Sustainable Wastewater Treatment and the Circular Economy

1. Introduction to the Special Issue

At present, the issue of restricted resources and the pressure on the environment are more severe than ever [1,2]. These challenges demand innovative solutions to achieve sustainable development, especially in wastewater treatment [3,4]. A circular economy presents a promising pathway to address these challenges by integrating waste recovery and resource reuse [5,6,7,8]. In the context of wastewater treatment, the quantity of biomass waste, such as sewage sludge and organic byproducts, has risen sharply in recent years, resulting in serious environmental pollution and public health concerns [9]. Biomass waste from wastewater treatment poses a dual challenge: it is both a pollutant and a potential resource [10,11,12]. The improper disposal of this waste can lead to the contamination of soil and water resources, the release of greenhouse gasses, and increased treatment costs. Consequently, the sustainable management and utilization of biomass waste are critical [13,14]. By adopting the principles of the circular economy, this waste can be transformed into valuable resources, providing both environmental and economic benefits [15,16]. Thus, the utilization of biomass waste has practical significance and aligns with the goals of sustainable wastewater management.

Significant progress has been made in recent years in the research on, and the development of technology for, biomass waste recovery. The current approaches primarily focus on thermal conversion, as well as aerobic and anaerobic biotechnologies [8,10,17]. Thermal conversion methods, including pyrolysis and gasification, can produce energy-dense fuels such as biochar, syngas, and bio-oil [18,19]. These products can be used as renewable energy sources, contributing to energy security and reducing dependence on fossil fuels. Similarly, biological processes such as anaerobic digestion and composting can convert organic components of biomass waste into biogas or nutrient-rich fertilizers, fostering resource recovery and reducing environmental pollution [8,17]. Moreover, wastewater treatment biomass waste offers a unique opportunity to recover high-value compounds, such as phosphates, nitrogen, and even rare metals, which are essential for agriculture and industrial applications [10,20,21]. Despite these advancements, challenges remain in scaling up these technologies and integrating them into existing infrastructure [19,22]. Economic feasibility, policy support, and public awareness are critical factors that influence the adoption of circular practices in wastewater management [15,16]. Additionally, the environmental trade-offs of some technologies, such as emissions from thermal conversion, must be carefully managed [18,19].

Therefore, the sustainable utilization of biomass waste from wastewater treatment is crucial for advancing the principles of the circular economy. By leveraging innovative technologies and fostering interdisciplinary collaboration, wastewater treatment can be transformed from a resource-intensive process into a sustainable and resource-generating system. This paradigm shift will not only mitigate environmental pollution but also enhance energy recovery, improve resource efficiency, and ensure long-term sustainability. This Special Issue of Water focuses on the sustainable utilization of biomass waste from wastewater treatment to advance the development of a circular economy. Since the call for papers was announced on 14 December 2022, ten original papers were accepted for publication after a rigorous peer-review process (Contributions 1–10). The authors represent diverse countries, including China, Thailand, Spain, Australia, the USA, Mexico, and Egypt. These papers are categorized into four areas: (1) pollutant removal technologies and waterbody restoration; (2) sustainable energy and the advancement of the circular economy; (3) water resource management and the optimization of water-saving efficiency; and (4) ecological conservation within the context of agricultural pollution control. To provide a clearer understanding of this Special Issue, we summarize the key highlights of the published papers below.

