5. Anaerobic Digestion of Rice Straw for Biogas Production

Insoluble organic compounds, such as cellulose, protein, fat, and some insoluble forms of organic compounds, are decomposed by enzymes (produced by bacteria) and anaerobic bacteria. Small soluble organic molecules, which are produced in this stage, are the raw material for the bacteria in the next stage. Hydrolysis of carbohydrates can occur within a few hours, while protein and fat hydrolysis may take several days. However, lignocellulose and lignin substances decompose slowly and incompletely (Deublein and Steinhauser 2011). Facultative anaerobes consume dissolved oxygen in water, which results in reduction of redox potential that is favorable to the AD process. In this stage, carbohydrates are broken down into simple sugars; fats degrade into fatty acids; and proteins degrade into amino acids (Gerardi 2003; Eastman and Ferguson 1981).

Simple organic compounds which were created during hydrolysis stage will be transformed into volatile fatty acids (VFAs), long chain fatty acids, propionate, and butyrate by anaerobes (Jördening and Winter 2006). The concentration of H⁺ formed in this stage may affect the products of fermentation. High concentration of H⁺ reduces the production of acetate. In general, during this stage, simple sugars, fatty acids, and amino acids are fermented to form organic acids and alcohol (Gerardi 2003).

The products from the previous stage are the substrate for bacteria in the acetic acid-producing stage. The products from these intermediate substrates are H₂, CO₂, and acetate. Acetogenic bacteria grow together with methanogen bacteria in this stage.

During this stage, methane is created under completely anaerobic conditions. This reaction is considered an exothermic reaction. Stage 4 can be divided into two methane-generating processes: reduction of CH₃COO⁻ and conversion of H₂ with CO₂. Acetotrophic methanogens are responsible for the reduction of acetate (CH₃COO⁻) into methane, while hydrogenotrophic methanogens are responsible for converting H₂ and CO₂ into methane (Ziemiński and Frąc 2012). Some of the reactions during methanogenesis are described in Table 5.2.

Depending on the microorganisms involved, the anaerobic reactors operate in specifuied temperature regimes:

The activity of methane-forming bacteria is strongly influenced by temperature. In general, when the temperature increases, biogas production increases. For that reason, digesters are heated in colder regions, such as Europe. But at a temperature range of 40–50 °C, biogas production will decrease because this temperature range is not suitable for both mesophilic and thermophilic bacteria. If the temperature is above 60 °C, biogas production decreases and it stops completely at 65 °C or higher. Biogas production is at its maximum rate when the temperature is kept at 35 °C (Chandra et al. 2012), while optimum temperatures for methane production range from 35 to 40 °C (Lianhua et al. 2010).

The most appropriate temperature is 30–40 °C. Low temperatures, abrupt changes in the system, or both weaken methanogen activities (Yadvika et al. 2004; Nozhevnikova et al. 1999).

Temperature is a very important variable for efficient anaerobic digestion of rice straw. Maximized methane production and economic input depend on it.

McCarty (1964) determined that the biological process of AD works optimally in a pH environment of 6.6–7.6. However, Yadvika et al. (2004) and Gerardi (2003) reported that the optimum pH range for AD is at 6.8–7.2. At a pH lower than 5.5, acidogenic bacteria are still active but methanogenic bacteria are inhibited. Inhibition of methanogenic bacteria lowers the methane content of the biogas; therefore, low pH conditions are avoided. During the AD process, pH will usually drop lower than 6.6 if there is excessive accumulation of fatty acids in the acidogenesis stage. Decrease in pH is caused by either overloading of substrates or because the toxins in the feedstock inhibit the activity of methanogens. In this case, substrate feed must be stopped so that acid production will stop or decrease and acetogens and methanogens will be able to degrade excess acid that was produced. Another solution is to use lime for neutralizing the acid and increase the pH to an optimum range. An increase in pH (greater than 8.0) also has an inhibitory effect on AD. At pH 9.0, the methanogenesis process completely stops (Clark and Speece 1971).

Alkalinity is a measure of the capacity of a solution to neutralize acid. Bicarbonate (HCO₃ ⁻), carbonate (CO₃ ²⁻), and hydroxide (OH⁻) are the ions that are used to increase alkaline conditions in the digester. Alkalinity is also considered as the buffering capacity of a solution and is essential for controlling and maintaining a stable pH for the anaerobic digestion process. High alkalinity conditions (1500–3000 mg CaCO₃ L⁻¹) enhance the pH stability of the anaerobic digestion process (Gerardi 2003).

