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Floating Treatment Wetland Mesocosm Experiments: Impact of hydraulic regime and organic material in mats Expérience Mésocosme Marais Flottant : impact du régime hydraulique et de la présence de matière organique dans le radeau Karine E. Borne*, Victor Adamu, Vincent Gagnon, Yves Andrès, Florent Chazarenc Ecole des Mines de Nantes, Department of Energy Systems and Environment, GEPEA, 4 Rue Alfred Kastler 44300 Nantes, France *corresponding author: karine.borne@gmail.com

RÉSUMÉ L’objectif de la présente étude était de définir dans quelles mesures la performance de traitement d’un Marais Flottant (MF) est impactée par le régime hydraulique (dynamique versus statique) et la présence de matière organique (MO) dans les éléments constitutifs du radeau. Ceci a été étudié dans une expérience mésocosme alimentée par une eau de ruissellement synthétique. Les résultats montrent que des MF avec (MForg) ou sans (MFpet) MO constituant le radeau fournissent des traitements additionnels similaires pour l’orthophosphate (46 et 39% d’enlèvement massique (EM) médian supplémentaire par rapport au control (bac sans MF)). Le traitement s’est effectué tant en conditions dynamiques que statiques. Seul MForg a montré un EM des nitrates significatif, très probablement dû à son matelas en fibre de coco (source de carbone organique) intensifiant la dénitrification, et ceci principalement en mode statique. Aucune MO n’a été apportée à l’effluent synthétique ce qui est rarement le cas des effluents réels. MFpet pourrait très probablement permettre l’enlèvement des nitrates par dénitrification en condition réelle mais MForg serait plus efficace pour des ruissellements peu chargés en MO. Aucune différence significative n’a été observée entre les MF et le control pour l’enlèvement des matières en suspension (MES) et l’ammonium (NH4). Suffisamment de microorganismes ont dû se développer sur les parois des bacs permettant la nitrification de NH4, principalement enlevé pendant le mode statique. Le développement racinaire limité des MF n’aurait permis qu’un piégeage restreint des MES. Les MF semblent cependant avoir limité la re-suspension des MES, observée dans le control. Des EM de MES similaires ont été observés en mode statique et dynamique pour les MF.

ABSTRACT The aim of the present study was to define to which extent the treatment performance of Floating Treatment Wetlands (FTWs) is impacted by the hydraulic regime (dynamic versus static conditions) and the presence of organic material. This was investigated in a mesocmsm experiment using synthetic stormwater. The results indicate that FTWs with (FTWorg) or without (FTWpet) organic mat provide similar additional PO4 treatment compared to a Control (additional 46 and 39% mass removal efficiencies (MRE), respectively). Similar MREs were achieved in static and dynamic regime. NO3 was significantly removed only by FTWorg whose organic mat most probably enhanced denitrification, and this mainly during the static mode. It is worth mentioning that the synthetic stormwater didn’t comprise any organic carbon which is rarely the case in the field. FTWpet would most probably enable denitrification in a real application however FTWorg would be more efficient at removing NO3 for low organic carbon effluents. No overall significant difference was observed between the Control and the FTW treatments for NH4 and TSS. Enough microorganisms might have been able to develop on the walls of the tanks to nitrify NH4, which mainly happened during the static mode. The somehow limited roots’ development of the FTWs might have limited TSS entrapment into roots’ biofilm. However it seems that the planted treatment limited re-suspension, which was observed in the Control. TSS was similarly removed during the static and dynamic stage for the planted treatments.

KEYWORDS Floating treatment wetland, hydraulic impact, organic matter, nitrate, phosphorus

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1 INTRODUCTION Recent studies have shown that Floating Treatment Wetlands (FTWs) can efficiently remove suspended solids, metals and nutrients from contaminated waters (Borne, 2014; Borne et al., 2013; Ijaz et al., 2015; Ladislas et al., 2013; Van De Moortel et al., 2010; White and Cousins, 2013). A FTW is composed of a floating mat planted with emergent aquatic plants that extend their roots into the water column. Installed on a stormwater retention pond, this novel technology introduces wetland-like vegetation and therefore provides additional pollutant removal mechanisms such as direct plant uptake, adsorption, precipitation, nitrification, denitrification and entrapment into the roots’ network. While overall good removal efficiencies have been reported in the literature to date, no insight on the hydraulic impact on the FTWs’ performance has been investigated. However stormwater ponds are often alternately subject to static and dynamic stages in between and during storm events, respectively. Some characteristics of the FTW itself, like the type of mat (with or without organic material (known to impact NO3

- removal), can also influence the performance removal. These parameters are important factors to investigate from a design perspective.

