cont

286 mL

All co-

(J. Li et al.,

TS-8%, rice of

digestion

2014)

straw to

CH4/L/d

proportions

dairy

ay* in

improved

manure

the first

the biogas

ration of

stage of

production,

5:5 on a

stabilizat

except 9:1.

mass basis,

ion

TCL-Treatment cycle length, OLR-Organic loading rate, S/I ratio = substrate to inoculum ratio, TS-Total solids, VS- Volatile solids, VSS- volatile suspended solids (VSS), * reported biogas yield was converted to methane yield with the conversion factor of 0.55 (methane content-55%).

37

Table 5: Summary of experimental conditions and methane production of wheat residues Substrat

Co-

Pre-

e

substra treatm

Mo

Operating

Methane Remarks

de

conditions

producti

Reference

te

ent

on

Wheat

Cattle

Size

Bat

Mesophilic,

380 mL

Pre-treatment

(Krishania et

straw

manure

reducti

ch

cattle

of CH4/g

improved the

al., 2013)

on

manure-60%,

VS

CH4 yield

(2-3

total solids -

mm),

10%,

(Ca(OH

inoculum of

)2-

10 %, 3%

Na2CO3

Ca(OH)2 +

)

3% Na2CO3, Time- 48 h

Wheat

Cattle

Size

Bat

Mesophilic,

370 mL

The increase of

(Krishania et

straw

manure

reducti

ch

cattle manure

of CH4/g

CH4 yield by

al., 2013)

on

of 60%, total

VS

94% with alkali

(2-3

solids -10%,

pre-treatment

mm),

inoculum -

of wheat straw

alkali

10%, NaOH 2%

Wheat

Cattle

Briquet

Co

Thermophilic, 351 mL

Co-digestion

(Xavier et

straw

manure

ting-20

nt

wheat straw –

improvement

al., 2015)

of CH4/g

38

mm

9%, cattle

VS

manure-91%

the methane yield by 33%

Wheat

Chicke

Size

Bat

Mesophilic,T

345 mL

straw

n

reducti

ch

S-8%, wheat

of CH4 /g with chicken

manure

on 2-

straw to

of

manure

3cm

chicken

VSremoved

improved the

manure ratio -

Co-digestion

(Zhang et al., 2014)

stability

50:50 on a dry basis Wheat

Cattle

Size

Bat

WS:CM ratio

320 mL

Pre-treatment

(Song and

straw

manure

reducti

ch

40:60,

of CH4/g

and co-

Zhang,

on 20-

inoculum-200

VS

digestion at 40:

2015)

30 mm

gr, total

60 improved the

and

solids-8%,

CH4 production

H2O2

3% H2O2

Wheat

Size

Bat

Mesophilic,

307-335

Combined size

(Reilly et al.,

straw

reducti

ch

S/I ratio 0.66

NmL of

reduction and

2015)

based on VS

CH4/g

enzymatic

VS

treatment

on (3,2, 1.25 mm)

improved the

and

CH4 production

enzyma

39

tic

Wheat

-

straw

Size

Bat

Mesophilic,

301 to

Cost effective

(Reilly et al.,

reducti

ch

S/I ratio 0.66

320 mL

pre-treatment

2015)

based on VS

of CH4/g

with particle

1.25

VS for

size reduction

mm)

all

to 3 mm

and

particle

compared to

alkali

sizes

enzymatic pre-

on (3,2,

treatment Wheat

Sewage

Size

Co

Mesophilic,

296 mL

Co-digestion of

(Peng et al.,

straw

sludge

reducti

nt

OLR-2 g

of

sewage sludge

2016)

on

VS/L/day,

CH4/kg

and digestate

- 3 mm

recirculation

VS

liquor

of digestate

recirculation improved the CH4 production

Wheat straw

-

Steam

Bat

Mesophilic,

293-323

Impregnation

(Ferreira et

explosi

ch

S/I ratio-0.5

mL of

had negligible

al., 2014)

based on VS

CH4/g

effect on

VS

methane

on and water impreg

production

nation

40

Wheat

-

straw

Steam

Bat

Mesophilic,

288-296

Steam

(Ferreira et

explosi

ch

S/I ratio-0.5

mL of

explosion

al., 2014)

based on VS

CH4/g

improved the

VS

methane

on

production by 24-27% Wheat

-

straw

Size

Bat

Mesophilic

280 mL

Pre-treatment

(Nkemka

reducti

ch

I/S ratio -2.

