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To face the ongoing issues related to global warming, a circular economy approach should be pursued, rethinking the waste management system and the recovery of organic waste. The main organic waste streams are Food Waste (FW) and municipal Sewage Sludge (SS). In the spirit of circularity, a commingled treatment of FW and SS could be a viable solution. To this end, the present work aims to review the technical and environmental aspects of the co-treatment of FW and SS through biological and thermal processes. Firstly, a detailed characterization of the two substrates is presented as well as the current and future treatment technologies. Then, the technical feasibility and the environmental impacts of conventional biological co-treatments of FW and SS (i.e., composting, anaerobic digestion, and a combination of them), as well as innovative thermal ones (i.e., incineration, gasification, pyrolysis, and hydrothermal carbonization), is summarized. The outcomes of this work could contribute to achieving a more sustainable way to approach organic waste treatment and to help policy-making authorities move toward sustainable planning. [ABSTRACT FROM AUTHOR]

Hydrothermal carbonization (HTC) has emerged as a pivotal technology in the battle against climate change and fosters circular economies. Operating within a unique reaction environment characterized by water as a solvent and moderate temperatures at self-generated pressures, HTC efficiently converts biomass residues into valuable bio-based products. Despite HTC's potential—from the management of challenging biomass wastes to the synthesis of advanced carbons and the implementation of biorefineries—it encounters hurdles transitioning from academic exploration to industrial implementation. Gaps persist, from a general comprehension of reaction intricacies to the difficulty of large-scale integration with wastewater treatments, to the management of process water, to the absence of standardized assessment techniques for HTC products. Addressing these challenges demands collaboration to bridge the many scientific sectors touched by HTC. Thus, this article reviews the current state of some hot topics considered crucial for HTC development: It emphasizes the role of HTC as a cornerstone for waste management and biorefineries, highlighting potentialities and challenges for its development. In particular, it surveys fundamental research aspects, delving into reaction pathways, predictive models, analytical techniques, and HTC modifications while exploring HTC's crucial technological applications and challenges, with a peculiar focus on combined HTC, wastewater integration, and plant energy efficiency. [ABSTRACT FROM AUTHOR]

This study assesses the status of hydrothermal carbonization (HTC) technology and identifies barriers hindering its commercial viability. Conducting a global survey among HTC companies (with a total of 24 surveys sent), the research evaluates the current landscape, challenges, and future prospects of large-scale HTC operations. Furthermore, it presents a detailed global inventory of existing HTC facilities, illustrating geographical distribution and trends in application. Most of the companies are located in Europe, followed by Asia and North America. With substantial participation from HTC companies, exceeding 62% in the survey (15 companies), the study provides a comprehensive overview of diverse companies, their business models, regulatory challenges, and the overall state of HTC technology. The majority of companies in this study, approximately 80%, offer services in the field of waste management. This paper also explores the potential of HTC in transforming waste management practices, carbon sequestration methodologies, and the development of new materials. Employing a thorough SWOT analysis, the paper advocates for a broader adoption of HTC, emphasizing its transformative capacity in fostering sustainable management of urban, industrial, and agricultural residues, promoting circular economy principles, mitigating climate change, and offering a robust foundation for informed decision-making and sustainable development strategies. [ABSTRACT FROM AUTHOR]

The COVID-19 pandemic has led to the increased use of disposable face masks worldwide, resulting in a surge of potentially infectious waste. This waste must be safely managed and disposed of to prevent the spread of the virus. To address this issue, a preliminary study explored the use of hydrothermal carbonization (HTC) as a potential method for converting surgical mask waste into value-added carbonaceous materials. The HTC treatments were conducted at 220 °C for 3 h with or without the addition of acetic acid. The resulting hydrochar was characterized using several techniques, including thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and N2-physisorption analyzers. The study found that the masks formed a melt with reduced mass (−15%) and volume (up to −75%) under the applied conditions. The carbon content and higher heating value (HHV) of the produced hydrochars were higher than those of the original masks (+5%). Furthermore, when acetic acid was added during the HTC experiment, a new crystal phase, terephthalic acid, was produced. This acid is a precursor in surgical mask production. The study suggests that hydrothermal carbonization could potentially achieve sanitization and volume reduction in non-renewable and non-biodegradable surgical masks while also producing a solid fuel or a raw material for terephthalic acid production. This approach offers an innovative and sustainable solution to manage the waste generated by the increased use of disposable face masks during the pandemic. [ABSTRACT FROM AUTHOR]

