Valorisation of food waste via hydrothermal carbonisation and techno-economic feasibility assessment

Unlocking biochar impacts on abiotic stress dynamics: a systematic review of soil quality and crop improvement

Global agricultural challenges, especially soil degradation caused by abiotic stresses, significantly reduce crop productivity and require innovative solutions. Biochar (BC), a biodegradable product derived from agricultural and forestry residues, has been proven to significantly enhance soil quality. Although its benefits for improving soil properties are well-documented, the potential of BC to mitigate various abiotic stresses-such as drought, salinity, and heavy metal toxicity-and its effect on plant traits need further exploration. This review aims to elucidate BC production by highlighting primary feedstock's and synthesis techniques, and examining its role in boosting soil decomposition efficiency and fertility, which are pivotal for sustainable crop growth. This review also discuss how BC can enhance the nutritional and chemical properties of soil under different abiotic stress conditions, emphasizing its capacity to foster crop growth and development in adverse environments. Furthermore, this article serves as a comprehensive resource for agricultural researchers in understanding the importance of BC in promoting sustainable agriculture, and addressing environmental challenges. Ultimately, this review highlights critical knowledge gaps and proposes future research avenues on the bio-protective properties of BC against various abiotic stresses, paving the way for the commercialization of BC applications on a large scale with cutting-edge technologies.

Hydrothermal carbonization of combined food waste: A critical evaluation of emergent products

Hydrothermal carbonization (HTC) increasingly appears as an eco-friendly method for managing food waste (FW). In this work, a combination of FW was subjected to HTC, and products were critically evaluated. This involved a lab-scale pressure reactor and optimization of HTC conditions: temperature (220-340 °C) and residence time (90-260 min) via central composite design type of response surface methodology (CCD-RSM). Results showed varying temperatures and residence time to impact the hydrochar (HC) and hydrothermal carbonization aqueous phase (HTC-AP) properties. Although HC produced through HTC exhibited lower ash content (<2%) despite higher fixed carbon (>55 %) with respect to the raw FW, the heating value of HC ranged from 19.2 to 32.5 MJ/kg. Temperature primarily influenced FW conversion, affecting carbonaceous properties. Saturated fatty acids (SFA) were found to be predominant in the HTC-AP under all tested operating conditions (77.3, 48.4, and 37.1 wt% for HTC at 340, 280, and 220 °C in 180 min, respectively). Total phosphorus recovery in HC and HTC-AP respectively peaked at 340 °C and 220 °C in 180 min. The study concludes that HTC holds promise for energy-dense biofuel production, nutrient recovery, and fostering a circular economy.

Evaluation of energy potential of MSW in the Tamale metropolis, Ghana: An assessment of solid waste characteristics and energy content

In the present investigation, a thorough examination of waste composition was undertaken in the Tamale Metropolis, Ghana. The methodology employed was carefully designed, incorporating both commercial and residential sectors as well as various socioeconomic classes. Source-based sampling of waste materials covered a range of locations, including households, markets, hotels, and restaurants, while socioeconomic-based categorization followed the zoning of the metropolis. Waste audit sampling involved collecting a total of 90 batches of solid waste over the three-month period from August to October 2023. The primary objectives were to physically characterize the waste and evaluate its chemical properties through proximate and ultimate analysis of waste fractions, aiming to determine the waste-to-energy potential and compatibility with waste-to-energy technologies. The study revealed that the Tamale Metropolis generates 176.1 tons/day of Municipal Solid Waste (MSW) at a rate of 0.47 kg/capita/day.Organic matter constituted the majority (44.9%), followed by inert, miscellaneous, and plastic waste at 20% each. The remaining 14.3% comprised paper, metal, glass, leather, and textile components. Notably, the hospitality sector exhibited the highest organic matter content at 62.3%, contrasting with average households and markets at 44.9% and 13.1%, respectively. Moisture content ranged from 5.4% to 12.6%, volatile solids from 21.8% to 77.2%, and gross calorific values from 7.9 MJ/kg to 28.9 MJ/kg. Household waste demonstrated the highest energy content at 6693.8 kcal/kg, followed by hotels/restaurants (2003.94 kcal/kg) and markets (1883.62 kcal/kg). This study's findings offer valuable insights that can inform the development of effective policies by regulatory bodies such as the Public Utility Regulatory Commission (PURC) and the Ghana Ministry of Energy, as well as city authorities like the Tamale Metropolitan Assembly and the Environmental Protection Agency. These insights can be used to address waste and energy challenges in the metropolis.Implications: The findings reveal critical insights with substantial implications for stakeholders and regulatory bodies. It emphasizes the waste-to-energy potential of various waste fractions, highlighting the need for sustainable municipal waste management strategies. Particularly, the high organic content in the hospitality sector presents significant energy recovery opportunities. For bodies like the Public Utility Regulatory Commission and the Ghana Ministry of Energy, this data is crucial for developing policies that support waste-to-energy technologies. Additionally, the Tamale Metropolitan Assembly and the Environmental Protection Agency can use these insights to improve local waste management practices, customizing interventions to address specific sectoral waste composition. The study also underlines the importance of ongoing monitoring to ensure the effectiveness of these strategies over time.

Hydrothermal Liquefaction: How the Holistic Approach by Nature Will Help Solve the Environmental Conundrum

Hydrothermal liquefaction (HTL) represents a beacon of scientific innovation, which unlocks nature's alchemical wonders while reshaping the waste-to-energy platform. This transformative technology offers sustainable solutions for converting a variety of waste materials to valuable energy products and chemicals-thus addressing environmental concerns, inefficiencies, and high costs associated with conventional waste-management practices. By operating under high temperature and pressure conditions, HTL efficiently reduces waste volume, mitigates harmful pollutant release, and extracts valuable energy from organic waste materials. This comprehensive review delves into the intricacies of the HTL process and explores its applications. Key process parameters, diverse feedstocks, various reactor designs, and recent advancements in HTL technology are thoroughly discussed. Diverse applications of HTL products are examined, and their economic viability toward integration in the market is assessed. Knowledge gaps and opportunities for further exploration are accordingly identified, with a focus on optimizing and scaling up the HTL process for commercial applications. In conclusion, HTL holds great promise as a sustainable technology for waste management, chemical synthesis, and energy production, thus making a significant contribution to a more sustainable future. Its potential to foster a circular economy and its versatility in producing valuable products underscore its transformative role in shaping a more sustainable world.

Life cycle assessment of a novel strategy based on hydrothermal carbonization for nutrient and energy recovery from food waste

In this work, a novel strategy for food waste valorization was evaluated from an environmental life-cycle perspective. A system based on acid-assisted hydrothermal carbonization of food waste combined with the exploitation of hydrochar by combustion and process water through nutrient recovery stage and subsequent anaerobic digestion, was assessed and compared with stand-alone anaerobic digestion as the reference system. This combination of processes aims to recover both nutrients in a stage of struvite precipitation from process water and energy through hydrochar and biogas combustion. Both systems were modeled in Aspen Plus® to identify and quantify their most relevant input and output flows and subsequently evaluate their environmental performance through the life cycle assessment methodology. The novel combined system was found to generally involve a more favorable environmental performance than the reference stand-alone configuration, which would be closely linked to the substitution of hydrochar for fossil fuels. In addition, the impacts associated with soil application of the struvite produced in the integrated process would also be reduced compared to the use of the digestate generated in the stand-alone anaerobic digestion process. Following these results and the evolving regulatory framework for biomass waste management, mainly in the field of nutrient recovery, combined process based on acid-assisted hydrothermal treatment plus nutrient recovery stage and anaerobic digestion is concluded to be a promising circular economy concept for food waste valorization.

