From Organic Wastes to Bioenergy, Biofuels, and Value-Added Products for Urban Sustainability and Circular Economy: A Review

Abstract

Energy is a crucial factor for urban development. Cities have a crucial role in climate change, as they use 2/3 of the world’s energy, producing 70% of greenhouse gas (GHG) emissions. In order to reduce the large ecological footprint of the utilization of conversional energy sources (coal, gas, and oil) and enhance a nation’s energy independence (security), it is crucial to find alternative fuels. Biomass residues are characterized as a sustainable and carbon-neutral energy source. Hence, this review describes a critical assessment of not only the quality characteristics of several waste and biomass residues for bioenergy production and biofuels but also the value-added products that could be produced from wastes to enhance industry (e.g., pharmaceutical, cosmetics, packaging industry, etc.). Furthermore, the challenges and potential solutions of waste utilization for bioenergy production and the transformation of value-added products for urban sustainability are also explored. Despite the high-quality characteristics and the availability of these wastes, several critical factors should be taken into account. Biomass residues could contribute to sustainable development goals (SDG), such as sustainable cities and communities, clean energy, responsible consumption and production, the economic growth of a country, and, as a result, urban development.

1. Introduction

The management of solid waste (e.g., food waste, organic waste, paper/cardboard, textiles, plastic non-biodegradable waste, etc.) in an urban society is a challenge and a global concern, even today. In addition, the management of biomass waste resulting from agriculture, animal husbandry (biodegradable substrates—manure), and industry (e.g., spent brewer’s grains, winery, low-quality compost from municipal solid waste, etc.) remains an essential unsolved issue. The growing volume and complexity of waste management is a major concern for society, with many negative effects on the environment and society. It is imperative to develop environmentally friendly waste management techniques in order to assess the environmental and socioeconomic impacts of solid waste. Substantial energy and resources are needed to create new products from scratch.

The utilization of the above-mentioned waste constitutes a promising renewable, sustainable source of energy for every city, when combined with modern technologies to optimize their conversion into sustainable energy and value-added products and technologies to reduce gas emissions. In addition, the transition of the above-mentioned wastes to new value-added products (e.g., in the pharmaceutical industry, etc.) is an important tool for the economic development of an area. The practices of waste to energy (WtE) and waste to value-added products can minimize the ecological impact and reduce the needed costs for the production of a new product.

Moreover, due to the continuous growth of the population, industrialization, technological evolution, and urbanization, global energy consumption has significantly increased. Bioenergy significantly contributes to sustainable energy transitions and climate targets. For urban development and the reduction of the ecological footprint of the cities, it is crucial to implement the Sustainable Development Goals, SDGs (7: affordable and clean energy, 11: sustainable towns and urban areas, 12: attentive consumption and production, 13: climate action) [1], circular economy, WtE, and zero waste practices.

Biomass waste has emerged as a promising source of sustainable energy, as it does not affect land, food, or water needs. Physicochemical as well as biological methods of energy production from biomass have developed, particularly in recent years.

Several studies have been conducted regarding the energy recovery of municipal solid waste (MSW), biomass wastes, animal manure, and sludges. Zhou and Zhang [2] suggest incineration compared to landfills in the Greater Bay Area of China, as they estimated that by combusting MSW, it could generate 31,346 GWh electricity by 2030 and 77,748 GWh by 2060. Mahmudul et al. [3] explored Australia’s potential for sustainable energy production from household food waste, finding that processing 10% FDW could produce 1.22 to 35.4 GWh/year, reducing GHG emissions and positively impacting the nation’s economy. A recent study showed that agricultural biomass residues (such as maize stalk/husk, rice husk/straw, wheat straw/husk, sweet potato peelings, groundnuts shells/husks/straw, straw beans, banana stem, peels, leaves, sugar cane bagasse, tops/leaves, coffee and cocoa husks, cotton stalk) and MSW could cover a significant part of energy demand (580 PJ and 26 PJ, respectively, in 2020, in Cameroon) and mitigate climate change through the CO₂ capture (1,600,000,000 kgCO₂) of biogas production [4]. For the first time in published research, Jin et al. [5] showed how food waste can be effectively converted to acetone-butanol-ethanol (ABE) through continuous integrated cell fermentation, even in the face of low yield and expensive feedstock. A novel, efficient method for recovering sludge energy (using free nitrous acid) into medium-chain fatty acids was performed [6]. Cevrim and Caner [7] studied the biomass source potential in Erzurum (Turkey) from animal wastes between 2002 and 2021. The findings demonstrated that it could collect 9,582,132 tons/year of manure and produce 251,977,679 m³ CH₄/year. Another study [8] showed that by treating rice straw with CO₂ nanobubbles, anaerobic digestion (AD) can increase methane production. In addition, membranes (porous and non-porous) are a successful viable method for purification, separation, and reaction in the synthesis of biofuels, which enhances process intensification, while recovering catalysts and alcohol improves process costs and promotes a circular economy. Heat and power generation from rice straw and microalgae and syngas production with CO₂ capture using gasification and solar energy were investigated [9]. The Russia–Ukraine war has caused changes in the energy system’s configuration. The Energy Research Center (Ukraine) states that there are two approaches to improving system stability: to find and send all equipment that is readily available to Ukraine, especially transformers, and to outfit communities’ vital infrastructures with alternative energy sources, WtE, to meet the necessary thermal and electrical energy. The results revealed that the wastes that are used for bioenergy production in Ukraine are limited, but there are some successful practices in Ukraine of waste management (Illintsi Territorial region, Vinnytsia) that should follow other communities to achieve the integration of the National Waste Management practices in Ukraine 2030, developing bioenergy recovery guidelines from wastes. This study proposed that organic unused wastes be used for biogas and Refuse Derived Fuel (RDF) fuel production to boost the community’s energy independence, reduce GHG emissions, and reuse these wastes for new sustainable value-added products (e.g., fertilizers) [10].

A recent review of methods for converting various organic wastes into energy and new products is rare in the literature. There is a need to bring together the recent progress of various methods of converting waste into useful products and energy.

Therefore, the goal of this work is to investigate the potential applications of solid organic wastes (biomass residues, MSW, animal manure, and their combination) for bioenergy recovery and production of several sustainable value-added materials with a literature review (from 2000 to 2024) of the current waste management practices of the cities, from these wastes using several traditional and new methods (such as gasification, pyrolysis, anaerobic digestion, immobilized-cell fermentation, dark fermentation, transesterification, hydrothermal carbonization (HTC), co-hydrothermal carbonization, combustion, co-combustion, digestion, co-digestion, liquefication, co-liquefication, pyrolysis, co-pyrolysis, co-torrefaction, etc.) in order to achieve energy security, green biofuel for heating, cooling and for vehicles, eliminate gas emissions and wastes volume of a country, implementing simultaneously a better waste management system (circular economy, waste to energy, zero waste practice) and SDG goals for a sustainable urban development and environment protection. Additionally, the paper explores recent technological achievements, such as integrated systems, nanomaterials, green catalysts, and additives for synergistic effects, and the consequences of such methods. This review will shed light on technological methods supporting a long-term plan for converting waste into sustainable bioenergy and value-added products.

2. Methodological Approach

The Scopus database was used in the current review study. The search string was refined for the ‘Article’, ‘Review’, Book chapter’, and ‘Book’ document types between 2020 and 2024 (18 February 2024).

Exclusion criteria: Document type: conference paper, conference review, editorial, letter, erratum, note, short survey, reports, retracted, data paper, review, thesis, Timeframe: before 2020, Language: other than English, Study focus: other than energy recovery from biomass residues and solid wastes.

Applied search keywords: energy recovery, bioenergy, wastes, biomass residues, energy production. Excluded keywords: wastewater, battery, seawater.

Subject areas: energy & environmental science.

Scopus advanced query: (TITLE-ABS-KEY (energy AND recovery) OR TITLE-ABS-KEY (bioenergy) AND TITLE-ABS-KEY (wastes) AND TITLE-ABS-KEY (biomass AND residues) OR TITLE-ABS-KEY (energy AND production) AND TITLE-ABS-KEY (sustainable) AND NOT TITLE-ABS-KEY (wastewater) AND NOT TITLE-ABS-KEY (battery) AND NOT TITLE-ABS-KEY (seawater)) AND PUBYEAR > 2019 AND PUBYEAR < 2025 AND (LIMIT-TO (LANGUAGE, “English”)) AND (EXCLUDE (DOCTYPE, “cp”) OR EXCLUDE (DOCTYPE, “cr”) OR EXCLUDE (DOCTYPE, “ed”) OR EXCLUDE (DOCTYPE, “sh”) OR EXCLUDE (DOCTYPE, “ab”) OR EXCLUDE (DOCTYPE, “rp”) OR EXCLUDE (DOCTYPE, “no”) OR EXCLUDE (DOCTYPE, “le”) OR EXCLUDE (DOCTYPE, “er”) OR EXCLUDE (DOCTYPE, “tb”) OR EXCLUDE (DOCTYPE, “dp”) OR LIMIT-TO (DOCTYPE, “ar”)) AND (EXCLUDE (SUBJAREA, “MATH”) OR EXCLUDE (SUBJAREA, “COMP”) OR EXCLUDE (SUBJAREA, “PHYS”) OR LIMIT-TO (SUBJAREA, “ENER”) OR LIMIT-TO (SUBJAREA, “ENVI”)).

3. Waste to Bioenergy, Biofuels, and Value-Added Products

3.1. Bioenergy, Biofuels, and Value-Added Products from Urban Wastes/Municipal Solid Wastes

Organic solid waste is a promising source for circular economy principles that promote conversion into bioenergy and sustainable new value-added materials. Using empirical models (Landfill Gas Emissions Model, LandGEM 3.02) and several statistical data of the Kanuru (in Vijayawada landfill, solid wastes from Nandigama, Tiruvuru, and Vijayawada), such as the MSW generation and population (0.569 kg/capita/day in 2021 with 1,396,853 population, and 0.81 kg/capita/day with 6,328,761 population in 2040), the calorific value of the MSW (year 2016: 3216 kcal/kg), the ash content (range of 30.2 to 47.7%), and the moisture content (27.4–51.9%). Ramprasad et al. [11] estimated methane and CO₂ emissions and calculated the potential H₂ generation for clean energy. A maximum of 43.3 Gg/y of renewable H₂ during 2042 might be produced at the Kanuru landfill site using the steam reforming reaction. In landfills, a gas recovery system with an efficiency of about 80% is crucial for energy recovery.

Dodo and Ashigwuike [12] performed a comprehensive analysis of the physicochemical properties and grid electricity prospects of Abuja’s (Nigeria) MSW. According to this study, more than 69% of the 257,500 tons of MSW (with an average net calorific value: 18.1 MJ/kg) produced annually (0.53 kg/person/day) can generate energy, power, and grid power of 2274.42 MWh, 28.43 MW, and 19.19 MW, respectively, and save 67.5 million metric tons/year of CO₂ emissions.

Vasileiadou et al. [13] performed a comprehensive analysis regarding the combustion features, highest amount of gas emissions, empirical chemical formulas, secondary waste formation, fouling and slagging, kinetic and thermodynamic analysis, and several case study scenarios for energy that could be covered, in Greece and in Europe, of 4 different MSW: food waste (FDW), green waste (GNW), paper waste (PAP), and organic fraction of MSW (OFMSW). These wastes were examined separately and in 12 different blends of lignite (LIGA). Additionally, several prediction models have been developed and compared with the literature. Raw MSWs revealed better quality characteristics (gross calorific value, GCV:FDW: 18.9 MJ/kg > OFMSW: 16.6 kJ/kg > PAP: 16.0 kJ/kg > GNW: 12.2 MJ/kg > LIGA: 12.7 MJ/kg, ash content per produced megajoule: FDW: 0.0025 kg/MJ > PAP: 0.0042 kg/MJ > OFMSW: 0.0067 kg/MJ > GNW: 0.0144 kg/MJ > LIGA: 0.0307 kg/MJ, lower maximum CO₂/MJ, NO/MJ and SO₂/MJ emission factors per produced megajoule, MJ) than lignite (reference sample). The findings demonstrated that MSW as a raw alternative fuel and MSW blended with lignite are both viable alternative options for energy production. In addition, these wastes could cover a significant amount of energy demand: 0.91 Mtoe/year, up to 5% in Greece and 36.8 Mtoe/year, up to 12.1% in Europe, respectively. (Basic used variables: waste generation: Europe 1.18 kg/capita/year in 2017, 1.30 kg/capita/year in 2030, and 1.45 kg/capita/year in 2060. In Greece, it was 503.7 kg/capita/year in 2017, and it is expected to be 491.1 in 2030, and 547.7 in 2060. The primary energy production (in 2017) was 758.2 Mtoe/year in Europe, and 7.5 Mtoe/year in Greece. The population (in 2017) was 511.8 million in Europe and 10.77 million in Greece, and it is expected to increase by +13% in 2030 and +36% in 2060). However, further studies should be performed on the denitrogenation process and the reduction process of chlorine. Cesaro et al. [14] used dark fermentation and formic acid pretreatment of the OFMSW (collected from an Italian municipality) for increased energy production and biochemicals. Results showed that H₂ yield was 31.6 mL/gVS when OFMSW was pretreated with 5% formic acid accumulating metabolites (e.g., acetic acid, butyric acid), and ethanol was recovered. The study reveals that acid concentration significantly influences the biological conversion of OFMSW, and adjusting operating temperature and treatment time can optimize processes for sustainable energy carriers or building blocks.

AD of energy crops, residues, and waste is gaining interest for its potential to reduce GHG emissions and promote sustainable energy development. Food waste is organic waste originating from a variety of places, for instance, food processing facilities, restaurants, households, and commercial and institutional establishments. Oliveira et al. [15] assessed the effect of forced continuous aeration pretreatment and aerobic storage time on food waste’s biochemical methane potential. The finding illustrates a rise in CH₄ yield concerning the total volatile solids (TVS) of FDW (425 NmL CH₄/g TVS) compared to samples without pre-processing (375 NmL CH₄/g TVS). After applying forced and continuous aeration pretreatment to food waste for 4 days, produced 456 NmL CH₄/g TVS for the leachate, which is 1.22 times more than when food waste was not stored. One alternative way to boost CH₄ production from food waste is to apply an aeration pretreatment prior to AD. Another study [16], transformed FDW into multiple products using 2 anaerobic processes: 1. anaerobic fermentation (AF) at 55 °C, pH 5.8, 20.1 d hydraulic retention time, and then 2. AD and open mixed cultures. Sustainable production was made possible by this multiproduct strategy by producing bioproducts (ethanol and short-chain fatty acids) and bioenergy (CH₄ and H₂). Since Australia does not yet have as many large-scale AD plants as those in the USA and Germany, Mahmudul et al. [3] studied the viability of clean energy generation using the AD process from domestic FDW in Australia. Between 2008–09 and 2017–18, Australia’s primary energy consumption increased by an average of 0.9%, from 5843.6 PJ to 6171.7 PJ. Using only 10% FDW from Australia could produce 1.22 GWh–35.4 GWh of energy per year (0.54–15.7 million AUD) with a reduction of about 639,850 tons GHG emissions. Therefore, FDW is a very effective source of sustainable energy that could also have a positive impact on the nation’s economy and GHG emissions.

Jin et al. [5] successfully converted FDW to acetone, butanol, and ethanol (ABE) using continuous immobilized cell fermentation, overcoming challenges like high feedstock costs and reduced efficiency, using Clostridium saccharoperbutylacetonicum deltptabuk as raw material. The reunification high-butanol generation strain Clostridium saccharoperbutylacetonicum deltptabuk was used as feedstock, achieving 19.65 g/L ABE with 0.43 yield and 4.56 g/L/h with 23-fold better productivity compared to batch fermentation.

The findings of Phuthongkhao et al. [17] showed that paper sludge waste could successfully be converted to carbonaceous hydrochar via HTC under controlled conditions. According to the results, paper sludge waste can be transformed into high-quality solid fuel and substituted for lignite.

