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Stormwater nitrogen removal performance of a floating treatment wetland Karine E. Borne, Chris C. Tanner and Elizabeth A. Fassman-Beck
ABSTRACT The nitrogen (N) removal efficiency and effluent quality of two parallel stormwater retention ponds, one retrofitted with a floating treatment wetland (FTW) and one without any vegetation, was compared in a field trial. This study shows that inclusion of FTWs in stormwater retention ponds has potential to moderately improve N removal. Median FTW outlet event mean concentrations (EMCs) were lower than median inlet and control pond outlet EMCs for all species of N, except for NH4-N. Performance was statistically better from late spring to end autumn due to higher organic nitrogen (ON) removal and denitrification in presence of the FTW. Low dissolved oxygen (DO), higher temperature and increased organic matter (OM) and microbial activity below the FTW, likely facilitated the higher denitrification rates observed over this period. Greater sediment N
Karine E. Borne (corresponding author) Elizabeth A. Fassman-Beck Faculty of Engineering, Department of Civil & Environmental Engineering, University of Auckland, Private Bag 92019 Auckland Mail Centre, Auckland 1142, New Zealand E-mail: [email protected] Chris C. Tanner National Institute of Water & Atmospheric Research, PO Box 11-115, Hamilton, New Zealand
accumulation in the FTW pond also contributed to its higher overall N removal. Higher OM availability in the FTW pond due to release of root exudates and supply of detritus from plant die-back may have contributed to floc formation in the water column, increasing particulate ON settlement. Enhanced ON mineralisation may also be responsible but was probably limited in summer due to the low DO induced by the FTW. Direct uptake by the plants appears to be of less importance. Key words
| floating treatment wetland, nitrogen, plant, pond, sediment, stormwater
INTRODUCTION A novel approach to improve the water quality performance of stormwater retention basins is to retrofit with a floating treatment wetland (FTW). FTWs are composed of a floating mat planted with emergent aquatic plants that extend their roots into the water column. Experience with FTWs for urban stormwater treatment is limited (Headley & Tanner ). Previous studies have demonstrated good nutrient removal efficiencies for influent concentrations ranging from 6 to 230 mg/L total nitrogen (TN) (Stewart et al. ; Hubbard ; Van De Moortel et al. ; De Stefani et al. ). This range of concentration is significantly higher than the typical urban stormwater runoff concentrations (median of inlet event mean concentrations (EMC) ranging from 0.75 to 2.37 mg/L TN (Geosyntec Consultants Inc. and Wright Water Engineers Inc. )). Winston et al. () reported improved TN removal after retrofitting existing stormwater retention ponds with FTWs, but the difference compared to the pre-retrofit period was not statistically significant. Additional full-scale studies are needed to better doi: 10.2166/wst.2013.410
understand and quantify the potential treatment benefits of using FTWs to treat urban stormwater runoff. The main objective of this study was to quantify the nitrogen (N) removal improvement in a stormwater retention basin retrofitted with a FTW and compare this to N removal in a conventional unvegetated retention pond. The relative magnitude of the different N removal processes (direct uptake by plants, immobilisation of the pollutant as sediment below the FTW, and nitrification/denitrification processes) was also investigated.
METHODS Experimental site The experimental site is located at Silverdale about Zealand, along a highway approximately 1.7 ha (75%
a stormwater retention pond 35 km north of Auckland, New interchange. The catchment is impervious). To allow a side by
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side performance comparison, the retention pond was divided into two parallel straight-walled sections (∼100 m2 each) with centrally located V-notch weirs at each end. A forebay (∼100 m2) was common to both sections and had a permanent pool of 1.09 m. Each partitioned section had a permanent water depth of 0.75 m with infiltration losses expected to be negligible due to the thick clay base. Inlets and outlets, and overall partition geometry had the same dimensions, and inflows and outflows of both ponds exhibited similar hydrographs with equivalent water volumes. Inflows and outflows of each partition section were measured continuously using a pressure transducer and weir arrangement. Detailed methodology for hydrologic monitoring is described elsewhere (Borne et al. ). An approximately 50 m2 FTW was installed on 8 December 2010 (summer in the southern hemisphere) in one partition (FTW pond), while the other partition (control pond) served as a control. The FTW comprised a rectangular 200 mm thick mat of tangled polyester fibre injected with patches of buoyant polyurethane foam (Biohaven™ Floating Islands, Waterclean Technologies, Kaiwaka, New Zealand) planted with Carex virgata (∼17 plants/m2). Water quality sampling and analysis Continuous temperature and dissolved oxygen (DO) monitoring was carried out below the FTW and in the control pond at 15 min intervals using a D-Opto logger (Zebra-Tech Ltd, Nelson, NZ). Three ISCO 3700 automatic samplers were installed in the forebay and upstream of each outlet weir to collect storm event samples. Within 24 h of each storm event, the samples were transported to the University of Auckland laboratory to make flow-weighted composite samples which were sent immediately for analysis to an external laboratory (Watercare Ltd, Auckland, New Zealand, an International Accreditation New Zealand (IANZ) laboratory). The analysis of each composite sample gave the EMC for each sampling station. Ammoniacal nitrogen (NH4-N ¼ NHþ 4 þ NH3) was analysed by colorimetry according to Methods for the Examination of Waters and Associated Materials (), and nitrite N (NO2-N) and nitrate N (NO3N) by ion chromatography according to Method 4110 B (APHA ). TN was analysed by persulphate digestion according to Method 4500-P J (APHA ) (modified to digest samples for 15 mins at 120 C-recovery verified with comparison data) and automated cadmium reduction method (Method 4500-NO3 F (APHA )). Organic nitrogen (ON) was calculated as TN minus dissolved inorganic N (DIN) species (NH4-N þ NO2-N þ NO3-N). W
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For individual storm events, pollutant mass removal efficiency (MRE) for each pond was calculated as: MRE (%) ¼
ðVin EMCin Þ (Vout EMCout ) 100 (Vin EMCin )
(1)
where Vin and Vout are volume of runoff in and out, respectively, and EMCin and EMCout are event mean concentrations of inlet and outlet samples, respectively. When EMCs were below the method detection limit (MDL), the MRE was calculated using the MDL value. The water quality data were statistically analysed to compare paired influent and effluent concentrations, paired FTW effluent and control effluent concentrations and paired FTW and control ponds’ MREs. The difference between each set of paired data was tested for normality using the Shapiro–Wilk test. If data were normally distributed, a paired Student’s t-test was performed. Otherwise, a Wilcoxon signed rank test was used. Correlation analysis was performed with a Pearson test if data were normal or lognormal; otherwise a Spearman test was used. Each significant correlation was graphically verified with scatter plots. All statistical tests were performed using SPSS statistics 19 (IBM) software. Detailed total suspended solids (TSS) data presented elsewhere (Borne et al. ) have been used in correlation analyses with nitrogen data to try to identify interactions and common removal processes. Plant sampling and analysis Plant biomass assessments were performed eight times: the day the FTW was installed and then at approximately 3monthly intervals. Eleven removable 30 × 30 cm squares of the planted mat inserted into an enamel-coated aluminium frame were incorporated into the FTW to allow easy access to measure roots. These removable inserts (sampling points P1 to P11) utilised the same matrix materials as the remainder of the FTW and were planted with a single Carex virgata plant. The FTW was divided into 11 sub-plots (plot 1 to 11) each represented by one of the removable inserts (sampling point P1 to P11). Similarly to Tanner & Headley (), root and shoot biomass was estimated for each sub-plot by determining the root/shoot density per plot (based on the sampling point measurements) and the dry weight per root/shoot estimated from regression Equations (2) and (3) (r 2 ¼ 0.687 and 0.856 for shoot and root dry weight, respectively) based on the estimated ‘majority’ (90 percentile) length, for shoots, and volume (calculated with the mean