Converting food waste into soil amendments for improving soil sustainability and crop productivity: A review

One-third of the annual food produced globally is wasted and much of the food waste (FW) is unutilized; however, FW can be valorized into value-added industrial products such as biofuel, chemicals, and biomaterials. Converting FW into soil amendments such as compost, vermicompost, anaerobic digestate, biofertilizer, biochar, and engineered biochar is one of the best nutrient recovery and FW reuse approaches. The soil application of FW-based amendments can improve soil fertility, increase crop production, and reduce contaminants by altering soil's chemical, physical, microbial, and faunal properties. However, the efficiency of the amendment for improving ecosystem sustainability depends on the type of FW, conversion method, application rate, soil type, and crop type. Engineered biochar/biochar composite materials produced using FW have been identified as promising amendments for soil remediation, reducing commercial fertilizer usage, and increasing soil nutrient use efficiency. The development of quality standards and implementation of policies and regulations at all stages of the food supply chain are necessary to manage (reduce and re-use) FW.

A Review on the Challenges and Choices for Food Waste Valorization: Environmental and Economic Impacts

Valorization of food waste (FW) is instrumental for reducing the environmental and economic burden of FW and transitioning to a circular economy. The FW valorization process has widely been studied to produce various end-use products and summarize them; however, their economic, environmental, and social aspects are limited. This study synthesizes some of the valorization methods used for FW management and produces value-added products for various applications, and also discusses the technological advances and their environmental, economic, and social aspects. Globally, 1.3 billion tonnes of edible food is lost or wasted each year, during which about 3.3 billion tonnes of greenhouse gas is emitted. The environmental (-347 to 2969 kg CO₂ equiv/tonne FW) and economic (-100 to $138/tonne FW) impacts of FW depend on the multiple parameters of food chains and waste management systems. Although enormous efforts are underway to reduce FW as well as valorize unavoidable FW to reduce environmental and economic loss, it seems the transdisciplinary approach/initiative would be essential to minimize FW as well as abate the environmental impacts of FW. A joint effort from stakeholders is the key to reducing FW and the efficient and effective valorization of FW to improve its sustainability. However, any initiative in reducing food waste should consider a broader sustainability check to avoid risks to investment and the environment.

Food and fruit waste valorisation for pectin recovery: Recent process technologies and future prospects

Pectin possesses a dual property of resistance and flexibility and thus has diverse commercial value which has generated research interest on this versatile biopolymer. Formulated products using pectin could be useful in food, pharma, foam, plasticiser and paper substitute industries. Pectin is structurally tailor-made for greater bioactivity and diverse applications. Sustainable biorefinery leaves greener footprints while producing high-value bioproducts like pectin. The essential oils and polyphenols obtained as byproducts from a pectin-based biorefinery are useful in cosmetics, toiletries and fragrance industries. Pectin can be extracted from organic sources following eco-friendly strategies, and the extraction techniques, structural alterations and the applications are continually being upgraded and standardized. Pectin has great applications in diverse areas, and its green synthesis is a welcome development. In future, growing industrial application of pectin is anticipated as research orients on biopolymers, biotechnologies and renewable source-based processes. As the world is gradually adopting greener strategies in sync with the global sustainable development goal, active involvement of policy makers and public participation are prime. Governance and policy framing are essential in the transition of the world economy towards circularity since green circular bioeconomy is ill-understood among the public in general and within the administrative circles in particular. Concerted efforts by researchers, investors, innovators, and policy and decision makers to integrate biorefinery technologies as loops within loop of biological structures and bioprocesses is suggested. The review focusses on generation of the different nature of food wastes including fruits and vegetables with cauterization of their components. It discusses the innovative extraction and biotransformation approaches for these waste conversions into value-added products at cost-effective and eco-friendly way. This article compiles numerous effective and efficient and green way pectin extraction techniques with their advantages with varying success in an integrated manner.

Development of a Continuous Hydrothermal Treatment Process for Efficient Dewatering of Industrial Wastewater Sludge

Sludges from the papermaking industry represent a challenging residue stream that is difficult to dewater using conventional processes. The successful development and scale-up of innovative processes from lab- to pilot- to industrial-scale are required to tackle challenges for waste treatment, including paper sludges. Biological paper sludge was treated via a mild hydrothermal carbonization process (TORWASH®) to improve dewaterability of the sludge, including long-duration, continuous testing. Initial lab-scale experiments indicated the optimal treatment temperature for sludge dewatering was 190 °C. Dewaterability improved with increasing temperature, but the obtained solid yield decreased. Scaling-up to a continuous flow pilot plant required a temperature of 200 °C to achieve optimum dewatering. Pilot-scale hydrothermal treatment and dewatering resulted in solid cakes with an average dry matter content of 38% and a solid yield of 39%. This study demonstrates the benefits of hydrothermal carbonization for the dewatering of biological paper sludge without the use of dewatering aids such as fiber sludge or polyelectrolytes. The results also demonstrate the successful adaptation of a lab-scale batch process to a pilot-scale continuous flow process for hydrothermal carbonization of industrial wastewater sludge.

Hydrothermal carbonization of food waste as sustainable energy conversion path

Every day, a large amount of food waste (FW) is released into the environment, causing financial loss and unpredictable consequences in the world, highlighting the urgency of finding a suitable approach to treating FW. As moisture content makes up 75% of the FW, hydrothermal carbonization (HTC) is a beneficial process for the treatment of FW since it does not require extensive drying. Moreover, the process is considered favorable for carbon sequestration to mitigate climate change in comparison with other processes because the majority of the carbon in FW is integrated into hydrochar. In this work, the reaction mechanism and factors affecting the HTC of FW are scrutinized. Moreover, the physicochemical properties of products after the HTC of FW are critically presented. In general, HTC of FW is considered a promising approach aiming to attain simultaneously-two core benefits on economy and energy in the sustainable development strategy.

Carbon material produced by hydrothermal carbonisation of food waste as an electrode material for supercapacitor application: A circular economy approach

This study aims to use landfill leachate (LL) as an aqueous medium during hydrothermal carbonisation (HTC) of food waste to produce hydrochar (FWH-LL-C), which could be used as an electrode material in energy storage devices. The structural properties and electrochemical performance of the hydrochar were compared to that obtained using distilled water as a reaction medium (FWH-DW-C). The results showed that there is a difference in Brunauer-Emmett-Teller (BET) surface area of FWH-LL-C (220 m² gm^-1) and FWH-DW-C (319 m² gm^-1). The electrochemical properties were comparable, with FWH-LL-C having 227 F g^-1 specific capacitance at 1 A g^-1 current density and FWH-DW-C having 235 F g^-1 specific capacitance at 1 A g^-1 current density. Furthermore, at a power density of 634 W kg^-1, FWH-DW-C achieved the highest energy density of 14.4 Wh kg^-1. The energy retention capacity of the electrode was 98% which indicate that the material has an excellent energy storage capacity. The findings suggested that LL could be used as an alternative source of aqueous media during the HTC of food waste to produce hydrochar which could be used as an effective electrode material in supercapacitors.

