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28 Bioconversion of Food Waste to Wealth – Circular Bioeconomy Approach

stages are converted into animal feed, chemicals, and fuels, resulting low disposal

rate of up to 5%. However, waste generated in retailing stage and consumer end has

lower recycling rate due to various reasons such as logistics, poor traceability, health,

and safety issues. Anaerobic digestion is the preferable method for treating and

recycling impure and low-quality food waste generated from homes, restaurants,

schools, and hospital cafeterias [15]. Low-quality and contaminated food waste can

be treated by anaerobic digestion to produce methane for energy. Among all types

of food wastes, restaurant and household food wastes have high methane potential

due to high level of lipids and easily digestible carbohydrates. Moreover, recovering

bioenergy from the food waste through anaerobic digestion involves less cost.

In anaerobic digestion, 40–60% of food waste solids are degraded and energy

recovered in the form of biogas (methane 60–70%, carbon dioxide 30–40%, traces

of hydrogen, hydrogen sulfide, and other gases), and the remaining nutrient-rich

solid residue can be used for land application or require further disposal [15, 17].

The digested solid residue can be used as a biofertilizer. However, if the heavy

metal contents in the solid residue exceed the limit, it need to be further disposed

by incineration or landfill. Anaerobic digestion involves following four phases: (i)

hydrolysis, (ii) acidogenesis, (iii) acetogenesis, and (iv) methanogenesis. Hydrolysis

is the first step in anaerobic digestion process. In this phase, complex organic

matters in the food wastes such as proteins, carbohydrates, and lipids are broken

down into amino acids, simple monomers, and fatty acids by extracellular enzymes

of hydrolytic bacteria, i.e. protease, amylase, cellulase, and lipase. During the second

stage called acidogenesis phase, fermentative bacteria decomposes the monomers

into volatile fatty acids including lactic acid, pyruvic acid, acetic acid, and formic

acid. Then in the acetogenesis phase, lactic and pyruvic acids are digested into acetic

acid and hydrogen. Then in the last stage called methanogenesis also known as gas

production phase, hydrogen and acetic acids are transformed into methane and

carbon dioxide [18]. In methanogenesis phase, pH of the substrate influences the

volume of methane gas production. Higher pH disintegrates the carbon dioxide in

the biomass and this enhances the methane concentration in biogas. The generated

biogas rich in methane content has higher energy value. Biogas can be utilized as a

fuel for internal combustion engines to generate electricity. Compressed biogas can

be a petroleum gas alternate for vehicle fuel.

Extensive research has been conducted on anaerobic digestion of food waste man-

agement on few decades. Ahamed et al. [19] compared the incineration, anaerobic

digestion, and conversion of food waste to biodiesel system. Anaerobic digestion is

most preferable if implemented in local environment when applicable. On the basis

of cost analysis, in case the oil content is greater than 5%, food waste-to-energy

biodiesel system is preferred and anaerobic digestion otherwise. In general view,

food waste-to-energy biodiesel can be chosen over incineration. Kim et al. [20]

developed modified three-stage anaerobic fermentation system which consists of

semi-anaerobic hydrolysis/acidogenesis, anaerobic acidogenesis, and anaerobic

methanogenesis. This three-stage reactor system showed higher methane yield

by increasing the rate of hydrolysis, acidogenesis, and methanogenesis without

affecting the pH of the substrate. Pineapple processing waste and pineapple on-farm