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diameter and ‘majority’ length) for the roots: Shoot biomass dry weight (mg) ¼ 0:6204 shoot majority length (cm)1:4105
(2)
Root biomass dry weight (mg) ¼ 158:47 root volume (cm3 )0:8707
(3)
TN concentration analysis in plant tissue was performed after 9 months of growth and then at approximately 3-monthly intervals. Six plants from the same batch as those originally planted on the FTW were analysed for TN in December 2010 as a baseline. Analyses were performed by an IANZ laboratory (Landcare Research, Palmerston North, NZ). Samples were digested using a Kjeldahl wet oxidation process as described by Blakemore et al. () and TN content was determined by colorimetry using the indophenol reaction with sodium salicylate and hypochlorite and flow injection analysis as described in Lachat Instruments (). TN accumulation in plant tissues was calculated individually for shoots and roots in each sub-plot using Equation (4) and then summed to estimate the total amount accumulated in the vegetation for the whole FTW: TN accumulation (g) ¼ biomass (kg) tissue TN concentration (g=kg)
(4)
The term ‘root’ used in this paper refers to the plant tissues growing below the FTW mat. Roots growing within the mat matrix were not analysed or measured. Sediment sampling and analysis Eight grab sediment samples per sampling mission were collected in each pond on 5 March, 11 June and 5 November 2012. Samples were collected in polypropylene containers and kept in a cooler before being sent by courier post for the same-day delivery to Watercare Ltd. TN concentration was analysed by colorimetry and discrete analyser as per Soil Sampling and Methods of Analysis (Canadian Society of Soil Science ). Weight lost on ignition (LOI), which corresponds to the organic matter or carbon content of the sample, was determined by ashing 2 to 4 g of sample at 550 C for 12 hours (Chagué-Goff ; Wang et al. ). The sediment data for each mission were statistically analysed to compare the mean concentrations in each pond W
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and detect changes over time. Each data set (comprising eight TN concentrations per sampling mission per pond) was tested for normality using the Shapiro–Wilk test. If data were normal or log-normal, comparisons were performed using Student’s t-test. Non-normal data were analysed with a Mann–Whitney test. Correlation analysis was performed using a Pearson test when data were normally or log-normally distributed; otherwise a Spearman test was used.
RESULTS AND DISCUSSION Storm events analysis Over a period of 1 year (May 2011–June 2012), 17 storm events were sampled. NH4-N and NO2-N EMC were generally low with 0.020, 0.021 and 0.021 mg/L (median values for NH4-N) and 0.006, 0.003, 0.002 mg/L (median values for NO2-N) for the inlet and control and FTW pond outlets, respectively. FTW median EMCout was lower than median EMCin and control pond median EMCout for most species of N, except for NH4-N where both median EMCout values were equal (0.021 mg/L). Nevertheless these differences were not statistically significant. Over the full monitoring period (17 storm events), the median ON, DIN and TN EMCin were respectively reduced from 0.692, 0.349 and 0.99 mg/L to 0.619, 0.22 and 0.82 in the control pond and to 0.472, 0.203 and 0.71 mg/L in the FTW pond. Although differences between the FTW and control ponds’ EMCout over the 1 year period were not statistically significant, FTW TN EMCout showed a statistically stronger difference (p ¼ 0.003) to the EMCin than did the control ponds’ EMCout (p ¼ 0.013). Similarly, FTW DIN EMCout exhibited a significant difference from EMCin (p ¼ 0.046) while no differences existed for the control pond. During storm events, measured inflow and outflow volumes were similar. High MREs were thus directly attributed to reductions in EMCout rather than reductions in flow volume between inlets and outlets. Median MREs for ON, NH4-N, NOx-N, DIN and TN, respectively, were 10.8, 5.5, 37.5, 44.2 and 29.1% for the FTW pond and 6.2, 18.3, 35.2, 37.4, 19.8% for the control pond. Although not statistically significant, FTW TN MREs were around 12% higher than the control pond MREs (median of the differences between paired MREs). Both ponds’ NOx-N EMCout values were negatively correlated with the sampling date (r ¼ 0.692, p ¼ 0.002 and r ¼ 0.767, p ¼ 0.0002 for the FTW and control ponds,
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respectively, Figure 1(a)). A weaker correlation existed between TN EMCout and the sampling date (r ¼ 0.502, p ¼ 0.04 and r ¼ 0.489, p ¼ 0.032 for the FTW and control ponds, respectively, Figure 1(b)). A correlation was not found between NOx-N or TN EMCin and the sampling date. The FTW NOx-N MREs were positively correlated with the sampling date (Spearman’s rho ¼ 0.502, p ¼ 0.04), while no correlation was found for the control pond NOxN MREs or TN MREs of either pond. This suggests that NOx-N treatment improved over time especially in the FTW pond and was noticeably better from 1 November 2011 until beginning of June 2012 (corresponding to late spring to end autumn in the southern hemisphere), with all MREs becoming positive and increasing over time. From winter to late spring, the discernibly lower TN EMCout of the FTW pond compared to the control pond (although not statistically significant – Figure 2(a)) were mainly attributed to higher ON MRE (Figure 3(a)). While ON EMCs were positively correlated with TSS EMCs at the outlet of the FTW pond (r ¼ 0.553, p ¼ 0.021), no correlation existed at the inlet or control pond outlet. This suggests that a higher proportion of particulate-bound ON was present at the outlet of the FTW pond than at the inlet or control pond outlet. Whatever the amount of dissolved ON entered the ponds, it was thus more efficiently removed in presence of the FTW. Higher ON MRE may also be attributed to higher particulate ON settlement in the FTW (‘Sediment analysis’
Figure 1
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section). FTW ON EMCout was nevertheless higher than EMCin and control EMCout twice during this period. This also happened once over summer (mid-February) after noticeable shoot die-back occurred (see ‘Plant sampling’ section). Nietch et al. () reported that wetlands may be potential sources of N, especially ON, to receiving waters, as a result of shoot turnover and leaching of soluble ON. As NH4-N EMCin values (median of 0.019 mg/L) were significantly lower than ON EMCin values (median of 0.703 mg/L), a modest difference between FTW and control ON MREs may result in a high difference between NH4-N amounts produced in both ponds. No significant increase of NH4-N (compared to the amount of ON removed) was observed between inlet and outlets of either pond (Figure 2(a)). This suggests that nitrification, plant uptake and/or ammonia volatilisation (especially in the control pond, which exhibited elevated pH partly due to algal photosynthesis (Borne et al. )) have occurred over winter and spring in both ponds. The higher NOx-N median EMCout of the FTW pond compared to the control pond over this period (Figure 2(a)) can be explained by increased mineralisation of ON leading to increased production of NH4-N and subsequent nitrification in the FTW pond under conditions of limited denitrification. A significantly improved TN removal performance occurred in the pond with FTW in summer–autumn. Over this period, the FTW pond exhibited significantly lower
NOx-N (a) and TN (b) EMCs over time for the control and the FTW ponds (exponential regression represents the Pearson correlation for natural log transformed data, i.e. it shows the correlation between ln(EMC) and time).
