How the Treatment Works
Understanding floating wetland performance starts with understanding the treatment mechanism. Without this context, the variability in published results can look like inconsistency. It's not. It's a biological system responding to local conditions.
The primary treatment engine in a floating wetland is not the plant. It's the biofilm. Root systems that grow down through the buoyant mat and into the water column generate between 4.6 and 9.3 square meters of submerged biological surface for every square meter of mat (Tanner & Headley, 2011; cited in Neal & Lloyd, 2018). That surface is colonized by microbial communities that form a structured biofilm performing the biochemical work of nutrient transformation.
Clemson University Extension reports that approximately 80% of nutrient removal occurs through this root zone biofilm activity, with direct plant uptake accounting for the remaining 20% (Escamilla, Scaroni & White, 2024). Within the biofilm, microbial communities self-organize into layered gradients -- aerobic bacteria near the oxygen-rich outer surface performing nitrification, anaerobic bacteria in the inner layers performing denitrification -- enabling simultaneous processing of both oxidized and reduced nitrogen species on the same root.
Beyond nutrient transformation, the root zone provides physical filtration. Suspended solids are trapped by the dense root network, promoting flocculation and sedimentation. This is why TSS removal is typically the most consistent performance metric across studies -- the physical mechanism is less dependent on biological maturity than the nutrient cycling pathways.
Performance variability across studies reflects real differences in the factors that govern biological treatment: root maturity, surface coverage percentage, placement relative to water flow, plant species and density, water temperature and growing season length, influent pollutant concentrations, and pond design characteristics. These aren't flaws in the technology. They're design parameters.
Approximately 80% of floating wetland nutrient removal is driven by root zone biofilm activity, not direct plant uptake. Performance varies based on coverage, placement, root maturity, and local conditions -- all of which are design decisions.
The complete biological cascade -- from biofilm microbial gradients through invertebrate colonization to food chain assembly -- is detailed in The Underwater Ecosystem: How Floating Wetlands Build a Food Chain. This page focuses on the treatment performance numbers.
Nutrient Removal: The Field Data
This section presents results from field-scale studies -- real ponds, real stormwater, real monitoring programs. Mesocosm experiments are valuable for isolating variables, but field data is what engineers and managers need to make decisions. The numbers here represent what floating wetlands have actually measured in practice.
The Range
Clemson University Extension reports total nitrogen removal rates of 70-97% across studies (Escamilla, Scaroni & White, 2024). That wide range reflects real differences in application type, coverage level, climate, and study design. It sets the frame: floating wetlands can deliver substantial nutrient removal, but what you get depends on how you design the system.
North Carolina: Four Ponds, Pre- and Post-Retrofit
The strongest single performance dataset comes from two studies at North Carolina State University, monitoring four wet ponds before and after FTW installation (NC State Extension, 2024; summarizing Winston et al., 2013 and Landon, 2023). The ponds ranged from 0.12 to 2.1 acres, with FTW coverage from 1% to 18% and both random and strategic placement designs.
| Pond | Coverage | Placement | TN Removal | TP Removal | TSS Removal |
|---|---|---|---|---|---|
| Pre-Retrofit (no FTWs) | |||||
| Pond 1 | -- | -- | 36% | 36% | 92% |
| Pond 2 | -- | -- | 59% | 57% | 89% |
| Pond 3 | -- | -- | 28% | 36% | 16% |
| Average | -- | -- | 41% | 43% | 66% |
| Post-Retrofit (with FTWs) | |||||
| Pond 1 | 9% | Random | 48% | 39% | 78% |
| Pond 2 | 18% | Random | 88% | 88% | 95% |
| Pond 3 | 3.5% | Strategic | 52% | 46% | 45% |
| Pond 4 | 1% | Strategic | 42% | 77% | 89% |
| Average | -- | -- | 58% | 63% | 77% |
Several patterns in this data matter. Post-retrofit averages improved across all three parameters: total nitrogen removal went from 41% to 58%, total phosphorus from 43% to 63%, and total suspended solids from 66% to 77%. But the individual pond results tell a more nuanced story.
Pond 2, with 18% coverage and random placement, produced the strongest overall numbers: 88% TN, 88% TP, and 95% TSS removal. But Pond 4, with just 1% coverage placed strategically near the outlet, achieved 77% TP and 89% TSS removal. That is a fraction of the coverage producing comparable phosphorus and sediment performance -- because placement forced stormwater through the root zone rather than allowing it to bypass the treatment area.