Other study [6], presented a novel efficient method of recovering sludge energy (using free nitrous acid) into medium-chain fatty acids. Applying 1.78 mg N/L without nitrous acid pre-processing to sewage sludge (SS) resulted in a maximum medium-chain fatty acids (MCFA) yield in the anaerobic fermentation of the sludge that was found to be 10.6 times higher than the control. Appropriate doses of free nitrous acid pretreatment (0.71 to 1.78 mg N/L) greatly increased the carbon flow from sewage sludge into MCFA in the fermentation system. However, because of its toxicity to living cells, its direct addition significantly reduced the generation of complete materials (e.g., complex alcohols, carboxylates) in sludge (dropping to 8.3%–13.9%). The outcomes are encouraging for achieving sustainable SS utilization. In addition, SS could be used to enhance the production of SCFAs through pretreatment using ferrate strengthened with percarbonate [18]. About 3670 mg COD/L SCFAs under ideal pretreatment (enhanced 551% compared to the control). As valuable renewable energy and chemical sources, SCFAs highlight the significance of optimizing SCFA production for viable waste management practices.

Several methods have been used in order to evaluate the quantity of MSW in a country. For instance, Alidoosti et al. [19] proposed a model (numerous objective optimization based on mixed integer nonlinear programming) for a viable MSW system to optimally extract various bioenergies that take into account all three dimensions of economic, environmental, and social sustainability under uncertain conditions. A variety of techniques based on interactive fuzzy programming were used to address uncertainty in this network. The data was collected from Arad Kooh in Iran, and the General Algebraic Modeling System (GAMS) modeling language was utilized. The suggested solution method resulted in the production of bioenergy through treatment technologies.

In general, it can be concluded that the quantity and quality of MSW physicochemical characteristics are increased in countries with a lower–middle income (e.g., Greece) than in lower income economies countries (e.g., Nigeria). Moreover, the MSW could contribute to covering a considerable percentage of energy demand. In addition, the quality (type) and the quantity of the acid treatment that is implemented in the anaerobic digestion (or anaerobic fermentation) process of MSW for CH₄ production (or for MCFA products) are crucial factors and affect the conversion process, and as a result, the biofuel (or value-added products) yield. In addition, total management (collection, storage, pretreatment, conversion to bioenergy or to ABE products) of domestic FDW should be implemented by municipal local authorities in countries that have not yet implemented a sustainable waste management system for sustainable cities. Last but not least, sewage sludge management (e.g., anaerobic fermentation) could contribute to short- and medium-chain fatty acids (SCFAs and MCFA).

Table 1. Municipal solid wastes to sustainable bioenergy, biofuels, and value-added products.

Biomass Residue Source	Method	Enhance Treatment/Additives	Biofuel/Result	Refs.
MSW	Hydrogen production, steam reforming reaction (Vijayawada landfill, solid wastes from Nandigama, Tiruvuru, and Vijayawada), 0.569 kg/capita/day in 2021 with 1,396,853 population, 0.81 kg/capita/day and 6,328,761, in 2040, 3216 kcal/kg, Landfill Gas Emissions Model, LandGEM 3.02	-	Maximum of 43.3 Gg/y Hydrogen production during 2042	[11]
MSW	Physicochemical analysis, grid electricity prospect of Abuja’s, average net calorific value: 18.1 MJ/kg, 0.53 kg/person/day (Nigeria)	-	Energy, reduced CO2 emissions, More than 69% of 257,500 tons/y could produce energy, power, and grid power of 2274.42 MWh, 28.43 MW, and 19.19 MW, respectively, and save 67.5 million metric CO2 t/year.	[12]
MSW	Combustion, all cities in Greater Bay Area of China, 2 scenarios 15% and 30% efficiency		Best: 31,346 GWh by 2030 & 77,748 GWh by 2060 electricity	[2]
4 types of raw MSW: FDW, GNW, PAP, & OFMSW were examined separately	Physicochemical, kinetic, thermodynamic, environmental impact, modelling, energy cover for Greece and Europe, empirical chemical formulas, Maximum potential emission factors, waste generation: Europe 1.18 kg/capita/year in 2017, 1.30 kg/capita/year in 2030, and 1.45 kg/capita/year in 2060. In Greece, 503.7 kg/capita/year in 2017, 491.1 in 2030, and 547.7 in 2060. Primary energy production (in 2017): Europe 758.2 Mtoe/year, Greece 7.5 Mtoe/year. Population (in millions, 2017): Europe 511.8, Greece 10.77 (+13% in 2030, +36% in 2060)	-	Enhanced energy GCV (avg. 15.9 MJ/kg): FDW: 18.9 MJ/kg > OFMSW: 16.6 kJ/kg > PAP: 16.0 kJ/kg > GNW: 12.2 MJ/kg > LIGA: 12.7 MJ/kg, reduced ash/MJ: FDW: 0.0025 kg/MJ > PAP: 0.0042 kg/MJ > OFMSW: 0.0067 kg/MJ > GNW: 0.0144 kg/MJ > LIGA: 0.0307 kg/MJ, reduced CO2, NO and SO2 emissions per MJ, could cover energy demand (0.91 Mtoe/y, up to 12.1% in Greece. 36.8 Mtoe/y, 5% in Europe), a sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[13]
MSW	Model (multi-objective possibilistic mixed-integer non-linear programming), interactive fuzzy programming was used to address uncertainty in network, General Algebraic Modeling System (GAMS) software Arad Kooh, Iran	-	Economic, environmental, and social sustainability	[19]
Organic fraction of municipal solid waste	Dark fermentation	Formic acid pretreatment	H2 yield 31.6 mL/gVS with 5% formic acid pretreatment, biochemicals	[14]
FDW	Biochemical methane potential	Forced continuous aeration pretreatment	CH4, Enhanced TVS of pretreated FDW: 425 NmL CH4/g TVS (samples without pre-processing: 375 NmL CH4/g TVS	[15]
FDW	Continuous immobilized-cell fermentation	-	ABE, 19.65 g/L ABE, 23-fold better productivity compared to the batch fermentation	[5]
FDW	2 anaerobic processes (anaerobic fermentation, AF and then anaerobic digestion, AD) & open mixed cultures	-	Bioproducts (ethanol and Short-chain fatty acids) and bioenergy (CH4 and H2)	[16]
Household FDW	6 states, Australia’s potential for sustainable energy, primary energy production 2017–18: 6171.7 PJ, population (2017): 24.4 million, AD, in 2016–17: 2.5 Mt of domestic FDW, 2 kWh of electricity and 7.7 MJ of heat can be generated per m3 of biogas	-	Energy, reduced CO2 emissions, using 10% FDW from Australian could produce 1.22 GWh–35.4 GWh/y, reduction of about 639,850 tons GHG emissions	[3]
Paper sludge wastes	HTC under controlled conditions	-	High quality carbonaceous hydrochar, substitute the use of lignite	[17]
SS	Fermentation, and several analytical methods TSS (total suspended solids), VSS (volatile suspended solids), etc. (Tianjin, China)	Ferrate strengthened with percarbonate pretreatment	Increased SCFAs production (under optimal pretreatment: 3670.2 mg COD/L SCFAs production)	[18]
SS	Novel efficient method using free nitrous acid	Free nitrous acid pretreatment	Medium-chain fatty acids	[6]

presents the main results of the abovementioned studies that transformed MSW into bioenergy and value-added products.

3.2. Bioenergy, Biofuels, and Value-Added Products from Solid Animal Waste (Manure)

Biogas production via anaerobic digestion presents several economic and sustainable environmental benefits. Several studies use AD to produce biogas, and they are presented below.

A spatial and economic analysis of biogas production in Khyber Pakhtunkhwa’s southern areas (Pakistan), originating from livestock farm manure, was performed [20]. According to the analysis, it is feasible to build 868 biogas plants, which would produce 909.34 MW_e electric power. A large number of biodigestors are used in China for residential use. Another study [21] showed that the hydrothermal liquefaction (HTL) of beef cattle manure could increase bioenergy production in limited feed varieties.

Cevrim and Caner [7] studied the biomass source potential in Erzurum (Turkey) from animal wastes between 2002 and 2021. The results showed that it is possible to collect 9,582,132 tons/year of manure and produce 251,977,679 m³ CH₄. The potential energy from animal waste could cover 96% of Erzurum province’s annual electricity consumption.

Latifi et al. [22] studied the potential for methane recovery (by AD) from the organic matter of Iranian slaughterhouse wastes (SHW). The results showed that about 111 million m³ per year of CH₄ could be produced by combined heat–power (CHP) plants (about 1000 GWh of electricity) and reduce Iran’s CO₂ emissions by 482,000 tons.

Madrigal et al. [23] studied the impact of biochar made of animal manure (bovine) as a sustainable additive to enhance the AD of cheese whey (CWP). Bovine manure and pure CWP contain increased levels of organic substances, which, if released into the environment, could pose serious environmental problems. AD can mitigate these effects. More than 163 mL CH₄/g VS_add impeded from direct AD of CWP. Adding biochar seems to be one method of avoiding the inhibition of AD. The results showed that the biochemical methane potential (BMP) of the CWP and 2 g biochar/gVS base revealed 358 mL CH₄/g VS_add.

Filho et al. [24] evaluated the possibility of producing biogas from organic waste using BMP bench-test biodigesters at the Supply and Logistics Center of Pernambuco. The biogas produced by inoculating waste with ruminant manure was found to be the lowest, whereas treatments involving sludge and mixtures yielded the highest volumes. Based on the biogas generated, an estimated 359 kWh/d of electric power could be produced.

Ziala Village, Bangladesh, is well known for producing biogas and managing cow dung. Large volumes of organic residues and biogas are utilized as organic fertilizer and fuel in Ziala Village. Renewable energy transfer plants improve cooking environments, reduce firewood collection time, and preserve forest resources. This leads to improved environmental conditions and socio-economic profiles [25].

Sustainable and economically feasible activated carbons from biocollagenic waste (leather industry) were introduced by Cabrera-Codony et al. [26], through chemical activation at various temperatures and weight ratios in order to be used for biogas enhancement. The resultant microporous adsorbents had a maximum siloxane adsorption capacity of 500 mg/g, and favorable textural and chemical characteristics (total pore volume of 0.76 cm³/g, Brunauer–Emmett–Teller BET specific surface area: 1600 m²/g). These materials were found to be effective for applications that reduce gaseous pollutants.

Physicochemical parameters of abattoir waste, fecal sludge, and vegetable and fruit wastes were examined [27]. The outcomes demonstrated that these wastes could be utilized for enhanced soil for agricultural productivity and biogas production.

Generally, it can be concluded that hydrothermal liquefaction of animal manures could increase bioenergy production. Moreover, the potential energy from animal waste could cover a high percentage of the annual electricity consumption and reduce CO₂ emissions. Last but not least, slaughterhouse waste could contribute to enhanced bioenergy production. This type of animal waste is used in not fully industrial countries like Pakistan, Bangladesh, Turkey, and African countries, e.g., Burkina Faso. One important issue regarding such wastes is the possibility of extracting high value-added products before using them as fuel.

Table 2. Solid animal waste to sustainable bioenergy, biofuels, and value-added products.

Waste Source	Method	Enhance Treatment/Additives	Biofuel/Result	Refs.
Animal wastes (manure)	Biogas source analysis, in Erzurum (Turkey), between 2002 and 2021, biogas heating value: 22.7 MJ/m3, methane heating value: 36 MJ/m3, efficiency 35%, electricity consumption 919,749.00 MWhe	-	CH4 production, possible to produce 251,977,679 m3 CH4/y, cover 96% of Erzurum province’s annual electricity consumption	[7]
Livestock farm manure	Spatial and economic analysis	-	Feasible to build biogas plants	[20]
Beef cattle manure	HTL at 200–300 °C, 60 min	-	Increase energy recovery in higher temperature, biocrude oil: 30–35 MJ/kg	[21]
Slaughterhouse wastes (SHW)	Combined heat–power (CHP) plants	-	CH4 production and CO2 emissions reduction, ~111 million m3/y CH4 could produce into CHP plants (~1000 GWh electricity), and reduce Iran’s CO2 emissions by 482,000 tons.	[22]
Pure cheese whey (CWP)	AD	Bovine manure	Biochar/CH4, (358 mL CH4/g VSadd, 2 g biochar/gVS)	[23]
Inoculating waste with ruminant manures, sludge and mixtures	BMP bench-test biodigesters	-	Biogas production (359 kWh.d−1 of electric power could be produced)	[24]
Cow dung	Biogas plant, samples from 12 dairy farms, Ziala Village at Tala Sub-District in Satkhira		Biogas production	[25]
Biocollagenic waste (leather industry)	Chemical activation at various temperatures and weight ratios	-	Low-cost and sustainable activated carbons, microporous adsorbents.	[26]
Faecal sludge (FS), abattoir waste, and fruit and vegetable waste	Physicochemical analysis, in Bobo-Dioulasso, Burkina Faso (Africa)	-	Enhanced soil, biogas production (90% fecal sludge & 10% of fruits and vegetables waste: 29.4 L/kg of biogas, settled sludge and semi-solid material: 54.4 L/kg of biogas with 51% of CH4).	[27]

illustrates the main results of the above-mentioned studies that transomed animal wastes to bioenergy, biofuels, and value-added products.

3.3. Bioenergy, Biofuels, and Value-Added Products from Biomass Waste/Residues

Biomass waste transformation into sustainable fuels and biochemicals is a strategic approach to reducing fossil feedstock use, reducing waste accumulation, not depleting resources for food consumption, reducing environmental impact, lessening financial losses, and making a feasible transition to a sustainable society. Moreover, thermochemical analytical experimental methods (e.g., proximate analysis, gross calorific value determination, ultimate analysis, scanning electron microscopy/energy-dispersive X-ray spectrometry, thermogravimetric, and differential thermogravimetric analysis, etc.) are useful tools for the evaluation, characterization, and categorization of fuels. Moreover, the results of the thermochemical analytical experimental methods could help (in combination with other statistical data, such as energy consumption, population, etc.) to calculate the energy cover of a country. In the last two decades, there has been an increase in interest in the research community due to the energy crisis and environmental pollution. Researchers utilized these methods in order to find sustainable alternative biofuels from wastes. Several investigations have been conducted concerning this shift. Sustainable energy generation from olive stones (OLS) and extracted olive pomace (EOP) from an olive oil mill was studied by Vasileiadou et al. [28] with several thermochemical experimental analyses and several calculated methods in order to characterize these wastes for potential alternative solid biofuels and estimate the possible energy cover. Thermodynamic and kinetic modeling, maximum emission factors, and empirical chemical formulas were performed. Moreover, several case studies were examined for sustainable practices of oil (olive) industry solid wastes for potential energy cover. Solid mill wastes resulting from oil (olive) production revealed attractive properties as solid biofuels. In comparison to lignite, they have nearly twice the value on gross calorific value (~21 MJ/kg), eight times less ash production (<7 wt.%) (secondary waste), and lower activation energy, CO_x, NO_x and SO_x emissions. Moreover, they showed lower activation energy, which is translated to a faster combustion rate. The results of this study showed that these wastes could be used effectively as a main source for bioenergy production in small-scale applications or as auxiliary/secondary fuels (e.g., with lignite blends, substituting part of lignite with eco-friendly fuels) [28]. The combustion of 100% brewers’ spent grain and BSG co-combustion with lignite, in several proportions, were studied as alternative agro-industrial waste management and sustainable energy production [29]. More specifically, the author, via multiple experimental analyses (ultimate analysis, higher heating value analysis, proximate analysis, environmental impact analysis regarding secondary solid wastes, ash elemental analysis, thermogravimetric analysis and synergistic effect, chlorides, and sulfate sulfur determination, thermodynamic analysis, kinetic analysis, empirical chemical formulas determination, maximum potential CO₂, SO₂, and NO emission factors, case studies for sustainable management), suggested an alternative sustainable management of BSG for energy recovery, as new studies have shown that BSG should not be used as animal feed due to microbial activity. In addition, Vasileiadou [30] investigated grape marc (winery solid waste) as a potential sustainable biofuel and as a supplemental fuel for lignite. The study found that grape marc is a good potential substitute biofuel because of its superior fuel qualities (e.g., enhanced energy content: more than 19 MJ/kg, low activation energy: 65 kJ/mol, low ash content: less than 5.7 wt.%, low gas emission per produced megajoule: 98.5 gCO₂ per produced MJ, 0.5 gSO₂ per produced MJ, and 0.5 gNO per produced MJ, etc.). Moreover, the quality of the solid composite fuels was enhanced in blends (containing lignite in 30%, 50%, and 70%) as the proportion of grape marc increased. Forestry waste residues of breadfruit tree (BFT), Macaranga spp. (MCG), Acacia mangium (ACM), and fig tree leaves (FGL) of Malaysia were examined using several thermochemical analyses for energy recovery. According to the results, the ACM may produce more pyrolysis bio-oils and bio-gases, while the FGL’s large residual mass makes it the most suitable for producing biochar [31]. Şen [32] evaluated the possibilities for the generation of bioenergy for heating and CO₂ emissions implementation from the palletization of greenhouse post-harvest wastes (using leaf and stem wastes of pepper, tomato, and eggplant). Tomato, pepper, and eggplant pellets had a gross calorific values ranging from 17.3 to 17.8 MJ/kg. The findings of the pellet biofuels were compared with ISO and EU standards, as well as with sawdust. According to the findings, 10 tons/hectare of waste could be produced, which would provide about 50 MWh of energy. In addition, these wastes could produce over 6.5 tons/hectare CO₂ and be utilized in the greenhouse during the cold season for photosynthesis. Diaz et al. [33] examined the potential of biomass residues/wastes (such as grape residues, potato residues, tomato grape residues, potato residues, tomato residues, and banana resides and banana resides, resulting both from the fields and from industry) using statistical data for renewable energy sources in the Canary Islands. The calculated findings revealed that the annual production of these residues in the Canary Islands is about 235,000 tons (banana agriculture: ~111,000 tn/year, tomato agriculture: ~53,000 tn/year, potato agriculture: ~40,000 tn/year, and grape agriculture: ~30,000 tn/year) and could generate about 1.39 petajoule (PJ) of energy (~4.79% of the current energy usage in the Canary Islands). Another theoretical estimation of potential energy was performed by Fitri et al. [34] who studied several biomass residues of the West Nusa Tenggara region in Indonesia, such as corn kernels, corn and paddy straw, rice, coffee, and cacao. The overall calculation of the total energy of the analyzed agricultural wastes was found to be almost 42.4 PJ. Del Valle et al. [35] used one of the major rural areas in China to study straw management decisions, focusing on climate, agricultural kinds, agricultural situations, economic costs, social networks, government planning, and individual attributes. The results demonstrated that increasing the use of straw for bioenergy and sustainable development can be achieved through a variety of targets for straw utilization, balanced policies, and actions pertaining to agricultural mechanization, land consolidation, and spatial planning. The development and enhancement of crop straw management initiatives can be guided by these findings. Last but not least, Holmatov et al. [36] studied worldwide bioethanol production and GHG emission savings from crop-based lignocellulosic biomass by using 123 crop residues in 192 countries. 20 case studies (optimistic and realistic) were examined. The results showed that bioethanol production ranges from 7.1 to 34.0 EJ/year, which could replace 7 to 31% of oil products for transportation, saving 338 Mt to 1836 Mt of CO₂ emissions.