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Figure 2
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Nitrogen inlet and outlet EMC of the FTW and control ponds from winter to late spring (a) and late spring to end autumn (b).
Figure 3
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MREs (%) of both ponds from winter to late spring (a) and from late spring to end autumn (b).
TN EMCout (p ¼ 0.028, Figure 2(b)) and higher TN MREs (p ¼ 0.033, Figure 3(b)) than the control pond. Enhanced mineralisation and/or particulate ON settlement, and denitrification are likely to be the main causes as suggested by greater reduction of ON and NOx-N EMC in the FTW pond than in the control pond over this period (Figures 2(b)). The difference between the FTW and control ponds’ ON median MREs was greater from late spring to end autumn
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(8%) than from winter to late spring (1%) (Figures 3(a) and 3(b)). This can be explained by enhanced mineralisation and/or particulate ON settlement in the FTW pond. Higher water temperature (median of 19.9 C compared to 13.5 C from winter to late spring) combined with higher microbial biomass present in the FTW root biofilms are favourable conditions for increased mineralisation rate (White & Reddy ; Kadlec & Reddy ; Reddy & Delaune ). Although mineralisation can occur under both W
W
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aerobic and anaerobic conditions, it is likely to have been limited over summer due to the lower DO induced by the FTW over this period (average of 0.9 mg/L DO, std. dev. 1.5 mg/L) (Reddy & Delaune ). Higher release of plant detritus (dead tissues and lysates) due to plant die-back, mid-summer to end autumn, may have contributed to the formation of flocs in the water column (Neto et al. ), increasing particulate ON settlement (section ‘Sediment analysis’). From late spring to end autumn, FTW NOx-N EMCout (mainly comprising NO3-N) showed a statistically stronger difference (p ¼ 0.007) than the control pond EMCout (p ¼ 0.025) to EMCin, implying higher denitrification in the FTW pond. DO monitored below the FTW was lower from late spring to end autumn, than in winter, ranging from 0 to 11.8 mg/L with a median of 1.1 mg/L over this period of time. Higher organic carbon content (as evidenced by higher LOI over summer–autumn 2012 in the sediment of the FTW pond, Figure 5) and associated microbial activity consuming DO probably promoted anoxic conditions. Lower water aeration due to the physical barrier provided by the FTW and increase of the water temperature over summer can also explain this DO depletion. Such conditions in the FTW pond are likely to have enhanced denitrification rates compared to the control pond. Unlike the FTW pond, the control pond showed statistically lower NH4-N EMCout compared to EMCin (p ¼ 0.031), probably due to less variability in EMCout distribution (std. dev. ¼ 0.008 and 0.012 mg/L for the control and FTW ponds, respectively). Plant sampling The FTW was planted in early summer, December 2010, which allowed a fast growth of the above- and below-mat vegetation (Figure 4). The biomass stopped increasing after the end of the first summer and some die-back occurred during autumn. Rapid shoot biomass growth over the next spring resulted in dense vegetation. Plant die-back was observed while sampling at the end of summer (February 2012). Potential causes could be toxic inputs (e.g. inadvertent herbicide entry in runoff), or toxicity generated within the stormwater pond (e.g. due to severe deoxygenation beneath the FTW). DO monitored below the FTW was the lowest over January and February 2012, ranging from 0 to 5.6 mg/L with a median of 0.16 mg/L, and water temperatures ranged from 18.9 to 23.7 C. The production of toxic products due to anaerobic processes beneath the mat appears to be the most plausible cause of the die-back W
Figure 4
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Biomass (a) and total nitrogen accumulated in FTW’s vegetation (b), and outlet EMCs of the FTW pond over time (b) (Sum ¼ summer, Aut ¼ autumn, Win ¼ winter, Spr ¼ spring).
(Lamers et al. ; Reddy & Delaune ). Live and dead shoots were trimmed at the beginning of March 2012 to leave 30 cm long shoots and were removed from the FTW. Plants slowly re-established in scattered areas. In order to speed up the re-establishment, the areas which were free of shoots were re-planted to a density of 5 plants/m2 in mid-July 2012. TN accumulation in shoots represented from 34 to 80% of the total amount of TN accumulated in plant tissues (first sampling mission when no roots were present below the mat and 3-month period after shoots were harvested not taken into account). The highest accumulated TN was recorded at the end of spring when shoot biomass was highest. The storm event analysis reveals that NOx-N MRE of the FTW pond improved over time (positive correlation with time), which resulted in a decrease of NOx-N EMCout, mainly comprising NO 3 (Figure 1(a), section ‘Storm events analysis’). The main forms of N taken up by plants are NHþ 4 and NO3 (Cronk & Fennessy ). Even if direct uptake by the plants was responsible for a part of NOx-N removal during the growing season, the improvement in NOx-N MRE and decrease of NOx-N EMCout were most noticeable up until the start of June 2012 (beginning of winter), including February 2012 when shoots were decaying (Figure 4). This suggests that plant uptake is
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unlikely to have played an important role in NOx-N removal during this period. Sediment analysis Mean TN concentrations were 1,811 and 1,520 mg/kg dry weight for the FTW pond and control pond, respectively, at the end of summer (March 2012) (not significantly different). A greater increase of TN occurred in the sediment of the FTW pond over autumn (mean concentration increase of 676 and 418 mg/kg N for FTW and control pond, respectively) probably partly due to increased settlement of particulate ON. TN concentrations were positively correlated with %LOI in the FTW pond only (r ¼ 0.697, p ¼ 0.0002). This suggests that the accumulation of N in the FTW pond was closely linked to the OM content of the sediment and was probably composed of ON. This accords with the dominant role of ON storage in natural wetlands (Reddy & Delaune ; Kadlec & Wallace ). Release of OM by decaying plants over summer–autumn may have increased flocculation of particulate ON and subsequent settlement as evidenced by higher TN concentration and %LOI in the sediment of the FTW pond. Conversely % LOI decreased in the control pond over this period (Figure 5). This resulted in a statistically significant difference (p < 0.05) of 550 mg/kg dry weight between the ponds’ mean sediment TN concentration in June 2012 (Figure 5). Over the following winter and spring, TN concentration decreased in sediments in both ponds with higher loss in the
Figure 5
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LOI (%) and TN (mg/kg) in sediments of the FTW and control ponds (Sum ¼ summer, Aut ¼ autumn, Win ¼ winter, Spr ¼ spring, error bars represent ± standard deviation).