2. Overview of the Contributions to This Special Issue

This Special Issue showcases significant advancements in pollutant removal technologies and waterbody restoration, emphasizing the innovative methodologies used to address critical challenges in wastewater treatment. Wang et al. (Contribution 1) investigate an iron-based metal–organic framework (MIL-101(Fe)–Na₂CO₃) adsorbent for the rapid removal of hexavalent chromium (Cr(VI)) from wastewater and elucidate the roles of ion branching and domain-limiting effects in the removal process. The primary objective of this study was to develop an efficient, stable, and cost-effective decontamination strategy for addressing Cr(VI) contamination in water. By modifying the surface of MIL-101(Fe) and optimizing the ratio of metal centers to organic ligands, as well as the synthesis conditions, this study demonstrates that the adsorbent can completely remove Cr(VI) within 20 min, achieving a maximum adsorption capacity of 20 mg/g. the experimental results indicate that the adsorption process follows the Langmuir adsorption isotherm and a pseudo-second-order kinetic model, driven primarily by electro-adsorption and monolayer adsorption mechanisms. Additionally, thermodynamic analyses revealed that the reaction is spontaneous, endothermic, and accompanied by an increase in entropy. The study also found that after alkali (NaOH) elution and acid (HCl) regeneration, MIL-101(Fe)–Na₂CO₃ retained a high adsorption efficiency over multiple cycles, indicating its strong potential for practical applications. Compared with other adsorbents, this material offers substantial advantages in both adsorption capacity and selectivity to Cr(VI), effectively mitigating the interference of coexisting anions. In summary, the MIL-101(Fe)–Na₂CO₃ adsorbent developed in this study shows promising applications in wastewater treatment and provides new insights and technological support for efficient heavy metal remediation. Qin et al. (Contribution 2) investigate a newly isolated halotolerant Bordetella strain and its ability to degrade p-nitrophenol (PNP) in high-salinity wastewater, facilitated by co-metabolites. The primary objective of this study was to overcome the low PNP degradation efficiency in high-salinity wastewater, while exploring novel strategies to enhance biodegradation. The researchers isolated a new Bordetella sp. from seafood-processing wastewater and employed adaptive acclimation, demonstrating a remarkable PNP degradation capacity under high-salinity conditions. Under optimal conditions (30 °C, pH 8.0, 3% NaCl, and an aeration rate of 0.3 m³/m³·min), the strain achieved an 85.9% degradation of PNP (initial concentration 350 mg/L) within 72 h. The addition of 30 mg/L pantothenic acid as a co-metabolite further enhanced the PNP degradation rate by 82.5%. GC/MS analyses verified hydroquinone as the key intermediate, indicating that the PNP degradation pathway proceeds via hydroquinone cleavage. Kinetic studies indicated that PNP degradation follows a first-order kinetic model and that elevated PNP concentrations inhibit the strain’s degradation efficiency. This study demonstrates that Bordetella sp. exhibits notable halotolerance and PNP degradation capacity, offering an efficient solution for the biological treatment of high-salinity wastewater. Meanwhile, the use of co-metabolites (such as pantothenic acid) offers new possibilities to further optimize the degradation efficiency, thereby laying a solid foundation for future research and applications in industrial wastewater treatment. Liu et al. (Contribution 3) review the status, technological progress, and future challenges of hydrothermal carbonization (HTC) in wastewater treatment, particularly its potential for achieving “carbon peaking” and “carbon neutrality.” HTC technology employs a high-temperature, high-pressure hydrothermal environment to convert organic waste into value-added carbon materials, thereby reducing energy consumption and pollutant emissions, and improving carbonization efficiency. The article highlights that HTC technology exhibits remarkable decontamination capabilities in wastewater treatment, including the efficient adsorption of heavy metals, organics, and anions, and plays a particularly prominent role in improving water quality. The hydrochar produced via HTC possesses a large specific surface area, abundant pore structures, and diverse surface functional groups, which can be further modified to improve its pollutant removal performance. Despite the wide-ranging application prospects of HTC technology, challenges persist regarding process optimization, cost management, and environmental impact assessments. Future research should concentrate on developing efficient catalysts, refining reaction mechanisms, and advancing the commercialization of HTC technology with the aid of policy support. In addition, international collaboration remains pivotal for expanding HTC technology applications, addressing global water challenges, and realizing carbon neutrality. Overall, HTC technology offers a novel approach to water environment management, fostering a circular economy and sustainable development while providing tangible support in alleviating water resource crises and enhancing water quality. These studies advance the scientific understanding and practical implementation of cutting-edge wastewater treatment technologies, offering innovative pathways for sustainable waterbody restoration and environmental management.