Direct combustion is a thermochemical method that burns biomass residue fuels in the presence of excess air. Brunerová et al. [37], investigate how Vietnam manages its sugarcane processing waste by using high-pressure briquetting technology (a hydraulic high-pressure briquetting press Briklis, type BrikStar 30-12), according to technical European Committee for Standardization standards (e.g., EN ISO 17831-2, EN ISO 18122, EN ISO 17225-1, EN 15234-1), to direct combustion. Reduced ash content (0.97 wt.%) and an increased calorific value (18.4 MJ/kg) were found, making it suitable for direct combustion practices. The bagasse’s structural performance was also positive, indicating its potential for energy recovery and sustainable technologies. Proto et al. [38] use a 30 kW combustor fitted with a multicyclone filtration system to study emissions behavior of different pruning residues (wood biomasses). Regarding emissions and energy yield, the results showed that olive residues were the most promising biomass. Wastes produced by fruits (jackfruit), such as seeds (JS) and peels (JP), have potential as a bioenergy source. Alves et al. [39] evaluated the suitability of combusting these wastes for sustainable energy generation. The combustion characteristics of JS and JP showed favorable burning performance, high heating value (~16.5 MJ/kg) and volatiles (75–81%), and reduced CO_x and SO_x radiations. This study contributes to the establishment of these wastes as sustainable energy sources.

Le et al. [40] studied the potential for sustainable fuel (bioethanol) and chemical production (lignin, silica, and nutrients) from paddy wastes (rice straw) at a pilot-scale biorefinery. Up to 96% silica and 79% pure lignin were obtained from the alkaline pretreatment of black liquor. The solid matter remaining after distillation served as a source of N₂ and was utilized for saccharification and fermentation in a manner analogous to corn steep alcohol with 1.6 weight percent ethanol production (160 h). The final liquid waste was repurposed for acidification. This zero-waste biorefinery model showed an energy efficiency of 0.53, encouraging the integration of the biomass industry and sustainable agricultural development.

Several biomass wastes (potato peel wastes, Conocarpus wastes, Eucalyptus pruning wastes, sugarcane baggage, microalgal biomass, banana peel wastes, maize stalks, pigeon pea) have been used in several studies conducted in recent years in pyrolysis (the breakdown of organic compounds without oxygen). Potato is one of the major cash crops worldwide, generating from 15 to 40 wt.% peel waste depending on the peeling method used (steam, abrasion, lye, etc.). Potato wastes are promising for bio-derived chemicals and biofuels. Daimary et al. [41] studied potato peels as a viable material for bio-oil, biochar, and a green catalyst for biodiesel production. The byproduct, biochar, can be converted into bio-based mixed metal oxides and carbonates. The optimal pyrolysis temperature for this process is 500 °C, resulting in high bio-char (~30%) and bio-oil (~24%) production. The synthesized catalyst revealed strong catalytic activity with a high potassium content, achieving a maximum oil conversion of 97.5% under optimized parameters (60 °C, 2 h, catalyst 3 wt.%, and 9:1 methanol:oil ratio). Large volumes of potato peel waste (PPW) are produced by the potato processing industry. This process turns potato waste into valuable, recyclable, and environmentally friendly products. Wallikhani et al. [42] studied bioenergy and biochar generation from eucalyptus and Conocarpus pruning residues using pyrolysis in Khuzestan (Iran) and found that by utilizing these wastes, they could generate 167,510 tons of biochar and 312 GWh of electricity. Biochar production from maize stalks and pigeon peas at different pyrolysis temperatures (400, 500, 600 °C) was studied [43]. Elevated pyrolysis temperatures lead to increased levels of essential nutrients (Ca, Mg, S, K, etc.) but lower N content. Lower temperature-produced biochar is suitable for controlling fertilizer nutrients and removing soil contaminants. Higher temperature-produced biochar results in materials that are comparable to environmental remediation, activated carbon, and reduced polycyclic aromatic hydrocarbons. Thermogravimetric analysis and kinetic investigation of sesame stalk non isothermal pyrolysis (nitrogen inert) were performed by Huang et al. [44] at a 5–20 K/min heating rate, 350–900 °C. The results showed an average activation energy of 140 to 150 kJ/mol for stage II (629–800 K) and 125–136 kJ/mol for stage I (450 to 629 K). The results of the study indicate that waste from sesame stalks is a suitable renewable feedstock for producing bioenergy, and the findings of the kinetics and thermodynamics modeling calculations may also be used to develop future pyrolysis reactors that are appropriate for using sesame stalk biomass.

Catalytic pyrolysis has been used by several researchers. More specifically, sugarcane baggage (SCB) was studied [45] using several analytical methods (e.g., Pyrolysis Gas Chromatography/Mass Spectrometry Py-GC MS, thermogravimetric analysis TGA, X-ray Fluorescence XRF, and X-ray fluorescence) to determine the physicochemical properties of SCB. Different mechanisms were involved in catalytic pyrolysis, and the SCB results evidenced that the activation energy changed as the transition coefficient shifted. When zeolite catalysts ZSM-5 was added at a weight percentage of 20%, the amount of furfurals and hydrocarbons enhanced by about 4 wt.% and 3 wt.%, respectively, while acid and phenols decreased by about 9 wt.% and 15 wt.%, respectively. The results showed that the characteristics of the pyrolysis products were enhanced by the addition of zeolite catalysts, and oxygenated compounds decreased. Under ideal circumstances, up to 2.4 g/L of bioethanol was found. Lee et al. [46] synthesized a biochar-nickel composite using Microcystis aeruginosa waste, a toxic microalgal biomass, as a catalyst for syngas production in CO₂-feeding pyrolysis.

Kwon et al. [47] investigated the pyrolysis of banana peel waste to improve waste management and energy recovery. Carbon dioxide was utilized instead of N₂ environment, and syngas production was examined. Pyrolysis using CO₂ accelerates the thermal cracking of volatile pyrolysates, promoting CO formation at 420 °C. By using no catalyst, this method also increases the biocrude aromaticity and dehydrogenation of liquid pyrolysates. According to this study, pyrolysis using CO₂ is an ingenious and strategic thermochemical practice for valuing household scraps.

Qureshi et al. [48] studied cotton stalk acidified in 5 wt.% hydrochloric acid (HCL) (to eliminate cotton stalk’s undesired amorphous material) to generate clean energy using pyrolysis and combustion at 4 leaching times from 0–180 min, under N₂ environment from 0–500 °C, and air from 500–900 °C. Several experiments (e.g., ultimate analysis, X-ray diffraction XRD, proximate analysis, scanning electron microscopy and energy dispersive X-ray spectroscopy SEM-EDS, kinetic analysis, Fourier Transform Infrared Spectroscopy FTIR, energy content, thermogravimetric, etc.) were performed at various rates of procedure. The results showed that cotton stalk leaching for 180 min perfectly infused HCL and helped to increase gross calorific value and fixed carbon. Moreover, thermogravimetric analysis showed increased conversion efficiency. According to SEM results, the acidulation process was enhanced, which resulted in larger pores.

Hydrothermal carbonization (HTC) is a promising thermochemical conversion method as it transforms biomass with high moisture content (wet biomass) into bioenergy and biochemicals under hot, compressed water without a pre-drying process. Yard wastes, wood residues, and industrial wastes (agave bagasse, apple wastes, and BSG) were used in the HTC process recently. More specifically, Zhang et al. [49] used wet biomass waste (yard waste) via HTC with and without N₂ pressurization to improve carbon storage in hydrochar using less water. Improvements in carbon usage and energy storage were achieved by decreasing the amount of process water, which also boosted the hydrochar’s calorific value (26.3 MJ/kg) and carbon recovery (~60%). Moreover, the pressurization enhanced hydrochar stability but reduced yield and increased production costs. In a zero-residue bio-economy, HTC was found [50] to be a profitable method for recovering industrial apple waste. Retaining energy (82 to 96%) and carbon (80 to 93%) in solids creates CO₂ neutral solid fuels with an energy content of 30 MJ/kg. Solid by-products enhance soil quality and generate aqueous streams with saturated fatty acids and phenolic compounds, which are economically advantageous. Weber et al. [51] compare HTC and steam explosion (SE) for the pretreatment of beverage industry semi-solid wastes, brewers’ spent grain (BSG), and agave bagasse for biogas production. The CH₄ yield was 162 to 173 mL/g COD (Chemical Oxygen Demand) for HTC preprocessing, and 316 to 362 mL/g COD for SE preprocessing. Agave bagasse yielded the maximum COD in the liquid stage when HTC was applied.

Another study [52] investigated the conversion of wood residue into biofuels using HTL and found that pretreatment with 4% NaOH boosted recovery and glucose yield (90 g/L) compared to unwrought wood. During the pretreatment stage, glucose produced 246 kJ of energy, and during the liquefaction stage, the Net Energy Ratio was 63% for glucose. These findings could help find new waste conversion techniques for sustainable biofuel production using continuous flow HTL.

Transesterification of oil extracted from biomasses is one of the most common practices for biodiesel production. Grain bran is an agricultural residue produced from rice processing. Most of the studies used KOH as an alkanine catalyst for transesterification. Lourenço et al. [53] used vegetable oil from grain bran using homogeneous basic transesterification. Except for kinematic velocity, the produced biodiesels met EN (European Standards), ASTM (American Society for Testing and Materials), and ABNT (Brazilian National Standards Organization) standards. Further studies should be performed in order to use rice oil for biodiesel production. Ash and indium oxide (In₂O₃) were also used as catalysts in biodiesel production. Bananas pseudostem was used [54] for biochar production, which was further processed for heterogeneous catalyst synthesis or sustainable high alkaline solution preparation. Waste cooking oil was converted into biodiesel using synthesized ash as a heterogeneous catalyst. About 97.6% of the leftover cooking oil was converted into FAME, and the lignin recovery was ~43 wt.%. The hydrothermal reaction was utilized to recover lignin from bamboo leaves using an ash-based alkaline solution. This approach provides a sustainable way to transform waste into biochemicals and biofuels. A current study [55] using a membrane reactor to generate viable biodiesel from inedible Cordia myxa seed oil (37 wt.%), and transesterification was catalyzed by a green heterogenous indium oxide (In₂O₃) nano catalyst with Boerhavia diffusa leaf extract. The biodiesel yield reached 95 weight percent (7:1 methanol to oil, 0.8 wt.% catalyst, 82.5 °C, 180 min.). The catalyst also showed reusability for up to 5 transesterification cycles. The biodiesel was found to contain 5, 8-octadecenoic acid (as the main fatty acid methyl ester, FAME), as detected using nuclear magnetic resonance and Fourier transform infrared spectroscopy. The biodiesel fuel quality characteristics were found to be similar to EN-14214 and ASTM D 6571 standards. Due to their multiple benefits (widely available, non-corrosive, non-toxic, biodegradable, low-cost material, and eliminating wastewater production), biomass-derived heterogeneous catalysts have been examined extensively in recent years. In other words, these green catalysts aid in overcoming the drawbacks of traditional catalysts, like toxicity, leaching, microporosity, environmental unfriendliness, and a lack of active sites.

Several biomass wastes, such as pine needle forest biomass, rice straw, lime fruit wastes, organic residues generated in grasslands, açaí seeds, hemp, watermelon, sugarcane wastes, and palm agro-industrial residues (date cake, trunk, leaves, pedicels, seeds, and leaf sheath), were used in the biochemical conversion of anaerobic digestion. More specifically, Mahajan et al. [56] investigate the structural alterations in lignocellulosic complex in pine needle forest biomass after preprocessing, such as acid-base-acid treatment, steam explosion, and milling. The results showed that AD of the pretreated biomass revealed 21.4% enhanced CH₄ than those from untreated pine litter. Moreover, coalescent materials and lignin droplets that might be employed as possible nanocomposites were found to be present on the biomass surface. AD of rice straw using CO₂ nanobubble treatment enhances the degradation of amorphous cellulose, enhances the predicted factional enzyme by 14% in the process of hydrolytic acidification, and methane production by 4.2–7.8% [8]. After citrus is processed to extract juice, a huge amount of the fruit mass is disposed of as waste. This waste material has the potential to be an important source of cellulosic biomass for the recovery of bioenergy (biogas). However, the high concentration of soluble sugars in citrus wastes, particularly d-limonene, can hinder the conversion process. A study by Ogundare and Olukanni [57], found that pretreatment of lime fruit waste (using hexane as the solvent in a solid–liquid extraction practice) can decrease the impact of d-limonene. For a duration of 28 days, mesophilic conditions were used to digest pretreated and untreated lime waste substrates in batches. The study found that untreated lime waste revealed an almost 67 mL/g VS biogas yield, while the pretreated waste showed more than 93 mL/g VS (40% enhanced). The findings suggest a viable biogas recovery option for lime waste. The nitrogen content was high in citrus and grapevine. Therefore, it might be essential to use a deNOx system when olive waste shows high energy content and low emissions. Moreover, olive pruning residue is the most common wood fuel in Europe, holding significant opportunities for sustainable development. The organic residues generated in grasslands were studied by Achinas and Euverink [58] via a new simplified theoretical model in order to determine the theoretical biogas potential (AD process) for a better waste management system. The results showed that this method can be used effectively for several feedstock materials for dimensioning reactors in AD processes. High-rich methane biogas recovery was performed [59] using anaerobic digestion of açaí seeds (dry regime, mesophilic temperature). The findings showed that more than 6 L of biogas were generated, and the experimental methane yield was about 156.65 mL/g TS (greater compared to the theoretical one, ~116 mL/g TS). Hemp, watermelon, and sugarcane wastes were used separately in another study [60], as feedstock for bioethanol production using different treatments (pH, temperature). pH 4.5, and 35 °C are the optimum values for bioethanol generation. The study’s findings demonstrated that producing bioethanol from lignocellulosic biomass through biological transformation is an environmentally friendly practice that could be used instead of petroleum products. According to [61], a biorefinery platform can be used to manufacture second generation ethanol, methane, and lignin on a commercial scale by using different date palm agro-industrial residues, such as the date cake, trunk, leaves, pedicels, seeds, and leaf sheath. Liquid hot water, catalyzed ethanol organosolv, and ethanol organosolv pretreatments were used. The results showed a great capacity for ethanol, lignin production, and methane production. Another study [62], used BSG for bioethanol and biogas production, which resulted in enhanced ethanol yield (45%) and enhanced biogas (raw BSG: 379 ± 19 mL biogas/g, defatted BSG: 235 ± 21 mL biogas/g, and stillage: 168 ± 39 mL biogas/g).