of CH4/g

improved the

and Murto,

VS

methane yield

2013)

on-1020 mm,

by 57%

steam, enzyma tic Wheat

Size

Bat

Mesophilic,

268 N

Size reduction

(Reilly et al.,

straw

reducti

ch

S/I ratio 0.66

mL of

had negligible

2015)

based on VS

CH4/g

effect on

VS

methane

on(3,2, 1.25 mm)

production

Wheat

Cattle

Size

Bat

Mesophilic,

241 mL

Co-digestion of

(Krishania et

straw

manure

reducti

ch

cattle manure

CH4/g

wheat straw and al., 2013)

on

of 60%,total

VS

cattle manure

(2-3

solids -10%,

improved the

mm)

inoculum-

methane

41

10%,

production

Wheat

Dairy

Size

Bat

Mesophilic,

234 mL

Co-digestion of (Wang et al.,

straw

manure

reducti

ch

S/I ratio-0.5,

of CH4/g

wheat straw

(DM)

on

DM/CM ratio

VS

with two

and

- 2-3

of 50:50

manure

chicken

cm

based on VS

improved the

manure

biogas

(CM)

production

2012)

compared with single manure Wheat

-

straw

Size

Bat

Mesophilic,

232-245

Size reduction

(Ferreira et

reducti

ch

S/I ratio-0.5

mL of

had negligible

al., 2014)

based on VS

CH4/g

effect on

VS

methane

on 3-5 cm and < 1 mm

production

Wheat

Dairy

Size

Seq

Psychrophilic

193 mL

Psychrophilic

(Saady and

straw

Manure

reducti

uen

(20º C )

of CH4/g

dry AD is as

Massé,

on

tial

S/R-1.7,

VS

efficient

2015)

bat

OLR-3.7, TS-

compared to

ch

27%,

mesophilic dry

TCL-21 days

AD.

Wheat

Urea

Size

Bat

Mesophilic,

165 mL

Alkali pre-

(Chandra et

42

straw

reducti

ch

S/I ratio 1

of CH4/g

treatment

on and

based on VS,

VS

improved the

alkali

C/N ratio

methane

25.0,

production by

NaOH-4%,

111.6%

al., 2012c)

Wheat

Cattle

Size

Co

Temperatures

130 to

Co-digestion

(Risberg et

straw

manure

reducti

nt

- 37,44, 55 º

210 N

with cattle

al., 2013)

on-10

C,

mL of

manure and pre-

mm and

OLR-0.28 g

CH4/g

treatment by

steam

VS/L/day, a

VS

Steam

explosi

steam

explosion had

on

explosion at

not improved

210º C, 10

the methane

min, retention

production

time-25 days Wheat

Cattle

Size

Bat

Mesophilic,

125 mL

Acid pre-

(Krishania et

straw

manure

reducti

ch

cattle

of CH4/g

treatment

al., 2013)

on

manure-

VS

reduced the

(2-3

60%,total

mm)

solids -10%,

and

inoculum -

acid

10%, 2%

CH4 production

H2SO4 -121º

43

C, time-30 min, pressure of 100 kPa Wheat

Urea

straw

Size

Mesophilic,

94 mL of

Hydrothermal

(Chandra et

reducti

S/I ratio- 1

CH4/g

pre-treatment

al., 2012c)

on and

(based on

VS

improved the

hydroth

VS), C/N

methane

ermal

ratio 25.0,

production by

temp-200 ºC,

20.0%

10 min, 1.5 Mpa Wheat

-

straw

Size

Bat

Mesophilic,

67 mL of

Cellulose and

(Liew et al.,

reducti

ch

TS=22%,S/I

CH4/g

hemicelluloses

2012)

ratio=2

VS

are main

on-9 mm

contributors for methane yield

Wheat

Dairy

Size

Se

Mesophilic,

10,519

Improved

(J. Li et al.,

straw

manure

reducti

mi-

TS-8%,wheat

mL of

biogas

2014)

on-2-3

con

straw to dairy

CH4*

production,

cm

t

manure ration

after 47

except of 9:1

of 5:5 on a

days of

ratio

mass basis,

digestion

working

44

volume -800 mL Wheat

Goat

Size

Bat

Mesophilic,

7,020

Co-digestion

(Zhang et al.,

straw

manure

reducti

ch

GM: Wheat

mL of

with goat

2013)

