Page 199 - Advances in Eco-Fuels for a Sustainable Environment

Anaerobic digestion of various feedstocks for second-generation biofuel production 163

pretreatment with NaOH or KOH is performed at ambient or marginally higher tem-
peratures around 40°C with a long reaction time [33]. When ammonia is used as an
alkali agent, higher temperature and pressure conditions need to be applied in order to
reduce the pretreatment time and also to prevent an ammonia freeze explosion [34].

6.3.3 Biochemical methods
In biochemical methods, any living organism can be used for degradation of the sub-
strate. Fungi, enzymes, and cellulose-degrading bacteria can be used. Preacidification
followed by multistage fermentation increases the production of biogas. This method
is defined by the environment and thus requires little energy and is beneficial, but we
cannot control this process. They are slow and require time to be supplemented [35].
Biological pretreatment is carried out by using a biological agent with various feed-
stocks. The microorganisms such as soft-rot fungi, brown-rot fungi, and white-rot
fungi are commonly used in the biological pretreatment that reduces lignin and hemi-
cellulose content in the substrate. White soft-rot fungi attack both cellulose and lignin
whereas brown-rot fungi mostly attack cellulose. In this way, various pretreatment
methods are used to increase the production of biogas [36].

6.4 Factors affecting anaerobic digestion

In this process, several factors that affect the anaerobic digestion process are discussed
here such as temperature, pressure, pH, retention period, solid-to-water content, and
carbon-to-nitrogen ratio.

6.4.1 Temperature
In the anaerobic digestion process, temperature plays a crucial role in microbial met-
abolic activities and the overall digestion rate, explicitly the rates of hydrolysis and
methanogenesis. The anaerobic digestion process is carried out within a wide range
of temperatures. Temperature is one of the most influencing operational parameters,
which is divided into two groups, namely mesophilic and thermophilic. The meso-
philic type of digester operates between 30°C and 42°C, most preferably at 37°C.
The thermophilic type of digester’s operational range is 43–55°C. In practice, the most
used digesters are designed to operate at a mesophilic range, mostly at 37°C, whereas
few are designed for a thermophilic temperature range of 52–55°C.The reason why
mesophilic digesters are preferred is that they are more stable, require less process
heat, and have a lower toxicity risk from ammonia nitrogen, whereas higher loadings
are allowed in thermophilic digestion processes, potentially with higher conversion
efficiencies, reduced hydraulic retention time, and pathogen disinfection [37].
Mesophilic anaerobic digestion is a commonly used system that has a more stable
operation but gives a lower biogas production rate. One more disadvantage of meso-
philic digestion is that, even though it operates at the stable rate, it does not cut down
the pathogen concentrations sufficiently to produce biosolids of the Class A type,