Hydrothermal treatment: An efficient food waste disposal technology

The quantities of food waste (FW) are increasing yearly. Proper disposal of FW is essential for reusing value-added products, environmental protection, and human health. Based on the typical characteristics of high moisture content and high organic content of FW, hydrothermal treatment (HTT), as a novel thermochemical treatment technology, plays unique effects in the disposal and utilization of FW. The HTT of FW has attracted more and more attention in recent years, however, there are few conclusive reviews about the progress of the HTT of FW. HTT is an excellent approach to converting energy-rich materials into energy-dense fuels and valuable chemicals. This process can handle biomass with relatively high moisture content and allows efficient heat integration. This mini-review presents the current knowledge of recent advances in HTT of FW. The effects of HTT temperature and duration on organic nutritional compositions (including carbohydrates, starch, lipids, protein, cellulose, hemicellulose, lignin, etc.) and physicochemical properties (including pH, elemental composition, functional groups, fuel properties, etc.) and structural properties of FW are evaluated. The compositions of FW can degrade during HTT so that the physical and chemical properties of FW can be changed. The application and economic analyses of HTT in FW are summarized. Finally, the analyses of challenges and future perspectives on HTT of FW have shown that industrial reactors should be built effectively, and techno-economic analysis, overall energy balance, and life cycle assessment of the HTT process are necessary. The mini-review offers new approaches and perspectives for the efficient reuse of food waste.

Improved energy recovery from food waste through hydrothermal carbonization and anaerobic digestion

Here we studied energy valorization of food waste by hydrothermal carbonization coupled with anaerobic digestion. Hydrothermal treatment was carried out at 200 °C and 230 °C for 1 h, obtaining hydrochar with properties suitable for solid biofuel according to ISO/TS 17225-8. The increase in temperature improved the fuel properties of hydrochar (higher heating value 20.3 and 23.7 MJ kg^-1, fuel ratio 0.33 and 0.37, energy density 1.07 and 1.25). The anaerobic digestion of process water achieved methane yields around 150 mL CH₄ STP g^-1 COD_added and made it possible to remove some specific recalcitrant compounds, such as 2-methylpyridine and 2-ethyl-3-methylpyrazine. Energy recovery from hydrochar and process water seems to be an interesting alternative way to sustain the process energetically and economically, despite the significant energy inputs required for hydrothermal carbonization.

Profitability analysis of thermochemical processes for biomass-waste valorization: a comparison of dry vs wet treatments

Herein pyrolysis, torrefaction and hydrothermal carbonization of olive tree pruning were compared from an economic perspective. For this economic comparison a hypothetical industrial plant of 1250 kg/h of capacity was selected, and the profitability analysis was performed through the discounted cash flow method. A baseline scenario was defined, which serves for basis of later comparison. Results show that under these circumstances, none of the alternatives are profitable, with net present values between -37 M€ and -45 M€. Therefore, different scenarios were studied regarding either the reduction of the associate costs or the improvement of the revenues to analyze the negative economic outputs obtained in the baseline scenario. From the revenues side, breakeven prices for the different solid products between 1.14 and 1.35 €/kg are needed to reach profitability. To reach such values, either subsidies from governments or greater selling product prices are required. When examining the associated costs share, the energy consumption is the main cost factor (representing between 70 and 90% of the total, depending on the technology). This means that a variation on the rest of the parameters will not significantly affect the overall performance. Covering the total investment needed for the plants would still present negative net present values (around -34 M€ for the three alternatives). Similarly, even if the price of electricity could be reduced to 0.02 €/kWh, none of the alternatives would reach profitability. This study reveals the importance of finding economic solutions to evolve towards circular economy societies.

Potential Greenhouse Gas Mitigation for Converting High Moisture Food Waste into Bio-Coal from Hydrothermal Carbonisation in India, Europe and China

Hydrothermal carbonisation is a promising technology for greenhouse gas (GHG) mitigation through landfill avoidance and power generation, as it can convert high-moisture wastes into bio-coal which can be used for coal substitution. The GHG mitigation potential associated with landfill avoidance of high-moisture food waste (FW) generated in India, China and the EU was calculated and the potential for coal substitution to replace either grid energy, hard coal, or lignite consumption were determined. Different HTC processing conditions were evaluated including temperature and residence times and their effect on energy consumption and energy recovery. The greatest mitigation potential was observed at lower HTC temperatures and shorter residence times with the bio-coal replacing lignite. China had the greatest total mitigation potential (194 MT CO2 eq), whereas India had the greatest mitigation per kg of FW (1.2 kgCO2/kg FW). Significant proportions of overall lignite consumption could be substituted in India (12.4%) and China (7.1%), while sizable levels of methane could be mitigated in India (12.5%), China (19.3%), and the EU (7.2%). GHG savings from conversion of high-moisture FW into bio-coal and subsequent coal replacement has significant potential for reducing total GHG emissions and represents in India (3%), China (2.4%), and the EU (1%).

Study on the process wastewater reuse and valorisation during hydrothermal co-carbonization of food and yard waste

The commercial success of hydrothermal carbonization (HTC) is contingent on seeking solutions for the downstream wastewater (process water) generated during the process which is still regarded largely as a nuisance. In the present study, the reusability and valorization strategy of process wastewater generated during co-HTC of organic fraction of municipal solid waste (food and yard waste) at 220 °C for 1 h was established. The process wastewater was anaerobically digested in the first part to determine its methane-generating capacity; and in the second part, it was recirculated up to five times to understand the evolution of physicochemical and fuel characteristics of hydrochar. The process water was characterized by the presence of high total organic carbon (up to 40 g/L) and chemical oxygen demand (up to 96 g/L). The decreasing trend of heavy metals with increasing recirculation suggested possible adsorption/immobilization mechanism taking place to the hydrochar surface. The process water generated from co-HTC condition has anaerobic biodegradability of 72% while experimental and theoretical methane yield observed were 224 mL/g COD and 308 mL/g COD, respectively. The presence of high organic and ionic species in recirculated process water promoted the overall carbonization process which was evident from the increased energy yield (86 to 92%), carbon content (68 to 71%) and calorific value (20 to 27 MJ/kg). The recirculation also enhanced overall combustion characteristics of hydrochar as analyzed by thermogravimetric analysis. The recirculation strategy enhanced fuel properties of hydrochar while making sure upstream and downstream water related burden is reduced (as illustrated by life cycle analysis) to create a cleaner production system for renewable solid biofuels production.

Full recycling of high-value resources from cabbage waste by multi-stage utilization

Cabbage waste (CW) was recycled for generating some potential high-value products by a multi-stage treatment technology. A novel multi-stage utilization process was successfully proposed which consisted of low-temperature extraction, medium-temperature thermolysis, and high-temperature activation. Plant extracts that contain fatty acids, alcohol, furan, and esters were first extracted from raw cabbage waste by ethanol at 70 °C. Pyrolytic oil was obtained by cabbage waste pyrolysis at different medium temperature conditions. The produced carbon residue was further activated at high temperature for environmental purification such as VOCs removal. The performance of this process was characterized by N₂ isothermal adsorption, Fourier transform infrared spectrometer (FTIR), thermogravimetric analysis (TG) and gas chromatography-mass spectrometry (GC-MS). Experimental results showed that the optimum temperatures for extraction, pyrolysis, and activation were 70 °C, 520 °C and 700 °C, respectively. Phenolic-rich pyrolysis solution with 50% phenolic contents could be obtained with the potential application of botanical pesticide. The produced biochar had a BET surface area of as high as 891.12 m²/g. The yields of biochar, pyrolytic liquid, and pyrolytic gas were 43.86%, 17.47%, 38.67%, respectively, and the process energy efficiency was over 42.7%. Applicability and feasibility of this process were also discussed in the aspects of energy quality balance, economy, and environment. The proposed multi-stage thermal-chemical process could be used as a full recycling method for biomass waste.