The redox potential is a measure of oxidation capacity or reducing capacity. Biogas is produced effectively in an anaerobic environment where the redox potential must be less than −150 mV. Redox always reaches a negative value (less than −100 mV) under anaerobic conditions (Wiese and König 2009).

In general, the use of substrates including oxygen, nitrate and sulfate promotes oxidation that can significantly change the redox potential and cause changes in pH. Redox potential can be used to predict impending changes in pH of the digester (Wiese and König 2009).

Methane begins to form and CO₂ and H₂ are converted into CH₄ when the redox value is less than −250 mV; H₂O and H₂S are produced when the redox value is less than −150 mV (Laanbroek 1990).

High salinity and ammonium concentrations have detrimental effects on biological processes such as anaerobic digestion (Fang et al. 2011; Chen et al. 2008; Reinhart and Townsend 1998; Kargi and Dincer 1996). High salt concentrations dehydrate bacterial cells due to osmotic pressure (Alhraishawi and Alani 2018). Salt toxicity is determined mostly by the type of cation the salt has.

Feedstock inflow to anaerobic digesters usually contains light metal ions, namely, sodium, potassium, calcium, and magnesium. These cations may also be liberated during the AD process (Chen et al. 2007). A study by Albraishawi and Alani (2018) on codigestion of food waste demonstrated that increasing salt concentrations (0, 16, 30, and 60 g NaCl L⁻¹) has a negative effect on the volume of biogas produced (45, 21, 5, and 2 ml d⁻¹). In terms of methane yield, a study by Lee et al. (2009) on anaerobic digestion of leachate from a food waste recycling facility showed that low-salt concentrations (0.5 and 2 g NaCl L⁻¹) increase methane yield, but higher salt concentrations (5 and 10 g NaCl L⁻¹) resulted in a decrease in methane yield (36 and 41% reduction).

Anwar et al. (2016) showed that methane yield inhibition in anaerobic digestion of food waste is negligible at salt concentrations of 8 g NaCl L⁻¹, but salt concentrations greater than 8 g Nacl L⁻¹ resulted in a sharp decline in methane yield. The cubic regression model y = 0.508 + 2.401x – 0.369x² + 0.033x³ was derived from this experiment to describe sodium salt inhibition, where y is the methane yield and x is the sodium salt concentration. This model predicted experimental results with a small discrepancy of 10%. A study by Ogata et al. (2016) on the effect of salt on biogas production of leachate in a waste landfill showed that methane production decreased while carbon dioxide production was unchanged at a salt concentration of 35 ms cm⁻¹ (approximately 19 mg L⁻¹). A salt content of 80 ms cm⁻¹ (approximately 44 mg L⁻¹) decreased production of both methane and carbon dioxide. Based on these studies on salt inhibition of the anaerobic digestion process, it can be inferred that low salt concentrations in the AD reactant mixture (up to 2 g L⁻¹ NaCl) increase methane yield, but higher salt concentrations (greater than 5 g L⁻¹ NaCl) decrease methane yield.

The quality of biogas produced by anaerobic digestion is determined by the growth of the community of bacteria in the digester. The optimal carbon to nitrogen (C/N) ratio for bacteria to grow is in the range of 20–30 because the bacteria use up carbon 20 to 30 times quicker than nitrogen (Bardiya and Gaur 1997; Malik et al. 1987). If the C/N ratio is higher than optimal, the decomposition rate will be slower. When the C/N ratio is low, the accumulation of ammonia can occur, which can inhibit the activity of bacteria. Some substrates for AD with different C/N ratios are shown in Table 5.4.

The difference in C/N ratios show that plant materials have a high C/N ratio and animal manures have a low one. To achieve the optimum C/N ratio of 20/1–30/1, plant material and animal manures are codigested.

The C/N ratio is a critical factor in the anaerobic digestion process, which shows the balance of nutrients of input materials. Depending on the type of paddy rice, untreated rice straw has a low concentration of total N content, even less than 1% of dry basis (Dinuccio et al. 2010; Lei et al. 2010). The typical C/N ratios for untreated rice straw are approximately from 40 to 80 (Tran et al. 2014; Arvanitoyannis and Tsherkezou 2008; Ghosh and Bhattacharyya 1999). The effect of the C/N ratio on methane yield is illustrated in Fig. 5.2, which demonstrates that methane yield increases when C/N ratio reaches the optimum range (20/1–30/1) and decreases when the C/N ratio rises beyond the optimum.

