Clean Technologies doi: 10.3390/cleantechnol8030077

Authors: Prashant Sharan Lucky Yerimah Manvendra Dubey Harshul Thakkar Mohamed Mehana Troy Semelsberger Michael Heidlage Rajinder Singh

Given the large natural gas (NG) reserves of the Intermountain West (I-WEST) region in the USA, it can emerge as a leader in hydrogen (H2) production. Currently, H2 production via steam methane reforming (SMR) of NG releases carbon dioxide (CO2) and the natural gas infrastructure has fugitive NG and H2 losses during production, conversion and transportation. Integrated carbon capture and sequestration (CCS) is a promising approach for producing hydrogen and CO2 from the SMR process for industrial uses including power, chemicals and fuels. However, the NG losses and regional water availability can be limiting factors for H2 production. H2 production assessments are often made at the global scale and neglect regional factors such as abundant gas and limited water in the I-WEST. We demonstrate that a regional SMR process unit sitting near NG wells offers opportunities to significantly reduce fugitive NG losses. We show that regional H2 production by SMR has a lower emissions profile than widespread natural gas combustion in the I-WEST and reduces the H2 production cost as well. Replacing the I-WEST transportation sector with H2 fuel cell vehicles and using 100% H2-powered electricity can provide substantial reductions in water consumption and fuel costs. This is better than blending H2 with NG which is more expensive. The captured CO2 can be effectively used for enhanced oil recovery in I-WEST. Finally, the potential of utilizing produced, brackish and treated impaired water sources is assessed to meet the water needs for H2 production in the I-WEST.

Clean Technologies doi: 10.3390/cleantechnol8030076

Authors: Ojo Olufisayo Riaan Stopforth

This study presents an experimental engine map investigation of waste swine oil biodiesel (WSO B100) in a single-cylinder, four-stroke variable compression ratio compression ignition engine, quantifying the coupled effects of compression ratio and load on brake thermal efficiency, brake-specific fuel consumption, torque, brake power, and regulated emissions of NOx, CO, HC, and CO2. Compression ratios of 12, 14, 16, and 18 were evaluated at dynamometer loads of 25%, 50%, and 75% under steady-state operation. The study’s primary contribution is a structured compression ratio–load mapping framework that produces consistent performance emission response surfaces and, supported by statistical modeling and sensitivity analysis, resolves main and interaction effects to identify operating regions that balance efficiency and emissions. Methodological traceability is strengthened by attaching fuel energy and mass flow calculations to batch-specific fuel properties, including viscosity and density, and by using calorimetry-derived heating value in efficiency calculations. Increasing the compression ratio from 12 to 18 improved brake thermal efficiency by 3–10% at low load and reduced brake-specific fuel consumption, while NOx increased by 20–30% across the load range. Increasing load raised brake thermal efficiency from 29% at 25% load to 42% at 75% load and reduced brake-specific fuel consumption from 309 to 215 g/kWh; NOx peaked at 488 ppm at 75% load and compression ratio 18. CO and HC decreased with both load and compression ratio, reaching minima of 0.15% and 30 ppm, whereas CO2 increased primarily with load. Relative to diesel, WSO biodiesel showed 8–12% higher brake-specific fuel consumption and 2–4% lower peak brake thermal efficiency, but achieved substantial CO and HC reductions. Generally, WSO biodiesel operates effectively across a wide compression ratio range with broadly comparable performance to diesel. However, increased NOx and reduced low-load efficiency indicate the need for targeted calibration or emission control.

Clean Technologies doi: 10.3390/cleantechnol8030075

Authors: Zhigang Shi Shiwei Xia Peng He Lin Zhang Nuochen Wang Yu Wang

Compared with previous analytical models that mostly neglect air–lining coupled heat transfer, the proposed model innovatively introduces this mechanism and achieves a maximum error reduction of 0.5% against experimental data. The analytical solution of the model is obtained by using the Green’s function method. The reliability and accuracy of this model are confirmed through comparisons with existing experimental data. Research indicates that adjusting the tunnel air temperature improves the ground heat exchanger’s heat exchange efficiency more significantly than modifying the thermal conductivity of the lining. In the tested range, as the flow velocity increases, its influence on the heat transfer effect gradually weakens. The simulation results indicate that under summer operating conditions, only approximately 5–8% of the heat transferred by the ground heat exchanger is dissipated to the tunnel air-side environment, while the vast majority (92–95%) is conducted to the surrounding rock.

Clean Technologies doi: 10.3390/cleantechnol8030074

Authors: Irfan Basturk Ibrahim Ozdemir Hande Gulcan Selda Murat Hocaoglu Recep Partal Burak Bozcelik Charuka Meegoda Harsha Ratnaweera Zakhar Maletskyi

Accurate and rapid determination of moisture content in waste sludge is essential for optimizing dewatering processes, reducing disposal costs, and minimizing environmental impact. This study investigates the use of Fourier Transform Near-Infrared (FT-NIR) spectroscopy combined with Partial Least Squares Regression (PLS-R) for predicting the moisture content of dewatered sludge. A total of 96 sludge samples, with dry matter contents ranging from 12.4% to 24.6%, were collected from two treatment plants. FT-NIR spectra were acquired over the 800–2500 nm range, and chemometric models were developed to correlate spectral information with gravimetrically determined moisture content. The optimized PLS-R model demonstrated strong predictive performance, achieving a cross-validated coefficient of determination (R2CV) of 0.87, a root mean square error of cross-validation (RMSECV) of 0.92%, and a residual predictive deviation (RPD) of 2.73. Independent test set validation confirmed the robustness of the model (R2Test = 0.88, RMSEP = 0.88%, RPD = 2.92), supported by strong calibration results (R2CT = 0.95, RMSEE = 0.60%, RPD = 4.46). Principal component analysis indicated that spectral variability observed in sludge samples was primarily associated with wastewater treatment plant (WWTP)-specific characteristics, reflecting moisture–organic matter interactions. These results demonstrate that FT-NIR spectroscopy is a promising tool for sludge moisture prediction.

Clean Technologies doi: 10.3390/cleantechnol8030073

Authors: Igor Kogut Juliane Alberts Bianca-Michaela Wölfling Stephan Hussy Daniel Polak Maciej Szwast

Domestic wastewater is a chemically complex and highly variable mixture of pollutants generated by everyday household activities, yet its contribution to environmental contamination is still frequently underestimated and only 56% of wastewater worldwide is being treated. This review provides a structured and quantitative assessment of major domestic wastewater pollutant groups, their principal exposure pathways, and current and emerging treatment technologies. Beyond a conventional narrative synthesis, the review derives per capita annual emission estimates from published data and uses these to compare pollutant groups by mass flow and environmental relevance. The analysis shows that high-volume household inputs, particularly sodium chloride from domestic water softening, toilet paper, personal-care products, detergents, and cleaning agents, can contribute substantially to overall pollutant loads, whereas lower-mass contaminants such as pharmaceuticals, antibiotics, PFAS, heavy metals, and microplastics remain critical because of their persistence, biological activity, and incomplete removal during treatment. The review further highlights that conventional wastewater treatment systems are often poorly equipped to remove many of these emerging contaminants effectively, especially under decentralised or only partially advanced treatment conditions. Advanced and hybrid technologies, including membrane bioreactors, nanofiltration, reverse osmosis, adsorption, photocatalysis, and electrochemical processes, offer clear potential, but their broader implementation remains constrained by cost, energy demand, fouling, and concentrate management. Overall, the added value of this review lies in linking mass-based pollutant prioritisation with treatment performance, thereby providing a more systematic basis for identifying dominant household emission pathways and for guiding targeted mitigation and technology selection in future wastewater management.

Clean Technologies doi: 10.3390/cleantechnol8030071

Authors: Milena Senjak Pejić Mladenka Novaković Bežanović Mirna Radović Igor Peško Maja Petrović

The construction sector is a major consumer of raw materials and a significant source of greenhouse gas emissions, necessitating data-driven approaches to support circular economy implementation and sustainable project management. This study develops an integrated framework combining machine learning-based material stock prediction, carbon footprint assessment, and Environmental, Social, and Governance (ESG) performance evaluation for construction projects. A dataset of 128 residential buildings was compiled from official use-permit documentation. After dimensionality reduction using variance filtering and Spearman correlation analysis, 25 regression algorithms were evaluated to estimate quantities of concrete, reinforcement, and brick products. The K-Nearest Neighbor (KNN) Regressor achieved the best predictive performance, with mean absolute percentage errors of 10.64% for concrete, 10.23% for reinforcement, and 16.05% for brick products. Predicted material quantities were used to calculate CO2 emissions across materialization, demolition, and disposal phases under linear and circular scenarios. The results indicate that circular economy implementation significantly reduces total emissions, particularly for concrete, with reductions of up to 97% under idealized full-substitution conditions, representing an upper-bound estimate. ESG assessment using the Delphi method identified environmental indicators as the most significant sustainability dimension. The proposed framework enables early-stage emission estimation and supports informed decision-making toward low-carbon and resource-efficient construction practices.

Clean Technologies doi: 10.3390/cleantechnol8030072

Authors: Heloísa Schneider Rolando Chamy César Valderrama Andrés Morales Fernanda Farías Sergi Vinardell

Chile is widely regarded as a key global player in green hydrogen production due to its exceptional renewable energy potential, which enables low-carbon and competitive production costs. This article provides a comprehensive review of Chile’s green hydrogen sector, evaluating the transition from early strategic goals to the current phase of industrial scaling. It offers an integrated analysis of the regulatory framework, infrastructure deployment, and the techno-economic variables essential for integrating Chilean derivatives into global markets. The country has established a supportive framework through its National Green Hydrogen Strategy (NGHS), which sets out goals of 25 GW of installed electrolyzer capacity and USD 2.5 billion in annual exports by 2030. Despite these ambitious targets, actual deployment remains in the early stages, with only 3.9 GW currently in the implementation phase and a lack of fully operational industrial-scale facilities. Furthermore, initial NGHS projections suggested a levelized cost of hydrogen (LCOH) of USD 1.3–1.4/kg by 2030. However, current calculations point to a more complex reality of approximately USD 3.1/kg due to infrastructure bottlenecks and global supply chain pressures. While Chile’s renewable resources ensure low production-stage emissions, the absence of explicit regulatory carbon targets underscores the need for comprehensive life-cycle assessments encompassing manufacturing and global distribution. Overall, this review concludes that Chile should overcome persistent regulatory and logistical constraints to consolidate a robust and internationally competitive green hydrogen sector, aligned with its 2050 carbon neutrality objectives.

Clean Technologies doi: 10.3390/cleantechnol8030070

Authors: Junnan Li Jingxin Zhang Chen Yu Shiqi Hou Peng Li Kaifeng Yu Xu Guo Fei Dou Xinglin Zhang Yiliang He

Rapid global urbanization is driving a surge in solid waste generation, while conventional treatment systems face both environmental risks and operational uncertainty. Digital twins, which enable real-time mapping between physical assets and virtual spaces, offer a computable and verifiable route toward low-carbon, resource-efficient, and intelligent waste management when deeply integrated with the Internet of Things, big data, and artificial intelligence. This study develops a comprehensive review tracing the digital twins from static geometric mirroring to dynamic, cognitive co-symbiosis, and summarizes a multidimensional architecture spanning physical, virtual, data, service, and connectivity layers, together with coupling mechanisms involving IoT sensing, federated learning, multimodal big data, and large model agents. The study aims to provide a theoretical framework and methodological references for advancing digital twin-enabled solid waste valorization. Building on this framework, we examine recent progress in three representative application scenarios for solid waste treatment, and identify key technical bottlenecks, including heterogeneous data fusion, model generalization across facilities and contexts, and real-time computation under constrained resources. We highlight the need for standardization, uncertainty quantification, cybersecurity, and lifecycle evaluation to support reliable prediction, optimization, and decision-making in real operations. Finally, we discuss future directions such as edge intelligence and the integration of city-scale material and energy networks.

Clean Technologies doi: 10.3390/cleantechnol8030069

Authors: Juan Carlos Sainz-Hernández Prabhaharan Renganathan Edgar Omar Rueda Puente

Eggshell waste generated by the poultry processing industry represents a significant yet underutilized biogenic resource with substantial potential for sustainable agricultural and environmental applications. Globally, several million metric tons of eggshell residues are produced annually, consisting predominantly of calcium carbonate (CaCO3) in the form of calcite, along with minor quantities of organic matrices and trace minerals. These physicochemical characteristics make eggshells a promising renewable alternative to conventional mineral sources for use as fertilizers, soil amendments, and biomaterials. Recent studies have shown that finely ground eggshell powder (ESP) is an effective liming material that can regulate soil chemical conditions and improve agronomic performance under acidic soil conditions. Furthermore, eggshell-derived materials have been incorporated into composting systems, biochar composites, and nanostructured fertilizers to enhance nutrient dynamics, immobilization of contaminants, and microbial activity. Advances in nanotechnology have facilitated the synthesis of nano-calcium carbonate (NCC) and nanohydroxyapatite (nHAP) fertilizers with improved nutrient supply and controlled-release properties. However, challenges associated with nanosafety evaluation, large-scale processing technologies, regulatory harmonization, and long-term field validation remain. Therefore, this review critically synthesizes the structural, biochemical, and physicochemical properties of eggshells and eggshell membranes, examines their applications in sustainable agriculture and environmental remediation, and identifies the key research priorities required to advance eggshell valorization within circular bioeconomy strategies.

Clean Technologies doi: 10.3390/cleantechnol8030068

Authors: Maria Avrami Christina Lazaratou Zacharias Frontistis Athanasia Tekerlekopoulou Vasilios Georgakilas Dimitris Vayenas

Propyl-paraben (PrP) is a common preservative found in cosmetics and pharmaceutical products. It is classified as a category 1 endocrine-disrupting compound, which highlights the importance of efficiently removing it from water during treatment processes. This study investigates the potential of using Leptolyngbya sp. dominated cyanobacterial biomass residues, in both their raw and hydrothermally treated (hydrochar) forms, for the removal of PrP from aqueous media. Batch and fixed-bed column experiments were carried out under varying conditions to assess adsorption kinetics and equilibrium behavior. Both raw biomass and hydrochar exhibited satisfactory PrP removal, achieving maximum adsorption capacities of 224.58 and 258.55 mg/g respectively, at 10 mg/L initial PrP concentration and 23.33 mg/L adsorbent dosage. Equilibrium data were best described by the Freundlich isotherm model, indicating a heterogeneous surface and multilayer adsorption. The kinetic analysis revealed that the adsorption behavior, for both adsorbents, was best described by the pseudo-second-order model, while the thermodynamic evaluation revealed negative ΔH° and ΔS° values, confirming an exothermic, physisorption-driven process. The adsorption mechanism was further investigated through surface characterization techniques, including Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, N2 physisorption, and zeta potential analysis. The findings demonstrate the potential of microalgal biomass as a low-cost, sustainable biosorbent, for emerging contaminants, reinforcing its role in advanced water treatment and circular economy strategies.

Clean Technologies doi: 10.3390/cleantechnol8030067

Authors: Cristina-Gabriela Grigoraș Andrei-Ionuț Simion Lidia Favier

Emerging compounds in water, ranging from dyes to pharmaceuticals, negatively impact living organisms and challenge the industries responsible for their release. These pollutants exhibit chemical persistence and resistance to conventional treatment processes. Adsorption is considered an effective and accessible approach, particularly when low-cost and renewable materials are employed. The Problem-Intervention-Comparison-Outcome (PICO) framework and Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines were followed. A structured search of Scopus was conducted to identify English-language original peer-reviewed articles published between 2016 and 2025 addressing the use of fruit waste (FW)-derived adsorbents for water decontamination. After independent screening, 528 studies were included. Risk of bias was assessed qualitatively. Due to substantial heterogeneity in materials, contaminants, and experimental designs, findings were synthesized narratively. FW-derived adsorbents were evaluated in terms of synthesis routes, physicochemical characteristics, adsorption mechanisms, kinetic and equilibrium behavior, process optimization and regeneration performance. Correlations were observed between surface functionalization, material properties and contaminant-specific removal efficiency, while limitations were noted for multi-component systems, regeneration stability, standardization and scale-up. By integrating material design with process-level considerations, this review outlines priorities for advancing FW valorization toward practical and sustainable water treatment applications.

Clean Technologies doi: 10.3390/cleantechnol8030066

Authors: Luigi Madeo Pietro Figliuzzi Assunta Perri Anastasia Macario Carlo Siciliano Pierantonio De Luca

This study investigates the use of type A zeolite as a filtering material for the removal of toxic and carcinogenic compounds from cigarette smoke, which contains nicotine and other harmful substances produced by tobacco combustion. The aim is to evaluate the effectiveness of zeolite in reducing exposure to secondhand smoke, with particular attention to health and environmental impacts. The zeolite was characterized using SEM-EDS, XRD, DSC, and TGA to determine its morphology, chemical composition, crystalline structure, and thermal stability. An experimental setup was designed to simulate realistic smoking conditions and test filter efficiency based on the active mass. The system allowed identification of harmful substances trapped in the filter and those remaining in the air. Performance was assessed through gravimetric analysis and GC-MS, enabling identification of adsorbed and non-adsorbed compounds. Results demonstrate significant efficiency in selective removal of toxic components. Finally, filter performance was compared with carbon nanotubes, tested under the same experimental protocol.