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control pond (mean concentration decrease of 375 (∼15% loss) and 513 mg/kg N (∼26% loss) for the FTW and control pond, respectively). N release from the sediment is known to occur when the carbon to nitrogen ratio (C:N) is below 25, under aerobic conditions (Reddy & Delaune ). Mean C: N ratio decreased from ∼50 in both ponds’ sediment end of summer to ∼39 at the end of autumn due to C assimilation by microbes and N accumulation in the sediment. Further C assimilation during winter may have reduced the C:N ratio below 25, inducing N release in both ponds. The increase of C:N ratio to ∼55 mid-spring 2012 suggests input of additional C and/or N losses through mineralisation/nitrification/denitrification of ON in the FTW sediments. As N release happened in both ponds over winter/spring, the overall higher TN concentration in FTW sediment may be attributed to higher settlement of particulate N below the FTW.
CONCLUSIONS Over ∼1 year monitoring period, the pond retrofitted with a FTW provided moderately higher N removal than the conventional unvegetated retention pond. While performance was significantly better in summer–autumn, restricted denitrification over winter to late spring appeared to be the limiting factor for TN removal. Better performance from late spring to end autumn resulted from greater ON removal and denitrification in the FTW. Higher release of plant detritus due to plant die-back may have contributed to the formation of flocs in the water column, increasing particulate ON settlement. ON mineralisation is likely to have played a lesser role over summer, limited by the low DO induced by the FTW. Low DO, coupled with higher temperatures, and increased organic carbon availability and microbial activity (associated with root biofilms) below the FTW over this summer–autumn period are likely to have stimulated denitrification losses. Direct uptake by the plants appears to be of less importance. Sediment N accumulation also contributed to the overall N removal with greater accumulation in the FTW pond than the control pond. Results presented in this paper are based on data collected at only one site in New Zealand. Additional full-scale monitoring studies are warranted to confirm these results and assess FTWs in a wider range of conditions (different input load, climate conditions, sizes and coverage ratio, etc.) and over a long-term period.
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ACKNOWLEDGEMENTS The authors thank the Auckland Council and Auckland Motorway Alliance/New Zealand Transport Agency for their financial support as well as Waterclean Technologies/Kauri Park Group, manufacturer of the FTW, who provided technical support during the installation. Viewpoints expressed in this paper are those of the authors and do not reflect policy or otherwise of the funding agencies or supplier.
REFERENCES APHA Standard Methods for the Examination of Water and Wastewater. 21st edn. American Public Health Association/ American Water Works Association/Water Environment Federation, Washington, DC, USA. Blakemore, L. C., Searle, P. L. & Daly, B. K. Methods for Chemical Analysis of Soils. Report 80, New Zealand Soil Bureau Scientific, Lower Hutt. Borne, K. E., Fassman, E. A. & Tanner, C. C. Floating treatment wetland retrofit to improve stormwater pond performance for suspended solids, copper and zinc. Ecological Engineering 54, 173–182. Canadian Society of Soil Science Soil Sampling and Methods of Analysis. CRC Press, Boca Raton, FL, USA. Chagué-Goff, C. Assessing the removal efficiency of Zn, Cu, Fe and Pb in a treatment wetland using selective sequential extraction: A case study. Water, Air, and Soil Pollution 160, 161–179. Cronk, J. K. & Fennessy, S. Wetland Plants: Biology and Ecology. CRC Press, Boca Raton, FL, USA. De Stefani, G., Tocchetto, D., Salvato, M. & Borin, M. Performance of a floating treatment wetland for in-stream water amelioration in NE Italy. Hydrobiologia 674, 157–167. Geosyntec Consultants Inc. and Wright Water Engineers Inc. International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals. Water Environment Research Foundation, Federal Highway Administration, Environment and Water Resources Institute of the American Society of Civil Engineers. Headley, T. R. & Tanner, C. C. Constructed wetlands with floating emergent macrophytes: an innovative stormwater treatment technology. Critical Review in Environmental Science and Technology 42, 2261–2310.
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Hubbard, R. K. Floating vegetated mats for improving surface water quality. In: Emerging Environmental Technologies (V. Shah, ed.), Springer, New York, pp. 211–244. Kadlec, R. H. & Reddy, K. R. Temperature effects in treatment wetlands. Water Environment Research 73, 543– 557. Kadlec, R. H. & Wallace, S. Treatment Wetlands. 2nd edn. CRC Press, Boca Raton, FL, USA. Lachat Instruments. Quik Chem Method 13-107-06-2-D. Milwaukee, WI, USA. Lamers, L. P. M., Tomassen, H. B. M. & Roelofs, J. G. M. Sulfate-induced eutrophication and phytotoxicity in freshwater wetlands. Environmental Science and Technology 32, 199–205. Methods for the Examination of Waters and Associated Materials (MEWAN) HMSO, London. Neto, R. R., Mead, R. N., Louda, J. W. & Jaffé, R. Organic biogeochemistry of detrital flocculent material (floc) in a subtropical, coastal wetland. Biogeochemistry 77, 283–304. Nietch, C. T., Borst, M. & O’Shea, M. L. Stormwater treatment: Ponds vs. constructed wetlands. Engineering Foundation Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impact Mitigation, Snowmass Village, CO, USA, pp. 524–528. Reddy, K. R. & Delaune, R. D. Biogeochemistry of Wetlands : Science and Applications. CRC Press, Boca Raton, FL, USA. Stewart, F. M., Mulholland, T., Cunningham, A. B., Kania, B. G. & Osterlund, M. T. Floating islands as an alternative to constructed wetlands for treatment of excess nutrients from agricultural and municipal wastes - results of laboratory-scale tests. Land Contamination and Reclamation 16, 25–33. Tanner, C. C. & Headley, T. R. Components of floating emergent macrophyte treatment wetlands influencing removal of stormwater pollutants. Ecological Engineering 37, 474–486. Van De Moortel, A. M. K., Meers, E., De Pauw, N. & Tack, F. M. G. Effects of vegetation, season and temperature on the removal of pollutants in experimental floating treatment wetlands. Water, Air, and Soil Pollution 212, 281–297. Wang, Q., Li, Y. & Wang, Y. Optimizing the weight loss-onignition methodology to quantify organic and carbonate carbon of sediments from diverse sources. Environmental Monitoring and Assessment 174 (1–4), 241–257. White, J. R. & Reddy, K. R. Influence of phosphorus loading on organic nitrogen mineralization of Everglades soils. Soil Science Society of America Journal 64, 1525–1534. Winston, R. J., Hunt, W. F., Kennedy, S. G., Merriman, L. S., Chandler, J. & Brown, D. Evaluation of floating treatment wetlands as retrofits to existing stormwater retention ponds. Ecological Engineering 54, 254–265.
