Environmental Research 237 (2023) 116943 Contents lists available at ScienceDirect Environmental Research journal homepage: www.elsevier.com/locate/envres Unlocking integrated waste biorefinery approach by predicting calorific value of waste biomass M. Waqas a, *, A.S. Nizami b, A.S. Aburiazaiza c, F. Jabeen d, O.A. Arikan e, A. Anees b, F. Hussain b, M.H. Javed b, M. Rehan f a Department of Environmental Sciences, Kohat University of Science and Technology, 26000, Kohat, Pakistan Sustainable Development Study Centre, Government College University, Lahore, 54000, Pakistan Department of Environmental Sciences, Faculty of Meteorology, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia d Department of Environmental Sciences, Abdul Wali Khan University, Mardan, Pakistan e Department of Environmental Engineering, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey f Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia b c A R T I C L E I N F O A B S T R A C T Keywords: Waste biomass Proximate analysis Bomb calorimeter High heating values Waste-to-energy Value added products The current study analyzed the high heating values (HHVs) of various waste biomass materials intending to the effective management and more sustainable consumption of waste as clean energy source. Various biomass waste samples including date leaves, date branches, coconut leaves, grass, cooked macaroni, salad, fruit and vegetable peels, vegetable scraps, cooked food waste, paper waste, tea waste, and cardboard were characterized for proximate analysis. The results revealed that all the waste biomass were rich in organic matter (OM). The total OM for all waste biomass ranged from 79.39% to 98.17%. Likewise, the results showed that all the waste biomass resulted in lower ash content and high fixed carbon content associated with high fuel quality. Based on proximate analysis, various empirical equations (HHV=28.296-0.2887(A)-656.2/VM, HHV=18.297-0.4128(A)+35.8/FC and HHV=22.3418-0.1136(FC)-0.3983(A)) have been tested to predict HHVs. It was observed that the heterogeneous nature of various biomass waste considerably affects the HHVs and hence has different fuel characteristics. Similarly, the HHVs of waste biomass were also determined experimentally using the bomb calorimeter, and it was observed that among all the selected waste biomass, the highest HHVs (21.19 MJ kg−1) resulted in cooked food waste followed by cooked macaroni (20.25 MJ kg−1). The comparison revealed that experimental HHVs for the selected waste biomass were slightly deviated from the predicted HHVs. Based on HHVs, various thermochemical and biochemical technologies were critically overviewed to assess the suitability of waste biomass to energy products. It has been emphasized that valorizing waste-to-energy technologies provides the dual benefits of sustainable management and production of cleaner energy to reduce fossil fuels dependency. However, the key bottleneck in commercializing waste-to-energy systems requires proper waste collection, sorting, and continuous feedstock supply. Moreover, related stakeholders should be involved in designing and executing the decision-making process to facilitate the global recognition of waste biorefinery concept. 1. Introduction The global population in 1960 was about 3 billion that increased to about 7 billion in 2021, and is projected to reach 8.1 billion by 2025 (Kumar and Samadder, 2017). The rapid population growth coupled with economic development has resulted in significant growth in industrialization and urbanization. Depending on the economic status, rate of urbanization, and consumption patterns of various materials have resulted in the significant generation of municipal solid waste (MSW) with various compositions (Waqas, 2019). The World Bank reported that by 2025, the global production of MSW will reach about 2200 million tons annually (Hameed et al., 2021). Moreover, it has been estimated that by the end of this century, the global energy demands of the growing population are expected to increase six folds than current demand (Kothari et al., 2010). In most of the developing countries, the current energy supply is still far lower than the required energy. Currently, fossil fuel is the primary source of the global energy supply and fulfills about 84% of the total power requirements (Ouda et al., 2016). However, the shortage of fossil * Corresponding author. E-mail address: waqaskhan@kust.edu.pk (M. Waqas). https://doi.org/10.1016/j.envres.2023.116943 Received 20 May 2023; Received in revised form 8 August 2023; Accepted 19 August 2023 Available online 22 August 2023 0013-9351/© 2023 Elsevier Inc. All rights reserved. M. Waqas et al. Environmental Research 237 (2023) 116943 which was the focus of this study. The key concern related to utilization of biomass as energy resource is the energy density that is usually lower as compared to fossil fuels (coals). The main reason behinds the lower energy content is the moisture content (MC) that may sometimes found in high levels in various biomass waste especially in case of food waste and green biomass waste (Özyuğuran and Yaman, 2017). Furthermore, there exist various unknown factors that affects the HHVs of biomass and these factors add more difficulty in plotting the energy potential of biomass (Friedel et al., 2005). In addition, the experimental methods for the estimation of the high heating values (HHVs) are sometime time consuming, expensive and may have the chances of typical experimental errors. Similarly, the HHVs of biomass resources cannot be calculated only from the heats of formation of the products because of the heterogeneous nature and the relevant bond energies of various compounds that cannot be estimated accurately (Friedel et al., 2005; Özyuğuran and Yaman, 2017). As it is well understood that proximate analysis only needs several easily available laboratory equipments such as oven for the determination of MC and furnace for the determination of volatile matter and ash contents which is easily available in any ordinary research laboratory. Hence based on proximate analysis, various empirical equations can be used for modelling of calorific value of biomass (Özyuğuran and Yaman, 2017). This research aims to (i) predict the HHVs of waste biomass based on the proximate analysis using various linear and non-linear empirical equations, (ii) determine the HHVs using bomb calorimeter according to ASTM D2015 standard method and investigate the comparisons with the predicted results and (iii) highlight the options under the integrated waste biorefinery concept to manage waste biomass effectively and encourage a shift to more sustainable consumption of waste as a source of clean energy. Furthermore, it has been emphasized that the abundant availability of waste as a source of sustainable energy can reduce the energy conflicts across the countries, facilitate the introduction of new technologies through waste-to-energy, reduce the water, air, and soil pollution, and the loss of natural resources for fulfilling the energy demands. Therefore, based on its sustainability, the transition to waste-toenergy technologies should be encouraged, and investment should be increased from various sectors on renewable waste biomass resources. List of abbreviations High heating values HHVs Organic matter OM Moisture content MC Municipal solid waste MSW Fixed carbon FC Organic waste OW Hydrothermal carbonization HTC Methane CH4 Carbon dioxide CO2 Ammonia NH3 Volatile fatty acids VFAs Chemical oxygen demand COD Hydrogen H2 Greenhouse gases GHGs fuel reservoirs and negative impacts such as climate change have threatened world stability. Therefore, the world needs alternative energy sources to meet future energy demands and mitigate global climate change (Waqas et al., 2018a). Hence, it is time to realize the energy potential of various resources, such as waste biomass, as a reliable option for future energy sources that will be environmentally sustainable and economically viable. All the produced waste from agricultural (animal and plant waste), residential (garden and kitchen waste), and industrial (dairy wastes, paper, sugar refinery, slaughterhouses, confectionary waste, pulp) sectors are directly disposed of dumpsites without any material recovery (Waqas et al., 2018b) that poses serious threats to the environment and public health (Fig. S1). Subsequently, the waste dumping sites are becoming an overwhelming source of soil and water contamination, emissions of greenhouse gases (GHGs), disease spreading vectors, ruthless odors and harmful leachate (Waqas, 2019). Hence, utilizing these waste as an energy source could provide the dual benefits of sustainable management and meet the growing energy demands. Moreover, biomass waste is the dominant fraction of the produced waste by contributing up to 60% of the total waste. Biomass waste generated from various sources having high moisture, lignin, cellulose, hemicellulose, and high biodegradability makes them a promising feedstock for energy production (Sansaniwal et al., 2017; Nizami et al., 2017a). Based on its physiochemical nature, utilizing waste biomass as an energy resource is getting significant attention and is currently ranked 4th energy source, contributing to about 10% of global energy demands (Raheem et al., 2016). It has been estimated that biomass waste utilization as an energy source and biofuel applications will be increased by up to 27% (Hassan et al., 2016). Biomass also contributes about 56% of energy demands among the other renewable energy sources such as wind, solar, geothermal, and hydropower (Hameed et al., 2021). Aside from these, the easy availability as compared to natural gas and petroleum, biomass is also the major and essential energy source by providing more than 33% of the total energy demand in most developing countries (Buragohain et al., 2010). The International Renewable Energy Agency (IRENA) established Global Renewable Energy Roadmap that by 2030, biomass will be the most significant renewable resource by providing about 60% of the total global energy supply (IRENA, 2018). Considering environmental integrity and sustainable development, many researchers focused on renewable energy options for fulfilling the growing energy demands through waste-to-energy technologies with minimal environmental and societal impacts. In this regard, various countries across the globe have shown remarkable research and development achievements on renewable energy technologies and the future potential of waste biomass resources for sustainable energy production, 2. Materials and methods 2.1. Feedstock collection and sample preparation The various types of biomass waste selected to predict the HHVs (MJ kg−1) were considerably different regarding their origin and fuel characteristics. To collect biomass feedstock, a small survey was carried out at the main campus of King Abdul Aziz University Jeddah-Kingdom of Saudi Arabia to determine the characteristics and composition of the produced waste. Among the waste biomass, various type of food waste was collected from the main cafeteria of King Abdul Aziz University. However, the food waste was collected regularly for one week to get a homogenous waste sample, as the food menu differed daily. Similarly, the other types of biomass, such as paper, cardboard, branches, leaves, grass clippings, and used tea, were also collected from the university’s main campus (Fig. S2). The feedstock selection was based on their composition and availability. After the feedstock collection, fractional characterization was carried out gravimetrically, and the percentages of various fractions were estimated. Likewise, the biomass feedstock was air-dried in open trays to avoid the biological decay of the freshly collected wet samples (Fig. S2). Similarly, the air-dried samples were mixed thoroughly and grinded into small particles less than 1 inch before further analysis. 