Clean Technologies doi: 10.3390/cleantechnol8030065

Authors: Lucía Pérez-Gandarillas Berta Galán Javier R. Viguri

Maritime transport is energy-efficient but remains heavily dependent on fossil fuels. Renewable electricity-based ammonia (e-NH3) has emerged as a promising alternative, particularly through small-scale, modular production. Assessing its economic viability is essential for future adoption, and techno-economic analysis offers a structured way to evaluate its feasibility. This study investigates the cost performance of a small-scale offshore e-NH3 plant of 2.4 tons per day (tpd) at the Port of Santander, Spain, based on nitrogen obtained via membrane separation and hydrogen from electrolysis of pretreated seawater. The results are based on process simulation outcomes obtained using ASPEN v14, and the detailed cost breakdown is derived from modular costing methodologies applied to preliminary process designs and sensitivity analyses of the levelized cost of ammonia (LCOA) with respect to the main variables. A comparative review of LCOA values reported in the literature for offshore and onshore e-NH3 plants is provided. An estimated CAPEX of 5.99 M EUR (equivalent to 0.53 M EUR/y), OPEX of 1.58 M EUR/y, and an LCOA of 2408 EUR/tNH3 are obtained, with equipment investment and operating costs identified as the most influential parameters. The results highlight the need for supraregional techno-economic studies considering optimal offshore wind availability within a collaborative interregional framework.

Clean Technologies doi: 10.3390/cleantechnol8030064

Authors: Maria N. Berradre Cristina Arroqui Idoya Fernández-Pan María José Beriain Francisco C. Ibañez Paloma Vírseda

The valorization of agro-industrial by-products through sustainable extraction of bio-compounds is a key challenge within circular economy and clean-processing frameworks, as large volumes of tomato and artichoke residues are generated by the food industry. This study evaluated the impact of non-thermal technologies on the recovery of biocompounds from tomato peels and blanched artichoke bracts using single green solvents instead of solvent mixtures. Ultrasound-assisted extraction (sonication), high-pressure processing (pressurization), and dual processing (pressurization + sonication) were compared with conventional extraction. Ethanol was used for lycopene extraction, while water was employed for inulin-type fructan recovery. Lycopene, total phenolic content, antioxidant activity, and inulin-type fructans were quantified. Non-thermal treatments significantly influenced extraction yields (p < 0.05). The dual processing provided the highest lycopene and inulin-type fructan contents (1440.09 ± 0.71 µg/g DW and 5.17 ± 0.51 g/100 g DW, respectively) and enhanced antioxidant activity in tomato peels and blanched artichoke bracts (25.50 ± 0.20% and 66.11 ± 2.03%), and phenolic co-extraction (1783.2 ± 215.3 μg GAE/g DW and 27.68 ± 1.29 mg GAE/g DW) outperformed individual technologies and conventional extraction. Compared with the conventional process, dual processing improved the extraction yields of lycopene (20.60 ± 0.44%) and inulin (26.40 ± 13.95%). The findings prove that non-thermal processes, particularly when combined, intensify mass transfer and enable efficient extraction using green solvents, offering a sustainable strategy for recovering bioactive compounds from tomato and artichoke by-products.

Clean Technologies doi: 10.3390/cleantechnol8030063

Authors: Xin Chen Xiaofeng Ling Zhen Xu Yuanfen Xia

The continuous increase in atmospheric CO2 concentration exacerbates global climate change, making carbon reduction an urgent global priority. Carbonic anhydrase (CA), a highly efficient biocatalyst that converts CO2 into bicarbonate, demonstrates significant potential for carbon capture and resource utilization. However, the stability and catalytic efficiency of native CA in industrial environments are limited, particularly its poor thermal tolerance under flue gas conditions and its sensitivity to impurities, hindering its direct large-scale application. This review systematically summarizes recent advances in modifying microbial CA through protein engineering (e.g., directed evolution, rational design) and immobilization techniques, which have markedly enhanced its thermal stability, adaptability, and reusability. Among these, the integration of machine learning with high-throughput experimentation has emerged as a transformative strategy for CA engineering. Furthermore, we outline CA-driven pathways for CO2 conversion into high-value chemicals and bioenergy. Finally, future prospects are discussed, including interdisciplinary integration, computational modeling coupled with experimental validation, and comprehensive life-cycle and techno-economic assessments, to facilitate the scaled application of engineered microbial CA in carbon neutrality pathways. Collectively, this review highlights the critical role of engineered CA in bridging biocatalysis with industrial carbon management, offering a viable and sustainable pathway toward carbon neutrality.

Clean Technologies doi: 10.3390/cleantechnol8030062

Authors: Hugo Martínez Ángeles Cesar Augusto Navarro Rubio José Gabriel Ríos Moreno Margarita G. Garcia-Barajas Roberto Valentín Carrillo-Serrano Mariano Garduño Aparicio Saúl Obregón-Biosca Mario Trejo Perea

The transition toward low-carbon and circular Municipal Solid Waste (MSW) systems requires integrated evaluation approaches that consider environmental performance, technological maturity, and governance capacity. This study presents a structured, systematic review of MSW disposal and treatment practices published between 2018 and 2026, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. A total of 71 studies were included and analyzed. Due to heterogeneity in methodologies, system boundaries, and reported indicators, no formal meta-analysis was conducted. Instead, the review provides a comparative and qualitative synthesis of key environmental indicators and structural determinants. Results indicate a transition from open dumping toward engineered landfills and advanced treatment technologies, including waste-to-energy and biological processes. Open dumping is consistently associated with high greenhouse gas emissions and environmental risks, while engineered systems improve containment and enable partial resource recovery. The findings highlight that environmental performance is not determined solely by technology but by the interaction between infrastructure design, operational quality, governance capacity, and economic conditions. The proposed analytical framework supports context-sensitive waste management strategies aligned with circular economy principles and climate mitigation objectives.

Clean Technologies doi: 10.3390/cleantechnol8030061

Authors: Eleni Dalaka Gabriel Vasilakis Markos Bilbilai Dimitris Karayannis Maria Sanida Ioannis Politis Panagiota Diamantopoulou Seraphim Papanikolaou Georgios Theodorou

Polyunsaturated fatty acids, particularly γ-linolenic acid, are recognized for their therapeutic and nutritional properties. Zygomycetes, such as Cunninghamellaelegans, represent a promising microbial platform for sustainable gamma-linolenic acid (GLA) production as an alternative to conventional sources. Despite this potential, the immunomodulatory activity of metabolites from C. elegans has not been previously explored. In this study, C. elegans was cultivated on hydrolysates from discarded residues of Pleurotus spp. cultures (DRPC-HL), optimized to release assimilable compounds, promoting valorization of low-value biomass within a circular bioeconomy. Dry mycelial biomass, lipid-free biomass, and intracellular lipids from these cultures, alongside previously reported C. elegans cultures grown under nitrogen-excess (N-Xs) and nitrogen-limited (N-Lim) conditions, were tested on THP-1-derived macrophages, under lipopolysaccharide (LPS)-induced inflammatory conditions. Following in vitro gastrointestinal digestion, dry biomass and lipid-free dry biomass fractions upregulated the anti-inflammatory cytokine IL10 and downregulated IL1B and TNF, particularly from N-Xs and DRPC-HL cultures. Lipids mainly enhanced IL10 expression, especially when derived from N-Xs cultures. No changes were observed in upstream regulators (TLR2, TLR4, NFKB1, RELA), suggesting a feasible post-receptor immunomodulatory action. Overall, these findings highlight the dual value of fungal bioproducts derived from agro-industrial residues, combining sustainable bioprocessing with bioactive compound generation, supporting environmentally friendly microbial platforms for industrial applications.

Clean Technologies doi: 10.3390/cleantechnol8030060

Authors: Hamza Labjouj Loubna El Joumri Najoua Labjar Ghita Amine Benabdallah Samir Elouaham Hamid Nasrellah Brahim Bihadassen Houda Labjar El Abass El Ouardi Souad El Hajjaji

With the rapid growth of the medicinal cannabis sector, there is a growing concern regarding its environmental impact and sustainability. In recent years, life cycle assessment (LCA) studies on medicinal cannabis cultivation and processing have been conducted since 2021. However, there is a lack of comprehensive LCA studies that include all stages of medicinal cannabis cultivation and processing. In this systematic review, various LCA studies conducted from 2021 to 2025 using the ISO 14040/44 methodology are reviewed and discussed in terms of their goal and scope, life cycle inventory (LCI), life cycle impact assessment (LCIA), and result interpretation. Various environmental impact indicators are considered in this review, such as greenhouse gas emissions, energy demand, water usage, eutrophication, acidification, and resource depletion. All of these impact indicators point to a significant environmental impact of indoor cultivation in terms of greenhouse gas emissions, which vary from 2.3 × 103 to 5.2 × 103 kg CO2 eq kg−1 of dried cannabis product. Nevertheless, it is important to note that this is significantly influenced by regional electricity sources. Low-carbon-based electricity sources, especially hydro-based sources, can reduce emissions to a significant level. Cultivation outdoors presents significantly lower emissions of (60–110 kg CO2 eq kg−1), but fertilizers and substrates used in cultivation contribute significantly to emissions. Also, outdoor plants use 22.7 L plant−1 d−1 water at peak growth, while indoor plants use 9–11 L plant−1 d−1 water. Improvements in the life cycle of cannabis cultivation can be achieved through renewable energy use, water and fertilizers, substrate use and reuse, and inventories for post-harvesting activities like drying and extraction. Botanical parameters including genotype, planting density, and harvesting frequency are identified as significant but under-characterized determinants of LCA outcomes. Ethical and legal barriers are shown to be structural drivers of the LCA data gap. A SWOT analysis contextualizes the opportunities and constraints of the sector. Future research should focus on cradle-to-grave LCA and incorporate socio-economic factors for sustainability in the medicinal cannabis sector.

Clean Technologies doi: 10.3390/cleantechnol8020059

Authors: Ann-Christine Johansson Rebecka Nordsvahn André Selander Torun Hammar Jesper Eman Magdalena Juntikka

Sustainable structural composites can significantly lower vehicle-related emissions. To evaluate the recycling of different composite materials, laboratory-scale pyrolysis was conducted and assessed both technically and environmentally. Two demonstrators were studied: a truck side skirt made from natural flax and hemp fibres with polypropylene (PP), and a car front header composed of glass fibres and PP. Additional materials examined included thermoplastic composites containing polyamide 6 (PA6), bio-based polyamide 11 (PA11) and thermoset polyester. Results showed that material type strongly influenced the pyrolysis outcome, product composition and recycling potential. Glass fibres could be recovered and reused as reinforced fibres, while natural fibres could be recovered as biooil for potential use in biofuel production. Polymers were recovered as pyrolysis products that, depending on their composition, can be used in different applications, from recovering monomers from PA6 to producing hydrocarbons that may replace naphtha (from PP) or aromatics (from polyester) in the petrochemical industry. Life cycle assessment (LCA) findings revealed that the climate impact of composite recycling is primarily driven by the environmental burdens of the recycling process itself and by the ability of recovered materials and chemicals to substitute conventional fossil-based alternatives. Efficient recycling pathways are therefore essential to maximising environmental benefits.

Clean Technologies doi: 10.3390/cleantechnol8020058

Authors: Mariana Murrieta-Melchor Stephany Isabel Vallarta-Serrano Edgar Santoyo-Castelazo Sergio Alberto Navarro-Tuch

As the world faces the challenge of mitigating climate change, energy- and emissions-intensive industrial processes must be addressed urgently worldwide. The cement production industry accounts for over 8% of global greenhouse gas (GHG) emissions from calcination and fuel use. Mexico, a middle-income economy, has rising cement demand for infrastructure and commercial growth. Thus, this study analysed national cement production, the primary emitting manufacturing industry in the country, under a business-as-usual (BAU) and two alternative scenarios, using a top-down approach to model energy consumption and GHG emissions by 2050. These scenarios follow the projection of national cement production, estimated using socio-economic indicators, which are considered the main drivers of cement demand, reaching 97.3 Mt. A qualitative analysis evaluates the strengths, weaknesses, opportunities, and threats (SWOT) of implementing emission-reduction strategies. The analysis showed that the BAU scenario might reach 66.5 Mt CO2e by 2050, while the most ambitious scenario reduced direct emissions by 80.1% through carbon capture, clinker-to-cement reduction, thermal energy intensity reduction, and the use of municipal solid waste as an alternative fuel. However, incorporating these strategies in Mexico requires a more active role and investment support from key stakeholders.

Clean Technologies doi: 10.3390/cleantechnol8020056

Authors: Manuel Hernández-Escaño Rafael Borja José Carlos García-Gómez Francisco Raposo

The accumulation of dead leaves from the Mediterranean seagrass Posidonia oceanica on beaches is a natural process that results in the formation of banquettes and, in some areas, spherical debris known as aegagropiles. These structures provide essential ecosystem functions, particularly coastal protection against erosion. Despite their ecological importance, accumulated Posidonia oceanica biomass is often perceived as undesirable waste by stakeholders such as beach managers, local authorities, and tourists, leading to its systematic removal. This review summarises the chemical characteristics of this marine biomass and assesses its environmental and socioeconomic impact. Additionally, some different valorisation pathways for this biomass waste are examined, including animal feeding, bioactive compound extraction, development of biochar, biofertilisers, and compost, production of biosorbents, biocomposites and building materials, and also energy generation. The findings highlight the significant potential of P. oceanica residues within circular economy strategies and underscore the need for improved management practices that recognise their ecological value.

Clean Technologies doi: 10.3390/cleantechnol8020057

Authors: Ahmed Mohammed Inuwa Victor Oluwafemi Fatokun Emmanuel Kweinor Tetteh Sudesh Rathilal Usman Mohammed Aliyu

The sustainable valorization of food waste is essential for advancing the circular bioeconomy and reducing the environmental impacts of organic waste disposal. This study presents an integrated approach combining hydrothermal carbonization (HTC) and anaerobic digestion (AD) to recover renewable energy and valuable resources from food waste. The process was simulated in Aspen Plus® version 14.1 using thermochemical and biochemical reaction models to evaluate the effects of feed moisture (60–85%) and HTC temperature (180–280 °C) on performance. Integration of HTC and AD increased overall energy recovery by 26–38% compared to standalone AD, with a feed moisture of 85%, organic loading of 4 kg VS m−3 d−1, and mesophilic/thermophilic temperatures of 35 and 55 °C. Improvements resulted from higher methane yield (0.42 m3 CH4 kg−1 VS) from HTC liquor and energy-rich hydrochar (25–29 MJ kg−1). The techno-economic assessment indicated a net energy ratio of 2.3, an Internal Rate of Return (IRR) of 18.6%, and a 4.8-year payback period, confirming economic viability. Sensitivity analysis highlighted energy prices and feedstock costs as key drivers, while Monte Carlo simulation demonstrated stability under ±20% uncertainty. Optimal conditions (HTC at 220 °C, 65% moisture, and 100 kg h−1 solid loading) significantly enhanced profitability and carbon efficiency. Overall, the integrated HTC–AD process offers a technically, economically, and environmentally sustainable route for converting food waste into renewable energy and biochar, supporting circular bioeconomy and net-zero energy goals.

Clean Technologies doi: 10.3390/cleantechnol8020055

Authors: Haibo Liu Kai Chen Yali Zhao Weiwei Xu Qiang Li

Air flotation is widely used in wastewater treatment for the removal of emulsified oils and suspended solids. The complex flow disturbances generated during the flotation process play a critical role in determining separation efficiency. This study employs the volume-of-fluid (VOF) method within the OpenFOAM framework to simulate the aggregation and rising behavior of microbubbles (40–100 μm) and oil droplets under various perturbation conditions. The effects of different airflow disturbance patterns on the flotation dynamics of oil–gas compounds are systematically investigated. Results show that negative pulsation promotes the rising of bubble–oil aggregates, whereas positive pulsation hinders their coalescence and upward motion. Furthermore, recirculation vortices induced by surface disturbances increase the residence time of oil–gas compounds in the water column, thereby affecting overall separation performance. The findings demonstrate that introducing vertical upward flow and bilateral oblique upward airflow can enhance flotation efficiency. This work provides insights into optimizing airflow configurations for improved oil removal in wastewater treatment applications.

Clean Technologies doi: 10.3390/cleantechnol8020054

Authors: Kazhi Hawrami Bassel Eissa Abdulrahman Shahin Elvin Hajiyev Hossein Emadi Marshall Watson

Freshwater demand for cementing operations in the Delaware Basin continues to increase with expanding unconventional development, creating a high demand for an alternative source of water. This study develops a chemistry screening and operational framework to evaluate the reusability potential in cementing operations in the Delaware Basin. A three-tier screening system for the produced-water samples was established by using the major-ion chemistry, total dissolved solids (TDS), pH, and saturation index (SI) thresholds derived from the cement literature and American Petroleum Institute (API) guidelines. The results of the geochemical screening aid in classifying the water samples into four suitability categories: Excellent/Preferred, Good/Suitable, Moderate/Marginal, and Poor/Unsuitable. The results suggest that the samples obtained from the Loving, Pecos, Reeves, Eddy and Lea counties meet the criteria for reuse in cementing operations with minimal conditioning. To assess the feasibility of operational use, a probabilistic forecasting model was developed to predict the cement water demand in 2026 for the basin. Linear regression of historical drilling trends between 2015 and 2025 showcased that approximately 3595 new wells will be drilled, with an average well depth of 21,778 ft. To evaluate whether the produced-water volumes in the basin are adequate for reuse in cementing, a Monte Carlo simulation (10,000 iterations) estimated an annual cementing water requirement centered at 6.16 MMbbl/year (P50). Produced-water availability from wells classified as Excellent/Preferred was also modeled probabilistically, using uncertainty in the water–oil ratio (WOR), estimated ultimate recovery (EUR), and forecast duration. These results demonstrate the potential for produced-water reuse to reduce freshwater demand for cementing operations in the Delaware Basin.