Biomass-derived versatile activated carbon removes both heavy metals and dye molecules from wastewater with near-unity efficiency: Mechanism and kinetics

Due to the ever-increasing industrialization, it is critical to protect the environment and conserve water resources by developing efficient wastewater treatment methods. Traditional methods that simultaneously remove heavy metal ions and complex dyes are too expensive and tedious to commercialize. This work demonstrates the versatility, effectiveness, and potential of a biomass-derived adsorbent (from a mangrove fruit of Rhizophora mucronata) synthesized using a simple route for rapid adsorption of complex dyes and heavy metals with an efficiency of near unity. The cartridges were prepared using activated carbon that removes both dye molecules and heavy metal ions simultaneously from wastewater, corroborating its applicability/feasibility to treat wastewater. Owing to the high surface area (1061.5 m²g^-1) and the pore volume (0.5325 cm³g^-1), the adsorbent showed >99% removal efficiency in just 12 min of exposure to wastewater. The cartridge exhibits >90% removal efficiency of both dyes and heavy metals from its mixed feed solution. The Langmuir and Freundlich models successfully explained the adsorption kinetics. These developed cartridges are versatile, rapid, efficient, and promising candidates for environmental remediation.

Fate of nutrients during hydrothermal treatment of food waste

Hydrothermal carbonization was evaluated as a food waste valorization strategy to obtain hydrochar and recover nutrients. In the hydrothermal treatment, the temperature (170-230 °C), reaction time (5-60 min), and addition of HCl (0.1-0.5 M) during the reaction were analyzed. Compared to the feedstock, hydrochar showed an increase in fixed carbon (greater than 45%) and a decrease in ash content (<7%), along with a higher heating value (18.6-26.2 MJ/kg), which would allow for its application as a biofuel for industry according to ISO/TS 17225-8. The hydrochar obtained using plain carbonization showed 75% P and 40% N of the feedstock content, whereas the HCl-mediated treatment (0.5 M) solubilized most of the P, K, and N in the process water (98% P as PO₄-P, 98% K, and the total N content as NH₄-N (16%) and organic-N) operating at 170 °C for 60 min.

On-line analysis of the correlation between gasification characteristics and microstructure of woody biowaste after hydrothermal carbonization

Woody biowaste is a component which is difficult to be converted among multiple solid waste (MSW) during the hydrothermal carbonization (HTC). In this paper, poplar sawdust was pretreated by HTC to study the correlation between microstructure and gasification characteristics. The results showed the fixed carbon and higher calorific value increased from 13.44 % and 19.41 MJ/kg to 41.47 % and 25.85 MJ/kg after HTC, respectively. The cold gas efficiency of hydrochars prepared at 220 °C was the highest of 93.57 % compared with that of raw material of 76.65 %. It was found that carbon structure had a greater influence on hydrochars gasification characteristics than pore structure. The crystallinity of hydrochars had a good correlation with the total yield and H₂/CO of syngas, which can provide guidance for HTC pretreatment of woody biowaste and MSW.

Hydrothermal carbonization of simulated food waste for recovery of fatty acids and nutrients

We conducted Hydrothermal carbonization (HTC) of simulated food waste under different reaction conditions (180 to 220 °C, 15 and 30 min), with the aim of recovering both fatty acids from the hydrochar and nutrients from the aqueous-phase products. HTC of the simulated food waste produced hydrochar that retained up to 78% of the original fatty acids. These retained fatty acids were extracted from the hydrochar using ethanol, a food-grade solvent, and gave a net recovery of fatty acid of ∼ 50%. The HTC process partitioned more than 50 wt% of the phosphorus and around 38 wt% of the nitrogen into the aqueous-phase products. A reaction path consistent with decarboxylation predominated during HTC under all of the reaction conditions investigated. A path consistent with dehydration was also observed, but only for the more severe reaction conditions. This work illustrates the potential that HTC has for valorization of food waste.

Effects of Lipase Addition, Hydrothermal Processing, Their Combination, and Co-Digestion with Crude Glycerol on Food Waste Anaerobic Digestion

To enhance anaerobic fermentation during food waste (FW) digestion, pretreatments can be applied or the FW can be co-digested with other waste. In this study, lipase addition (LA), hydrothermal pretreatment (HTP), and a combination of both methods (HL) were applied to hydrolyze organic matter in FW. Furthermore, the effects of crude glycerol (CG), which provided 5%, 10%, and 15% of the volatile solids (VS) as co-substrate (denoted as CG5, CG10, and CG15, respectively), on the anaerobic digestion of FW were assessed. With an increasing proportion of CG in the co-digestion experiment, CG10 showed higher methane production, while CG15 negatively affected the anaerobic digestion (AD) performance owing to propionic acid accumulation acidifying the reactors and inhibiting methanogen growth. As the pretreatments partially decomposed hard-to-degrade substances in advance, pretreated FW showed a stronger methane production ability compared with raw FW, especially using the HL method, which was significantly better than co-digestion. HL pretreatment was shown to be a promising option for enhancing the methane potential value (1.773 NL CH4/g VS) according to the modified Gompertz model.

Study of the Potential Uses of Hydrochar from Grape Pomace and Walnut Shells Generated from Hydrothermal Carbonization as an Alternative for the Revalorization of Agri-Waste in Chile

A myriad of resources and efforts have been devoted to assessing the possibilities of using locally sourced biomass to produce energy, reduce CO2 emissions, and, in turn, lower dependance on petroleum. Grape pomace (GP) and walnut shells (WS) are organic waste generated in Chile. Within the last decade, the potential benefits and application of biomass have received significant attention, both in terms of producing functionalized carbon materials, and the various potential applications in the field of energy storage and environmental protection. The proposed research motivation is on the development of carbonous materials through thermal decomposition processes. Few researchers have addressed the idea of developing a multipurpose carbonaceous matrix from hydrochar, and there remains a need for an efficient method to obtain hydrochar specially from grape pomace. Hence, the general objective of this research is to study the potential of grape pomace and walnut shells treated with hydrothermal carbonization (HTC) as an alternative low-cost and efficient carbonous matrix. Proximate and elemental analysis was determined to distinguish the nature of the feedstock along with the hydrochar produced. Yield and reaction severity were also studied to study the impacts of temperature and residence time for both feedstocks. Successful results from the proposed work have broad applications for increasing the sustainability biomass applications, contributing to a positive economic impact.

Effect of eggshell- and homo-type Ni/Al2O3 catalysts on the pyrolysis of food waste under CO2 atmosphere

This study highlights the potential of pyrolysis of food waste (FW) with Ni-based catalysts under CO₂ atmosphere as an environmentally benign disposal technique. FW was pyrolyzed with homo-type Ni/Al₂O₃ (Ni-HO) or eggshell-type Ni/Al₂O₃ (Ni-EG) catalysts under flowing CO₂ (50 mL/min) at temperatures from 500 to 700 °C for 1 h. A higher gas yield (42.05 wt%) and a lower condensable yield (36.28 wt%) were achieved for catalytic pyrolysis with Ni-EG than with Ni-HO (34.94 wt% and 40.06 wt%, respectively). In particular, the maximum volumetric content of H₂ (21.48%) and CO (28.43%) and the lowest content of C₂-C₄ (19.22%) were obtained using the Ni-EG. The formation of cyclic species (e.g., benzene derivatives) in bio-oil was also effectively suppressed (24.87%) when the Ni-EG catalyst and CO₂ medium were concurrently utilized for the FW pyrolysis. Accordingly, the simultaneous use of the Ni-EG catalyst and CO₂ contributed to altering the carbon distribution of the pyrolytic products from condensable species to value-added gaseous products by facilitating ring-opening reactions and free radical mechanisms. This study should suggest that CO₂-assisted catalytic pyrolysis over the Ni-EG catalyst would be an eco-friendly and sustainable strategy for disposal of FW which also provides a clean and high-quality source of energy.