This Special Issue presents significant advances in sustainable energy and the circular economy, emphasizing innovative wastewater management and energy recovery technologies. Suksaroj et al. (Contribution 4) investigates the potential of biogas codigestion technology to simultaneously treat palm oil mill effluent (POME) and empty fruit bunch (EFB) pressed wastewater, thereby advancing the circular economy. The study aims to optimize wastewater management, improve resource utilization, and foster sustainable clean energy generation in the palm oil production process. The article outlines the experimental approach, in which both batch and semi-continuous fermentation systems are used to examine how varying wastewater mixing ratios and hydraulic retention times (HRT) influence methane production. The results show that under a mixing ratio of 45% POME, 50% inoculum, and 5% EFB wastewater at an HRT of 25 days, total biogas and methane productions reached 18,679 mL/L and 6778 mL/L, respectively, with a methane content at 62%. In addition, a COD removal rate of 67% indicates the effective conversion of organic matter in the wastewater into biogas. The study also included an economic assessment, demonstrating that this codigestion strategy provides a higher internal rate of return and a shorter payback period, rendering it more economically viable compared to traditional processes. The resulting sludge complies with organic compost standards and can be used to enhance soil quality, thereby achieving the high-value utilization of waste. In summary, the proposed biogas codigestion technology delivers substantial environmental and economic benefits while offering practical guidance for sustainable development in the palm oil industry. This research may serve as a reference for other sectors dealing with high-pollution wastewater treatment and energy conversion. Huang (Contribution 5) provide a comprehensive overview of the status of, and emerging trends in, anaerobic digestion (AD) in waste stream treatment, emphasizing its pivotal role in meeting “Dual Carbon” targets. Anaerobic digestion harnesses microbial activity to transform organic waste into biogas and nutrient-rich digestate, thereby reducing greenhouse gas emissions, improving self-sufficiency in energy use, and contributing to the circular economy. The article reviews global advances in anaerobic digestion, encompassing multi-stage anaerobic reactors, membrane bioreactors, and photosynthetically assisted upgrading techniques, all of which have substantially increased biogas yields and enhanced overall system efficiency. Furthermore, the article discusses how co-digestion can optimize the carbon-to-nitrogen ratio and stabilize system operations. In terms of policy, Europe’s Green Deal and China’s “Dual Carbon” strategy have greatly accelerated the adoption of AD technologies, particularly in wastewater treatment and agricultural waste management. Despite these advancements, the technology continues to encounter significant challenges, such as high initial capital costs, volatile fatty acid (VFA) buildup, and ammonia inhibition. In addition, developing countries face greater barriers to adoption, attributable to insufficient technical support and incomplete policy frameworks. The article proposes that future efforts emphasize technological refinement, cost minimization, and stronger policy incentives, alongside increasing the public acceptance of AD projects. Overall, anaerobic digestion presents substantial potential for waste stream management and resource recovery, acting as a vital instrument for attaining carbon neutrality and sustainable development, and offering scientific support for policy and industrial applications. These studies contribute valuable insights into sustainable waste management and energy recovery, offering scalable solutions for advancing the circular economy and achieving carbon neutrality goals.