In order to produce bioethanol with NaOH (Sodium hydroxide) pretreatment, Ranjithkumar et al. [63] investigated the efficient use of 10 different textile mill wastes from cotton spinning. Cotton spring wastes were found to have a cellulose content of 55–86 wt.%. In optimal conditions (5 °C, 12% NaOH, 3 h), cellulose content was 98% and crystallinity reduction was 88%. Saccharification under ideal conditions produced a maximum of 65% ethanol at 60 h and 82% hydrolysis efficiency. Microalgae, tiny cell factories, are being explored as an alternative renewable energy source due to their sustainable lipid accumulation under nutrient starvation. Pardilhó et al. [64] conclude that marine macroalgae waste is a useful supplementary raw material for sustainable bioethanol production by thermal acid hydrolysis, studying 3 variables: acid concentration 0.1–2.5% v/v H₂SO₄, 10 to 60 min reaction time, and biomass:acid ratio of 5–15% w/v on efficiency and sugar concentration. Because the yeast did not consume all available sugars, the biomass:acid ratio increase resulted in a decrease in the bioethanol concentration. For the production of bioethanol from marine macroalgae waste, the condition involving a biomass:acid ratio of 10% (w/v) seems to be the most suitable, allowing both a higher biomass conversion yield to sugars and a higher bioethanol yield.

Lapo et al. [65] investigate the recovery of rare earth elements (REE) (Tb3+, Nd3+, Y3+, Eu3+, and Dy3+) utilizing banana wastes, primarily banana peel, pseudo-stem, and rachis. The adsorbent materials were studied using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, zeta potential, and scanning electron microscopy with an energy dispersive X-ray probe (SEM-EDX). The results showed that banana rachis had the most potent adsorption capacity. By utilizing ethylenediaminetetraacetic acid (EDTA) as the desorbing solution, 97% of the REE was recovered from the adsorbent, suggesting that banana rachis is a promising renewable bioresource with a high adsorption capacity and a moderate processing cost for recovering REE.

Using cutting-edge environmentally friendly solvents, such as natural deep eutectic solvents (glycerol-tartaric acid, xylitol-tartaric acid, glycerol-citric acid, and xylitol-citric acid) and acetosolv, and intense ultrasonication, sugarcane bagasse was sustainably recycled into nanofibrillated cellulose recently [66]. Without significantly losing cellulose, the combination of glycerol-citric acid and acetsolv produced the maximum solubilization of lignin and hemicellulose. A remarkable 84.4% yield of cellulose from sugarcane bagasse was produced. Using intense ultrasonication helped to successfully disintegrate all of the extracted cellulosic fibers, accelerating the production of nanofibrillated cellulose (≥90%).

Finore et al. [67] explore sustainable methods for producing hemicellulolytic catalysts from Thermoanaerobacterium thermostercoris strain buff, using industrial processing residues of tomatoes, fennel, potato, and carrot as carbon sources. Moreover, the bioconversion reaction of the stems and leaves of cardoons and the polymer fraction from the rhizome of giant reeds was examined using hemicellulolytic enzymes derived from T. thermostercoris. The findings demonstrated that residual biomass from the agricultural industry could be used to produce T. thermostercoris cell biomass and its cellulolytic enzymes in a sustainable manner. Furthermore, it was found that T. thermostercoris could produce biohydrogen and bioethanol directly from raw vegetable wastes, such as the residues of energy crops.

The continuous production of hydrocarbon fuels from waste oils (including linseed, cottonseed, olive, palm, and sea buckthorn oils) through the photoenzymatic decarboxylation of free fatty acids was studied [68]. Photoenzymatic decarboxylation (based on visible light) is a technology at mild conditions that is an alternative to the hydrotreating of oils. At concentrations ranging from 8.97 to 15.3 mM, different oils were transformed into C1-shortened alka(e)nes. Achieving a 204.3 kJ/L/h energy production rate proved to be highly efficient in producing high-grade biofuel. The hydrolysis and decarboxylation of sustainable oil wastes in a cascade system without the addition of organic solvents resulted in the sustainable and continuous production of hydrocarbon fuel.

Generally, there are a huge amount of biomass wastes (crops, residues, forest, industrial biomass origin solid wastes, etc.) that can be transformed into bioenergy or other value-added products using several technologies (AD, HTC, gasification, pyrolysis, combustion, etc.). The scientific community still seeks the optimum treatments/additives or the optimum biomass waste blends for a positive synergistic effect (for capturing Cl and S in the ash and not releasing them as emissions) for enhanced bioenergy yield and reduced emissions. Catalysts such as nanocatalysts, zeolite catalysts, nickel/biochar composite catalysts, ash catalysts, and several pretreatment methods, e.g., using alkaline, HCL, NaOH, and H₂SO₄ seem to be some of the feasible options for dealing with this problem.

Table 3. Biomass wastes/residues into bioenergy, biofuels, and value-added products.

Waste Source	Method	Enhance Treatment/ Additives	Biofuel/Results	Refs.
Rice straw	Hydrolytic acidification of AD	CO2 micro-nanobubble, N2- & H2- nanobubble technology	Biogas/Methane (CH4), Nanobubbles (especially CO2 nanobubbles) enhanced H2 yield (74–94 times)	[8]
Olive stones (OLS) and lignite blends, extracted olive pomace (EOP) (olive oil solid wastes from the oil industry)	100% olive stones, 100% extracted olive pomace, and several blends with lignite, thermochemical analyses, thermal characterization, kinetics, thermodynamic analysis, and several scenarios for sustainable practices	-	Energy generation, sustainable management, energy cover, energy cover, low ash content (<7 wt.%), high GCV (~21 MJ/kg), and low activation energy. Sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[28]
BSG alone (raw), BSG blends with lignite in several mixes	Combustion, co-combustion, thermochemical methods, thermal analysis, proximate analysis, ultimate analysis, empirical chemical formulas, case studies for sustainable management, environmental impact, maximum emission factors, kinetic and thermodynamic analysis, ash elemental analysis	Raw BSG, BSG blends with lignite	Energy generation, alternative use of BSG, GCV: 19.05 MJ/kg, ash < 5 wt.%, max gCO2/MJ: 91.9, max gSO2/MJ:1 (lower than lignite), but max potential gNO/MJ emissions: 4.8 (higher than lignite), deNOx system maybe is needed, sustainable management, sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[29]
Winery solid waste: grape marc (GM)	Combustion, thermochemical methods, kinetic and thermodynamic analysis, ash analysis, etc.	-	GCV: 19.3 MJ/kg, ash: 5.74 wt.%, CaO: 75.5%, MgO:6.8%. promising alternative sustainable solid biofuels (enhanced fuel characteristics, low COx, NOx, SOx emissions per Megajoule)	[30]
Forestry waste residues breadfruit tree (BFT), Macaranga spp. (MCG), Acacia mangium (ACM), and fig tree (FGL) leaves	Thermochemical analyses	-	Energy recovery, CM for pyrolysis bio-oils and bio-gases, FGL for biochar production	[31]
Greenhouse post-harvest wastes (stem and leaves of pepper, tomato, and eggplant)	Palletization, thermochemical analysis	-	Bioenergy and CO2 for greenhouse during cold season to photosynthesis	[32]
Grape residues, potato residues, tomato residues, and banana residues	Potential of biomass residues/wastes and potential energy	-	Renewable energy production for the Canary Islands	[33]
Several biomass residues (paddy straw, rice, corn straw and kernels, coffee, cacao)	collected data from various sources: Statistics Indonesia, One Data WNT, Department of Agriculture and Plantation of WNT, Department of Energy and Resources, and journal articles	-	Renewable energy production for the West Nusa Tenggara region (Indonesia)	[34]
Straw	Study on management decisions in rural regions (China)	-	Bioenergy, and sustainable management	[35]
Non-crop based lignocellulosic biomass by using 123 crop residues in 192 countries	20 case studies optimistic and realistic	-	Bioethanol production, GHG emission saving	[36]
Sugarcane	Direct combustion process	High-pressure briquetting technology	Energy content 18.4 MJ/kg, ash 0.97 wt.%, sustainable waste management of sugarcane processing	[37]
Different pruning residues: citrus, grapevine, olive (wood biomasses)	Combustion (boiler 30 kW, multicyclone filter bags), emission behavior (Italy)	-	LHV: avg. 18–18.7 MJ/kg, grapevine and citrus: high N content	[38]
Jackfruit peels and seeds waste	Combustion	-	Bioenergy, high heating value (~16.5 MJ/kg), low CO, CO2 and SO2 emissions	[39]
Paddy wastes (rice straw)	A pilot-scale biorefinery, gasification system	Alkaline pretreatment	Sustainable fuel (bioethanol) and chemical production (pure lignin up to 79%, silica up to 96%, and nutrient), energy efficiency 0.529 > energy efficiency of current process: 0.449, zero-waste biorefinery practice	[40]
Potato peel wastes (PPW)	Fixed bed pyrolysis reactor (450–550 °C, 30 °C/min heating rate, residence time 30 min)	-	Bio-oil, biochar, and a green catalyst (Potato peel pyrolyzed calcined biochar catalyst) for biodiesel synthesis	[41]
Conocarpus and Eucalyptus pruning wastes	Pyrolysis	-	Biochar (167,510 tons) and bioenergy production (312 GWh)	[42]
Maize stalks and pigeon pea	Pyrolysis at 400, 500, 600 °C	-	Lower temperature-produced biochar: suitable for controlling fertilizer nutrients and removing soil contaminants. Higher temperature-produced biochar: comparable to environmental remediation and activated carbon, and reduced polycyclic aromatic hydrocarbons	[43]
Waste from sesame stalks	Thermogravimetric and kinetic analysis, pyrolysis	-	Energy generation, multi-stage reactions in pyrolysis of sesame stalk seems	[44]
Sugarcane baggage (SCB)	Pyrolysis, several analytical methods (e.g., Py-GC-MS, TGA, XRF, and X-ray fluorescence)	Zeolite catalysts	Enhanced pyrolysis products with zeolite catalysts	[45]
Microcystis aeruginosa waste (a toxic microalgal biomass)	Synthesis of nickel/biochar composite, CO2-feeding pyrolysis	Nickel/biochar composite (as catalyst in CO2-feeding pyrolysis)	Synthesis of nickel/biochar composite that used as catalyst for syngas production in CO2-feeding pyrolysis	[46]
Banana peel waste	Pyrolysis	CO2 environment	Syngas production, CO formation at 420 °C	[47]
Cotton stalk	Pyrolysis and combustion, thermochemical analysis	Acidified 5% wt. HCL	Clean energy generation (eliminate cotton stalk’s undesired amorphous material)	[48]
Wet biomass waste (yard waste)	HTC with and without N2 pressurization	-	Improved carbon storage in hydrochar, less water consumption	[49]
Industrial apple waste	HTC	-	By retaining energy (82–96%) and carbon (80–93%) in solids, it creates CO2 neutral solid fuels with ~30 MJ/kg	[50]
Brewers’ spent grain (BSG), agave bagasse	On-site biogas production, hydrothermal carbonization	HTC and steam explosion (SE) for the pretreatment, and combination	biogas production, 162 to 173 mL/g COD for HTC preprocessing, 316 to 362 mL/g COD for SE pre-processing	[51]
Wood residue	HTL	Pretreatment with 4% NaOH	Sustainable biofuels, pretreatment enhanced by 1.8 times glucose production	[52]
Rice oil	Homogeneous basic transesterification	Vegetable oil from grain bran (as a catalyst)	Biodiesel production	[53]
Pseudostem of bananas, bamboo leaves, waste cooking oil	Heterogenous catalyst, an alkaline solution (from the ash) was used for lignin recovery from bamboo leaves by hydrothermal reaction	Ash (as heterogeneous catalyst in biodiesel production from waste cooking oil)	Biodiesel production, highly alkaline solution, biochar production, 97.6% conversion (of cooking oil waste) into FAME, lignin recovery ~43 wt.%	[54]
Cordia myxa seed oil	Membrane reactor to generate viable biodiesel, transesterification	Green heterogenous indium oxide (In2O3) nano catalyst with Boerhavia diffusa leaf extract	Enhanced biodiesel yield (up to 95 wt.%), similar to fuels with quality standards ASTM D 6571 & EN14214	[55]
Pine needle forest biomass	AD, structural alterations in lignocellulosic complex	Steam explosion, milling, and acid-base-acid treatment	21.4% enhanced methane production with pretreatment compared to the untreated, the biomass surface was found to contain coalescent materials and lignin droplets that could be utilized as potential nanocomposites	[56]
Lime fruit waste	Digestion (untreated and pretreated lime waste) in batches under mesophilic conditions (28 d)	Pretreatment using hexane as the solvent in a solid–liquid extraction process	Enhanced (40%) biogas in pretreated waste 93.2 mL/g VS	[57]
Organic residues generated in grasslands	AD	-	Better waste management system	[58]
Açaí seeds	AD (dry regime, mesophilic temperature)		CH4 yield 156.65 mL/g TS, enhanced biogas production	[59]
Hemp, watermelon, and sugarcane wastes	Biological transformation Anaerobic fermentation	Different treatment pH and temperature	Bioethanol production (optimum values: pH 4.5, and 35 °C)	[60]
Palm agro-industrial residues (date cake, trunk, leaves, pedicels, seeds, and leaf sheath)	Biorefinery platform, 2 scenarios: I. Maximum lignin generation, II. Maximum bioenergy.	Liquid hot water, ethanol organosolv, and catalyzed ethanol organosolv pretreatments	Enhanced methane, ethanol, and lignin production. I. ethanol: 807 mL/kg, CH4: 903 L/kg, lignin: 528 g/kg residue. II. Energy equal to 1553 mL of gasoline	[61]
BSG	Dehydration and the recovery of used oil	acid pretreatment enzymatic hydrolysis with CellicCTec2 and fermentation with S. cerevisiae	Enhanced bioethanol and biogas production [raw BSG: 379 ± 19 mL biogas/g, defatted BSG: 235 ± 21 mL biogas/g, and stillage: 168 ± 39 mL biogas/g]	[62]
10 different textile mill waste from cotton spinning	Saccharification	NaOH pretreatment	Bioethanol production, maximum 65% ethanol in optimal conditions	[63]
Marine macroalgae waste	Thermal acid hydrolysis, response surface methodology (RSM)	Acid concentration (0.1–2.5% v/v H2SO4)	Enhanced bioethanol production, 2.4 g/L	[64]
Banana wastes, primarily banana peel, pseudo-stem, and rachis	Rare earth elements (REE) (Nd3+, Eu3+, Y3+, Dy3+, and Tb3+) recovery from aqueous solutions study, the adsorbent materials were characterized using analytical techniques	Ethylenediaminetetraacetic acid (EDTA) as the desorbing solution	97% of the REE was recovered from the adsorbent, suggesting that banana rachis is a promising renewable bioresource	[65]
Sugarcane bagasse	Cutting-edge environmentally friendly solvents and intense ultrasonication	-	Nanofibrillated cellulose.	[66]
Industrial processing residues of tomatoes, fennel, potato, and carrot as carbon sources	Thermoanaerobacterium thermostercoris	Hemicellulolytic enzymes from T. thermostercoris was from giant reed rhizome and cardoon leaves and stems	T. thermostercoris cell biomass production, cellulolytic enzymes, T. thermostercoris could produce biohydrogen and bioethanol directly from raw	[67]
Waste oils: palm, olive, linseed, sea buckthorn, cottonseed oils	Photoenzymatic decarboxylation of free fatty acids, hydrolysis and decarboxylation using a cascade system	Without using organic solvents	High quality HC biofuels	[68]

presents the main results of the transformation of biomass residues (wastes) into bioenergy, biofuels, and value-added products.