The current water crisis necessitates the development of new materials for wastewater treatment. A variety of nanomaterials are continuously being investigated for their potential as adsorbents for environmental remediation. Researchers intend to develop a low-cost, simple, and sustainable material that can cater to removal of pollutants. Biochar derived from biowaste is a potential candidate for the existing problem of water pollution. The review focuses on the various aspects of biochar, such as its sources, preparation methods, mechanism, applications for wastewater treatment, and its regeneration. Compared with other adsorbents, biochar is considered as an environmentally friendly, sustainable, and cost-effective substitute for waste management, climate protection, soil improvement, wastewater treatment, etc. The special properties of biochar such as porosity, surface area, surface charge, and functional groups can be easily modified by various chemical methods, resulting in improved adsorption properties. Therefore, in view of the increasing environmental pollution and the problems encountered by researchers in treating pollutants, biochar is of great importance. This review also highlights the challenges and prospective areas that can be explored and studied in more detail in the future. [ABSTRACT FROM AUTHOR]

Municipal solid waste treatment and disposal have become one of the major concerns in waste management due to the excessive production of waste and higher levels of pollution. To address these challenges and protect the environment in sustainable ways, the hydrothermal pretreatment (HTP) technique coupled with anaerobic digestion (AD) becomes a preferred alternative technology that can be used for municipal solid waste stabilization and the production of renewable energy. However, the impact of HTP parameters such as temperature, retention time, pH, and solid content on the fermentation of TWAS is yet to be well studied and analyzed. Hence this study was conducted to review the effect of hydrothermal pretreatment of thickened waste-activated sludge (TWAS) on fermentation and anaerobic digestion processes. Many studies reported that fermentation of TWAS at pretreatment temperature ranges from 160 °C to 180 °C resulted in a 50% increase in volatile fatty acid (VFA) yields compared to no pretreatment. However, for the AD process, HTP in the range of 175 °C to 200 °C with a 30–60 min retention time was considered the optimal condition for higher biogas production, with 30% increase in biodegradability and greater than 55% increase in biogas production. Even though there is a direct relationship between increased HTP temperature and the hydrolysis of TWAS, a pretreatment temperature range beyond 200 °C alters the biogas production. The solid content (SC) of sludge plays a crucial role in HTP, where in practice up to 16% SC has been utilized for HTP. Further, a combined alkaline-HTP enhances the process performance. [ABSTRACT FROM AUTHOR]

Municipal sewage sludge is the residual material produced as a waste of municipal wastewater purification. It is a sophisticated multi-component material, hard to handle. For many years, it has been landfilled, incinerated, and widely used in agriculture practice. When unproperly discharged, it is very polluting and unhealthy. The rapidly increasing global amount of municipal sewage sludge produced annually depends on urbanization, degree of development, and lifestyle. Some diffused traditional practices were banned or became economically unfeasible or unacceptable by the communities. In contrast, it has been established that MSS contains valuable resources, which can be utilized as energy and fertilizer. The objective of the review was to prove that resource recovery is beneficially affordable using modern approaches and proper technologies and to estimate the required resources and time. The open sources of information were deeply mined, critically examined, and selected to derive the necessary information regarding each network segment, from the source to the final point, where the municipal sewage sludge is produced and disposed of. We found that developed and some developing countries are involved with ambitious and costly plans for remediation, the modernization of regulations, collecting and purification systems, and beneficial waste management using a modern approach. We also found that the activated sludge process is the leading technology for wastewater purification, and anaerobic digestion is the leading technology for downstream waste. However, biological technologies appear inadequate and hydrothermal carbonization, already applicable at full scale, is the best candidate for playing a significant role in managing municipal sewage sludge produced by big towns and small villages. [ABSTRACT FROM AUTHOR]

Sewage sludge hydrochars (SSHs), which are produced by hydrothermal carbonization (HTC), offer a high calorific value to be applied as a biofuel. However, HTC is a complex processand the properties of the resulting product depend heavily on the process conditions and feedstock composition. In this work, we have applied artificial neural networks (ANNs) to contribute to the production of tailored SSHs for a specific application and with optimum properties. We collected data from the published literature covering the years 2014–2021, which was then fed into different ANN models where the input data (HTC temperature, process time, and the elemental content of hydrochars) were used to predict output parameters (higher heating value, (HHV) and solid yield (%)). The proposed ANN models were successful in accurately predicting both HHV and contents of C and H. While the model NN1 (based on C, H, O content) exhibited HHV predicting performance with R2 = 0.974, another model, NN2, was also able to predict HHV with R2 = 0.936 using only C and H as input. Moreover, the inverse model of NN3 (based on H, O content, and HHV) could predict C content with an R2 of 0.939. [ABSTRACT FROM AUTHOR]

