Wheat straw pretreated with 4% NaOH (g/g TS) at 37 C for 5
days showed an increase in methane yield of 112% (from 78 to
166 L/kg VS) compared with untreated material in AD [46].
Sambusiti et al. [152] used 10% NaOH (g/g TS) to treat wheat
straw at 40 C for 24 h, and achieved a 47% increase in the
biochemical methane potential. They also found NaOH pretreatment
improved lignin solubilization and the hydrolysis of cellulose
and hemicellulose. Using solid-state NaOH to pretreat rice
straw for biogas production, He et al. [34,154] found that 3.2e
58.1% higher biogas yields were obtained from 4 to 10% (g/g TS)
NaOH-treated rice straw, and 10% NaOH pretreatment led to
approximately 54%, 16%, and 45% removal of hemicellulose, cellulose
and lignin, respectively. They found that the ester bonds of
ligninecarbohydrate complexes were destroyed through alkaline
hydrolysis and the intermolecular linkages and functional groups
of lignin, cellulose, and hemicellulose were either broken down
or destroyed. The cellulosic crystal structure was changed, but
the crystallinity of cellulose increased during NaOH pretreatment.
Chen et al. [35] compared two different NaOH (5% NaOH
for 48 h) treatment schemes for rice straw (pretreatment of raw
rice straw and pretreatment of AD digestated straw) and found
that the NaOH treatment of AD digested rice straw increased the
chemical oxygen demand (COD) of rice straw from 2311 to
10,488 mg/L and the crystallinity index of cellulose. They also
indicated that the two treatment methods resulted in similar
biogas yields of about 380 L/kg TSadded (30% higher than that of
the untreated material), whilel NaOH treatment on the AD
digested rice straw saved 50% NaOH uses. The pre-treatment of
straw with alkaline was believed to be economical and feasible
for biogas production.