2.2. Proximate analysis The proximate analysis comprised of the total moisture content (MC 2 M. Waqas et al. Environmental Research 237 (2023) 116943 %), organic matter content (OM %), ash content (%), and fixed carbon (FC %) was determined by following the standard protocols (APHA, 1998). The sample was dried in the oven (Model LDO-060E, Daihan Labtech, Korea) at 105 ◦ C for MC determination for 24 h. The percent loss in weight was calculated by using the following equation of O’Kelly (2004); MC (%) = dry weight of sample X 100 initial weight of sample accuracy and precision. Likewise, to ensure the repeatability of the experimental results, the obtained values that deviate by more than ±10% from the average value were discarded. Moreover, all the collected data were subjected to multiple variable regression analysis and analysis of variance (ANOVA) test using SPSS 16 (SPSS Inc., Chicago, IL, USA) statistical software to understand the effect of proximate analysis on HHVs of waste biomass. All the tests were performed at 5% probability level. (1) The dried waste biomass samples were kept in air-tight bottles to avoid further interaction with moisture in the air. Correspondingly for OM content determination, the oven dried samples were kept in the muffle furnace (Model 5300A30/F6010-TS, Thomas Scientific, Swedesboro, USA) at 550 ◦ C for 3 h. The percent weight loss was calculated as volatile, whereas the remaining mass after the burning in the furnace was estimated as ash content (%) according to the following equation (FCQAO: Federal Compost Quality Assurance Organization, 1994); Ash (%) = weight of ashed sample X 100 dry weight of sample 2.6. Life cycle assessment and interpretation The Life Cycle Assessment (LCA) was conducted to analyze the environmental impacts of processing one metric ton of biomass waste using the specified product system and process. The baseline for the assessment was taken from the Open LCA platform. Likewise, the gathered data from the Open LCA study provides a quantitative analysis of the impact factors associated with processing one ton of biomass. The LCA developed in this research focused on seven key environmental impacts, including global warming potential for 100 years (measured in kg CO2 eq.), human toxicity (carcinogenic and non-carcinogenic), ecotoxicity, and particulate matter (Patel et al., 2016). This quantitative information serves two essential purposes. Firstly, it helps in comprehending the environmental impacts caused by the system inputs. Secondly, it helps in identifying areas where future optimization efforts should be directed for mitigating the environmental footprint of the biomass processing. (2) The fixed carbon (FC %) in the collected waste biomass feedstock was determined using the equation presented by McClements (2005); FC (%) = 100 − (Ash%) 1.8 (3) 2.3. Prediction of the high heating values using proximate analysis The HHVs (MJ kg−1) of the waste biomass samples were predicted from the results of proximate analysis using various empirical equations containing linear and nonlinear terms developed by Özyuğuran and Yaman (2017). HHV = 28.296-0.2887(A)-656.2/VM (4) HHV = 18.297-0.4128(A)+35.8/FC (5) HHV = 22.3418-0.1136(FC)-0.3983(A) (6) 3. Results and discussion 3.1. Proximate analysis of the waste biomass The data in Table 1 shows the proximate analysis of various types of organic waste (OW). Based on the heterogeneous nature of the collected waste biomass, the results revealed that the proximate analysis showed wide ranges of variation. The findings of the MC determine using equation (1) revealed that among all the waste biomass samples, the maximum MC was observed for the vegetable components, where the highest MC was recorded for salad (91.91%), followed by vegetable 2.4. Prediction of high heating values through bomb calorimeter Table 1 Proximate analysis of various types of biomass waste samples. The waste biomass samples’ HHVs (MJ kg−1) were determined using Parr-6100 Bomb Calorimeter (Model 6100 Calorimeter with 1108P Oxygen Vessel of Alloy 20, United States) (Fig. S3). The working principle of the Parr-6100 Bomb Calorimeter is to determine the temperature or water temperature from the coal combustion. Therefore, the procedure for determining HHVs of any material using the Parr-6100 Bomb Calorimeter is carried out in various steps. First, the instrument is calibrated using 1 g of the benzoic acid tablet with a known calorific value. The benzoic acid is kept in the sample container, inserted in the cylinder chamber, and connected with a thread tied to the nichrome wire. The assembly was then put in the bomb vessel and filled with 25 bar oxygen. Following the oxygen filling, the bomb vessel was put in the water jacket, having 2 L of distilled water. Correspondingly, the jacket was put in the bomb calorimeter, and the bomb calorimeter, along with the digital bomb calorimeter, was turned on to calculate the HHVs (MJ kg−1). After the calibration analysis, the bomb vessel was removed, the oxygen remaining inside was removed, and the sample container and the vessel were cleaned for the actual sample. After calibration, 0.5 g–1 g of the sample was weighed, followed by the abovementioned procedure to determine HHVs (MJ kg−1). Sample Category Moisture Content (%) Organic Matters (%) Ash Content (%) Fixed Carbon (%) Date leaves 38.92 ± 5.45 92.60 ± 0.67 7.40 ± 0.67 Date branches 61.26 ± 5.50 94.20 ± 1.45 5.80 ± 1.45 Coconut leaves 56.16 ± 1.47 91.15 ± 0.97 8.85 ± 0.97 Grass 68.52 ± 2.06 87.43 ± 2.25 Cooked macaroni Salad 63.22 ± 0.72 98.17 ± 0.27 12.57 ± 2.25 1.83 ± 0.27 91.91 ± 1.14 93.18 ± 1.02 6.82 ± 1.02 82.54 ± 3.10 91.91 ± 2.68 8.09 ± 2.68 51.44 ± 0.37 52.33 ± 0.80 50.64 ± 0.54 48.57 ± 1.25 54.54 ± 0.15 51.77 ± 0.57 51.06 ± 1.49 87.22 ± 0.73 92.92 ± 2.70 7.08 ± 2.70 Fruit & Vegetables peels Vegetable scraps Cooked food waste Paper waste 55.81 ± 3.44 97.68 ± 0.56 2.32 ± 0.56 35.30 ± 0.04 96.87 ± 0.65 3.13 ± 0.65 2.5. Quality assurance and data analysis Tea waste 4.36 ± 0.24 79.39 ± 5.66 The proximate analysis and HHV determinations of the waste biomass samples were repeated several times to assess the results’ Cardboard 5.42 ± 0.03 92.44 ± 2.28 20.61 ± 5.66 7.56 ± 2.28 3 51.62 ± 1.50 54.26 ± 0.31 53.82 ± 0.36 44.11 ± 3.14 51.36 ± 1.27 M. Waqas et al. Environmental Research 237 (2023) 116943 scrape (87.22%) and fruit and vegetable peels (82.54%) (Table 1). Likewise, the MC in cooked food waste components was 63.22% for cooked macaroni and 55.81% for cooked food waste (Table 1). Compared to post-consumption, the pre-consumed food waste samples had significantly higher MC because they comprised wet fruit and vegetable peelings. Likewise, the diverse nature of post-consumed food wastes comprised a wide range of consumed food, including cooked rice, cooked vegetables, bread, meat, and noodle, resulting in comparatively lower MC (Ho and Chu, 2019). Furthermore, the MC in lawn-related waste such as date leaves, date branches, coconut leaves, and grass was 38.92%, 61.26%, 56.16%, and 68.52%, respectively (Table 1). However, it was observed that among all the waste biomass samples, the lowest MC was recorded for tea waste (4.36%) and cardboard (5.42%). The OM results demonstrated that all the waste biomass samples were rich in OM content. The total OM for all the waste biomass ranged from 79.39% to 98.17%. It was observed that the cooked food waste components showed the highest values (98.17%) for OM whereas the maximum OM contents were recorded for cooked macaroni followed by cooked food waste (97.68%) (Table 1). The current results findings align with those of Waqas et al. (2018b) and Ali et al. (2019), who found higher values of OM for food waste. Correspondingly, the paper waste also showed comparatively higher values of 96.87% OM. In addition, the OM in vegetable waste components such as salad, fruit and vegetable peels, and vegetable scrape was 93.18%, 91.91%, and 92.92%, respectively. Moreover, OM content for the selected lawn waste range between 87.43% for grass, 91.15% for coconut leaves, 92.60% for date leaves, and 94.20% for date branches (Table 1). Longjan and Dehouche (2018) reported that higher biodegradable OM content is required for various energy conversion technologies, whereas the feedstock with OM lower than 60% is not considered suitable feedstock for most of the biochemical processes. The findings of the ash content determined using equation (2) presented in the Table 1 showed that all the selected waste biomass resulted in lower ash content except tea waste and grass. The highest ash content (20.61%) was recorded for tea waste, followed by grass (12.57%). The accumulation of higher ash content in biomass samples is undesirable, making them unsuitable feedstock for energy conversion technologies (Motghare et al., 2016). The results showed that the lowest ash content (1.83%) was recorded for cooked macaroni followed