Production of solid biofuels from organic waste in developing countries: A review from sustainability and economic feasibility perspectives

Practising urban agriculture positively influences household organic waste management habits: A quantitative study from Florianópolis, Brazil

Proper household organic waste management practices are crucial to limit the negative environmental and health impacts that can result from inappropriate municipal waste treatment. The environmental impacts of organic waste have previously been described in the literature, and the main treatment strategy for managing such waste relies on technical facilities such as biodigesters. However, such technologies require significant financial investments, which could hinder their application in areas with lower economic power. Among the several available organic waste treatment strategies, composting for urban agricultural (UA) use has become increasingly popular. Although the literature suggests that UA can contribute to organic waste management by encouraging self-treatment practices, investigations into how practising UA can influence household waste management behaviours have been limited thus far. To this end, we analyse the role of practising UA along with other demographic variables, such as age, gender, education, income and housing conditions, in influencing citizens' household organic waste management behaviours. The city of Florianópolis, Brazil was selected as a case study since that municipality recently passed a new organic waste regulation law that supports the use of organic compost in UA. We surveyed 206 individuals regarding their household organic waste management habits in four behavioural areas: (i) separation, (ii) use of public services, (iii) self-treatment and (iv) reduction. The dataset comprises 102 individuals who were actively engaged in UA activities and 104 who were not involved in UA to compare habits of the two groups. The results show that UA practitioners are more likely to separate and self-treat their organic waste and use the derived compost for gardening activities. The use of public facilities for organic waste management is influenced by people's housing conditions. Respondents who lived in an apartment with no access to a garden logically had a lower willingness than did those with garden access to self-treat the organic waste produced. On the other hand, the results show that UA practitioners compost their own organic waste regardless of their housing conditions. The results show a strong, positive influence of practising UA on self-composting and thereby highlight the role of such practices in sensitizing urban residents to waste management issues and supporting local organic waste management strategies. Although the debate over the role of UA in organic waste management is still open, we reveal that highlighting this role could support a shift towards a circular approach to organic waste treatment.

Electric field as an activator of inoculated Bacillus clausii enhances humification during electric field-assisted aerobic composting

A novel electric field-assisted aerobic composting (EAC) method effectively facilitates compost disposal by applying a low electric field to conventional aerobic composting (CAC). The humification effect of inoculation with Bacillus clausii in the EAC system was better than that in the CAC system, so this study focused on the enhancement effect of microbial inoculation in the EAC system. Compared with EAC, EAC with microbial inoculation (AMI-EAC) increased the degradation of cellulose, hemicellulose, and lignin. Furthermore, AMI-EAC improved the humification index by 42.89 % relative to EAC. AMI-EAC also increased the relative abundance of Bacillus, enriched thermophilic and electroactive microorganisms, and enhanced the activity of associated degradative enzymes, which promoted the decomposition and humification of organic matter. Partial least squares-path model analysis showed that Bacillus inoculation during AMI-EAC enhanced the direct positive effect of microorganisms on enzyme activity and strengthened the positive impacts of substance degradation and enzyme activity on compost maturation. This study provided new insights for inoculating microbial agents to enhance composting efficiency in future engineering applications of EAC.

Dimensionality reduction of high-solid anaerobic digestion flow pattern: Flow velocity distribution model and control strategy

High-solid anaerobic digestion (HSAD) can be used to treat organic waste. However, the operating stability is limited by the hydraulic conditions; therefore, regulation is essential. The flow field contains a large amount of information that cannot be directly regulated. Dimension reduction research in HSAD could simplify hydrodynamic regulation by removing redundant information from the flow field to reduce the dimensionality. However, to date, no relevant studies have been conducted. In this study, dimension reduction research was conducted to simplify the regulation of the hydraulic conditions of HSAD by constructing a mathematical model based on the simulation results. The HSAD flow velocity distribution model was first proposed, which features the mixing efficiency of the reactor. In addition, an HSAD flow control model with a fitting error below 10% was proposed. The controlled hydraulic enhancement strategy for HSAD in this study provides new insights into mixing performance.