Wet organic waste treatment via hydrothermal processing: A critical review

There are several recent reviews published in the literature on hydrothermal carbonization, liquefaction and supercritical water gasification of lignocellulosic biomass and algae. The potential of hydrochar, bio-oil or synthesis gas production and applications have also been reviewed individually. The comprehensive review on the hydrothermal treatment of wet wastes (such as municipal solid waste, food waste, sewage sludge, algae) covering carbonization, liquefaction and supercritical water gasification, however, is missing in the literature which formed the basis of the current review paper. The current paper critically reviews the literature around the full spectrum of hydrothermal treatment for wet wastes and establishes a good comparison of the different hydrothermal treatment options for managing wet waste streams. Also, the role of catalysts as well as synthesis of catalysts using hydrothermal treatment of biomass has been critically reviewed. For the first time, efforts have also been made to summarize findings on modelling works as well as techno-economic assessments in the area of hydrothermal treatments of wet wastes. The study concludes with key findings, knowledge gaps and future recommendations to improve the productivity of hydrothermal treatment of wet wastes, helping improve the commercial viability and environmental sustainability.

Optimizing hydrothermal carbonization of olive tree pruning: A techno-economic analysis based on experimental results

In this study the optimization of the hydrothermal carbonization process for the conversion of olive tree pruning into biofuel is presented. To this end, a combined experimental-economic assessment is performed. Experimental data obtained at laboratory scale were used to estimate the economic performance of a hypothetical industrial scale plant. To evaluate the viability of the project, three different plant sizes according to their capacity were selected (1250-625-312.5 kg/h). The discounted cash flow method was applied for the profitability analysis. Different scenarios were analyzed considering the reduction of associate costs or the improvement of the revenues compared to the baseline case. Results indicate that with the sizes studied, none of the alternatives are profitable. Despite that, the larger capacity shows the best outcomes. In this case, minimum selling price of 0.39 €/kg for hydrochar is required to reach profitability. Lower plant sizes would require higher selling prices (i.e., 0.46 €/kg for 625 kg/h capacity and 0.59 €/kg for 312.5 kg/h capacity). Similarly, a reduction of 33% in the electrical energy consumption can make the plan be profitable for the larger capacity. Likewise, a reduction until 0.053 €/kWh in the electricity price must be reached for achieving profitability. Thus, importance of government incentives is revealed in this work given that the reduction of costs along with the improvement in the revenues for the selling of the product can make the project economically viable. Other parameters like the number of workers are also interesting to consider as for example the reduction by two units improves the NPV value in almost 600 k€ for all the plant sizes.

Minimizing hazardous impact of food waste in a circular economy - Advances in resource recovery through green strategies

Recent trends in food waste and its management have increasingly started to focus on treating it as a reusable resource. The hazardous impact of food waste such as the release of greenhouse gases, deterioration of water quality and contamination of land areas are a major threat posed by food waste. Under the circular economy principles, food waste can be used as a sustainable supply of high-value energy, fuel, and nutrients through green techniques such as anaerobic digestion, co-digestion, composting, enzymatic treatment, ultrasonic, hydrothermal carbonization. Recent advances made in anaerobic co-digestion are helping in tackling dual or even multiple waste streams at once with better product yields. Integrated approaches that employ pre-processing the food waste to remove obstacles such as volatile fractions, oils and other inhibitory components from the feedstock to enhance their bioconversion to reduce sugars. Research efforts are also progressing in optimizing the operational parameters such as temperature, pressure, pH and residence time to enhance further the output of products such as methane, hydrogen and other platform chemicals such as lactic acid, succinic acid and formic acid. This review brings together some of the recent progress made in the green strategies towards food waste valorization.

Optimization of biochar production based on environmental risk and remediation performance: Take kitchen waste for example

Two major obstacles that need to be addressed for environmental application of biochar include its environmental risk and remediation performance for target pollutants. In this study, kitchen waste was taken as an example to optimize the pyrolysis temperature for biochar production based on its heavy metal risk and Cd(II) remediation performance. The results showed that the pH and ash content of kitchen waste biochar (KWB) increased; however, the yield, H/C, and N/C decreased with increasing pyrolysis temperature. Total content of heavy metals in KWB got enriched after pyrolysis, while heavy metals' risk was reduced from moderate to low due to the transformation of directly toxic heavy metal fractions into potentially and/or non-toxic fractions. The equilibrium adsorption capacities of biochar for Cd(II) ranked as follows: 49.0 mg/g (600 °C), 46.5 mg/g (500 °C), 23.6 mg/g (400 °C), 18.2 mg/g (300 °C). KWB pyrolyzed at 500 °C was found to be the most suitable for green, efficient, and economic remediation of Cd(Ⅱ) contaminated water. SEM-EDS and XPS characterization results indicated that KWB removed Cd(II) via precipitation, complexation with carboxyl/hydroxyl, ion exchange with metal cations, and coordination with π-electrons. This study puts forward a new perspective for optimizing biochar production for environmental application.

Life cycle assessment on the treatment of organic waste streams by anaerobic digestion, hydrothermal carbonization and incineration

By means of source-segregation, organic waste streams are deterred from mass-burning and more sustainable treatment paths may be enabled. This study analyzed the environmental and economic performance of custom treatment paths for organic waste streams in Germany towards energy generation. The reference quantity was the production and export of 1 kWh of exergy, while treating a certain amount of waste. The methodologies of environmental life cycle assessment (eLCA) and levelized costs of exergy (LCOE) were applied. The considered treatment systems were (i) anaerobic digestion, (ii) hydrothermal carbonization (HTC) and (iii) incineration. The technologies were considered individually or in a cascade. The analyzed waste streams were the organic fraction of municipal solid waste (OFMSW), food waste and waste wood. For food waste and OFMSW, anaerobic digestion followed by the spreading of the digestate was characterized by overall beneficial environmental and economic properties. As the preferable treatment system was the same for both substrates, a separate collection of food waste was not considered worthwhile. For waste wood, the most environmentally friendly and economic option was direct incineration - either in a waste incinerator or in a lignite power plant. The implementation of an HTC process appeared viable for the treatment of OFMSW and food waste, both from an environmental and economic point of view. However, the energy intensive system must be outlined towards an environmentally friendly heat source. In this context, a cascading of anaerobic digestion with the HTC-process proofed to be an acceptable alternative.