The AD process is affected by the loading rate, represented by chemical oxygen demand per cubic meter per day (COD m⁻³ day⁻¹) or volatile solids per cubic meter per day (VS m⁻³ day⁻¹), and hydraulic retention time (HRT) of the fermentation mixture. A high organic loading rate will cause accumulation of fatty acids at stage 3, which negatively affects the production of methane. A low organic loading rate on the other hand will produce low volumes of biogas that will make AD operation uneconomical. The recommended organic loading rate for an anaerobic tank without medium is 1–4 kg V m⁻³ day⁻¹ (Eder et al. 2006). HRT is the time that the substrates stay inside the anaerobic digester reactor. HRT strongly depends on the substrates and it usually varies from 30 to 60 days. In general, 30 days are typical HRT for nonstirring digesters, the digesters with high decomposition rates can be reduced to an HRT of 10–20 days.

Biogas production happens in the absence of oxygen; therefore, the presence of oxygen will inhibit the process. In this case, oxygen is considered to be toxic to anaerobic bacteria. In addition, there are many substances that are potentially toxic to microorganisms. Toxic substances and their toxic doses are presented in Table 5.5.

The percentage of dry matter content in a digester suitable for biogas generation and solids reduction is about 9–10%. The concentration of dried matter up to 20% helps to save 50% of the volume of digesters but may lead to souring (reduction of pH) and consequently, reduction of biogas output. Monnet (2003) recommended that the organic dry matter content of batch digester should be adjusted to a range of from 5% to 10%.

Stirring maximizes contact of bacteria with organic waste to accelerate the digestion process. It also minimizes the solids deposition on the bottom of the digesters and avoids foaming and scum on the fermentation solution surface.

Without stirring, the substrate in the digester is usually stratified into three layers: the upper layer is the floating layer; the middle layer is fermentation solution; the bottom layer is the sediment layer (Fig. 5.3). Bacteria not distributed evenly in fermentation solution result in uneven contact of bacteria with the substrates. There are many “dead zones” in the digester where the bacteria density is very low and the decomposition is weak. Organic materials can accumulate and settle in those zones. Stirring can overcome the above disadvantages and enhance the decomposition process.

During the digestion process, it is necessary to mix the substrates in the digester, especially for the substrate from plant biomass, to avoid scum formation. Mixing increases the contact between bacteria and substrate and improves decomposition. Mixing can also prevent foaming and make the temperature uniform inside the digester. However, a fast mixing speed will break down the microbial population, thus it is best to stir at lower mixing speeds (Monnet 2003).

Fibrous substrates—especially straw, grass, weeds, and stalks—are difficult to decompose and must be treated before digestion. Pretreatment aims to reduce the crystallization of cellulose, increase the surface area of the substrate, and make cellulose more accessible to enzymes that convert carbohydrates into fermentable sugars. Pretreatment includes physical, chemical, biological, and their combinations (Alvira et al. 2010).

The anaerobic fermentation process can be enhanced if the digestate is recycled and mixed with substrate feed. The solids content of feeding materials should be adjusted to 5–10%.

Feeding material size is one of the factors affecting biogas production. Materials should not be too large because it will lead to digester blockage and will also make it difficult for bacteria to decompose. Small-sized materials will have larger surface areas and increased microbial activity, resulting in faster decomposition. According to Sharma et al. (1988),when substrates having five different particle sizes (0.088, 0.4, 1.0, 6.0, and 30.0 mm) were digested and their biogas output were compared, it was shown that the optimal size for producing biogas was 0.088 and 0.4 mm. Some other studies also suggested that a physical pretreatment method, such as grinding and chopping, can significantly reduce digester size design as compared with digesters using untreated substrates,without reducing the biogas production (Moorhead and Nordstedt 1993; Gollakota and Meher 1988).

These plants include widely developed and used technologies such as fixed-dome, Indian, and floating drum digester types shown in Fig. 5.4a–c, respectively. Features of these digester types are described in Table 5.6.

Pretreatment of the feedstock can increase its solubility, consequently increasing biogas production and enhancing reduction of volatiles and solids content. Pretreatment is especially helpful in the digestion of biomass substrates as these substances tend to have high cellulose or lignin content. Additives can increase the production rate of the reactor or increase the startup speed, but their additional cost must always be balanced against improvements in efficiency (Ward et al. 2008; Ngan 2012).

