Floating Treatment Systems, Report
Agricultural, non point source contributors face increasing pressure to remediate or reduce nutrient rich runoff
as concerns over deteriorating water quality increase. Constructed wetlands, both surface and subsurface flow,
are often used to remediate nutrient rich waters, but the large land area required for installation sometimes limits
their applicability. Floating mat treatment systems (FMSs) are potential alternatives to constructed wetland
systems for remediation of nutrient rich water. They provide nutrient-processing functions similar to wetlands
with large root surface area exposed in the water column. The floating plant root system provides adequate area
for microbial colonization, filtration, sedimentation, and plant nutrient uptake. An additional benefit is easier
harvest of plant mass for additional nutrient removal. Ease of harvest in not typically considered when
constructed wetlands are designed; rather they are designed for a specific function. Floating mat systems can
potentially provide similar functions, as well as facilitating quick installation, rapid establishment, and simpler
harvest. Any nutrients fixed either in plant roots or shoots are easily removed from the aquatic system as plants
are harvested. This harvested tissue may then be used as a nutrient source if properly composted. Floating mat
systems exhibit great potential for nutrient remediation.
The goal of this research was to assess the potential of FMSs for remediating agrichemicals prior to
entry into water bodies. The experiment is divided into two treatment classes, the first consists of smaller scale
(one 88 ft3 and two 51 ft3 units) channels and the second consists of larger-scale (one 500 ft3 and one 918 ft3)
ponds. Plants from each species examined were seated in floating mats were placed in the flow-through
channels and ponds April 14, 2008 and fertilizer treatments were started May 2, 2008, thus during the first
portion of the establishment period no additional nutrients were applied. Canna flaccida and Juncus effusus
were established in the vegetative channel treatments. Canna flaccida, Juncus effusus, Eleocharis montana,
Agrostis sp. were established in floating mats in the pond treatments. Sampling began 3 days after initiation of
fertilizer addition. Sampling involved measuring dissolved oxygen, pH, and conductivity of the water,
measuring shoot height and width, root length and lateral width, flowering status from three plants per treatment
unit, and collecting one water sample at a particle point in time from each of the ponds and vegetated channels.
Water samples were analyzed for anions (Cl, NO2, NO3, PO4, and SO4) via IC analysis with a Dionex
AS10 IC with AS50 auto-sampler (Dionex Corp., Sunnyvale, CA), total organic carbon (dissolved carbon from
organic sources that is available for microbial metabolic functions) via NPOC/TN analysis using a Shimadzu
TOC-V CPH total organic carbon analyzer with TNM-1 total nitrogen measuring unit (Shimadzu Scientific
Instruments, Kyoto, Japan), and ions (total P, K, Ca, Mg, Zn, Cu, Mn, Fe, S, Na, B, and Al) were analyzed via
inductively coupled plasma emission spectrophotometer (ICP-ES, 61E Thermo Jarrell Ash, Franklin, MA).
Final water samples and harvest of selected plant tissues occurred Sept. 18, 2008. Roots and shoots of each
2 of 6
species were dried at 80 ºC, weighed, and ground in a Wiley mill (Swedesboro, NJ) to pass through a 40-mesh
(0.425-mm) screen. Nitrogen concentration was determined using 100 mg of tissue and assayed using an
Elementar Vario Macro Nitrogen combustion analyzer (Mt. Laurel, NJ) with tissue analysis procedures
described by Clemson University’s Agricultural Service Laboratory (Anonymous, 2000), and P was assayed by
wet acid digestion procedure using the nitric acid and hydrogen peroxide method (Anonymous, 2000; Mills and
Jones, 1996). Phosphorus, K, Ca, Mg, Zn, Cu, Mn Fe, S, Na, B, and Al concentrations were in plant tissues
determined by ICP-ES.
The FMSs are able to reduce both nitrogen and phosphorus export from the treatment system (Figure 1).
