Editorial: Resource Utilization of Solid Waste and Circular Economy

1. Introduction

The rapid pace of industrialization and urbanization worldwide has led to an unprecedented increase in the generation of solid waste, posing serious challenges to environmental sustainability and resource security [1]. Traditional disposal practices such as landfilling and incineration not only consume valuable land and energy but also release hazardous pollutants, undermining global efforts toward carbon neutrality and sustainable development [2,3]. Against this backdrop, the concept of a circular economy, which promotes the reduction, reuse, and recycling of waste, has emerged as a transformative paradigm for closing material loops and enhancing resource efficiency [4].

Solid waste, once considered an environmental liability, is now increasingly viewed as a valuable secondary resource [5]. Technological innovations in materials science, chemical engineering, environmental management, and digital intelligence have paved the way for the efficient valorization of various types of solid waste—including industrial by-products, municipal waste, biomass residues, and e-waste—into construction materials, energy carriers, functional materials, and other high-value products [6,7,8]. However, despite these promising developments, the transition toward a fully circular economy remains constrained by technical, economic, regulatory, and social barriers.

This Special Issue of Applied Sciences, titled “Resource Utilization of Solid Waste and Circular Economy”, aims to present the latest research progress in solid waste valorization and circular economy practices. It brings together 11 contributions from multidisciplinary teams across the globe, exploring innovative materials, green technologies, system optimization methods, and policy-oriented strategies that support waste minimization and sustainable resource use.

2. An Overview of Published Articles

2.1. Valorization of Solid Waste in Construction Materials

Several studies in this Special Issue explore the conversion of industrial waste into functional construction materials. Yu et al. investigated the effects of water-reducing agents on the mechanical performance of foamed phosphogypsum, a promising material for lightweight and sustainable construction (Contribution 1). Similarly, Haddad et al. assessed the use of concrete washing fines in mortar mixtures, highlighting their impact on microstructure and carbon footprint (Contribution 2). Zhou et al. developed a dual chemical–physical approach for curing soluble phosphorus and fluorine in phosphogypsum, providing insights into stabilization mechanisms and potential field applications (Contribution 6). Meanwhile, Souza Oliveira et al. introduced a clinker-free cement concept that eliminates Portland clinker altogether, showcasing a low-carbon alternative to traditional cementitious materials (Contribution 5).

2.2. Waste-to-Energy and Carbonization Processes

A noteworthy contribution by Yang et al. presents a method to repurpose graphite waste from Acheson furnaces as anode materials for lithium-ion batteries (Contribution 3). This study not only enhances the value of carbonaceous waste but also advances sustainable battery design through circular materials engineering. Cavali et al. reported on the co-hydrothermal carbonization of sawdust and sewage sludge, producing hydrochar with effective adsorption properties while assessing the ecotoxicity of process water (Contribution 7). Complementing this, Yu et al. provided a comprehensive review of hydrothermal oxidation technologies for sludge treatment, discussing operational challenges, pollutant removal efficiency, and energy recovery potential (Contribution 4).

2.3. Circular Economy and Digital Waste Management

Beyond material transformation, this Special Issue also examines system-level strategies. AlJaber et al. analyzed circular economy implementation in the building sector, emphasizing stakeholder roles, feasibility factors, and lifecycle considerations (Contribution 8). Pinho et al. addressed wood waste management in the furniture industry, outlining pathways for revalorization and regulatory alignment toward circularity (Contribution 9). Digital technologies are playing a crucial role in modern circular economy systems. Vesga Ferreira et al. proposed a low-cost capacitive sensing system coupled with machine learning algorithms for automatic waste classification (Contribution 11). This study demonstrates how AI-powered sensing can reduce human intervention and improve recycling accuracy. Finally, Moreno-Bermedo et al. explored the reuse of tea waste as a catalyst support in the electro-Fenton process for removing emerging organic contaminants (Contribution 10). This work exemplifies the potential of biomass residues in environmental remediation within circular economy frameworks.

3. Conclusions and Outlook

The contributions gathered in this Special Issue showcase a broad spectrum of innovative approaches to solid waste utilization, spanning from construction materials and energy systems to digital waste management and pollutant removal technologies. These studies reflect a growing global consensus on the urgency of transitioning from a linear to a circular economy, wherein waste is not an end product but a resource to be continuously reintegrated into productive cycles.

While significant technical progress has been achieved, challenges persist in scaling up laboratory findings, ensuring economic feasibility, aligning regulatory frameworks, and fostering cross-sector collaboration. Future research should focus on:

• Integrating life cycle assessment (LCA) and techno-economic analysis in material design;
• Enhancing digitalization for smart waste monitoring and logistics;
• Strengthening policy tools and public engagement to promote behavioral shifts;
• Developing standards and certification systems for waste-derived products.

We hope that this Special Issue will serve as a valuable resource for researchers, engineers, policymakers, and industry practitioners striving to build more sustainable, resource-efficient, and resilient societies through the lens of circular economy and solid waste valorization.