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Waste Valorization, Green Technologies and Circular Economy

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A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Environmental Sciences".

Deadline for manuscript submissions: closed (20 July 2025) | Viewed by 12787

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Dr. Amanda Laca Pérez

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Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain
Interests: enzymatic hydrolysis; fungi; biological treatment; lignocellulosic biomass; cellulose; hemicellulose; lignin
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Dr. Yolanda Patiño

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Catalysis, Reactors and Control Research Group (CRC), Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain
Interests: emerging pollutants; adsorption process; electrochemical degradation; sludge valorization
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Dr. Daniel Sol

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Guest Editor

Department of Chemical and Environmental Engineering, University of Oviedo, C/Julián Clavería s/n, 33006 Oviedo, Spain
Interests: environmental technologies; material science; waste management; water quality

Special Issue Information

Dear Colleagues,

Global waste generation is continuing to grow today, which entails the need to develop new strategies to convert residues into valuable materials. This affects various different sectors; for example, it is well known that one third of the world’s food produced for human consumption becomes waste every year, whereas sewage sludge management requires important costs, both in economic and environmental terms. In this context, waste valorization is an excellent option to embrace the strategy of the circular economy and sustainable development.

In view of the current situation, this Special Issue aims to compile the latest research on waste valorization topics, including but not limited to the following:

Development of green technologies to waste management;
Sludge valorization;
Wastewater reuse;
Obtention of bioactive compounds from food industry residues;
Environmental assessment of waste management;
Physical, chemical, and biological treatments for waste valorization.

Dr. Amanda Laca Pérez
Dr. Yolanda Patiño
Dr. Daniel Sol
Guest Editors

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Keywords

waste valorization
life cycle assessment
circular economy
recycling
sustainable development

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Published Papers (9 papers)

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Research

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25 pages, 1824 KiB

Open AccessArticle

Measuring the Circularity of Bio-Based Fertilizers: Applying the BIORADAR Product Circularity Monitoring Framework

by Hasler Iglesias, Ana Paredes Ortiz, Ángeles Pereira, David Fernández-Gutiérrez and Andrés J. Lara-Guillén

Appl. Sci. 2025, 15(14), 7701; https://doi.org/10.3390/app15147701 - 9 Jul 2025

Viewed by 227

Abstract

The transition to the circular economy (CE) is one of the EU’s current strategic policies to improve its competitiveness and sustainability. While the EU has developed a framework for monitoring overall progress toward the CE, there are gaps in monitoring specific priority sectors, such as the bioeconomy. In order to support industry and policymakers in this sector, this paper presents the application of the BIORADAR’s product circularity monitoring framework to five bio-based fertilizers. The framework is composed of two publicly available indicators: the circular index and the circularity indicator of nutrient; and two new indicator proposals: the biodegradable content and the nutrient slow-release index. Making use of life cycle inventories and supplementary data from the scientific literature, these four indicators were calculated for algae biomass, compost, feather meal, spent mushroom substrate, and wood vinegar. The framework proved to be useful for measuring the circularity at the product level for bio-based fertilizers, especially shedding light on the virgin non-renewable materials consumption, waste generation, biodegradability, nutrient recovery process efficiency, and nutrient release speed. It constitutes the first approach to measuring circularity tailored to bio-based fertilizer. By incorporating it into eco-design, innovation, and managerial decision-making processes, key stakeholders can rely on guiding metrics to support their transition toward higher circularity levels. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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14 pages, 1053 KiB

Open AccessArticle

Agro-Food and Lignocellulosic Urban Wastes as Sugar-Rich Substrates for Multi-Product Oil-Based Biorefineries

by Alberto Rodríguez-López, María José Negro, José Luis Fernández-Rojo, Ignacio Ballesteros and Antonio D. Moreno

