Effects of Manganese Sand Proportion on Nitrogen and Phosphorus Removal Performance and Microbial Community in Constructed Wetlands

The optimal utilization of manganese sand as a substrate is crucial for enhancing the performance of constructed wetlands (CWs) in the advanced treatment of secondary effluent. This study systematically investigated the effects of manganese sand mass ratios (0%, 20%, and 60%) on nutrient removal and microbial community in vertical flow CWs planted with Acorus calamus L. and Canna indica L. The results showed that ammonia nitrogen (NH4+-N) removal was highly efficient (94.5–96.0%) and unaffected by the manganese sand proportion. In contrast, total nitrogen (TN) and total phosphorus (TP) removal were significantly enhanced at the 20% ratio, synchronously reaching optimum levels (TN: 25.3%; TP: 97%). A unique coral-like porous structure was observed on the manganese sand at this proportion. Microbial analysis indicated the 20% unit supported the highest microbial diversity (Shannon index = 9.24) and was enriched with key functional genera like the denitrifying bacterium Pseudomonas. These studies confirm that a 20% manganese sand proportion enhances nutrient removal by optimizing the substrate microstructure and shaping a beneficial microbial community, providing a scientific basis for the parameter optimization of manganese sand-enhanced CWs.