3.4. Bioenergy, Biofuels, and Value-Added Products from Mixed Feedstock (Biomass Residues, Wastes, Manure Etc.), Catalyst, or/and Integrated Energy Systems

Increased research interest currently exists regarding mixed sustainable feedstocks and integrated systems for energy generation.

Gasification (thermochemical conversion) of several feedstock processes has been implemented recently by several studies. More specifically, co-processing synthetic MSW and gypsum from drywall wastes (1:1) using gasification has the potential to yield economic feasibility and enhanced energy recovery. 10 g of synthetic MSW and 10 g of gypsum from drywall wastes were mixed and tested at 800 and 900 °C, in two environments: oxidizing (2.1 slpm of O₂/N₂ mixture with 10%O₂) and pyrolysis (2.1 slpm of N₂). The results showed lower char yields and higher syngas generation due to the synergistic effect and CaSO₄ char oxidation [69]. A novel idea was proposed [70] to enhance wood waste hydrochar gasification using the co-hydrothermal carbonization of FDW digestate. Findings showed that the hydrochar’s gasification activity was greatly increased up to 7.2 folds (at 900 °C) and the gasification reaction period was shortened by the addition of food waste digestate. Moreover, the hydrochar produced by co-hydrothermal carbonization exhibited a high concentration of metal components (e.g., up to 124 mg/g Ca) and surface functional groups.

Pyrolysis (thermochemical conversion) has also been used by several researchers in the last few years using mixed feedstocks, such as sludges with glucose, and municipal mixed wastes. More specifically, Tiwari et al. [71] used thermo-kinetic methods to characterize peels from kaner fruit, seeds from kaner fruit, and yellow oleander (Cascabela thevetia). The results demonstrated that these biomass wastes could be effectively used for bioenergy production through pyrolysis. In a recent study [72], pyrolysis was evaluated to produce pyrolysis oil (py-oil) from FDW, with and without different wastes (bones, chopsticks, plastic, and eggshell). The optimal yield was 37.5 wt.% from pure FDW when performed at 20 °C/min, 400 °C, and 20 min. Waste like eggshells, polypropylene, and bones improved the organic fraction of py-liquid from 6.5 to 10.7 wt.%. Treatment with eggshells and activated biochar catalysts highly increased hydrocarbon production (GCV: 36–44.4 MJ/kg). The resulting pyrolysis oil has the potential to be used as a traditional liquid fuel. SS due to the fact that it has several nitrogen species, could be used in a sustainable way by decreasing sugar and co-pyrolyzing the Maillard reaction. Among the three blended samples (DSS75Glc25, DSS50Glc50, and DSS25Glc75), the best appropriate glucose ratio was 50 wt.% for nitrogen heterocyclic compound (NHC) production [73]. Another study [74] investigates the use of CO₂ in catalytic (using Ni/SiO₂) pyrolysis of tea waste (TW) to enhance syngas formation. CO was formed in the presence of CO₂ as a result of a homogenous reaction with volatile organic compounds (VOCs) resulting from TW pyrolysis. Catalytic pyrolysis enhanced syngas formation at low temperatures, resulting in 28 times more H₂ and CO generation. Compared to biochar produced by pyrolysis in N₂, CO₂ can be used as a reactive gas medium to produce biochar with a 34–35 weight percent yield, competitive porosity, and surface area.

Another study [75] explored microalgae co-hydrothermal liquefaction with fecal sludge at several proportions (0:100, 100:0, 75:25, 25:75, 50:50) for high-quality biocrude production. Microalgal planting and processing are expensive and energy-consuming procedures. For these reasons, the research illustrates a sustainable approach for substituting part of microalgae with an eco-friendly, low-cost alternative fecal sludge. A microalgae-fecal sludge ratio of 25:75 reveals the highest biocrude production (38% biocrude yield, GCV ~34 MJ/kg, and reduced nitrogen content of 2.8%). Moreover, economic benefits were found, as fecal sludge was a low-cost substitution. Operating a co-HTL facility next to an algae-based wastewater treatment plant and an FS disposal plant could lower the overall costs. Moreover, solar panels could be used to lower costs. Furthermore, reusing carbon and nutrients from biochar residues from co-HTL could enhance a sustainable bioenergy system.

Other studies have used combined AD and HTC systems for energy recovery. For instance, Allegue et al. [76] studied a closed-loop integrated biorefinery using FDW and purple phototrophic bacteria and AD after thermal hydrolysis. Thermal hydrolysis, AD, and photofermentation are combined in this process to recover bioenergy resources and create value-added products. Anaerobic digestibility and biogas production are both enhanced by thermal hydrolysis. This environmentally friendly, energy-efficient method is appropriate for combined heat and power plants because it reduces waste disposal by 78.6%. By using a purple phototrophic bacteria-based mixed culture for phototrophic treatment of the hydrolysate, biomass expansion with 65% wt.% protein was achieved. Additionally, the system produces polyhydroxyalkanoates and H₂, which together account for a total valorization of 16.9% of the raw food waste’s initial total solids. The best option can be selected with the nitrogen composition of the food waste modified so that low nitrogen promotes the production of hydrogen and polyhydroxyalkanoate (PHA). On the other hand, increased protein synthesis results from high nitrogen levels. The potential of AD and HTC to improve the efficiency and energy production of garden waste was investigated by Wang et al. [77]. AD achieved the highest heating value. The combined system achieved an energy production of 90.2% (12% CH₄ and 78.2% biochar) on leaves (with AD, 21 d), 81.2% (66.8% biochar and 14.4% CH₄) on branches and grass (AD, 14 d), and reduced secondary waste. This research improves the efficiency of energy utilization and decreases the production of secondary waste, offering important new insights into AD in conjunction with HTC technology.

Another study [78] produced biodiesel from waste seed oil from Citrus aurantium by utilizing recyclable zirconium oxide nanoparticles made from Alternanthera pungens aqueous leaf extract. At 87.5 °C, reaction period 120 min, methanol:oil ratio 6:1, catalyst of 0.5 wt.%, response surface methodology, the maximum yield of 94% was attained, demonstrating the potential of this waste to provide sustainable energy and support the circular bioeconomy practice. A novel method combining transesterification and CO₂-assisted pyrolysis was put forth by Cho et al. [79] in an effort to recover as much energy and value-added product as possible from swine manure, including syngas, biodiesel, and biochar. The biodiesel yield was increased (>94%) due to silica and swine manure biochar at 400 and 220 °C, respectively. Biochar acts as an alkaline catalyst. The residual solid after transesterification was then valorized with pyrolysis with CO₂ (as a co-reactant) for a more sustainable method. A Ni/SiO₂ catalyst was used: 2 wt.% and 5 wt.%. The combined effects of CO₂ and the Ni/SiO₂ catalyst greatly increased the formation of syngas. The results confirm that swine manure is a valuable resource for fuel and chemicals.

Agricultural wastes from corn cob and spelt husk were pretreated using ultrasound-assisted ozone [80] yielding high purity lignin (~92% with 95% purity) and cellulose pulp (~84% with 78% purity). Better quality separation of the components of biomass improves value-added product conversion downstream, increasing cost effectiveness and sustainability. The study emphasizes the potential of biomass lignocellulose in production of energy, fuels, and chemicals.

Polyethylene terephthalate (PET) vials and dried Daniella oliveri leaf (weigh radio 9:1) were used to develop hybrid biochar in a low-temperature self-regulated reactor. The results revealed promise for energy recovery and a better waste management system [81].

A huge amount of research interest has been found in recent years regarding anaerobic (co-)digestion technology for transforming mixed wastes into enhanced sustainable biofuels. The combined biological process of FDW was developed by [82] using semi-continuous anaerobic co-digestion (ACoD) of wheat straw, FDW, and cattle manure (CM) to examine the connection between the mixtures and C:N load at several organic load rates (2–3.6 kg VS/m³·d). The optimal mono-digested biomass surpassed 565.5 LN/kg VS_in at a 2.4 kg VS/m³·d organic load rate. The ideal mixture ratio for co-digested substrates was FDW:CM 75:25, where the biogas yield increased by 62%, 39.89%, 91.26%, 130.9%, and 119.97% for organic loading rates ranging from 2, 2.4, 2.8, 3.2, and 3.6 kg VS/m³·d, respectively. According to another study [83], the spent mushroom substrate (SMS) was utilized in AD to estimate improved biomethane production. The research demonstrates that fungal pre-processing applying Pleurotus ostreatus (PO) to individual and mixed agricultural wastes, rice straw (RS), pearl millet straw (PMS), and wheat straw (WS) in equal ratios, together with the biochar addition (5, 7, and 10%), can shorten the pretreatment period and enhance biomethane yield. The addition of biochar reduces the overall pretreatment time in comparison to non-biochar materials. The bio-methane yield (187 mL/gVS) rose about 83% from SMS and combination WS + PMS + RS with 10% biochar, which is translated to increased CH₄ production by 9.4%, 22.2%, and 57.1% times than individuals (those without biochar addition) SMS of PMS, RS, and WS, respectively. This study demonstrates how biochar could enhance energy production, reduce the biological pre-processing period, and eliminate the need for lignocellulosic biomass in order to produce energy (bio-methane). Duarte et al. [84] proposed an approach to evaluating the energy content of livestock manure and mixed SS by integrating agro-food biowastes, which include non-edible crops, manures, vegetable/fruit wastes, fish canning industry wastes, and coffee wastes. The findings demonstrate improved energy performance (from 30 to 250% for livestock manure and 62 to 539% for mixed SS), which encourages a circular bioeconomy and wise use of AD in both rural and urban development. Cow dung and cassava wastes (peels, stem, and mill effluent) in several doses were used effectively for bioelectricity and biogas production via the AD approach [85]. Srivastava et al. [86] evaluated anaerobic defatted microalgae residue (Desmodesmus GS12, and Chlorella CG12) co-digestion with rice straw (RS), with a C/N ratio of 30, for sustainable development. The results showed that this practice shows potential for sustainable biorefinery development, increasing the biomethane yield by 49.87% (382 mL/g-VS, CG12 + RS) and 22.26% (311 mL/g-VS, GS12 + RS) compared to the control. Because of their intricate structures, microalgae cells are resistant to AD, which suggests pretreatment methods for improved biomass solubilization and methane yield, with solar energy being under investigation.

Vassalle et al. [87] used solar pretreatment for biomass solubilization for enhanced methane production (AD) of sewage co-digestion with microalgal biomass, comparing an upflow anaerobic sludge blanket (UASB) reactor fed only raw sewage and another UASB reactor that co-digested with microalgal sewage. Anaerobic-aerobic treatment solutions, including UASB reactors and high-rate algal ponds, have proven efficient for removing contaminants and micropollutants, as well as energy recovery from sewage co-digestion with microalgal. The solar pretreatment achieved 32% organic matter solubilization and 45% methane yield in the anaerobic co-digestion of raw sewage compared to mono-digestion.

Bioenergy production from dried household food waste was achieved through bioconversion based on microbiological processes [88]. Dried household food waste (FORBI) was fermented with ethanol and H₂, or extracted with water, resulting in sugar-rich liquid fractions and solid residues. Effluents were used for methane production through AD. For alcoholic fermentation, mono-cultures and co-cultures of C5 and C6 yeasts were utilized, and for H₂ production, mixed acidogenic consortia were utilized. The maximum ethanol yield was 0.16 g per kg of this waste for separate waste fermentation, while the highest hydrogen yield was 210.44 ± 4.02 L H₂/kg TS of this waste for 1% solids loading and addition of cellulolytic enzymes. Direct AD of dried household food waste or its fractions resulted in reduced energy generation than that obtained when fermentation and subsequent AD were performed. According to Ref. [89], Methanosarcina mazei improved with palm oil mill effluent (POME) sludge from pineapple peel could increase biohythane gas (H₂, CH₄, and CO₂) production using a single-stage AD procedure using mesophilic batch conditions for sustainable energy recovery. Co-digestion of daily animal manure and a variety of Salix (lignocellulosic energy crops, 6 types) were studied in order to enhance compressed biomethane gas (CBG) production [90]. According to the energy performance results, Salix might be a viable product for co-digestion with dairy manure. The production of bioenergy from rice husk (RH) and melon husk (MH) co-digested (for 200 days, at different RH and MH ratios) with cow dung (CD) (as inoculant) was investigated by Mohammed et al. [91]. A mixture-process variable design was used, with NaOH concentrations ranging from 8 to 9% and total solids ranging from 8% to 10%. The highest biogas yield was found at RH100:MH0 while RH0:MH100 revealed the lowest yield. Cucina et al. [92] used an AD plant to co-digest 3 bioplastic wastes (two polylactic-acid based and one starch-based) with SS. The bioplastics’ biomethane potential was 135 ± 23 NLCH₄ kg/Volatile Solids. The AD of biodegradable plastics could be a viable method to enhance biogas yield and minimize leakage. The outcomes showed that this treatment could have a positive impact on the sustainable life chain of bioplastics, demonstrating the potential of AD. In another study, Thompson et al. [93] co-digested seaweeds—Caribbean pelagic sargassum (PS)—and FDW at different ratios for biogas and biofertilizer production. The results showed that hydrothermal preprocessing enhanced the hydrolysis of food wastes and organic Caribbean pelagic sargassum of mono-substrate digestion by approximately 200% and 10%, respectively, compared to untreated samples. Co-pretreated FDW with Caribbean PS at a 25:75 weight ratio could produce 292.18 ± 8.70 mL/gVS CH₄ yield. Hasan et al. [94] evaluate seasonal variation of fruits, vegetables, and agrowastes (FVA) generated in wholesale market wastes in New Delhi, India. Anaerobic co-digestion (mesophilic conditions for 30 d) with sewage sludge and cattle manure of organic fraction for biogas potential. Four different mixes were formulated based on the different seasons. The experimental results were compared to the modeling results (modified Gompertz model). The results showed a positive synergistic effect of co-digestion. The synergistic impact of pig manure (PM) co-digesting sargassum (Sar) biomass was also evaluated by Rivera-Hernández et al. [95] for maximum biochemical methane potential BMP. Several blends were examined (0Sar:100PM, 100Sar:0PM, 50Sar:50PM, 65Sar:35PM, 30Sar:70PM). According to the results, co-digestion significantly improved BMP (79–160.4%) compared to mono-digestion procedures. The best (BMP ~441 mLCH₄/gVS_Fed) was found in 50Sar:50PM blend, with a 16.8 C:N ratio. D’ Silva et al. [96] investigate the fruit and vegetable wastes (FVW) anaerobic co-digestion (AcoD) with cow dung (CD) and dry fell leaves (DFL) in the absence of an active inoculum. 12 different mixtures were created with the percentage of DFL and FVW changing while CD content remained at 6% total solids and the temperature at 37 °C. The reactor generated the highest amount of biogas yield when the DFL: FVW ratio was 100:0 (809 ± 96 mL/g VS_input). On the other hand, a reactor with a DFL to FVW ratio of 40:60 demonstrated a maximum CH₄ production of 388 ± 131 mL/g VS_input while conserving 2 wt.% of additional water compared to the former. Due to its advantageous features, the suggested co-digestion strategy may find widespread use.

