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5.3.1.3 Biological Pretreatment
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.
N. V. C. Ngan et al.
5.3.1.3 Biological Pretreatment
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.
N. V. C. Ngan et al.