Rice straw particle size reduction breaks the cell walls and makes the organic substrate more readily available for microbes to decompose (Zhang and Zhang 1999). Size reduction of rice straw increases surface area and breaks down its polymer structure, thereby increasing hydrolysis yield and hydrolysis rate during digestion (Hendriks and Zeeman 2009). Gharpuray et al. (1983) verified that pretreatment of wheat straw by ball-milling was found to be effective in increasing specific surface area (2.3 m² g⁻¹ for pretreated substrate compared to 0.64 m² g⁻¹ for raw straw). Fiber degradation and methane yield are enhanced when particle size is reduced from 100 mm to 2 (Mshandete et al. 2006). Consequently, methane yield increased (5–25%) and digestion time was reduced (23–59%) (Hendriks and Zeeman 2009). A study by Zhang and Zhang (1999) revealed that rice straw cut in 25-mm lengths has higher methane (198 L kg⁻¹ VS) versus uncut rice straw. However, addition of a milling step in the AD process is expensive due to its high energy requirements (Hendriks and Zeeman 2009).

Møller et al. (2004) observed, for rice straw AD, the increase in methane yield from 30-mm lengths (145 L kg⁻¹ VS added) compared to 1-mm lengths (161 L kg⁻¹ VS added) was significant after 60 days of digestion. Increased bio-degradability of rice straw has been demonstrated when methane production yield was found 7.9 and 13% higher, respectively, for sizes of 0.30 and 0.75 mm compared to 1.5 mm (Chandra et al. 2015). Sharma et al. (1988) reported that methane production tends to increase by from 43.5 to 52.0% when the particle size was gradually reduced from 30 mm to 0.088 mm. However, rice straw sizes under 0.5 mm could be detrimental to the methane production process due to excessive accumulation of volatile fatty acids (VFAs) that will reduce pH in biogas reactor, causing inhibition of anaerobic organisms.

Methane production from rice straw with sizes of 2, 5, and 10 mm illustrates that there is no significant difference among treatments, but compared to the untreated straw, methane production is from 8.3 to 9.3% higher. Comparing the methane yields of AD of rice straw of different sizes (1, 10, 20 cm, and original size) that was soaked in AD slurry for 5 days, it was found that there is no substantial difference in methane yields (166, 121, 166, and 168 L kg⁻¹ VS added, respectively) between treatments (Fig. 5.5).

Chopping and grinding rice straw into small particles less than 1 mm in size and mixing with kitchen waste and pig manure could produce methane from 205 to 384 L kg⁻¹ VS added (Ye et al. 2013) whereas 2–3 cm of chopped rice straw with added micronutrients (nickel and cobalt at 10 and 15 mg kg⁻¹, respectively) increased biogas production by 37 and 46% in mesophilic and thermophilic digesters, respectively.

Chemical pretreatment methods mainly include acid and/or alkaline pretreatments.

Biological pretreatment of feedstock for AD has attracted interest because it requires less energy input as compared with physical pretreatment and is less costly than chemical pretreatment, which costs more because of the expensive chemicals required. Hence, the biological route seems to be the most promising because it is an eco-friendly process and there is no inhibition during the process (Liu et al. 2014).

Rice straw is one of the most important biomass energy sources, largely because of its abundance. According to the Vietnam Statistical Yearbook (2018), around 23.63 million tons of rice straw are produced annually in the Mekong Delta, but more than 80% of it is burned on-site (Nguyen and Tran 2015). Typically, rice straw has a complex polymer crystal structure that is formed by the physical and chemical bonds among the cellulose, hemicellulose, and lignin components, which renders it difficult for anaerobic bacteria to utilize these components for biogas production (Sun et al. 2015). This becomes a major limitation to rice straw’s efficient utilization.

Generally, methane yields from agricultural biomass are lower compared to conventional substrates, but agricultural biomass is an inexpensive option. Biological pretreatment methods can reduce anaerobic digestion duration, enhance feedstock digestibility, and increase gas production rate. This is because the lignocellulosic components of the straw are degraded into simple substances and made easy to digest in AD, especially when using microorganisms with strong lignocellulose degradation ability. The key to the success of biological pretreatment is to find microorganisms that have exceptional lignin degradation ability and to determine the optimum digestion conditions for these microorganisms. Examples of biological pretreatment methods are: microaerobic treatments, ensiling or composting, separation of digestion stages, and fungi pretreatments.

Biological pretreatment of rice straw using fungi is a comparatively eco-friendly approach of enhancing degradability when compared with chemical pretreatment, which requires expensive chemicals, high energy inputs, and toxic substance removal, (Carrere et al. 2016). Several fungi species are used for pretreatment of lignocellulosic biomass for anaerobic digestion and most of them are the white-rot fungus (Ceriporiopsis subvermispora). A study by Zhao et al. (2014a, b) using white-rot fungus as the pretreatment agent increased methane yield by 5–15% as compared with untreated biomass.

