Page 42 - Energy from Toxic Organic Waste for Heat and Power Generation

30 Energy from Toxic Organic Waste for Heat and Power Generation

decomposition and amount of gas production. Bacteria are more effective
in a limited range of temperatures. Any change in these properties can ad-
versely affect the biogas production.

Temperature
The research work reported by Takizawa et al. [46] reveals that there are dif-
ferent temperature ranges at which anaerobic digestion can be carried out,
such as psychrophilic (< 30°C), mesophilic (30–40°C), and thermophilic
(50–60°C). Mesophilic and thermophilic are the two types of anaerobic
bacteria most commonly found in the digesters. Mesophilic bacteria have
the optimum output at temperatures of 25–40°C and thermophilic bacteria
survive best at 50–65°C [47, 48].
Angelidaki and Ahring [49] have mentioned in their research that the
length of the digestion period is dependent on temperature. Angelidaki
and Ahring [49], and Garba [50] have also reported that the increase in
biogas yield and reduced concentration of slurry were observed at 55°C of
digester temperature when ammonia (NH 3 ) was in a high concentration.
Methanogens are very sensitive to sudden thermal changes; therefore, any
drastic change in temperature must be avoided.

Pretreatment
Pretreatment is a process that breaks the complex organic structures of bio-
mass into simpler molecules, which are then more susceptible to microbial
degradation. Dar and Tandon [51], Singh et al. [52], and Wang et al. [53]
reported several pretreatment processes of biomass feedstock, such as alkali
or acid treatment, predigestion of fresh substrate, thermochemical pretreat-
ment, and ultrasonic pretreatment.
Dar and Tandon [51] observed an improvement of about 31%–42% mi-
crobial digestibility and an increase in biogas production, when alkali-treated
(1% NaOH for 7 days) biomass was added as a supplement to the digestible
feedstock.
Singh et al. [52] reported on their investigation that the predigestion
of fresh slurry in a batch reactor at temperatures of 30–35°C for 2 days,
and using the same slurry as the feedstock for anaerobic digestion, could
increase the biogas production by 17%–19% having a methane content of
68%–86%. Wang et al. [53] reported that, when they adopted an ultrasonic
pretreatment to treat the waste-activated sludge for 30 min, it increased the
methane production up to 64%.