Evaluation of purification capacity and net contribution of five aquatic plant-substrate combinations to polluted waters in Beijing

Practical guidance for selecting plant-substrate combinations in constructed wetlands treating polluted urban waters is limited. This study employed a simulated pollution experiment to assess the purification capacity and net contribution of total nitrogen (TN), total phosphorus (TP), ammonia (NH₄⁺), and chemical oxygen demand (COD) in polluted waters using five commonly used aquatic plants in Beijing, namely Phragmites australis , Typha orientalis , Iris pseudacorus , Scirpus validus , Lythrum salicaria , combined with five substrates including crushed stone, gravel, volcanic rock, ceramic granule, and zeolite. The results showed that: (1) The 25 plant-substrate combined systems effectively removed pollutants, with average removal efficiencies of TN, TP, NH₄⁺ and COD exceeding 63, 21, 79 and 82%, respectively. (2) The top-performing combinations were identified via entropy weight method. (3) The combined plant-substrate system exhibited synergistic effects, outperforming pure plants and pure substrates in pollutant removal. And (4) The contribution of plants and substrates in the combined system was comparable in TN removal, substrates played a dominant role in TP removal, while plants were the main contributors in NH₄⁺ and COD removal. This study provides practical guidance for plant-substrate selection in constructed wetlands for Beijing’s polluted urban waters, clearly differentiating their pollutant removal performance.