First received 11 March 2013; accepted in revised form 3 June 2013
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Stormwater nitrogen removal performance of a floating treatment wetland Karine E. Borne, Chris C. Tanner and Elizabeth A. Fassman-Beck
ABSTRACT The nitrogen (N) removal efficiency and effluent quality of two parallel stormwater retention ponds, one retrofitted with a floating treatment wetland (FTW) and one without any vegetation, was compared in a field trial. This study shows that inclusion of FTWs in stormwater retention ponds has potential to moderately improve N removal. Median FTW outlet event mean concentrations (EMCs) were lower than median inlet and control pond outlet EMCs for all species of N, except for NH4-N. Performance was statistically better from late spring to end autumn due to higher organic nitrogen (ON) removal and denitrification in presence of the FTW. Low dissolved oxygen (DO), higher temperature and increased organic matter (OM) and microbial activity below the FTW, likely facilitated the higher denitrification rates observed over this period. Greater sediment N
Karine E. Borne (corresponding author) Elizabeth A. Fassman-Beck Faculty of Engineering, Department of Civil & Environmental Engineering, University of Auckland, Private Bag 92019 Auckland Mail Centre, Auckland 1142, New Zealand E-mail: [email protected] Chris C. Tanner National Institute of Water & Atmospheric Research, PO Box 11-115, Hamilton, New Zealand
accumulation in the FTW pond also contributed to its higher overall N removal. Higher OM availability in the FTW pond due to release of root exudates and supply of detritus from plant die-back may have contributed to floc formation in the water column, increasing particulate ON settlement. Enhanced ON mineralisation may also be responsible but was probably limited in summer due to the low DO induced by the FTW. Direct uptake by the plants appears to be of less importance. Key words
| floating treatment wetland, nitrogen, plant, pond, sediment, stormwater
INTRODUCTION A novel approach to improve the water quality performance of stormwater retention basins is to retrofit with a floating treatment wetland (FTW). FTWs are composed of a floating mat planted with emergent aquatic plants that extend their roots into the water column. Experience with FTWs for urban stormwater treatment is limited (Headley & Tanner ). Previous studies have demonstrated good nutrient removal efficiencies for influent concentrations ranging from 6 to 230 mg/L total nitrogen (TN) (Stewart et al. ; Hubbard ; Van De Moortel et al. ; De Stefani et al. ). This range of concentration is significantly higher than the typical urban stormwater runoff concentrations (median of inlet event mean concentrations (EMC) ranging from 0.75 to 2.37 mg/L TN (Geosyntec Consultants Inc. and Wright Water Engineers Inc. )). Winston et al. () reported improved TN removal after retrofitting existing stormwater retention ponds with FTWs, but the difference compared to the pre-retrofit period was not statistically significant. Additional full-scale studies are needed to better doi: 10.2166/wst.2013.410
understand and quantify the potential treatment benefits of using FTWs to treat urban stormwater runoff. The main objective of this study was to quantify the nitrogen (N) removal improvement in a stormwater retention basin retrofitted with a FTW and compare this to N removal in a conventional unvegetated retention pond. The relative magnitude of the different N removal processes (direct uptake by plants, immobilisation of the pollutant as sediment below the FTW, and nitrification/denitrification processes) was also investigated.
METHODS Experimental site The experimental site is located at Silverdale about Zealand, along a highway approximately 1.7 ha (75%
a stormwater retention pond 35 km north of Auckland, New interchange. The catchment is impervious). To allow a side by
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side performance comparison, the retention pond was divided into two parallel straight-walled sections (∼100 m2 each) with centrally located V-notch weirs at each end. A forebay (∼100 m2) was common to both sections and had a permanent pool of 1.09 m. Each partitioned section had a permanent water depth of 0.75 m with infiltration losses expected to be negligible due to the thick clay base. Inlets and outlets, and overall partition geometry had the same dimensions, and inflows and outflows of both ponds exhibited similar hydrographs with equivalent water volumes. Inflows and outflows of each partition section were measured continuously using a pressure transducer and weir arrangement. Detailed methodology for hydrologic monitoring is described elsewhere (Borne et al. ). An approximately 50 m2 FTW was installed on 8 December 2010 (summer in the southern hemisphere) in one partition (FTW pond), while the other partition (control pond) served as a control. The FTW comprised a rectangular 200 mm thick mat of tangled polyester fibre injected with patches of buoyant polyurethane foam (Biohaven™ Floating Islands, Waterclean Technologies, Kaiwaka, New Zealand) planted with Carex virgata (∼17 plants/m2). Water quality sampling and analysis Continuous temperature and dissolved oxygen (DO) monitoring was carried out below the FTW and in the control pond at 15 min intervals using a D-Opto logger (Zebra-Tech Ltd, Nelson, NZ). Three ISCO 3700 automatic samplers were installed in the forebay and upstream of each outlet weir to collect storm event samples. Within 24 h of each storm event, the samples were transported to the University of Auckland laboratory to make flow-weighted composite samples which were sent immediately for analysis to an external laboratory (Watercare Ltd, Auckland, New Zealand, an International Accreditation New Zealand (IANZ) laboratory). The analysis of each composite sample gave the EMC for each sampling station. Ammoniacal nitrogen (NH4-N ¼ NHþ 4 þ NH3) was analysed by colorimetry according to Methods for the Examination of Waters and Associated Materials (), and nitrite N (NO2-N) and nitrate N (NO3N) by ion chromatography according to Method 4110 B (APHA ). TN was analysed by persulphate digestion according to Method 4500-P J (APHA ) (modified to digest samples for 15 mins at 120 C-recovery verified with comparison data) and automated cadmium reduction method (Method 4500-NO3 F (APHA )). Organic nitrogen (ON) was calculated as TN minus dissolved inorganic N (DIN) species (NH4-N þ NO2-N þ NO3-N). W
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For individual storm events, pollutant mass removal efficiency (MRE) for each pond was calculated as: MRE (%) ¼
ðVin EMCin Þ (Vout EMCout ) 100 (Vin EMCin )
(1)
where Vin and Vout are volume of runoff in and out, respectively, and EMCin and EMCout are event mean concentrations of inlet and outlet samples, respectively. When EMCs were below the method detection limit (MDL), the MRE was calculated using the MDL value. The water quality data were statistically analysed to compare paired influent and effluent concentrations, paired FTW effluent and control effluent concentrations and paired FTW and control ponds’ MREs. The difference between each set of paired data was tested for normality using the Shapiro–Wilk test. If data were normally distributed, a paired Student’s t-test was performed. Otherwise, a Wilcoxon signed rank test was used. Correlation analysis was performed with a Pearson test if data were normal or lognormal; otherwise a Spearman test was used. Each significant correlation was graphically verified with scatter plots. All statistical tests were performed using SPSS statistics 19 (IBM) software. Detailed total suspended solids (TSS) data presented elsewhere (Borne et al. ) have been used in correlation analyses with nitrogen data to try to identify interactions and common removal processes. Plant sampling and analysis Plant biomass assessments were performed eight times: the day the FTW was installed and then at approximately 3monthly intervals. Eleven removable 30 × 30 cm squares of the planted mat inserted into an enamel-coated aluminium frame were incorporated into the FTW to allow easy access to measure roots. These removable inserts (sampling points P1 to P11) utilised the same matrix materials as the remainder of the FTW and were planted with a single Carex virgata plant. The FTW was divided into 11 sub-plots (plot 1 to 11) each represented by one of the removable inserts (sampling point P1 to P11). Similarly to Tanner & Headley (), root and shoot biomass was estimated for each sub-plot by determining the root/shoot density per plot (based on the sampling point measurements) and the dry weight per root/shoot estimated from regression Equations (2) and (3) (r 2 ¼ 0.687 and 0.856 for shoot and root dry weight, respectively) based on the estimated ‘majority’ (90 percentile) length, for shoots, and volume (calculated with the mean