Meanwhile, Pond 1 at 9% random coverage showed only modest improvement over its pre-retrofit baseline. More coverage scattered randomly across the pond did not outperform less coverage placed with intent.
Australia: A Full-Year Urban Stormwater Dataset
A 12-month field study in Queensland, Australia monitored a floating treatment wetland receiving stormwater from a 7.46-hectare urban residential catchment (Walker, Tondera & Lucke, 2017). Overall removal performance was 80% for TSS, 53% for TP, and 17% for TN at a FTW footprint of just 0.14% of the contributing catchment area.
The low TN result is worth explaining rather than hiding. Influent nitrogen concentrations were often very low to begin with, and researchers noted that percentage-based removal metrics understate system activity when incoming concentrations are already near background levels. The strong TSS and TP results from this same study confirm the treatment mechanisms were active.
New Zealand: Side-by-Side Comparison
A field trial near Auckland compared two geometrically similar stormwater retention ponds receiving the same inflows -- one retrofitted with a floating treatment wetland, one without (Borne, Fassman & Tanner, 2014). The FTW pond showed 27% lower total phosphorus outlet concentrations than the conventional pond. Researchers identified sedimentation enhancement and physical entrapment in the root zone as the primary phosphorus removal pathways, rather than direct plant uptake.
What the Data Tells Us Collectively
TSS removal is the most consistent benefit across studies (typically 70-95%). TP removal is strong and measurable (40-80%+). TN removal is the most variable (17-97%) and depends heavily on coverage, retention time, and influent concentrations. Higher coverage and strategic placement produce dramatically better results. Biological systems need time to mature -- performance improves over seasons as root systems and biofilm communities develop.
Coverage, Placement, and Design Optimization
The NC State dataset makes a critical point: the same technology produces dramatically different results depending on design decisions. Coverage and placement are the two variables that matter most, and the research provides clear guidance on both.
How Much Coverage?
Clemson University Extension recommends 5-10% surface coverage for water quality improvement in stormwater ponds (Escamilla, Scaroni & White, 2024). North Carolina's Department of Environmental Quality requires a minimum of 5% coverage for FTW retrofit nutrient credits (NC State Extension, 2024). For fisheries applications, measurable benefits have been documented at just 2.3% (Neal & Lloyd, 2018).
A meta-analysis of eight international FTW field studies confirmed coverage as one of the top variables driving treatment performance, along with pond loading ratio and planting density (Tirpak et al., 2022). More coverage means more root surface, more biofilm, and more treatment capacity. But coverage alone doesn't tell the whole story.
Where You Put Them Matters as Much as How Many
The NC State research produced one of the most actionable findings in the floating wetland literature: strategic placement can compensate for lower coverage. Winston et al. (2013) found that 9% random coverage produced no statistically significant improvement in nutrient removal, while Landon (2023) showed that 1-3.5% strategic coverage -- FTWs placed near the outlet and across the flow path -- produced significant reductions across multiple parameters (NC State Extension, 2024).
The mechanism is straightforward. Random placement allows stormwater to flow around and under the FTW modules without contacting the root zone. Strategic placement forces water through the roots by spanning the flow path, which increases hydraulic retention time and maximizes the contact between polluted water and the biofilm treatment surface. Staggered positioning across the flow path impedes short-circuiting, the single biggest factor that undermines pond treatment performance.
Design Criteria
NC State Extension provides detailed design guidance based on their four-pond monitoring program (NC State Extension, 2024). The key parameters: pond depth should be 3-6 feet in the FTW placement zone (shallower than 3 feet risks plants rooting into the bottom sediment; deeper than 6 feet allows water to flow under the roots). Plant density should target approximately one plant per 2 square feet, maintaining at least 85% plant coverage on the mats. Multi-species plantings have shown improved growth and nutrient uptake compared to monocultures. And total FTW coverage should not exceed 50% of the pond surface to avoid depleting dissolved oxygen in the water column.
Strategic FTW placement near pond outlets and across the flow path produces significant pollutant removal at coverage levels as low as 1-3.5%, while random placement may require 18%+ coverage for comparable results. Design intent -- not just quantity -- determines performance.
What Floating Wetlands Cost (and How They Compare)
Until recently, the cost literature for floating treatment wetlands was thin enough to make financial comparisons difficult. That changed in 2025 with the publication of the first large-scale international cost analysis.
