Does common reed (phragmites spp ) contribute to the removal of phosphorous and nitrogen from domestic wastewater in constructed subsurface flow wetlands

Does common reed (Phragmites spp.) contribute to the removal of phosphorous and nitrogen from domestic wastewater in constructed subsurface flow wetlands? L A Tuana, *, I Smetsb and G Wyseurec a Department of Environment and Natural Resources, Can Tho University, Campus II, 3/2 Street, CanTho City, Vietnam (*E-mail: latuan@ctu.edu.vn) b Department of Chemical Engineering, Chemical and Biochemical Process Technology and Control Section, Katholieke Universiteit Leuven, Belgium c Division for Soil and Water Management, Faculty of Biosciences Engineering, Katholieke Universiteit Leuven, Belgium ABSTRACT Common reed (Phragmites spp.), or ‘reed’, is widely used as dominant vegetation in constructed treatment wetlands With respect to their role in removing phosphorous and nitrogen, this paper presents the result of two parallel experiments The first experiment was aimed at surveying the biomass increase of reed over time and the percentage of accumulated total phosphorous (P) and total Kjeldahl nitrogen (N) in their biomass In the second experiment the P and N balances were examined in pots The wastewater used in the experiments was taken from different treatment stages of an experimental subsurface flow wetland located at the Can Tho University, Vietnam The experiment results showed that between 24% and 60% of the total P and between 22 and 77% of the total Kjeldahl N were removed mainly contributed by two mechanisms: through adsorption in the sand (23 to 48% P and 0.8 to 1% N) and through uptake in common reed plant and root systems (1 to 12% P and 22 to 76% N) These removal percentages fit the results of previous reports It may be concluded that reed as a macrophyte species plays a certain treatment role in constructed wetlands, but predominantly for nitrogen removal Keywords: Phosphorous, nitrogen, common reed, constructed subsurface flow wetland, domestic wastewater INTRODUCTION In general, a constructed subsurface flow wetland (CSFW) consists of a suitable depth of a porous medium with a lining at the bottom and the side in order to prevent seepage to the environment In this porous medium wetland plants are often grown The sand and root system in CSFW systems are colonised by a layer of attached micro-organisms that form a biofilm (Rousseau et al 2004) The wastewater flows through the porous medium of sand and rhizosphere (if wetland plants are present) and it is treated by a combination of physico- and biochemical processes Finally, the treated wastewater flows out of the bed In a subsurface flow wetland, the water is always kept below the surface The CSFW systems have a number of alternative names, such as: vegetated submerged bed, root zone method, microbial rock reed filter, and plant-rock filter systems There are two basic types of CSFW, that is, horizontal-flow systems and verticalflow systems, based on the flow direction through the soil and gravel layers In the flat Mekong River Delta (MD) of Vietnam, it is easier to construct horizontal-flow systems than vertical systems since the groundwater level near the surface limits the vertical dimensions of wetlands Stottmeister et al (2003) state that suitable plants in constructed wetland should provide oxygen into the root zone, take up nutrients and degrade pollutants When focusing on the literature dealing with macrophyte wetland plant based horizontal subsurface flow constructed wetlands, the results reported on the different nutrient removal percentages and on the proposed (dominant) removal mechanisms, greatly vary Billore et al (1999) reported that Phragmites Karka plants in Central India removed 78% of nitrogen (N) and 58–65% of phosphorous (P) from a horizontal subsurface flow gravel bed constructed wetland system after five months of operation Windham and Ehrenfeld (2003) have observed the rates of nitrate reduction (dissimilatory and assimilatory) were 300% larger in Phragmites australis sediments, and also Phragmites spp., that is, common reed, was found to contribute significantly to the N removal process by plant uptake through the root system (Lee & Scholz, 2006) However, Tanner (2001) assessed the mass balance of N and P accumulation in