Appl. Sci. 2025, 15(13), 7240; https://doi.org/10.3390/app15137240 - 27 Jun 2025

Viewed by 292

Abstract

The effective use of biowaste resources becomes crucial for the development of bioprocessing alternatives to current oil- and chemical-based value chains. Targeting the development of multi-product biorefinery approaches benefits the viability and profitability of these process schemes. Certain oleaginous microorganisms, such as oleaginous red yeast, can co-produce industrially relevant bio-based products. This work aims to explore the use of industrial and urban waste as cost-effective feedstock for producing microbial oil and carotenoids using Rhodosporidium toruloides. The soluble fraction, resulting from homogenization, crushing, and centrifugation of discarded vegetable waste, was used as substrate under a pulse-feeding strategy with a concentrated enzymatic hydrolysate from municipal forestry residue obtained after steam explosion pretreatment (190 °C, 10 min, and 40 mg H₂SO₄/g residue). Additionally, the initial nutrient content was investigated to enhance process productivity values. The promising results of these cultivation strategies yield a final cell concentration of 36.4–55.5 g/L dry cell weight (DCW), with an intracellular lipid content of up to 42–45% (w/w) and 665–736 µg/g DCW of carotenoids. These results demonstrate the potential for optimizing the use of waste resources to provide effective alternative uses to current biowaste management practices, also contributing to the market of industrially relevant products with lower environmental impacts. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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17 pages, 3038 KiB

Open AccessArticle

Integrated Biowaste Management by Composting at a University Campus: Process Monitoring and Quality Assessment

by Cristina Álvarez-Alonso, María Dolores Pérez-Murcia, Encarnación Martínez-Sabater, Ignacio Irigoyen, Joseba Sánchez-Arizmendiarrieta, Ramón Plana, Marga López, Isabel Nogués and María Ángeles Bustamante

Appl. Sci. 2025, 15(6), 2910; https://doi.org/10.3390/app15062910 - 7 Mar 2025

Viewed by 928

Abstract

The sustainable management of biowaste, mainly food and pruning waste, is currently a challenge due to the increase in its production. The CaMPuSTAJE program, which has been implemented on the campus of the Public University of Navarre (UPNA) since 2019, is an excellent example of how the institution is addressing its strategic interests in sustainable waste management. The principal aim of this program is to manage the biowastes generated by the campus canteens through a simple community composting facility, involving UPNA students and graduates. This program aims to promote experiential learning and applied research in sustainability and circular economy, managing their own waste in a circular and local way. Thus, four composting sets of the CaMPuSTAJE program were evaluated by monitoring the process and the main chemical properties of the composting samples. Also, final composts were fully characterized to ensure the process reproducibility and efficiency and the absence of any hazard in the end-products. The final composts showed a significant agronomic quality, had low content of potentially toxic elements, and were free from phytotoxicity, thus being able to be reintroduced as an organic amendment at the university campus itself. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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14 pages, 2755 KiB

Open AccessArticle

Integrated Process for Ballot Bin Waste Valorization for High-Quality Cellulose Acetate Recovery

by Niccolò Lamanna, Giovanni Zuccante, Massimiliano Rossetti, Andrea Franzetti, Carlo Santoro and Luca Zoia

Appl. Sci. 2024, 14(23), 10955; https://doi.org/10.3390/app142310955 - 26 Nov 2024

Viewed by 1333

Abstract

Cigarette butt littering poses a significant environmental challenge, with billions of butts discarded each year, fouling ecosystems with slow-to-decompose cellulose acetate filters that absorb and release harmful compounds. In response, an innovative, sustainable approach for valorizing ballot bin waste (BBW) by extracting high-quality cellulose acetate from cigarette butts was investigated. This green approach eliminates the need for hazardous acids and toxic solvents, resulting in a yield of 30% (w/w) and a degree of substitution (DS) of 2.0–2.5, which is comparable to pure cellulose acetate. The following four essential processes are involved in this process: filter separation, water washing to remove impurities, ethanol purification, and acetone precipitation of the cellulose acetate. This approach not only mitigates environmental harm, but also supports circular economy goals by transforming waste into valuable resources. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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12 pages, 763 KiB

Open AccessArticle

Solid-State Fermentation Initiated by Pleurotus ostreatus of a Cottonseed Cake and Lathyrus clymenum Pericarp Mixture: Impact on Nutritional Profile and Gossypol Content

by Christos Eliopoulos, Ioanna Langousi, Eleni Kougia, Georgia Saxami, Giorgos Markou, Serkos A. Haroutounian and Dimitrios Arapoglou

