Page 196 - Advances in Eco-Fuels for a Sustainable Environment

160 Advances in Eco-Fuels for a Sustainable Environment

large amount of kitchen waste from household and residential areas. Statistics say that
larger households that accommodate more people tend to produce more waste than
their smaller counterparts. However, single-person households have been reported
to waste the most food on a per capita basis. Similarly, older people waste as much
avoidable food as younger people (1.2kg per person per week), a fact that contradicts
accepted wisdom in our society. A lot of kitchen waste is generated every year that
needs to be disposed of in order to save our environment. Anaerobic digestion is
an organized biological decaying process of the kitchen waste that allows the efficient
capturing and utilization of biogas for energy generation. This biogas is approximately
60% methane and 40% CO 2 . People in rural areas to adopt biogas technologies to meet
their household cooking and lighting needs, involves Khadi and Village Industries,
Janata and Deenbandhu biogas plants [17]. These biomasses are a great source for bio-
methanation and production of various liquid fuels such as biodiesel and transporta-
tion oils. Anaerobic digestion is one of the main processes to convert the biomass into
biogas [18]. Biogas is converted into syngas by partial oxidation or steam reforming.
The syngas is further converted into liquid fuels by using various methods such as the
Fischer-Tropsch process and with methanation the syngas is converted into methanol,
ethanol, and various biofuels [19].

6.2 Stages of anaerobic digestion

Anaerobic digestion (AD) has been deemed to be an energy-efficient and promising
method to dispose of waste produced from a municipal waste plant for its high value-
added products and low operating cost. Anaerobic digestion is generally categorized
into four stages: (1) hydrolytic of complex organic compounds, (2) acidogenesis,
(3) acetogenesis, and (4) methanogenesis.

6.2.1 Hydrolysis

Biomass consists of large organic polymers. These complex organic polymers are
converted into simple soluble molecules through different groups of facultative or fer-
mentative bacteria by emitting extracellular enzymes. The procedure of dissolving the
small molecules and breaking these chains into a solution is called hydrolysis. Hydro-
lysis of these polymeric components with high molecular weight is necessary for the
bacteria inanaerobic digesterstoaccessthe energypotential ofthematerial.Lipasescon-
vert lipids to long-chain fatty acids; proteases convert proteins to amino acids; and the
polysaccharides such as cellulose, starch, and pectin are hydrolyzed to monosaccharides
by cellulases, amylases, and pectinases. These constituent parts such as fatty acids,
amino acids, and sugars (monosaccharides) are then utilized by other bacteria [7, 20].

6.2.2 Acidogenesis
The process of acidogenesis resembles the way milk sours. It is a biological process
that is carried out by acidogenic (fermentative) bacteria, resulting in the additional
breakdown of the remaining components. In this stage, volatile fatty acids such
as acetic acid (CH 3 COOH), propionic acid (CH 3 CH 2 COOH), butyric acid