In an era where clean water is increasingly precious and the threats posed by pollution are escalating, addressing the contamination of freshwater systems has become paramount. One insidious form of pollution that has recently garnered attention is micro-polluted water—characterized by low concentrations of carbon, nitrogen, and phosphorus compounds. These dissolved pollutants, though present in minute amounts, pose a significant threat to aquatic ecosystems by contributing to phenomena such as harmful algal blooms, mass fish mortality, and a consequential decline in biodiversity. The challenge lies in the difficulty that conventional wastewater treatment plants face in efficiently removing these diluted contaminants, often turning their treated effluents into unexpected sources of micro-polluted water.
Current strategies to mitigate this subtle yet impactful pollution are under scrutiny, and constructed wetlands (CWs) have emerged as a promising nature-inspired solution. These engineered ecosystems leverage the synergistic interaction of plants, microbial communities, and soil substrates to emulate the water-cleaning functions inherent in natural wetlands. Despite the demonstrated effectiveness in controlled or experimental settings, there remains a knowledge gap regarding their real-world performance at full scales. Because small-scale models may not fully capture complex environmental variables and operational dynamics, understanding the functioning of full-scale constructed wetlands is critical for practical application.
A recent comprehensive review led by Professor Haiming Wu from Shandong University has systematically analyzed 78 full-scale constructed wetlands to assess their pollutant removal performance and determine the key factors influencing their efficacy. The intent was not only to delineate the operational mechanics underlying these systems but also to provide actionable insights to enhance their pollutant capture capabilities. This extensive meta-analysis highlights the variations in pollutant profiles depending on water sources; agricultural runoff, for example, is rich in dissolved organic carbon measured as chemical oxygen demand (COD), whereas polluted river waters contain higher levels of nitrogen, phosphorus, alongside heavy metals and pharmaceutical residues—each challenging the purification potential of CWs differently.
Significantly, the direction and rate of water flow within these wetlands were identified as fundamental determinants of system efficiency. Horizontally flowing water across the surface tends to facilitate pollutant removal by maximizing contact with plants and microbial biofilms, whereas vertical flows introduce different substrate interactions. A balancing act is required because while slower hydraulic retention times increase pollutant degradation, excessively slow flows promote anaerobic conditions that can generate foul odors and potentially limit processing efficiency. This nuanced understanding underscores the importance of hydraulic design tailored to specific pollution profiles.
The biological dimension of constructed wetlands is equally paramount. Rapidly growing macrophytes not only uptake dissolved nutrients directly but also influence microbial communities that are essential for biodegradation of contaminants. The stoichiometric balance between carbon and nitrogen within the inflowing water emerges as a critical chemical factor. Adequate carbon sources are necessary to sustain microbial denitrification processes, thereby facilitating the removal of nitrogenous compounds. Additionally, electron donors such as manganese naturally present in substrates serve as catalytic agents in reducing heavy metals and organic pollutants, emphasizing the role of substrate composition in wetland design.
Oxygen availability is another cornerstone of wetland efficacy. Dissolved oxygen supports aerobic microbial growth which is critical for biodegradation pathways and metal precipitation. Enhancing oxygen levels through passive aeration techniques or engineered inputs can boost the removal of both nutrients and toxic heavy metals, leading to marked improvements in effluent quality. This highlights the importance of active management strategies, including regular harvesting of wetland biomass—a practice that removes sequestered pollutants and promotes sustained system productivity.
Augmentation of constructed wetland substrates using biochar, metal ore wastes, and agricultural by-products has surfaced as a potent strategy to bolster polluting compound immobilization and breakdown. Biochar, in particular, offers a porous, high-surface-area medium that enhances microbial habitat and adsorptive capacity. Modified biochar variants tailored to specific pollutant classes are being explored to refine this approach further. However, challenges remain concerning the economic feasibility and long-term sustainability of such substrate enhancements, particularly at the scales required for municipal or agricultural wastewater treatment.
The presence of emerging contaminants, such as per- and polyfluoroalkyl substances (PFAS), introduces additional complexities. These persistent chemical pollutants resist traditional degradation pathways and pose serious ecological and human health risks. Understanding and innovating CW designs capable of effectively treating such recalcitrant compounds is a frontier area of research necessitating interdisciplinary collaboration across environmental engineering, chemistry, and ecology.
Looking ahead, Prof. Wu advocates for the development of predictive modeling frameworks that integrate hydrological parameters, substrate chemistry, and biological dynamics to optimize wetland configurations. These models aim at minimizing physical footprint while maximizing pollutant removal through tailored species selection and hydraulic regimes. Such sophisticated simulation tools would enable environmental managers to anticipate system behavior under varied environmental conditions, thereby enhancing operational reliability and cost-efficiency.
The wide appeal of constructed wetlands stems from their low capital and operational costs coupled with their environmental sustainability. By harnessing natural processes, CWs provide a viable alternative or complement to energy-intensive and chemically dependent conventional wastewater treatment technologies. Moreover, the ecosystem services rendered by wetlands—including habitat provision, carbon sequestration, and aesthetic benefits—render them multifaceted assets within urban and rural landscapes alike.
Nevertheless, to fully realize the potential of full-scale constructed wetlands in safeguarding freshwaters from micro-pollution, dedicated investment in research and infrastructure is indispensable. Collaborative efforts involving academic researchers, public utilities, and policymakers will be vital to surmount technical challenges and scale innovations. Holistically, the growing body of knowledge encapsulated in Prof. Wu’s review chart a hopeful path towards cleaner water bodies and resilient aquatic ecosystems.
As water scarcity intensifies and anthropogenic stressors proliferate, the urgency to deploy robust, cost-effective, and ecologically harmonious solutions is undeniable. Constructed wetlands stand poised at the intersection of nature and technology as a beacon for sustainable water management. Continued exploration, refinement, and outreach will be key to transforming these engineered wetlands from promising prototypes into mainstream environmental guardians.
Subject of Research:
Not applicable
Article Title:
Recent advances on micro-polluted water remediation by full-scale constructed wetlands: Pollutant removal performance, key influencing factors, and enhancing strategies
News Publication Date:
28-Mar-2025
Web References:
DOI: 10.1016/j.jes.2025.03.049
References:
Wu, H., et al. (2025). Recent advances on micro-polluted water remediation by full-scale constructed wetlands: Pollutant removal performance, key influencing factors, and enhancing strategies. Journal of Environmental Sciences, 159.
Image Credits:
“Experimental Wetlands” by born1945 on Flickr
Keywords:
Environmental sciences, Pollution, Water pollution, Environmental management, Environmental monitoring, Environmental policy