Increased research interest has been found in the last few years in the combination of bioprocesses and integrated systems (such as gasification with pyrolysis, thermal with biological technologies, and solar with WtE systems) for bioenergy generation. More specifically, Niedzialkoski et al. [97] studied the combination of bioprocesses (vermicomposting, composting, fraction separation, hydration, and AD) to make greater use of agricultural-industrial poultry wastes. The combination of bioprocesses in poultry waste management offers a sustainable alternative for nutrient recycling and energy recovery, resulting in a high-quality organic fertilizer with a germination index above 100% and greater energy recovery (461.8 L CH₄/kg VS_add). For many years, the US military has utilized open air burn pits (OBPs) for garbage disposal (such as paper, food waste, plastic, etc.). OBPs can be replaced with gasification and pyrolysis technologies. Tovkach et al. [98] developed a structured multi-step decision-making method using 3 gasification and 3 pyrolysis technologies for the treatment of solid waste to bioenergy on forward operating bases (FOBs) that are decentralized of 3 dimensions: 120-person, 1200-person, and 12,000-person. The results showed that the military could probably need to set up a microgrid on each FOB in order to integrate various WtE technologies into a cohesive system. Moreover, the findings indicate that choosing the right technology depends largely on the size of the community and the stakeholders’ functional perspectives. Sette et al. [99] focus on the valorization of grape and apple residues resulting from the wine and cider industries by gasifying/pyrolyzing and extracting bioactive compounds and residual solids to produce value-added chemical materials (e.g., biochar). The grape stalk extract showed the highest polyphenol content and antioxidant capacity. The resulting biochars can be used to reinforce soil structure and generate activated carbon for use in fuel processes. Khan et al. [100] proposed an innovative design for an integrated solar—WtE factory to maximize efficiency. More specifically, a solar thermal system warms the steam produced by the MSW combustor even more to improve its quality before it enters the steam turbine. Furthermore, the flue gas powers an iso-butane organic Rankine cycle, which generates electricity that is utilized by a reverse osmosis system for clean water generation and a proton exchange membrane electrolyzer for H₂ production.

Moura et al. [101] applied the sustainable value approach to compare several types of waste (biomass agro-industrial residues, paper mill sludge, slurries, and effluents, pulp, piggery effluents, and OFMSW) and conversion technologies to produce energy and biofuels (electricity and heat by combustion, H₂ by dark fermentation, bio-oils by pyrolysis or HTL, synthesis gas, and biogas by gasification). Based on 14 criteria, including material and energy inputs and outputs, process technology, and stakeholder adoption, the numerator ‘Functional Performance’ of the Sustainable Value Index (SVI) was calculated. The technologies’ performance was categorized by assigning values to each criterion, such as the level of satisfaction (S) and relative importance (φ). The final SVI ranking showed gasification > combustion > AD > (trans) esterification > fermentation to ethanol and pyrolysis > hydrothermal liquefaction > dark fermentation, respectively, as the most proper waste processes. Process and input criteria had a bigger impact on thermochemical conversions than did output and social acceptance criteria for biochemical conversions. The SVI proved to be an effective tool for evaluation and decision-making.

Several researchers have recently calculated energy potential using integrated sustainable technologies and mixed biofuels in specific regions for sustainable cities. More specifically, Madhesh et al. [102] investigate a novel cascade system of Organic Rankine Cycle (ORC) and Kalina for cooling and green hydrogen using a combination of MSW biogas production and solar energy. Four different regions with different climatic conditions (Delhi, Guwahati, Chennai, and Mumbai) were examined in India in order to find the optimal case study. In terms of energy utilization and efficiency, the organic Rankine cycle—vapor compressor refrigeration system (ORC-VCRS) system is inferior to the cascade ORC—Kalina system. Although the ORC-VCRS system has a marginally higher energy efficiency, the cascade Kalina-ORC system is a more appealing long-term solution. In order to effectively combine biogas and solar sources for a future sustainable energy transition, the study emphasizes the great promise of artificial intelligence-enhanced renewable energy cooling systems. A recent study [103] also combined AD with solar energy as a solution for livestock farms. Due to its widespread spread across the globe, pig manure has been regarded as a model substrate. Economic analysis for the hybrid anaerobic digestion–solar system on 5 locations (Laixi in China, Soria in Spain, Iowa in USA, Santa Catarina in Brazil, Odense in Denmark) was performed. This novel system of solar hybrid panels could deliver enough power for biofuel production (biogas, biomethane), ensuring higher energy efficiency with lower production costs, in isolated or non-isolated communities and cities.

Khalil end Dincer [104], developed an integrated hydrogen and renewable energy systems (solar photovoltaics, wind plants, and biomass-based systems) for sustainable cities: bioethanol and biogas production (combined electricity and heat production), hydrogen production—via a proton exchange membrane electrolyzer. 4 countries (Canada, S. Africa, Netherlands, and Denmark) were studied. All regions showed good potential for energy production. Large quantities of biomass residues (MSW and solid woody biomass) are available in Kalundborg (Denmark) for bioethanol and biogas production.

Yong et al. [105] explored the economic and environmental benefits of producing biofertilizers and biogas from OFMSW in Malaysia. It reveals that organic waste contributes 45% of the total MSW. Utilizing 50% of this waste via AD could generate electricity (3941 MWh/d) and biofertilizer (2500 t/d), reducing landfilling region and CO₂ emissions (2735 t/d CO₂ emissions, 1128 m²/d of landfilling area, and leachate of 481 m³/d can be evaded). This approach aligns with sustainable waste management practices.

Organic solid waste and fecal sludge were co-liquefied (at 320 °C for 60 min) to produce low phenolic naphtha-rich biocrude [106]. The water:ethanol co-solvent (0:1, 1:0, 1:1, 3:1, 1:3) and sourcing (1:1, 1:0, 0:1) were optimized. With a calorific value of about 37.5 MJ/kg, the findings revealed that the greatest biocrude production (55.7%) was obtained at 1:1 water:ethanol co-solvent, 50% OSW, and 50% FS, suggesting that it could ultimately take the place of petrocrude (42 to 49 MJ/kg). The biocrude’s composition (hydrocarbon: 25%, organic acid: 15%, ester: 58%, and phenol: less than 1%) guarantees the least number of phenolic compounds, preventing phenolic toxicity. The fact that the highest API value was 23.3 emphasizes how similar it is to the standard petroleum API gravity (medium lighter crude). With energy recovery and consumption ratios of 88% and 0.38, respectively, a metric ton of feedstock could bring in US$ 550 in revenue, indicating the possibility of sustainable energy production.

Yang et al. [107] utilize integrated pyrolysis and AD of grass biomass to produce biomethane, biogas, biochar, and biooil. According to the results, as the AD duration grew from 3 to 15 days, the methane yield rose from about 33 mL/g VS to 250 mL/g VS. Biochar was produced from about 29% to 35%, while the biooil and pyrolysis gas slowly decreased. A net calorific value of 2 MJ/kg and the highest energy efficiency, about 72%, were attained at a 12-day AD time. Food sludge co-torrefaction with 6 lignocellulose biowaste using microwave-assisted for biochar production and nutrient recovery was performed by Zheng et al. [108]. The maximum GCV (19.6 MJ/kg), lower ash level, higher carbon content, and enhanced biochar quality were achieved by blending sludge with macadamia husk at 25:75 db%. Biochar showed high combustion efficiency, thermal stability, and combustion characteristics, making it a great coal alternative for producing electricity. Green and sustainable waste management is promoted by the enhanced protein and carbohydrate content of the food sludge supernatant, which was co-torrefied with biowaste. This allowed for recycling back into the activated sludge unit. Another study [109] used potato peel wastes for sustainable bioconversion into biogas and ethanol using organosolv pretreatment using 50–75% v/v ethanol solution with/without catalyst (1 wt.% H₂SO₄). Catalyzed organosolv pretreatment with 50% v/v ethanol at 120 °C produced a high hydrolysate yield of 539.8 g glucose per kg dry PPW, which was subsequently followed by enzymatic hydrolysis. The hydrolysate was successfully fermented to produce 224.2 g ethanol/kg dry PPW. The liquid portion of the pretreatment was recovered, and the unhydrolyzed particles from enzymatic hydrolysis were digested anaerobically to recover further energy. The AD yielded 57.9 L of CH₄ for every kilogram of dry PPW. Consequently, 8112 kJ/kg of the energy were produced of dry PPW by the biorefinery, which included solvent recovery, ethanolic organosolv pretreatment, ethanolic fermentation, AD, and enzymatic hydrolysis of wastes.

Syngas production with CO₂ capture using gasification and solar energy, heat, and power generation from system 1: rice straw and system 2: microalgae was investigated [9] through energy, exergy, technological, economic, and environmental evaluation. System 1 has the potential to produce higher H₂ at a lower amount of O₂ carries, has higher energy efficiency (~4.3%), is more environmentally friendly and sustainable, and has a lower exergetic product cost than system 2. Using supercritical water gasification, Ruya et al. [110] investigated H₂ generation using a palm oil mill and empty fruit bunch waste. The study examined the effects of alternative H₂ separation processes, H₂S adsorption, and CH₄ steam reformers on gross hydrogen generation and performance of the system. Achieving a significant amount of biomass (25 wt.% empty fruit bunch and palm oil mill effluent) resulted in 98% increased net H₂ output, 70% energy performance (with no reformer), and 58.3% (with reformer). Sotoodeh et al. [111] introduced an integrated WtE multi-system for heating, cooling, power, and fuel production that contains a steam gasifier, Brayton waste heat recovery, organic Rankine, absorption refrigeration systems, domestic heating systems, and H₂ production. An absorption chiller’s condenser and absorber are used to retrieve the heat that was rejected for thermoelectric generators, which enhances power generation by 12% on average. The study reveals that gasification temperature significantly impacts system performance, with energetic and exergetic efficiencies of 52.3% and 41.3%, respectively, and the steam gasification subsystem has the maximum exergy destruction rate (great opportunity for increasing energy efficiency).

By using bio-iron nanoparticles and lignocellulosic biomass hydrolysate, Sakthi Vignesh et al. [112] were able to enhance microalgal biomass production and, consequently, the lipid content. Nitrogen starvation increased the lipid content and allowed FAME to recover. It was found that the bio-iron nanoparticle-mediated FAME conversion was a good option for enhancing biodiesel production.

The industry produces huge quantities of waste that could be reused to transform them into value-added products. Remón et al. [113] convert almond wastes produced from almond industries into value-added products (high-purity xylo- and cello- oligomer, small oxygenates) via carbon-neutral Ru CNF (carbon nanofibers) catalysts reinforced by carbon nanofibers and hydrothermal hydrogenation.

With a focus on specific areas of Northest Europe (the Netherlands and Belgium), the impact of using roadside grass clippings as a substitute source on the environment (digested and co-digested with pig manure) for biogas production was investigated [114]. The results showed that co-digestion of roadside grass clippings with pig manure is more eco-friendly than mono-digestion or composting. The potential economic growth and their export potential of 25 biowastes (Low-Income Potential: animal manure of goat and sheep, cashew nutshell, cotton stalk, non-edible seed oil, neem seeds, ceper seed oil, olive residue, coconut oil cake, corn cob; Medium-Income Potential: rice straw, sugar cane, porcine, and husk, wheat husk and straw, sunflower waste cooking oil, soya bean oil and cake, long wood sticks; High-Income Potential: food and vegetable waste, grass waste, cellulose, fat fraction from leather fleshing waste, MSW, poultry fattening and chicken fat, pine sawdust, duck tallow, animal manure, vegetable oils) of 7 biomass-enriched countries (USA, Brazil, Argentina, India, Indonesia, Pakistan, China) were assessed by Ayub et al. [115] using the innovative economic Product Space Model. With a focus on affordable, clean, and sufficient energy, the study offers useful implications from economic, social, and environmental angles. The results of this study showed that Pakistan has a wealth of biomass resources that could be used for bioenergy. In 2019, the United States, China, and India produced the most sophisticated products. Canola oil, among all biowastes, has the greatest potential for profit and contributes fairly to worldwide export revenue (financial advantages of its export). The United States exports bagasse effectively; it could be a sustainable source for energy production in these countries due to the significant quantities produced in these nations. Based on the global export value, soya bean oil cake is the most exportable biomass feedstock. As a result, it projects that nations that produce more soy bean oil cakes will reap significant benefits.

Kowalski et al. [116] described the design procedures of the Polish Śmiłowo Eco Park, which promotes a business strategy that encloses the whole life cycle of the products. This park is an ecological industrial park that greatly boosts environmental and economic efficiency among co-operating companies by promoting efficient waste, energy, water, and material exchange. Utilizing new technologies, modernizing current processes, reducing and reusing waste, recycling, recovering materials and energy, substituting wastes for raw materials, and thermally treating wastes and using them as sustainable biofuels are some of the strategies used to prevent pollution of the environment. At Śmiłowo Eco-Park, 300,000 t meat waste is utilized, 110,000 t of meat bone meal biofuel is produced, 120,000 t of pig manure is used as fertilizer, 460,000 GJ of bioenergy is generated, and 92,000 t CO₂ emissions are eliminated.

Several thermochemical conversion methods have been used in recent years using mixed feedstock wastes and biomass residues. More specifically, Vasileiadou et al. [28] showed that blending olive stone with lignite and blending extracted olive pomace with lignite in several proportions (30%, 50%, and 70%) could produce alternative high-quality biofuels. Moreover, via co-combustion, reduce the use of fossil fuels, as part of the fuel is substituting lignite with sustainable biomass residue. This waste-to-energy practice could contribute to sustainable bioenergy generation, and sustainable waste management. Moreover, for the first time in the literature, Vasileiadou [29] studied blends of brewers’ spent grain (resulted from a beer industry) and their co-combustion with lignite (in several proportions, 30–50–70%) for bioenergy production using several thermochemical experimental methods (thermal analysis, thermogravimetric analysis, proximate analysis, ultimate analysis, ion chromatography, ash elemental analysis, etc.), and several calculated methods, by using the experimental results, such as empirical chemical formulas, kinetic and thermodynamic analysis, environmental impact, maximum emission factors. Moreover, several case studies of energy cover were developed. The results showed that BSG blends revealed much higher fuel characteristics than lignite (reference sample), similar Cl content with lignite (~0.03 wt.%), reduced activation energy, but increased N content. Synergistic effects were found. More specifically, BSG70 LIG30: more than 18 MJ/kg, ash: 0.0080 kg ash/MJ (−74% ash production compared to lignite), BSG50 LIG50: about 16 MJ/kg, 0.0132 kg ash per MJ, and BSG30 LIG70: more than 14 MJ/kg, 0.0197 kg ash/MJ [lignite: 12.7 MJ/kg, ash: 0.0307 kg ash/MJ]. De Souza et al. [117] evaluated the use of coffee processing residues with forest eucalyptus wood for pellet production, focusing on energy generation via thermochemical analytical methods. Six combinations of coffee residues blended with forest biomass were created using varying ratios. Biomass blend pellets showed a high energy content of 16.51–17.08 MJ/kg. Pellets with eucalyptus/parchment/coffee husk (ECPCH) revealed 11.60 GJ/m³, higher durability of 98.2%, Type B according to European standards. The results showed better energy characteristics with biomass blends compared to pellets made with coffee husks and silver skin. The biomass blends also improved the bulk density, mechanical durability, and combustibility of pellets. A synergistic effect was also shown [30] during the devolatilization and decarbonization stages of grape marc (GM) co-combustion with lignite. Compared to lignite, GM blends had a higher energy content, lower ash content, lower emissions per produced MJ, lower activation energy, and lower maximum possible emission factors per produced MJ. The ashes from GM blends with lignite were found to be Type C (high melting temperature), which means fewer ash-related combustion problems. The experimental results showed that co-combusting grape marc with lignite is viable. In another study [13], four types of municipal solid wastes (food wastes, green wastes, organic fraction of municipal solid wastes, and paper wastes) were blended with lignite in three proportions (30:70, 50:50, and 70:30) for potential alternative fuels. 12 blends were examined experimentally via in-depth analytical and calculated methods (ash elemental analysis, gross calorific value, ion chromatography, proximate analysis, ultimate analysis, thermogravimetric analysis, kinetic modeling and thermodynamic analysis, maximum emission factors, environmental footprint index, etc.). The findings showed that these blends are an attractive option for energy production, as most of them revealed enhanced fuel characteristics compared to lignite (reference), and a better waste management system. Another study [118] used coal in blends in the co-incineration process. The results showed that coal co-incineration with 11% tire waste in cement plants, with or without raw mills in function, could enhance energy recovery, and reduce CO₂ emissions. Deep Singh et al. [119] determine India’s energy demand by using MSW, agricultural residues, and animal manure. The average MSW calorific value was 1751 kcal/kg, and the biomass potential was 20 MJ/m³. The findings showed that utilizing these wastes can produce 1.29 × 10³ PJ of biogas and 7.79 × 10² PJ of cellulosic ethanol per year. The bioenergy potential of livestock waste and agricultural residues in 602 rural districts of India was calculated [120]. The results indicate that livestock wastes could produce about 37% of the energy demand (AD processes). The combination of livestock wastes and crop residues could cover about 55.6% of energy demand in rural districts. Chaudhary et al. [121] examine a self-sufficient rural bio-energy design in Uttar Pradesh, India, estimating a biogas production of 400,329 m³/year from animal and agricultural waste, allowing for the establishment of biogas plants in centralized (20–100 m³) or decentralized (2–20 m³) systems. Several case studies were examined: (a) combined thermal energy and electricity production, (b) for direct cooking, and (c) depending on energy needs, a maximum amount of electricity production (up to 800,658 kWh) and thermal energy (1,200,988 kWh) potential. A quantity of more than 162,500 kg/year of firewood, 443,000 kg/year of dung cake, about 71,000 kg/year of charcoal, and about 13,500 kg of liquified petroleum gas might be swapped out with the generated raw biogas, which is enhanced if the biogas is upgraded. In case c, where all wastes are co-digested, a maximum surplus of approximately 222,000 m³/year of upgraded biogas, and about 170,000 m³/year of raw biogas, is possible.

