The combustible industry is a fossil fuel industry like the coal processing industry. The waste arising from these industries is rich in hydrocarbon. On earth, the resources from fossils are costly and high in demand, and coal conversion technologies show adverse effects on contaminants’ atmosphere [58]. The animal-based agroindustrial sector releases a high amount of organic waste into the environment. This organic waste mainly consists of animal by-products like blood, paunch, fat and protein, bones of head and limb. It can spread human diseases like Creutzfeld-Jacob and bovine spongiform encephalopathy [25]. Considering these issues, researchers focused on converting industrial waste residues into energy resources (Table 3.4).
3.3.1.5.3 Municipal Solid Waste
Municipal solid waste possesses paper and household wastes, non-degradable plastic waste, and non-flammable materials such as glass and aluminum from various residences [59], [60]. According to several variables, the MSW constitution varies according to household waste, seasonality, housing standards, and the prevalence of measures to reduce waste [60]. Municipal solid waste leads to a significant pollution concern worldwide [61]. All are regulated by dumping rubbish at municipal landfills, recycling to produce valuable products, and incineration for energy generation [59].
Unregulated OF-MSW’s (organic fraction of municipal solid waste) decomposition contributes to global warming and soil, water, and air pollution [60, 62]. Ebrahimian et al. [61] stated that OF-MSW could process high-value fuels and products to control these issues. Enterobacter aerogenes is one such promising bacterial organism process OF-MSW’s into various biofuel products such as 2, 3-butanediol (octane booster), bioethanol, acetic acid, and biohydrogen, and PHAs (polyhydroxyalkanoates), biomethane. The recovery of the carbon sources present in OF-MSW provided a considerable amount of energy (8236.9 kJ) and an environmentally sustainable bioplastic [61].
3.3.1.5.4 Food Waste
Food waste is recognized as a fascinating source of bioenergy [63]. Biodegradation of food residues is utilized for energy resources and the use of supplements (Table 3.5). It is a productive solution to enhance food waste (FW) consumption. Many nations prefer food waste to biogas production as a viable biomass [64, 65].
3.3.1.5.5 Sewage Waste
Sewage is a type of wastewater that arises from industries, municipal areas, and domestic practices that possess 99% moisture content [70]. It has the foremost responsibility for environmental and health issues [71]. The treatment of wastewater expels out semi-solid slurry referred to as sewage sludge rich in proteins, carbohydrates, detergents, phenols, and lipids (Table 3.6). Sewage sludge generated from the wastewater treating industries is higher at present [72]. Since sludge possesses organic matter and minerals it can act as an energy resource (Table 3.6). They also include various toxic substances of phenol derivatives, aromatic hydrocarbons, Chlorine derivatives, chemical pesticides, and other harmful components [70]. Scum is a kind of basin sedimented at the bottom of the wastewater treatment plants (Table 3.6). It consists of various hydrophobic compounds such as oil, grease, low-density concrete, algal debris, and either presence or absence of foams and bubbles [73]. Scum possesses a high calorific value, a large quantity of unsaturated fatty acid ester (57.5-64.1%), and lipid content compared to sewage sludge [74]. The fatty acid methyl esters (FAMEs) produced from scum are rich in oleic acid methyl ester, whereas sewage sludge constitutes palmitic acid methyl ester as an abundant source. By these aspects, Wang et al. [74] reported that sewage scum could be sufficient biomass for biodiesel production (Table 3.6).
Table 3.5 List of food waste-based biomass, the process of conversion, and biofuel products.
| Food waste biomass | Process of conversion | Energy products | Reactor | Reference |
| Sludge from a pilot-scale high-rate activated sludge systems co-treating municipal wastewater and food waste | Anaerobic digestion at the mesophilic condition | Biogas | Laboratory-scale mesophilic digester | [66] |
| Thermal-acid pretreated sugarcane bagasse (SB) and untreated food waste (FW) with waste active sludge (WAS) | Anaerobic digestion | Biogas | Six different batch reactor under ambient condition | [67] |
| Food waste | Anaerobic digestion with activated persulfate | Biogas | - | [65] |
| Food waste (FW) and waste active sludge (WAS) | Anaerobic co-digestion with biological co-pretreatment | Biogas | - | [68] |
| Agri-food waste - Pellets of tomato waste and grape marc | Combustion | Biofuel | Domestic boiler | [1] |
| Pasteurized food wastes and dairy cattle manure | Thermophilic anaerobic digestion | Biogas | Batch and large volume laboratory digesters | [69] |
Table 3.6 List of sewage biomass, the process of conversion, and biofuel products.
| Types of sewage waste | Biomass | Process of conversion | Biofuel products | Reactor | Reference |
| Sewage sludge | Process water (PW) of dewatered waste activated sludge(DWAS) with primary sewage sludge (PSS) | Hydrothermal carbonization (HTC) and Anaerobic co-digestion (AD) at thermophilic and mesophilic conditions | Hydrochar (Solid fuel) and methane-rich biogas | Mesophilic digesters / Zipperclave 4L stainless steel thermostated reactor / Borosilicate glass digesters | [75] |
| Wet sewage sludge with 85% water content | In situ wet-transesterification | Biodiesel (BD) | Bench-scale (10L) reactor |