(GM)

on

straw ratios of CH4 * in

manure

2-3 cm

30: 70 ,TS-

improved the

55 days

8%, working

stability

volume -700 mL VSS-Volatile suspended solids, BMP- Biochemical methane potential, TS-Total solids; VSVolatile solids, OLR-Organic overloading rate, * reported biogas yield was converted to methane yield with the conversion factor of 0.55 (methane content-55%).

Table 6: Summary of experimental conditions and methane production of maize (corn) residues Substra

Co-

Pre-

Mo

Operating

Methan

te

substr

treatmen

de

Conditions

e

ate

t

Remarks

Ref

Co-digestion

(Zhang et al.,

product ion

Corn

Chicke

Size

Batc Mesophilic,

383 mL

45

stalk

n

reduction

manur

2-3cm

h

e

TS-8%,

CH4 /g

improved the

cornstalk to

of

stability

chicken

VSremove

manure ratio

d

2014)

of 50:50 on dry matter basis Corn

-

straw

Size

Batc Mesophilic,

325 mL

Pre-treatment

(Fu et al.,

reduction

h

of

improved

2014)

I/S ratio-0.5

- 5mm,

based on TS, CH4/g

(TMP) the

thermoph

Shaking

hydrolysis

ilic (55o

speed-130

and reduced

C) micro-

rpm,

lag phase

aerobic

oxygen load

time

pre-

- 5mL/g of

treatment

VS

of VS

(TMP) Corn

Chicke

Size

Batc Mesophilic,

281 mL

Biodegradabil (Y. Li et al.,

Stover

n

reduction

h

of

ity of 62%

manur

-< 30 mm

e

C/N ratio -

2014)

20, Loading- CH4/g 3 g VS/L,

of VS

S/I ratio 0.5

46

Corn

Vermi

Size

Batc Mesophilic,

259 mL

Co-digestion

(Chen et al.,

stalk

compo

reduction

h

Inoculum-

of

with

2010)

st

corn

400 g Vermi

CH4/g

vermicompost

stalk-10-

compost -

TS

improved the

20 mm;

40% , TS-

biodegradabil

vermi

6%

ity

compost0.8mm Corn

-

Stover

Size

Batc Mesophilic,

256 mL

Ammonia

(Yuan et al.,

reduction

h

4%

of CH4

pre-treatment

2015)

-

NH3,70%

/g VS

improved the

<5mm,a

moisture

biogas

mmonia

content,

production by

pre-

Inoculum-

26.70%

treatment

15 [MLSS] g/l,

Corn

Blue

Size

Con

Mesophilic,

234 mL

Co-digestion

(Zhong et al.,

straw

algae

reduction

t

C/N ratio -

of CH4

with corn

2013)

- 5 to 10

20, OLR- 6

/g VS

straw

mm

g VS/L,

improved the

HRT-10

methane

days

production by 46%.

47

Corn

Chicke

Size

Con

Mesophilic,

223 mL

Stable

(Y. Li et al.,

Stover

n

reduction

t

C/N ratio -

of

methane

2014)

(CS)

manur

-< 30 mm

20, CM:CS - CH4/g

production at

e

1:1.4,

OLR of 4 g

(CM)

TS- 12%,

of VS

VS/L

OLR -4 g VS/L Maize

Poultry Size

Batc Mesophilic,

188 mL

Co-digestion

(Cuetos et al.,

residues

blood

h

200 rpm, I/S

of

of maize

2013)

ratio-1-2

CH4/g

leaves with

maize to

VS

poultry blood

reduction -3mm

poultry

improved the

blood

methane yield

mixture 70:30 on VS basis Maize

Poultry Size

Sem Mesophilic,

165 mL

VFAs

(Cuetos et al.,

residues

blood

reduction

i-

HRT-

of

accumulation

2013)

-3mm

cont

days36.

