Phytoremediation of wastewater using a field-scale floating wetland system

Phytoremediation is an environmentally friendly, cost-effective, and sustainable technology that uses plants to clean up contaminated soil, water, and air. Compared to traditional wastewater treatment methods – which are often energy-intensive and expensive – phytoremediation techniques use low-cost, readily available local materials, have minimal upfront capital investment, are simple to maintain and operate, have little to no energy input, and provide multiple co-benefits (e.g., habitat for wildlife, improvement of local aesthetics, and biomass harvest for composting and biofuel). This study evaluated the effectiveness of a field-scale floating wetland system in reducing concentrations of nutrients and algal toxins (microcystin), using native aquatic plants installed in the equalization basin of a wastewater treatment plant. The floating wetland system was deployed in late spring and, through summer and fall, we monitored nutrient levels, microcystin concentrations, physico-chemical parameters, and plant biomass. A 78% reduction in microcystin was achieved during peak plant growth, and the relative abundance of cyanobacteria decreased from 27.7% to 4.5% during this period. Nutrient assimilation (and plant biomass production) was higher in systems with mixed plants (polyculture), with nutrient reduction reaching peak values of 2968 mg/m² for NH₄⁺, 1767 mg/m² for PO₄³⁻, and 12 mg/m² for NOx during the study. Environmental factors such as pH and water temperature also affected nutrient assimilation, with varying effects on both polyculture and monoculture systems. Precipitation was also a key factor influencing microcystin reduction rates, while microcystin toxicity had no significant effect. In order to evaluate the role of microbes in the phytoremediation process, we also performed microbial analysis of wastewater samples and root biofilms, including 16S rRNA gene sequencing. This characterization of the bacterial community revealed significantly higher microbial diversity in the rhizosphere compared to the water. Proteobacteria dominated the rhizosphere (47%–52%) while cyanobacteria dominated the water (30%). The polyculture system had greater abundance of beneficial microbial taxa and metabolic pathways, which was associated with higher plant growth and enhanced nutrient assimilation.