Constructed Wetlands for Wastewater Treatment: Optimization of Design and Mechanisms for Enhanced Pollutant Removal

Constructed wetlands (CWs) represent a nature-based, economically viable solution for the comprehensive treatment of diverse wastewater contaminants. By leveraging synergistic physical, chemical, and biological processes—including sedimentation, filtration, adsorption, redox-mediated transformations, plant uptake, and microbial degradation under both aerobic and anaerobic conditions—CWs offer an integrated platform for sustainable pollutant attenuation. This review work critically evaluates the treatment performance of various CW configurations—including free water surface, horizontal and vertical subsurface flow, hybrid, baffled, and aerated systems—with an emphasis on their design and operational optimization. Key parameters such as hydraulic retention time, hydraulic loading rate, substrate porosity, water depth, and aspect ratio are examined for their influence on pollutant removal efficiency. The ecological functions of vegetation and microbial consortia are highlighted, particularly their roles in contaminant biotransformation and nutrient cycling. Emerging innovations in substrate modification, artificial aeration, and the application of advanced modeling tools—such as HYDRUS, computational fluid dynamics (CFD), genetic algorithms, and artificial neural networks (ANNs)—are discussed as pathways for performance enhancement. Despite their numerous advantages, CWs face critical challenges, including substantial land requirements, media clogging, seasonal variability, and the absence of standardized design frameworks. Future research should prioritize real-time system monitoring, micropollutant (MP) attenuation, and integrated modeling approaches to enhance the scalability, adaptability, and long-term resilience of CWs. This review aims to provide a comprehensive and forward-looking foundation for advancing the design, implementation, and optimization of CW technologies in sustainable wastewater management.