Field-scale application of floating wetlands in a wastewater treatment plant
The vast majority of studies on the use of artificial floating islands (AFIs) for remediation of water contamination has been conducted in laboratory settings (microcosms and mesocosms). As a result, our mechanistic understanding of the effectiveness of AFIs in natural settings is limited. To address this knowledge gap, we constructed a field-scale AFI system installed with two native wetland aquatic plants – Carex comosa (bristly sedge) and Eleocharis obtusa (blunt spike-rush), in the equalization basin of the Plymouth Wastewater Treatment Plant, located in north-central Ohio, USA (figure above).
The Plymouth WTP (figure above) has been in operation for 14 years. This facility receives an approximate daily influx of 950 m3 of residential raw sewage, which goes into a system comprising two sequencing batch reactors (SBRs) for biological treatment of total suspended solids (TSS), biological oxygen demand (BOD), Escherichia coli (E. coli), and nutrients. After treatment, the effluent is discharged into the west branch of the Huron River. During heavy precipitation when the flow rate exceeds the plant’s treatment capacity, this basin stores combined sewage overflow (CSO), which contains stormwater and untreated wastewater. Subsequently, after each precipitation event, the water retained in the equalization basin is gradually introduced into the plant as standard sewage. The basin has an area of 30,000 m2 and holds a permanent pool of water with an average depth of 1 m and minimum depth of 0.6 m. The return line (outlet), directing water from the equalization basin to the SBRs, is located at the southeast corner of the equalization basin (figure above). The supply line (inlet), receiving overflow influent, sits 20 meters west from the return line.
We installed a total of twelve AFI units at the southeast corner of the equalization basin, situated between the supply line (inlet) and the return line (outlet), on Apr 26, 2022. These AFI units were organized into four types: three units for the exclusive installation of C. comosa (C1, C2, and C3 in the figure above), three for E. obtusa (E1, E2, and E3 in the figure above), three for a mixed polyculture of both species (CE1, CE2, and CE3 in the figure above), and the remaining three units were deployed without plants (B1, B2, and B3 in the figure above). The photo below the deployment schematic was taken on August 9, 2022, fifteen weeks after the initial deployment. This AFI deployment achieved approximately 5% effective coverage over an area of 80 m2. We installed two AFI units near the supply line (inlet) and designated them as plant nurseries (N1 and N2 in the deployment schematic – figure above). Plant tissues were collected bi-weekly from these nurseries and used for biomass analysis. Harvesting of the plants in the linear experimental systems took place at the end of the study, on November 16, 2022.
Environmental conditions at the equalization basin
Water temperature during the study showed dramatic variation, from the highest of 28.1°C on Aug 24 to the lowest of 3.3°C on Nov 16 (figure above). This wide temperature range covered the peak growth conditions for the aquatic plants, as well as the optimal temperature range for cyanobacteria’s active growth (exceeding 20 °C from Aug 16 to Sep 21) and dormancy (below 10°C on Oct 19 and Nov 16). Temperature is a critical environmental factor that affects AFI’s ability to remove pollutants such as nutrients, total organic carbon, and heavy metals. Elevated temperatures accelerate chemical reactions involved in the conversion and decomposition of compounds such as total nitrogen (TN) and total phosphorus (TP), and enhance microbial activities within plant root systems, although they also reduce oxygen solubility, affecting the microbial composition of the root biofilm.
Throughout the monitoring period, dissolved oxygen (DO) concentrations in the equalization basin remained low, ranging from 0.6 mg/L to 2.5 mg/L, with the lowest levels observed in August, coinciding with the highest temperatures observed in summer and consequently the lowest oxygen solubility. This low DO level was likely due to the lack of aeration in the equalization basin, compounded by high levels of biodegradable organic matter in the sludge, which resulted in increased oxygen consumption by bacterial activity.
The pH in the equalization basin was consistently alkaline throughout the monitoring period, ranging from 7.3 to 9.2 (figure above). The highest pH was observed on Aug 16, with the lowest on Oct 26. Research has shown that aquatic plants exhibit significantly reduced growth rates at lower pH levels. Furthermore, pH affects the composition of phytoplankton species in aquatic systems. Under conditions ranging from neutral to weak alkaline, it has been reported that Microcystis aeruginosa, the major contributor to HABs in freshwater, becomes dominant and outcompetes the less harmful green algae.
During the study, ammonium (NH₄⁺) concentrations remained low, ranging from 0.02 mg/L to 0.05 mg/L (figure above), notably lower than the typical range found in residential wastewater (1 mg/L to 13 mg/L). This low level may be attributed to the alkaline environment in the equalization basin. Conversely, orthophosphate (PO₄³⁻) levels displayed notable fluctuations, from a minimum of 0.2 mg/L on Aug 16 to over 2 mg/L between Sep 7 and Oct 5 (figure above). Similar temporal variations were also observed in total oxidized nitrogen (NO2⁻ + NO3⁻), which showed elevated levels exceeding 5 mg/L in September (figure above).
Freshwater harmful algal blooms (HABs), resulting from the excessive proliferation of cyanobacteria, present a substantial and pervasive environmental challenge affecting all 50 states in the US and most freshwater systems globally. A major concern with these blooms is the production of cyanotoxins and other harmful metabolites by certain cyanobacterial species, which pose risks to the health of humans, animals, and aquatic ecosystems. Among these cyanotoxins, microcystin (MC) stands as the most widespread and persistent, leading to severe health effects in humans, such as pneumonia, liver failure, and increased cancer risk from both short- and long-term exposure. One of the objectives of our study was to evaluate the efficiency of AFIs in removing microcystin from wastewater prior to treatment by the WTP.
The figure above shows MC concentrations throughout the experiment at the inlet, outlet, and control sites (see figure 3 for the location of these sampling sites). MC concentrations at all sampling sites were significantly correlated with temperature, with high values during August (above 8 µg/L), and lower values toward late fall (below 1 µg/L). Pink rectangles highlight three instances where the outlet MC concentration was higher than the inlet. Grey bars are 4-day accumulated rainfall. Several instances of heavy precipitation exceeding 3 cm/day were recorded during the monitoring period, with significant rainfall intensities observed on Sep 4 (4.22 cm/day), Sep 19 (4.70 cm/day), and Nov 11 (3.07 cm/day) – figure above.