Dr John Awad and Dr. Divina Navarro, CSIRO

Constructed Floating Wetlands (CFWs), which are also called floating treatment wetlands (FTW), free-floating wetlands (FFW) or artificial floating islands (AFI), are a relatively novel nature-based water treatment technology that has seen a sharp increase in adoption during the last 20 years. CFWs have been used for both stormwater and wastewater treatment, as well as for provision of habitat and for aesthetic enhancement.

Pilot floating wetland systems at a) urban stormwater stream channel1 and 2) wastewater lagoon2

What are CFWs?

CFWs mimic the appearance of natural floating islands but are enhanced with the advantages of a wetland system's biological and biogeochemical processes. CFWs provide water treatment functions commonly found in both pond and conventional constructed wetland systems. Unlike a conventional constructed wetland, CFWs use a buoyant structure to support planting of the vegetation, allowing the roots to grow freely into the water body, deriving nutrients from the water in which it is floating. The large surface area of plant roots also provides a habitat for microorganisms (biofilms) which facilitate nutrient removal through phytodepuration and capture of suspended particles within the water source. As they float on the water surface, CFWs can be retrofitted into existing waterbodies without impacting flood storage capacity.

Schematic diagram of a constructed floating wetland (Source[1])

What are the potential applications of CFWs?

Commonly reported benefits of CFWs include their efficacy in removing nutrients (phosphorus and nitrogen-containing compounds) from stormwaters, and wastewaters (industrial and domestic). CFWs can attain removal rates up to 98% for total phosphorous (TP) and greater than 90% for total nitrogen (TN) from diverse water sources[1, 2]. It can also be used for removing heavy metals such as cadmium (Cd), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni) and zinc (Zn)[3].
In addition to the above treatment benefits, a wide range of investigations have been conducted on the capacities of CFWs for removal of organic compounds present in various waters and wastewaters. These include removal of pharmaceutical and personal care product compounds (PPCPs), pesticides and fire-retardant chemicals.

Our recent research has shown the capability of three hydroponically grown wetland species (Phragmites australis, Baumea articulata and Juncus krausii) to bioaccumulate and translocate two per-and polyfluoroalkyl substances (PFAS), namely perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) [4]. These are commonly found in fire-fighting foams, and have been detected in stormwater. Consequently, these plants could serve as promising candidates for integration into a CFW system.

Percentage removal of PFOS and PFOA by various wetland plant species (Source[4])

There are many reported benefits of CFWs that extend from water treatment for removal of pollutants and water quality improvements. These benefits include enrichment of aquatic biodiversity, wave energy dissipation, landscape amenity enhancement and increased socio-economic value.

What are the key factors influencing the CFW’s performance?

A wide range of pollutant removal rates have been reported for CFWs in the field. The CFWs performance and the ability of nutrients, heavy metals, and organic compounds to accrue and translocate within the plant depend on a range of factors. This includes plant type(s) used, management of the CFW system (e.g., rate of plant growth harvesting), water conditions (temperature, flow rate, turbidity, aerobic, anoxic, anaerobic) and climate. Further, the treatment performance of a CFW highly depends on the hydrodynamic flow conditions around the plant root systems (i.e., the fraction of water in the system that passes through a root zone; and the residence time distribution within those root zones).
Where do we go from here?

One of the challenges in evaluating CFWs is understanding the impact of the technology in terms of treatment efficiencies and reaching defined and expected performance targets. Assessments appear to be often based on quantitative determination of key elements and/or compounds (and derivative compounds) that have accumulated in plant tissues and/or based on experimental configurations by which removal rates from test waters are examined. To date, few studies have been conducted to examine treatment efficiency from a mass balance approach where the key elements and/or compounds are quantified at the inflow of the water body being treated, followed by quantification across different phases of the water body (water, sediment, sludge), and finally evaluating untreated discharges from the water body, i.e., pollutants not removed following the CFW treatment.

Currently, controlled pilot CFWs have been installed in an urban stormwater stream channel at Salisbury, South Australia1 and in a wastewater lagoon at Cowes Wastewater Treatment Plant, Victoria2. These trials will allow us to assess long-term overall performance of the CFW treatment. Seasonal performance variations would be also considered.

1 This project is a joint initiative between Salisbury Water, University of South Australia, Bygen, Clarity Aquatic, Covey Associates and CSIRO with R&D funding from Salisbury Water.

2 This Project is a joint initiative between Westernport Water, Deakin University, Clarity Aquatic, Covey Associates and CSIRO with funding from the Victorian Government, Intelligent Water Networks and Yarra Valley Water.

For more information, feel free to contact Dr. John Awad (john.awad@csiro.au) and Dr. Divina Navarro (divina.navarro@csiro.au) at CSIRO. If you’re coming to the Ecoforum/SustRem 2023, make sure to attend the presentations on CFWs by Dr. John Awad.

References:

[1] J. Ayres, J. Awad, C. Walker, D. Page, J. van Leeuwen, S. Beecham, Constructed Floating Wetlands for the Treatment of Surface Waters and Industrial Wastewaters, in: N. Pachova, P. Velasco, A. Torrens, V. Jegatheesan (Eds.), Regional Perspectives of Nature-based Solutions for Water: Benefits and Challenges, Springer International Publishing, Cham, 2022, pp. 35-66. https://doi.org/10.1007/978-3-031-18412-3_3.
[2] J. Awad, G. Hewa, B.R. Myers, C. Walker, T. Lucke, B. Akyol, X. Duan, Investigation of the potential of native wetland plants for removal of nutrients from synthetic stormwater and domestic wastewater, Ecological Engineering 179 (2022) 106642. https://doi.org/https://doi.org/10.1016/j.ecoleng.2022.106642.
[3] M. Afzal, K. Rehman, G. Shabir, R. Tahseen, A. Ijaz, A.J. Hashmat, H. Brix, Large-scale remediation of oil-contaminated water using floating treatment wetlands, npj Clean Water 2(1) (2019) 3. https://doi.org/10.1038/s41545-018-0025-7.
[4] J. Awad, G. Brunetti, A. Juhasz, M. Williams, D. Navarro, B. Drigo, J. Bougoure, J. Vanderzalm, S. Beecham, Application of native plants in constructed floating wetlands as a passive remediation approach for PFAS-impacted surface water, Journal of Hazardous Materials 429 (2022) 128326. https://doi.org/https://doi.org/10.1016/j.jhazmat.2022.128326.