The aim of the present study was to define to which extent the treatment performance of FTWs is impacted by the hydraulic regime (dynamic versus static conditions) and the presence of organic material in the FTW. This was investigated in experiments comprising mesocosm tanks (volume ~0.34 m3 at 0.55 m operational water depth) connected to mixing tanks from which synthetic stormwater was dispensed. The focus was on nitrogen (NH4

+, NO3-), phosphorus (PO4

3-), and total suspended solids (TSS). Monitoring was performed over summer 2015.

2 METHODS 2.1 Mesocosms Batch mesocosm experiments were conducted in Nantes, France (47°16'N, 1°31'O) from end June to beginning of August 2015. Experiments were carried out with nine polyethylene (PE) tanks, 0.6m X 1.06m (width x length) surface opening at 0.55m operational water depth, equivalent to a volume of ~0.34m3. These nine PE tanks were divided in three sets of three tanks. One set comprised a feeding, a mesocosm and an effluent tank. Synthetic stormwater solution was created in the feeding tank (elevated at 1.1m height) and then dispensed by gravity to the mesocosm tank. The effluent leaving the mesocosm tank was collected in an effluent tank (Figure 1).

To investigate the influence of different types of the floating wetlands, water quality responses were compared for three different treatments, including a control. The floating wetland treatments were 0.36m2 (45cm (width) x 80cm (length)) planted mats. One floating treatment wetland mat was mainly made of polyethylene terephthalate (PET) (hereafter referred as FTWpet). The 15cm porous FTWpet mat served as a growing medium for the plants (Carex riparia) and was half submerged. Patches of expended foam provided buoyancy. The other floating treatment wetland comprised a ~5cm thick organic mat (coconut coir mat) (hereafter referred as FTWorg). FTWorg comprised a 3cm thick rigid honeycomb structured polyethylene (PE) panel on which the coconut coir mat pre-planted with Carex acutiformis was installed. Polyurethane floats attached underneath the PE panel provided buoyancy. The coconut coir mat served as a growing medium for the plants and was almost totally submerged. FTWpet and FTWorg were held in nursery tanks for 11 and 5 months, respectively, prior the start of the experiment, with periodic additions of nutrients. The control tank was shaded by a rigid PE plate suspended above the water surface and covered by a white geotextile to limit water temperature increase. The space between the FTWs and the tanks’ wall was also shaded by a plate (Figure 1). This avoided proliferation of planktonic and algae in the tanks.

Each treatment was monitored 4 times (i.e. 4 runs). An experimental run was composed of a dynamic stage and a static stage. During the dynamic stage the feeding tanks were filled and drained twice (i.e. 2 flushes) to simulate a short intense storm event (~1h runoff duration). In order to force the incoming water to flow horizontally below the floating mats, perforated stilling boxes were installed at the inlet side of the mesocosm tanks (Figure 1). The dynamic stage was followed by a static stage during which no influent was added to the mesocosms during 7 days.

Concentrated artificial stormwater was diluted in each feeding tank to have an initial concentration of key pollutants reflective of nutrient rich agricultural stormwater runoff (Kato et al., 2009; Lang et al., 2013; Maynard et al., 2009; Tanner and Sukias, 2011; Zhang et al., 2007): 1.2 mg/L ammonium (NH4-N), 20 mg/L nitrate (NO3-N), 0.6 mg/L ortophosphate (PO4-P), 170 mg/L total suspended solids (TSS).

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Prior to the start of the experimental runs, two acclimation runs were performed to expose the different treatments to the artificial stormwater.

