Advancing the Frontiers of Wastewater Treatment—Synthesis and Future Perspectives in State-of-the-Art Techniques

Water scarcity, exacerbated by population growth, industrialization, and climate change, demands innovative and sustainable solutions [1,2,3]. Wastewater treatment stands as a cornerstone in addressing this global crisis, transforming polluted effluents into valuable water resources [4,5]. This Special Issue presents cutting-edge research addressing the complex challenges of modern wastewater treatment, covering novel materials, process optimization, mechanistic insights, predictive modeling, and more. The field of wastewater treatment is undergoing a dynamic transformation, shaped by several key trends.

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Advanced material development. Significant research efforts have focused on engineering functional materials, such as modified biochars, nanocomposites, and specialized adsorbents, tailored for pollutant removal (e.g., heavy metals, nutrients, pharmaceuticals, etc.) [6,7,8,9,10]. Surface functionalization and hybrid material design enhance selectivity and capacity. Beyond mere pollutant removal, technologies are increasingly targeting the recovery of valuable resources like nutrients (N, P), energy, and embedded materials (e.g., metals from waste streams), promoting circular economy principles [11,12,13,14].

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Sustainable wastewater treatment technology. Understanding the complex microbial consortia in treatment systems (e.g., aerobic granular sludge, biofilms in wetlands) and manipulating community structure for enhanced nutrient removal and stability is becoming a major research thrust [15,16,17]. Persistent and potentially hazardous pollutants, including microplastics, pharmaceutical residues, antibiotic resistance genes (ARGs), and industrial solvents, remain a research priority [18,19,20,21]. Engineered natural systems, such as constructed wetlands, continue to be optimized for their cost-effectiveness and ecological benefits, particularly in decentralized treatment and non-point source pollution control [22,23,24,25].

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Process intensification and hybridization. The integration of physical, chemical, and biological processes (e.g., coupling adsorption with biological degradation or combining membranes with advanced oxidation) enhances the efficiency and resilience of the water treatment systems, particularly for recalcitrant compounds [26,27,28,29,30,31]. The application of sophisticated machine learning (ML) algorithms and statistical models for predicting treatment performance, pollutant fate, material behavior, and system optimization is rapidly growing, enabling smarter design and operation [32,33,34,35].

Despite significant progress in the field, several critical challenges remain.

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Translating innovation into practical applications. Scaling up the synthesis of novel materials cost-effectively and ensuring their long-term stability, regenerability, and safe disposal, and translating laboratory-scale innovations into reliable, large-scale implementation remain key barriers. Furthermore, many promising technologies lack rigorous, long-term field validation under realistic, variable operating conditions, hindering confidence and adoption. Complex processes like Advanced Oxidation Processes (AOPs) face issues such as catalyst deactivation and high resource demands, limiting real-world adoption.

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Complexity for effective prediction and control. The inherent complexity of wastewater systems and pollutant behavior presents fundamental scientific and engineering challenges. Elucidating precise mechanisms of pollutant removal within heterogeneous matrices containing contaminant mixtures requires deeper investigation. For example, the full lifecycle impact of microfibers and microplastics within treatment plants—including their fragmentation, interactions with other pollutants, and effects on microbial communities and sludge properties—is still poorly understood. Developing robust and generalizable predictive models capable of accurately forecasting treatment performance, pollutant fate, and system response under dynamic conditions (e.g., shock loads, seasonal changes) is hampered by insufficient data and modeling limitations.

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Ensuring holistic sustainability and impact management. Truly sustainable water treatment extends beyond simple removal efficiency, assessing and optimizing the broader environmental and economic footprint. Comprehensive techno-economic analyses (TEA) and lifecycle assessments (LCA) comparing novel technologies (especially those focused on resource recovery) to conventional methods are often incomplete, making holistic sustainability claims difficult. Ultimately, achieving sustainability requires integrating solutions that minimize negative environmental consequences while maximizing resource recovery and economic viability.

As a result, this Special Issue addresses these gaps through contributions in three core areas:

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Engineered adsorbents and functional materials. Feng et al. provide a review focusing on improving persulfate (PS) activation strategies, emphasizing the role of chelating/reducing agents in overcoming inherent limitations like Fe(III) accumulation/sluggish reaction rates for efficient and sustainable contaminant destruction. Struhs et al. and Mirkouei et al. explore nutrient recovery (N, P) from aquaculture effluent using modified biochar and contribute to closing the loop in fish farming. The focus on a specific real-world application (Idaho aquaculture) provides valuable field-relevant data. Geng et al. identify acid-modified rice husk biochar as an optimal adsorbent for polycyclic aromatic hydrocarbons (PAHs) and a microbial carrier for Rhodococcus sp. DG1. Their study details adsorption interactions (π-π/n-π EDA, H-bonding, hydrophobicity) and demonstrates field applicability potential with significantly enhanced phenanthrene degradation. They also highlight the crucial impact of immobilization methods (adsorption vs. adsorption–embedding) on microbial activity and nutrient access, a key consideration for hybrid technologies. Niu et al. present a multifunctional, sustainable adsorbent derived from iron-rich microalgae, capable of high Cu²⁺ adsorption, efficient oil/water emulsion separation, and photo-Fenton antifouling activity, offering a solution for complex wastewaters. The integrated functionality (adsorption, separation, self-cleaning) addresses the need for versatile materials.