Nitrogen removal is consistent with both inflow and effluent in both the pond and vegetated channel treatments
(Figure 1A and 1B). Average nitrogen removal in constructed wetland systems ranges from 0 to 84.2% with
average concentrations ranging from 0.7 to 55.0 mg/L NO3-N (Vymazal, 2007). Nitrogen loading rates for this
experiment were in the very low range, and that consistent nitrogen removal achieved was slightly better than in
other constructed wetland treatment systems.
Phosphorus removal was also consistent and effluent P concentrations averaged below 0.03 ± 0.01 mg/L
total P for both the pond and vegetated channel treatments over the 4.5 months sampling period (Figure 1C and
1D, Table 1). The range of P concentrations entering other constructed wetland treatment systems ranges from
0.7 to 10.5 mg/L PO4-P and PT (Taylor et al, 2006; Vymazal, 2007) with average effluent concentrations
ranging from 0.02 to 5.15 mg/L PO4-P and PT. Previous research in our lab has indicated that constructed
wetland systems are not effective at remediating phosphorus when inflow concentrations are below 1 ppm.
However, even with loading concentrations of PT into the pond and vegetated channel treatments averaging
0.075 ± 0.01, effluent PT were consistently reduced by 67% to less tan 0.03 mg/L PT within effluent, over the
sampling period (Figure 1C and 1D). Other researchers have shown much greater variability in phosphorus
remediation both with effluent concentrations achieved and seasonal efficacy. These results indicate that FMSs
may have great potential when used to “polish” nutrient rich water. Further work with FMS may reinforce their
usefulness in these low nutrient environments where desired outflows of P are < 50 ppb, and where other
remediation means would be cost prohibitive.
Tissue concentrations of nutrients among plant species exposed were variable (Figure 2). Juncus effusus
was the most effective species at taking up both N and P (Table 2, Figure 2). Canna flaccida and Agrostis were
the next most efficient. Great variability existed among species between the pond and vegetated channel
treatment, with all species consistently taking up more nutrients and exhibiting better growth patterns and
overall aesthetic quality (data not shown) in the pond treatments. The Eleocharis removed the least levels of N
and P. The Juncus exhibits the greatest potential of the species screened for use in FMS. However, mixtures of
plant species may be more effective than a monoculture, thus more work should be conducted examining
remediation efficacy in FMSs with individual species and species mixtures.
Overall, the FMSs were highly effective remediation systems; especially for removing P. More stringent
P effluent quality criteria (P concentrations < 50 ppb) are the hardest to attain economically. These FMSs were
easy to install, maintain, and harvest, and may prove to be an economically feasible treatment technology for
polishing water quality to very low P effluent concentrations.
Sarah A. White, Ph.D.
Assistant Professor of Horticulture
December 5, 2008
3 of 6
Table 1. Daily loading rate1 and concentration of nitrogen and phosphorus flowing into and leaving the pond
and vegetated channel floating mat treatment systems.
Average (standard error) nutrient loading rate of 2 pond experimental units or 3 vegetated channel experimental units.
Table 2. Average1 phosphorus fixed per unit area by Canna flaccida and Juncus effusus after exposure to
nutrient rich water from May through September 2008.
Average (standard error) phosphorus retained by plants 3 vegetated channel experimental units normalized by treatment area.
4 of 6
Figure 1. Nitrogen and phosphorus concentrations1 entering (control) and leaving (ponds or veg2)
floating mat treatment systems.
Control have only one sample per date; pond (n=2) and veg (n=3) treatments are composite samples for each date.
Average data points ± standard error of the average.
Figure 2. Nitrogen and phosphorus mass1 in shoots and roots of various aquatic plants
established in both pond and vegetated channels in the floating mat treatment systems.
5 of 6
Three plant replicates per bar, each bar represents the average ± standard error of the average
6 of 6
Milton D. Taylor, Sarah A. White, Stewart L. Chandler, Stephen J. Klaine, and Ted Whitwell. Nutrient
Management of Nursery Runoff Water using Constructed Wetland Systems. 2007 HortTechnology,
16: 610 - 614.
Vymazal, J., 2007. Removal of nutrients in various types of constructed wetlands. The Science of the
Total Environment, 380, 48-65.