CSIRO Cost Analysis: 11 International Projects
Researchers from CSIRO and the University of South Australia analyzed 11 constructed floating wetland schemes across Australia, the United States, Canada, and Pakistan (Awad et al., 2025). The analysis covered schemes ranging from 55 to 3,926 square meters and included both capital expenditure (CAPEX) and operating expenditure (OPEX).
Capital costs ranged from US$15 to $2,537 per square meter, reflecting significant differences in design complexity, labor costs, and location. Annual operating costs ranged from $0.50 to $181 per square meter. The wide ranges underscore that floating wetlands can be built at very different price points depending on local conditions and project requirements.
For nutrient removal specifically, the cost per kilogram of nitrogen removed by plant uptake was $10 to $120 -- consistently lower than the cost of phosphorus removal, which ranged from $15 to $3,250 per kilogram (Awad et al., 2025). Two findings stood out. First, scale matters: larger installations reduced the cost per kilogram of nutrients removed, making them more economical over time. Second, climate matters: wetlands in warmer regions with longer growing seasons delivered better cost-effectiveness due to higher pollutant removal rates.
The researchers concluded that floating wetlands are competitive with other engineering treatment options, especially for nitrogen removal (Awad et al., 2025).
The Land Cost Advantage
One of the most significant economic advantages of floating wetlands is invisible in a cost-per-square-meter analysis: they don't require additional land. Constructed surface-flow wetlands typically require 4-8% of the contributing catchment area as dedicated treatment land. Bioretention systems require 1-2%. For municipalities where stormwater retention ponds already exist, floating wetlands retrofit the existing infrastructure using the existing pond surface. There is no land acquisition, no earthmoving, no loss of developable acreage. In urban and suburban settings where land costs represent the single largest barrier to new treatment infrastructure, this advantage can dwarf the cost of the wetland modules themselves.
Breakeven Against Chemical Treatment
For ponds currently managed with ongoing chemical treatment programs (algaecides, phosphorus binders, dyes), Clemson Extension reports that floating wetland installations break even against those recurring costs within 4 to 13 years (Escamilla, Scaroni & White, 2024). After breakeven, the comparison shifts permanently: chemical programs require perpetual purchasing with no compound benefit, while the biological treatment capacity of a floating wetland system continues to strengthen as root systems expand and biofilm communities mature.
CSIRO's analysis of 11 international projects found nitrogen removal costs of $10-$120/kg, competitive with conventional treatment. Floating wetlands require no additional land and break even against chemical treatment programs within 4-13 years, after which biological performance continues to compound while chemical costs continue to accumulate.
Installation and Maintenance
Floating wetland performance depends on getting the installation right. NC State Extension's four-pond monitoring program produced detailed, field-tested guidance for practitioners (NC State Extension, 2024).
Site Requirements
The pond must be accessible for both initial installation and periodic maintenance. Depth in the FTW placement zone should be 3-6 feet. Shallower areas risk plant roots reaching the bottom sediment and anchoring to the substrate, which prevents the mat from rising with water levels and can kill the vegetation. Areas deeper than 6 feet allow stormwater to flow beneath the root system, bypassing treatment. Ponds with existing forebay structures and well-positioned outlets provide the best retrofit opportunities.
Mat and Plant Selection
Several commercial FTW products have established track records, including Beemats, BioHaven floating islands, and Biomatrix systems (NC State Extension, 2024). Plant density should target approximately one plant per 2 square feet, with at least 85% plant coverage maintained on the mats. Multi-species native plantings are recommended over monocultures -- research has shown improved plant growth and nutrient uptake with species diversity. Plant species should be selected for the local climate and growing conditions, with emphasis on proven wetland species appropriate to the region.
Anchoring
FTW mats require anchoring to prevent drifting, which would compromise the strategic placement that drives performance. Bottom anchoring works in ponds with stable substrate; bank anchoring is preferred where shoreline access allows. The anchoring system must accommodate water level fluctuations -- the defining advantage of floating wetlands over rooted constructed wetlands is that mats rise and fall with the water surface. Anchors should be positioned so FTWs don't block outlets, spillways, or emergency overflow structures.
