Structure and activity of microbial communities in response to environmental, operational, and design factors in constructed wetlands

Constructed wetlands are nature-like engineering systems implemented to treat wastewater through physical, chemical, and biological processes under controlled environments. In constructed wetlands, pollutant removal is primarily accomplished by microbial processes such as nitrification, denitrification, anammox, comammox, organic matter degradation, and phosphate removal. This review discusses the effects of environmental, operational, and design factors on the structure, diversity, and activity of microbial communities, providing useful information to improve constructed wetlands' performance. Factors such as temperature, pH, depth, substrate, availability of organic carbon, and presence of plants affect microenvironments, and thus the microbial communities within a constructed wetland, promoting different microbial metabolic pathways. In this review, special attention is given to the effect of these factors on different pollutant (nitrogen, phosphorus, and organic matter) removal pathways and performance within different CW configurations, and to the related microorganisms that have been identified by molecular techniques in these systems. A greater understanding of the critical factors that can be manipulated or controlled to shift the dominance of different microbial groups and thereby enhance microbial activity and improve constructed wetlands' performance is still needed. Moreover, precise experiments manipulating critical factors and assessing microbial behavior as well as the performance of constructed wetlands may prove useful in developing strategies to optimize constructed wetlands' efficiency.