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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	[26]
	Sewage sludge	Pyrolysis	Ash	Membrane bioreactor based on wastewater treatment and sewage sludge from a domestic wastewater treatment plant.	[76]
	A liquid fraction from hydrothermal carbonization of sewage sludge	Anaerobic digestion (AD)	Methane	-	[77]
	Sewage sludge	Microwave pre-treatment and semi-continuous anaerobic digestion	Methane	Pyrex reactor under a mesophilic condition in semi-continuous mode	[71]
	Sewage sludge and marine and freshwater microalgae (Isochrysis galbana and Selenastrum capricornutum)	Co-digestion under mesophilic and thermophilic conditions	Biogas	Semi-continuous reactors for acclimation and batch reactors for co-digestion	[78]
Sewage scum	Lipids extracted by solvent-less separation from wet sewage scum	Direct esterification	Biodiesel (Fatty acid methyl esters (FAMEs))	Three sequential batch reactors using a counter-current flow of reactants	[79]