by cooked food waste (2.32%) (Table 1). Based on previously published reports, the current investigations declared that high OM and low ash content are desirable factors that make any feedstock materials suitable for energy conversion technologies. In addition, the findings of the FC estimated using equation (3) also showed a slight variation in terms of FC for all the selected waste biomass except tea waste and grass, having FC below 50%. The lowest values (44.11%) for FC were recorded for tea waste, followed by grass (48.57%) (Table 1). It has been observed that these wastes with low fixed carbon showed high ash contents as compared to other waste biomass. In this regard, Özyuğuran and Yaman (2017) plotted the relationship between FC and ash content, where high ash content and low FC content are associated with the poor fuel quality of biomass. Likewise, the FC for all other waste biomass range from 50.64% to 54.26% (Table 1). It has been observed that the utmost values for FC (54.54%) was recorded for cooked macaroni followed by food waste (54.26%). The overall results depict that among all the waste samples, the highest FC was recorded for food waste components, followed by paper, vegetable, and lawn waste (Table 1). However, the FC in date branches was recorded at 52.33% among the lawn waste. The woody structure of date branches contains lignin-cellulosic materials with high OM and FC contents. Heating values have also termed the heat of combustion. Hence in this study, the HHVs of each type of waste biomass were determined to establish their potential role in waste-to-energy systems. The results given in Table 2 showed the HHVs experimental using Bomb Calorimeter and predicted using equation (4) (HHV = 28.296-0.2887 (A)-656.2/VM), equation (5) (HHV = 18.297-0.4128(A)+35.8/FC), and equation (6) (HHV = 22.3418-0.1136(FC)-0.3983(A) of all the selected waste biomass. It was observed that the heterogeneous nature of various types of waste considerably changed the HHVs and hence have different fuel characteristics. The experimental HHVs determination using a bomb calorimeter showed that higher values for energy contents were recorded for dry basis compared to wet basis (Table 2). The key factor was the high MC in the freshly collected waste that significantly affected the HHVs. For experimental HHV, it was observed that among all the selected waste biomass, the higher HHVs were recorded for the cooked food waste components, where the highest HHVs (21.19 MJ kg−1) resulted in cooked food waste followed by cooked macaroni (20.25 MJ kg−1). Similarly, the vegetable components of the selected OW also showed comparatively similar results, where the HHVs of 20.01 MJ kg−1 were recorded for vegetable scrapes, 19.70 MJ kg−1 for salads, and 18.14 MJ kg−1 for vegetables and fruit peels (Table 2). Among the other types of selected waste, the HHVs for coconut leaves and paper waste were 19.19 and 19.16 MJ kg−1. Furthermore, the HHVs for selected lawn waste components ranged between 15.23 and 19.19 MJ kg−1. The waste biomass was observed to be rich in organic contents, resulting in high HHVs. Waste biomass is a rich source of various organic compounds such as carbohydrates, proteins, fats, and oil and hence can be effectively subjected to various biochemical conversion processes to yield energy products (Waqas, 2019). During the biochemical and thermal treatment of OW, various processes resulted in the breakdown of complex organic compounds, such as the bond breakage of glycoside to release oligosaccharides and monosaccharides, which are highly acquired for various energy production processes (Waqas, 2019). Hence the organic-rich nature with high biodegradability makes waste biomass a promising feedstock for various waste-to-energy technologies. Table 2 Experimental and predicted high heating values for the collected waste biomass samples. Sample Category Date leaves Date branches Coconut leaves Grass Cooked macaroni Salad Fruit & Vegetables peels Vegetable scraps Cooked food waste Paper waste 3.2. High heating values of waste biomass Tea waste HHVs of waste biomass are the key indicator of the energy contained in that specific biomass. It is the energy produced in the form of heat upon the complete combustion of biomass (Motghare et al., 2016). Cardboard 4 Experimental Predicted HHV wet basis HHV dry basis HHV dry basis Equation#4 Equation#5 Equation#6 12.46 ± 1.33 7.95 ± 0.70 9.99 ± 0.36 5.76 ± 1.18 11.55 ± 1.67 15.55 ± 2.85 8.45 ± 1.57 15.23 ± 0.24 13.79 ± 0.19 19.19 ± 0.40 16.58 ± 0.24 20.26 ± 1.01 19.70 ± 2.71 18.14 ± 3.69 18.99 ± 0.25 19.59 ± 0.54 18.45 ± 0.37 17.02 ± 0.87 21.06 ± 0.10 19.21 ± 0.38 18.73 ± 1.01 15.94 ± 0.27 16.59 ± 0.59 15.35 ± 0.39 13.85 ± 0.91 18.20 ± 0.11 16.17 ± 0.41 15.66 ± 1.09 13.55 ± 0.23 14.09 ± 0.49 13.07 ± 0.33 11.82 ± 0.75 15.42 ± 0.09 13.74 ± 0.34 13.32 ± 0.90 16.49 ± 1.92 17.45 ± 1.74 13.98 ± 2.14 11.24 ± 1.52 13.45 ± 1.35 20.01 ± 1.43 21.19 ± 0.73 19.16 ± 2.59 14.95 ± 0.44 15.13 ± 0.87 19.11 ± 1.01 20.59 ± 0.21 20.59 ± 0.24 13.82 ± 2.31 18.93 ± 0.86 16.07 ± 1.09 17.67 ± 0.23 17.67 ± 0.27 10.60 ± 2.27 15.87 ± 0.92 18.66 ± 0.90 15.25 ± 0.19 14.98 ± 0.22 11.12 ± 1.90 13.50 ± 0.76 M. Waqas et al. Environmental Research 237 (2023) 116943 In addition to the experimental HHVs determination, mathematical equations (equations (4)–(6)) developed by Özyuğuran and Yaman (2017) have been adopted to predict the HHV of the biomass samples from their proximate analysis results. The HHVs of waste biomass using various linear and non-linear equations as coefficients of proximate analysis are also listed in Table 2. Equation (4) (HHV = 28.296-0.2887 (A)-656.2/VM) contains linear and non-linear terms and includes the factor of ash content and volatile contents. Likewise, equation (5) (HHV = 18.297-0.4128(A)+35.8/FC) also has linear and non-linear terms and includes the factor of ash content and fixed carbon. However, equation (6) (HHV = 22.3418-0.1136(FC)-0.3983(A) comprised only linear terms and included fixed carbon and ash content. Nhuchhen and Salam (2012) also used non-linear equations comprised of VM FC−1, ash VM−1, FC ash−1, FC VM−1, VM ash−1, and ash FC−1 and their combinations to predict HHVs. The results demonstrated that proximate analysis of waste biomass significantly affects HHVs (Tables 1 and 2). As the prediction of HHVs using equations (4)–(6) through proximate analysis is carried out on a dry basis, the comparison has been made for experimental HHVs determination on a dry basis. The comparison of experimental and predicted HHVs for the selected waste biomass was found to be deviated by −20 to 5%, and the graphical representation is given in Figs. 1–3. The difference between the experimental and predicted HHVs ranged between 0.56 and 6 MJ kg−1 for all the selected waste biomass except for equation (6), which gave slightly lower HHV for few waste biomass (Table 2). Özyuğuran and Yaman (2017) also reported similar that the experimental HHVs for various biomass differed slightly from the predicted HHVs. obtained standard error for the equations (4) and (5) was 0.105 and 0.095 respectively. The findings of the current investigation are in line with those of Huang and Lo (2020) who determined HHV of various biomass waste using various linear and non-linear equations and found the values of coefficient of determination (R2) ranged between 0.937 and 0.993. However, a slight variation has been observed for the equation (6) (HHV = 22.3418-0.1136(FC)-0.3983(A) in term of coefficient of determination (R2) and standard error as compared to equations (4) and (5). For equation (6), the recorded coefficient of determination (R2) was 0.879 with the standard error of 1.542 (Table 3). This slight variation as compared to equations (4) and (5) can be attributed to the variation in the biomass species utilized in the current investigation with high heterogeneity in their physiochemical characteristics. Similar results have also been observed by Gillespie et al. (2013) who predict HHV based on proximate analysis of biomass materials and observed the R2 value to about 0.88. Likewise out findings are also in close proximity with those of Özyuğuran and Yaman (2017) who predicted HHV of various biomass waste using various equations and found the R2 value between 0.827 and 0.837. Moreover, the ANOVA results presented in the given Table 3 also showed significant results for both the experimental and predicted HHV of various waste biomass sample. The overall results of multiple variable regression analysis and ANOVA showed that all studied proximate analysis parameters significantly affect the HHV of the studied waste biomass samples. The results of current study are in line with those of Ahmad et al. (2019), Ibikunle et al. (2022) and Kujawska et al. (2023) who reported that the heterogeneous nature of waste biomass resulted in high HHVs and hence can be effectively subjected to various waste-to-energy conversion technologies to yield energy products. 3.3. Regression analysis for the experimental and predicted high heating values 3.4. Impact of moisture content on high heating values of waste biomass The data in Table 3 showed the results of the multiple variable regression analysis and ANOVA for the experimental and predicted high heating values (MJ kg−1) against the proximate analysis of for the collected waste biomass samples. The development of regression model to predict the HHV of using proximate analysis has been well reported in literature (Qian et al., 2018). The regression analysis showed that for experimental HHV, the coefficient of determination (R2) was 0.884 whereas the standard error was 2.004. Based on the obtained results, high correlation were observed between the proximate analysis of waste biomass and experimental HHV (Table 3). On other hand, the HHV using equation (4) (HHV = 28.296-0.2887(A)-656.2/VM) and equation (5) (HHV = 18.297-0.4128(A)+35.8/FC) resulted highly closed coefficient of determination (R2) of 0.978 and 0.998 (Table 3). Likewise the In the current study, the selected waste biomass was subjected to HHVs determination both on fresh and dry basis using a bomb calorimeter, and the results are given in Table 2 and also plotted in Fig. 4. The results showed that freshly collected wet biomass resulted in lower HHVs however, after drying at 105 ◦ C for 24 h, the HHVs values enhanced significantly. Fig. 4 demonstrates that drying the waste biomass to remove the moisture momentously impacts the HHVs. Conversely, the waste biomass with lower MC showed very slight variation in HHVs for both wet and dry basis (Tables 1 and 2). For instance, the MC in tea waste and cardboard was 4.36 and 5.42%, respectively, whereas the HHVs on wet and dry basis were 11.24 and 14.95 MJ kg−1, and 13.45 and 15.13 MJ kg−1, respectively (Table 2). Özyuğuran and Fig. 1. Comparison of experimental and predicted high heating values using equation 4. 