Field-scale artificial floating islands reduces cyanotoxin from residential raw sewage treatment basin

Abstract

Microcystin (MC) is the most prevalent and persistent cyanotoxin associated with freshwater harmful algal blooms (HABs), posing substantial health risks to both humans and aquatic ecosystems. Artificial floating islands (AFIs) offer a promising solution to MC reduction. In this study, we evaluated the effectiveness of a field-scale AFI system in reducing MC concentrations, using two native aquatic plants, Carex comosa (bristly sedge) and Eleocharis obtusa (blunt spike-rush), installed in the equalization basin of a wastewater treatment plant for the preliminary treatment of residential raw sewage. Over a three-month period during late summer and fall, we monitored MC concentrations, physico-chemical parameters, nutrient levels, and plant biomass. The results showed that the AFI system achieved up to a 77.9 % reduction in MC during peak plant growth. Additionally, the relative abundance of cyanobacteria decreased from 27.7 % to 4.5 % following AFI treatment during this period. We identified precipitation as a key factor influencing MC reduction rates in natural settings (r = − 0.56, p-value = 0.06 between accumulated precipitation and reduction in MC concentrations). Despite the presence of MC, plant growth and MC reduction were not significantly hindered by MC toxicity (r = 0.01, p-value = 0.99). While temperature and nutrient concentrations did not directly affect MC reduction rates, they did influence plant biomass production, which in turn impacted AFI performance. C. comosa exhibited greater potential for MC reduction compared to E. obtusa, due to its faster growth and higher biomass production. This study is the first investigation of field-scale AFIs targeting MC as the primary pollutant, emphasizing their feasibility as a sustainable and effective strategy for managing cyanotoxin during HABs.