Polishing wastewater effluent using plants: floating plants perform better than submerged plants in both nutrient removal and reduction of greenhouse gas emission

While research on aquatic plants used in treatment wetlands is abundant, little is known about the use of plants in hydroponic ecological wastewater treatment, and its simultaneous effect on greenhouse gas (GHG) emissions. Here, we assess the effectiveness of floating and submerged plants in removing nutrients and preventing GHG emissions from wastewater effluent. We grew two species of floating plants, Azolla filiculoides and Lemna minor, and two species of submerged plants, Ceratophyllum demersum and Callitriche platycarpa, on a batch of domestic wastewater effluent without any solid substrate. In these systems, we monitored nitrogen and phosphorus removal and fluxes of CO2, CH4 and N2O, for 2 weeks. In general, floating plants produced the most biomass, whereas submerged plants were rapidly overgrown by filamentous algae. Floating plants removed nutrients most efficiently; both floating species removed 100% of the phosphate while Lemna also removed 97–100% of the inorganic nitrogen, as opposed to a removal of 81–88% in submerged plants with algae treatments. Moreover, aquaria covered by floating plants had roughly three times higher GHG uptake than the treatments with submerged plants or controls without plants. Thus, effluent polishing by floating plants can be a promising avenue for climate-smart wastewater polishing. HIGHLIGHTS Floating plants rapidly reached high biomass, while submerged plants were overgrown by algae.; Nutrient uptake by floating plants was responsible for most N and P removal, while algae removed N and P in the submerged plant treatment.; Lemna was most efficient in removing N and P (up to 100% removal).; All treatments resulted in net greenhouse gas uptake. Small peaks in N2O and CH4 emissions were fully compensated by CO2 uptake.;