Clean Technologies doi: 10.3390/cleantechnol8020053

Authors: Raúl Bahamonde Soria Jefferson Estupiñan Irma Gonza Monserrat Naranjo Billy D. Chinchin-Piñan Lucia E. Manangón Katherine Vaca Martha Romero-Bastidas Henry Pupiales Verónica Taco Patricia Luis

Improving the absorption of visible light in photocatalysts could enhance photocatalytic reactions and reduce energy consumption, particularly in sunny regions like Ecuador. This study reports the synthesis of ZnO and ZnO nanoparticles doped with 1.5 at.% Er, 5 at.% Al, and 1.5 at.% Er, 5 at.% Al using the sol–gel method. The effect of doping on the structure, morphology, absorption spectra, and photocatalytic properties was analyzed by XRD, SEM, EDS, and UV-Vis spectrophotometry. XRD confirmed the presence of the wurtzite ZnO structure, and UV-Vis diffuse reflection spectra showed a red shift in the band gap for doped ZnO compared to pristine ZnO. Photocatalytic activity was evaluated through the degradation of methyl orange (MO) under artificial visible light and natural sunlight in Quito, Ecuador. ZnO doped with Er/Al nanoparticles exhibited significantly enhanced photocatalytic performance under solar light, suggesting the potential for replacing artificial light and reducing operating costs in photocatalytic processes. Moreover, all doped samples retained the antibacterial properties of ZnO against B. subtilis, and Er/Al co-doping improved the inhibition of E. coli compared to undoped ZnO.

Clean Technologies doi: 10.3390/cleantechnol8020052

Authors: Héctor Rivera Diana Pinto Heidis Cano

This article presents a systematic literature review on methane (CH4) emissions from municipal solid waste (MSW) disposal sites and their implications for footprint outcomes. This review followed a PRISMA 2020 screening logic using Scopus and ScienceDirect (2019–2024); English and Spanish; subject areas: engineering and environmental, earth sciences), yielding a final sample of 30 studies for qualitative synthesis. This review focuses on how landfill CH4 is quantified and how system boundaries and functional units shape reported CO2 results. Evidence indicates that reported CH4 estimates are sensitive to methodological choices and key assumptions and site-context drivers (degradable organic carbon (DOC)/model first-order decay (FOD) and constant k, the methane correction factor (MCF), gas collection, oxidation, waste composition, landfill age/type, and climate), limiting direct comparability between studies. Mitigation and waste-to-energy pathways (capture/utilization, anaerobic digestion, and incineration) are summarized in terms of the reported climate benefits. Finally, reporting gaps are identified, and the minimum information set is outlined to improve the reproducibility of landfill-related carbon footprint estimates for planning and future research.

Clean Technologies doi: 10.3390/cleantechnol8020051

Authors: Megan Rand Wheeler Brandi Everett Victor Prybutok

Artificial intelligence presents a critical paradox for clean technology: while enabling unprecedented environmental optimization, AI deployment demands massive resource inputs that threaten to offset benefits. As global AI infrastructure investment approaches $500 billion annually, data center electricity consumption is projected to exceed 1000 TWh by 2030. We conducted a systematic literature review of 73 peer-reviewed empirical studies (2021–2025) to develop an Environmental Asset-Cost Framework categorizing AI’s impacts across five asset categories (energy optimization, production enhancement, green innovation, resource conservation, precision applications) and five cost categories (energy consumption, water use, e-waste, infrastructure, supply chain extraction). Our analysis reveals three critical insights: First, AI’s environmental impact follows a synthesized S-curve heuristic—a pattern derived from convergent but methodologically diverse evidence strands—characterized by initial emission reductions (0–2 years), mid-term rebound effects (2–5 years), and conditionally projected long-term optimization (5+ years). Second, geographical context creates 10–60× variation in outcomes; regions with high renewable electricity and water abundance achieve net benefits within 2–3 years, while fossil fuel-heavy, water-stressed regions may never reach net positive outcomes. Third, the rebound effect is predictable and manageable through strategic interventions. Our framework provides actionable deployment guidance, demonstrating that achieving AI’s net environmental benefits requires renewable energy infrastructure development before AI deployment, alternative cooling technologies, and policy frameworks incorporating temporal dynamics.

Clean Technologies doi: 10.3390/cleantechnol8020050

Authors: Hamad Ahmed Al-Ali Koji Tokimatsu

Sustainable hydrogen production in water-scarce regions poses critical environmental challenges due to limited freshwater availability and the energy intensity of seawater treatment. This study examines the environmental trade-offs of providing water for hydrogen production via seawater desalination (with or without brine treatment) or freshwater purification, using a comprehensive life cycle assessment (LCA) framework. The assessment centers on three water-stressed countries: the United Arab Emirates (UAE), Spain, and Australia. Results reveal clear trade-offs between freshwater conservation and marine environmental pressures. Brine treatment reduces nutrient-related marine impacts but increases energy-related burdens, particularly under fossil-dominated electricity systems. Water sourcing for electrolysis coupled with energy-intensive desalination systems generally exhibits higher environmental pressures than alternative configurations, whereas freshwater-based supply for hydrogen production pathways shows lower burdens in several impact categories but raise concerns regarding freshwater resource use. Sensitivity analysis confirms that system performance is strongly influenced by water demand and electricity characteristics, highlighting the importance of aligning hydrogen deployment strategies with regional energy and water conditions. Overall, the findings demonstrate that water sourcing decisions play a critical role in shaping the environmental sustainability of hydrogen systems in water-stressed regions and must be evaluated through integrated water–energy planning.

Clean Technologies doi: 10.3390/cleantechnol8020049

Authors: Feyisayo O. Adepoju Vadim A. Shevyrin Elena G. Kovaleva Alicia C. Mondragón Alberto Cepeda José Manuel Miranda

Natural deep eutectic solvents (NADESs) have emerged as promising green alternatives to conventional solvents for the extraction of bioactive compounds from plant materials. In this study, eight natural deep eutectic solvents were synthesized and evaluated for their efficiency in extracting betulin from birch bark. Extraction yield was assessed using high-performance liquid chromatography with ultraviolet detection. Among the tested systems, N3 (choline chloride and urea in a 1:1 molar) and N4 (choline chloride and fructose in a 1:1 molar) were the most effective, yielding 101.26 ± 0.03 and 243.32 ± 0.26 mg betulin per gram of dry extract, respectively. Fourier transform infrared spectroscopy analysis confirmed the structural similarity of the N4 extract to pure betulin. In addition to increased extraction performance, the N4 extract demonstrated the greatest antioxidant activity (DPPH (1,1-diphenyl-2-picrylhydrazyl): 63% and ABTS (2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid)): 97% inhibition) and total phenolic content (12.12 mg GAE/g extract), and betulin yield was strongly correlated with total phenolic content (TPC) and antioxidant activity (FRAP (ferric ion reducing antioxidant power), DPPH, and ABTS), indicating the preservation of bioactivity. These findings underscore the potential of NADESs as sustainable solvents for the extraction of bioactive compounds from birch bark, supporting greener extraction technologies for biomass valorization and natural product processing.

Clean Technologies doi: 10.3390/cleantechnol8020048

Authors: Ahsan Mehtab Hong-Seok Mun Eddiemar B. Lagua Hae-Rang Park Jin-Gu Kang Young-Hwa Kim Md Kamrul Hasan Md Sharifuzzaman Sang-Bum Ryu Chul-Ju Yang

Cold thermal energy storage (CTES) has emerged as a critical clean-energy technology for enhancing postharvest management in rural agricultural supply chains, where losses often exceed 20–40% due to inadequate cooling infrastructure and unreliable electricity. This review synthesizes the recent literature on CTES systems, including ice-, chilled-water-, and phase-change material (PCM)-based storage, with a focus on smart-farm integration, IoT-based monitoring, predictive control, and solar photovoltaic (PV) energy coupling. Trends in village-level cold rooms, micro-dairy milk cooling, and fruit–vegetable storage are critically examined, highlighting efficiency, resilience, and scalability relative to battery-dominant and conventional refrigeration systems. Current research gaps are identified in multi-scale modeling, PCM stability, state-of-charge estimation, techno-economic optimization, and AI-based operational strategies. Addressing these gaps is essential to realizing sustainable, low-carbon, and energy-efficient rural cold chains.

Clean Technologies doi: 10.3390/cleantechnol8020047

Authors: Shunchang Yang Na Wu Pratap Pullammanappallil

This study explores a circular bioeconomy strategy for microbrewery waste by characterizing and valorizing its primary waste streams: sugar mash water (A), spent yeast with hops (B), spent yeast without hops (C), and alkaline cleaning wastewater (D). The biochemical methane potential of the acidic organic blend (E, from A-C) was assessed under mesophilic (38 °C) and thermophilic (55 °C) conditions, revealing significant substrate-specific temperature sensitivity. The highly acidic blend E (pH 4.16) was effectively neutralized to pH 7.0 using the on-site alkaline wash water (D, pH 12.03). Mesophilic anaerobic digestion of the neutralized blend achieved a high methane yield of approximately 500 mL/g VS. Furthermore, the alkaline wash water successfully served as an in situ CO2 scrubber, upgrading biogas to ~100% methane content. This integrated approach demonstrates a viable, closed-loop pathway for microbreweries to achieve simultaneous energy recovery from organic wastes and chemical-free treatment of acidic and alkaline effluents. The findings also highlight the importance of substrate-specific thermal management and provide a robust framework for microbreweries to achieve energy independence and internal CO2 neutralization–wastewater treatment.

Clean Technologies doi: 10.3390/cleantechnol8020046

Authors: Bo Shen Xiaoyan Zheng Lili Zheng Yang Yang Dao Xiao Zhanwu Sheng Yiqiang Wang Binling Ai

The initial carbon-to-nitrogen (C/N) ratio is a fundamental parameter for aerobic composting, with a generally recommended optimal range of 25:1 to 30:1. However, in practical applications, the optimal C/N ratio often deviates from the recommended value. We attribute this discrepancy to the limitations of traditional stoichiometric methods in assessing the bioavailability of carbon and nitrogen sources. This study investigated how carbon bioavailability governs composting efficiency and product quality. Laboratory-scale aerobic composting experiments were conducted using six types of raw crop straws and two physically pretreated straws, representing a biodegradability gradient. Results demonstrated that carbon bioavailability significantly modulated the composting performance. Substrates rich in labile carbon pool (LCP), such as wheat straw and extruded cassava plant residue, demonstrated superior thermogenesis, humification, and seed germination indices compared to those dominated by recalcitrant carbon pool (RCP), such as untreated cassava plant residue. Principal component analysis confirmed a strong positive correlation between LCP content and key quality indicators. Microbiological analysis revealed that carbon source variations shaped bacterial succession: Bacteroidota abundance correlated positively with LCP, driving rapid initial degradation, whereas Pseudomonadota were more abundant in RCP-rich treatments, suggesting a role in complex polymer breakdown. This study confirmed that carbon bioavailability, rather than the bulk C/N ratio alone, is a critical limiting factor. This finding logically extends to the role of nitrogen bioavailability, suggesting that a “biochemical C/N ratio”—accounting for the lability of both carbon and nitrogen—could be a more accurate predictor of aerobic composting performance.

Clean Technologies doi: 10.3390/cleantechnol8020045

Authors: Hanyi Lin Qing Tong Sheng Zhou Cuiping Liao

Developing far-offshore wind power integrated with hydrogen production represents a critical pathway for China’s energy decarbonization. However, the investment prospects of off-grid offshore wind-to-hydrogen projects remain highly uncertain due to volatile technology costs and hydrogen prices, complicating the evaluation of project value and optimal timing. To address the oversimplified treatment of electrolyzer operation and the limited consideration of alkaline electrolyzers in the existing studies, this paper proposes an integrated assessment framework that combines time-series operational simulation with real options analysis. A detailed dynamic model of an alkaline (ALK)–proton exchange membrane (PEM) hybrid configuration is developed to simulate the coordinated hydrogen production under fluctuating wind power. Technical learning effects and stochastic hydrogen price processes are incorporated, and the least-squares Monte Carlo method is applied to determine the optimal investment strategies. A case study of a planned far-offshore wind farm in Guangdong indicates that, compared with a standalone PEM configuration, the hybrid configuration reduces the levelized hydrogen cost by about 15%, increases the investment value by up to 17 times under slow technological progress, and brings forward the optimal investment year by five years, from 2039 to 2034. Sensitivity analysis shows that expected hydrogen prices and discount rates dominate the investment outcomes.

Clean Technologies doi: 10.3390/cleantechnol8020044

Authors: Andrzej Piotrowicz Janusz Kolczyński Mirosław Kostrzewa Wojciech Kaczmarek Bogdan Samojeden

Microwave-assisted depolymerization (MD) of heterogeneous postconsumer plastics was carried out in a quarter-scale reactor to evaluate product composition and the influence of feedstock type on oil quantity and quality. Various waste streams, including: PS, PP, ABS materials, keyboard housings, textile plastics, PCBs, and mixed electronic components, were processed in 3–6 kg batches using magnetron powers up to 2 × 1.55 kW. All experiments yielded a condensed liquid fraction, with color intensity correlating with aromatic content. FTIR spectroscopy showed that all oils consisted of hydrocarbon matrices dominated by aliphatic C-H stretching bands (2956–2850 cm−1). Aromatic contributions varied significantly: PS produced oils rich in aromatic OOP C-H bands (900–650 cm−1), PP yielded predominantly aliphatic oils with minor aromatic features, and ABS or electronics materials produced mixed aliphatic–aromatic profiles. Textile oils additionally exhibited carbonyl and O-H bands, indicating oxygenated decomposition products. Fractional distillation separated the oils into low-boiling aliphatic (<250 °C) and heavier aromatic (250–350 °C) fractions. These results suggest that MD reliably converts diverse plastic wastes into hydrocarbon oils whose spectroscopic characteristics reflect both feedstock composition and thermal pathways intrinsic to microwave heating.

Clean Technologies doi: 10.3390/cleantechnol8020043

Authors: Vu-Mai-Linh Nguyen Adama Ndao Jean-François Blais Kokou Adjallé

This study explores the potential of using paper pulp mill sludge (PPMS) as an economical substrate for producing high-value protease enzymes with an indigenous bacterial strain, Bacillus tropicus P4. Isolated directly from PPMS, B. tropicus P4 showed high protease-producing ability, approximately 134 U/mL after 48 h—more than three times the yield of the benchmark strain (B. megaterium). Among various additives tested to boost enzyme production, Tween 80 emerged as the most effective, increasing enzyme activity by more than threefold compared to the control. Scale-up experiments in bioreactors of 5 L and 150 L confirmed that B. tropicus P4 maintains high protease yields under typical cultivation conditions with minimal modifications, specifically the addition of Tween 80 (1%) and increased total solids concentration (25 g/L). In the 5 L bioreactor, enzyme production peaked at approximately 755 U/mL within 24 h, while the 150 L bioreactor consistently achieved high enzyme activity (~848 U/mL). These results support the feasibility of a simple and scalable approach for converting industrial sludge into high-value protease enzymes, contributing to resource recovery and circular bioeconomy strategies.

Clean Technologies doi: 10.3390/cleantechnol8020042

Authors: Ifigeneia Nikolaidou Josep Oriol Pou Maria Auset

The environmental crisis and the growing need to reduce solid waste make it imperative to adopt integrated, scientifically sound, and environmentally friendly solid waste management practices in order to ensure a sustainable future. This study presents an alternative waste management proposal in accordance with the standards set out in the European Waste Directive (Directive 2018/850/EC) in order to lessen greenhouse gas emissions. The primary objective is to develop a circular waste management system that uses waste as feedstock for the production of biofuel in order to meet Catalonia’s energy needs and, at the same time, reduce its environmental footprint. Waste that is highly biodegradable and rich in organic matter cannot be disposed of in landfills, according to order TED/834/2023, and is therefore used to produce biogas through anaerobic digestion (AD) or to produce compost. In addition, gas emissions from landfills, which are rich in methane, are also collected and used for biogas production. Plans for biogas production at landfills and at an anaerobic digestion biogas plant, and for compost production from organic waste, were implemented using SuperPro Designer simulation software. The research has shown that this approach to solid waste management offers positive results in terms of energy due to biogas production, in terms of the environment due to waste reduction and compost production, and in terms of the economy due to a 25% increase in the efficiency of the biogas plant.

Clean Technologies doi: 10.3390/cleantechnol8020041

Authors: Hyung-kyu Lee Go-eun Kim Seong-ho Jang Ho-min Kim Byung-gil Jung Young-chae Song Won-ki Lee

Soluble cutting fluids (SCFs) from metalworking processes pose significant treatment challenges. Here, SCFs were treated using a monopolar electrochemical (EC) system, using turning scrap generated from metalworking operations as the anode. The system was operated for 60 min under various conditions, including different anode materials, electrolyte addition, aeration, and initial pH. Treatment performance was evaluated in terms of chemical oxygen demand (CODCr) and total organic carbon (TOC) removal efficiencies and specific energy consumption (SEC) for CODCr removal. The Al scrap (20 g/L) showed the optimal overall performance, achieving CODCr and TOC removal efficiencies of 29.28% and 25.62%, respectively, with an SEC comparable to that of the Al electrode. Electrolyte addition improved the energy efficiency under all conditions, with NaNO3 10 mM yielding the lowest SEC (0.57 kWh/kg-CODCr), and aeration negatively affected both removal efficiency and energy consumption. Although acidic conditions (pH 2) resulted in high apparent removal, most of the reduction occurred during pre-treatment pH adjustment, and the highest energy efficiency was achieved at pH 7 (0.47 kWh/kg-CODCr). These results demonstrate that Al turning scrap is a promising alternative anode material for electrochemical treatment of SCFs with optimized electrolyte addition and operating pH enabling improved energy efficiency.

