EFFECTS OF POLYCULTURE ON NUTRIENT REMOVAL BY ARTIFICIAL FLOATING ISLANDS FROM RESIDENTIAL RAW SEWAGE

Nutrient pollution is one of the most concerned environmental problems associated with harmful algal blooms (HABs), posing substantial health risks to both humans and aquatic ecosystems. Artificial floating islands (AFIs) present a promising solution to nutrient removal. It is one phytoremediation technology that is growing in popularity and has been used in many water bodies, including rivers, lakes, and reservoirs. At the current stage of AFI studies, evaluating the feasibility of upscaling AFI systems from lab-scale to field-scale is of most importance. In this study, we implemented a field-scale AFI system with two native aquatic plants, Carex comosa (bristly sedge) and Eleocharis obtusa (blunt spike-rush), in the equalization basin of a wastewater treatment plant for preliminary treatment of residential raw sewage. We monitored the physicochemical parameters, plant biometrics, and nutrient concentrations for over six months. The primary purpose of this study was to evaluate the effectiveness of these field-scale AFIs in nutrient removal under natural conditions. Moreover, it has been reported that polyculture – the practice of mixing multiple aquatic plant species on one AFI unit – could potentially improve the performance of AFIs. However, we have limited evidence and data supporting this hypothesis, especially at field-scale. Therefore, we compared two treatments of plant deployment – monoculture and polyculture, to analyze whether the practice of polyculture provides improved AFI performance. Our preliminary results indicated that polyculture of the two tested plant species caused a competition for space between the two species, resulting in improved growth in C. comosa but suppressed growth in E. obtusa. However, polyculture did not necessarily increase the nutrient storage within plant tissues. Considering the overall nutrient removal by each AFI units, polyculture presented comparable nutrient removal with monocultured AFIs, but did not present significantly improved performance. We concluded that combining efficient plant species, e.g., C. comosa, with cost-effective plant species, or simply reducing the plant density, could result in similar AFI performance, but largely reduce the expenses of field-scale AFIs for nutrient pollution management.