A systematic review of coastal zone integrated waste management for sustainability strategies

Coastal areas stand out because of their rich biodiversity and high tourist potential due to their privileged geographical position. However, one of the main problems in these areas is the generation of waste and its management, which must consider technical and sustainable criteria. This work aims to conduct a systematic review of the scientific literature on integrated solid waste management (ISWM) by considering scientific publications on the scientific basis for the proposal of sustainability strategies in the context of use and efficiency. The overall method comprises i) Search strategy, merging and processing of the databases (Scopus and Web of Science); ii) Evolution of coastal zone waste management; iii) Systematic reviews on coastal landfills and ISWM in the context of the circular economy; and iv) Quantitative synthesis in integrated waste management. The results show 282 studies focused on coastal landfills and 59 papers on ISWM with the application of circular economy criteria. Systematic reviews allowed for the definition of criteria for the selection of favorable sites, such as i) sites far from the coastline, ii) impermeable soils at their base to avoid contamination of aquifers, iii) use of remote sensing and geographic information system tools for continuous monitoring, iv) mitigation of possible contamination of ecosystems, v) planning the possibility of restoration (reforestation) and protection of the environment. In coastal zones, it is necessary to apply the ISWM approach to avoid landfill flooding and protect the marine environment, reducing rubbish and waste on beaches and oceans. Therefore, applying the circular economy in ISWM is critical to sustainability in coastal environments, with the planet's natural processes and variations due to climate change.

Optimization and characterization of hybrid bio-briquettes produced from the mixture of sawdust, sugarcane bagasse, and paddy straw

Biomass briquetting is a viable densification technique that converts waste biomass materials into useful products and alternative energy. This work explores the characteristics and optimization of hybrid bio-briquette production by combining crop residues (paddy straw) and solid biomass materials (sawdust and sugarcane bagasse). A total number of 20 briquettes were fabricated with three input factors: sawdust (SD), sugarcane bagasse (SB), and paddy straw (PS) based on the faced-centered central composite design (FCCCD) approach in the laboratory to investigate the calorific value (CV) and ash content (AC). The bomb calorimeter technique was used to evaluate the briquette's calorific value and ash content. The proposed work focused on optimizing the briquette input parameters (SD, SB, and PS) and output responses (CV and AC) using analysis of variance (ANOVA) and response surface methodology (RSM) and hybrid artificial neural network-integrated with multi-objective genetic algorithms (ANN-MOGA). This study shows that the MOGA-ANN-based model results in the best value of CV (17.07 MJ/kg) and AC (1.95%) with optimal input parameters SD (39.99 g), SB (29.02 g), and PS (69.02 g). The optimal results observed from the MOGA-ANN model have also been validated experimentally. The Fourier transform infrared (FTIR) spectroscopy investigation reveals that biomass briquettes are the sustainable and environment-friendly option of fossil fuels for power generation and indoor cooking. The study suggests a strategy for minimizing agro-waste, which may be converted into future fuel in the form of briquettes.

Optimization of organic solid waste composting process through iron-related additives: A systematic review

Composting is an environmentally friendly method that facilitates the biodegradation of organic solid waste, ultimately transforming it into stable end-products suitable for various applications. The element iron (Fe) exhibits flexibility in form and valence. The typical Fe-related additives include zero-valent-iron, iron oxides, ferric and ferrous ion salts, which can be targeted to drive composting process through different mechanisms and are of keen interest to academics. Therefore, this review integrated relevant literature from recent years to provide more comprehensive overview about the influence and mechanisms of various Fe-related additives on composting process, including organic components conversion, humus formation and sequestration, changes in biological factors, stability and safety of composting end-products. Meanwhile, it was recommended that further research be conducted on the deep action mechanisms, biochemical pathways, budget balance analysis, products stability and application during organic solid waste composting with Fe-related additives. This review provided guidance for the subsequent targeted application of Fe-related additives in compost, thereby facilitating cost reduction and promoting circular economy objectives.