Ocak and Acar [122] studied theoretically the potential of biofuels from agricultural and livestock wastes in the Marmara Region, Turkey. The findings demonstrated that these wastes could meet more than 50% of the area under study’s electricity needs. Direct combustion and the production of biogas from animal waste were found to be cost effective.

Another study [123] performed an economic feasibility of two WtE scenarios (I. implementing transesterification and AD, and II. AD) using SHW for sustainable waste management in Saudi Arabia. Wastes of approximately 43% of the cattle and 12% of the sheep body are produced. Massive volumes of waste are dumped in landfills without any kind of energy or material recovery. According to this study, the amount of solid waste produced by slaughtering activities was estimated to be 0.08 million tons in 2016 and might reach 0.2 million tons by 2030. In 2030, scenario I’s and scenario II’s potential revenue contributions to the national economic circle will be 288 MSAR and 319 MSAR, respectively.

Through the development of a mathematical formulation to determine the ideal network design for treatment facilities for both urban (municipal solid waste, MSW) and rural wastes (cattle manure), Argentina’s integrated urban-rural waste management was studied [124] emphasizing reduced carbon emissions and high energy regeneration. Three different scenarios were examined in order to determine the best approach for integrated processes for MSW and manure treatment. Anaerobic co-digestion is preferred in the centralized configuration, with a blend of 70% MSW and 30% manure. Transporting garbage from remote locations is justified by the high methane yield. On the other hand, AD for decentralized models revealed suitability for the biggest communities (with more than 5 tons of organic waste per day) and feedlots (with more than 30 tons of manure per day). The results demonstrated that MSW and animal manure co-digestion are attractive strategies for sustainable waste management and energy production. d’Espiney et al. [125] evaluated the bioenergy potential of 3 types of residues (municipal wastes, agricultural residues, and forestry residues) with a case study of the region of Lafões in Portugal with biochemical and thermochemical routes. In comparison to the thermochemical route (543 TJ/year), the biochemical conversion route revealed a 765 TJ/year energy potential.

A few studies have used stimulation software programs for bioenergy generation. More specifically, Kaushal et al. [126] explore cow dung, FDW co-digestion with algae via Stat-Ease Design-Expert Software-13 in batch-type digesters to increase biogas yield. The findings showed that the biogas yield and hydrolysis were improved by co-digestion and pretreatment. De Lorena Diniz Chaves et al. [127] performed a system dynamic modeling analysis for sustainable refuse-derived fuel generation in Espírito Santo, Brazil. The analysis estimates the availability of recyclable waste streams for RDF production and discusses possible intervention strategies. The model was simulated over a 20-year period, and the results show that with partial implementation of the proposed solid waste management policy, there will be sufficient waste streams for RDF generation. The study also highlights the indirect environmental benefits of sustainable RDF production in the cement industry. Compared to a directly fired biomass power plant, the integrated gasification combined cycle (IGCC) configuration conserves more energy. Because of its high energy performance (more than 50%) and possibilities for improvements through the use of solar energy, IGCC is becoming more and more popular. Ansari et al. [128] using ASPEN Plus simulation software, assessed crop wastes for energy and biofertilizer generation. According to the results, temperature and pressure have an effect on gas yield, whereas biomass composition affects the yield of biofertilizer. The solar biomass-based IGCC system is highly efficient, producing 0.55 MW of electricity at a low cost compared to other conversion systems. Altan et al. [129] investigated the possibility of using biological and thermal technologies to produce energy from livestock waste. Four mixed wastes were analyzed: sheep, cattle, chicken, and goat waste. Several processes were applied: combustion, gasification, pyrolysis (thermal processes at 550 and 750 °C), along with anaerobic digestion (biochemical process). A defined design algorithm with important parameters was utilized (ASPEN Plus stimulation) in 7 geographical areas (Marmara, Aegean, Black Sea, Mediterranean, Central Anatolia, EA, SEA) in Turkey. The results showed that combustion was the most efficient (0.43 MW_e/t), followed by gasification (0.34 MW_e/t, 21% lower than combustion) and pyrolysis (0.30 MW_e/t at 750 °C, and 0.15 MW_e/t at 550 °C). AD showed a 0.21 MW_e/t recovery potential. Energy recovery from livestock waste contributes to waste to energy practice, reducing emissions, and promoting sustainability.

Novel technologies (such as fuzzy programming, and machine learning) have recently been used in bioenergy production. Constructing installations to use agricultural and livestock waste (wheat, maize, and rice straw, hen and cow manure) from the Iranian region of Fars were studied [130] in order to create a supply chain bioenergy network. To manage the uncertainty, a strategy relying on the fuzzy programming method was utilized. The group best–worst method and the fuzzy goal programming technique were combined to solve the mathematical model. The results show that the network generates approximately 87% of its total revenue from the sale of bioelectricity. The establishment of facilities, the procurement of supplies, and production account for the majority of the network’s total costs.

Conventional HTL is time-consuming and requires significant effort to produce high-quality bio-oil (e.g., low N content). Li et al. [131] used Machine Learning (ML) algorithms to enhance the energy of bio-oil production and reduce the nitrogen content from the HTL of biomass containing high humidity (e.g., sludge, algae, food waste, and manure), taking into account associated elements in HTL such as solvents, process parameters, and elemental and biochemical compositions of biomass. The results indicated that for the multi-task prediction (bio-oil yield, N oil, and energy recovery ER_oil), the random forest (RF) algorithm was the most superior (avg. R2 = 0.8). Since the modeling and experimental results matched, the experiment validation was successful.

Generally, it can be concluded that in the last five years, there has been a huge research interest in mixed feedstocks of organic wastes and intergraded energy systems for bioenergy and value-added product generation. By mixing feedstock, optimum fuel quality characteristics can be achieved. Several different alternative mixed fuels (e.g., MSW & biomass residues, MSW & sludges & biomass residues) could be created in order to be available in every place, season, etc. In addition, multiple combinations of sustainable energy systems (e.g., AD with pyrolysis, gasification with pyrolysis, integrated exchange membrane electrolyzers with renewable energy systems, such as solar photovoltaics, wind plants, and biomass-based systems, AD and HTC) could be used for enhanced efficiency and reduced energy losses. Catalytic treatment is essential for enhanced bioenergy yield. Simulations, and machine learning can help to generate the optimum fuel (for every country) with enhanced quality characteristics, create the optimum integrated energy system with enhanced efficiency, reduced energy losses, and reduced emissions for sustainable cities.

Multiple advantages can be achieved by utilizing all the above-mentioned organic solid wastes, such as: (1) the availability of biofuel, as multiple sustainable sources with similar quality fuel characteristics can be generated from several wastes (not dependent on one feedstock); (2) reduced costs, as energy comes from wastes (not consuming fossil fuels, natural gas, etc.); (3) reduced CO₂ emissions, as biomass (residue) is considered as carbon-neutral fuel [4,30]; (4) reduced SO_x emissions, as several organic wastes showed limited sulfur content [30]; (5) reduced SO_x, HCl as synergistic effects exist in blends (capture the sulfur and Cl in the ash of the fuel) [29,30,69]; (6) reduced plant costs, as in many cases, these wastes could be used in the existing coal-fired power plants with small adjustments; (7) significant energy cover enhanced the energy independence (security) of a country, as these wastes (biofuels) resulted from the country [4,13,28,122]; (8) reduced secondary wastes (sustainable waste management) [123]; (9) enhanced the local (energy) market; etc.

Table 4. Bioenergy, biofuels, and value-added products produced from mixed feedstocks, or integrated energy systems.