Figure 1 Feeding, mesocosm and effluent tanks set up

2.2 Sampling and analysis 2.2.1 Water

During the dynamic stage of a run, samples were collected before and after each flush in the feeding tank and effluent tank, respectively, (i.e. 2 samples for each tank corresponding to both flushes). Samples collected in the same tank were then mixed to get inlet (for feeding tanks) or outlet (for effluent tanks) composite samples representative of the total input and output pollutant loads during the dynamic stage. Samples were also collected in the mesocosm tanks to assess the mass of pollutants already present and left in the mesocosm tanks at the start and end of the dynamic stage, respectively. During the static stage of a run, samples were collected in the mesocosm tanks 0.5, 1, 3 and 7 days after the end of the dynamic stage. This represents a total of 30 samples collected per run. Dissolved oxygen (DO) was measured using a multiparameter probe (HI 9829, Hanna instrumens) in the mesocosm tanks before and after the dynamic stage and then 0.5, 1, 3 and 7 days after the end of the dynamic stage.

Samples were collected in the middle of each tank (through a hole in the middle of the FTWs for the mesocosm tanks-Figure 1) using 90cm PE bailers. NH4-N, NO3-N, PO4-P and TSS were analysed as per Standard Method 4500-NH3 D, 4500-NO3 D, 4500-P E and 2540-D, respectively (APHA, 2011). This represents a total of 96 analyses per run. Over the 384 analyses performed for this experiment, four were removed from the data set as they were extreme outliers suggesting potential contamination.

For individual runs, the dynamic stage, the static stage and the overall pollutant mass removal efficiencies (DMRE, SMRE,TMRE, respectively) were calculated as:

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where VFeed, VMes1, VEff, VMes2 and VMes3 are water volume entering the mesocosm tank, in the mesocosm tank before the dynamic stage, leaving the mesocosm tank, in the mesocosm tank at the end of the dynamic stage and at the end of the static stage, respectively.

and where CFeed, CMes1, CEff, CMes2 and CMes3 are concentrations of samples collected in the feeding tank, in the mesocosm tank before the dynamic stage, in the effluent tank, in the mesocosm tank at the end of the dynamic stage and at the end of the static stage, respectively.

The water quality data were statistically analysed to compare planted treatment’s TMREs to the Control’s TMREs. Data were tested for normality using the Shapiro-Wilk test. If data were normally distributed, a one-way Anova with Dunnett Multiple Comparisons with a Control was performed. Otherwise, a Mann-Whitney test with correction for multiple comparisons was used. For each treatment, a significant difference between the mean SMRE and DMRE was assessed with a Student T-test (for normal data) or a Mann-Whitney test (for non-normal data). All tests were achieved using the software Minitab 17 (Minitab Inc.).

2.2.2 Plant

Plant biomass measurement was performed at the start and end of the experiment. The shoot and root lengths were measured for each FTW from the upper (for shoots) and lower (for roots) surface of the mat. The length below which about 90% of roots and shoots occurred was reported as the majority length. The term “root” used in this paper refers to the plant tissues growing below the FTW mats. Roots and rhizomes growing within the mat matrix were not measured.

3 RESULTS AND DISCUSSION 3.1.1 Phosphorus

Median PO4-P overall Mass Removal Efficiencies (TMRE) were 52, 45 and 5% for FTWorg, FTWpet and the Control, respectively (Figure 2-a). Both planted treatments exhibited statistically significant greater TMRE than the Control (p=0.013 and 0.003 for FTWpet and FTWorg, respectively) however no difference existed between FTWorg and FTWpet. This suggests that the FTWs provided additional removal mechanisms, e.g. plant uptake or sorption, independent of the plant or mat type (i.e. with or without organic material). For the planted treatments, PO4-P was removed during the dynamic and the static stage with the same order of magnitude, however the MRE variability was greater for the dynamic stage exhibiting greater interquartile range (Figure 2-a). PO4-P sorption appears to be the most plausible removal mechanism during this stage, given its short duration (~1 hour). It might have been subject to available sorption sites (on plant roots or particles) and contact time duration which may be responsible of the observed variability.

3.1.2 Nitrates

Median NO3-N TMREs were 18, 5 and -6% for FTWorg, FTWpet and the Control, respectively. FTWorg TMREs were greater than FTWpet and Control TMREs mainly due to greater NO3 removal during the static stage (Figure 2-b). Overall low DO conditions prevailed in the treatments during the static mode (mean of 1.1, 2.2 and 0.8 mg/L DO in FTWpet, Control and FTWorg) suggesting potential for denitrification. Under anoxic conditions, the additional organic matter provided by the thick coconut mat in FTWorg might have contributed to greater denitrification than in FTWpet and the Control, leading to greater SMREs. It is worth mentioning that the synthetic stormwater didn’t comprise any organic carbon which is rarely the case in the field. FTWpet would most probably enable denitrification in a real application however FTWorg would be more efficient at removing NO3 for low organic carbon effluents.