Clean Technologies doi: 10.3390/cleantechnol8020040

Authors: José Cabrera-Escobar Carlos Mauricio Carrillo Rosero César Hernán Arroba Arroba Santiago Paúl Cabrera Anda Catherine Cabrera-Escobar Raúl Cabrera-Escobar

This study presents a two-dimensional computational fluid dynamics analysis of a vertical-axis Savonius-type wind turbine under atmospheric conditions representative of an educational environment located in the Ecuadorian Andean region. Unlike previous studies conducted under sea-level meteorological conditions, this research is performed under high-altitude conditions (2723 m a.s.l.). The unsteady flow around the rotor was simulated using a two-dimensional approach based on the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations, discretized with the finite volume method and coupled with the k–ω Shear Stress Transport (SST) turbulence model. The rotor rotation was modeled using sliding mesh technique, employing a second-order implicit time scheme to ensure numerical stability and adequate temporal resolution. The numerical model was configured for a tip speed ratio of 0.8 and a wind speed of 3.9 m/s. The time step was defined based on a constant angular advancement of the rotor per time iteration, ensuring numerical stability and adequate temporal resolution. The aerodynamic torque was obtained by integrating the pressure and viscous forces acting on the blades, allowing the calculation of the mechanical power generated and the power coefficient. The results showed a periodic and stable torque behavior after the initial transient cycles, yielding an average torque of 0.7687 N·m and a mechanical power of 5.17 W, while the power coefficient reached a value of 0.2102. Analysis of the flow fields revealed the formation of a low-velocity wake downstream of the rotor, regions of high turbulent kinetic energy associated with periodic vortex shedding, and a significant pressure difference between the advancing and returning blades, confirming that turbine operation is dominated by drag forces. The numerical results were validated through comparison with previous studies, showing good agreement and demonstrating the reliability of the proposed Computational Fluid Dynamics (CFD) approach. This study highlights the potential of Savonius turbines for low-power applications in urban and educational environments, as well as the usefulness of CFD as a tool for evaluating and optimizing their aerodynamic performance.

Clean Technologies doi: 10.3390/cleantechnol8020039

Authors: Kamyar Shirvanimoghaddam Agnieszka Krzyszczak-Turczyn Ilona Sadok Bożena Czech Omid Zabihi Minoo Naebe

The global demand for high-quality food is rising due to the increasing population, necessitating intensive farming practices that often involve the extensive use of pesticides, which can accumulate in soils and enter the food chain. This study explores the use of synthesized and commercial graphenes for the removal of kresoxim-methyl (KM), a common strobilurin fungicide, from soil. Adding only 1 wt% of graphene to soil enhanced its partitioning capacity from about 4.77 mg/g for unamended soil to 9.57 mg/g, indicating effective immobilization and reduced environmental risk. The adsorption efficacy was notably higher in materials rich in oxygen-containing functional groups and with a large surface area, highlighting the significance of surface characteristics and porosity. The adsorption followed pseudo-second-order kinetics, underscoring the importance of surface heterogeneity in KM adsorption.

Clean Technologies doi: 10.3390/cleantechnol8020038

Authors: María-Isabel Aguilar Mercedes Lloréns Víctor-Francisco Meseguer Juan-Francisco Ortuño Ana-Belén Pérez-Marín Rafael Valentín

Adsorption is an effective method frequently used for removing contaminants, including dyes, from liquid effluents. This study uses silk fibroin nanoparticles produced by the Bombyx mori moth as an adsorbent material to remove methylene blue dye from aqueous solutions. Batch tests were carried out to examine the effect of pH and temperature on methylene blue adsorption and to obtain kinetic and equilibrium data. The experimental data were fitted to different kinetic models (pseudo-first-order, pseudo-second-order, Elovich, intraparticular diffusion and Bangham) and isotherm models (Langmuir, Freundlich, Sips and Redlich–Peterson). The experimental data can be best explained by the pseudo-second-order and Bangham kinetic models. The adsorption capacity increases with temperature so adsorption is an endothermic process. The maximum adsorption capacities achieved in the experiments were 122 mg·g−1, 132 mg·g−1, and 155 mg·g−1 at temperatures of 10 °C, 25 °C, and 40 °C, respectively. Among the models studied, the ones that best describe the equilibrium data are Freundlich and Redlich–Peterson models.

Clean Technologies doi: 10.3390/cleantechnol8020037

Authors: Jesica Vilchez Cairo Tessa Yazmin Sanchez Grandez Danai Noelia Hidalgo Cabrera Luis Fernando Medrano Canchari Julio Rodrigo Tornero Loayza Doris Esenarro Carlos Manuel Cavani Grau Miguel Ramón Cobeñas Cabrera

The natural resources and local communities of Madre de Dios, Peru, face severe environmental degradation due to illegal mining, deforestation, and the expansion of agricultural activities, threatening one of the most ecologically sensitive regions of the Amazon. This research proposes a low-carbon and bioclimatic architectural design for a Sustainable Interpretation and Research Center dedicated to the conservation of the ecosystems of Manu National Park. The study is based on an analysis of the surrounding environment in terms of flora, fauna, and climate, applying bioclimatic strategies focused on sustainability and supported by specialized digital tools (Revit 2024, Canva, Global Mapper 2024, SketchUp 2024, Photoshop 2022, and Illustrator 2022). The project presents a bioclimatic architectural design that integrates constructive techniques ensuring thermal comfort in a warm-humid climate, while promoting the use of clean technologies such as photovoltaic solar systems generating 15,571.8 kWh per year and a rainwater harvesting system collecting 70,675 L annually. The infrastructure is built with bamboo and locally sourced wood, renewable materials that ensure durability and low environmental impact. In addition, the design includes the reforestation of 17.92% of the total area and 3.46% of public spaces, incorporating native species such as Brazil nut, rosewood, and capirona to reinforce local biodiversity. Overall, this research demonstrates how low-carbon construction, renewable materials, and bioclimatic design can contribute to sustainable development, environmental awareness, and the preservation of natural ecosystems in tropical regions.

Clean Technologies doi: 10.3390/cleantechnol8020036

Authors: Ibrahim Elnaml Louay N. Mohammad Heather Dylla Moses Akentuna Samuel Cooper

The U.S. transportation section contributed a third of the national Greenhouse Gas (GHG) emissions in 2022. As such, the Louisiana Department of Transportation and Development (DOTD) initiated federally funded efforts to create Life Cycle Assessment (LCA) models for pavement systems. The objective of this study was to quantify the holistic, cradle-to-grave environmental impacts of asphalt pavements containing Reclaimed Asphalt Pavement (RAP) and Warm Mix Asphalt (WMA) technologies using a closed-loop recycling assumption based on 100% RAP recovery at the end-of-life stage, consistent with current practice in Louisiana. Five field sections in service for up to 16 years were collected from DOTD’s LaPave database. The LCA framework followed ISO 14040 and included definition of cradle-to-grave system boundaries, a functional unit based on in-service pavement sections, inventory data derived from public databases and field performance records, and use-phase modeling based on pavement–vehicle interaction. Public datasets were used to quantify GHG emissions across all life cycle phases. Results indicated WMA additives reduced production and construction GHG emissions by 5%. An RAP increase by 1% decreased material/construction GHG emissions by approximately 0.9%; however, it potentially increased use-phase emissions due to roughness. Mixtures combining WMA and RAP emitted the lowest GHG among the studied mixtures, which promotes integrating sustainable pavement strategies.

Clean Technologies doi: 10.3390/cleantechnol8020035

Authors: José Rey Catarina Nobre Bruna Rijo Andrei Longo Paulo Brito Cecilia Mateos-Pedrero

Renewable hydrogen purification is a critical yet often underemphasised step in enabling its use as a clean energy carrier. Hydrogen produced from biomass-based thermochemical and biological routes typically contains CO2, CO, CH4, H2S, and other impurities that must be removed to meet stringent requirements for fuel cell, industrial, and grid-injection applications. This review provides a critical and up-to-date assessment of renewable hydrogen purification technologies, focusing on their suitability for variable and impurity-rich renewable hydrogen streams. Established benchmark technologies, including pressure swing adsorption and cryogenic separation, are described, with emphasis on their operating principles, material innovations, and process integration strategies. Recent advancements in inorganic, polymeric, and mixed-matrix membranes are highlighted, with particular focus on how advanced porous materials enhance selectivity, permeability, and flexibility. Additionally, a comparative techno-economic assessment is presented, evaluating each purification method based on technology readiness level, capital and maintenance costs, energy efficiency, and operational lifespan. By incorporating recent research trends, this approach facilitates the selection and design of purification systems that are not only efficient and scalable but also cost-effective, tailored to both decentralised and centralised renewable hydrogen production.

Clean Technologies doi: 10.3390/cleantechnol8020034

Authors: Bruna Thomazinho França Filipe Paradela Marta Martins Ana Luísa Fernando Alberto Reis Paula Costa

The transition to sustainable energy systems has intensified the search for renewable alternatives to reduce greenhouse gas emissions and reliance on fossil fuels. In this context, microalgae have emerged as a promising third-generation feedstock for biofuel production due to their rapid development, high lipid content, and ability to grow in wastewater without competing with freshwater resources. In this study, the hydrotreatment of biocrudes derived from C. vulgaris, T. obliquus, and a mixed microalgal culture cultivated in domestic wastewater is investigated. Catalytic upgrading was applied using sulphided CoMo/Al2O3 (sCoMo) and Pt/Al2O3 catalysts. The results demonstrated that catalytic upgrading enhanced the upgraded bio-oils’ quality compared to non-catalysed reactions, confirming the crucial role of catalysts in improving bio-oil properties. Compared with the Pt catalyst, sCoMo produced higher yields of upgraded bio-oil, greater enrichment in carbon and hydrogen, and higher heating value (HHV), while effectively enhancing nitrogen and oxygen removal. However, when compared with the non-sulphided CoMo, the sulphiding treatment did not significantly improve denitrogenation and treated oil yields. The highest fraction of components within the jet fuel boiling range (37.7%) was obtained using a Pt catalyst, while the non-catalysed process yielded the lowest (26.6%). In this sense, catalytic upgrading of microalgae-based biocrude represents an important step towards the production of advanced and environmentally sustainable fuels.

Clean Technologies doi: 10.3390/cleantechnol8020033

Authors: Ilias Diamantis Gabriel Vasilakis Seraphim Papanikolaou Nikolaos Stoforos Panagiota Diamantopoulou

The present study investigates the potential of olive mill wastewater (OMW), supplemented with expired commercial glucose syrup, as a sustainable substrate for the submerged cultivation of Tuber spp. wild mushrooms. OMW contains considerable quantities of phenolic compounds, making it both a challenging pollutant and a promising nutrient source. To assess fungal performance under increasing phenolic stress, culture media were prepared with varying OMW concentrations (0–75% v/v on agar; 0–50% v/v in liquid media), while glucose was adjusted to ~30 g/L using expired glucose syrup. A sequential experimental approach was followed, beginning with Petri dish screenings on substrate/strain selection (measuring the mycelial growth rate; Kr, mm/day), progressing to 25-day shake flask fermentations and subsequently scaling up the most promising strain (Tuber mesentericum) in a controlled stirred-tank bioreactor. Throughout cultivation, substrate consumption (glucose, phenolics), pH evolution and decolorization were evaluated, while the resulting biomass was analyzed for polysaccharides, β-glucans, proteins, lipids, fatty acids, antioxidants, phenolic acids and triterpenoids content. Results showed that increasing OMW concentration enhanced tolerance and metabolic activity in selected Tuber species, with T. mesentericum exhibiting the highest resilience and achieving comparable or higher biomass yields in OMW-based media than in glucose (control). Phenolic removal exceeded 60% in flasks and 50% in the bioreactor, confirming simultaneous bioremediation capacity. Bioreactor cultivation demonstrated efficient substrate utilization and biomass production, while OMW-grown biomass presented high lipid content, enriched with unsaturated fatty acids, high β-glucan levels and increased antioxidant and phenolic profiles. Overall, this study demonstrates that OMW (supplemented with expired glucose syrup) can serve as a cost-effective and environmentally beneficial substrate for Tuber biomass production with dietary and antioxidant properties, offering an alternative source to mushroom carposomes, as well as supporting the circular bioeconomy strategies within olive oil processing industries.

Clean Technologies doi: 10.3390/cleantechnol8020032

Authors: Michael Sturm Daphne Argyropoulou Pieter Ronsse Anika Korzin Dennis Schober Erika Myers Antonis Eleftheriou Ioannis Lelekis Andriani Galani Katrin Schuhen

This study evaluated the performance of a pilot unit for the combined removal of microplastics and total suspended solids at the municipal wastewater treatment plant of Mykonos, Greece. The pilot unit was installed downstream of the two-stage conventional activated sludge line and operated in semi-continuous mode to demonstrate its function under real effluent conditions. Across five experimental loops, influent microplastics concentrations ranged from 633 to 5843 microplastics/L, while effluent values were reduced to 96–263 microplastics/L, corresponding to an average removal efficiency of 86 ± 8%. In parallel, total suspended solids decreased by 95 ± 3%, turbidity by 93 ± 7%, and chemical oxygen demand by 70 ± 20%, while pH and conductivity remained stable. Influent water showed pronounced variability in chemical oxygen demand, total suspended solids, and turbidity due to irregular wastewater deliveries, yet the pilot consistently stabilized the effluent quality. A correlation analysis revealed strong associations between turbidity, total suspended solids, and chemical oxygen demand in the influent, while effluent data indicated close links between microplastics removal and particulate reduction. These findings confirm the robustness of the organosilane-based agglomeration process and highlight its potential as an advanced treatment stage to reduce MP emissions, improve effluent stability, and mitigate environmental risks in receiving environments such as the Mediterranean Sea.

Clean Technologies doi: 10.3390/cleantechnol8020031

Authors: Barbara Malsegna Andrea Di Giuliano Greta D’Antonio Katia Gallucci

This work investigated hydrotalcite-derived sorbents for CO2 capture at 350 °C, 10 or 14 bar, and 38.5 vol% CO2 in wet or dry gas flow under dynamic Pressure Swing Adsorption (PSA) in a packed-bed laboratory reactor. The chosen conditions enabled a preliminary assessment of the suitability of hydrotalcite-derived sorbents for Sorption-Enhanced-Water-Gas-Shift (SEWGS), a promising process for producing pure hydrogen from syngas. Two starting sorbents were considered: derived from commercial hydrotalcite, and from hydrotalcite synthesized by low-supersaturation. Both sorbents were doped with 20 wt% K2CO3. In addition, a hydrotalcite bifunctional catalyst-sorbent for SEWGS was studied. K2CO3-doping and higher pressure significantly improved the CO2-sorption capacity; the highest value (1.51 mmolCO2∙g−1) was measured under wet conditions at 14 bar. The bifunctional material showed good, stable CO2 sorption capacity (1.39 mmolCO2∙gsolid−1 on average out of five PSA cycles under wet conditions at 14 bar). Materials derived from commercial hydrotalcite doped with K2CO3 showed promising performances for future industrial SEWGS applications.

Clean Technologies doi: 10.3390/cleantechnol8020030

Authors: Musiliu A. Liadi Tawakalt O. Ayodele Abodunrin Tijani Ibrahim A. Bello Niloy Chandra Sarker C. Igathinathane Hammed M. Ademola

Mycelium-based composites (MBCs) have emerged as promising biofabricated materials that align with circular economy and clean technology goals by utilizing fungal networks to transform lignocellulosic residues into functional, biodegradable composites. Despite the MBC’s potentials, the intrinsic nature of the fungal strain, substrate physico-chemical composition and engineering property variability remain significant hurdles that should be critically surmounted. Substrate treatment is central to determining growth kinetics, microstructural uniformity, and mechanical performance in MBC production. This review highlights recent advancements in physical, chemical, biological, and hybrid pretreatment methods, including comminution, pasteurization, alkali hydrolysis, enzymatic conditioning, microwave-assisted hydrolysis, ultrasound pretreatment, steam explosion, plasma activation, and irradiation. These technologies collectively enhance substrate digestibility, aeration, and permeability while reducing contamination. Optimization parameters—temperature, pH, C:N ratio, moisture content, particle size, porosity, and aeration—are examined as critical process levers influencing hyphal density, bonding efficiency, and composite uniformity. Evidence suggests that properly engineered substrate treatments accelerate colonization, strengthen hyphal networks, and significantly improve compressive, tensile, and flexural material properties. The review discusses emerging process control tools such as AI-assisted modeling, micro-CT porosity analysis, and sensor-integrated bioreactors that enable reproducible and energy-efficient fabrication. Collectively, the findings position substrate engineering as a foundational technology for scaling high-performance mycelium composites and advancing sustainable material innovation.

Clean Technologies doi: 10.3390/cleantechnol8020029

Authors: Steffi Wünsche Vico Tenberg Arulselvan Ponnudurai Erik Temmel Heike Lorenz

In recent years, sustainability and the concept of a circular economy have grown in importance within almost all industrial sectors. Especially in the chemical industry, recycling of polymer waste streams has become an important pathway to avoid plastic waste being landfilled or incinerated. Additionally, traditional carbon sources, such as fossil fuels, can be substituted with streams of recycled polymer. For example, polyethylene terephthalate (PET), which is utilized in plastic bottles and textiles, may be recycled via glycolysis. This depolymerization yields the monomer bis(2-hydroxyethyl) terephthalate (BHET). This study focuses on the thermal behavior and stability of BHET, both in pure form as well as in the presence of ethylene glycol (EG), as it results from PET glycolysis. For this, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), high-performance liquid chromatography (HPLC), powder X-ray diffraction (PXRD), and thermogravimetry (TG) were utilized. The results exhibited pure BHET polymerizing to PET at temperatures above 120 °C, while further increasing temperatures increased the reaction kinetics. Additionally, no reaction was observed in BHET/EG mixtures at any temperature investigated, which can be attributed to the presence of EG shifting the equilibrium of the reaction towards the BHET, thus inhibiting polymerization. Based on these results and the determined BHET/EG (solubility) phase diagram, potential purification strategies based on crystallization are proposed.

