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162 Advances in Eco-Fuels for a Sustainable Environment

partially solubilized. Bien et al. studied that the methane production was increased by
25% with an increase in thermal pretreated sewage sludge (170–180°C) compared to
untreated sludge. The chemical pretreatment technique comprises ozonolysis, oxida-
tion, alkaline, and acid pretreatment. Application of these chemical pretreatments
results in higher solubilization and biodegradation of the chemical composition of bio-
mass. Most et al. investigated that acid-pretreated pig slurry (H 2 SO 4 ) produced a 20%
higher methane yield than untreated slurry. Mechanical pretreatment also aims to
reduce the particle size by applying force by various methods to improve the transfer
of heat and mass for the digestion process [27].
In the production of biogas, the principal parameter which plays a significant role
in the face of the particles. The microbial activity is increased by the reduction of the
size of the particles, which involves an increase in the use of the substrate. Sharma
et al. studied the supreme quantity of biogas produced from 0.40, and a size of a par-
ticle of 0.088mm indicated that the effect of the size of the particles on the forest res-
idues and agricultural having various sizes (30, 0, 6, 0, 1, 0, 0, 40 and 0.088mm).
Ultrasonic waves can be used to crush the cell membrane of the substrate, and nec-
essary nutrients will increase the consumption of the substrate faster, increasing the
biogas production. However, ultrasonic wave generation is costly [28]. Ultrasonic pre-
treatment produces favorable conditions for biodegradation by disintegrating the par-
ticles and enhancing the reduction of biogas. During anaerobic digestion, pretreatment
dramatically reduces volatile solids and increases daily biogas production. Braguglia
et al. [29] examined that ultrasonic-pretreated waste sludge provided 30% higher bio-
gas production than untreated sludge.

6.3.2 Chemical methods
Chemicals are used to break down the bonds to transform the substrate into a soluble
one for increasing the biodegradable production of biogas. The alkalis such as NaOH
or KOH and acids such as sulfuric acid can be used, depending upon the substrate and
pH of the solution [10]. The rate of digestion will depend upon the substrate. Alkalis
also increase the surface area of digestion. However, this can produce some chemical
agents harmful to the environment. Also, the cost of chemicals is more [28].
Chemical pretreatment uses oxidants, acids, or alkalis to break down the complex
organic compounds present in biomass to simpler structures [30]. The most promising
function of chemical pretreatment of lignocellulosic biomass is the demolition of firm
organic structure biomass by cutting the lignin-carbohydrate link and cellulose trans-
parent matrix or the hydrolysis of hemicellulose. Strong acid treatment using sulfuric
acid and hydrochloric acid at high concentration tends to highly obstruct the anaerobic
digestion process through the production of unwanted byproducts such as furfural and
derivatives [31]. Because of the excessive degradation of the complex substrate during
the treatment with strong acids, it results in a loss from it from fermentable sugar.
Hence, dilute acids are commonly preferred for acid pretreatment. Moreover, acid pre-
treatment is coupled with higher temperatures that are more than 100°C. Alkali pre-
treatment is mostly preferred over acid pretreatment because it can enhance better
circumstances for anaerobic digestion by preventing a drop in pH value [32]. Alkali