Special Issue on Waste Valorization, Green Technologies and Circular Economy I

1. Introduction

The rapid increase in the global population has significantly intensified waste generation, leading to critical environmental and resource management challenges. Consequently, traditional linear economic models based on the “take–make–dispose” approach are no longer sustainable. In recent years, the concepts of waste valorization, green technologies, and the circular economy have gained increasing attention as integrated strategies for sustainable development [1].

Waste valorization refers to the process of transforming residues into valuable products such as energy, chemicals, and secondary raw materials. Rather than being treated as an endpoint, waste is increasingly recognized as a resource that can be reintegrated into production cycles. It is well established that waste valorization plays a central role in the circular economy systems by enabling the recovery of value from diverse waste streams—including food, agricultural, and industrial residues—while simultaneously reducing environmental impacts and promoting resource efficiency [2].

Green technologies are fundamental drivers of waste valorization processes, providing environmentally friendly and resource-efficient solutions for the treatment of solid and liquid wastes. This concept encompasses a wide range of biological, physical, and chemical processes, such as anaerobic digestion, fermentation, membrane separation, electrochemical treatments, and supercritical fluid techniques, among others [3,4]. Nevertheless, their environmental performance is context-dependent and must be assessed in terms of energy demand, emissions, and life cycle impacts. In this context, digital tools and process optimization are increasingly improving the efficiency and sustainability of waste-to-resource strategies, thereby supporting the transition toward circular economy frameworks [5,6].

The circular economy model integrates waste valorization and green technologies into a systemic approach aimed at closing material and energy loops. This framework promotes the reuse, recycling, and regeneration of resources, extending product life cycles and reducing waste generation [7].

Recent studies demonstrate that circular economy strategies are essential for reducing greenhouse gas emissions, improving resource efficiency, and fostering sustainable industrial systems. Furthermore, innovative frameworks are being developed to optimize circular systems, enabling the transformation of waste streams into valuable inputs across interconnected sectors [8]. In the current context, this Special Issue aims to compile the latest research addressing key challenges in wastewater and solid waste treatment, with a particular focus on waste valorization, green technologies, and circular economy approaches. This Editorial includes a review of the 10 papers published in the Special Issue.

2. Valorization of Urban and Agro-Industrial Waste Streams

The valorization of urban and agro-industrial waste streams serves as a key pillar of the circular economy model, enabling the transformation of environmental burdens into high-value opportunities through sustainable technologies and advanced biological processes. In urban contexts, an innovative green chemical approach facilitated the recovery of high-quality cellulose acetate from discarded cigarette butts with a 30% yield, effectively removing heavy metals and toxic residues like nicotine (contribution 1). Additionally, decentralized community composting programs, such as the "CaMPuSTAJE" initiative, successfully converted university canteen food waste and campus pruning debris into stable, pathogen-free organic amendments that meet European safety standards for local landscaping (contribution 2). Furthermore, municipal forestry residues and discarded horticultural vegetables provided sugar-rich substrates for the cultivation of oleaginous yeast (Rhodosporidium toruloides), producing microbial oils and carotenoids such as β-carotene for industrial applications (contribution 3).

Within the agro-industrial sector, retail meat waste was efficiently fractionated via enzymatic hydrolysis using proteases and lipases to recover digestible protein hydrolysates, collagen, and unsaturated fatty acids, with yields reaching up to 0.8 kg of product per kg of dry waste (contribution 4). In addition, cottonseed and legume by-products were nutritionally upgraded through solid-state fermentation with Pleurotus ostreatus, which increased protein content by nearly 35% and dramatically reduced toxic gossypol levels by over 9-fold, making the resulting material a safe animal feed supplement (contribution 5). Finally, the integration of aquaculture, urban, and swine farm effluents into hydroponic and aquaponic systems facilitated a nutrient recovery of up to 95% for nitrogen, phosphorus, and potassium, demonstrating water-use efficiency significantly superior to conventional agriculture while ensuring the production of safe and stable food crops such as strawberries, lettuce, and papaya (contribution 6).