Clean Technologies doi: 10.3390/cleantechnol8020028

Authors: Manuel L. Aguado Francisco Vázquez S. Fernando F. Calatrava Arturo F. Chica Mª Ángeles Martín

Biological wastewater treatment relies primarily on activated sludge and anaerobic digestion for the removal of organic matter. In urban wastewater treatment plants discharging into eutrophication-sensitive environments, the simultaneous removal of carbon, nitrogen, and phosphorus is required to meet increasingly stringent discharge limits. Under these conditions, the transformation of complex organic matter into volatile fatty acids (VFAs) represents a more efficient strategy than complete mineralization, as biodegradable carbon is essential to sustain biological nitrogen and phosphorus removal processes. In this study, an anaerobic sequencing batch reactor was operated under acidogenic conditions to promote the conversion of organic matter into VFAs. For the first time, this study demonstrates how temperature-controlled acidogenic pretreatment can reliably supply biodegradable carbon to support efficient downstream nitrogen and phosphorus removal in municipal wastewater treatment. A kinetic model was developed to describe the temporal evolution of the different carbon fractions involved in anaerobic digestion, including biodegradable and non-biodegradable organic matter, intermediate compounds, short-chain volatile fatty acids, and biogas. The model assumes first-order kinetics and constant biomass concentration and was successfully validated against experimental data, with deviations below 10%. Estimated kinetic constants exhibited a strong temperature dependence, particularly for hydrolysis and acidogenic pathways, whereas methanogenic steps showed lower sensitivity. Overall, the results demonstrate that temperature is a key operational parameter governing acidogenic performance and carbon transformation pathway. The simple and novel proposed kinetic model provides a useful tool for predicting VFA production and optimizing anaerobic pretreatment strategies aimed at enhancing downstream nutrient removal processes. Optimizing SBR operation for nutrient removal also offers sustainability benefits by improving resource efficiency and reducing energy and chemical inputs.

Clean Technologies doi: 10.3390/cleantechnol8020027

Authors: Fausto de Souza Pagan Marcos Vinícius Mateus Thiago Vinicius Ribeiro Soeira Mário Sérgio da Luz Deusmaque Carneiro Ferreira Rodrigo Moruzzi André Luiz Andrade Simões Julio Cesar de Souza Inácio Gonçalves

Humic substances (HSs) pose a significant challenge to safe drinking-water production due to their ubiquity, limited removal by conventional methods, and their role in forming toxic disinfection by-products, reinforcing the need for more efficient, energy-favorable, and scalable treatment technologies. This study developed and evaluated a compact hydrodynamic cavitation (HC) system that simultaneously induces coagulation and generates microbubbles for flotation-based HS removal. For the first time, HC is explored as a multifunctional unit capable of integrating rapid mixing, coagulant destabilization, and flotation within a single device. Optimal coagulation conditions were established at pH 5.0 and 9.5 mg L−1 of ferric chloride. Process optimization using a Rotated Central Composite Design demonstrated that inlet pressure, flotation time, and initial HS concentration were the dominant operational factors, enabling the HC system to achieve a maximum removal efficiency of 81.9%. Five Venturi geometries with divergent angles of 4°, 8°, 11°, 14°, and 90° were investigated, with the 8° Venturi exhibiting superior performance due to stable microbubble formation and effective coagulant dispersion, as confirmed by CFD analyses. Comparative tests with a conventional Flotest unit showed that achieving similar efficiencies required at least 30% saturated water. In contrast, the HC system delivered equivalent removal in continuous flow without external air saturation. These findings demonstrate the potential of HC as an integrated coagulation–flotation core and highlight its promise as a compact, energy-efficient, and scalable technology for natural organic matter removal in water treatment.

Clean Technologies doi: 10.3390/cleantechnol8020026

Authors: Myo Myo Khaing Yutong Chai Soheil Asgarpour Shunde Yin

The global shift towards cleaner energy has accelerated the application of hydrogen as a clean fuel. Retrofitting and reusing existing natural gas (NG) pipeline infrastructure is a cost-effective way to enable bulk deployment of hydrogen. This study investigates the technical feasibility of retrofitting and rehabilitating NG pipelines for hydrogen transport. Material compatibility, especially hydrogen embrittlement, fatigue resistance, and permeability in steel pipes and weld joints, is examined in the analysis. Retrofitting approaches such as internal coatings, flow regulation, and pipeline pressure adjustments are reviewed in the context of current engineering standards. Structural integrity assessments, using established codes, are conducted to evaluate post-retrofit performance and safety. This is a literature-based technical assessment using existing codes and standards, such as CSA Z662 and ASME B31.12, combined with industry case studies and experimental insights to evaluate the readiness of legacy pipelines for hydrogen service. This paper provides a foundational framework for assessing the safe reuse of legacy pipeline systems for pure or blended hydrogen transport. It sets the stage for further techno-economic analysis in future research.

Clean Technologies doi: 10.3390/cleantechnol8010025

Authors: Mohammad Tarikuzzaman Shaurav Alam Muhammad Aamir Iqbal Md Reazul Islam Zannatul Ferdous Tulona Joan G. Lynam

The sustainable conversion of agricultural waste biomass, particularly crop residues such as corn stover, into high-value products is vital for reducing their open-field burning and mitigating environmental hazards. The hydrothermal carbonization (HTC) process integrated with natural deep eutectic solvents (NADES) presents an alternative approach for valorizing biomass into lignin-rich solid fuels and fermentable sugars for bioethanol production. In this study, corn stover was subjected to HTC using deionized (DI) water, a xylose-based NADES (ChCl:Xy:W), and an oxalic acid-based NADES (ChCl:OA:W) in a 150–300 °C temperature range to optimize both solid fuel and sugar stream yields. Characterization, including fiber analysis, SEM, FTIR, EDS, and bomb calorimetry, was conducted to evaluate structural, compositional, and energetic transformations. The results explored the HTC process, restructuring the biomass, promoting extensive hemicellulose solubilization and cellulose depolymerization, as well as substantially enriching lignin and polymerized compounds with increasing temperature. In addition, the DI water at 300 °C generated a lignin-rich residue, the Xy-based NADES effectively removed ash and extractives, and the OA-based NADES produced the most carbon-dense hydrochar with the highest calorific value. Collectively, these findings demonstrate that solvent-assisted HTC may be employed as a possible strategy for the valorization of agricultural residues into high-energy solid fuels.

Clean Technologies doi: 10.3390/cleantechnol8010024

Authors: Mohammad Alyassin Saffa Izzati Kaderi Grant M. Campbell Helen Masey O’Neill Michael R. Bedford

Arabinoxylans (AX) and their oligosaccharides (AXOS) have potential as functional ingredients. The emergence of biorefineries, leading to more Distillers Dried Grains with Solubles (DDGS) entering the animal feed market, encourages commercial production of AX products. Extracting AX from the two components of DDGS offers the opportunity to increase the biorefinery’s product portfolio and reduce costs. This paper explores AX extraction from solubles and wet grain, using a Gunt pilot-scale bioethanol plant to produce the two streams. After fermentation and distillation, solids were separated from the liquid to give Wet Distillers Grain (WDG), from which alkaline hydrogen peroxide extraction of water-unextractable AX (WUAX) was performed. The water-extractable AX (WEAX) was recovered from the solubles by ultrafiltration and ethanol precipitation. Both extracts were tested for suitability for AXOS production and characterised for their functionality. 10 kg of wheat yielded 3.2 litres of ethanol at 90% purity, 85 g of WUAX (51.6% purity, 110 kDa) and 92 g of WEAX (74.2% purity, 70 kDa). Enzymatic conversion of WEAX into oligosaccharides was 53%, whereas WUAX was unsusceptible to enzyme hydrolysis. Both AX fractions showed interactions with starch that could increase the shelf life of bakery products. AX-based products could be produced from a range of agricultural and biorefinery waste or low value streams, with the global market potentially > £1 billion per annum.

Clean Technologies doi: 10.3390/cleantechnol8010023

Authors: Siavash Davoodi Bhabananda Biswas Laurence N. Warr Balu R. Thombare Ravi Naidu

Anthropogenic CO2 emissions have accelerated climate change, prompting the need for effective capture technologies. Adsorption using clay-based sorbents offers an eco-friendly alternative, although performance often requires enhancement. This study explored mechanochemical modification of two halloysite-rich kaolin clay samples—iron-poor (Hal) and iron-rich (HalFe)—using locust bean gum and quillaja saponin and compared their CO2 uptake with the calcined counterparts (CHal, CHalFe). All samples were characterized using standard techniques, and their CO2 uptake was measured volumetrically across 0.1–20 bar and 15–35 °C. Modified sorbents showed enhanced mesoporosity and binding sites, increasing CO2 uptake by up to 26% at 20 bar (11.85 mg/g) and 125% at 1 bar (2.25 mg/g). Calcination, however, reduced surface area and sorption capacity. Isosteric heat values remained within the physisorption range, as supported by FTIR, XRF, and XPS, which showed no bulk carbonate formation. These sorbents show lower CO2 uptakes than conventional ones. Yet their low costs, abundance, biocompatibility, and solvent-free synthesis indicate strong potential for large-scale applications, especially for low-pressure implementations such as landfills. Further detailed studies on kinetics, thermodynamics, and sorbent regeneration are needed. Spent sorbents can potentially be repurposed for subsequent use in other applications, e.g., water treatment, construction materials, thereby minimizing waste production and supporting circular economy principles.

Clean Technologies doi: 10.3390/cleantechnol8010022

Authors: Rita de Cássia Freire Soares da Silva Thaís Cavalcante de Souza Charles Bronzo Barbosa Farias Ivison Amaro da Silva Joyce Alves de Oliveira Attilio Converti Renata Laranjeiras Gouveia Leonie Asfora Sarubbo

The widespread use of petroleum derivatives in industrial settings poses a challenge due to their toxicity and the difficulty of removing them from tanks, pipes, and equipment. Conventional degreasers are generally expensive, toxic, and harmful to workers’ health and the environment. In this study, an environmentally friendly biodetergent formulated from natural ingredients was produced in a pilot plant with 480 L h−1 capacity, in 250 L homogenizers, at 3500 rpm and 80 °C, and its performance evaluated under different operating conditions. Furthermore, the biodetergent efficiency was compared with that of commercial degreasers commonly used in industrial settings. Stability tests indicated 100% stable emulsion with 2.0% fatty alcohol and 1.0% stabilizing gum after one week of storage. In application tests, the biodetergent promoted up to 100% removal of heavy fuel oil (OCB1) and diesel from metal surfaces, both in concentrated and (1:1 v/v) diluted forms. In direct comparisons, the product performed equally or better than commercial degreasers, notably removing >95% of OCB1 in 10 min and maintaining efficiency after multiple reuse cycles. Unlike acidic or solvent-based formulations, the biodetergent did not induce corrosion on pieces or release toxic vapors when applied to heated surfaces. In summary, the developed bioproduct demonstrated industrial scalability and high efficiency, constituting a sustainable alternative for petrochemical cleaning operations in onshore and offshore environments.

Clean Technologies doi: 10.3390/cleantechnol8010021

Authors: Bijan Pouryousefi Markhali Adam Farahani Matheus Campos Duarte Pooja Kaur Chaggar Kazem Javan Mariam Darestani

Per- and polyfluoroalkyl substances (PFASs) are persistent and mobile contaminants of global concern, and, while granular activated carbon (GAC) is widely used for their removal, it is limited by the high regeneration and disposal costs. This study investigates surface-modified clinoptilolite zeolites as low-cost and thermally regenerable alternatives to GAC for PFAS removal from water. Natural clinoptilolite was modified through acid washing, ion exchange with Fe3+ or La3+, grafting with aminosilane (APTES) or hydrophobic silane (DTMS), dual APTES + DTMS grafting, and graphene oxide coating. The adsorption performance was evaluated for perfluorooctanoic acid (PFOA, C8) and perfluorobutanoic acid (PFBA, C4) at 100 µg L−1 in single- and mixed-solute systems, with an additional high-concentration PFOA test (1 mg L−1). PFAS concentrations were quantified by liquid chromatography–tandem mass spectrometry (LC–MS/MS) using a SCIEX 7500 QTRAP system coupled to a Waters ACQUITY UPLC I-Class. Raw zeolite showed limited PFOA removal (4%), whereas dual-functionalized APTES + DTMS zeolites achieved up to 93% removal, comparable to GAC (97%) and superior to single-silane or metal-exchanged variants. At lower concentrations, modified zeolites effectively removed PFOA but showed limited PFBA removal (<25%), highlighting ongoing challenges for short-chain PFASs. Overall, the results demonstrate that dual-functionalized clinoptilolite zeolites represent a promising and scalable platform for PFAS remediation, particularly for mid- to long-chain compounds, provided that strategies for enhancing short-chain PFAS binding are further developed.

Clean Technologies doi: 10.3390/cleantechnol8010020

Authors: Ana S. S. Sousa Ana S. Oliveira Paula M. L. Castro Catarina L. Amorim

Meat-processing wastewater (MPWW) is rich in nutrients and organic matter. This study assessed its potential as feedstock for microalgal biomass production while enabling wastewater treatment. In batch assays, the microalgae-based consortium grew in raw MPWW, and its synergy with the native wastewater microbial community enhanced the chemical oxygen demand (COD) removal rate. If suspended solids were pre-removed from wastewater, COD removing rates improved from 828.5 ± 60.5 to 1097.5 ± 22.2 mg L−1 d−1. In a raceway system operated in fed-batch mode with sieved and sedimented MPWW, COD removal was consistently achieved across feeding cycles, despite the variability in wastewater composition, reaching rates of up to 806.3 ± 0.0 mg L−1 d−1. Total nitrogen also decreased in most cycles. Microalgal biomass, estimated from total photosynthetic pigment’s concentration, increased from 0.4 to 17.9 µg mL−1. The microalgae-based consortium became more diverse over time, harboring at the end, additional eukaryotic taxa such as protozoan grazers and fungi (e.g., Heterolobosea class and Trichosporonaceae and Dipodascaceae families), although their roles in removal processes remain unknown. This study highlights the potential use of real MPWW as feedstock for microalgal-based biomass production with concomitant carbon/nutrient load reduction, aligning its implementation with circular economy percepts.

Clean Technologies doi: 10.3390/cleantechnol8010019

Authors: Cristian A. Diaz Kiara Montiel-Centeno Jhonny Villarroel-Rocha Deicy Barrera Anthony Dorhauer Carlos Wexler Karim Sapag

The efficient storage of strategic gases—CH4, CO2, and H2—remains a critical challenge due to the need for high pressures or cryogenic temperatures to achieve sufficient storage densities, often resulting in energy- and cost-intensive processes. Adsorption-based storage using porous materials offers a promising alternative. In particular, ordered mesoporous carbons, such as CMK-8 and CMK-9, are attractive due to their mechanical, thermal, and chemical stability, as well as their highly tunable textural properties. Surface functionalization can further enhance gas uptake, though the effect is often gas-specific. This study investigates the adsorption performance of four carbon materials: pristine CMK-8 and CMK-9, and their oxygen-functionalized counterparts produced via HNO3 treatment. The adsorption capacities for CH4, CO2, and H2 were evaluated through a combination of experimental gas adsorption measurements and molecular simulations. The results reveal structure–property relationships between surface chemistry and gas-specific adsorption behavior, with implications for the rational design of carbon-based materials for gas storage.

Clean Technologies doi: 10.3390/cleantechnol8010018

Authors: Sodiq B. Yusuf Michael R. Maughan Armando G. McDonald

Biobased composites from fast growing hemp have drawn significant attention because they are inexpensive, biodegradable, sustainable, promote the circular economy, and have good mechanical properties. This proof-of concept study focused on utilizing low value hemp hurd (H), a byproduct of hemp fiber production, as a reinforcement for use in biocomposite materials. The H was characterized by particle size, surface area and chemical composition. Mixtures of 30–50% H and 70–50% phenol-resorcinol-formaldehyde (PRF) resin were blended and subsequently extruded on a single screw extruder. The uncured (wet) blends were evaluated for their rheological properties and showed pseudoplastic behavior. The extruded biocomposites were cured and their water absorption, flexural strength/modulus, and thermal properties were determined. The water absorption properties increased with H content 17% after 12 days for 30 H to 44% for 50 H. The biocomposites containing 40% H had a flexural strength of 41 MPa, while lower values were obtained at 50% and 30% H. These results show that underutilized H can be valorized in extrudable biocomposites.

Clean Technologies doi: 10.3390/cleantechnol8010017

Authors: Khadija Sarquah Satyanarayana Narra Gesa Beck Nana Sarfo Agyemang Derkyi

Municipal solid waste challenges (MSW) and concerns about fossil fuel dependence motivate efforts to recover energy from waste, including refuse-derived fuel (RDF). Techno-economic assessment (TEA) evaluates the feasibility of systems by quantifying investment performance. However, most RDF-TEA studies typically rely on isolated sensitivity analyses. That provides limited insight into interaction effects in emerging markets. This study maps the multivariable feasibility of RDF production from MSW in Ghana under realistic economic conditions. Using a pilot-calibrated case study, the assessment integrates discounted cash flow analysis with response surface methodology–design of experiment (RSM-DoE). A central composite design evaluates interaction effects among operational and economic variables for a system capacity of 2875 tonnes RDF/year. The results indicate economic viability with a net present value (NPV) of USD 892,556.44, a payback period (PBP) of 6.61 years and a levelised production cost (LPC) of USD 18.96/tonne. The RSM models show high explanatory power (R2, R2adj, R2pred > 90%). Sensitivity results demonstrate that support mechanisms can significantly reduce LPC and PBP while preserving investment viability. The study quantifies the feasibility thresholds and the support instruments within the RDF design levers. It further provides a transferable framework for assessing deployment and upscaling in emerging markets. The findings highlight the need for structured pricing mechanisms and regulatory support for the long-term sustainability of RDF as an AF.