Following a recycling or continuous recycling process, there is always waste with no material or market value that can be converted into energy or other fossil fuel substitutes. The present study aimed to evaluate the management of organic waste policy and to predict the trend of organic waste generation in Albania. The research used an appropriate Box–Jenkins Auto Regressive Integrated Moving Average (ARIMA) to determine the quantification of organic waste to be generated. The main results obtained can support the decision-making process in the planning, change and short-term implementation of organic waste management, and the information provided is very useful in collecting, transporting, storing and managing waste in Albanian cities (Tirana, Durrës, Kukës, Berat, Shkodra, Dibër, Gjirokastër and Elbasan). Furthermore, the high percentage of the organic waste generation until 2025 constitutes good premises to raising public awareness related to their energy recovery. [ABSTRACT FROM AUTHOR]

Hydrothermal carbonisation (HTC) has been identified as a potential route for digestate enhancement producing a solid hydrochar and a process water rich in organic carbon. This study compares the treatment of four dissimilar digestates from anaerobic digestion (AD) of agricultural residue (AGR); sewage sludge (SS); residual municipal solid waste (MSW), and vegetable, garden, and fruit waste (VGF). HTC experiments were performed at 150, 200 and 250 °C for 1 h using 10%, 20%, and 30% solid loadings of a fixed water mass. The effect of temperature and solid loading to the properties of biocoal and biochemical methane potential (BMP) of process waters are investigated. Results show that the behaviour of digestate during HTC is feedstock dependent and the hydrochar produced is a poor-quality solid fuel. The AGR digestate produced the greatest higher heating value (HHV) of 24 MJ/kg, however its biocoal properties are poor due to slagging and fouling propensities. The SS digestate process water produced the highest amount of biogas at 200 °C and 30% solid loading. This study concludes that solely treating digestate via HTC enhances biogas production and that hydrochar be investigated for its use as a soil amender. [ABSTRACT FROM AUTHOR]

Biochar (BC) addition is a novel and promising method for biogas yield increase. Brewer's spent grain (BSG) is an abundant organic waste with a large potential for biogas production. In this research, for the first time, we test the feasibility of increasing biogas yield and rate from BSG digestion by adding BC, which was produced from BSG via torrefaction (low-temperature pyrolysis). Furthermore, we explore the digestion of BSG with the presence BCs produced from BSG via torrefaction (low-temperature pyrolysis). The proposed approach creates two alternative waste-to-energy and waste-to-carbon type utilization pathways for BSG: (1) digestion of BSG waste to produce biogas and (2) torrefaction of BSG to produce BC used for digestion. Torrefaction extended the short utility lifetime of BSG waste turned into BC. BSG was digested in the presence of BC with BC to BSG + BC weight ratio from 0 to 50%. The study was conducted during 21 days under mesophilic conditions in n = 3 trials. The content of dry mass 17.6% in all variants was constant. The kinetics results for pure BSG (0% BC) were: reaction rate constant (k) 1.535 d−1, maximum production of biogas (B0) 92.3 dm3∙kg−1d.o.m. (d.o.m. = dry organic matter), and biogas production rate (r), 103.1 dm3∙kg−1d.o.m.∙d−1. his preliminary research showed that the highest (p < 0.05) r, 227 dm3∙kg−1d.o.m.∙d−1 was due to the 5% BC addition. This production rate was significantly higher (p < 0.05) compared with all other treatments (0, 1, 3, 8, 10, 20, 30, and 50% BC dose). Due to the high variability observed between replicates, no significant differences could be detected between all the assays amended with BC and the variant 0% BC. However, a significant decrease of B0 from 85.1 to 61.0 dm3∙kg−1d.o.m. in variants with the high biochar addition (20–50% BC) was observed in relation to 5% BC (122 dm3∙kg−1d.o.m.), suggesting that BC overdose inhibits biogas production from the BSG + BC mixture. The reaction rate constant (k) was not improved by BC, and the addition of 10% and 20% BC even decreased k relatively to the 0% variant. A significant decrease of k was also observed for the doses of 10%, 20%, and 30% when compared with the 5% BC (1.89 d−1) assays. [ABSTRACT FROM AUTHOR]