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diameter and ‘majority’ length) for the roots: Shoot biomass dry weight (mg) ¼ 0:6204 shoot majority length (cm)1:4105
(2)
Root biomass dry weight (mg) ¼ 158:47 root volume (cm3 )0:8707
(3)
TN concentration analysis in plant tissue was performed after 9 months of growth and then at approximately 3-monthly intervals. Six plants from the same batch as those originally planted on the FTW were analysed for TN in December 2010 as a baseline. Analyses were performed by an IANZ laboratory (Landcare Research, Palmerston North, NZ). Samples were digested using a Kjeldahl wet oxidation process as described by Blakemore et al. () and TN content was determined by colorimetry using the indophenol reaction with sodium salicylate and hypochlorite and flow injection analysis as described in Lachat Instruments (). TN accumulation in plant tissues was calculated individually for shoots and roots in each sub-plot using Equation (4) and then summed to estimate the total amount accumulated in the vegetation for the whole FTW: TN accumulation (g) ¼ biomass (kg) tissue TN concentration (g=kg)
(4)
The term ‘root’ used in this paper refers to the plant tissues growing below the FTW mat. Roots growing within the mat matrix were not analysed or measured. Sediment sampling and analysis Eight grab sediment samples per sampling mission were collected in each pond on 5 March, 11 June and 5 November 2012. Samples were collected in polypropylene containers and kept in a cooler before being sent by courier post for the same-day delivery to Watercare Ltd. TN concentration was analysed by colorimetry and discrete analyser as per Soil Sampling and Methods of Analysis (Canadian Society of Soil Science ). Weight lost on ignition (LOI), which corresponds to the organic matter or carbon content of the sample, was determined by ashing 2 to 4 g of sample at 550 C for 12 hours (Chagué-Goff ; Wang et al. ). The sediment data for each mission were statistically analysed to compare the mean concentrations in each pond W
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and detect changes over time. Each data set (comprising eight TN concentrations per sampling mission per pond) was tested for normality using the Shapiro–Wilk test. If data were normal or log-normal, comparisons were performed using Student’s t-test. Non-normal data were analysed with a Mann–Whitney test. Correlation analysis was performed using a Pearson test when data were normally or log-normally distributed; otherwise a Spearman test was used.
RESULTS AND DISCUSSION Storm events analysis Over a period of 1 year (May 2011–June 2012), 17 storm events were sampled. NH4-N and NO2-N EMC were generally low with 0.020, 0.021 and 0.021 mg/L (median values for NH4-N) and 0.006, 0.003, 0.002 mg/L (median values for NO2-N) for the inlet and control and FTW pond outlets, respectively. FTW median EMCout was lower than median EMCin and control pond median EMCout for most species of N, except for NH4-N where both median EMCout values were equal (0.021 mg/L). Nevertheless these differences were not statistically significant. Over the full monitoring period (17 storm events), the median ON, DIN and TN EMCin were respectively reduced from 0.692, 0.349 and 0.99 mg/L to 0.619, 0.22 and 0.82 in the control pond and to 0.472, 0.203 and 0.71 mg/L in the FTW pond. Although differences between the FTW and control ponds’ EMCout over the 1 year period were not statistically significant, FTW TN EMCout showed a statistically stronger difference (p ¼ 0.003) to the EMCin than did the control ponds’ EMCout (p ¼ 0.013). Similarly, FTW DIN EMCout exhibited a significant difference from EMCin (p ¼ 0.046) while no differences existed for the control pond. During storm events, measured inflow and outflow volumes were similar. High MREs were thus directly attributed to reductions in EMCout rather than reductions in flow volume between inlets and outlets. Median MREs for ON, NH4-N, NOx-N, DIN and TN, respectively, were 10.8, 5.5, 37.5, 44.2 and 29.1% for the FTW pond and 6.2, 18.3, 35.2, 37.4, 19.8% for the control pond. Although not statistically significant, FTW TN MREs were around 12% higher than the control pond MREs (median of the differences between paired MREs). Both ponds’ NOx-N EMCout values were negatively correlated with the sampling date (r ¼ 0.692, p ¼ 0.002 and r ¼ 0.767, p ¼ 0.0002 for the FTW and control ponds,
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respectively, Figure 1(a)). A weaker correlation existed between TN EMCout and the sampling date (r ¼ 0.502, p ¼ 0.04 and r ¼ 0.489, p ¼ 0.032 for the FTW and control ponds, respectively, Figure 1(b)). A correlation was not found between NOx-N or TN EMCin and the sampling date. The FTW NOx-N MREs were positively correlated with the sampling date (Spearman’s rho ¼ 0.502, p ¼ 0.04), while no correlation was found for the control pond NOxN MREs or TN MREs of either pond. This suggests that NOx-N treatment improved over time especially in the FTW pond and was noticeably better from 1 November 2011 until beginning of June 2012 (corresponding to late spring to end autumn in the southern hemisphere), with all MREs becoming positive and increasing over time. From winter to late spring, the discernibly lower TN EMCout of the FTW pond compared to the control pond (although not statistically significant – Figure 2(a)) were mainly attributed to higher ON MRE (Figure 3(a)). While ON EMCs were positively correlated with TSS EMCs at the outlet of the FTW pond (r ¼ 0.553, p ¼ 0.021), no correlation existed at the inlet or control pond outlet. This suggests that a higher proportion of particulate-bound ON was present at the outlet of the FTW pond than at the inlet or control pond outlet. Whatever the amount of dissolved ON entered the ponds, it was thus more efficiently removed in presence of the FTW. Higher ON MRE may also be attributed to higher particulate ON settlement in the FTW (‘Sediment analysis’
Figure 1
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section). FTW ON EMCout was nevertheless higher than EMCin and control EMCout twice during this period. This also happened once over summer (mid-February) after noticeable shoot die-back occurred (see ‘Plant sampling’ section). Nietch et al. () reported that wetlands may be potential sources of N, especially ON, to receiving waters, as a result of shoot turnover and leaching of soluble ON. As NH4-N EMCin values (median of 0.019 mg/L) were significantly lower than ON EMCin values (median of 0.703 mg/L), a modest difference between FTW and control ON MREs may result in a high difference between NH4-N amounts produced in both ponds. No significant increase of NH4-N (compared to the amount of ON removed) was observed between inlet and outlets of either pond (Figure 2(a)). This suggests that nitrification, plant uptake and/or ammonia volatilisation (especially in the control pond, which exhibited elevated pH partly due to algal photosynthesis (Borne et al. )) have occurred over winter and spring in both ponds. The higher NOx-N median EMCout of the FTW pond compared to the control pond over this period (Figure 2(a)) can be explained by increased mineralisation of ON leading to increased production of NH4-N and subsequent nitrification in the FTW pond under conditions of limited denitrification. A significantly improved TN removal performance occurred in the pond with FTW in summer–autumn. Over this period, the FTW pond exhibited significantly lower
NOx-N (a) and TN (b) EMCs over time for the control and the FTW ponds (exponential regression represents the Pearson correlation for natural log transformed data, i.e. it shows the correlation between ln(EMC) and time).