5 M. Waqas et al. Environmental Research 237 (2023) 116943 Fig. 2. Comparison of experimental and predicted high heating values using equation 5. Fig. 3. Comparison of experimental and predicted high heating values using equation 6. technologies and are highly recommended for feedstock with a high percentage of non-biodegradable complex organic contents and low MC (Table 4). Among these, incineration is considered the easiest and most widely adopted technology for treating various types of waste (Shi et al., 2016). Likewise, other technologies such as pyrolysis and gasification are also in practice to treat waste thermally, however in comparison to incineration, both pyrolysis and gasification require technical skills for operation (Table S1). Several commercially available incineration, pyrolysis, and gasification plants are in operation across the globe for treating various types of MSW (Ionescu et al., 2013). The typical reaction conditions and products from thermal treatment processes are shown in Table 4. The key difference between incineration, pyrolysis, and gasification technology is the operating temperature and oxygen supply. The quality and use of the final products from these technologies mainly depend on these two factors. Moreover, thermochemical technologies are applied to waste biomass with high energy content due to significant combustible materials (Waqas et al., 2018c). This section overviews the feasibility of various thermochemical technologies for converting waste to value-added products and their properties. Yaman (2017) reported that the HHVs of biomass is highly affected by the MC that may sometimes reach to a high levels in various biomass waste such as food waste and green biomass waste and hence lead to lower HHVs. Likewise, Motghare et al. (2016) also explored the impact of MC on HHV of biomass. They observed that the dried biomass resulted in high volatile content and high HHV as compared to wet biomass. The obtained results clarify that the simple treatment of removing the MC is adequate to enhance the HHVs. Hence, simple pretreatment could be highly helpful for processing freshly collected waste biomass for energy conversion technologies. 4. Opportunities for waste biomass as a source of sustainable bioenergy production 4.1. Thermal technologies Currently, several thermochemical technologies such as pyrolysis, gasification, incineration, and hydrothermal carbonization (HTC) are in practice to treat and convert OW to various value added products (gas, solid and liquid fuel, and heat) (Yuan et al., 2021). However, specific technology is selected based on the characteristics of the waste and the potential application of the desired waste-derived products. Generally, pyrolysis, gasification, and incineration are termed dry thermochemical 4.1.1. Pyrolysis Pyrolysis is the thermal treatment process without oxygen at 400–800 ◦ C. The process is operated for the thermal degradation of 6 M. Waqas et al. Environmental Research 237 (2023) 116943 Table 3 Multiple variable regression analysis and Analysis of variance for the experimental and predicted high heating values for the collected waste biomass samples. Multiple variable regression analysis results HHV determination Experimental HHV Equation (4): HHV = 28.296-0.2887(A)-656.2/VM Equation (5): HHV = 18.297-0.4128(A)+35.8/FC Equation (6): HHV = 22.3418-0.1136(FC)-0.3983(A) Analysis of variance (ANOVA) results HHV determination Experimental HHV Equation (4): HHV = 28.296-0.2887(A)-656.2/VM Equation (5): (HHV = 18.297-0.4128(A)+35.8/FC) Equation (6): (HHV = 22.3418-0.1136(FC)-0.3983(A) Regression Residual Total Regression Residual Total Regression Residual Total Regression Residual Total R 0.844 0.999 R2 0.865 0.978 Adjusted R2 0.885 0.997 Standard Error 2.004 0.105 0.999 0.998 0.998 0.095 0.892 0.879 0.864 1.542 Sum of Squares 28.767 40.587 69.354 40.433 .100 40.533 44.594 .083 44.677 19.718 21.417 41.135 df 2 9 11 2 9 11 2 9 11 2 9 11 Mean Square 14.383 4.510 F 3.189 Sig. .040 20.216 .011 1812.911 .000 22.297 .009 2426.510 .000 9.859 2.380 4.143 .041 Predictors (Constant) variables for all the analysis were all the parameters of proximate analysis (MC, OM, Ash, and FC). Dependent variables were experimental and predicted high heating values. Fig. 4. Effect of moisture content on high heating values of waste biomass samples. organic materials to produce char, liquid fuel, and gases however the yield and their quality are mainly dependent on the feedstock, particle size residence time, heating rate, and temperature (Kalyani and Pandey, 2014; Lombardi et al., 2015). Therefore, specific types of feedstock such as wood waste, plastic, tires, and OW are highly desired for getting the resultant products of good quality (Fig. S4). Likewise, for operating conditions, the pyrolysis of OW at a lower temperature (400–550 ◦ C) mainly yields biochar, whereas pyrolysis at a high temperature (above 550 ◦ C) majorly results in liquid oil (Fig. S4). Compared to other technologies, such as landfilling and incineration, pyrolysis of OW is considered cost-effective and environmentally friendly because of the low energy input and less emissions (Liu et al., 2017; Yao et al., 2018). Significant investigations on utilizing OW for pyrolysis have analyzed the conversion mechanism of this waste-to-energy products. The major reactions during biomass pyrolysis to yield various products comprised cracking, decarboxylation, hydrocracking, hydrodeoxygenation, and hydrogenation (Cha et al., 2016; Nzihou et al., 2019). However, based on operating parameters and final products, pyrolysis is divided into various types: slow, fast, flash, and vacuum pyrolysis (Waqas, 2019). Slow pyrolysis is the thermochemical process of treating the waste at 350–800 ◦ C with a slow heating rate (5–9 ◦ C min−1) and high residence time (hours to days). This process is also termed carbonization, resulting in high biochar and less gas and liquid oil (Zhao et al., 2018). The long residence time and slow heating rate favored secondary reactions by providing sufficient time to complete and yield high biochar and less gas and bio-oil. Slow pyrolysis is advantageous for treating waste with high organic contaminants and pathogens, such as MSW and animal waste, to convert them to biochar (Lou et al., 2016; Gupta et al., 2018). Likewise, fast pyrolysis is the thermal process carried out at elevated temperature (about 500–800 ◦ C) with short residence time (less than 10 s) and a heating rate from 300 to 800 ◦ C min−1. The key products of fast pyrolysis are bio-oil (up to 75% weight basis), biochar (up to 12% weight basis), and certain gases (Lee et al., 2013; Liu et al., 2017). The obtained bio-oil 7 M. Waqas et al. Environmental Research 237 (2023) 116943 Table 4 Operating parameters, process efficiency and products of various waste to energy technologies. Study WTE technology option Feedstock Ouda et al. (2016); Nizami et al. (2017a,b) Pyrolysis Kobayashi et al. (2008); Kirubakaran et al. (2009); Sadef et al. (2016) Ouda et al. (2016); Nizami et al. (2017a,b) Gasification Municipal solid waste, Agricultural waste, Wood Organic waste, Inorganic waste Murphy and Power. (2006); Albertson et al. (2006); Nizami et al. (2017a,b) Ge et al. (2006); Bai et al. (2008); Waqas et al. (2018a,b,c,d) Working principal Operating Temperature (oC) Process efficiency Thermo-chemical degradation of organic materials in the absence of oxygen 400–800 Thermo-chemical degradation organic materials under Partial oxidation/ controlled supply of oxygen Full oxidative combustion Incineration General waste Anaerobic digestion Organic waste, Food waste, Animal manure Bio-chemical conversion of organic materials in the absence of oxygen Ethanol production Organic waste, Wood waste Bio-chemical conversion of organic materials Products Energy production (Mega Watt/ ton/day) Operational cost per ton waste (USD) Solid & liquid Gas Up to 17% Char H2O, CO, H2, N2 0.01–0.014 17–25 800–1600 Up to 32% Ash, slag CO2, H2, CH4, N2, CO, H2O 0.04–0.045 19–30 850–1200 Up to 25% Fly ash, bottom ash, slag 0.01–0.02 14–22 – Up to 30% Digestate CO2, O2, H2O, N2 CH4, CO2 0.015–0.02 0.1–0.14 35–37 Up to Liquid Ethanol – – can be utilized as an efficient energy source based on the energy content. In addition, fast pyrolysis minimizes residence time and enhances thermal cracking at a specific temperature to convert OW to bio-oil (Lee et al., 2013). However, owing to the low thermal conductivity of the process, the low particle size (2–3 mm) is required to achieve significant heat transfer at the particle interface of waste biomass during the fast pyrolysis (Waqas et al., 2018a). Likewise, flash pyrolysis is termed the improved version of fast pyrolysis. Flash pyrolysis is carried out at the temperature range of 800–1100 ◦ C with a very short residence time (less than 2 s) and high heating time (above 1000 ◦ C) (Lee et al., 2013). The short residence time, high operating temperature, and heating rate increase bio-oil and low biochar yield momentously. For certain feedstock such as sewage sludge, flash pyrolysis technology is the feasible option for large-scale valorization to produce bio-oil (Alvarez et al., 2015). However, the key challenge in commercializing flash pyrolysis technology is the reactor configuration for waste biomass that meet the extremely low residence time and high heating rate (Canabarro et al., 2013). The fourth type of pyrolysis is vacuum pyrolysis, during which the thermochemical degradation of OW is carried out in the absence of oxygen at the temperature range of 300–600 ◦ C under low pressure of ≤0.02 MPa (Lee et al., 2013). Generally, the heating rate is somehow similar to that of slow pyrolysis. However, the residence time is low, and bio-oil yield is high due to the avoidance of secondary reactions due to the rapid removal of organic vapors produced during primary pyrolysis. However, the complex nature coupled with the high cost of