Clean Technologies doi: 10.3390/cleantechnol8010016

Authors: Linda Sandgren Sri Ram Gnanesh Erik Johansson Victoria Van Camp Magnus Karlberg Mats Näsström Roland Larsson

Copper is a strategic raw material and an important component in electric motors, widely used across industries because of its excellent conductivity and recyclability. It plays an important role in the transformation from fossil fuel-based systems to green, electrified systems. However, substantial material losses continue throughout the lifecycle of electric motors, even with copper’s intrinsic capacity for circularity. Also, copper’s increasing demand, which is driven by the emergence of electric vehicles, industrial electrification, and renewable energy infrastructure, poses questions regarding its sustainable supply. The recovery of secondary copper sources from end-of-life (EoL) products is becoming more and more important in this context. However, it is still difficult to achieve circularity of copper, especially from industrial electric motors. This study investigates the challenges of closing the loop for copper during the lifecycle of motors in industrial applications. Based on an examination of EoL strategies, material flow insights, and practical investigation, the research pinpoints significant inefficiencies in the current processes. The widespread use of scraping as an approach of end-of-life management is one significant issue. Most of the electric motors are not built to separate their components, which makes both mechanical and manual disassembly difficult. The quality of recovered copper is thus compromised by the dominance of mixed metal shredding methods in the recycling step. This study highlights the need for systemic changes in addition to technical solutions to address copper circularity issues. It requires a focus on circularity in designing, giving disassembly and metal recovery a priority. This study focuses on circularity and its technological challenges in a value chain of copper. It not only identifies different processes such as supply chain disconnections and design constraints, but it also suggests workable solutions to close the copper flow loop in the electric motor sector. Copper quality and recovery is ultimately a problem involving design, technology, and cooperation, in addition to resources. This study supports the transition to a more sustainable and circular electric motor industry by offering a basis for directing such changes in industry practices and prospective EU regulations.

Clean Technologies doi: 10.3390/cleantechnol8010015

Authors: Caela Kleynhans Hendrik G. Brink Nils Haneklaus Willie Nicol

This study evaluated low-cost food waste anaerobic digestion (FWAD) designed for African urban informal settlements, where electricity and process control are limited. Eight small-scale reactors were operated under varying mixing, pH control, and temperature conditions to assess the feasibility of stable operation with minimal input. Results showed no significant difference in methane yield between continuously mixed and minimally mixed (48-hourly) systems, nor between reactors with continuous pH dosing and those adjusted every 48 h (ANOVA p > 0.05 for all comparisons). The highest mean methane yield, 0.267 L CH4 g VS−1, was achieved by the minimally mixed reactor with 48-hourly pH control at 30 °C, while the controlled reactor at 37 °C produced a comparable 0.247 L CH4 g VS−1. Total methane production was similar at both temperatures, although gas generation was faster during the first 24 h at 37 °C. Compared to gas recovery achieved by extended batch operation following semi-continuous feeding, 58–73% of total methane was produced within the 48-h cycle, suggesting conversion could increase by 30–40% with extended liquid retention. Microbial analyses showed compositional differences but consistent performance, indicating functional redundancy within the microbial consortia. These results confirm the capacity of FWAD for stable, efficient biogas production without continuous energy input.

Clean Technologies doi: 10.3390/cleantechnol8010014

Authors: Giannis Pachakis Sofia Mai Elli Maria Barampouti Dimitris Malamis

The increasing atmospheric concentration of carbon dioxide (CO2) poses a critical threat to global climate stability, highlighting the need for efficient carbon capture technologies. While amine-based solvents such as monoethanolamine (MEA) are widely used for industrial CO2 capture, they are subject to limitations such as high energy requirements for regeneration, solvent degradation, and environmental concerns. This study investigates potassium carbonate/bicarbonate system as an alternative solution for CO2 absorption. The absorption mechanism and reaction kinetics of potassium carbonate in the presence of bicarbonates were reviewed. A rate-based model was developed in Aspen Plus, using literature kinetics, to simulate CO2 absorption using 20 wt% potassium carbonate (K2CO3) solution with 10% carbonate-to-bicarbonate conversion under different industrial conditions. Three flue gas compositions were evaluated: cement industry, biomass combustion, and anaerobic digestion, each at 3000 m3/h flow rate. The simulation was conducted to determine minimum column height and solvent loading requirements with a target output of 90% CO2 removal from the gas streams. Results demonstrated that potassium carbonate systems successfully achieved the target removal efficiency across all scenarios. Column heights ranged from 18 to 25 m, with molar K2CO3/CO2 ratios between 1.41 and 4.00. The biomass combustion scenario proved most favorable due to lower CO2 concentration and effective heat integration. While requiring higher column heights (18–25 m) compared to MEA systems (6–12 m) and greater solvent mass flow rates, potassium carbonate demonstrated technical feasibility for CO2 capture. The findings of this study provide a foundation for technoeconomic evaluation of potassium carbonate systems versus amine-based technologies for industrial carbon capture applications.

Clean Technologies doi: 10.3390/cleantechnol8010013

Authors: Ronja Wagner-Wenz Frederik Funk Regine Peter Tobias Necke Fabian Brückner Maximilian Philipp Markus Engelhart Anke Weidenkaff Emanuel Ionescu

A treatment process was developed for effluents from direct physical lithium-ion battery (LIB) recycling with a focus on the removal of organic contaminants. The high chemical oxygen demand to biological oxygen demand ratio (COD/BOD5) of 3.9–4.6 indicates that biological treatment is not feasible. Therefore, three advanced oxidation processes were evaluated: UV/H2O2 oxidation, the Fenton process and electrochemical oxidation. Two target scenarios were considered, namely compliance with the limit for discharge into the sewer system (COD = 800 mg/L) and compliance with the stricter limit for direct discharge into surface waters (COD = 200 mg/L). Under the investigated conditions, UV/H2O2 oxidation and the Fenton process did not meet the required discharge limits and exhibited high chemical consumption. In contrast, electrochemical oxidation achieved both discharge criteria with a lower energy demand, requiring 32.8 kWh/kgCODremoved for sewer discharge and 95.3 kWh/kgCODremoved for direct discharge. An economic assessment further identified electrochemical oxidation as the most cost-effective option, with treatment costs of EUR 6.63/m3, compared to EUR 17.31/m3 for UV/H2O2 oxidation and EUR 28.66/m3 for the Fenton process. Overall, electrochemical oxidation proved to be the most efficient and environmentally favorable technology for treating wastewater from LIB recycling, enabling compliance with strict discharge regulations while minimizing the chemical and energy demand.

Clean Technologies doi: 10.3390/cleantechnol8010012

Authors: James Niffenegger Kaitlin Brunik Katie Peterson Andrew Simms Tristen Stewart Jessica Cross Michael Lawson

Electrochemical ocean alkalinity enhancement is a form of marine carbon dioxide removal, a rapidly growing industry that is powered by efficient onshore or offshore energy sources. As more and larger deployments are being planned, it is important to consider how variable energy sources like tidal energy can impact plant performance and costs. An open-source Python-based generalizable model for electrodialysis-based ocean alkalinity enhancement has been developed that can capture key system-level insights of the electrochemistry, ocean chemistry, acid disposal, and co-product creation of these plants under various conditions. The model additionally accounts for hybrid energy system performance profiles and costs via the National Laboratory of the Rockies’ H2Integrate tool. The model was used to analyze an example theoretical plant deployment in North Admiralty Inlet, including how the plant is impacted by the available energy sources in the region and the scale at which plant costs are covered by the co-products it generates, such as recycled concrete aggregates, without requiring carbon credits. The results show that the example plant could be profitable without carbon credits at commercial scales of 100,000 to 1 million tons of carbon dioxide removal per year, so long as it uses low-cost electricity sources and either sells acid or recovers recycled concrete aggregates with about 1 molar acid concentrations, though more research is needed to confirm these results.

Clean Technologies doi: 10.3390/cleantechnol8010011

Authors: Olga Ioannou Fieke Konijnenberg

Façades account for approximately 15–20% of a building’s embodied carbon, making them a key target for material decarbonization. While bio-composites are increasingly explored for façade insulation, cladding systems remain dominated by carbon-intensive materials such as aluminum and fiber-reinforced polymers (FRPs). This paper presents findings from a study investigating the use of food-waste-derived bulk fillers in bio-composite materials for façade cladding applications. Several food-waste streams, including hazelnut and pistachio shells, date seeds, avocado and mango pits, tea leaves, and brewing waste, were processed into fine powders (<0.125 μm) and combined with a furan-based biobased thermoset resin to produce flat composite sheets. The samples were evaluated through mechanical testing (flexural strength, stiffness, and impact resistance), water absorption, freeze–thaw durability, and optical microscopy to assess microstructural characteristics before and after testing. The results reveal substantial performance differences between waste streams. In particular, hazelnut and pistachio shell fillers produced bio-composites suitable for façade cladding, achieving flexural strengths of 62.6 MPa and 53.6 MPa and impact strengths of 3.42 kJ/m2 and 1.39 kJ/m2, respectively. These findings demonstrate the potential of food-waste-based bio-composites as low-carbon façade cladding materials and highlight future opportunities for optimization of processing, supply chains, and material design.

Clean Technologies doi: 10.3390/cleantechnol8010010

Authors: Mladen Bošnjaković Robert Santa Antonija Vučić Zoran Crnac

In the original publication [...]

Clean Technologies doi: 10.3390/cleantechnol8010009

Authors: Milena Espinosa César Afonso Bárbara Saraiva Davide Vione Annabel Fernandes

The textile industry is one of the largest consumers of water worldwide and generates highly complex and pollutant-rich textile wastewater (TWW). Due to its high load of recalcitrant organic compounds, dyes, salts, and heavy metals, TWW represents a major environmental concern and a challenge for conventional treatment processes. Among advanced alternatives, electrooxidation (EO) and membrane technologies have shown great potential for the efficient removal of dyes, organic matter, and salts. This review provides a critical overview of the application of EO and membrane processes for TWW treatment, highlighting their mechanisms, advantages, limitations, and performance in real industrial scenarios. Special attention is given to the integration of EO and membrane processes as combined or hybrid systems, which have demonstrated synergistic effects in pollutant degradation, fouling reduction, and water recovery. Challenges such as energy consumption, durability of electrode and membrane materials, fouling, and concentrate management are also addressed. Finally, future perspectives are proposed, emphasizing the need to optimize hybrid configurations and ensure cost-effectiveness, scalability, and environmental sustainability, thereby contributing to the development of circular water management strategies in the textile sector.

Clean Technologies doi: 10.3390/cleantechnol8010008

Authors: Lorenzo Maria Ruggeri Carlo Maffei Domenico Prisa Francesco Crea Damiano Spagnuolo

The increasing environmental impact of synthetic antifouling paints has stimulated the search for natural, eco-friendly alternatives. In this study, alcoholic and aqueous extracts of the macroalgae Ulva ohnoi and Asparagopsis taxiformis were evaluated for their antifouling potential on aluminum substrates representative of boat hulls. Extracts were applied to aluminum plates coated with gelcoat under three different surface conditions (non-worn, worn, highly worn). The treated panels were submerged at 5 m and biofilm and fouling development was monitored every 96 h using digital imaging and quantitative segmentation. All treated surfaces exhibited significantly lower fouling colonization than the untreated control (p < 0.001). Among treatments, the aqueous extract of A. taxiformis produced the lowest degree of colonization across all surface conditions, while U. ohnoi extracts showed moderate antifouling activity. Increased surface wear enhanced overall colonization but did not suppress extract efficacy. These results demonstrate that both algal species possess active compounds capable of inhibiting early biofilm formation on marine substrates. Although less potent than conventional biocidal coatings, their biodegradability and absence of ecotoxicity represent a substantial environmental advantage. Future studies should focus on the chemical characterization of active metabolites, the formulation of hybrid bio-based coatings, and long-term field testing under dynamic marine conditions.

Clean Technologies doi: 10.3390/cleantechnol8010007

Authors: Cornel Aramă Cristian-Ioan Leahu

Currently, the general focus of engine-produced pollution reduction lies in exhaust gas aftertreatment methods. This paper attempts a paradigm shift in the field by applying the pre-combustion treatment technologies by fumigation method, which consists of introducing an aqueous solution into the engine intake, which could lead to a significant reduction in polluting emissions. Common and inexpensive substances used (sodium borate, citric acid, podium carbonate, hydrogen peroxide, potassium permanganate, and ammonium nitrate) in tests are not ordinarily known to be combustible. The key to the research is understanding the thermochemical phenomena during combustion. The method used was to formulate hypotheses regarding thermochemical reactions and validate them by measuring parameters and pollutant emissions (CO, CO2, NO, NO2, NOx, and smoke) of a single-cylinder engine mounted on the test stand. The results indicate that chemical fumigation leads to a significant reduction, specifically a decrease in CO by 145 ppm and NOx (NO2 and NO) by 55 ppm at an engine speed of 1500 rpm. All substances fumigated into the engine intake increased the exhaust gas temperature. The highest increase is nearly 150 °C at 1500 rpm, while the least pronounced rise is 50 °C at 3500 rpm. Additionally, a decarbonization process of a passenger car engine is presented, carried out by applying the fumigation method simultaneously with potassium permanganate and ammonium nitrate. In this case, the results showed that the opacity index decreased to 0.01 m−1.

Clean Technologies doi: 10.3390/cleantechnol8010006

Authors: Claudia Bassano Mattia Micciancio Paolo Deiana Gabriele Calì Enrico Maggio Leonardo Colelli Giorgio Vilardi

The European Union’s enhanced greenhouse gas (GHG) reduction targets for 2030 make the large-scale deployment of carbon capture and storage (CCS) technologies essential to achieve deep decarbonization goals. Within this context, this study aims to advance CCS research by developing and testing a pilot-scale system that integrates gasification for syngas and power production with CO2 absorption and solvent regeneration. The work focuses on improving and validating the operability of a pilot plant section designed for CO2 capture, capable of processing up to 40 kg CO2 per day through a 6 m absorber and stripper column. Experimental campaigns were carried out using different amine-based absorbents under varied operating conditions and liquid-to-gas (L/G) ratios to evaluate capture efficiency, stability, and regeneration performance. The physical properties of regenerated and CO2-saturated solvents (density, viscosity, pH, and CO2 loading) were analyzed as potential indicators for monitoring solvent absorption capacity. In parallel, a process simulation and optimization study was developed in Aspen Plus, implementing a split-flow configuration to enhance energy efficiency. The combined experimental and modeling results provide insights into the optimization of solvent-based CO2 capture processes at pilot scale, supporting the development of next-generation capture systems for low-carbon energy applications.

Clean Technologies doi: 10.3390/cleantechnol8010005

Authors: Kyriaki Kiskira Sofia Plakantonaki Nikitas Gerolimos Konstantinos Kalkanis Emmanouela Sfyroera Fernando Coelho Georgios Priniotakis

The global shift toward renewable energy and circular economy models requires industrial systems that minimize waste and recover value across entire life cycles. This review synthesizes recent advances in by-product recovery technologies supporting renewable energy and circular industrial processes. Thermal, biological, chemical/electrochemical, and biotechnological routes are analyzed across battery and e-waste recycling, bioenergy, wastewater, and agri-food sectors, with emphasis on integration through Life Cycle Assessment (LCA), techno-economic analysis (TEA), and multi-criteria decision analysis (MCDA) coupled to process simulation, digital twins, and artificial intelligence tools. Policy and economic frameworks, including the European Green Deal and the Critical Raw Materials Act, are examined in relation to technology readiness and environmental performance. Hybrid recovery systems, such as pyro-hydro-bio configurations, enable higher resource efficiency and reduced environmental impact compared with stand-alone routes. Across all technologies, major hotspots include electricity demand, reagent use, gas handling, and concentrate management, while process integration, heat recovery, and realistic substitution credits significantly improve life cycle outcomes. Harmonized LCA-TEA-MCDA frameworks and digitalized optimization emerge as essential tools for scaling sustainable, resource-efficient, and low-impact industrial ecosystems consistent with circular economy and renewable energy objectives.

Clean Technologies doi: 10.3390/cleantechnol8010004

Authors: Carmen María Álvez-Medina Sergio Nogales-Delgado Beatriz Ledesma Cano Vicente Montes-Jiménez Silvia Román Suero

Fisheries and aquaculture residues pose escalating environmental challenges due to their high moisture content, nutrient loads, and pollutant potential when improperly managed. Conventional valorization routes, such as fishmeal, fish oil, and silage, offer partial mitigation but remain limited in scalability, conversion efficiency, and environmental performance. In this study, fish processing residues were subjected to hydrothermal carbonization (HTC) under controlled subcritical conditions (180–220 °C), along with a high-severity catalytic run (325 °C) using sodium bicarbonate (NaHCO3) as an additive. The latter condition exceeded the typical HTC range and entered the subcritical hydrothermal liquefaction (HTL) regime. The resulting solid, liquid, and gaseous fractions were comprehensively characterized to assess their energy potential, chemical composition, and reactivity. Hydrochars achieved higher heating values (HHVs) ranging from 14.2 to 25.7 MJ/kg. These results underscore their suitability as renewable solid fuels. The gas products were dominated by CO2 under standard HTC conditions. In contrast, the catalytic run in the subcritical HTL regime achieved a hydrogen enrichment of up to 30 vol.%, demonstrating the efficacy of NaHCO3 in promoting the water-gas shift reaction. Subsequent air gasification confirmed the high reactivity of the hydrochars, producing syngas enriched in H2 and CO at elevated temperatures. Overall, this study demonstrates a scalable multiproduct valorization route for fishery residues, supporting circular bioeconomy strategies and contributing to the achievement of UN Sustainable Development Goals (SDGs 7, 12, and 13).

Clean Technologies doi: 10.3390/cleantechnol8010003

Authors: Anna M. Pryor Peter A. Gostomski Carlo R. Carere

Dilute methane (CH4) emissions from dairy barns (<500 ppm) are a challenging agricultural greenhouse-gas source to abate via biofiltration because its poor solubility makes gas–liquid mass transfer a primary limitation in biotrickling filters (BTFs). Here, we evaluated lab-scale BTFs for treating dairy-relevant CH4 concentrations and tested two enhancement strategies: (1) aerosolised nutrient delivery to improve liquid distribution and (2) reduced liquid addition rates to increase gas–liquid mass-transfer efficiency. Liquid-fed BTFs and aerosol-fed BTFs (ABTFs) packed with scoria or glass beads were compared. Aerosolised nutrients reduced the elimination capacity (EC) compared to biotrickling delivery. Switching from liquid to aerosol decreased an initial EC of ~30 g m−3 h−1 by 35% at 2500 ppm CH4, and the original EC was not recoverable. Slower liquid addition consistently improved CH4 removal for both delivery techniques. In a glass bead ABTF at 2500 ppm CH4, the EC increased from 5.5 to 12.4 g m−3 h−1 when the liquid coalescence rate decreased from 0.79 to 0.006 cm h−1. In a scoria ABTF, a 1.5-fold increase in EC was observed as the rate decreased from 2.36 to 0.15 cm h−1. Below a threshold liquid addition rate in the scoria BTF, the EC dropped ~33%, likely due to uneven wetting or high pH conditions. Therefore, optimising liquid delivery can significantly enhance BTF performance for agricultural CH4 mitigation.