soft-stem bulrush and showed that only to 11% of the N removal and to 13% 138 Southeast Asian Water Environment of the P removal recorded from wastewaters applied to the wetlands could be ascribed to plant uptake and accumulation Therefore, the aim of this paper is to further investigate the role of the reed in the uptake (and, hence, removal) of N and P MATERIALS AND METHODS To test the hypothesis that N and P are removed by the reed root system within the sand bed, two parallel experiments have been setup in a constructed subsurface wetland (CSFW) This experimental CSFW is located at Campus I of Can Tho University and was built in April 2003 to treat domestic wastewater from dormitories (Figure 1) The main part of the system is a sand tank (12.0 × 1.8 × 1.5 m) In this tank, river sand (average porosity of 47%) is filled up to a thickness of 1.2 m Common reed, a very common and easy growing plant in the Mekong River Delta, is planted with a density of 25 plants per m2 On the bottom of the sand tank, there are sampling valves, coded as WL3, WL4, WL5, WL6 and WL7 The WL1 and WL8 positions are in the influent and effluent tanks of the system, respectively Water sampling has been carried out each two-week continuously since 20 August 2003 up to now Water sampling has been carried out each two-week continuously since 20 August 2003 up to December 2006 The 4-year average pollutant parameters as a function of their wetland location showed a high removal rate of N and P (Figures and 3) Striking were the different shapes of the curves, which suggests different mechanisms The effluent meets Vietnamese industrial wastewater discharge standards (TCVN 5945-1995), where Level A is the discharge standard for water bodies used as source for domestic water supply and Level B is discharge standard for water bodies used for navigation, irrigation, bathing, aquatic breeding and cultivation and so on Influent wastewater Common Reed 220 100 2m WL1 2m WL2 WL3 WL5 WL4 1m WL8 WL7 WL6 1m 12 m 16 m Experiment 1: Reed growths and its % of dried P and N to Time → Experiment 2: Reed growths and P and N balances → Figure Constructed subsurface flow wetland in Can Tho and experimental set up 20.00 18.00 Avg Ptotal (mg/L) 16.00 y = 33.974e-0.5928x R2 = 0.9606 14.00 12.00 10.00 8.00 6.00 Level B 4.00 Level A 2.00 0.00 WL1 WL2 WL3 WL4 WL5 WL6 WL7 WL8 Figure The average changes in total phosphorous in the water at the constructed wetland sampling locations during 4-year operation Does common reed (Phragmites spp.) contribute to the removal of phosphorous and nitrogen 139 70.00 Level B 60.00 Avg TKN (mg/L) y = -0.4397x - 2.195x + 59.417 R2 = 0.9669 50.00 40.00 30.00 Level A 20.00 10.00 0.00 WL1 WL2 WL3 WL4 WL5 WL6 WL7 WL8 Figure The average changes in TKN in the water at the constructed wetland sampling locations during 4-year operation In the first experiment the biomass of the reed and the sand were sampled to determine the removal of total phosphorous (P) and total Kjeldahl nitrogen (N) by the reed There were 35 plastic bags, 40-cm in height and 25-cm in average diameter, filled with sand, each one containing three 5-cm reed stems The reed stems were taken from the same place and were cut to a similar size The bottoms of the bags were closed but at 1/3 of the height drainage holes were provided to collect the overflow of water At each of the water sampling points (from WL3 to WL7 in Figure 1) bags were grown Every time step (i.e., every 10 days) a bag at each sampling point was sacrificed to harvest the reed In the second (parallel) experiment with similar containers, but now called pots, the overflowing water was collected (Figure 4) A small pipe was attached at the drainage holes, again located at 1/3 of the height from the bottom for collecting the water Reed shoots were planted at 20 cm depth from the sand surface At WL3, WL5 and WL7 three replicates of sand pots with reeds were positioned The plastic pots were 40-cm in height and 25-cm in average diameter Reed Wastewater in 20 cm 26 cm Sand sampling Wastewater out 13 cm 40 cm Covering cloth 25 cm Water collecting bottle Figure Experimental set up for P and N balance with sand and reed in pot Throughout the experiment, every days, L of water from the respective sampling points was fed to the bags and pots The purpose was to establish