vacuum pyrolysis is the major constraint in commercializing vacuum pyrolysis technology because huge vessels and piping arrangements are required to achieve vacuum conditions. Owing to its wide adaptability, pyrolysis is considered a welldeveloped thermochemical technology for treating OW with several advantages such as (i) feedstock selection is more flexible because this technology can directly treat various types of OW, (ii) based on the desired products, the process can be optimized by changing the operating condition, and (iii) environmental friendly technology with minimal gases emission. Furthermore, various researchers well reported the pyrolysis of specific types of wastes and possible commercial use of the end products as a potential energy source. For instance, in 1987 in CO2 Germany, a plant with a capacity of 110 tons day−1 successfully generated electricity through pyrolysis MSW (Lombardi et al., 2015). Likewise, the successful implementation of pyrolysis technology for treating MSW has been well reported in several countries such as Toyohashi, Japan (295 tons day−1), France (191 tons day−1), Hamm, Germany (275 tons day−1), UK (22 tons day−1) (Panepinto et al., 2014). Likewise, Baggio et al. (2008) reported that the net conversion efficiency of the pyrolysis of MSW for gas production is 28%–30%. They also reported that produced gas could be effectively utilized for energy recovery by using gas turbines. As discussed that pyrolysis has several advantages over other thermal technologies in treating specific waste streams to produce valuable products however, significant research still needs to be conducted on energy recovery from waste biomass through pyrolysis at a commercial scale. 4.1.2. Gasification Gasification is another thermochemical technology carried out at an elevated temperature and controlled oxygen supply during which the organic compounds are converted into syngas. As the main gasification product, syngas combines various types of gases, including methane (CH4), H2, CO2, and CO (Table 4). In addition, the products are also comprised of CXHY compounds and heavier hydrocarbons (tars) that usually liquefy at 250–300 ◦ C temperature range. Moreover, gasification is also used to generate feedstock for other liquid and chemical fuels (Yap and Nixon, 2015). Biomass gasification has received significant attention in recent decades due the global attention towards mitigating climate change and the hike in the oil price (Sikarwar et al., 2016). The key benefits of gasification over typical combustion and incineration are associated with cost efficiency and balanced connection among operational conditions such as reactors types and temperature conditions to obtain gases for energy utilization (Table S1). Generally, this technology has been widely used in coal however recently, gasification has been under consideration for energy recovery from waste biomass (Arafat and Jijakli, 2013). During the process of ideal gasification, no solid residues (biochar) are produced however, in real cases, the gasification process usually ends up with a low biochar yield (Kambo and Dutta, 2015). The process of biomass degradation to produce syngas during the gasification process is comprised of 4 steps 8 M. Waqas et al. Environmental Research 237 (2023) 116943 including; (i) drying carried out below 250 ◦ C, (ii) pyrolysis conducted at 250–500 ◦ C temperature range, (iii) partial combustion, and (iv) reduction. The operating temperature for reduction and partial combustion touches about 800–1200 ◦ C. Oxygen, air, steam, or their mixture are effective oxidizing agents during the process. Therefore, the temperature is considered a critical parameter affecting syngas yield, biochar and tar yield. The yield of tar and biochar decreases with increasing gasification temperature (Zhang et al., 2019a,b). As discussed that the key product of gasification is syngas (up to 85%) with a low amount of biochar (up to 10%) and oil and tar (up to 5%) (Gasco et al., 2017). This showed that as compared to other thermochemical technologies, such as pyrolysis, the biochar yield in gasification is lower in gasification (Cha et al., 2016). This is because partial combustion converts the major portion of organic biomass to gases. The waste biomass conversion process is carried out in various components in the gasification system. These include a feed system, primary heat exchanger, primary reformer, gas filter, and syngas cooler. Before entering to feed system, the waste biomass is grinded or milled depending on the size and nature of the biomass. The grinded materials are fed through gravity to the metering bin above the feed system and subsequently enter the feed system. The feed materials are then conveyed through a sealing mechanism that acts as a pressure seal on the system’s front end. This mechanism keeps the air out of the reformer and prevents the syngas from backing up into the feed system. Correspondingly, the feedstock materials are then conveyed into the primary heat exchanger. The primary heat exchanger performs two key functions: biomass feedstocks are conveyed to the convection section where pre-heating, evaporation of water, and devolitization occurs. Secondly, the syngas that leaves the primary reformer is hot enough, giving their heat energy to the primary heat exchanger and cooling down to the desired process temperature before proceeding to the gas filter. Likewise, in the primary reformer, the conveying syngas and partially gasified (reformed) biomass pass through the primary heat exchanger’s convection section to the primary reformer’s radiant coil section. In the primary reformer, high-temperature steam reforming occurs. The gas filter receives syngas, and the syngas and char are passed through the filter element to collect and remove the dried solid residues (char). The char is collected in the bin, where it can be disposed of or reused. After passing through the gas filter, the air-cooled heat exchanger receives the clean syngas to reduce the gas temperature to the optimum level for fuel uses or other power generation. Several researchers explored the operation of gasification systems to treat processed organic and MSW around the globe (Panepinto et al., 2014). For instance, 85 gasification plants were operating, treating tremendous MSW to produce energy in Japan. Likewise, several other countries, such as Germany, the UK, the USA, Italy, Iceland, and Norway, also used gasification technology to manage MSW (Panepinto et al., 2014). Gasification technology is popular because it produces less CO2 emission than incinerators of the same capacity (Narnaware and Panwar, 2022). Moreover, it has also been reported that advanced gasifiers are designed to have minimal soil and water contamination (Defra, 2013). Recent advancements show that gasification technology is considered as a promising global waste-to-energy technology (Ouda et al., 2016). Zaman (2010) reported that among the thermal technologies, gasification, and pyrolysis are promising technologies to treat MSW and recover energy in an environmentally friendly way. Furthermore, gasification and pyrolysis significantly reduce the overall water volume by up to 95% and require less air input than incineration (Yap and Nixon, 2015). Though gasification is considered an effective waste-to-energy technology in terms of energy recovery and environmental efficiency however still need extensive research to establish large-scale gasification systems for energy recovery from MSW (Luz et al., 2015). The key challenges in the process efficiency of gasifiers and gas cleaning are the MSW’s heterogeneous nature and high MC, which must be effectively addressed for large-scale gasification systems. 4.1.3. Incineration Incineration is the thermal technology of burning biomass feedstock at high temperatures (800–1200 ◦ C) in the presence of sufficient air supply (Table 4). The produced heat from incineration can be successfully utilized for running turbines to generate energy. Moreover, the heat can also be used in heat exchangers operation. Initially, the sole purpose of incineration was to reduce the overall volume of the waste and protect the public and environmental health from the toxic impacts of various hazardous wastes instead of energy recovery (Brunner and Rechberger, 2015). However, with the advancements in air pollution control strategies, incineration is now considered a promising technology for treating various types of waste (Ouda et al., 2016). The dual benefits of the process are the volume and mass reduction by up to 90%, along with the generation of heat and electricity (Nixon et al., 2013; Lombardi et al., 2015). By processing one ton of MSW, the incineration process generates about 544 KWh of energy and about 180 kg of solid residues (Zaman, 2010). Furthermore, in cold countries, the heat produced from incinerators can be supplied to the central heating system. Likewise, the heat is also supplied to run various industries, such as the paper mill, and sometimes to produce electricity (Brunner and Rechberger, 2015). In addition to the size reduction and heat/power generation, the recent research studies also highlighted other advantages of incineration, including the utilization of bottom ash in cement production and road construction and extraction of various mineral substances (Meylan and Spoerri, 2014; Allegrini et al., 2014). Based on the technological advancement to recover valuable metals and minerals from bottom ash after incineration will significantly increase the public perception of this waste-to-energy technology (Morf et al., 2013). However, in the least developed countries, incineration is considered an effective and economical technology for managing MSW without pretreatment to produce heat/electricity (Table S1). In addition, the other main advantage of incineration is the complete mineralization of organic substances and the destruction of any living organisms present in MSW so that end products are free of harmful substances (Brunner and Rechberger, 2015). Tan et al. (2014) reported that the high incineration temperature makes the process suitable for decomposing non-biodegradable MSW. However, sometimes supplementary fuels are used during the incineration of MSW, but when the latent heat of vaporization of the MSW range between 1000 kcal kg−1 and 1700 kcal kg−1 or above. For effective incineration operations to recover