Environmental Impact Evaluation of Current Municipal Solid Waste Treatments in India Using Life Cycle Assessment

An environmental life cycle assessment was conducted to compare proposed municipal solid waste treatment systems with the existing system in Visakhapatnam, India. Five waste alternative treatment systems, including open dumping of municipal solid waste (S1), landfill without gas recovery [LFWGR] (S2), landfill with gas recovery (S3), anaerobic digestion + LFWGR (S4), and incineration + LFWGR (S5). EASETECHTM was considered for assessment using ReCiPE Midpoint (Heuristic) world environmental impact assessment method. Global warming potential (GWP), terrestrial acidification (TA), freshwater eutrophication (FEW), marine water eutrophication (ME), human toxicity (HTP), terrestrial ecotoxicity (TE), freshwater ecotoxicity (FWT), and marine ecotoxicity (MET) impacts were determined for each option. The existing MSW disposal practice in Visakhapatnam city (baseline scenario, S1) has the highest GWP (1107 kg CO2 eq.), which can potentially be reduced to 68.2%, 81.5%, 98.2%, and 94.5% by alternative waste management scenarios S2, S3, S4 and S5, respectively. Scenario S4, involving the use of anaerobic digestion of food waste and residues dumped in engineered landfill without energy recovery was found to be the option with the highest mitigation potential of most of the impacts, and it contributes to significant environmental benefits in terms of ecological footprints in a low-income country such as India. Sensitivity analysis was conducted to confirm the reasonable legitimacy of data used for the determination of the impacts.

Analysis and optimisation of a novel 'almond-refinery' concept: Simultaneous production of biofuels and value-added chemicals by hydrothermal treatment of almond hulls

For the first time, this work investigates the achievability of developing a biorefinery concept around almond hulls by hydrothermal treatment (HTT), thoroughly scrutinising the influence of the temperature (200-300 °C), pressure (100-180 bar), time (20-180 min) and solid loading (5-25 wt%). This process allowed the conversion of almond hulls into four main products: gas (2-13%), bio-oil (2-12%), aqueous (4-69%) and hydro-char (17-89%). The gas consisted of a mix of H₂, CO₂, CO and CH₄ with a LHV fluctuating from 1 to 13 MJ/m³ STP. The bio-oil comprised a mixture of alkanes, aldehydes, ketones, phenols, furans, benzenes and nitrogen compounds with a HHV between 21 and 31 MJ/kg. The solid product resembled an energetic hydro-char material (HHV 21-31 MJ/kg), while the aqueous effluent comprised a mixture of value-added chemicals, including saccharides and small oxygenated compounds. The production of biofuels can be maximised at 256 °C and 100 bar, using a 5 wt% solid loading for 157 min, conditions at which 43% of the original feedstock can be converted into an elevated energy-filled bio-oil (11% yield, 30 MJ/kg), along with a high energetic hydro-char (32% yield, 29 MJ/kg). Regarding value-added chemicals, up to 10% of the almond hulls can be converted into a bio-oil with a high proportion (45%) of phenolic species at 250 °C and 144 bar with a solid loading of 5 wt% for 167 min. In comparison, a sugar-rich (81 C-wt%) solution can be produced in high yield (54%), by treating a 24 wt% suspension at 252 °C and 180 bar for 153 min. Therefore, the versatility, novelty and intrinsic green and holistic nature of this 'almond-refinery' concept exemplify a landmark achievement in future energy and chemicals production from biomass, which might help render the complete bio-refinery for almond hulls more cost-effectively and ecologically feasible.

Biohydrogen production from catalytic conversion of food waste via steam and air gasification using eggshell- and homo-type Ni/Al2O3 catalysts

Steam and air gasification with 5 wt% Ni/Al₂O₃ eggshell (Ni-EG) and homo (Ni-H) catalysts were performed for the first time to produce biohydrogen from food waste. The steam gasification produced comparably higher gas yield than air gasification. In non-catalytic experiments, steam gasification generated a higher volume percent of H₂, whereas more CO, CO₂, CH₄, and C₂-C₄ were produced in air gasification. Ni-EG demonstrated higher potential to obtain H₂-rich gases with a low C₂-C₄ content compared to that obtained by Ni-H, particularly in steam gasification at 800 °C, which produced gaseous products with 59.48 vol% H₂. The long-term activity of both catalysts in steam gasification was evaluated, and Ni-EG exhibited higher stability than Ni-H. The ideal distribution of Ni species on the outer region of γ-Al₂O₃ pellets in Ni-EG resulted in higher activity, stability, and selectivity than Ni-H in both steam and air gasification.

Experimental studies of hydrothermal liquefaction of kitchen waste with H+, OH- and Fe3+ additives for bio-oil upgrading

Kitchen waste (KW) has gradually become a prominent problem in municipal solid waste treatment. Hydrothermal liquefaction (HTL) is a promising method used to make fuel oil from food and KW. However, the upgrading of bio-oil is particularly important for the sake of industrial reuse. In this study, the KW from university restaurants was subjected to HTL experiments in order to study theoretical feasibility. With the change of conversion temperature and residence time, the optimal conversion working conditions in this study were determined according to the quality and yield of the bio-oil. Moreover, the bio-oil upgrading effects of different additives (hydrogen chloride, sodium hydroxide, and iron(III) chloride) on the HTL of KW were studied. Alkaline additives have an inhibitory effect on the bio-oil yield and positive effect on coke yield. Acidic additives and iron (Fe)-containing additives can promote bio-oil yield. As an important aspect of upgrading, the effect on the nitrogen content of bio-oil with additives was revealed. The alkaline and Fe-containing additives have little effect on reducing the viscosity of the bio-oil while with the appropriate ratio (2.5 mol•kg^-1) of acidic additives to the raw material, the static and dynamic fluidity of the oil phase products are reduced to about 0.1 Pa•s.

Co-hydrothermal carbonization of food waste with yard waste for solid biofuel production: Hydrochar characterization and its pelletization

In this study, Co-HTC of food waste with yard waste was conducted for biofuel pellets production, and also to understand any possible synergy between two feedstock types. The calorific value of blended raw feedstock was 13.5 MJ/kg which increased to 27.6 MJ/kg after Co-HTC at 220 °C for 1 h. Energy yield and fuel ratio calculated was 45% and 0.65 respectively. Hydrochar produced demonstrated a stable combustion profile as compared to reactive combustion profile for raw samples. The blend of food and yard waste hydrochar was easily pelletized, and its pellets showed improvement in mechanical properties as compared to pellets made from mono-substrate((food waste) hydrochar. Pellets produced from the blend of food and yard waste hydrochar showed higher energy (46.4 MJ/m³) and mass density (1679 kg/m³) as compare to the pellet produced from food waste hydrochar alone. Tensile strength obtained for the blended hydrochar pellet was 2.64 MPa while same for the pellets produced from food waste hydrochar alone was 1.30 MPa. In addition to improving hydrophobicity, soften lignin from yard waste also helped in binding the food waste hydrochar particles together within the pellets matrix during heated pelletization. The results presented in the study indicated that in the presence of all favorable conditions, there is a potential that approximately 11% of the global coal consumption could be replaced by the combustion of hydrochar produced from food and yard waste globally.

Co-hydrothermal carbonization of food waste-woody sawdust blend: Interaction effects on the hydrochar properties and nutrients characteristics

The influence of co-hydrothermal carbonization (co-HTC) on the hydrochar properties and nutrients distribution derived from food waste (FW) and woody sawdust (WS) blend was assessed. The carbon retention, surface functional groups and morphology features involved in hydrochar were evaluated to study the interaction effects. Results suggested that hydrochar yield consistently decreased with increase of both FW ratio and HTC temperature. C retention from 260 °C hydrochar was low (approximately 65%), but more microsphere structures was formed due to the enhanced carbonization degree of hydrochar. Hydrochar obtained at high FW blend ratio and temperature resulted in weaken oxygen-containing groups like OH and CO with enhanced CC and C(O, N). 10.43-60.45% of N and 82-94% of P were retained in hydrochar. NH₄⁺-N (6.63%-15.63%) and organic nitrogen (70.4%-87.7%) were identified as main N-containing species in liquid phase, while total P content (14-166 mg/L) depended more on FW ratio.