Clean Technologies doi: 10.3390/cleantechnol8010002

Authors: Natalija Velić Marija Stjepanović Marta Ostojčić Helena Švarc Ivica Strelec Sandra Budžaki

The objective of this study was to valorise eggshell waste (ESW) by investigating its biosorption properties and evaluating its efficiency as a sustainable biosorbent for the removal of the synthetic dye Congo Red (CR) from model CR solutions and synthetic wastewater with the addition of CR. Batch biosorption experiments were conducted to investigate the influence of several factors on the biosorption process, including ESW concentration (1–15 g L−1), contact time (1–360 min), temperature (15, 25, 35, 45 °C) and initial CR concentration (10–100 mg L−1). Desorption experiments were performed using ultrapure water, 0.1 M NaCl, 50% ethanol, 0.1 M HCl, or 0.1 M NaOH as solvents. A higher ESW concentration improved CR removal, but the amount of CR adsorbed on ESW decreased. The dye uptake by ESW was increased with prolonged contact time and temperature increase. When the effect of CR initial concentration was investigated, the results indicated that the process is concentration-dependent and that overall, CR uptake by ESW was higher in synthetic wastewater than in the model dye solution. The biosorption process was better described by the Langmuir isotherm model than by the Freundlich model, indicating monolayer adsorption. Kinetic analysis showed that the pseudo-second-order model provided a better fit than the pseudo-first-order model. Desorption of CR from ESW under the applied experimental conditions was generally low (0.67–27.13%).

Clean Technologies doi: 10.3390/cleantechnol8010001

Authors: Sirous Yasseri Maryam Shourideh Hamid Bahai

A system dynamics approach is described to explore the path of Carbon Capture diffusion. The proposed model, in principle, follows the Bass diffusion of innovation theory and includes all major influencing factors. The primary contribution of this paper is the modification of Bass’s model to reflect parameters affecting the adoption of Carbon capture and storage technology. Consequently, it differs from other extensions to Bass’s model. The underpinning of this work is the system dynamics (SD) approach, which can open a pathway for further research into CCS acceptance. The proposed model’s behaviour is illustrated for various transition pathways of the technology, for different regimes. By modifying the proposed model, the paper also allows consideration of various capturing technologies on their merit. The proposed framework enables the examination of the impact of intervention policies on the adoption of CCS by individual investors. The purpose is to identify the parameters of these policies to support the under-resourced CCS technology and reduce the need for government participation. It is worth noting that the SD is primarily a descriptive method used for scenario analysis to illustrate what the future would look like.

Clean Technologies doi: 10.3390/cleantechnol7040115

Authors: Emily Cook Magnus de Witt

The Arctic is warming three to four times faster than the global average. This is transforming global maritime routes, thereby increasing shipping and resource extraction in Alaska. This surge requires sustainable energy solutions as policy trends towards stricter emissions standards. This article assesses the potential of Geothermal-to-X (GtX) technologies in establishing clean refueling infrastructure across Alaska, using its untapped geothermal resources. GtX uses electrolysis to split water into hydrogen and oxygen, a process powered by geothermal energy. Hydrogen and its X products, such as green methane or green ammonia, can be stored as fuels and are largely recognized as the key to a carbon-free future to address the growing energy demand. This study assesses the technical, economic, strategic, and geological feasibility of GtX refueling hubs in Alaska. Five locations were denoted as potential candidates and beckon future research. This study concludes that Unalaska is the most viable initial GtX hub given the highest Multi Criteria Decision Analysis (MCDA) score from its combination of a high-quality geothermal resource, an existing and accessible deepwater port, and a sizable local energy demand. The goal of this study is to provide an accessible and comprehensive resource for stakeholders and policymakers, outlining an energy future with sustainable maritime development, powered by affordable and secure energy.

Clean Technologies doi: 10.3390/cleantechnol7040114

Authors: Emilio Ritoré Carmen Arnaiz José Morillo Agata Egea-Corbacho José Usero

Soil contamination by petroleum hydrocarbons represents a significant environmental challenge, especially in industrial and urban areas. This study evaluates the use of three industrial liquid by-products—sludge dewatering sidestream (SD), leftover yeast (LY), and secondary clarifier effluent (SC)—as biostimulant agents for the bioremediation of soils contaminated with gasoline and diesel mixtures. The novelty lies in applying these waste streams within a circular economy framework, with the added advantage that they can be injected directly into the subsurface. Microcosm tests were conducted over 20 weeks, analyzing the degradation of total petroleum hydrocarbons (TPHs) and their aliphatic and aromatic fractions using gas chromatography. The results show that all by-products improved biodegradation compared to natural attenuation. LY was the most effective, achieving 73.2% TPH removal, followed by SD (70.6%) and SC (65.4%). The greatest degradation was observed in short-chain hydrocarbons (C6–C16), while compounds with higher molecular weight (C21–C35) were more recalcitrant. In addition, aliphatic hydrocarbons showed greater degradability than aromatics in heavy fractions. Kinetic analysis revealed that the second-order model best fitted the experimental data, with higher correlation coefficients (R2) and more representative half-lives. Catalase enzyme activity also increased in soils treated with LY and SD, indicating higher microbial activity.

Clean Technologies doi: 10.3390/cleantechnol7040113

Authors: Basmah Bushra Paul J. Wood Diganta B. Das

This study investigates the removal of copper and zinc at environmentally relevant concentrations from aqueous solutions using barista coffee waste in both standalone and blended forms (with rice husk biochar). A fixed-bed horizontal column adsorption study was conducted to determine the effects of contact time, adsorbent type, and initial metal concentration on the removal efficiency. As far as we are aware, this study is the first to focus on eliminating low concentrations in accordance with World Health Organization (WHO) guideline levels, employing a horizontal fixed-bed column setup. Adsorption equilibrium was achieved around six hours after initiation and resulted in a high percentage of metal removal (up to 96.71%). Ground coffee waste performed better for lower initial metal concentrations (2.5 ppm copper and 10 ppm zinc), although a mixture of coffee waste and biochar performed better at concentrations greater than 5 ppm for copper and 25 ppm for zinc. Experimental results were applied to the Thomas model to determine the efficiency of the adsorbents. Results indicated it was linear with a good correlation coefficient (R2 = 0.94). The experimental data also fitted the pseudo-first-order reaction kinetic with a higher correlation coefficient (R2 = 0.93) than the second-order reaction kinetics. The experimental and calculated values were very similar for the first-order reaction kinetic. The metal adsorption was affected by both external mass transfer and intra-particle diffusion mechanisms. This study developed an engineered solution to remove heavy metals from wastewater using widely available ground coffee waste as an effective adsorbent.

Clean Technologies doi: 10.3390/cleantechnol7040112

Authors: Alejandro Delgado-Cortez Carlos Castillo-Zacarias Isaías Juárez-Ramírez Sergio Arturo Galindo-Rodríguez Catalina Rivas-Morales Catalina Leos-Rivas Ezequiel Viveros-Valdez

Oranges are widely consumed worldwide and are highly valued both for their nutritional properties and their economic importance. In Mexico, particularly in the northeastern citrus-producing region, large amounts of peel are generated during industrial processing, representing a significant source of agro-industrial waste. This byproduct is naturally rich in compounds of interest, including flavonoids, polyphenols, and pectin, which motivates the development of sustainable recovery strategies. In this work, orange peel biomass was valorized using ultrasound-assisted extraction in combination with deep eutectic solvents (DESs). Among the evaluated formulations, the choline chloride–lactic acid DES at a 1:10 molar ratio produced the highest overall extraction yield (43.88% by dry weight/mass). The 2:1 formulation, however, was the most efficient for the recovery of phenolic compounds, reaching 4.12 mg GAE/g, and exhibited the greatest antioxidant activity (2.55 mmol Trolox/g) and the strongest antimicrobial response against clinically relevant microorganisms. This same DES ratio enabled the highest quantification of key phenolics such as naringin (1150.29 µg/g), caffeic acid (139.41 µg/g), and ferulic acid (379.96 µg/g). For polysaccharide extraction, the 1:1 DES ratio was the most effective, achieving a pectin yield of 48.24%. Overall, the findings demonstrate that DES, particularly when combined with ultrasound, offers a green and efficient approach for the integrated recovery of pectin, phenolic antioxidants, and antimicrobial compounds from citrus byproducts, contributing to environmentally sustainable biorefinery strategies.

Clean Technologies doi: 10.3390/cleantechnol7040111

Authors: K. Yamini Laurence G. Dyer Bogale Tadesse Richard D. Alorro

The increasing emphasis on green chemistry has led numerous researchers to focus on environmentally friendly solvents for mineral extraction. Among them, deep eutectic solvents (DESs) have garnered significant attention due to their eco-friendly, non-toxic, and biodegradable properties. These solvents possess comparable physicochemical properties to conventional ionic liquids but are more cost-effective and environmentally friendly. While DESs have been widely studied for extracting metals from synthetic minerals and end-of-life products, its use with primary ores and associated wastes remains relatively unexplored. This study aims to bridge that gap by assessing the effectiveness of choline chloride- and ethylene glycol-based DESs in extracting rare earth elements from primary feedstocks with varied grades and mineralogy, including sub-economic ores, monazite flotation tailings, and acid-crack and leach residue. The study also examines the practical challenges in preparing DES and assesses the applicability of the solvents for primary materials. By examining both solvent preparation challenges and the variable responses of different feed materials, this work provides a high-level scoping analysis to better understand the suitability and limitations of DES for primary resource extraction. This study highlights the challenges with physical properties and mineral breakdown in using DES.

Clean Technologies doi: 10.3390/cleantechnol7040110

Authors: Sergi Vilajuana Llorente José Ignacio Rapha Magnus Daniel Kallinger José Luis Domínguez-García

Floating offshore wind is now receiving much attention as an expansion to bottom-fixed, especially in deep waters with large wind resources. In this regard, improving the performance and efficiency of floating offshore wind farms (FOWFs) is currently a highly addressed topic. The inter-array (IA) cable connection is a key aspect to be optimised. Due to floating offshore wind (FOW) particularities such as dynamic cable designs, higher power capacities, and challenging installation, IA cabling is expected to be a primary cost driver for commercial-scale FOWFs. Therefore, IA cabling optimisation can lead to large cost reductions. In this work, an optimisation with an adaptive particle swarm optimisation (PSO) algorithm for such wind farms is proposed, considering the floating substructures’ horizontal translations and its impact on the dynamic cable length. The method provides an optimised IA connection, reducing acquisition costs and power losses by using a clustered minimum spanning tree (MST) as an initial solution and improving it with the PSO algorithm. The PSO achieves a reduction in the levelised cost of energy (LCOE) between 0.018% (0.022 EUR/MWh) and 0.10% (0.12 EUR/MWh) and a reduction in cable acquisition costs between 0.18% (0.3 M EUR) and 1.34% (3.8 M EUR) compared to the initial solution, showing great potential for future commercial-sized FOWFs.

Clean Technologies doi: 10.3390/cleantechnol7040109

Authors: Nisreen Salem Kamalpreet Kaur Brar Ali Asgarian Kulwinder Kaur Sara Magdouli Nancy N. Perreault

Carbon dioxide (CO2) is the most significant anthropogenic greenhouse gas (GHG), accounting for approximately 81% of total emissions, with methane (CH4), nitrous oxide (N2O), and fluorinated gases contributing the remainder. Rising atmospheric CO2 concentrations, driven primarily by fossil fuel combustion, industrial processes, and transportation, have surpassed the Earth’s natural sequestration capacity, intensifying climate change impacts. Carbon Capture, Utilization, and Storage (CCUS) offers a portfolio of solutions to mitigate these emissions, encompassing pre-combustion, post-combustion, oxy-fuel combustion, and direct air capture (DAC) technologies. This review synthesizes advancements in CO2 capture materials including liquid absorbents (amines, amino acids, ionic liquids, hydroxides/carbonates), solid adsorbents (metal–organic frameworks, zeolites, carbon-based materials, metal oxides), hybrid sorbents, and emerging hydrogel-based systems and their integration with utilization and storage routes. Special emphasis is given to CO2 mineralization using mine tailings, steel slag, fly ash, and bauxite residue, as well as biological mineralization employing carbonic anhydrase (CA) immobilized in hydrogels. The techno-economic performance of these pathways is compared, highlighting that while high-capacity sorbents offer scalability, hydrogels and biomineralization excel in low-temperature regeneration and integration with waste valorization. Challenges remain in cost reduction, material stability under industrial flue gas conditions, and integration with renewable energy systems. The review concludes that hybrid, cross-technology CCUS configurations combining complementary capture, utilization, and storage strategies will be essential to meeting 2030 and 2050 climate targets.

Clean Technologies doi: 10.3390/cleantechnol7040108

Authors: Tilen Jernejc Gorazd Bombek Ignacijo Biluš Luka Kevorkijan Luka Lešnik

Solid waste presents a very large problem in the developed world. Waste plastics, which make up a large part of solid waste, have high energy value, which is discarded if they are not treated properly. Most of the plastic found in solid waste is produced from petrochemical material, so it can be used in resource recovery processes to produce various materials. One promising resource recovery process is the pyrolysis process, from which pyrolytic oil, gas, and solid residue are obtained. Pyrolytic oils have properties that are similar to conventional fossil fuels, and are promising fuels for use in heat engines or heating applications. In the present work, HDPE plastic in the form of plastic bottles caps was collected from solid waste and used in a thermal pyrolysis process for the production of pyrolytic oil. The obtained oil was characterised, and the obtained results were compared to conventional fuels. The obtained oil was used further in an oil burner fuel injection application, in which the spray breakup characteristics were monitored and analysed using VisiSize particle characterisation systems. The obtained results were compared to those of conventional fuel. The results indicate that the difference in fuel properties influences the spray breakup process slightly, but the differences are rather small. This indicates that from a spray development perspective, pyrolytic oil can be used as a substitute for conventional fuels in oil burners.

Clean Technologies doi: 10.3390/cleantechnol7040107

Authors: Hector Garcia-Gonzalez Pablo Menendez-Cabo

Underground mining environments present elevated occupational health risks, primarily due to substantial exposure to diesel exhaust emissions within confined and poorly ventilated spaces. This study assesses the real-world performance of two advanced retrofit emission control systems—Proventia NOxBuster and Purifilter—installed on underground mining trucks operating in a Spanish mine. Emissions of carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2) were quantified using a Testo 350 multigas analyser, while ultrafine particle (UFP) concentrations were measured with an Engine Exhaust Particle Sizer (EEPS-3090) equipped with a thermodiluter. Controlled tests under both idling and acceleration conditions revealed substantial reductions in pollutant emissions: CO decreased by 60–98%, NO by 51–92%, and NO2 by 20–87%, depending on the system and operational phase. UFP concentrations during idling dropped by approximately 90%, from 542,000 particles/cm3 in untreated trucks to below 50,000 particles/cm3 in retrofitted vehicles. Under acceleration, the Proventia NOxBuster achieved reductions exceeding 95%. Conversely, Purifilter-equipped trucks exhibited a counterintuitive increase in UFPs within the 5.6–40 nm range, potentially due to ammonia slip events during selective catalytic reduction (SCR). Despite these discrepancies, both systems demonstrated considerable mitigation potential, albeit highly dependent on exhaust temperature (optimal: 200–450 °C), urea dosing precision, and maintenance protocols. This work underscores the necessity of in situ performance verification, regulatory vigilance, and targeted intervention strategies to protect underground workers effectively. Further investigation is warranted into the long-term health benefits, system durability, and nanoparticle emission dynamics under variable load conditions.

Clean Technologies doi: 10.3390/cleantechnol7040106

Authors: Dan-Teodor Bălănescu Vlad-Mario Homutescu Marius-Vasile Atanasiu

The depletion of fossil fuel reserves and the pollution produced by fuel combustion are major concerns in the energy generation sector. Due to this, waste heat recovery has become a stringent objective in this domain. The current study pursues this objective with regard to gas–steam combined cycle power plants, which are currently viewed as the most advanced technology in fossil fuel power generation. The proposed solution for waste heat recovery is to add an organic Rankine cycle (ORC) power system to the gas–steam combined cycle power plant with a Solar Centaur 40 gas turbine, produced by Solar Turbines, a Caterpillar Company (San Diego, CA, USA). The ORC power system is placed along the path of the flue gas, downstream of the heat recovery steam generator of the combined cycle power plant. R1336mzz (Z), R1233zd (E), and R601a were investigated as working fluids. The performance of the ORC system was analyzed as a function of the degree of superheat. The superheating process was proven to be disadvantageous since it led to performance deterioration. The numerical study showed that the overall efficiency of the combined cycle power plant increased up to 0.014 (1.4%) as a consequence of adding the ORC system, which itself achieves a maximum efficiency of 0.133 (13.3%). The annual fuel (natural gas) savings achievable under these conditions were roughly estimated at 398,185 Nm3/year, equating to annual fuel cost savings of approximately 269,000 EUR/year and an 810 t/year reduction in CO2 emissions.

