3.2 Biochemical Conversion Technologies
79
matters contained within the cells [69, 80]. The major chemical methods are (1) acid
pre-treatment, (2) alkaline pre-treatment, (3) ozonation and (4) advanced oxidation
methods [79, 81]. However, alkaline pre-treatment is usually the preferred chemical
method since it is more compatible with the subsequent anaerobic digestion and is
one of the least costly alternatives [80, 82]. The main reactions that occur during
alkali treatment are solvation and saponification, which result in the swelling of
the particulate organics, making the cellular substances more susceptible to enzymatic attack and thus improving the biodegradability of the solid and liquid phases
[80, 82, 83].
Acid pre-treatment can break down both cellulose and lignin and thus are more
suitable for lignocellulosic substrates. The hydrolytic microbes are also capable of
acclimating to acidic conditions [80]. The main reaction that occurs during acid
treatment is the hydrolysis of bio-polymers (cellulose, hemicellulose and lignin)
into monomers and oligomers, thus it could result in an increase in the rate of
digestion as the hydrolysis step has been partially completed in the pre-treatment
[80, 84]. The main disadvantages of acid pre-treatment include the possibility of
formation of inhibitory by-products such as hydroxymethylfurfural (HMF) at strong
acidic conditions, loss of fermentable sugars due to the increased degradation of
complex substrates, high costs of acids used for the pre-treatment and bases used for
neutralizing the acidic substrates before AD [80].
Ozonation is an oxidative treatment process to enhance biomass hydrolysis,
however, the production of ozone requires a high energy input [85]. During ozonation,
biomass flocs break down into finely dispersed particles. Other oxidation processes
include Wet Air Oxidation (WAO) operating at 150–300 °C and 20–200 bar, and
Fenton’s peroxidation occurring at ambient conditions using Fe
2+ and H 2 O 2 to create
reactive hydroxyl radicals [86].
3.2.1.3 Combined Thermal and Chemical Pre-treatments
Combined thermal and chemical pre-treatment has the advantage of avoiding the
necessity of high temperatures and was reported to produce better solubilization
compared to either chemical or thermal pre-treatment [73, 87]. It is an effective
technique for breaking down the microbial cells or compounds that are difficult to
hydrolyze to easy biodegradable compounds [88]. Acids and alkalis are commonly
used catalysts in the thermal/chemical hydrolysis of organic macromolecules.
3.2.1.4 Mechanical Pre-treatments
Mechanical pre-treatment methods are based on the disruption of microbial cells by
shear stress generated by pressure, translational or rotational energy [73, 77]. The
shear forces could decrease the particle size and increase the surface area of microbial
cells available for enzymatic degradation [78]. Mechanical pre-treatments are helpful
79
matters contained within the cells [69, 80]. The major chemical methods are (1) acid
pre-treatment, (2) alkaline pre-treatment, (3) ozonation and (4) advanced oxidation
methods [79, 81]. However, alkaline pre-treatment is usually the preferred chemical
method since it is more compatible with the subsequent anaerobic digestion and is
one of the least costly alternatives [80, 82]. The main reactions that occur during
alkali treatment are solvation and saponification, which result in the swelling of
the particulate organics, making the cellular substances more susceptible to enzymatic attack and thus improving the biodegradability of the solid and liquid phases
[80, 82, 83].
Acid pre-treatment can break down both cellulose and lignin and thus are more
suitable for lignocellulosic substrates. The hydrolytic microbes are also capable of
acclimating to acidic conditions [80]. The main reaction that occurs during acid
treatment is the hydrolysis of bio-polymers (cellulose, hemicellulose and lignin)
into monomers and oligomers, thus it could result in an increase in the rate of
digestion as the hydrolysis step has been partially completed in the pre-treatment
[80, 84]. The main disadvantages of acid pre-treatment include the possibility of
formation of inhibitory by-products such as hydroxymethylfurfural (HMF) at strong
acidic conditions, loss of fermentable sugars due to the increased degradation of
complex substrates, high costs of acids used for the pre-treatment and bases used for
neutralizing the acidic substrates before AD [80].
Ozonation is an oxidative treatment process to enhance biomass hydrolysis,
however, the production of ozone requires a high energy input [85]. During ozonation,
biomass flocs break down into finely dispersed particles. Other oxidation processes
include Wet Air Oxidation (WAO) operating at 150–300 °C and 20–200 bar, and
Fenton’s peroxidation occurring at ambient conditions using Fe
2+ and H 2 O 2 to create
reactive hydroxyl radicals [86].
3.2.1.3 Combined Thermal and Chemical Pre-treatments
Combined thermal and chemical pre-treatment has the advantage of avoiding the
necessity of high temperatures and was reported to produce better solubilization
compared to either chemical or thermal pre-treatment [73, 87]. It is an effective
technique for breaking down the microbial cells or compounds that are difficult to
hydrolyze to easy biodegradable compounds [88]. Acids and alkalis are commonly
used catalysts in the thermal/chemical hydrolysis of organic macromolecules.
3.2.1.4 Mechanical Pre-treatments
Mechanical pre-treatment methods are based on the disruption of microbial cells by
shear stress generated by pressure, translational or rotational energy [73, 77]. The
shear forces could decrease the particle size and increase the surface area of microbial
cells available for enzymatic degradation [78]. Mechanical pre-treatments are helpful