Co-hydrothermal carbonization of water hyacinth and polyvinyl chloride: Optimization of process parameters and characterization of hydrochar

The co-hydrothermal carbonization (co-HTC) of water hyacinth (WH) and polyvinyl chloride (PVC) was investigated and the response surface methodology, which could deduce the interactions among process parameters and establish reliable mathematical models forecasting the behavior of output variables, was implemented to optimize process parameters, including reaction temperature (200-260 °C), residence time (30-90 min) and WH/PVC mixing ratios (0.5-2). Statistical analysis revealed that reaction temperature was the predominant parameter affecting hydrochar dechlorination efficiency, yield, calorific value, energetic recovery efficiency and electricity consumption. The predicted condition of 200-30-0.5 could simultaneously acquire the optimal energetic recovery efficiency and electricity consumption for producing unit HHV, corresponding to 94.96% and 13.81. The characterization results identified that hydrochar could harvest lower H/C and O/C ratios as well as superior inorganics removal ability. Overall, the co-HTC of WH and PVC could definitely be a promising alternative to bridge the gap from solid wastes to renewable fuels.

Hydrochars as Emerging Biofuels: Recent Advances and Application of Artificial Neural Networks for the Prediction of Heating Values

In this study, the growing scientific field of alternative biofuels was examined, with respect to hydrochars produced from renewable biomasses. Hydrochars are the solid products of hydrothermal carbonization (HTC) and their properties depend on the initial biomass and the temperature and duration of treatment. The basic (Scopus) and advanced (Citespace) analysis of literature showed that this is a dynamic research area, with several sub-fields of intense activity. The focus of researchers on sewage sludge and food waste as hydrochar precursors was highlighted and reviewed. It was established that hydrochars have improved behavior as fuels compared to these feedstocks. Food waste can be particularly useful in co-hydrothermal carbonization with ash-rich materials. In the case of sewage sludge, simultaneous P recovery from the HTC wastewater may add more value to the process. For both feedstocks, results from large-scale HTC are practically non-existent. Following the review, related data from the years 2014–2020 were retrieved and fitted into four different artificial neural networks (ANNs). Based on the elemental content, HTC temperature and time (as inputs), the higher heating values (HHVs) and yields (as outputs) could be successfully predicted, regardless of original biomass used for hydrochar production. ANN3 (based on C, O, H content, and HTC temperature) showed the optimum HHV predicting performance (R2 0.917, root mean square error 1.124), however, hydrochars’ HHVs could also be satisfactorily predicted by the C content alone (ANN1, R2 0.897, root mean square error 1.289).

From waste to fuel: Energy recovery from household food waste via its bioconversion to energy carriers based on microbiological processes

In the present study the bioconversion of dried household food waste (FORBI) to energy carriers was investigated aiming to its sustainable management and valorization. FORBI was either directly fermented towards ethanol and hydrogen or was previously subjected to extraction with water resulting to a liquid fraction (extract) rich in sugars and a solid residue, which were then fermented separately. Subsequently, the effluents were assessed as substrates for methane production via anaerobic digestion (AD). Mono-cultures and co-cultures of C5 and C6 yeasts were used for the alcoholic fermentation whereas for the production of hydrogen, mixed acidogenic consortia were used. Taking into account the optimum yields of biofuels, the amount of recoverable energy was estimated based for each different approach. The maximum ethanol yield was 0.16 g ethanol per kg of FORBI and it was achieved for separate fermentation of liquid and solid fractions of the waste. The highest hydrogen yield that was observed was 210.44 L ± 4.02 H₂/kg TS FORBI for 1% solids loading and supplementation with cellulolytic enzymes. Direct AD of either the whole FORBI or its individual fractions led to lower overall energy recovery, compared to that obtained when fermentation and subsequent AD were applied. The recoverable energy was estimated for the different exploitation approaches of the waste. The maximum achieved recoverable energy was 21.49 ± 0.57 MJ/kg.

Experimental and Computational Evaluation of Heavy Metal Cation Adsorption for Molecular Design of Hydrothermal Char

A model hydrochar was synthesized from glucose at 180 °C and its Cu(II) sorption capacity was studied experimentally and computationally as an example of molecular-level adsorbent design. The sorption capacity of the glucose hydrochar was less than detection limits (3 mg g−1) and increased significantly with simple alkali treatments with hydroxide and carbonate salts of K and Na. Sorption capacity depended on the salt used for alkali treatment, with hydroxides leading to greater improvement than carbonates and K+ more than Na+. Subsequent zeta potential and infrared spectroscopy analysis implicated the importance of electrostatic interactions in Cu(II) sorption to the hydrochar surface. Computational modeling using Density Functional Theory (DFT) rationalized the binding as electrostatic interactions with carboxylate groups; similarly, DFT calculations were consistent with the finding that K+ was more effective than Na+ at activating the hydrochar. Based on this finding, custom-synthesized hydrochars were synthesized from glucose-acrylic acid and glucose-vinyl sulfonic acid precursors, with subsequent improvements in Cu(II) adsorption capacity. The performance of these hydrochars was compared with ion exchange resins, with the finding that Cu(II)-binding site stoichiometry is superior in the hydrochars compared with the resins, offering potential for future improvements in hydrochar design.

Hydochar and biochar: Production, physicochemical properties and techno-economic analysis

Utilization of natural, abundant, and renewable resources for the production of carbon materials with simple and energy-efficient processes is an upsurge interest. The production processes and resultant biochar can address widespread concerns such as climate change, energy crisis, and environmental pollution. The properties of produced chars (biochar/hydrochar) depend on the production methods, feedstock, and operating parameters, which significantly affect their use for various applications. In this review, production, physiochemical properties, and techno-economic analysis of chars are summarized. This review provides the fundamentals and reaction mechanism of char production methodologies. Physicochemical properties based on chemical composition, functional groups, structure, porosity, and shapes have been compared. The effects of operating parameters on the physicochemical properties of chars are discussed. In addition, this review offers insights on new directions for char production and research in the future, based on the updated and detailed investigation of energy balance with economy of char production methodologies.

High-Value Compounds in Fruit, Vegetable and Cereal Byproducts: An Overview of Potential Sustainable Reuse and Exploitation

Food waste (FW) represents a global and ever-growing issue that is attracting more attention due to its environmental, ethical, social and economic implications. Although a valuable quantity of bioactive components is still present in the residuals, nowadays most FW is destined for animal feeding, landfill disposal, composting and incineration. Aiming to valorize and recycle food byproducts, the development of novel and sustainable strategies to reduce the annual food loss appears an urgent need. In particular, plant byproducts are a plentiful source of high-value compounds that may be exploited as natural antioxidants, preservatives and supplements in the food industry, pharmaceuticals and cosmetics. In this review, a comprehensive overview of the main bioactive compounds in fruit, vegetable and cereal byproducts is provided. Additionally, the natural and suitable application of tailored enzymatic treatments and fermentation to recover high-value compounds from plant byproducts is discussed. Based on these promising strategies, a future expansion of green biotechnologies to revalorize the high quantity of byproducts is highly encouraging to reduce the food waste/losses and promote benefits on human health.