Biological pretreatment is normally done by soaking the straw in a natural microbial solution obtained from the effluent of an anaerobic digester, anoxic sediment from ponds or lakes, and wastewater. One study found that rice straw pretreated by soaking in anoxic sediment and digester effluent for 5 days produced 79–85% more biogas volume than straw soaked in tap water (Tran et al. 2017). Compared with untreated substrates, pretreatment using microbiological action increases the degradability of substrates. Yadav et al. (2019) verified that optimal conditions for the biological treatment of lignocellulose biomass of wheat straw by Chaetomium globosporum was found to be 36 °C, 31 days, and 81% moisture, resulting in a 2.9-fold increase in reducing sugar, 48% removal of lignin, and 31% increase in biogas yield.

Similarly, a study by Shen et al. (2018) on the effect of organic loading rate on anaerobic codigestion of rice straw and pig manure showed that after biological pretreatment, the substrate was optimally fermented at an organic loading rate of 2.5 kg COD m⁻³ day⁻¹. This pretreatment achieved the optimum volumetric methane production rate of 640 L CH₄ m⁻³ day⁻¹, and a methane yield of 456 L CH₄ kg⁻¹ COD removed, which were 62.4 and 37.8% higher than those of the control under the same organic loading rate.

Using aerobic and anaerobic fungi as pretreatment agents resulted in increased biodegradability of rice straw (Ghosh and Bhattacharyya 1999; Cann et al. 1994). Methane production from rice straw AD was increased 31–46% when pretreating it with white-rot fungus and brown-rot fungus (Polyporus ostreiformis using a straw-to-fungi ratio of 14:1 and then digested in batch reactors at 30 °C after a 3-week incubation period. Cann et al. (1994) reported that the anaerobic fungi from the rumen consistently increased the digestibility of rice straw when compared with fermenters where the fungi were inhibited. Haruta et al. (2002) also presented the enhancements by a microbial community formed by mixing rice straw, chicken feces, pig feces, cattle feces, and sugarcane dregs. This degraded 60% of the rice straw within 4 days.

Momayez et al. (2018) presented an investigation using effluent of biogas digestate to pretreat rice straw. The straw was pretreated at different temperatures (130, 60, and 190 °C) at different pretreatment times (30 and 60 min). The pretreated straw was subjected to different processes, including liquid anaerobic digestion (L-AD) and dry anaerobic digestion (D-AD). The highest methane yields were obtained through L-AD and D-AD of straw pretreated at 190 °C and 30 min, resulting in 24 and 26% increases in methane produced as compared with L-AD and D-AD using untreated straw. Mustafa et al. (2016) showed that rice straw pretreated with fungi (Pleurotus ostreatus and Trichoderma reesei) and used as feedstock for AD improved the methane yield by 120 and 78.3%, respectively, as compared with untreated straw. These points of view show that biological pretreatment studies have a huge potential, considering that there were only a few fungi species that were studied. There are still millions of fungi species that are yet to be studied.

The results of the previous research mentioned above are shown in Fig. 5.6.

This technology was developed at IRRI (RKB, accessed 2019) by Nguyen et al. (2016) using a hermetic bag (so called IRRI super bag) to make the digesters. This plastic batch-AD is shown in Fig. 5.8a, b. Rice straw and carabao dung are arranged in layers in the digester. Rice straw is spread on the first layer then covered by a dung layer. This is repeated with cattle dung on the top to cover all substrates.

Biogas yield was in the range of 211–779 L kg⁻¹ ODM. This experiment illustrated the following advantages of the IRRI super-bag AD: (1) the capital requirement is low with bags costing US$ 3 each, (2) floating rice straw is avoided; (3) it is portable; and (4) the digester contents are easy to unload after digestion is finished.

Based on the assessment and verification of AD industrial technology, the pilot of a two-stage AD system was designed and is being tested at IRRI. Figure 5.9 shows the schematic diagram of the system with the following characteristics:

Figure 5.10 shows the process route of the biogas power plant using rice straw feedstock, which is located at Fazilka in Punjab, India. Rice straw is fed into a chopper to be sheared off. The sheared rice straw is mixed with 25% cow dung in weight and pumped into the first stage 2000-m³ digester, which is maintained at 35 °C. Retention time (RT) of the substrate in the first anaerobic digestion (AD) stage is 20 days and then conveyed into the second-stage digester, also 2000 m³ and maintained at 35 °C with an RT of 20 days. The biogas is collected through a reservoir and then converted to electric power by a generator. The digestate after AD is used for processing organic fertilizers.