Page 197 - Advances in Eco-Fuels for a Sustainable Environment

Anaerobic digestion of various feedstocks for second-generation biofuel production 161

(CH 3 CH 2 CH 2 COOH), and valeric acid (CH 3 CH 2 CH 2 CH 2 COOH) are formed, along
with hydrogen, ethanol, and ammonia with some organic nitrogen, carbon dioxide,
and hydrogen sulfide as byproducts [21, 22].

6.2.3 Acetogenesis

The products of acidogenesis are further taken for degradation through acetogens
except for acetic acid, which is taken directly to the last stage. In this stage, the volatile
fatty acids having more than two carbon atoms (from the acidogenesis stage) are
converted into acetic acids, hydrogen, and carbon dioxide with the help of acetogens
through the reductive acetyl-CoA or Wood-Ljungdahl pathway [23].

6.2.4 Methanogenesis

The final biological process of methanogenesis is the end stage of anaerobic digestion.
The intermediate products such as acetic acid and hydrogen with a small amount of
carbon dioxide (CO 2 ) of the previous stages are converted into methane (CH 4 ), carbon
dioxide, and water through methanogens. These compositions make up the majority of
the biogas with around 55%–70% of methane formed from acetic acids using acetate
decarboxylation and the remaining 30%–40% of methane formed from carbon dioxide
reduction released from the digester. Methanogenesis is pH sensitive and strictly
restricted to occur in the range of pH 6.5–8. The remains in the digester are called
digestate, and are rich in nitrogen. This nitrogen-rich digestate can be used as an
organic fertilizer. The advantage of this digestate over a mineral fertilizer is the pres-
ence of organic carbon, which is an essential component of healthy soil. Digestate is
composed of a lignin-rich substantial fraction that can be converted into heat, gas, and
biooil by various thermochemical technologies such as the gasification process [24].

6.3 Pretreatment

Pretreatment is a process used for increasing the codigestion of tough materials such as
cellulose by microorganisms, which increases the production of biogas. It can be done
by degradation of the material or the substrate increasing its surface area that is
exposed to microorganism action or increasing its solubility for ease in microorganism
action. This can be done by physical, chemical, or biological methods [25].

6.3.1 Physical methods

This is done by heating and pressuring of the substrate, so it requires high energy. In
thermal pretreatment, heat is used, which breaks the links in compounds, increasing
digestion. Heating can also be done by steam explosion. However, sometimes merely
biodegradable material can lose volatile organics, resulting in a reduction of biogas
[26]. Thermal pretreatment includes microwave thermal treatment, steam explosion,
and conventional thermal hydrolysis. Production of biogas increases during the ther-
mal pretreatment, when the inorganic and organic compounds of the substrate are