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Figure 2
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Nitrogen inlet and outlet EMC of the FTW and control ponds from winter to late spring (a) and late spring to end autumn (b).
Figure 3
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MREs (%) of both ponds from winter to late spring (a) and from late spring to end autumn (b).
TN EMCout (p ¼ 0.028, Figure 2(b)) and higher TN MREs (p ¼ 0.033, Figure 3(b)) than the control pond. Enhanced mineralisation and/or particulate ON settlement, and denitrification are likely to be the main causes as suggested by greater reduction of ON and NOx-N EMC in the FTW pond than in the control pond over this period (Figures 2(b)). The difference between the FTW and control ponds’ ON median MREs was greater from late spring to end autumn
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(8%) than from winter to late spring (1%) (Figures 3(a) and 3(b)). This can be explained by enhanced mineralisation and/or particulate ON settlement in the FTW pond. Higher water temperature (median of 19.9 C compared to 13.5 C from winter to late spring) combined with higher microbial biomass present in the FTW root biofilms are favourable conditions for increased mineralisation rate (White & Reddy ; Kadlec & Reddy ; Reddy & Delaune ). Although mineralisation can occur under both W
W
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aerobic and anaerobic conditions, it is likely to have been limited over summer due to the lower DO induced by the FTW over this period (average of 0.9 mg/L DO, std. dev. 1.5 mg/L) (Reddy & Delaune ). Higher release of plant detritus (dead tissues and lysates) due to plant die-back, mid-summer to end autumn, may have contributed to the formation of flocs in the water column (Neto et al. ), increasing particulate ON settlement (section ‘Sediment analysis’). From late spring to end autumn, FTW NOx-N EMCout (mainly comprising NO3-N) showed a statistically stronger difference (p ¼ 0.007) than the control pond EMCout (p ¼ 0.025) to EMCin, implying higher denitrification in the FTW pond. DO monitored below the FTW was lower from late spring to end autumn, than in winter, ranging from 0 to 11.8 mg/L with a median of 1.1 mg/L over this period of time. Higher organic carbon content (as evidenced by higher LOI over summer–autumn 2012 in the sediment of the FTW pond, Figure 5) and associated microbial activity consuming DO probably promoted anoxic conditions. Lower water aeration due to the physical barrier provided by the FTW and increase of the water temperature over summer can also explain this DO depletion. Such conditions in the FTW pond are likely to have enhanced denitrification rates compared to the control pond. Unlike the FTW pond, the control pond showed statistically lower NH4-N EMCout compared to EMCin (p ¼ 0.031), probably due to less variability in EMCout distribution (std. dev. ¼ 0.008 and 0.012 mg/L for the control and FTW ponds, respectively). Plant sampling The FTW was planted in early summer, December 2010, which allowed a fast growth of the above- and below-mat vegetation (Figure 4). The biomass stopped increasing after the end of the first summer and some die-back occurred during autumn. Rapid shoot biomass growth over the next spring resulted in dense vegetation. Plant die-back was observed while sampling at the end of summer (February 2012). Potential causes could be toxic inputs (e.g. inadvertent herbicide entry in runoff), or toxicity generated within the stormwater pond (e.g. due to severe deoxygenation beneath the FTW). DO monitored below the FTW was the lowest over January and February 2012, ranging from 0 to 5.6 mg/L with a median of 0.16 mg/L, and water temperatures ranged from 18.9 to 23.7 C. The production of toxic products due to anaerobic processes beneath the mat appears to be the most plausible cause of the die-back W
Figure 4
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Biomass (a) and total nitrogen accumulated in FTW’s vegetation (b), and outlet EMCs of the FTW pond over time (b) (Sum ¼ summer, Aut ¼ autumn, Win ¼ winter, Spr ¼ spring).
(Lamers et al. ; Reddy & Delaune ). Live and dead shoots were trimmed at the beginning of March 2012 to leave 30 cm long shoots and were removed from the FTW. Plants slowly re-established in scattered areas. In order to speed up the re-establishment, the areas which were free of shoots were re-planted to a density of 5 plants/m2 in mid-July 2012. TN accumulation in shoots represented from 34 to 80% of the total amount of TN accumulated in plant tissues (first sampling mission when no roots were present below the mat and 3-month period after shoots were harvested not taken into account). The highest accumulated TN was recorded at the end of spring when shoot biomass was highest. The storm event analysis reveals that NOx-N MRE of the FTW pond improved over time (positive correlation with time), which resulted in a decrease of NOx-N EMCout, mainly comprising NO 3 (Figure 1(a), section ‘Storm events analysis’). The main forms of N taken up by plants are NHþ 4 and NO3 (Cronk & Fennessy ). Even if direct uptake by the plants was responsible for a part of NOx-N removal during the growing season, the improvement in NOx-N MRE and decrease of NOx-N EMCout were most noticeable up until the start of June 2012 (beginning of winter), including February 2012 when shoots were decaying (Figure 4). This suggests that plant uptake is
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unlikely to have played an important role in NOx-N removal during this period. Sediment analysis Mean TN concentrations were 1,811 and 1,520 mg/kg dry weight for the FTW pond and control pond, respectively, at the end of summer (March 2012) (not significantly different). A greater increase of TN occurred in the sediment of the FTW pond over autumn (mean concentration increase of 676 and 418 mg/kg N for FTW and control pond, respectively) probably partly due to increased settlement of particulate ON. TN concentrations were positively correlated with %LOI in the FTW pond only (r ¼ 0.697, p ¼ 0.0002). This suggests that the accumulation of N in the FTW pond was closely linked to the OM content of the sediment and was probably composed of ON. This accords with the dominant role of ON storage in natural wetlands (Reddy & Delaune ; Kadlec & Wallace ). Release of OM by decaying plants over summer–autumn may have increased flocculation of particulate ON and subsequent settlement as evidenced by higher TN concentration and %LOI in the sediment of the FTW pond. Conversely % LOI decreased in the control pond over this period (Figure 5). This resulted in a statistically significant difference (p < 0.05) of 550 mg/kg dry weight between the ponds’ mean sediment TN concentration in June 2012 (Figure 5). Over the following winter and spring, TN concentration decreased in sediments in both ponds with higher loss in the
Figure 5
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LOI (%) and TN (mg/kg) in sediments of the FTW and control ponds (Sum ¼ summer, Aut ¼ autumn, Win ¼ winter, Spr ¼ spring, error bars represent ± standard deviation).