Elaborating the role of rhamnolipids on the formation of humic substances during rice straw composting based on Fenton pretreatment and fungal inoculation

Composting is a green and sustainable way to dispose and reuse agricultural wastes, but the low degradation rate during composting hinders its application. This study was conducted to explore the effect of added surfactant rhamnolipids after Fenton pretreatment and inoculation of fungi (Aspergillus fumigatus) into the compost on the formation of humic substances (HS) during rice straw composting, and explored the effect of this method. The results showed that rhamnolipids speeded up the degradation of organic matter and HS formation during composting. Rhamnolipids promoted the generation of lignocellulose-degrading products after Fenton pretreatment and fungal inoculation. The differential products benzoic acid, ferulic acid, 2, 4-Di-tert-butylphenol and syringic acid were obtained. Additionally, key fungal species and modules were identified using multivariate statistical analysis. Reducing sugars, pH, and total nitrogen were the key environmental factors that affected HS formation. This study provides a theoretical basis for the high-quality transformation of agricultural wastes.

Towards a sustainable organic waste supply chain: A comparison of centralized and decentralized systems

Proper disposal of Municipal Solid (MSW) waste is an important issue as it causes land, air, and water pollution. Organic MSW provides a habitat environment to insects and often it spreads dangerous diseases. Major reasons identified behind this as the non-separation of MSW at the source and lack of facilities (bins) in the appropriate place for collection of wastes. The present study has proposed an integrated three-stage model to provide a solution to the problem of (i) allocation of the bin for waste collection, (ii) allocation and comparison of centralized and decentralized composting plants, and finally, (iii) vehicle routing for waste collection. The proposed generic model is applied to an Indian city, Bilaspur located in the state of Chhattisgarh. From the results, it is observed that the first stage model provides an optimal number of bins required and allocation of it at minimum cost. Taking it as input for the second stage model, it identifies the best locations for centralized and decentralized composting plants. The result also reveals that decentralized composting plants are more economical than centralized plants. Finally, the third stage of the model identifies the vehicle routing for the waste collection considering both centralized and decentralized plants to minimize the cost. Further, sensitivity analysis is carried out on collection rate and participation percentage parameters to draw additional insights for better management of MSW.

Torrefaction of walnut oil processing wastes by superheated steam: Effects on products characteristics

Walnut oil production waste (WOPW) is a by-product of walnut oil processing. The organic waste is rich in holocellulose and lignin, showing good potential to be converted by thermal process to valuable products. Superheated steam (SHS) torrefaction is a recently proposed thermal process enabling fast and unformal biomass heating, resulting in high-quality solid products as direct fuel. The potential of SHS to torrefy lipids and proteins (being rich in WOPW) is attractive for broader application of SHS torrefaction to upgrade more biomass wastes. SHS torrefaction was studied in this work to upgrade WOPW for solid products with different reaction temperatures (200, 250, 300 °C) and residence times (20, 40, 60 min). The lowest weight yield was 43.64 wt% under the severest treatment of 300 °C and 60 min, accompanied with the highest energy enhancement of 1.34 (reaching HHV of 27.03 MJ/kg). Response surface method is employed to reveal the effects of temperature and residence time. Residence time of 40 min under 300 °C was supposed to be an ideal condition to upgrade WOPW with HHV of 26.68 MJ/kg and in the range of coal from Van Krevelen diagram. Combustion indices (e.g., fuel ratio, combustion index, and volatile ignitability) indicated that the aforementioned torrefied WOPW had favourable properties as co-firing material. On the other hand, combustion behaviours analysis demonstrated that SHS torrefied WOPW could perform well as direct fuel. Aqueous effluent was also condensed and analyzed, where products from lipids and proteins were massively presented, giving an insight into the decomposition of those two constitutes undergoing SHS torrefaction.