Stochastic economic analysis of coal-alternative fuel production from municipal solid wastes employing hydrothermal carbonization in Zimbabwe

Hydrothermal carbonization (HTC) is a promising technology for converting high moisture municipal solid waste (MSW) to a safe low-chlorine hydrochar. The key objective is to assess the economic viability of an HTC based MSW management system in Zimbabwe. Previous studies have only used deterministic estimates of hydrochar production costs disregarding uncertainties in their model parameters. Herein, a probabilistic economic analysis is introduced to quantify the uncertainty concerning costs. The goal is to determine factors that will consolidate the venture to achieve a certain level of return. The effectiveness of different investment strategies, namely, a government or private sector-run operation will be tested using Monte Carlo simulations. Results indicate a 55% and 18% probability for a positive Net Present Value (NPV) for a state-run and private operation respectively. A specific investment cost of US$54 - 67 per Mg of MSW treated, a return on investment (ROI) of 5.4-29.0% and internal rate of return (IRR) of 5.2-22.9% can be expected if the project is undertaken by government. The private sector can expect an ROI of -0.8-18.2% at a 90% confidence level and a lower IRR of -2.1-16.2% from US$57 - 71 per Mg of MSW invested. Contingency costs are US$25 per Mg and US$38 per Mg of MSW for the government and private sector respectively at a 100% confidence interval. A 70% rise in collection tariffs or a minimum selling price of US$91/Mg of the hydrochar would ensure a positive NPV for the government-run operation.

Removal of aqueous-phase lead ions by dithiocarbamate-modified hydrochar

Hydrochar produced from agricultural and forestry wastes and its application into the environment are very attractive. Herein, a high-efficiency dithiocarbamate-modified hydrochar (DTHC) was prepared successfully and then applied to eliminate Pb(II) from aqueous solutions. DTHC was characterized by various techniques. It was found that dithiocarbamate and amine groups were successfully grafted onto the surface of hydrochar. The surface area of DTHC was 7.94 m²·g^-1, which was four folds less than pristine hydrochar (31.60 m²·g^-1), but its adsorption capacity obviously increased. Adsorption experiments showed that the Pb(II) adsorption process onto DTHC well accorded with pseudo-2nd-order kinetics and Langmuir isotherms. The highest Pb(II) uptake by DTHC at 293 K determined from the Langmuir model was 151.51 mg·g^-1. Fourier transform infrared spectra and X-ray photoelectron spectroscopy verified that dithiocarbamate, carboxylate, amine and sulfonate groups all facilitated the Pb(II) adsorption. The adsorption mechanism was ascribed to the inner-sphere surface complexation of Pb(II) by these groups and to the ion exchange between Pb(II) and Na(I). Thus, DTHC is an effective adsorbent for Pb(II) removal from water.

Engineered biochar derived from food waste digestate for activation of peroxymonosulfate to remove organic pollutants

The anaerobic digestion of food waste, can result in large amounts of solid waste digestate, often without methods of disposal. In this study, a biochar was prepared from food waste digestate (FWD) by pyrolysis, and its potential to activate peroxymonosulfate (PMS) for the removal of pollutants from a simulated textile wastewater was evaluated. The results showed that the addition of biochar (0.5 g/L) and PMS (1 mM) to wastewater could remove >99% of a representative azo dye pollutant (reactive brilliant red X-3B, 1 g/L) within 10 min. The efficiency of this removal process was attributed to the catalytic sites in the biochar (graphitic carbon and nitrogen, pyridinium nitrogen and CO structures) which could activate PMS to produce reactive oxygen species (¹O₂, O₂^-, OH and SO₄^-). The results obtained in this study confirmed the activation potential of the biochar derived from FWD on PMS, providing an alternative utilization strategy for anaerobic FWD.

Current advances on waste biomass transformation into value-added products

Ceaseless growth in human population led to high demand in everything. Currently, the world largely depends on petroleum-based "all material synthesis" scheme. On the other hand, depletion of fossil-based resources and their huge impact on environmental pollution have forced us to search for sustainable and eco-friendly alternative resources. In this context, the notion to utilize waste biomass could possibly provide environmental and economic benefits. This study was carefully designed to critically review state of the art in the transformation of waste biomass into value-added products. Even though extensive reviews on biomass utilization have been published in the past few years, the current study basically focused on new trends and prospective in this area. Here, global biomass potential, research developments and practices, novel biomass transformation approaches, and future perspectives were broadly discussed. More importantly, in addition to revising published researches, already implemented and ongoing large-scale projects on valorization of waste biomass have been assessed. Therefore, this study is believed to give crucial information on the current status and future direction of waste biomass utilization so as to accomplish the quest towards green economy.Key Points • Huge biomass potential and dramatically increase in R&D trends on waste biomass.• Selection of appropriate waste biomass valorization techniques. • Development of efficient and feasible waste biomass transformation technology. • Coproduction of low-value, high-volume and high-value, low volume products.

Changes in global trends in food waste composting: Research challenges and opportunities

Increasing food waste (FW) generation has put significant pressure on the environment and has increased the global financial costs of its appropriate management. Among the traditional organic waste recycling technologies (i.e., incineration, landfilling and anaerobic digestion), composting is an economically feasible and reliable technology for FW recycling regardless of its technical flaws and social issues. The global scenario of FW generation, technical advancement in FW composting and essential nutrient recovery from organic waste with waste recycling are discussed in this article. Recent research on various strategies to improve FW composting, including co-composting, the addition of organic/inorganic additives, the mitigation of gaseous emission, and microbiological variations are comprehensively explained. Subsequently, it is shown that the performing FW composting in an existing mechanical facility can improve organic waste degradation and produce value-added mature compost to save on costs and increase the technological feasibility and viability of FW composting to some extent.

Downstream augmentation of hydrothermal carbonization with anaerobic digestion for integrated biogas and hydrochar production from the organic fraction of municipal solid waste: A circular economy concept

Developing a treatment technology which minimizes the production of by-product (waste) is need of an hour. In this study, municipal yard waste (primary raw material) was microwave-pretreated before anaerobic digestion (AD) to improve biogas production. The anaerobically digested, Pretreated Yard Waste (PTY) and the Untreated Yard Waste (UTY) (waste/secondary raw material) was Hydrothermally Carbonized as a downstream treatment technique to produce energy rich hydrochar. The Hydrothermal carbonization (HTC) was conducted at a temperature of 180 °C and 200 °C for 6 h to produce carbon-rich hydrochar. Physicochemical, structural and combustion properties of PTY and UTY hydrochar were characterized and compared using a range of techniques to gain detailed insight into individual hydrochar samples. Microwave pretreatment of yard waste enhanced the biogas production from 264 ± 11 mL/g VS to 370 ± 14 mL/g VS. The carbon content and higher heating value of digestate increased considerably from 44 and 44.35% to 53-56% and15-16 MJ/kg to 21-23 MJ/kg, respectively after HTC. Thermal gravimetric analysis of the prepared hydrochar showed that the high-temperature carbonization increased the combustion properties of hydrochar. The hydrochar prepared from PTY showed enhanced physicochemical, structural and combustion properties as compared to hydrochar prepared from UTY. The finding asserted that the pretreatment of yard waste before AD not only improved biogas production but also improved yield with better quality hydrochar when its resulting digestate was hydrothermally carbonized. AD of yard waste yields biogas and HTC of the resulting digestate yields hydrochar; both are biofuel, hence, augmenting HTC as a downstream treatment process along with AD would result in the creation of near-zero loss process.