Appl. Sci. 2024, 14(12), 5066; https://doi.org/10.3390/app14125066 - 11 Jun 2024

Cited by 6 | Viewed by 1692

Abstract

Solid-State fermentation (SSF) is a valuable process used for the enhancement of the nutritional profile of agro-industrial by-products. The main objective of the present study concerns the exploitation of a mixture consisting of Cottonseed Cake (CSC) and Lathyrus clymenum pericarp (LCP) at a ratio of 80–20% w/w, which was utilized as substrate for the initiated by Pleurotus ostreatus SSF process. The final goal is the improvement of their nutritional value and the parallel reduction in their gossypol content. The obtained results revealed a statistically significant increase (p < 0.05) in protein content by 34.91%, while 1,3-1,6 β-glucans exceeded a 5-fold statistically significant increment (p < 0.05) at Day 11. Furthermore, lignin was reduced significantly (−26.71%) at Day 11. Free gossypol’s presence was lowered by 12.45%, while SSF presented a profound effect concerning the total gossypol level since the latter underwent a statistically significant reduction (p < 0.05) that exceeded 9-fold at Day 11. The study herein highlights SSF’s efficiency as a potential means to reduce free and total gossypol content with a parallel upgrade of its nutritional value. The fermentation outcome reveals its potential as a feed supplement and contributes to the reduction in the environmental footprint within the framework of a circular economy. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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11 pages, 4545 KiB

Open AccessArticle

Energy Recovery from Municipal Sewage Sludge: An Environmentally Friendly Source for the Production of Biochemicals

by Luigi di Bitonto, Antonella Angelini and Carlo Pastore

Appl. Sci. 2024, 14(12), 4974; https://doi.org/10.3390/app14124974 - 7 Jun 2024

Cited by 1 | Viewed by 1486

Abstract

In this work, a detailed analysis of the lipid component in primary sludge and sewage scum up-taken from several wastewater treatment plants located in southern Italy was carried out. Lipids in the primary sludge accounted for 200–250 mg/g of the total solids (TS), with calcium soaps as a main component (70–82%), while total lipids made up about 350–500 mg/g_TS in the sewage scum and consisted mainly of FFAs (45–60%) and calcium soaps (27–35%). In addition, estolides and 10-hydroxystearic acid were also quantified. A specific valorization process was then developed and tested for either primary sludge or sewage scum. In detail, lipids were first recovered, chemically activated by the addition of acids (calcium soaps were converted to free fatty acids) and finally reacted with methanol to obtain methyl esters. The lipid recovery from primary sludge and sewage scum was particularly efficient (recoverability of 92–99%). The conversion of the starting acids into FAMEs (yield > 98%) was achieved under very mild conditions (70 °C, 2 h) with AlCl₃·6H₂O as a catalyst. Biodiesel (according to EN14214), methyl 10-hydroxystearate and methyl estolides were efficiently isolated by distillation under vacuum. Finally, a feasibility study of the proposed processes was carried out to evaluate their possible integration into a wastewater treatment plant, critically analyzing both the positive aspects and the relative limitations. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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13 pages, 479 KiB

Open AccessArticle

Combined Pre-Treatment Technologies for Cleaning Biogas before Its Upgrading to Biomethane: An Italian Full-Scale Anaerobic Digester Case Study

by Adolfo Le Pera, Miriam Sellaro, Crescenzo Pellegrino, Carlo Limonti and Alessio Siciliano

Appl. Sci. 2024, 14(5), 2053; https://doi.org/10.3390/app14052053 - 29 Feb 2024

Cited by 4 | Viewed by 1654

Abstract

Biogas produced by anaerobic digestion contains different types of contaminants, and it is preferable to eliminate those contaminants before biogas’ energetic valorization or upgrading to biomethane as they are harmful to human health and detrimental to combustion engines. This study presents the biogas cleanup system optimized by an Italian full-scale anaerobic digester treating food waste (FW) and represented by micro-oxygenation, chemical scrubber, cooling, and activated carbon sections. The cleaned biogas is upgraded to biomethane using a membrane-based upgrading unit and injected into the natural gas network for transport sector use. H₂S and volatile organic compound (VOC) concentration in raw biogas was reduced from an annual average value of 1207 ppm_v and 895 mg/Nm³, respectively, to below 0.1 mg/Nm³ in the final biomethane. In the summer, the H₂S average content in raw biogas was 833 ppm_v due to a greater presence of low-sulfur-containing vegetables in FW, while in the winter it was an average of 1581 ppm_v due to a larger portion of protein-containing FW. On the other hand, raw biogas VOC content in the winter was an average of 1149 mg/Nm³, with respect to 661 mg/Nm³ in the summer, due to the greater consumption of citrus fruits containing high amount of terpene compounds. The concentration of other trace contaminants, such as HCl, NH_3, and siloxanes, was lowered from 17, 36, and 0.6 mg/Nm³ in raw biogas, respectively, to below 0.1 mg/Nm³ in the final biomethane. All the considerations and evaluations underlying the technological and plant engineering choices together with the individuation of the best operating conditions are discussed. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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21 pages, 1662 KiB

