According to research by Lisanne Hendriks, an ecologist at Radboud University, azolla and duckweed are the best floating aquatic plants for purifying wastewater. The plants absorb nitrogen and phosphorus from wastewater and so render it much cleaner. The phosphorus and other pollutants in the water can then be recovered and reused. The work is published in the journal Water Science & Technology.
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Source: Phys.org