Ongoing Maintenance
Vegetation harvesting is the most important maintenance activity because it permanently removes the nutrients stored in plant tissue from the water system. Without periodic harvesting, nutrients sequestered in plant biomass can re-enter the water column when plants senesce. Seasonal inspections should assess mat integrity, plant health, anchor condition, and coverage levels. Dead plants should be replaced to maintain the 85% coverage threshold, and invasive species that colonize the mats should be removed. The overall maintenance burden is low compared to chemical treatment programs or mechanical aeration systems, with no consumable inputs required beyond occasional plant replacement.
Successful installation requires 3-6 foot pond depth, strategic placement near outlets, multi-species native plantings at approximately one plant per two square feet, and periodic vegetation harvesting to permanently export nutrients from the system.
Regulatory Recognition
As field data accumulates, regulatory frameworks are beginning to formally recognize floating wetlands as creditable stormwater control measures. The regulatory landscape is still developing, but early adopters are providing the template.
North Carolina's Department of Environmental Quality awards formal nutrient removal credits for FTW retrofits on stormwater wet ponds (NC State Extension, 2024; citing NCDEQ, 2018). Under the state's Stormwater Control Measure Credit Document, a wet pond built to minimum design criteria receives a total nitrogen effluent credit of 1.22 mg/L and a total phosphorus credit of 0.15 mg/L. Adding FTWs at 5% or greater coverage improves those credits to 0.85 mg/L for TN and 0.09 mg/L for TP -- a meaningful reduction that can help municipalities meet water quality targets without building new infrastructure.
Florida has assigned a 12% treatment credit for floating wetlands toward watershed nitrogen and phosphorus nonpoint source reduction goals (NC State Extension, 2024; citing Wanielista et al., 2012). Other states are evaluating similar crediting frameworks as published field data expands the evidence base.
For municipalities facing Total Maximum Daily Load (TMDL) compliance requirements, FTWs represent a distinctive option: they retrofit existing retention pond infrastructure to improve nutrient removal without requiring additional land, new permitting for constructed treatment systems, or the ongoing chemical costs of alternative approaches. The regulatory framework is catching up to the research.
North Carolina awards formal nutrient credits for FTW retrofits at 5%+ coverage, improving wet pond TN effluent credits from 1.22 to 0.85 mg/L and TP from 0.15 to 0.09 mg/L. Florida provides a 12% treatment credit. As field data accumulates, additional states are expected to follow.
What Honest Assessment Looks Like
The data presented on this page supports a clear conclusion: properly designed floating treatment wetland installations improve pond water quality. The trend is consistent across studies, geographies, and application types. But honesty about what the research does and doesn't tell us builds more credibility than cherry-picking the strongest results.
Most published field studies span one to three years. Long-term datasets covering 10+ years of continuous operation are still limited, which means projections about multi-decade performance rely partly on the trajectory of biological maturation rather than completed observation. Cold climate performance data is thinner than warm and temperate datasets, reflecting both the geographic concentration of published studies and the real biological constraint that growing season length affects annual treatment capacity.
Total nitrogen removal is the most variable parameter in the literature and the hardest to guarantee for a specific installation. TN performance depends on a chain of biological processes -- nitrification followed by denitrification -- that require both aerobic and anaerobic conditions, sufficient contact time, and adequate influent concentrations to begin with. When incoming nitrogen is already low, percentage-based removal metrics can understate system activity. When coverage is insufficient or placement allows bypass flow, the biological chain gets interrupted.
The CSIRO research team put it directly: floating wetlands are not a silver bullet but should be considered as part of a broader suite of water treatment options (Awad et al., 2025). That framing is accurate and worth adopting.
What the data does support, consistently, is this: floating wetlands treated as engineered systems -- with intentional coverage levels, strategic placement, appropriate plant species, and regular maintenance -- produce measurable, significant improvements in pond nutrient and sediment removal. The installations that underperform tend to be undersized, randomly placed, or inadequately maintained. The technology works. The design decisions determine how well.
And unlike every chemical treatment alternative, the biological performance of a floating wetland system compounds over time. Root systems expand. Biofilm communities mature and diversify. Invertebrate populations establish. The treatment capacity in year five exceeds year one, and year ten exceeds year five. That trajectory doesn't exist with any product you pour into the water.
Floating wetland performance is a design outcome, not a guarantee. Properly designed installations consistently improve pond water quality, with biological treatment capacity that compounds over time as root systems and biofilm communities mature. The honest conclusion from the research is not that floating wetlands always work -- it's that they work when you design them to.