79
5.3 Current Technology Developments and Practices for Rice
Straw AD
5.3.1 Rice Straw Pretreatment for AD
Pretreatment methods to improve the anaerobic digestion process were presented in
many studies, such as Ngan (2012), Pilli et al. (2011), Hendriks and Zeeman (2009),
Neyens et al. (2003), Weemaes and Verstraete (1998), Stuckey and McCarty (1984),
and Haug et al. (1978). Pretreatment has been reported as an important step in the
methane production process (Alvira et al. 2010; Carvalheiro et al. 2008; Taherzadeh
and Karimi 2008). Pretreatment will change the structure of cellulose so that the
enzymes can easily convert high molecular weight molecules, such as carbohydrates into simple sugars (Mosier et al. 2005). Especially in the case of rice straw, a
biomass with a high-lignin pretreatment step is necessary to amplify the degradability of rice straw and speed up the anaerobic digestion process.
Rice straw pretreatment methods are classified as physical (particle size reduction), chemical (acid and alkali additions), and biological (fungi).
5.3.1.1 Physical Pretreatment: Effect of Particle Size of Rice Straw
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 m
2
g
−1
for pretreated substrate compared to 0.64 m
2
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
5 Anaerobic Digestion of Rice Straw for Biogas Production
5.3 Current Technology Developments and Practices for Rice
Straw AD
5.3.1 Rice Straw Pretreatment for AD
Pretreatment methods to improve the anaerobic digestion process were presented in
many studies, such as Ngan (2012), Pilli et al. (2011), Hendriks and Zeeman (2009),
Neyens et al. (2003), Weemaes and Verstraete (1998), Stuckey and McCarty (1984),
and Haug et al. (1978). Pretreatment has been reported as an important step in the
methane production process (Alvira et al. 2010; Carvalheiro et al. 2008; Taherzadeh
and Karimi 2008). Pretreatment will change the structure of cellulose so that the
enzymes can easily convert high molecular weight molecules, such as carbohydrates into simple sugars (Mosier et al. 2005). Especially in the case of rice straw, a
biomass with a high-lignin pretreatment step is necessary to amplify the degradability of rice straw and speed up the anaerobic digestion process.
Rice straw pretreatment methods are classified as physical (particle size reduction), chemical (acid and alkali additions), and biological (fungi).
5.3.1.1 Physical Pretreatment: Effect of Particle Size of Rice Straw
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 m
2
g
−1
for pretreated substrate compared to 0.64 m
2
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
5 Anaerobic Digestion of Rice Straw for Biogas Production