energy, the average HHV of MSW should be at least 1700 kcal kg−1 (World Bank, 1999). Likewise, International Energy Agency suggested that HHV values must be higher than 1900 kcal kg−1 for effective incineration (Melikoglu, 2013). However, the performance of incineration is affected by the MC and presence of inert substances that directly affect the energy content and combustibility of MSW. It is because the latent heat of vaporization decreases with increasing the waste stream’s MC, resulting in lower calorific values. In this regard, pretreatment, such as chemical, mechanical, thermal, or biological, of the waste feedstock is sometimes required to remove inert substances, excess moisture, and toxic elements (Lombardi et al., 2015). As it is well understood that the key advantages of incineration are the reduction of waste volume by up to 80% along with hear and/or power generation however being an old technology, sometimes, conventional incineration is associated with the production of several harmful emissions such as furans, dioxins and certain metals (Table S1). Moreover, sometimes the produce incombustible ash containing several heavy metals such as cadmium, mercury, and arsenic that pose serious health hazards (Zhu et al., 2020). However, considering the potential negative impacts of incineration, significant advancements have made the overall technology an effective and eco-friendly option. For example, 9 M. Waqas et al. Environmental Research 237 (2023) 116943 Leme et al. (2014) reported that in the United States (US), the emissions from waste incinerators are reduced to the extent that US-EPA declared incineration a cleaner technology to treat MSW and produce energy. carbons, and HHV value are significantly affected by the operating temperature of HTC. The increase in the operating temperature results in the reduction of H:C and O:C molar ratios and mass yield of solid carbons and hence increase the HHV (Li et al., 2021). The several economic and environmental benefits of HTC resulted in the exploration of this technology as a sustainable approach to convert biomass waste and other MSW to solid fuel (Berge et al., 2015). However, converting various types of biomass waste to hydrochar using HTC is still in the early stage. The heterogeneous nature of OW and the treatment options for the liquid outputs still require significant research development to be investigated. 4.1.4. Hydrothermal carbonization Hydrothermal carbonization (HTC) is a unique thermal technology and getting greater attention due to its potential to transform the types of waste, especially biomass waste, with a higher level of MC (80%– 90%) (Table S2). This process is also considered a wet process that stabilizes various types of waste to valuable products at constant pressure and a lower temperature, typically between 180 and 350 ◦ C (Bach and Skreiberg, 2016; Zhang et al., 2018a,b). The other thermochemical technologies, such as pyrolysis, gasification, and incineration, are mostly subjected to dry waste. However, the suitability of wet waste for the HTC is the high MC, especially in the OW that are available for the process. Numerous benefits of HTC comprised waste stabilization, short processing time, odors, and contamination-free procedure. In addition, the high-temperature treatment during the process eliminates pathogenic elements. The resultant product of HTC is termed hydrochar, and the characteristics of hydrochar are depended on operating parameters such as heating rate, residence time, and hydrothermal temperature (Wang et al., 2018; Li et al., 2021). The temperature of the process is termed the critical parameter affecting the properties of hydrochar because the temperature determines the ionic reactions to occur in a subcritical region due to changes in the water properties (Table S2). The first step in the process of HTC of biomass waste is hydrolysis, resulting in the depolymerization of hemicellulose into oligomers and monomers. Subsequently, etherification, acetalization, and aldol condensation produce solid hydrochar with optimum MC (Shi et al., 2016). However, increasing the residence time decreases the yield of the hydrochar. In addition, the heating rate is also a critical parameter affecting the hydrochar, where a low heating rate results in a higher yield of hydrochar. Sometimes the hydro char is confused with biochar; however, there exists the key difference that biochar is obtained as a result of dry carbonization technology (pyrolysis), whereas hydrochar obtained from HTC needs to be separated from the solution used for the process (Kambo and Dutta, 2015). Moreover, biochar and hydrochar also exhibit significant differences between their physical and chemical characteristics (Kambo and Dutta, 2015). HTC technology is getting significant attention due to its economically feasible and eco-friendly technology. The process requires inexpensive chemical reactor media with only water and inert gases for the reactor. In addition to the low cost, the elimination of pretreatment requirements, low temperature, and waste stabilization without toxic chemicals further enhance the suitability of HTC as a promising technology (Benavente et al., 2015; Hitzl et al., 2015). Likewise, compared to dry technologies, the hydrochar produced resulted from HTC possessing high HHV with less inorganic compounds. This is due to the solubility of organic and inorganic contents in pressurized subcritical water that releases minerals and salt from the OW (Kambo and Dutta, 2015). To substitute fossil fuels, the hydrochar with high HHV values possesses great potential to be used as a solid fuel source (Table S2). In addition, during the HTC of OW, about 60% of the total nitrogen is transferred to the liquid product as a result of the degradation of nitrogen-based compounds that had a benefit on the properties of the obtained hydrochar and resulted in minimal NOx emissions as compared combustion to raw biomass wastes (Theppitak et al., 2020). In addition, any fuel’s characteristics mainly depend on H:C and O:C ratios to assess their properties for energy application (Li et al., 2021). It has been reported that hydro char with low H:C and O:C ratios possess good fuel properties owing high degree of carbonization, stable carbon configuration, and high content of aromatic compounds (Zhou et al., 2019). Based on H:C and O:C ratios, the results findings of Li et al. (2021) reported that the atomic ratios of hydrochar were closely related to that of lignite and peat. However, O:C and H:C ratios, mass yield of solid 4.2. Biological technologies The decomposition of organic fractions of MSW through microbial activities is termed biological technology, which is documented as an environmentally viable method for energy recovery from MSW (Waqas, 2019). Waste containing high moisture and organic biodegradable content is usually preferred for biological technologies. The key technological methods for energy recovery from waste biomass under this category are ethanol production, anaerobic digestion, volatile fatty acids (VFAs) and biohydrogen production. 4.2.1. Ethanol production Numerous renewable biomass constituents are fermented using yeast to produce ethanol, either directly utilized as a fuel source or consumed as a feedstock to produce various other industrial products like ethylene (Table 4). Starchy raw materials like wheat, potato, and sorghum corn are the most commonly used feedstock for the fermentation to produce ethanol (Pietrzak and Kawa-Rygielska, 2014). OW such as banana and potato peel, mixed food waste, kitchen waste, and bread have been effectively considered feedstocks for bioethanol production. However, to increase the digestibility of starch and cellulose, several feedstocks, such as food waste, need pretreatment such as thermal and enzymatic processes and alkali and acid treatment due to their complex lignocellulosic nature (Pham et al., 2015). The standard technique is enzymatic hydrolysis among the pretreatment approaches, which is a two-step procedure to produce shortchained dextrins by starch liquefaction carried out by a-amylase (EC 3.2.1.1). In the first step, 1,4-glycosidic bond breakage occurs at the amylose and amylopectin chain; while in the second step, monomeric sugar (glucose) is produced dextrin saccharified through glucoamylase (EC 3.2.1.3). However, various enzymes such as proteases, pullulans, and cellulases are added up as a nutrient source for microbes (yeast) to produce fermentable sugars and free amino nitrogen (Sapinska et al., 2013). After hydrolysis, the obtained mash is exposed to ethanol fermentation via inoculation by yeast. After fermentation, distillation is carried out to obtain refined ethanol, and this process of ethanol manufacturing is recognized as separate hydrolysis and fermentation (SHF). Various studies reported the yield of ethanol of 29.1 g L−1 and 32.2 g L−1 produced from the food waste treated with amylases (Uncu and Cekmecelioglu, 2011), respectively. Likewise, the fermentation of waste bread was compared for various pretreatment and without treatment methods and proved that fermentation with pretreatment methods, for instance, enzymatic pre-hydrolysis, sonification, and microwave irradiation improved the yield of ethanol to about 12–35 g kg−1 as compared to the fermentation without pretreatment. However, using granular starch hydrolyzing enzymes to convert waste bread to ethanol directly resulted in a high yield (354 g kg−1 of raw material) (Pietrzak and Kawa-Rygielska, 2014). 