28.4 Techniques for Bioconversion of Food Waste Toward Circular Bioeconomy Approach

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waste were found to be a promising feed in the production of biogas. Anaerobic

digestion of fish processing waste decreases the problems associated with landfill

and incineration. Higher concentration of lipids and insoluble proteins in fish

processing waste inhibits the anaerobic digestion and is recommended to combine

with other substrates.

28.4.1.1

Factors Influencing Anaerobic Digestion

Factors that affect the design and performance of anaerobic digestion process are

substrate characteristics which include moisture content, volatile solid content,

nutrient content, particle size, and biodegradability [17]. Biodegradability of the

substrate is indicated by methane yield. Even though anaerobic digestion is suc-

cessfully employed in animal manure, agricultural and industrial waste treatment

challenges associated with the implementation of anaerobic digestion in food waste

management are volatile fatty acid accumulation, foaming, low buffer capacity, and

process instability. Lack of process control and optimization leads to generation of

harmful intermediate substances which reduce the system stability and methane

yield. Carbohydrates in the food waste are easily digested at the early stages of

anaerobic digestion process, resulting in a considerable pH drop, which often leads

to digester instability and system failure. To prevent this, anaerobic digestion system

is to be performed on limited organic load with enough buffering capacity [21].

To improve the digestion efficiency and system performance, food waste can

be co-digested with animal manure, sewage sludge, or lignocellulosic biomass.

Simultaneous digestion of two substrates has the advantage of balancing C/N

(carbon/nitrogen) ratio, diluting the inhibitors, and enhancing methane produc-

tion [22]. C/N ratio is an important factor for nutrient balance and stability of

microorganisms involved in anaerobic digestion. Animal manure is found to be a

suitable substrate for co-digestion due to its high buffering capacity. Furthermore,

co-digestion of food waste with animal manure can provide an excellent environ-

ment for anaerobic microbes. Co-digestion of cow manure with food processing

industry waste (7 : 3 wt/wt, wet basis) at thermophilic temperature enhances the

methane production more than twofold [23]. Solid-state anaerobic fermentation

of spent hay (high C/N ratio) co-digested with soybean processing waste (low

C/N ratio) in the ratio of 25 : 75 enhances the methane production [22]. Higher

alkalinity and huge quantity of active microorganisms present in the sewage sludge

also recommended as a co-substrate; however, if the light and heavy metal ions in

the sewage sludge are too high, it may inhibit the digestion process. Co-digestion

of microalgae with food waste improved the digestion and maximized the methane

production [21]. Co-digestion of rendering waste with potato pulp improved the

methane yield by inhibiting the accumulation of digestion intermediate products

such as volatile fatty acids, long-chain fatty acids, and free ammonia [24].

Foaming is another complex phenomenon. When it occurs, the produced biogas

gets dispersed in the liquid slurry and it increases the volume of digestate which in

turn reduces the volume of digester. The following reasons are responsible for foam-

ing: (i) higher levels of surface-active agents such as proteins, fatty acids, detergents,

and other compounds present in the food waste; (ii) generation of surface-active