Biogas could be defined as an end product of anaerobic digestion where a biochemical process is held during
which complex organic matter is decomposed in the absence of oxygen, by various types of anaerobic
microorganisms1
. Biogas consists mainly of methane (CH4), carbon dioxide (CO2), and several other traces of gas
and impurities. The gas components of biogas are specific to the plant and substrates type, and the composition of
the biogas can only be partially controlled. However, the anaerobic digestion of biomass improves with longer time
of exposure, where toward the end of the residence time, the concentration of methane increases disproportionately2
.
1.1. Co-digestion of substrates and organic loading rate
Mixing organic matters by two or more substrates, which commonly known as co-digestion, may give a
synergistic effect that can result in the higher production rate of the biogas2
. This action improves the biogas
production rate and methane concentration in the fermentation process and relies heavily on the organic loading rate
(OLR) of co-substrates engage to the process. The most common applications of co-digestion can be found in
agricultural biogas plants by using a basic substrate such as animal manure and by adding minor amounts of
additional substrates3
.
In anaerobic digestion process, OLR affects the stability of the fermentation process and the rate of gas
production by providing digestible substrates for the growth of microorganisms4
. It is an important factor that
indicates on how much organic dry matter can be fed into the digester per volume and time unit, and has been
defined as the amount of time that the biomass is retained within the digester5
. With a precautious handling, an
increase in the OLR might improve the biogas production by avoiding an outrageous OLR feeding which might
impair the fermentation process and cause a wash-out of microorganisms.
1.2. Agricultural lignocellulosic biomass residues
Agricultural sector provides abundant biomass residues, and it has variously been estimated that these wastes can
account for over 30 % of worldwide agricultural productivity6
. Corn residues which are produced from this sector,
which cover the husk, stover, and cob, are lignocellulosic biomass which has been commonly used as one of the
substrates for biogas production worldwide. With the high content of cellulose and hemicelluloses, corn residues are
considered to be a good substrate for the biogas production.
Lignocelluloses are basically composed of carbohydrates (cellulose and hemicelluloses), lignin, and extraneous
materials7
. However, the compact crystalline structure where lignin physically protects the cellulose and
hemicelluloses parts makes these materials more resistant to anaerobic digestion8
. In the anaerobic digestion
process, if a substrate is well enclosed in lignin structures, the type of disintegration of the substrate becomes
important. The structure should be disrupted or defibrated rather than cut, because they are refractory to
decomposition under anaerobic conditions3
. Without prior treatments, a slow hydrolysis might occur and biogas
production could become low with a long retention time required to produce sufficient amount of biogas9
.
Avicenna et al. / Procedia Chemistry 14 ( 2015 ) 91 – 100 93
1.3. Sodium hydroxide (NaOH) pretreatment
The pretreatment methods of lignocellulosic-containing substrates involve the use of sodium hydroxide (NaOH).
It is the most popular alkali used in alkaline pretreatment, and has been extensively studied to improve biogas yield
from lignocellulosic biomass10. A study by Chandra et al.11 with the pretreatment of wheat straw with NaOH showed
that NaOH pretreated substrate produced 87.5 % higher biogas production and 111.6 % higher methane production
compared to the untreated wheat straw substrate. Another study conducted by Taherdanak and Zilouei12 in the
utilization of NaOH for wheat pretreatment showed that the best improvement in the yield of methane production
was achieved by pretreatment at 75 °C for 60 min, giving a methane yield of 404 mL · g–1 VS. Zheng et al.10
provided a conclusive statement that NaOH is the most cost effective and widely used alkali for lignocellulosic
biomass pretreatment. However, its utilization should be handled with care, as it might cause Na+ ion inhibition of
the fermentation process, especially methanogenesis