In summary, these strategies turn waste into valuable resources, reducing environmental impact while creating economic opportunities. Their success will depend on scalability, supportive policies, and integration into local systems to advance a truly circular and sustainable economy.

3. Energy from Waste

Recent research in Italy demonstrated advanced methodologies for transforming urban waste, specifically food waste and municipal sewage sludge, into high-value renewable energy and biochemicals. In a full-scale anaerobic digestion plant in Cosenza, a multi-stage cleanup system—comprising micro-oxygenation, chemical scrubbing, cooling, and activated carbon—successfully purified raw biogas by removing harmful trace contaminants such as hydrogen sulfide (H₂S) and volatile organic compounds (VOCs). These contaminants exhibited seasonal fluctuations, with H₂S levels peaking in winter due to protein-rich food waste and VOCs increasing during the same period because of higher citrus fruit consumption. This optimized pre-treatment allowed the production of biomethane with >98% purity, which is suitable for the Italian gas grid and the transport sector (contribution 7). Simultaneously, studies in the Apulia region highlighted the potential of sewage sludge and scum as lipid-rich feedstocks, achieving lipid recovery rates of 92–99% through methods like hexane extraction or thermal centrifugation. Using an AlCl₃ catalyst under mild conditions, these lipids were converted into biodiesel (FAMEs) with a yield exceeding 98%. Furthermore, this process facilitated the recovery of high-value by-products like methyl estolides and methyl 10-hydroxystearate for use as biolubricants, with the valorization of sewage scum estimated to generate a significant profit of approximately 76 € per ton (contribution 8). Collectively, these findings illustrate a robust circular economy framework that converts hazardous municipal waste into sustainable energy vectors and industrial chemicals.

4. Assessment and Management of Circularity

The transition towards a circular bioeconomy is being accelerated through the integration of advanced computational optimization and product-level monitoring frameworks designed to enhance waste governance and resource efficiency. The integration of Multi-Criteria Decision Analysis (MCDA) and Artificial Intelligence (AI) serves as a transformative approach to optimize the governance of reverse supply chains for solid waste (RSCSW), employing hybrid models such as neural networks and genetic algorithms to improve operational efficiency, reduce costs, and enhance waste traceability (contribution 9). Complementing these systemic governance strategies, the BIORADAR framework provides essential micro-level metrics to measure the circularity of specific products, such as bio-based fertilizers, through indicators like the Circular Index and the Circularity Indicator of Nutrient (CIN) (contribution 10). Together, these methodologies address existing knowledge gaps by providing policymakers and managers with the necessary tools to navigate regulatory barriers, validate sustainability claims, and implement evidence-based practices that promote the reintegration of waste into production cycles.

5. Looking Ahead

Overall, the transition from linear to circular systems is expected to accelerate in the coming years, driven by technological innovation, policy support, and increasing environmental challenges. Future efforts should focus on scaling up resource recovery solutions, improving process efficiency, and ensuring their adaptability across different sectors and waste streams. These developments are closely aligned with global sustainability frameworks such as the United Nations Sustainable Development Goals, particularly those related to responsible consumption and production (SDG 12), climate action (SDG 13), clean energy (SDG 7), and sustainable cities and communities (SDG 11).

At the European level, these advancements are strongly supported by strategic initiatives such as the European Commission’s European Green Deal and the Circular Economy Action Plan, which promote resource efficiency, waste reduction, and the development of sustainable industrial systems. In this context, future progress will depend on strengthening the link between research, industry, and policy, as well as on the implementation of robust monitoring frameworks to assess circularity performance.

Ultimately, the consolidation of these approaches will contribute to more resilient and resource-efficient systems, where waste is consistently redefined as a valuable input. This will not only support environmental sustainability and climate neutrality targets in Europe, but also enhance economic competitiveness and social well-being in line with global sustainability agendas.