Citation: Wilkinson, S.R.; Naeth, M.A.; Dhar, A. Potential of Macrophytes for Wastewater Remediation with Constructed Floating Wetlands in Cold Climates. Water 2023, 15, 2479. https:// doi.org/10.3390/w15132479 Academic Editor: Terry Lucke Received: 1 June 2023 Revised: 28 June 2023 Accepted: 3 July 2023 Published: 6 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Potential of Macrophytes for Wastewater Remediation with Constructed Floating Wetlands in Cold Climates Sarah R. Wilkinson, M. Anne Naeth * and Amalesh Dhar Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada * Correspondence: anaeth@ualberta.ca Abstract: Nature-based, low technology wastewater treatment systems can benefit small and remote communities. Adding a constructed floating wetland (CFW) to waste stabilization ponds can enhance treatment efficacy at low cost, depending on appropriate macrophytes. In cold climates, harsh growing conditions may limit CFW success, requiring research under-ambient field conditions. Seven native macrophytes were assessed for the growth, biomass production, and root and shoot uptake of potential contaminants of concern from municipal wastewater in a facultative stabilization pond in Alberta, Canada. All macrophytes established. Scirpus microcarpus had high nitrogen and phosphorus in roots and shoots and phytoextraction potential. Metal and trace elements were highest in Glyceria grandis, Beckmannia syzigachne, and Scirpus microcarpus, mostly greater in roots than shoots, indicating phytostabilization. Tissue contaminant concentrations did not always indicate high contaminant accumulation in the CFW. Total uptake per unit area was greatest for Glyceria grandis, although chromium and molybdenum were greatest in Beckmannia syzigachne and Carex aquatilis, respectively. Beckmannia syzigachne and Scirpus microcarpus have potential for phytoremediation if biomass per unit area is increased. Species variability is high for contaminant accumulation and biomass; in unpredictable climates and wastewaters with suites of contaminants, different macrophytes for wetland water treatment systems are recommended. Keywords: constructed wetland; nitrogen; phosphorus; metals; plant species selection 1. Introduction Wastewater stabilization ponds are frequently used to treat municipal and industrial wastewater in small and remote communities. Stabilization ponds can effectively reduce suspended particles, biochemical oxygen demand, coliform bacteria, nutrients, metals and micropollutants [1]. Common contaminants in municipal wastewater include nitrogen, phosphorus, lead, nickel, mercury, cadmium, chromium, copper, and zinc [2–5]. Adding a constructed floating wetland (CFW) to a stabilization pond can further assist in contami- nant removal during wastewater treatment [6–9], providing the same benefits as natural wetlands for water management and purification. These floating platforms form terrestrial habitat in aquatic environments while helping improve quality of the water in which they float. In contrast to traditional constructed wetlands, floating ones are not impacted by fluctuations in water and plant roots are permanently in contact with the water. Planted with native macrophytes, phytoremediation is the main mechanism of water treatment and includes the physical filtering of suspended sediments, water aeration, direct uptake of con- taminants, and a suite of rhizosphere facilitated processes including biofilm establishment and immobilization and the mineralization of contaminants. All the mechanisms involved in water treatment with CFWs are not completely under- stood [10]. The indirect role plants play may far exceed their direct role in the removal of contaminants [11]. For example, filtering by plant roots reduces biological oxygen demand and total suspended solids [6], and anerobic waste stabilization ponds, common in the arctic [12], may benefit from aeration and oxidation by plants. While phytoremediation Water 2023, 15, 2479. https://doi.org/10.3390/w15132479 https://www.mdpi.com/journal/water