Research progress on microbial enhancement of pollutant removal in constructed wetlands

Microorganisms play a pivotal role in the removal of pollutants within constructed wetlands. Rational design, efficient operation, and appropriate enhancement measures can optimize the microbial community structure, improve microbial activity, and consequently enhance the pollutant removal efficiency of these engineered ecosystems. This article summarized the pathways through which microorganisms remove conventional pollutants in constructed wetlands, and analyzed the influences of various factors, including different substrate types, vegetation, carbon and oxygen levels, hydraulic retention time (HRT), and organic loading, on the microbial populations, abundance, and activity within constructed wetlands. Further discussion was made on how variations in microbial communities affect the overall efficacy of pollutant removal. The findings indicate that substrates with a large specific surface area and high porosity provide extensive habitat space for microbial colonization and growth. Constructed wetlands employing multi-layer substrate structures demonstrate a higher proportion of microbial abundance and more pronounced spatial distribution heterogeneity compared to those with single-layer substrates. Specifically, the incorporation of iron, biochar, or iron-carbon composite substrates can facilitate synergistic nitrogen removal by promoting both autotrophic and heterotrophic denitrification processes. The presence of plants significantly enhances microbial abundance and diversity in constructed wetlands relative to unplanted systems. Variations in root oxygen release capacity and the composition of root exudates among different plant species directly influence the structure and richness of associated microbial communities. Operational strategies such as addition of plant-based carbon sources, high-molecular-weight slow-release carbon sources, step-feeding of influent, intermittent aeration, and tidal flow regimes can optimize the distribution of carbon sources and oxygen, thereby refining the microbial community structure. An appropriately managed hydraulic retention time is crucial for maintaining a dynamic balance among key microbial processes within the wetland system, including organic matter degradation, nitrification, and denitrification. The introduction of exogenous microorganisms can effectively augment both the quantity and metabolic activity of microbial consortia in constructed wetlands. To optimize the design and operation of constructed wetlands, a multifaceted approach is recommended. This includes the rational combination of substrate materials, strategic selection and configuration of wetland plants, precise regulation of carbon and oxygen availability, careful control of hydraulic retention time, judicious addition of exogenous microbial inoculants, and the development of hybrid or integrated constructed wetland systems. These measures collectively aim to optimize the microbial community structure, enhance microbial metabolic activity, and ultimately strengthen the capacity of microorganisms for pollutant removal. In conclusion, this paper provides a perspective on the underlying mechanisms and potential practical applications of microbial enhancement strategies for improved pollutant removal in constructed wetlands. Future research should focus on elucidating the complex interactions within these engineered microbial ecosystems to facilitate more robust and efficient wetland design for diverse environmental remediation contexts.