the terms of the phosphorus and nitrogen balances as follows: Pin = (Pout + Psand + Preed + Prest ) Nin = (Nout + Nsand + Nreed + Nrest ) where P and N stand for total phosphorous and nitrogen; subscripts ‘in’, ‘out’, ‘sand’, ‘reed’, ‘rest’ mean the concentration in the applied input of water, overflow outgoing water, kept in sand, absorbed/consumed in reed biomass and the rest which was mostly stored in the micro-organism communities and remaining wastewater in the system at the end of the testing period, respectively In the above balance equations, the losses of P and N in the system under the form of gas 140 Southeast Asian Water Environment emissions were ignored At each water application Pin and Nin were measured and every 10 days, destructive sampling was done of a bag from the first experiment in order to establish the Preed, and Nreed as well as the ΣPsand and ΣNsand The latter were measured by the difference between the P and N in the sand at the beginning and at the end of the experiment Samples of the sand were taken randomly from near surface, middle and bottom depths in the centre of each container From the parallel experiment in the pots, the Pout and Nout were determined Finally, the ΣPrest and ΣNrest were determined from the balance equations (1) and (2) The ΣPrest and ΣNrest are the (i) remaining parts of total phosphorus and total nitrogen existing in the bacterial body and in the water stored in the pot and (ii) the Kjeldahl nitrogen that has been transformed to nitrite or nitrate At the end period of the experiment, all the reed plants including their stems, leaves and root systems were removed gently from each pot and weighed Before analyzing P and N in the reed plant, their root system was cleaned, other solids were removed by feather duster and then washed carefully to remove bacteria by water spray Total phosphorous and nitrogen in the sand were measured by the Methods of Soil Analysis (Page et al 1982) The total phosphorous in reed plant tissues were analyzed by dry ashing and the Vanadomolydate colorimetric method The total Kjeldahl nitrogen as the sum of ammonia nitrogen and organic nitrogen were determined by Vapodest equipment (Gerhardt GmbH, Germany) Data were graphically compared and an ANOVA analysis was performed at the significance level of α = 0.05 RESULTS AND DISCUSSION The average concentrations of P and N in the partly treated wastewater along the positions in the wetland system are shown in Figure The N amount in the domestic wastewater flowing into the system was on average 61.2 mg/L during the year observation period The N content of the outflow (WL8) was on average 9.9 mg/L Eighty days after planting, reed plants were developed well in pots as is evidenced in Figure 40 PTotal (mg/L) TKN (mg/L) 35 P and N (mg/L) 30 25 20 15 10 WL3 WL4 WL5 WL6 WL7 Reed weight increase (mg) Figure Average concentrations of P and N at the sampling points along the wetland from which the influent to the bags and pots were taken 100 90 80 70 60 50 40 30 20 10 WL3 WL4 WL5 WL6 WL7 10 20 30 40 50 Days 60 70 80 Figure Reed development over time After 30 days, it was found that the reed biomass weight increase was 10.1 g, 11.2 g and 9.7 g at WL3, WL5 and WL7, respectively These growths could be seen easily through the stem, leaf and shoot development The evolution of the P and Does common reed (Phragmites spp.) contribute to the removal of phosphorous and nitrogen 141 N concentrations in the reed in time is presented in Figure 7a, b The performed ANOVA analysis showed that the differences in the concentration of P and N in the reed biomass at the different locations WL3, WL5 and WL7 were smaller than the significant level of 5% From the P balance presented in Figure 8a, the P removals at three positions are mainly in the sand and outflow The reed absorbed only a small amount of P The lost P can partly be explained as lost in the reed root processing procedure By washing the roots some of the absorbed P can be lost However, by comparing the total phosphorous concentration in and out of the pots, it is found that the total phosphorus removal percentage of P was high, among 50%−79% or in average 62% This removal performance is in line with the results by Billore et al (1999) and Reed et al (1995) However, since a large portion of P was stored