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9 Biodegradation of Plastics by Microorganisms

aerobic digestion, unlike the anaerobic digestion stage. The aerobic biodegradation

process helps improve the environment of human beings and animals, thereby con-

trolling the pathogens. Comparatively better and more complete digestion of solid

wastes can be obtained through aerobic processes with a reduction of more than 50%

accumulation in most cases.

9.3.2.2

Anaerobic Biodegradation

In the anaerobic biodegradation, organic contaminants are quickly biodegraded

by microorganisms, under anaerobic conditions. This process produces methane,

water, CO2, minerals, and salt [15]. Anaerobic degradation occurs in a situation

when there is a dominance of anaerobic microbes over the aerobic ones. Several

anaerobic bacteria use sulfate, iron, nitrate, manganese, and CO2 as their electron

acceptors, thus breaking down the organic compounds to smaller complexes. The

anaerobic process is widely known for the biodegradable waste treatment and

treatment of wastewater sludge because it helps to reduce the mass and volume of

input.

Various bacteria, including acetic acid and methane forming bacteria, are engaged

in the anaerobic degradation of plastics. These acetic acid and methane forming bac-

teria feed upon the primary feed stock that undergoes several progressions changing

it into intermediate molecules containing hydrogen, sugar, and acetic acid before

being transformed into biogas at the end. To date, scientist found four major chemi-

cal and biological stages of anaerobic degradation, namely, hydrolysis, acidogenesis,

acetogenesis, and methanogenesis.

9.3.3

Mechanism of Microbial Degradation of Plastic

Microorganisms have different enzymes that enable plastic to be used as a substrate.

For this purpose, they are best suited or eco-friendly degradation of plastics. Microor-

ganisms break down polymers bond through redox enzymes. The nature of enzymes

and their catalytic activity vary depending on the microbial species and strains. Dif-

ferent types of microbial enzymes can degrade different types of polymers. As for

example, Bacillus spp. and Brevibacillus spp., both can produce protease, which can

degrade various polymers [16]. Fungus contains laccase that can degrade lignin and

also oxidize aromatic and non-aromatic compounds. Microbial enzymes control the

biodegradation of polymers in an efficient and eco-friendly way.

Biological degradation of plastics waste product depends on so many factors.

They include molecular weight, surface area, functional groups, hydrophilicity,

hydrophobicity, chemical structure, crystallinity, and melting point of plastics.

Molecular weight of polymers also influences the digestibility cycle of plastic.

If the molecular weight of the polymer is higher, the degradation potential is

lesser, due to reduction in solubility and degradation rate. There are four key

steps involved in the microbial digestion of polymers namely, bio-deterioration,

assimilation, bio-fragmentation, and mineralization. They are briefly described

in Figure 9.3. The phase of degradation can be altered by the superficial degrada-

tion. Bio-deterioration largely affects the superficial degradation process. Growth

of microbial biofilms causes severe physical and chemical degradation on the