Both FTWs’ plant biomass increased significantly especially the shoots (Figure 4 and Figure 4) suggesting that direct uptake of NO3-N into both FTWs’ vegetation occurred. On-going analyses of plant tissues N content will help identifying whether this mechanism was responsible for greater NO3-N removal by FTWorg. However, the apparent limited difference in biomass growth between the planted treatments suggest that denitrification played a bigger role. The negative MREs in the Control and FTWpet during the static stage may result from the observed NH4-N removal (Figure 2-c) leading to the production of NO3-N under conditions of limited denitrification.

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Removal of NO3-N was low in all the treatments during the dynamic stage, with median DMREs ranging from 1.4 to 2 %. The duration of the dynamic stage might be too short to allow for denitrification.

3.1.3 Ammonium

The planted treatments NH4-N median TMREs were slightly greater than the Control with 43, 37 and 28% for FTWorg, FTWpet and the Control, respectively (Figure 2-c). However no statistical difference existed between the different treatments. This suggests that NH4-N was similarly removed with or without the presence of FTWs. Enough microorganisms might have been able to develop on the walls of the tanks to nitrify NH4-N. While most of NH4-N removal occurred during the static stage (SMREs ranging from 28 to 33%), a small portion was removed during the dynamic stage (median ranging from 3 to13% depending on the treatment). Longer period of time of the static stage compared to the dynamic stage most probably contributed to greater removal.

3.1.4 Total suspended solids

No significant difference existed between the different treatments regarding TSS removal. Median TMREs were 51, 46 and 41% for FTWorg, FTWpet and the Control, respectively (Figure 2-d). MREs were of the same order of magnitude (with similar variability) during the static and dynamic stages for both planted treatments. While median MREs were similar during the static and dynamic stages for the Control, this later exhibited a negative value resulting from observed high TSS concentration in the mesocosm’s water column at the end of the dynamic stage during the 3rd run. Settled TSS from the previous runs might have been re-suspended during run 3’s dynamic stage. This phenomenon was not observed in the planted treatments.

It is somehow surprising to see that the Control performed as well as the planted treatments for TSS as it has been reported in the literature that TSS entrapment into FTWs’ roots biofilm contributed to improved TSS removal (Borne et al., 2013; Tanner and Headley, 2011; Winston et al., 2013). It is worth mentioning that root length was rather small for FTWorg implying limited additional TSS removal potential. However FTWpet’s majority root length was ~25cm during the whole experiment, occupying ~60% of the peak flow water column height. FTWpet’s root density might have been too low, not providing a dense enough root network to act as an efficient physical filter.

Figure 2 Dynamic (DMRE), Static (SMRE) and Overall (TMRE) pollutant Mass Removal Efficiency for FTWpet,

FTWorg and the Control (N=4)

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Figure 3 FTWpet and FTWorg at the start and end of the experiment

Figure 4 FTWs’ shoots and roots majority length (cm) at the beginning and end of the experiment

4 CONCLUSION The results of the present study indicate that FTWs (with or without organic mat) provide similar additional PO4 treatment compared to a Control. This can be achieved in static and dynamic regime however the MRE’s variability appears to be greater for this later. NO3 was significantly removed only by the FTW comprising an organic mat, most probably enhancing denitrification, and this mainly during the static mode. It is worth mentioning that the synthetic stormwater didn’t comprise any organic carbon which is rarely the case in the field. FTWpet would most probably enable denitrification in a real application however FTWorg would be more efficient at removing NO3 for low organic carbon effluents. No overall significant difference was observed between the Control and the FTW treatments for NH4 and TSS. Enough microorganisms might have been able to develop on the walls of the tanks to nitrify NH4, which mainly happened during the static mode. The somehow limited root development of the FTWs might have limited TSS entrapment into roots’ biofilm resulting in little difference with the Control. However it seems that the planted treatment limited re-suspension, which was observed in the Control. TSS was similarly removed during the static and dynamic stage for the planted treatments.

ACKNOWLEDGEMENTS The authors thank Waterclean Technologies/Kauri Park Group and AquaTerra Solutions, who provided the FTWs and financial support. Viewpoints expressed in this paper are those of the authors and do not reflect policy or otherwise of suppliers.

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