4.2.2. Anaerobic digestion The process of degradation of OW in the absence of oxygen to produce biogas and other gases is termed anaerobic digestion (or bio methanation). In anaerobic digestion, the organic fraction of the 10 M. Waqas et al. Environmental Research 237 (2023) 116943 biodegradable MSW gets degraded and converted into CH4 through a series of stages (Fig. 5). The initial stage is called hydrolysis, in which the complex organic compounds of MSW, like carbohydrates, proteins, and fats, get converted into soluble organic materials such as sugars, amino and fatty acids (Fig. 5). Fermentation is the next stage of the anaerobic digestion process, in which the organic molecules break into acetic acid, H2, and CO2. The final stage is methanogenesis, in which CH4 formation takes place. The detailed process flow for converting OM into CH4 is shown in Fig. 5. The quality of the produced biogas is mainly based on the composition of the substrate and process parameters (Table S3). Biogas normally comprises 25%–50% CO2, 50%–75% CH4, and 1%– 15% of other gases, including H2S, ammonia (NH3), and water vapor (Surendra et al., 2014). Anaerobic digestion also produces digestate/slurry, which can be utilized as a soil conditioner and organic amendments to the agricultural lands (Pivato et al., 2016). The feedstock quality (vitamin, protein, and mineral content of waste) significantly affect the quality of solid products from anaerobic digestion (Ali et al., 2016). The potential occurrence of unwanted constituents in the feedstock, the resultant solid products from anaerobic digestion as fertilizer is prohibited according to European legislation (Browne et al., 2014). The processes of anaerobic digestion are primarily of two types, "dry" (containing 24–40% of dry matter fraction) and "wet" (containing 10%–15% of dry matter constituents) methods. Less solid products and more liquid waste are produced in the wet process, which requires less reactor volume than the dry method. The CH4 yield, reactor type (single or multi-stage), and procedures (dry or wet) are based on the quality of feedstock and required products (gas or digestate) (Table S3). It is estimated that anaerobic digestion can produce 2 to 4 times more CH4 per each ton of MSW in 3 weeks than landfill in 6–7 years (Saxena et al., 2019; Fazzino et al., 2021). Moreover, it has also been reported that 1 ton of MSW generates about 150 kg of CH4 from anaerobic digestion, considering 40% MC and 60% OM (Scarlat et al., 2015). However, long periods (20–40 days) of microbial activities are the major problem allied with this process (Pham et al., 2015). Sometimes, cations (like calcium, sodium, and potassium) and nitrogen-rich elements in the waste stream intensify the concentration of salt and NH3, making the procedure noxious for methanogenic activities (Rahmani et al., 2022; Mishra et al., 2022). Several studies suggest combining municipal solid waste with food waste, sewage sludge, and low nitrogen content waste to decrease the high NH3 concentration and increase biogas yield (Latifi et al., 2019). The CH4 yield of the organic fraction of MSW under different operating conditions reported by various researchers is summarized in Table S3. Most researchers used food waste and a suitable inoculum for maximum gas recovery. The excellence of the biogas produced using an anaerobic digestion process can be enhanced by eliminating CO2 and other trace gases as transport fuel which can substitute natural gas in industrial and domestic applications (Kasturirangan, 2014). Previously, anaerobic digestion was used to treat animal manure, domestic sewage, and agricultural waste. However, it is currently widely used to recover energy from waste biomass and various types of MSW, particularly in developing states generating waste with high moisture levels (Yap and Nixon, 2015). Biogas produced from anaerobic digestion is considered environmentally and economically sustainable (Abbas et al., 2017). 4.2.3. Volatile fatty acids As potential renewable carbon source, volatile fatty acids (VFAs) have numerous applications, including N removal, biodiesel production, and polymer synthesis produced during the treatment OW in AD (Zhou et al., 2018). In addition, the storage of VFAs is comparatively easier and safer than biogas. Furthermore, biogas is less economically valued at about 0.72 $ m−3 than the VFAs (about 130 $ ton−1), which makes it a more remarkable product from biomass waste fermentation (Waqas, 2019). At the first stage, acidogenic bacteria converted the hydrolysate monomers to H2, CO2, propionate, alcohol, acetate, and butyrate during the process of AD, followed by further conversion of butyrate, acetate, alcohols, and propionate to acetate by acetogenic pathways. The main products of VFAs of acidogenic fermentation are propionate, acetate, and butyrate (Jiang et al., 2013). The approaches for growing the VFAs production through AD include 1) refining the acidogenesis process, 2) abolishing the constraining factors, and 3) enhancing the rate of hydrolysis to yield soluble substrates targeted to enhance the carbon availability for its translation to VFAs. Therefore, during acidogenic fermentation, hydrolysis is measured as a rate-limiting step in acidogenic fermentation (Waqas, 2019). Although, the increase in hydrolysis rate is directly associated with operational constraints such as temperature, pH, and pretreatment of food waste before fermentation. Pretreatment of food waste in anaerobic fermentation increases the production of soluble chemical oxygen demand (COD), a crucial intermediate in connecting the acidogenesis and hydrolysis (Güelfo et al., 2011), making it a promising approach to raising VFAs production. Food waste can be pretreated with various methods like chemical (acid and alkaline), biological (enzymes), and physical (ultrasound, thermal, and microwave) methods. Acidogenic fermentation of food waste under thermal, enzymatic, and joint thermal-enzymatic pretreatment has been studied and found to increase VFA production and COD generation in all pretreatments. However, the combined thermal-enzymatic treatment was observed as an increased VFAs production. Moreover, the degradation of OW also needs highly effective microbial communities. Thus, the critical drivers to speed up the process are precise operational inoculums. To improve the VFAs production with reduced consumption, the methanogens activities must be inhibited through inhibitor addition, thermal pretreatment, and pH control. The promising pH for the methanogens activities ranges from 7.8 to 8.2, hence the digester pH required to be adjusted to favor the production of VFAs and constrain Fig. 5. Process, stages and products of anaerobic digestion of organic waste to yield biogas. 11 M. Waqas et al. Environmental Research 237 (2023) 116943 methanogenesis (Wang et al., 2014). Likewise, temperature is another variable that affects substrate hydrolysis, enzymatic activities, and microbial biomass. In addition, it has been reported that heating the inoculum to 100 ◦ C or above deactivate the non-pore formation and improve the yield of VFA (Yan et al., 2014). Generally, the mesophilic range (35 ◦ C) is measured as efficient and economical for VFA production (Jiang et al., 2013). However, the optimum temperature fluctuates by examining the VFAs’ composition. For example, acetate and propionate are generated at 35 ◦ C and 45 ◦ C in the fermentation of food waste, whereas butyrate is formed above 55 ◦ C, followed by propionate and acetate. type, and reactor configuration influence H2 fermentation from OW and identify significant gaps and variations in scientific data (De Gioannis et al., 2013). In the process of fermentation, the yield of H2 is also affected by waste composition (Alibardi and Cossu, 2016). For example, a high yield of H2 from carbohydrate-rich substrates has been reported compared to the lipid and protein-rich substrates (De Gioannis et al., 2013). However, for fermentative H2, macromolecules, the stored nutrients, must be fragmented into accessible forms, such as glucose and free amino nitrogen, before microbial consumption (Han et al., 2015). Thus, numerous pretreatment methods have been established for translating macromolecules into necessary components (Fig. 6) (De Gioannis et al., 2013). Enzymatic hydrolysis is a promising method that speeds up the hydrolysis process of food waste and releases nutrients. Several research studies have found that heat treatment results in high H2 production without harming H2 consuming bacteria compared to untreated food waste. It is documented that pretreatment is an important parameter in increasing the yield of H2 from OW (Elbeshbishy et al., 2011). Other factors like biodegradability, COD, particle size, nutrient concentration, OM, and MC influence H2 production. Likewise, it has also been reported that a high yield of H2 is obtained from food waste at optimum MC, and C:N ratio is up to 20 (Sreela-Or et al., 2011). Among the various types of organic biomasses, food waste is considered suitable feedstock for producing H2 due to the availability of indigenous microbial consortiums and high carbon content (Waqas et al., 2018d). 4.2.4. Biohydrogen production Various biotechnological processes, such as dark and photo fermentation tied with the AD, two stage H2+CH4 fermentation, and one-stage H2 fermentation, are used to produce biohydrogen (H2) from various types of waste biomass (Alibardi and Cossu, 2016). Biohydrogen can be utilized in various applications ranging from electricity generation to transport fuel as a clean and green fuel. Without producing GHGs on combustion, H2 has appeared among the best renewable biofuel since it has the highest energy potential. As it well understood that the increased industrialization, GHGs emissions, global warming, climate change, and other environmental issues are at extreme levels. Thus, H2 gas is considered green, eco-friendly, and less polluted than fossil fuels in the twenty-first century (Kumar and Sharma, 2014). In this regard, the use of OW to produce H2 is instantaneously resolving the problem of waste and renewable energy production. In biorefinery, the fundamental role of producing H2 from OW is favored by dark fermentation because of its low energy requirements (Fig. 6) (Alibardi et al., 2014). In addition, H2 is produced faster in dark fermentation as compared to photo fermentation. Similarly, the economic benefit of the process can be enhanced by using inexpensive existing feedstock like food waste. Moreover, the scientific community is concerned with designing new technologies to produce H2 from waste because of the utilization of H2 in electricity generation and rigorous environmental regulations towards cleaner energy (Enterprise Application Integration, 2009). Numerous factors like micro-nutrients availability, requirements of pretreatment approaches, type and origin of fermentation, substrate 4.3. Life cycle assessment of waste-to-energy technologies The data in the Table S4 showed the results of the LCA analysis across seven impact categories. Negative values in the table indicate environmental impact reductions, while positive values indicate net environmental loads. The LCA results of open dumping are given for comparing the environmental impacts with other waste-to-energy options. Notably, the highest contribution to the global warming impact category is attributed to the open dumping. The primary elements responsible for this global warming impact are CH4 non-fossil, and CO2. Likewise, various studies have reported diverse levels of toxic metals found in dumpsites (Abu-Daabes et al., 2013). Among the various waste-to-energy technologies, incineration contributes the most to the Fig. 6. Operating parameters and pretreatment of waste biomass for hydrogen production through dark fermentation. 