New Review of Nitrogen Removal Processes and Mechanisms in Constructed Wetlands

Constructed wetland systems (CWs) have increasingly become a preferred method for treating low-pollution wastewater due to their lower operational and maintenance costs and their alignment with green sustainable development goals. However, their pollutant removal efficiency, particularly regarding nitrogen removal, is often suboptimal (40% or more). Although various enhanced nitrogen removal processes in CWs have emerged, a comprehensive review of the contributions and influences of substrates, plants, and microorganisms on the nitrogen removal process is lacking. This paper addresses this gap by reviewing the nitrogen removal effects and mechanisms of enhanced water purification technologies in CWs. First, the paper discusses the role of the tidal flow operation mode in increasing dissolved oxygen, regulating microbial activity, and enhancing nitrogen removal processes. Second, this paper analyzes the advantages of multistage CWs (MLCWs) in adjusting different water quality characteristics and improving nitrogen removal rates (in some studies, the total nitrogen (TN) removal rate was increased by 2.7 to 3.7 times). The principle of substrate enhancement through material selection and electron donor provision mechanisms to strengthen nitrogen removal is also elucidated, e.g., the addition of sucrose and reed litter had comparable effects on the removal of TN (84.9%–93.5%) and nitrate (98.3%–99.8%). Additionally, the importance of plants in the nitrogen removal process is emphasized (2%–29% of TN removal improvement), highlighting the contribution of specific species selection and the planting of plant combinations. Finally, methods combining CWs with bioelectrochemical technology are introduced, with an emphasis on the superiority of this technology in improving nitrogen removal effectiveness. For example, the TN removal rate of MEC-CWs is 81.71%–96.82% higher than that of traditional CWs. All in all, this paper summarizes the mechanisms by which different enhancement technologies affect nitrogen removal, provides insights into the microorganisms that enhance nitrogen removal, and proposes strategies for optimizing nitrogen removal, offering theoretical support and guidance for further research and application of CWs.