Optimization and action mechanism of pollutant removal performance of unsaturated vertical flow constructed wetland (UVFCW) driven by substained-release carbon source.

Constructed wetland (CW) technology has attracted much attention due to its economical and environmentally friendly features. The low dissolved oxygen (DO) and low carbon/nitrogen (C/N) ratio in the wetland influent water affect the treatment performance of CW, resulting in a decrease in the removal efficiency of ammonia nitrogen (NH₄ ⁺-N) and nitrate nitrogen (NO₃ ^--N). In order to address this problem, this study optimized the pollutants removal performance of unsaturated vertical flow constructed wetland (UVFCW) by adding sustained-release carbon sources (corn cobs + polybutylene adipate terephthalate (PBAT)). The results showed that the sustained-release of carbon source increased the carbon source in UVFCW, thus increasing the abundance and activity of denitrifying microorganisms and enhancing the denitrification reaction, ultimately improving the removal of NO₃ ^--N, with its removal efficiency reaching up to 95.50%. The placement method of sustained-release carbon source mainly affected the distribution of carbon source and DO in water body, thus influencing the relative abundance of microorganisms, finally affecting the removal of pollutants. Among them, the removal efficiency of total nitrogen (TN), NO₃ ^--N, and total phosphorus (TP), and the relative abundance of denitrifying microorganisms in the CW_R-Cu (uniform placement of sustained-release carbon source) were significantly higher than those in the CW_R-Ca (centralized placement above) and CW_R-Cb (centralized placement below) (p < 0.05). The surface C:O (carbon:oxygen) ratio of sustained-release carbon source after water treatment showed a decreasing trend, and CW_R-Cu exhibited the greatest decrease in C:O ratio. In summary, CW_R-Cu achieved the highest utilization of the carbon source and produced the largest number of heterotrophic microorganisms. This study reveals that CW_R-Cu is a structural process for the efficient removal of nitrogen and phosphorus pollutants, and our findings provide theoretical basis and technical support for actual projects.