12 M. Waqas et al. Environmental Research 237 (2023) 116943 global warming impact, human toxicity, including both non-carcinogenic and carcinogenic effects. However, it is worth mentioned that the CO2 generated during incineration from the biogenic source is considered carbon neutral, corroborated by findings from other relevant studies (Müller et al., 2020). Gheewala et al. (2022) reported that the production of the value-added products through various technologies in the biorefinery system significantly pose environmental impacts however the eco-efficiency of the resultant product lead to considerable total product values. Furthermore, for various impact categories pyrolysis technology showed environmental benefits for eco and human toxicity. In addition to all these, another noteworthy category included in the LCA is the particulate matter due to growing awareness of its potential dangerous impacts towards human well-being (Brusseau et al., 2019). The results showed that pyrolysis technology demonstrated a lower environmental burden in term of particulate matter as compared to other technologies. The overall results revealed that waste valorization in energy conversion technologies demonstrates substantial environmental benefits along with valuable products in comparison to open dumping. However, LCA is not only the assessment tool for assessing the sustainability of any waste-to-energy technology. Hence, several other detailed sustainability assessments need to be explored to fully understand the impacts on economic returns, potential investment for industrial sector, product production and environmental impacts. The various sustainability assessments tools to study the impacts are net energy balance and net energy ratio, employment generation assessment and eco-efficiency assessment (Gheewala et al., 2022). of process circumstances resulting in harmful intermediate compound production, which causes low yield and reduced system stability. For example, the energy generation from biomass gasification bearings abundant substantial risks with high environmental and social influence. Partial burning and oxidation generate toxic gaseous constituents and elements like NOx, CO, SOx, and unstable organics, which are also hazardous for humans and the environment. Thus, seepage should be avoided in gasification; an active gas clean-up with an appropriate conditioning structure is a dynamic part. Reducing the possibilities of partial oxidation during gasification is also an efficient way to minimize the risk of harmful gas release. Improper disposal of ashes and toxic elements obtained after gasification creates conservational complications, which can be subsidized by properly dumping remainders. Similarly, several types of OW containing high protein and lipids lead to the production of long-chain fatty acids, NH3, and hydrogen sulfide during biochemical progression (Xu et al., 2018). The other technical encounters are scaling up the technology, refining end material, and valuing biomass that inspires the researchers to break through to find probable solutions. For instance, intensification has been a main challenge for solid-state fermentation for a long time. However, the current initiation of biochemical engineering resulted in designing many bioreactors with a large-scale capacity for waste treatment with online monitoring of numerous method parameters, including transfer of mass and heat. However, purification and product recovery are expensive. Thus, an indepth technical and economic feasibility study is required before scaling up the process. Furthermore, an individual technology cannot achieve the zero waste concept, nor can it competes with other renewable energy sources such as solar and wind. The technological solution to these challenges is the selection of conversion technologies according to the characterization and composition of the waste and their integration in waste biorefinery. 4.4. Techno-economic feasibilities and approaches to overcome limitations of waste-to-energy technologies Environmental quality is a growing concern for any community keeping under consideration the dependency on fossil fuels, impacts and increasing prices of fossil fuels and improper disposal of waste material. The environmental and economic benefits associated with waste-toenergy technologies improved public well-being, reduced pollution and dependency on fossil fuels, and new business with job creation. Waste-to-energy technologies have tremendous environmental value, reduced emissions of GHGs (Waqas et al., 2018a), groundwater and soil protection, energy saving, and landfill saving (Nizami et al., 2015, 2017b). Carbon dioxide (CO2) generation from landfill CH4 through thermal conversion is attributed to reduce the GHG emissions. From the global warming perspective, CO2 is 21 times less detrimental than CH4. Reductions in GHG emissions have extensive financial revenue through the claim of carbon credits. Similarly, process efficiency, infrastructure requirements, feedstock, commercializing, and end-use application are certain restrictions associated with each waste-to-energy technology. To overcome these challenges, all the characteristics of waste biomass need a detailed understanding of this emerging waste-to-energy area of research. Likewise, all types of waste are mixed and thrown, thus making it challenging to collect segregated waste, which needs proper campaigns to highlight the importance of segregation of OW at sources like food industries, urban planning departments, and households. Policy development regarding waste transportation to collection points, sorting, and methods for separating non-biodegradable material from biodegradable waste is essential to further smoothing the processing and use of the collected OW. Constant waste biomass supply is needed to start large industries for waste recycling. However, variation in the waste type region-wise is an important factor to consider, which may affect the sorting and separation of waste according to the waste generated, which is a barrier to the practical implementation of a large industry of sustaining waste. Small processing units could be installed with numerous waste producers like restaurants, parks, and residentials, which also minimizes the cost of transportation. Furthermore, every technology converting waste-to-energy products faces technical challenges, including control 4.5. A way forward toward the commercialization and practical applicability of waste-to-energy systems An extensive supply chain is required to develop sustainable waste biorefinery. Adequate feedstock availability around the year is the first main component in this supply chain to convert waste-to-energy products, and OW could contribute meaningfully to meeting this growing feedstock requirement. However, bioengineering tools must be developed to understand the feedstock composition and selection of suitable technology for fermentation. In addition, developing economically viable waste collection methods, sorting, transportation, storage, and pretreatment are highly required. Finally, a reliable and continuous feedstock supply is also required for biofuel production at commercial scale. The development of integrated biorefineries is highly associated with the cost-effective operation is necessary to improve the efficiency of biofuel production along with the effective use of the products (solid material, heat, and electricity) for economic processes. Biofuel distribution is another important step after the production of biofuel, and compatibility with available components of infrastructure like dispensing followed by transportation and storage along with quality control is an important factor in ensuring that the generated biofuel meets all the product specifications and standards. Biofuel consumption in energy, fuel, and other applications is the final stage. Replacement and implementation of conventionally used equipment with biofuelcompatible equipment of equal performance are necessary for customers’ satisfaction with biofuel use. Process economics is the final important deliberation in the whole supply chain. Favorable economics must convert waste into value-added products at each supply chain point to make it commercially successful. In this regard, the sustainability of biorefinery concept of biomass waste to energy needs to be evaluated using life-cycle assessment studies to access their economic affordability and technological possibility. 13 M. Waqas et al. Environmental Research 237 (2023) 116943 5. Conclusions and outlook References HHVs of waste biomass are essential to analyze while designing waste-to-energy systems. In this regard, various biomass waste samples were characterized for proximate analysis to assess their fuel characteristics. The results showed that all the waste biomass possesses high OM, fixed carbon, and low ash content except tea waste and grass. Based on the proximate analysis results, the HHVs of collected waste biomass were predicted through various empirical equations. The results showed that the heterogeneous nature of various types of biomass waste considerably affects the HHVs and hence could serve as suitable feedstock for various energy conversion technologies. Moreover, HHVs of waste biomass were also determined experimentally using a bomb calorimeter, and it was observed that among all the selected waste biomass, the higher HHVs were recorded for cooked food waste components. However, the results also revealed that MC significantly affects the HHVs where the wet biomass resulted in lower HHVs than dried biomass. Correspondingly, based on HHVs, various energy conversion technologies were critically overviewed to assess the suitability of waste biomass to valuable products such as biogas, liquid fuel, heat, and chemicals. 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