This Special Issue contributes significantly to water resource management and water-saving efficiency optimization, offering innovative strategies and practical solutions for sustainable development. Urrea Vivas et al. (Contribution 6) evaluate the feasibility of reusing the water at the El Salitre wastewater treatment plant in Bogotá, Colombia, employing both economic and technical assessments, and highlight its role as a model for circular economy practices. The study initially examines the existing status of the wastewater treatment plant and identifies the need for upgrades, focusing on secondary and tertiary treatments—such as activated sludge and UV disinfection—to improve the quality of the final effluent. The research proposes repurposing treated water for agricultural, industrial, and recreational uses, in alignment with the Colombian regulatory standards, thereby alleviating pressure on conventional water sources. Their economic assessment employs a cost–benefit analysis (ACB) and present net value (NPV) techniques, factoring in externalities like the environmental benefits of diminished river pollution. The findings reveal that reclaimed water can be produced at a cost of EUR 1.23/m³, offering an economically favorable alternative to traditional water sources and generating approximately EUR 6.52/m³ in additional profit for industrial users. In addition, the sustained utilization of reclaimed water could curtail reliance on natural water bodies, leading to tangible economic and environmental gains. The study underlines the importance of the circular economy’s principles in managing water resources, showcasing how water reuse can facilitate resource efficiency and foster sustainable development. Future directions call for a comprehensive evaluation of technological reliability and strategies to increase the public acceptance of reclaimed water, thereby aiding in the broader deployment of reuse strategies. Cai et al. (Contribution 7) apply a multilayer perception (MLP) neural network and use structural equation modeling (SEM) to investigate the complex factors that shape customers’ water-use behavior in hotels, culminating in the proposal of a representative hotel water-use behavior model. The study seeks to uncover the key drivers of water-use behavior and enhance water efficiency within the hotel industry, thereby informing the development of more targeted water conservation strategies. The article examines water-use behavior across diverse dimensions—such as individual attributes, water-saving awareness, and consumer practices. The research demonstrates that different factors distinctly affect specific activities—including washing, handwashing, drinking, showering, and toilet flushing—with gender exerting the largest influence on showering and flushing, whereas length of stay emerges as the key determinant for all water-use behaviors. In addition, the study indicates that education, income, and hotel type each exert varying degrees of influence on customers’ water-use habits. A total of 292 valid questionnaires were obtained, and both MLP and SEM analyses were employed to quantify the relative importance of each influencing factor. The findings reveal that individual characteristics significantly shape typical water-use behaviors, whereas consumer behavior exerts a comparatively lesser effect, guiding the future design of more effective water-saving policies and behavior-oriented interventions. In addition to offering theoretical underpinnings for water-saving measures in the hotel industry, this article establishes the groundwork for subsequent inquiries into water-use behavior in other service sectors and serves as a reference point for the development of predictive models aimed at alleviating water scarcity. Wang et al. (Contribution 8) examine the operational performance and water quality of centralized WWTPs in industrial parks along the Yellow River Basin, offering suggestions to enhance their efficiency and sustainability. The study centers on 63 centralized wastewater treatment plants in 54 national- or provincial-level industrial parks, evaluating factors such as treatment capacity, influent properties, energy demand, and operational expenses. The findings indicate that most WWTPs are small to medium in scale (1 × 10⁴–5 × 10⁴ m³/d), operating at an average hydraulic loading rate of 53.8% and failing to meet their design capacities. Aerobic biological processes prevail, with approximately 55.1% of facilities utilizing AAO (anaerobic–anoxic–oxic), AO (anaerobic–oxic), or oxidation ditch processes, while certain plants have introduced advanced treatments to comply with the ever-more stringent discharge regulations. The influent quality displays uneven distributions of COD, BOD, NH₃-N, TN, and TP, coupled with generally low COD and BOD levels, underscoring the presence of insufficient carbon sources that markedly constrain nitrogen- and phosphorus-removal efficiencies. Furthermore, energy consumption exceeds the national average (approximately 1.1 kWh/m³), which is mainly attributable to the complex and low-biodegradability profile of industrial wastewater. The article recommends bolstering wastewater segregation and pretreatment within industrial parks, refining treatment processes to optimize carbon utilization, and integrating energy-efficient technologies and renewable sources to reduce overall consumption, thus improving both economic feasibility and long-term sustainability in the Yellow River Basin. Collectively, these studies address critical aspects of water resource management by exploring water reuse, behavioral insights, and treatment plant optimization. Their findings provide a comprehensive approach to enhancing water-saving efficiency, reducing the environmental impacts, and advancing sustainable water management practices in diverse contexts.