Waste Source	Method	Enhance Treatment/Additives	Biofuel/Results	Refs.
Synthetic MSW & gypsum from drywall wastes	Gasification, thermogravimetric analysis and differential scanning calorimetry, lab-scale fixed bed reactor, 800 °C & 900 °C, 2 different environments	synthetic MSW-gypsum mixtures 1:1 mass ratio	Lower char yields and higher syngas generation, synergistic effect and CaSO4 char oxidation	[69]
Wood waste hydrochar & food waste digestate	Co-hydrothermal carbonization for enhance the gasification	Co-hydrothermal carbonization	Co-hydrothermal carbonization exhibited a high concentration of metal components, such as Ca and surface functional groups	[70]
Yellow oleander, fruit seeds & peels from Kaner	Τhermo-kinetic characterization, Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods	-	Activation energy of fruit peels from KAS and FWO: 184.8 kJ/mol & 182.3 kJ/mol, respectively.Activation energy fruit peels from KAS and FWO: 140.1 kJ/mol & 139.2 kJ/mol, respectively. Kaner fruit peels and seeds have a great deal of promise for pyrolysis-based bioenergy production	[71]
FDW	Pyrolysis pure FDW at 500 °C & co-pyrolysis	With & without different impurities (plastic, chopsticks, eggshell and bones), activated biochar catalysts	Eggshell treatment and activated biochar catalysts significantly boost hydrocarbon production, GCV: 36–44.4 MJ/kg, pyrolysis oil’s could be used as a traditional liquid fuel	[72]
SS and glucose, 3 blends DSS75Glc25, DSS50Glc50, and DSS25Glc75	Co-pyrolysis maillard reaction with reducing sugar, thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS)	-	Best appropriate glucose ratio: 50%, nitrogen heterocyclic compounds (NHCs)	[73]
Tea waste (TW)	Catalytic pyrolysis with CO2	Ni/SiO2	Enhanced 28 times more H2 and CO production	[74]
Microalgae and faecal sludge mixes	Co-hydrothermal liquefaction		Sustainable approach for substituting part of micro-algae with an eco-friendly, low-cost alternative fecal sludge	[75]
FDW	Closed-loop integrated biorefinery, purple phototrophic bacteria, combined thermal hydrolysis, AD, and photofermentation	Purple phototrophic bacteria-based mixed culture for phototrophic treatment of the hydrolysate	Bioenergy resources recover, value added products:H2, polyhydroxyalkanoates, PHA, protein synthesis from the high nitrogen	[76]
Garden waste	AD and HTC, combined system		Enhanced energy efficiency, reduced secondary wastes	[77]
Waste seed oil from Citrus aurantium	Biodiesel production (methanol:oil 6:1 ratio, reaction time 120 min, 87.5 °C, 0.5 wt.% catalyst)	Recyclable zirconium oxide nanoparticles made from Alternanthera pungens aqueous leaf extract	Sustainable bioenergy	[78]
Swine manure	transesterification and CO2-assisted pyrolysis	Ni/SiO2 catalyst(2 wt.% and 5 wt.%)	maximize energy recovery and value-added products (biodiesel, biochar, and syngas)	[79]
Agricultural residues from corn cob and spelt husk yielding	Biomass fractionation, analytical methods	Ultrasound assisted ozone	High purity lignin (~92% with 95% purity) and cellulose pulp (~84% with 78% purity)	[80]
Dried leaves of Daniella oliveri and polyethylene terephthalate (PET) bottles (weigh radio 9:1)	Low-temperature self-regulated reactor	-	Hybrid biochar production	[81]
Cattle manure (CM), FDW, wheat straw and mixes	Semi-continuous anaerobic co-digestion	-	Ideal mix W:CM 75:25 revealed 119.97% for OLRs 3.6 kg VS/m3·d	[82]
Spent mushroom substrate (SMS), mixed agro-waste wheat straw (WS), rice straw (RS), and pearl millet straw (PMS)	AD	Fungal pretreatment, biochar (5%, 7% and 10%)	Enhanced biomethane yield, biochar reduces the overall pretreatment time, avg. CH4 yield: 187 mL/gVS (~83% enhanced compared to untreated SMS of PMS + WS + RS)	[83]
Livestock manure, mixed SS, non-edible crops, manures, vegetable/fruit wastes, fish canning industry wastes, and coffee wastes	AD	-	Enhanced energy performance (from 30 to 250% for livestock manure and 62 to 539% for mixed SS)	[84]
Cow dung and cassava wastes (peels, stem, and mill effluent) in blends	AD	-	Effectively bioelectricity and biogas production	[85]
Defatted microalgae residue (Chlorella CG12 and Desmodesmus GS12), rice straw (RS)	Anaerobic co-digestion	-	Potential for sustainable biorefinery development, increasing biomethane yield 311 mL/g-VS (GS12 + RS) & 382 mL/g-VS (CG12 + RS)	[86]
Sewage with microalgal biomass	AD co-digestion	Solar pretreatment for biomass solubilization	Enhanced methane production	[87]
Dried household food waste (FORBI)	AD, bioconversion based on microbiological processes		Higher energy recovery was obtained when fermentation and subsequent AD performed	[88]
POME sludge from pineapple peel	AD single-stage system, mesophilic batch process conditions	-	Increased biohythane gas (H2, CH4, and CO2) production	[89]
Daily animal manure and a variety of Salix (lignocellulosic energy crops, 6 types)	Co-digestion, 1:1	SO2-catalyzed steam explosion	Enhanced compressed biomethane gas (CBG) production	[90]
Rice husk (RH), melon husk, (MH) and cow dung (CD),	Co-digestion		Highest biogas yield was found at RH100:MH0, the lowest yield revealed at RH0:MH100	[91]
3 bioplastics wastes: 1 starch-based and 2 polylactic-acid based with SS	AD co-digest	-	Bioplastics’ bio-methane potential: 135 ± 23 NL CH4 kg Volatile Solids−1, AD of bio-plastics can be a sustainable method (biogas production, and leakage reduction)	[92]
Seaweeds -Caribbean pelagic sargassum (PS)- and FDW blends	Co-digestion	Hydrothermal pretreatment	Enhanced biogas (292.18 ± 8.70 mL/gVS of co-pretreated pelagic Sargassum:FDW, 25:75) and biofertilizer production	[93]
SS and cattle manure mixes	Evaluating biogas potential, anaerobic co-digestion (mesophilic conditions for 30 d), seasonal variation of fruits, vegetables and agrowastes (FVA) with sewage sludge and cattle manure), New Delhi (India)	-	Radish leaves combined with waste activated sludge (WAS) showed the highest biogas yield (407.2 mL/g VSfed)	[94]
Sargassum (S) biomass with pig manure (PM) blends	Co-digestion	-	Enhanced biomethane potential, the highest BMP (~441 mL CH4/gVSFed) was found in 50S:50PM blend, (16.8 C:N)	[95]
Fruit and vegetable wastes (FVW), cow dung (CD) & dry fell leaves (DFL), 12 mixtures	AcoD	In the absence of an active inoculum	Co-digestion revealed several advantageous features, maximum methane yield of 388 ± 131 mL/g VSinput (blend: 40DFL:60FVW), maximum biogas yield: 809 ± 96 mL/g VSinput (blend: 100DFL: 0FVW)	[96]
Agro-industrial poultry wastes	Combination of bioprocesses: vermicomposting, composting, fraction separation, hydration, and AD	-	High-quality organic fertilizer, high energy recovery	[97]
Solid waste	3 gasification & 3 pyrolysis technologies	-	Solid waste to bioenergy, decentralized FOB of 3 sizes: 120-, 1200-, and 12,000-person	[98]
Apple and grape waste from the cider and wine industries	Extracting bioactive compounds, pyrolyzing/gasifying	-	Value-added chemical products: biochar, activated carbon, fuel applications, soil reinforcement	[99]
MSW	Integrated solar—waste to energy incineration plant, iso-butane organic Rankine cycle, proton exchange membrane electrolyzer, reverse osmosis system	-	H2 and clean water production (2.87 g/s & 26.96 kg/s, respectively rate production), efficiency: thermal 21.34% & exergy 16.64%	[100]
Biomass agro-industrial residues, paper mill sludge, slurries and effluents, pulp and, organic fractions of MSW and piggery effluents	Conversion technologies: electricity and heat by combustion, H2 by dark fermentation, bio-oils by pyrolysis or HTL, biogas and synthesis gas by gasification	-	Bioenergy & biofuels production,Final rank: gasification > combustion > AD > (trans)esterification > pyrolysis and fermentation to ethanol > hydrothermal liquefaction > dark fermentation	[101]
MSW	a novel cascade system of ORC and Kalina using a combination of MSW biogas production and solar energy, 4 regions (Delhi, Guwahati, Chennai, and Mumbai) in India	-	Green hydrogen production and cooling, biogas. Energy ratio 0.76, exergy efficiency 21.6%, total cost $58,677	[102]
Livestock farms	Hybrid AD—solar energy, economic analysis for the AD / solar system of 5 locations: Soria (Spain), Iowa (USA), Odense (Denmark), Santa Catarina (Brazil), Laixi (China)	-	Enhanced biomethane/biogas production, lower costs	[103]
MSW & solid woody biomass	An integrated hydrogen (proton exchange membrane electrolyzer) and renewable energy systems (solar photovoltaics, wind plants, and biomass-based systems),4 communities (Canada, S. Africa, Netherlands, and Denmark) were studied	-	Sustainable cities,Sustainable hydrogen, biogas, and bioethanol production	[104]
Organic municipal solid waste	Environmental and economic analysis in Malaysia (via AD)		Sustainable biogas and biofertilizer production. Utilizing 50% of the wastes: 3941 MWh/d electricity, 2500 t/d biofertilizer, and 2735 t/d reduced CO2 emissions	[105]
Fecal sludge and organic solid waste mixes	Co-liquefication (320 °C, 60 min)	-	Low phenolic naphtha-rich biocrude, low toxicity	[106]
Grass biomass	Integrated AD and pyrolysis	-	Enhanced biomethane, biogas, biochar, and biooil production	[107]
Food sludge and 6 lignocellulose biowaste	Co-torrefaction, using micro-wave-assisted		The maximum calorific value, lower ash level, higher carbon content, and enhanced biochar quality achieved by blending sludge with macadamia husk at 25:75 db%	[108]
Potato peel wastes (PPW)	Biorefinery which included ethanolic organosolv pretreatment, solvent recovery, enzymatic hydrolysis, ethanolic fermentation, and AD	Organosolv pretreatment using 50–75% (v/v) ethanol solution with/without catalyst (1 wt.% H2SO4)	Biogas, ethanol, 57.9 L CH4/kg dry PPW	[109]
Rice straw (system 1) and microalgae (system 2)	Gasification and solar energy, heat and power generation, energy, exergy, techno-economic and environmental analysis	-	Syngas production with CO2 capture, system 1 has the potential to produce higher H2 with higher energy efficiency	[9]
Palm oil mill effluent & empty fruit bunches	Supercritical water gasification	-	H2 production	[110]
Wastes	waste to energy integrated multi-system for power, heating, cooling and fuel production that contains steam gasifier, Brayton waste heat recovery, organic Rankine, absorption refrigeration systems, domestic heating systems and H2 production, Engineering Equation Solver (EES) software	-	WtE integrated system	[111]
Lignocellulosic biomass hydrolysate	Hydrolysate	Bio-iron nanoparticles	Enhance microalgal biomass production (enhanced lipid content), FAME recovery, enhanced biodiesel production	[112]
Almond wastes from almond industries	Carbon-neutral catalysts: Hydrothermal hydrogenation	Ru/CNF catalyst	Value-added liquids	[113]
Roadside grass clippings, pig manure	Roadside grass clippings as a substitute source on the environment (digested and co-digested with pig manure), Netherlands, and Belgium	-	Biogas production, co-digestion is an eco-friendly method	[114]
25 biowastes	innovative economic Product Space Model, 7 biomass-enriched countries (USA, Brazil, Argentina, India, Indonesia, Pakistan, China)	-	Economic, social, and environmental benefits of bioenergy production	[115]
Wastes, pig manure, meat waste, meat bone meal	Ecological industrial park Śmiłowo Eco-Park, (Poland), environmental and economic efficiency among companies, efficient waste, energy, water and material exchange	-	Sustainable waste management & bioenergy, reduced CO2 emissions	[116]
Olive stones (OLS) and lignite blends, extracted olive pomace (EOP) and lignite blends, in 30–50–70%(olive oil solid wastes from oil industry)	Several OLS blends with lignite, several EOP blends with lignite, thermochemical analyses, thermal characterization, kinetics, thermodynamicanalysis, and several scenarios for sustainable practices for energy cover	-	Energy generation, sustainable management, energy cover,GCV: OLS70 LIG30 > OLS50 LIG50 > EOP70 LIG30 > EOP50 LIG50 > EOP30 LIG70 > OLS30 LIG70 > LIGA, Ash content: OLS blends: 14.6 to 27.8 wt.% & EOP blends: 15.2 to 28.6 wt.%,sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[28]
BSG blends with lignite in several mixes (70–50–30 wt.%)	Co-combustion, thermochemical methods, thermal analysis, proximate analysis, ultimate analysis, empirical chemical formulas, case studies for sustainable management, environmental impact, maximum emission factors, kinetic and thermodynamic analysis, ash elemental analysis	BSG blends with lignite	Blends revealed higher GCV than lignite, lower ash per produced MJ, lower COx, SOx maximum potential emissions per MJ but higher potential NOx, better fuel characteristics, activation energy: lower in blends (synergy effect)Energy generation, alternative use of BSG, sustainable management, sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative, sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[29]
Forest biomass (coffee processing & eucalyptus wood) residues blends	Thermochemical methods, 6 blends in different proportion, pellet production	-	Bioenergy generation, eucalyptus-parchment-coffee husk ECPCH pellets: 11.6 GJ/m3, blends shod ~17 MJ/kg	[117]
3 grape marcblends with lignite (30–50–70 wt.%)	Co-combustion, thermochemical methods, kinetic and thermodynamic analysis, ash analysis, etc.	-	Ash per Megajoule (better>worst): GM: 0.0030 kg/MJ > GM70LIG30: 0.0082 kg/MJ > GM50LIG50: 0.0131 kg/MJ > GM30LIG70: 0.0205 kg/MJ > LIGA: 0.0307 kg/MJ, promising alternative sustainable solid biofuels (enhanced fuel characteristics, low COx, NOx, SOx emissions per Megajoule), sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[30]
MSW: FDW, GNW, PAP and OFMSW were blended (12 blends) with lignite, in 30–50–70 wt.%	Physicochemical, kinetic, thermodynamic, environmental impact, modelling, energy cover for Greece and Europe, empirical chemical formulas, Maximum potential emission factors	-	GCV: FDW blends 13.9 to 16.6 MJ/kg, GNW blends 12.4 to 13.1 MJ/kg, OFMSW blends 13.4 to 15.3 MJ/kg, PAP blends 13.8 to 17.5. MJ/kg, Enhanced energy, reduced emissions, energy cover, sustainable approach for substituting part of lignite with an eco-friendly, low-cost alternative	[13]
11% tire waste and coal	Co-incineration in cement plants	With or without raw mill in function	Enhanced energy recovery, reduced CO2 emissions	[118]
MSW, agricultural residues, and animal manure	Energy demand that can be met in India, avg. MSW calorific value: 1751 kcal/kg, biomass potential: 20 MJ/m3	-	1.29 × 103 PJ biogas/year, and 7.79 × 102 PJ cellulosic ethanol/year	[119]
Livestock waste and agriculture residues	Calculated methods, AD, 602 rural districts of India		Rural India needs 1927 TJ/day (2.75 MJ/capita/day) energy for cooking, livestock wastes could generate 715 TJ/day (1.02 MJ/capita/ day) ~37% of energy demand, using crop residue can generate 2296 TJ/day (3.27 MJ/capita/day)	[120]
Agricultural residues & animal waste	Case studies for estimating biogas potential Uttar Pradesh, (India), co-digestion	-	In case of co-digestion of all wastes, a maximum 170,000 m3/year of raw biogas and 222,000 m3/year of upgraded biogas is possible	[121]
Agricultural and livestock wastes	Theoretical study of potential biofuels and energy cover in Marmara Region, (Turkey)	-	Cover more than half of the electricity demand	[122]
SHW	Economic feasibility analysis, 2 WtE scenarios, transesterification and AD, Saudi Arabia. By 2030, national economic circle: 288 MSAR (scenario I) and 319 MSAR (scenario II)	-	Sustainable waste management in Saudi Arabia	[123]
Blends of rural wastes (cattle manure) and MSW urban wastes	3 case studies (scenarios)	-	Energy recovery, and reduced carbon emissions, Anaerobic co-digestion is preferred in the centralized solution, with a blend of 30% manure and 70% MSW, AD for decentralized designs is appropriate for larger cities (with more than 5 tons of organic waste/day) and feedlots (with more than 30 tons manure)	[124]
3 biomass residues (municipal wastes, agricultural residues, and forestry residues)	Case studies of the region Lafões (Portugal), biochemical and thermochemical routes	-	Bioenergy, biochemical conversion route revealed 765 TJ/year energy potential while thermochemical route 543 TJ/year	[125]
MSW & agricultural crop residues (coffee and cocoa husks, maize stalk/husk, wheat straw/husk rice husk/straw, sugar cane baggase, sweet potato peelings, groundnuts shells/husks/straw, straw beans, banana stem, peels, leaves, tops/leaves, cotton stalk)	Biogas production in Cameroon		Significant energy cover demand, (MSW: 26 PJ & agro-waste: 580 PJ, in 2020, in Cameroon), migrate climate change (1,600,000,000 kgCO2)	[4]
Cow dung, food waste and algae	Co-digestion in batch-type digester, Stat Ease Design Expert Software 13	KOH & sodium hydroxide	Increased biogas yield	[126]
Municipal and industrial waste	A system dynamic modeling analysis	-	Sustainable RDF production	[127]
Crop residues	Solar biomass-based IGCC system, ASPEN Plus simulation software		Co-production of power & biofertilizer	[128]
Livestock mixed wastes: cattle, sheep, goat, and chicken waste	Thermal and biological technologies: combustion, gasification, pyrolysis (at 550 and 750 °C), and AD, a defined design algorithm with important parameters, 7 geographical areas studied in Turkey, ASPEN Plus stimulation	-	Combustion was the most efficient (0.43 MWe/t), followed by gasification and pyrolysis. AD showed a 0.21 MWe/t recovery potential	[129]
Livestock & agricultural waste (such as maize straw, wheat straw, rice straw, cow and hen manure)	Building biogas plants were studiedfuzzy programming method was utilized to manage the uncertainty	-	bioenergy supply chain network, sale of bioelectricity accounts for ~87% of the network’s overall revenue	[130]
Biomass with high moisture (algae, sludge, manure, and food waste)	Machine Learning (ML) algorithms taking into account several factors (solvents, process parameters, elemental and biochemical compositions of biomass) from HTL	-	Enhanced energy of bio-oil, reduced Nitrogen content, random forest (RF) algorithm was the best one	[131]

presents the main results of the above-mentioned studies that utilized several mixed feedstocks, catalysts, and/or integrated energy systems to generate sustainable energy and value-added products.

4. Conclusions and Future Perspectives

Most countries in the world have not yet developed a system for dealing with crucial issues, such as national energy security, sustainable bioenergy (waste to energy practices, reduced carbon emissions), and a sustainable management system (circular economy, zero waste practices).

This review paper summarizes various waste-to-energy (WtE) practices that were used (from 2020 to 18 February 2024) by the scientific community for sustainable development. Three main categories of wastes were examined: municipal solid wastes, animal solid wastes, and biomass wastes.

The results showed that WtE practice is a feasible solution to the problem of sustainable urban development in communities. Several studies have effectively used physicochemical experimental analyses for the characterization and categorization of wastes. The results of this study showed that biomass wastes (forest biomass wastes, yard wastes, agave bagasse, brewers’ spent grain, jackfruit peels and seeds waste, textile mill waste from cotton spinning, potato residues, tomato residues, and banana residues, grape marc, straw, crop residues, etc.), animal wastes (e.g., manure, slaughterhouse wastes), and municipal solid wastes (e.g., food wastes, green wastes, organic fraction of municipal solid wastes, paper wastes) are viable sources of sustainable bioenergy production. Biomass residues are effectively transformed into bio-fuels and sustainable chemicals through various thermal, physicochemical, or biochemical conversion techniques, such as anaerobic digestion (AD), hydrothermal carbonization (HTC), combustion, gasification, and transesterification. By using sustainable green catalysts, e.g., utilization of green heterogenous indium oxide (nano catalyst) with Boerhavia diffusa leaf extract in the transesterification process for biodiesel production from Cordia myxa seed oil, a zeolite catalyst in sugarcane baggage pyrolysis, energy yield can be enhanced.

Increased research interest has been found regarding mixed sustainable feedstocks (such as wood waste with food wastes, agricultural residues, and animal waste, biomass waste with lignite, fruit and vegetable wastes with dung and dry fell leaves, sewage with microalgal biomass, MSW with solid woody biomass, MSW with lignite, rural wastes -cattle manure- with MSW urban wastes, etc.) and integrated systems for energy generation, such as gasification with solar energy, AD with HTC, integrated solar-WtE, integrated hydrogen and renewable energy systems (solar photovoltaics, wind plants, and biomass-based systems), solar biomass-based IGCC system, integrated solar-AD system, etc.

The shift from mono-(combustion, digestion, gasification, etc.) to co-(combustion, co-digestion, gasification, etc.) and to novel integrated sustainable energy systems (that combines with other renewable energy forms, e.g., wind, solar, etc.), taking into account the climatic and geo-morphological characteristics of the area, with the aim of energy security (the independence from other countries for energy coverage), the local green energy production (the use of biofuels in vehicles and in urban transport such as buses, taxis), the strengthening of the local economy and the labor market (agriculture, industry, trade in biofuels and waste products, etc.), can lead to a proper waste management (zero waste), reducing greenhouse gas emissions of the cities, with the ultimate goal of creating sustainable green cities in a country. There are cases in which this is already a reality, e.g., bioethanol production in Brazil.

Energy consumption and production are critical criteria for determining the level of industrial development in countries. Each country can calculate the amount occupied by these wastes and the energy they could produce from them, alone as alternative biofuels or in combination with each other, but also between various technologies and/or various pretreatments (e.g., gasification, combustion, pyrolysis, catalysis, etc.). Moreover, the combination with other technologies, such as solar, can lead to better results in areas with high sunshine or winds. In addition, the use of combined cooling, heating, and power (CCHP) production in the energy network is a sustainable solution for achieving efficient energy conversion by enhancing energy savings, flexibility in the operation of the system, and the efficient recovery of waste heat. In other words, specialized research should be carried out per region so that the most appropriate combination of the above-mentioned systems can be chosen, which will give the best result for every country, depending on the availability of biomass, weather conditions, etc.

Huge opportunities to reduce costs and greenhouse gas emissions could arise by replacing chemical products (e.g., fertilizers) with organic products (e.g., organic fertilizers, green catalysts, etc.), contributing to the local circular economy in less economically developed countries (such as Greece) that need strong efforts to reduce CO₂ emissions, in order to achieve the GHG emissions target (net zero emissions by 2050, according to the National Climate Law). The results of this review showed that the majority of the wastes can be effectively reused and transformed into high-value products such as biogas, biohydrogen, bioethanol, butanol, organic acids, proteins, activated carbons, and other industrial products.

Future studies should focus on the use of innovative tools, such as artificial intelligence, machine learning, fuel quality indicators, and simulation, in order to choose the alternative fuels with the best quality characteristics per site, seasonal variation, predict the performance of complex alternative biofuels, optimize system performance, assess the environmental footprint, and help find the best possible solutions for each rural community, town, country, and season, taking into account their special characteristics (climate, morphology, etc.). In addition, critical decisions should be made on the production, collection, transportation, and storage of waste and biomass residues for the possibility of energy production in decentralized collection points of this waste (e.g., landfills, concentration camps, industries, etc.) with the participation of each municipality and citizen in this achievement.