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Abstract

Constructed wetlands (CWs) serve as a sustainable and eco-friendly solution for wastewater treatment, particularly in the removal of eutrophication-causing pollutants. This review focuses on the comparative performance of Vertical Flow (VF) and Horizontal Flow (HF) Subsurface Flow (SSF) systems, assessing their efficiency in removing Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Nitrogen (TN), and Total Phosphorus (TP). VF systems demonstrate superior pollutant removal, particularly for nitrogen, due to enhanced aeration and efficient oxygenation processes. In addition, their compact design reduces land area requirements, making them advantageous in space-limited applications. Conversely, HF systems are more effective at supporting nutrient gradient development, particularly for phosphorus removal, but they require more land and exhibit slower treatment rates due to limited oxygenation. The review synthesizes current literature on the mechanisms of pollutant removal in these systems, emphasizing the role of phytoremediation plants and microbial interactions. Our analysis underscores that, while both VF and HF systems offer substantial environmental benefits, VF systems consistently outperform HF systems in terms of pollutant removal efficiency and spatial economy. This makes VF systems particularly valuable in settings where land availability is constrained and nitrogen reduction is prioritized. These findings highlight the critical role of VF-SSF systems in advancing wastewater treatment, positioning them as essential components in strategies to mitigate eutrophication and enhance environmental sustainability. Scientifically and academically, future research should focus on optimizing VF system designs, enhancing phosphorus removal, and ensuring resilience across diverse climatic conditions for long-term, global water management solutions.

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