CH4/g

caused

OLR-3.1 g

of VS

inhibition at

VS/L/day,

OLR of 3.1 g

TS-12.6%,

VS/L/day.

Maize-60%

48

based on VS

Corn

-

stover

Size

Batc Mesophilic

81 mL

Cellulose and

(Liew et al.,

reduction

h

TS=22%,S/I

of CH4 /

hemicellulose

2012)

ratio=2,

kg of

s are mainly

VS

contributed methane yield

Corn

Goat

Size

Batc Mesophilic,

8,812

Co-digestion

(Zhang et al.,

stalks

manur

reduction

h

GM: corn

mL of

improved the

2013)

e

2-3 cm

stalks ratio

CH4 * in biogas

is 70: 30

55 days

production

(GM)

,TS-8%, working volume -700 mL Corn

Dairy

Size

Sem Mesophilic,

10,685

Optimal

(J. Li et al.,

stalk

manur

reduction

i-

TS-8%, corn

mL of

biogas yield

2014)

e

-2-3 cm

cont

stalk to

CH4/g

obtained at

inuo dairy

TS* after corn stalk to

us

manure

47 days

dairy manure

ration of 5:5

of

ratio of 5:5

on a mass

digestio

(mass basis)

basis,

n

49

working volume -800 mL MLSS= mixed liquor suspended solids; HRT=Hydraulic retention time, TS= total solids, VSVolatile solids, * reported biogas yield was converted to methane yield with the conversion factor of 0.55 (methane content-55%).

Table 7: Energy potential and CO2 emission reduction scenario with AD of crop residues Crop

Surplus

CH4

Methane

Energy

Net energy

Coal

CO2

Net CO2

residue

quantity

potential

potential

potential

potential

substitution

emission

emission

(Mt)

(m3/ton)

(Mm3)

(×109 MJ)

(×109 MJ)

(Mt)

reduction

reduction

(Mt)

(Mt)

Paddy

43.50

231*

10,049

360

352

27.1

41

25

Wheat

28.40

221*

6300

226

220

16.9

26

15

Maize

9.00

258*

2328

84

81

6.3

9

6

18,677

670

653

52

76

46

Total

80.9

* Values adopted from (Chandra, 2015) and converted to methane production of fresh mass based on the assumption that crop residues are having TS=85% and VS=80%

50

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Fig. 1. Percentage (%) of gross area under different crops (Cardoen et al., 2015a) Fig. 2. Advantages of the AD system Fig. 3. Typical flow diagram of the AD process Fig. 4. Strategies to improve the methane production in AD Fig. 5. Influencing factors in AD of crop residues Fig. 6. Recirculation of liquid fraction of digestate

69

30

Area of crop cultivation (%)

25

20

15

10

5

0

Crop type

Fig.1

Crop residues

Collection/Transportation/Pre-processing.

Fig.2

70

Organic matter

Carbohydrates

Proteins

Lipids

Sugars

Aminoacids

Long chain fatty acids

Hydrolysis

Acidogenesis

Fig. 3 Propionate, Butyrate , Valerate, alcohols, lactate

Acetogenesis Acetate

Hydrogen +Carbon dioxide

Methanogenesis

71

Pre-treatment Physical Chemical Biological

Co-digestion Animal Manure Sewage sludge High protein substrates

Improved methane yield Fig. 4

72

Crop Variety

Type of harvesting (Mechanical/Manual)

Pre-treatment Physical Chemical Biological

Total Temperature solids pH

Biogas

Digestate Climatic conditions

Nutritional Composition Microbial population Balance

Maturity of crop

Fig.5

73

a

b

Feed In

Feed In

Biogas

Feed In Biogas

Biogas

Fig. 6

74

Highlights  The technological developments and influencing factors in anaerobic digestion of paddy, wheat, and maize crop residues were discussed .  The co-digestion of crop residues with nitrogen rich animal manures improves the efficiency of anaerobic digestion.  The fungal pre-treatment, aeration of crop residues, recirculation of liquid leachate improves the efficiency of the process.  The utilization of crop residues through anaerobic digestion results in the generation of net energy of 632 ×109 MJ/year that could avoid the 46 Mt of CO2 emissions /year from releasing into the atmosphere.

75