Open AccessArticle

A Green Technology Approach Using Enzymatic Hydrolysis to Valorize Meat Waste as a Way to Achieve a Circular Economy

by Miguel Angulo and Mª Carmen Márquez

Appl. Sci. 2023, 13(15), 8763; https://doi.org/10.3390/app13158763 - 29 Jul 2023

Cited by 9 | Viewed by 2734

Abstract

The retail meat industry produces a significant amount of waste, containing proteins, lipids, and other elements that could serve as the basis for other products. This work presents the results of research on the enzymatic hydrolysis of meat waste as a green technology to obtain products with added value as a substitute for other raw materials. pH, temperature, the protease/proteinic substrate ratio (Eo/So), and the lipolase/lipidic substrate ratio (Eo’/So’) were studied as process variables for hydrolysis of proteins and lipids, respectively. Hydrolysis for the recovery of proteins (as protein hydrolysates or collagen) was carried out with the protease Alcalase; pH around 8.0, temperature around 50 °C, and Eo/So around 0.16 AU/g were the optimum process variables’ values for obtaining high amounts of recovered proteins and peptides that are easily digestible and have a pleasant taste. The lipase Resinase was used to hydrolyze the lipids; a clear relationship was observed between Eo’/So’ and the amounts of recovered fatty acids. The optimum process variables’ values were found to be Eo’/So’ around 0.83 kLU/g, pH around 8.0 and temperature around 50 °C. Unsaturated fatty acids prevailed in the final product. For the simultaneous recovery of protein hydrolysates, collagen, and fatty acids, a combination of Alcalase and Resinase was used; the process variables examined included the optimal range of values for Eo/So and Eo’/So’, as well as pH and temperature that were suggested in research for both Alcalase and Resinase, separately. The results showed that the simultaneous process was mainly influenced by the Eo/So and Eo’/So’ ratios, instead of being influenced by the pH and temperature values which were less influential. For Eo/So = 0.16 AU/g, Eo’/So’ = 1.11 kLU/g, pH = 7.5, T = 50 °C, the maximum amounts of products (0.8 kg by kg of dry meat waste) were obtained more economically, where the whole of the proteins and lipids in meat waste were practically recovered. Therefore, in order to preserve a circular economy for retail meat waste, enzymatic hydrolysis is appealing and environmentally friendly. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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22 pages, 1166 KiB

Open AccessReview

Artificial Intelligence and MCDA in Circular Economy: Governance Strategies and Optimization for Reverse Supply Chains of Solid Waste

by Joel Joaquim de Santana Filho, Arminda do Paço and Pedro Dinis Gaspar

Appl. Sci. 2025, 15(9), 4758; https://doi.org/10.3390/app15094758 - 25 Apr 2025

Cited by 2 | Viewed by 1029

Abstract

The integration of multi-criteria decision analysis (MCDA) and Artificial Intelligence (AI) is revolutionizing the governance of reverse supply chains for solid waste (RSCSW) within a circular economy framework. However, the existing literature lacks a systematic assessment of the effectiveness of these methods compared to traditional waste management practices. This study conducts a systematic literature review (SLR), following PRISMA guidelines and the P.I.C.O. framework, to investigate how MCDA and AI can optimize governance, operational efficiency, and the sustainability of RSCSW. After collecting 1139 articles, 22 were selected and used for analysis. The results indicate that hybrid MCDA-AI models, employing techniques, such as TOPSIS, AHP, neural networks, and genetic algorithms, enhance decision-making automation, reduce costs, and improve waste traceability. Nevertheless, regulatory barriers and technological challenges still hinder large-scale adoption. This study proposes an innovative framework to address these gaps and drive evidence-based public policies. The findings provide guidelines for policymakers and managers, contributing to the Sustainable Development Goals (SDGs) agenda and advancements in circular economy governance. Full article

(This article belongs to the Special Issue Waste Valorization, Green Technologies and Circular Economy)

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