Circular economy model for developing countries: evidence from Bangladesh

From environmental and sustainable development perspective, circular economy model is rarely applied in developing countries compared to developed nations. The aim of this paper is to review the overall scenario of the circular economy (CE) model in Bangladesh toward sustainable development. The study relies on the descriptive analysis of both qualitative and quantitative data, collected mostly from secondary sources with some in-depth interviews of the experts in the relevant field. The overall environmental status of Bangladesh, prospects, practices, and challenges of the circular economy model were thoroughly discussed in this paper. Though there are prospects to switching towards CE, the study reveals that the CE model's applicability is very limited in Bangladesh, being exercised mostly through recycling processes in some industries. Most importantly, we attempted to explore what is holding the CE practice in Bangladesh back, and iterated some policy, technical, and public participation barriers existing in Bangladesh. This paper will benefit the policymakers in developing countries in general and Bangladesh in particular to look more into the matter and hope to present ideas for future researchers to work on the idea of CE in the context of particular sectors and subsectors of Bangladesh.

Waste to Energy in Developing Countries—A Rapid Review: Opportunities, Challenges, and Policies in Selected Countries of Sub-Saharan Africa and South Asia towards Sustainability

Daily per capita waste generation will increase by 40% and 19%, for developing and developed countries by 2050, respectively. The World Bank estimates that total waste generation is going to triple in Sub-Saharan Africa (SSA) and double in South Asia (SA) by 2050. This article conducts a rapid review and aims to demonstrate the current waste management scenario and the potential of waste to energy generation in the developing world, focusing on SSA and SA. Although many review articles related to waste to energy (WtE) in developing countries are available in the literature, a rapid review particularly focusing on countries in SSA and SA is rarely seen. An analysis of different WtE generation technologies, and current waste management practices in developing countries in SSA and SA are also presented. The analysis shows that about three-fourths of waste is openly dumped in developing countries of SSA and SA. In terms of waste composition, on average, about 48.70% and 51.16% of waste generated in developing economies of SSA and SA are organic. Opportunities to convert this waste into energy for developing countries are highlighted, with a case study of Bangladesh, a country in SA. Major challenges regarding the waste to energy (WtE) projects in the developing world are found to be the composition of waste, absence of waste separation scheme at source, ineffective waste collection method, lack of suitable WtE generation technology in place, lack of financial support and policies related to a WtE project, and absence of coordination between different governmental institutions.

Land use change in the Vietnamese Mekong Delta: New evidence from remote sensing

This paper presents the first attempt to capture a comprehensive spatial view of land use change in the Vietnamese Mekong Delta (VMD) for a long period, i.e., from 2000 to 2020. It is aimed at monitoring holistically the land use change and flooding situation in the region, addressing the reasons for land use change, and assessing the impacts of land use change on hydraulic aspects and farmer livelihoods during the last 21 years. MODIS products, in particular, are used to study the dynamics of land use and floods after demonstrating high validation with statistical data and radar satellites, with R² = 0.96 and R² ≥ 0.97 for land use and flood maps, respectively. The results show that rice cultivation is the most dominant land use type, accounting for 40% to 46% of the delta area, while aquaculture accounts for 10% to 22%, respectively. The total rice cultivation area increased from 3764 thousand hectares (thous. ha) in 2001 to 4343 thous. ha in 2015 based on the intensive development of triple rice cropping in the upper zone, then decreased to 3963 thous. ha in 2020. In contrast, aquaculture areas are farmed mainly in the coastal area and remained relatively steady, increasing slightly from 619 thous. ha in 2001 to 856 thous. ha in 2020. The massive construction of dikes for triple rice cropping in the upper zone appears to cause a significant impact on the annual flooding regime. Land use policies have influenced the changes in land use patterns, flooding situations, and the livelihoods of local farmers.