The Special Issue makes significant contributions to ecological conservation within the framework of agricultural pollution control, addressing complex challenges through the use of innovative methodologies. Zhang et al. (Contribution 9) introduce a precipitation forecasting method that consolidates CEEMD (complete ensemble empirical mode decomposition), ELM (extreme learning machine), and FFOA (whale optimization algorithm). The proposed model aims to overcome the shortcomings of conventional precipitation forecasting techniques when confronted with nonlinear and intricate time-series data. The central concept involves employing CEEMD to decompose precipitation time series into multiple intrinsic mode functions (IMFs), effectively filtering out noise and extracting informative features. Subsequently, the ELM model—recognized for its rapid training and robust generalization—performs the training and forecasting tasks. To increase its forecasting accuracy, the article integrates the FFOA algorithm to optimize the ELM parameters. The FFOA explores optimal solutions through a whale-foraging paradigm, thus mitigating the risk of converging on local optima. The experimental findings indicate that the CEEMD–ELM–FFOA model surpasses traditional stand-alone approaches in precipitation forecasting, particularly with respect to accuracy and robustness. This method not only adeptly manages the nonlinear characteristics of precipitation data, but also enhances the model’s predictive competence, demonstrating substantial practical utility. In conclusion, this integrated forecasting model provides a novel paradigm for precipitation forecasting, making it especially pertinent to the complex nonlinear dynamics of meteorological data. Ge et al. (Contribution 10) address the issue of nonpoint-source phosphorus pollution and its ecological ramifications while exploring the emerging trends in applying amendment materials to mitigate phosphorus runoff. Nonpoint-source phosphorus pollution, predominantly derived from agricultural practices (e.g., fertilization and soil erosion), constitutes a principal driver of water eutrophication. The article first analyzes the pathways by which phosphorus moves from soil to aquatic systems, addressing mechanisms such as the release, migration, and deposition of phosphorus. Phosphorus reaches aquatic environments through runoff and subsurface infiltration, resulting in water quality degradation, algal bloom, and broader ecological disturbances. The study underscores that deciphering the dynamics of phosphorus in soil and curbing its transfer to water systems are paramount for mitigating phosphorus losses. The article then highlights various amendment materials—including organic matter, mineral additives, and chemical remediation agents—that effectively diminish phosphorus loss through bolstering the adsorption of phosphorus by the soil, modifying phosphorus’ solubility, or facilitating the immobilization of phosphorus. The article concludes with a comparative assessment of each material’s strengths and limitations, advocating for further exploration of innovative amendment options and evaluations of their efficacy across various environmental scenarios. In sum, the article underscores the multifaceted nature of nonpoint-source phosphorus pollution and its implications for aquatic ecosystems, proposing amendment-based strategies to curb phosphorus runoff and offering critical insights and practical foundations for the prevention of water eutrophication. These studies provide valuable insights into predictive modeling and pollution control, advancing ecological conservation efforts. Their findings support sustainable agricultural practices and effective resource management, offering scalable solutions to mitigate the environmental impacts.

3. Conclusions

The advancements highlighted in this Special Issue underscore the transformative potential of integrating circular economy principles into wastewater treatment. By prioritizing the sustainable utilization of biomass waste and leveraging cutting-edge technologies such as anaerobic digestion, thermal conversion, and innovative filtration methods, the field is advancing towards reducing environmental impact and enhancing resource efficiency. These approaches not only mitigate pollution but also generate renewable energy and recover valuable nutrients, aligning with the global sustainability goals. The contributions further emphasize the critical role of interdisciplinary collaboration and policy support in overcoming challenges such as economic feasibility, public acceptance, and technological integration. The research on pollutant removal, sustainable energy recovery, water reuse, and agricultural pollution control demonstrates the multifaceted benefits of these innovations. From achieving carbon neutrality through optimized energy recovery systems to improving water-saving efficiencies in service sectors, these studies provide actionable insights that bridge the gap between scientific discovery and practical implementation. However, challenges remain in scaling these technologies, ensuring their cost-effectiveness, and addressing environmental trade-offs. Moving forward, greater focus on life-cycle assessments, robust regulatory frameworks, and public awareness campaigns will be pivotal in mainstreaming these solutions.

Ultimately, this Special Issue serves as a call to action for researchers, policymakers, and industry leaders to deepen their commitment to sustainable wastewater management. By transforming wastewater from a burden into a resource, we can build resilient systems that contribute to ecological conservation, energy security, and sustainable development, ensuring a healthier future for both the environment and society.