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Microbial processes, community dynamics, and biological treatment. Onyedibe et al. investigate the impact of polyester/denim microfibers (MFs) on aerobic granular sludge (AGS) systems. They provide vital mechanistic insights, linking decline to nutrient transport blockage and toxic leachates, and strongly advocate for upstream MF control. Zheng et al. explore algae-assisted nitrogen fixation and nitrification to diversify the sources of irrigation water. Yuan et al. focus on field-scale performance, examining nitrogen removal in lakeshore multicell constructed wetlands (MCWs) under seasonal nutrient shocks, emphasizing the role of forebay storage ponds in mitigating shock loads and identifying hydraulic factors as primary drivers of microbial community variation. They reveal how shock loads and periodic saturation create complex micro-environments that foster diverse nitrogen-cycling microbes, enhancing resilience and performance. They also expand the view to receiving environments, linking water level fluctuations (WLF) and vegetation cover to sediment microbial communities and N-cycling genes in Lake Erhai, demonstrating that WLF, by creating oxic–anoxic interfaces, filters for adaptable microbes and increases abundance of key functional genes. Mai et al. investigate cyanobacteria interactions and the management of blue-green algae outbreaks in reservoirs. Csutak et al. explore a novel application of yeast (C. parapsilosis CMGB-YT) to produce biosurfactants that enhance heavy metal removal (Pb, Cd) by Rhodotorula mucilaginosa from wastewater and inhibit pathogenic biofilm. They highlight the potential of microbial-derived surfactants for multi-contaminant management in complex waste streams.

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Modeling, risk assessment, and remediation strategies. Jung et al. provide a comprehensive review of the pollution issues associated with shipyards (heavy metals, petroleum hydrocarbons, and organic solvents) and explore effective strategies for soil and groundwater remediation. Cheng et al. apply a sophisticated Copula Bayesian Network model to Qilu Lake. It quantifies spatial and temporal pollution risks, identifies key drivers (agricultural runoff, water scarcity), and establishes early warning thresholds (total nitrogen TN, total phosphorus TP, COD) for “deteriorated” status. This provides a powerful tool in proactive lake management and data-driven decision-making, moving beyond simple descriptive monitoring. Hou et al. quantify the disproportionate impact of extreme rainfall (ER) events on surface runoff and diffuse nitrogen loss in a subtropical monsoon watershed. They reveal the limited effectiveness of natural systems (forests, pastures) in controlling losses during ER and emphasize the dominance of water-soluble ON-N and the amplifying role of antecedent soil moisture. Rodziewicz et al. investigate the effect of carbon dosing on nutrient removal in a rotating electro-biological disk contactor (REBDC) treating hydroponic tomato wastewater and evaluate the impact on effluent COD, finding that the rational solution enables high substrate utilization and nitrogen removal. Wang et al. bridge the gap between lab-scale remediation concepts and field implementation for complex industrial contaminants, providing critical validated field data and a practical framework for remediating groundwater contaminated by challenging organic solvents. Shi et al. introduce the novel IDISO metaheuristic algorithm, enhancing the DISO algorithm with nonlinear shrinking and Cauchy mutation, to optimize XGBoost model hyperparameters. Demonstrating superior performance on hydrochar element prediction, they provide a valuable computational tool for accelerating material design and characterization in the biochar/hydrochar domain. Zhao et al. highlight the detection of diverse ARGs in a reclaimed water river system, linking their distribution to environmental factors. This study contributes to understanding the drivers of antibiotic resistance in water reuse scenarios, essential for managing public health risks associated with water reclamation.

Building on the insights from this Special Issue and recognizing the persisting challenges, several future directions emerge. Regenerable and scalable functional materials with enhanced stability and specificity are still important. Promising avenues include improved modification techniques (e.g., co-doping, nanohybrids), biocomposite design, and stimuli-responsive materials (responsive to pH, light, or specific triggers). Correspondingly, advanced process understanding and hybridization promote the application of novel materials. For example, advanced in situ techniques should be applied to investigate real-time adsorption, degradation, and microbial interaction mechanisms. Systematic design of hybrid systems, combining physical (e.g., membranes, adsorption), chemical (e.g., tailored AOPs), and biological processes (e.g., AGS, CWs), holds potential for tailored treatment solutions. Enhancing system robustness against variability is another point to be considered, like integrating climate projection into the design and management of treatment systems and developing models predicting performance under extreme hydrology. Leveraging AI or ML and sensor networks for real-time system monitoring and control can help to improve the accuracy and convenience of the operation. Based on the above, accelerating translation to practice is the final target, and long-term pilot- and full-scale demonstrations of promising technologies across diverse geographies and wastewater types are essential. Collaborative platforms for sharing comprehensive datasets on treatment performance, material properties, and environmental conditions will support model generalizability and accelerate practical adoption. Overall, we hope this Special Issue inspires continued innovation and collaboration, paving the way for state-of-the-art wastewater treatment technologies to become standard practice in the pursuit of sustainable water management.