Organic solid waste upgrading under natural gas for valuable liquid products formation: Pilot demonstration of a highly integrated catalytic process

An integrated catalytic process for natural-gas-assisted OSW upgrading is fully demonstrated at a pilot scale, which combines the methanolysis, methano-refining and catalytic liquefaction processes through careful process design and catalyst tailoring. Three types of agricultural and forestry wastes including pelleted wood chip, crushed rice straw and crushed corn stover are used as representatives of OSW for at least a 1-month continuous operation. The products are comprehensively analyzed and the indices for quality control in terms of basic, compositional, and fuel properties, as well as elemental and distillation analyses, are monitored and proved to be satisfactory for practical use. Economic assessment and life cycle analysis are also performed. It is validated that the OSW upgrading process under a natural gas atmosphere exhibits favorable feasibility, stability, operational margins and environmental friendliness. This exploration manifests an alternative route for simultaneous energy supply and waste management with great economic and environmental advantages.

Thermochemical conversion of municipal solid waste into energy and hydrogen: a review

The rising global population is inducing a fast increase in the amount of municipal waste and, in turn, issues of rising cost and environmental pollution. Therefore, alternative treatments such as waste-to-energy should be developed in the context of the circular economy. Here, we review the conversion of municipal solid waste into energy using thermochemical methods such as gasification, combustion, pyrolysis and torrefaction. Energy yield depends on operating conditions and feedstock composition. For instance, torrefaction of municipal waste at 200 °C generates a heating value of 33.01 MJ/kg, while the co-pyrolysis of cereals and peanut waste yields a heating value of 31.44 MJ/kg at 540 °C. Gasification at 800 °C shows higher carbon conversion for plastics, of 94.48%, than for waste wood and grass pellets, of 70-75%. Integrating two or more thermochemical treatments is actually gaining high momentum due to higher energy yield. We also review reforming catalysts to enhance dihydrogen production, such as nickel on support materials such as CaTiO₃, SrTiO₃, BaTiO₃, Al₂O₃, TiO₃, MgO, ZrO₂. Techno-economic analysis, sensitivity analysis and life cycle assessment are discussed.

Co-cultivation of T. asperellum GDFS1009 and B. amyloliquefaciens 1841: Strategy to regulate the production of ligno-cellulolytic enzymes for the lignocellulose biomass degradation

The influence of fossil fuels on the environment focused on the development of new technology on biofuels. In this situation, lignocellulolytic hydrolysis enzymes such as Cellobiohydrolase, β-Glucosidase, Endoglucanase, cellulase and xylanase have broad applications in the biofuel production. The Trichoderma have used for the production of cellulase and xylanase to hydrolyze the lignocellulose. Hence, in the present study, co-culture has been employed to induce the production of polysaccharide hydrolyzing enzymes under both induction and repression conditions. The enzyme activity and its gene expression were induced by the co-culture of T. asperellum and B. amyloliquefaciens compared to the monoculture. Further, the co-culture upregulated the transcription regulatory genes and downregulated the repressor genes under both repressor and inducer conditions, respectively. The crude enzyme produced by the co-culture and monocultures using the optimized medium containing molasses, cornmeal and rice bran were further used to hydrolyze the pretreated corn Stover, rice straw, and wheat straw. These results indicate that the co-culture of T. asperellum and B. amyloliquefaciens is a promising and inexpensive method to advance the innovation on the continuous production of cellulase and xylanase under different circumstances for the bioconversion of lignocellulosic biomass into glucose for the bio-fuels.