Clean Technologies doi: 10.3390/cleantechnol7040105

Authors: Franco Hernan Gomez Maria Cristina Collivignarelli Stefano Bellazzi Kelly Cristina Torres Alessandro Abbà Sabrina Sorlini

The absence of domestic wastewater (DWW) treatment in impoverished rural communities of the global south remains a pressing challenge for both public health and environmental sustainability. This study presents a simplified and decentralized treatment chain at laboratory-scale designed under the principles of nature-based solutions (NBS) and the circular economy (CE), emphasizing the integration of the macrophyte Eichhornia crassipes (EC) and bioproducts derived from aloe vera waste (AVW) and soursop seed waste (SSW). The system comprises three sequential stages: (1) coagulation using AVW, which achieved up to 39.9% turbidity reduction; (2) a horizontal flow biofilter system (HFB) employing the aquatic macrophyte EC, which removed 97.9% of fecal coliforms, 82.4% of Escherichia coli, and 99.9% of heterotrophic bacteria; and (3) a tertiary treatment step employing adsorbent derived from SSW, which attained 99.7% methylene blue removal in preliminary tests and an average 97.5% turbidity reduction in DWW. The integrated configuration demonstrates a practical, effective, and replicable approach for decentralized domestic wastewater treatment, fostering local waste valorization, reducing reliance on commercial chemicals, and enhancing water quality in resource-limited rural areas, with potential for scaling to pilot applications in rural communities.

Clean Technologies doi: 10.3390/cleantechnol7040104

Authors: Pablo Menendez-Cabo Hector Garcia-Gonzalez

Diesel-powered machinery is the primary energy source in underground mining, exposing workers to hazardous diesel exhaust emissions. This study evaluates occupational exposure to diesel particulate matter (DPM) and gaseous pollutants (NO, NO2) at an underground mine before and after implementing Diesel Particulate Filters (DPF) and Selective Catalytic Reduction (SCR) in mining equipment. A comprehensive monitoring campaign was conducted, employing elemental carbon (EC) as a tracer for diesel particulate emissions and electrochemical sensors for gas measurements. Results show a substantial reduction in EC concentrations following the implementation of DPFs, with median EC exposure decreasing from 0.145 mg/m3 in 2021 to 0.034 mg/m3 in 2023, and the proportion of samples exceeding the occupational exposure limit (OEL) falling from 90% to 28%. Similarly, SCR implementation led to a 72% reduction in NO2 levels and a 77.5% decrease in NO concentrations in certain equipment; however, NO levels remained persistently high near loaders, suggesting that additional mitigation measures are required. These findings underscore the efficacy of DPF and SCR technologies in improving air quality and reducing occupational exposure in underground mining environments. Nevertheless, persistent NO concentrations and maintenance-related challenges highlight the need for a holistic emission control approach, integrating ventilation improvements, expanded DPF adoption, alternative propulsion systems, and enhanced maintenance protocols. This study provides critical insights into the effectiveness of advanced emission reduction strategies and informs future regulatory compliance efforts in the mining industry.

Clean Technologies doi: 10.3390/cleantechnol7040103

Authors: Benjamin Gazeau Atiq Zaman Roberto Minnuno Faiz Uddin Ahmed Shaikh

Plastic recycling is critical to transitioning toward a circular economy (CE), yet traceability systems for recycled plastics remain unevenly adopted. While effective traceability supports transparency, compliance, and supply chain accountability, its implementation is shaped not only by technological readiness but also by organisational behaviours and strategic priorities. This study explores how traceability adoption is influenced by company size, internal CE strategy, and perceptions of cost, risk, and regulatory demand. A survey of 65 Australian industry stakeholders reveals that 76% of companies with a CE strategy have implemented traceability systems, compared to 42% without. Larger firms report higher adoption rates than small and medium enterprises, largely due to resource advantages and differing interpretations of traceability’s value. Key barriers include high perceived costs, lack of standardised frameworks, and scepticism toward digital tools. Conversely, motivations such as reputational benefits, regulatory alignment, and inter-organisational trust were identified as enablers, alongside emerging technologies like blockchain and chemical tracers. The findings underscore the role of organisational context in shaping traceability practices and highlight the need for tailored interventions. Recommendations include financial incentives, harmonised standards, and sector-specific guidance that address not only technical gaps but behavioural and structural factors limiting uptake. Positioning traceability as an integrated organisational strategy may accelerate its adoption and support broader circular economy outcomes across the plastics value chain.

Clean Technologies doi: 10.3390/cleantechnol7040102

Authors: Yaxuan Peng Meiyan Wang

Chlorobenzoic acids (CBAs) are a group of chlorinated persistent environmental pollutants with hard biodegradability, high water solubility, and well-documented carcinogenic and endocrine-disrupting properties. Electrocatalytic hydrodechlorination (ECH) is a highly efficient method under mild conditions without harmful by-products, but the ECH process commonly requires adding precious metal catalysts such as palladium (Pd). To address the economic constraints and more effective utilization of Pd, a palladium/manganese dioxide (Pd/MnO2) composite catalyst was developed in this study by chemical deposition. This method utilized the excellent electrochemical activity of MnO2 as a carrier as well as the hydrogen storage and activation capacity of Pd. The test showed the optimal Pd loading was 7.5%, and the removal percent of 2,4-dichlorobenzoic acid (2,4-DCBA), a typical CBA, reached 97.3% using 0.5 g/L of Pd/MnO2 after 120 min of electrochemical reaction. Under these conditions, the dechlorination percent can also be as high as 89.6%. A higher current density enhanced the dechlorination efficiency but showed the lower current utilization efficiency. In practical applications, current density should be minimized on the premise of compliance with the water treatment requirement. Mechanistic studies showed that MnO2 synergistically promoted hydrolysis dissociation and hydrogen spillover and facilitated Pd-mediated adsorption of atomic hydrogen (H*) for dehydrogenation of 2,4-DCBA. The presence of MnO2 can effectively disperse the loaded Pd and reduce the amount of Pd via the above process. The catalyst exhibited excellent stability over multiple cycles, and the 2,4-DCBA removal could still reach more than 80% after the five cycles. This work establishes electrocatalytic strategies for effectively reducing Pd usage and maintaining high removal of typical CBAs to support CBA-related water treatment.

Clean Technologies doi: 10.3390/cleantechnol7040101

Authors: Homa Esfandiari Helene Muri Diogo Kramel

Global shipping is an essential, energy-efficient enabler of trade, yet it remains a hard-to-abate sector. With shipping demand projected to continue to rise in the coming decades, identifying scalable and sustainable fuel alternatives is critical. Biofuels, and particularly biomethanol, offer a promising option due to their compatibility with existing infrastructure. However, their sustainability critically hinges on land use impacts. From this Perspective, we argue that biomethanol derived from a dedicated crop could contribute to maritime decarbonisation, with ~71–77% well-to-wake greenhouse gases (GHG) reductions under cropland-only constraints. We further point to the fact that a wider adoption faces challenges such as higher costs, limited availability, and lower energy density relative to fossil fuels. Continued research and monitoring are essential to ensure that biofuel production does not inadvertently contribute to deforestation or biodiversity loss. We underscore the need for spatially sensitive biofuel deployment strategies that align maritime decarbonisation with land-system sustainability and climate objectives.

Clean Technologies doi: 10.3390/cleantechnol7040100

Authors: Reza Vahidzadeh Marta Domini Giorgio Bertanza

In recent years, industrial symbiosis (IS) has gained attention as a strategy to enhance circularity and to reduce the environmental impacts of solid waste management through resource reuse and recovery. Life Cycle Assessment (LCA) is increasingly used to evaluate the environmental performance of such inter-industry collaborations. Given the growing diversity of IS practices and LCA models, this updated review serves as a methodological reference, mapping existing approaches and identifying gaps to guide future research on the systematic assessment of circular strategies. Moreover, it investigates the environmental performance of IS approaches in the field, based on the LCA results of the analyzed case studies. We analyzed 48 peer-reviewed studies to examine how LCA has been applied to model and assess the environmental impacts and benefits of IS in the context of waste management. The literature revealed wide methodological variability, including differences in system boundaries, functional units, and impact categories, affecting comparability and consistency. Case studies confirm that IS can contribute to reducing environmental burdens, particularly with regard to climate change and resource depletion, though challenges remain in modelling the complex inter-organizational exchanges and accessing reliable data. Socio-economic aspects are increasingly considered but remain underrepresented. Future research should focus on methodological improvements, such as greater standardization and the better integration of indirect effects, to strengthen LCA in decision-making and to explore a wider range of scenarios reflecting different stakeholders, analytical perspectives, and the evolution of symbiotic systems over time.

Clean Technologies doi: 10.3390/cleantechnol7040099

Authors: Ramona Elena Tataru-Farmus María Harja Lucia Tonucci Francesca Coccia Michele Ciulla Liliana Lazar Gabriela Soreanu Igor Cretescu

CO2 emissions from various anthropogenic activities have led to serious global concerns (climate change and global warming), and, therefore, CO2 capture by sustainable methods is a priority research topic. One of the most widely used and cost-effective technologies for post-combustion CO2 capture (PCC) is the chemical absorption method, where potassium carbonate solution is proposed as a solvent (with or without the addition of promoters, such as amines). An ecological alternative, presented in this study, is the use of amino acids instead of amines as promoters—alanine (Ala), glycine (Gly) and sarcosine (Sar)—in concentrations of 25% by weight of K2CO3 + 5 or 10% by weight of amino acid salt, thus resulting in the so-called green solvents, which do not show high toxicity and inertness to biodegradability. The studies had as a first objective the characterization of the proposed green solvents, in terms of density and viscosity, and then the comparative testing of their efficiency for CO2 retention from gaseous fluxes containing high CO2 concentrations. The experiments were performed at temperatures of 298 K, 313 K, and 333 K at atmospheric pressure. The best performance was observed with K2CO3 + 5% Sar salt at 313 K, reaching an absorption capacity of 2.58 mol CO2/L solvent, which is a promising improvement over the reference solution based on K2CO3. Increasing the amino acid concentration to 10% generally led to a reduced performance, especially for sarcosine, probably due to an increase in solution viscosity or a possible kinetic inhibition. This study provides valuable experimental data supporting the ecological potential of amino acid-promoted potassium carbonate systems, paving the way for further development of chemisorption processes and their implementation on an industrial scale.

Clean Technologies doi: 10.3390/cleantechnol7040098

Authors: Dhimas Pramadhani Rita Kartika Sari Mahdi Mubarok Apri Heri Iswanto Antonios Papadopoulos Ioanna A. Papadopoulou Dimitrios I. Raptis Tati Karliati Muhammad Adly Rahandi Lubis

The reaction between polyols and diisocyanates forms polyurethane (PU) adhesives. However, these materials are derived from petroleum-based chemicals, whose availability is declining. As an environmentally friendly, renewable, and formaldehyde-free alternative, tannins offer a promising solution. This study aimed to characterize tannin-based polyurethane (TPU) adhesives modified with bio-polyol, analyze their performance, and determine optimal tannin extract formulation for use as a plywood adhesive, as the first step toward developing eco-friendly TPU adhesives. TPU adhesives were made using modified polyvinyl alcohol (PVOH) and tannins at concentration levels of 0%, 10%, 20%, 30%, 40%, and 50%. The analysis is carried out on raw materials, adhesives, and plywood. The results showed that adding tannin extracts had a significant effect on viscosity, tannin solids content, density, delamination, and dry and wet adhesion strength, but not for moisture content. Functional group analysis (FTIR) confirmed that both liquid and solid TPU adhesives contained urethane, hydroxyl, and isocyanate functional groups. The lowest DMA loss modulus was observed in TPU with tannin 20%. Additionally, the highest adhesion strength was achieved with 20% TPU, which correlated with increased wood failure. Based on these findings, PVOH/tannin 20% was considered an effective formula for TPU adhesives.

Clean Technologies doi: 10.3390/cleantechnol7040097

Authors: Jiangqi Qu Ruijun Ren Zhanhui Wu Jie Huang Qingjing Zhang

The rapid expansion of global aquaculture has led to wastewater enriched with nitrogen, phosphorus, organic matter, antibiotics, and heavy metals, posing serious risks such as eutrophication, ecological imbalance, and public health threats. Conventional physical, chemical, and biological treatments face limitations including high cost, secondary pollution, and insufficient efficiency, limiting sustainable wastewater management. Algal–bacterial symbiotic systems (ABSS) provide a sustainable alternative, coupling the metabolic complementarity of microalgae and bacteria for effective pollutant mitigation and concurrent biomass valorization. Immobilizing microbial consortia within carrier materials enhances system stability, tolerance to environmental changes, and scalability. This review systematically summarizes the pollution characteristics and ecological risks of aquaculture effluents, highlighting the limitations of conventional treatment methods. It focuses on the metabolic cooperation within ABSS, including nutrient cycling and pollutant degradation, the impact of environmental factors, and the role of immobilization carriers in enhancing system performance and biomass resource valorization. Despite their potential, ABSS still face challenges related to mass transfer limitations, complex microbial interactions, and difficulties in scale-up. Future research should focus on improving environmental adaptability, regulating microbial dynamics, designing intelligent and cost-effective carriers, and developing modular engineering systems to enable robust and scalable solutions for sustainable aquaculture wastewater treatment.

Clean Technologies doi: 10.3390/cleantechnol7040096

Authors: Alexis Sagastume Gutiérrez Juan José Cabello Eras Daniel David Otero Meza

Energy transition is crucial for climate change mitigation and Sustainable Development Goals (SDGs), and has been a key government focus in Colombia since 2022, which must carefully consider its energy roadmap. This study evaluates three potential scenarios for achieving nearly 100% renewable energy by 2035: replacing fossil fuels with biofuels, using hydrogen for transport and industrial heat, and relying entirely on renewable electricity. This paper discusses these scenarios’ technical, economic, and social challenges, including the need for substantial investments in renewable energy technologies and energy storage systems to replace fossil fuels. The discussion highlights the importance of balancing energy security, environmental concerns, and economic growth while addressing social priorities such as poverty eradication and access to healthcare and education. The results show that while the Colombian government’s energy transition goals are commendable, a rapid energy transition requires 4 to 8 times the government’s projected 34 billion USD investment, making it economically unfeasible. Notably, focusing on wind, photovoltaic, and green hydrogen systems, which need storage, is too costly. Furthermore, replacing fossil fuels in transport is impractical, though increasing biofuel production could partially substitute fossil fuels. Less energy-intensive alternatives like trains and waterway transport should be considered to reduce energy demand and carbon footprint.

Clean Technologies doi: 10.3390/cleantechnol7040095

Authors: An Liu Buer Qi Lisbeth Ku

The fashion and textile industry (FTI) is a significant contributor to greenhouse gas emissions, resource consumption, and waste generation, necessitating sustainable alternatives. Chitosan, a biodegradable and renewable biopolymer, has shown potential in reducing environmental impact throughout the textile lifecycle. However, existing studies often focus on isolated applications rather than its broader role in industrial sustainability. This review synthesises findings from 142 academic studies to assess chitosan’s applications in textile production, dyeing, finishing, and waste management, emphasising its impact on energy efficiency, carbon reduction, and resource circularity. Chitosan’s biodegradability, antimicrobial properties, and affinity for sustainable dyeing offer a viable alternative to synthetic materials while also enhancing wastewater treatment and eco-friendly finishing techniques. By evaluating its contributions to sustainable manufacturing, this review highlights its potential in supporting decarbonisation and circular economy transitions within the textile sector, while also identifying challenges for future research.

Clean Technologies doi: 10.3390/cleantechnol7040094

Authors: Plamen Stanchev Nikolay Hinov

Fuel cells are highly efficient electrochemical devices that convert the chemical energy of fuel directly into electrical energy, while generating minimal pollutant emissions. In recent decades, they have established themselves as a key technology for sustainable energy supply in the transport sector, stationary systems, and portable applications. In order to assess their real contribution to environmental protection and energy efficiency, a comprehensive analysis of their life cycle, Life Cycle Assessment (LCA) is necessary, covering all stages, from the extraction of raw materials and the production of components, through operation and maintenance, to decommissioning and recycling. Particular attention is paid to the environmental challenges associated with the extraction of platinum catalysts, the production of membranes, and waste management. Economic aspects, such as capital costs, the price of hydrogen, and maintenance costs, also have a significant impact on their widespread implementation. This manuscript presents detailed mathematical models that describe the electrochemical characteristics, energy and mass balances, degradation dynamics, and cost structures over the life cycle of fuel cells. The models focus on proton exchange membrane fuel cells (PEMFCs), with possible extensions to other types. LCA is applied to quantify environmental impacts, such as global warming potential (GWP), while the levelized cost of electricity (LCOE) is used to assess economic viability. Particular attention is paid to the sustainability challenges of platinum catalyst extraction, membrane production, and end-of-life material recovery. By integrating technical, environmental, and economic modeling, the paper provides a systematic perspective for optimizing fuel cell deployment within a circular economy.

Clean Technologies doi: 10.3390/cleantechnol7040093

Authors: Viral Ajay Modi Qiang Xu Sujing Wang

The Allam power cycle is a groundbreaking elevated-pressure power generation unit that utilizes oxygen and fossil fuels to generate low-cost electricity while capturing carbon dioxide (CO2) inherently. In this project, we utilize the CO2 generated from the Allam cycle as feedstock for a newly envisioned industrial complex dedicated to producing renewable hydrocarbons. The industrial complex (FAAR) comprises four subsystems: (i) a Fischer–Tropsch synthesis plant (FTSP), (ii) an alkaline water electrolysis plant (AWEP), (iii) an Allam power cycle plant (APCP), and (iv) a reverse water-gas shift plant (RWGSP). Through effective material, heat, and power integration, the FAAR complex, utilizing 57.1% renewable energy for its electricity needs, can poly-generate sustainable hydrocarbons (C1–C30), pure hydrogen, and oxygen with near-zero emissions from natural gas and water. Economic analysis indicates strong financial performance of the development, with an internal rate of return (IRR) of 18%, a discounted payback period of 8.7 years, and a profitability index of 2.39. The complex has been validated through rigorous modeling and simulation using Aspen Plus version 14, including sensitivity analysis.