in the sand, the removal efficiencies will decrease over time (b) 1.80 6.0E-03 1.60 WL3 WL4 WL5 WL6 WL7 1.40 5.0E-03 WL3 4.0E-03 WL4 3.0E-03 2.0E-03 WL5 WL6 1.0E-03 WL7 N in Reed (%) P in Reed (%) (a) 7.0E-03 1.20 1.00 0.80 0.60 0.40 0.20 0.0E+00 0.00 20 40 60 80 Days 20 40 60 80 Days Figure (a) Phosphorus (P) and (b) nitrogen (N) uptake in reed versus time Figure Phosphorous (a) and nitrogen (b) balance Figure 8b present the total N balance Interesting to see is that the reed took up a similar amount of nitrogen (about 600 mg) The excess nitrogen in the influent was included in Nrest terms, meaning that the siginificant part of N removal in WL3 and WL cannot be explained by uptake by reed nor by adsorption on the sand Hence, in pots watered by wastewater from WL3 and WL5 (i.e., near the influent tank with a high influent N concentration) the Nrest is high In pots watered by the wastewater from WL7, the Nrest is small (Figure 8b) The N removal percentage was acceptable, ranged 87%−92% or 90% in average The percentage was higher than the nitrogen removal ranges of US-EPA (1988), this can probably be contributed to the freshness of the applied sand material CONCLUSIONS Through two parallel experiments the biomass growth of reed plants was assessed and the mass balance of N and P was examined It can be concluded that reed as a macrophyte species plays only a minor role in the removal of phosphorous since the dominant removal mechanism was the adsorption/retention in the sand In contrast, common reed did contribute to the removal of Kjeldahl nitrogen in the constructed wetlands There existed, however, an upper limit in nitrogen uptake by the plant When dealing with highly concentrated domestic wastewaters, the excess nitrogen must be removed by other mechanisms These findings will form the scientific basis to design an effective and economically feasible constructed wetland for domestic wastewater treatment in a tropical region as the MD, Vietnam where capacities of the invested capitals and industrial management for a modern wastewater treatment plants are still limited 142 Southeast Asian Water Environment ACKNOWLEDGEMENTS The authors thank the VLIR-CTU project for financial support this research and thank all faculties and staff in the Department of Environmental Engineering, CTU for their helps during our experiments REFERENCES Billore S K., Singh N., Sharma J K., Dass P and Nelson R M (1999) Horizontal subsurface flow gravel bed constructed wetland with Phragmites Karka in Central India Water Sci Tech., 40, 163–171 Lee B H and Scholz M (2006) What is the role of Phragmites Australis in experimental constructed wetland filters treating urban runoff? Ecol Eng., 29, 87–95 Page A L., Miller R H and Keeney D R (1982) Methods of Soil Analysis Part – Chemical and Microbiological Properties American Society of Agronomy, Inc., Madison, WI, USA Reed S C., Crites R W and Middlelebrooks E J (1995) Natural Systems for Waste Management and Treatment McGraw–Hill, New York, pp 433 Rousseau D P L., Vanrolleghem P A and Pauw N D (2004) Model-based design of horizontal subsurface flow constructed treatment wetlands: a review Water Res., 38, 1484–1493 Stottmeister U., Wiessner A., Kuschk P., Kappelmeyer U., Kastner M., Bederski O., Muller R A and Moormann H (2003) Effects of plants and microorganisms in constructed wetlands for wastewater treatment Biotechnol Adv., 22, 93–117 Tanner C C (2001) Growth and nutrient dynamics of soft-stem bulrush in constructed wetlands treating nutrient-rich wastewaters Wetlands Ecol Manag., 9, 49–73 Windham L and Ehrenfeld J G (2003) Net impact of a plant invasion on nitrogen-cycling processes within a brackish tidal marsh Ecol Appl., 13, 883–896  ... through the stem, leaf and shoot development The evolution of the P and Does common reed (Phragmites spp.) contribute to the removal of phosphorous and nitrogen 141 N concentrations in the reed in. .. changes in TKN in the water at the constructed wetland sampling locations during 4-year operation In the first experiment the biomass of the reed and the sand were sampled to determine the removal of. .. from the first experiment in order to establish the Preed, and Nreed as well as the ΣPsand and ΣNsand The latter were measured by the difference between the P and N in the sand at the beginning and