Recent Advancements in the Synthesis and Application of Carbon-Based Catalysts in the ORR

Fuel cells are a promising alternative to non-renewable energy production industries such as petroleum and natural gas. The cathodic oxygen reduction reaction (ORR), which makes fuel cell technology possible, is sluggish under normal conditions. Thus, catalysts must be used to allow fuel cells to operate efficiently. Traditionally, platinum (Pt) catalysts are often utilized as they exhibit a highly efficient ORR with low overpotential values. However, Pt is an expensive and precious metal, posing economic problems for commercialization. Herein, advances in carbon-based catalysts are reviewed for their application in ORRs due to their abundance and low-cost syntheses. Various synthetic methods from different renewable sources are presented, and their catalytic properties are compared. Likewise, the effects of heteroatom and non-precious metal doping, surface area, and porosity on their performance are investigated. Carbon-based support materials are discussed in relation to their physical properties and the subsequent effect on Pt ORR performance. Lastly, advances in fuel cell electrolytes for various fuel cell types are presented. This review aims to provide valuable insight into current challenges in fuel cell performance and how they can be overcome using carbon-based materials and next generation electrolytes.

Eco-Restoration of Coal Mine Spoil: Biochar Application and Carbon Sequestration for Achieving UN Sustainable Development Goals 13 and 15

Open cast coal mining causes complete loss of carbon sink due to the destruction of vegetation and soil structure. In order to offset the destruction and to increase sequestration of carbon, afforestation is widely used to restore these mine spoils. The current field study was conducted to assess the ecosystem status, soil quality and C pool in an 8 years old reclaimed mine spoil (RMS), compared to a reference forest (RF) site and unamended mine spoil (UMS). Biochar (BC) prepared from invasive weed Calotropis procera was applied in this 8 year RMS at 30 t ha−1 (BC30) and 60 t ha−1 (BC60) to study its impact on RMS properties and C pool. Carbon fractionation was also conducted to estimate inorganic, coal and biogenic carbon pools. The C stock of 8 year old RMS was 30.98 Mg C ha−1 and sequestered 113.69 Mg C ha−1 CO2. BC30 and BC60 improved the C-stock of RMS by 31% and 45%, respectively, and increased the recalcitrant carbon by 65% (BC30) and 67% (BC60). Spoil physio-chemical properties such as pH, cation exchange capacity, moisture content and bulk density were improved by biochar application. The total soil carbon at BC30 (36.3 g C kg−1) and BC60 (40 g C kg−1) was found to be significantly high compared to RMS (21 g C kg−1) and comparable to RF (33 g C kg−1). Thus, eco-restoration of coal mine spoil and biochar application can be effective tools for coal mine reclamation and can help in achieving the UN sustainable development goal 13 (climate action) by increasing carbon sequestration and 15 (biodiversity protection) by promoting ecosystem development.

Non-Catalytic Dissolution of Biochar Obtained by Hydrothermal Carbonization of Sawdust in Hydrogen Donor Solvent

The production of fuel hydrocarbons from CO2-neutral raw materials is a promising task at present. The thermal dissolution of biochar obtained by the method of hydrothermal carbonization of sawdust was studied. The dissolution of biochar in tetralin (hydrogen donor solvent) was studied at different temperatures (350–450 °C) and with two types of dilution of the mixture with tetralin: 1/3 and 1/4. The process proceeded without a catalyst. It was found that the samples subjected to thermal dissolution at temperatures of 425–450 °C had the highest conversion and yield of liquid products. The reaction temperature also had a significant effect on the composition of liquid products. It was found that an increase in the reaction temperature led to a significant increase in benzenes, both in the direct and in the hexane fraction. A benzene yield of more than 50% was observed for both fractions at a temperature of 450 °C. It was also suggested that the possible positive effect of abietates on the homogenization of the reaction mixture contributed to high conversion in the process. The biochar/tetralin ratio effects the yield and composition of the liquid products as well. An increase in the tetralin concentration in the mixture during thermal dissolution led to an increase in the conversion and yield of hydrocarbon fractions for fuel purposes. This is undoubtedly due to the large amount of elemental hydrogen involved in the hydrogenation of the reaction mixture.