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control pond (mean concentration decrease of 375 (∼15% loss) and 513 mg/kg N (∼26% loss) for the FTW and control pond, respectively). N release from the sediment is known to occur when the carbon to nitrogen ratio (C:N) is below 25, under aerobic conditions (Reddy & Delaune ). Mean C: N ratio decreased from ∼50 in both ponds’ sediment end of summer to ∼39 at the end of autumn due to C assimilation by microbes and N accumulation in the sediment. Further C assimilation during winter may have reduced the C:N ratio below 25, inducing N release in both ponds. The increase of C:N ratio to ∼55 mid-spring 2012 suggests input of additional C and/or N losses through mineralisation/nitrification/denitrification of ON in the FTW sediments. As N release happened in both ponds over winter/spring, the overall higher TN concentration in FTW sediment may be attributed to higher settlement of particulate N below the FTW.
CONCLUSIONS Over ∼1 year monitoring period, the pond retrofitted with a FTW provided moderately higher N removal than the conventional unvegetated retention pond. While performance was significantly better in summer–autumn, restricted denitrification over winter to late spring appeared to be the limiting factor for TN removal. Better performance from late spring to end autumn resulted from greater ON removal and denitrification in the FTW. Higher release of plant detritus due to plant die-back may have contributed to the formation of flocs in the water column, increasing particulate ON settlement. ON mineralisation is likely to have played a lesser role over summer, limited by the low DO induced by the FTW. Low DO, coupled with higher temperatures, and increased organic carbon availability and microbial activity (associated with root biofilms) below the FTW over this summer–autumn period are likely to have stimulated denitrification losses. Direct uptake by the plants appears to be of less importance. Sediment N accumulation also contributed to the overall N removal with greater accumulation in the FTW pond than the control pond. Results presented in this paper are based on data collected at only one site in New Zealand. Additional full-scale monitoring studies are warranted to confirm these results and assess FTWs in a wider range of conditions (different input load, climate conditions, sizes and coverage ratio, etc.) and over a long-term period.
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ACKNOWLEDGEMENTS The authors thank the Auckland Council and Auckland Motorway Alliance/New Zealand Transport Agency for their financial support as well as Waterclean Technologies/Kauri Park Group, manufacturer of the FTW, who provided technical support during the installation. Viewpoints expressed in this paper are those of the authors and do not reflect policy or otherwise of the funding agencies or supplier.
REFERENCES APHA Standard Methods for the Examination of Water and Wastewater. 21st edn. American Public Health Association/ American Water Works Association/Water Environment Federation, Washington, DC, USA. Blakemore, L. C., Searle, P. L. & Daly, B. K. Methods for Chemical Analysis of Soils. Report 80, New Zealand Soil Bureau Scientific, Lower Hutt. Borne, K. E., Fassman, E. A. & Tanner, C. C. Floating treatment wetland retrofit to improve stormwater pond performance for suspended solids, copper and zinc. Ecological Engineering 54, 173–182. Canadian Society of Soil Science Soil Sampling and Methods of Analysis. CRC Press, Boca Raton, FL, USA. Chagué-Goff, C. Assessing the removal efficiency of Zn, Cu, Fe and Pb in a treatment wetland using selective sequential extraction: A case study. Water, Air, and Soil Pollution 160, 161–179. Cronk, J. K. & Fennessy, S. Wetland Plants: Biology and Ecology. CRC Press, Boca Raton, FL, USA. De Stefani, G., Tocchetto, D., Salvato, M. & Borin, M. Performance of a floating treatment wetland for in-stream water amelioration in NE Italy. Hydrobiologia 674, 157–167. Geosyntec Consultants Inc. and Wright Water Engineers Inc. International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals. Water Environment Research Foundation, Federal Highway Administration, Environment and Water Resources Institute of the American Society of Civil Engineers. Headley, T. R. & Tanner, C. C. Constructed wetlands with floating emergent macrophytes: an innovative stormwater treatment technology. Critical Review in Environmental Science and Technology 42, 2261–2310.
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Hubbard, R. K. Floating vegetated mats for improving surface water quality. In: Emerging Environmental Technologies (V. Shah, ed.), Springer, New York, pp. 211–244. Kadlec, R. H. & Reddy, K. R. Temperature effects in treatment wetlands. Water Environment Research 73, 543– 557. Kadlec, R. H. & Wallace, S. Treatment Wetlands. 2nd edn. CRC Press, Boca Raton, FL, USA. Lachat Instruments. Quik Chem Method 13-107-06-2-D. Milwaukee, WI, USA. Lamers, L. P. M., Tomassen, H. B. M. & Roelofs, J. G. M. Sulfate-induced eutrophication and phytotoxicity in freshwater wetlands. Environmental Science and Technology 32, 199–205. Methods for the Examination of Waters and Associated Materials (MEWAN) HMSO, London. Neto, R. R., Mead, R. N., Louda, J. W. & Jaffé, R. Organic biogeochemistry of detrital flocculent material (floc) in a subtropical, coastal wetland. Biogeochemistry 77, 283–304. Nietch, C. T., Borst, M. & O’Shea, M. L. Stormwater treatment: Ponds vs. constructed wetlands. Engineering Foundation Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impact Mitigation, Snowmass Village, CO, USA, pp. 524–528. Reddy, K. R. & Delaune, R. D. Biogeochemistry of Wetlands : Science and Applications. CRC Press, Boca Raton, FL, USA. Stewart, F. M., Mulholland, T., Cunningham, A. B., Kania, B. G. & Osterlund, M. T. Floating islands as an alternative to constructed wetlands for treatment of excess nutrients from agricultural and municipal wastes - results of laboratory-scale tests. Land Contamination and Reclamation 16, 25–33. Tanner, C. C. & Headley, T. R. Components of floating emergent macrophyte treatment wetlands influencing removal of stormwater pollutants. Ecological Engineering 37, 474–486. Van De Moortel, A. M. K., Meers, E., De Pauw, N. & Tack, F. M. G. Effects of vegetation, season and temperature on the removal of pollutants in experimental floating treatment wetlands. Water, Air, and Soil Pollution 212, 281–297. Wang, Q., Li, Y. & Wang, Y. Optimizing the weight loss-onignition methodology to quantify organic and carbonate carbon of sediments from diverse sources. Environmental Monitoring and Assessment 174 (1–4), 241–257. White, J. R. & Reddy, K. R. Influence of phosphorus loading on organic nitrogen mineralization of Everglades soils. Soil Science Society of America Journal 64, 1525–1534. Winston, R. J., Hunt, W. F., Kennedy, S. G., Merriman, L. S., Chandler, J. & Brown, D. Evaluation of floating treatment wetlands as retrofits to existing stormwater retention ponds. Ecological Engineering 54, 254–265.
First received 11 March 2013; accepted in revised form 3 June 2013
