An innovative constructed wetland system for small stream water restoration

Transcription

1 An innovative constructed wetland system for small stream water restoration S.J. Kim¹, ², S.W. Hong¹, ³, Y.S. Choi¹, W.K. Bae², and S.H. Lee¹ ¹Water Environment & Remediation Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, , Korea ( ²Department of Civil and Environmental Engineering, Hanyang University, 1271, Sa-1 dong, Sangnok-gu, Ansan, Kyeonggi-do, , Korea ( ³School of Civil, Urban & Geosystem Engineering, Seoul National University, Seoul, , Korea ( Abstract The newly designed wetland system for the restoration of polluted stream water combines a surface and subsurface flow. It is composed of two wetlands connected in series by the flow shifter in the middle of the constructed wetland (CW) system. The flow shifter, which converts the flow direction of a surface and subsurface within the system, was able to enhance the treatability of organic contaminants and total nitrogen (TN) via nitrification and denitrification. In this system, the suspended solids (SS) and total phosphorus (TP) removal efficiencies were considerably improved by the yellow-soil media and the flow shifter. Untreated SS and TP in an upper layer of the former wetland could be eliminated in the following wetland by filtration, precipitation and adsorption onto the yellow-soil media while they flowed into a lower layer of the latter one via the shifter. At a hydraulic loading rate (HLR) and hydraulic residence time (HRT) of 222 cm/day and 5.02 hr, respectively, the average removal efficiencies of COD and TN were 63.4 and 48%, respectively, during the summer period. These values are 2 to 4 times higher than those efficiencies from the previous experimental period without the shifter. Keywords constructed wetland; flow shifter; stream restoration; surface-subsurface flow; yellow-soil media Introduction In urban areas, some rivers and streams are impaired due to point and non-point sources which originate from a wide variety of human activities. According to the EPA reports (2002), these have originated from agricultural activities, hydrologic modifications, habitat modifications, urban runoff, storm sewers, municipal point sources and unknown sources. Dealing with these problems, the constructed wetland system is the one most applicable and cost effective treatment alternative. Kadlec and Hey (1994), in the Des Plaines River Wetlands Demonstration Project, illustrated the potential of CW for controlling the non-point source (NPS) pollutants such as nutrients and agricultural chemicals. In this project, up to 40 % of the average stream flow was treated using four CWs. Hunt et al. (1999) and Stone et al. (2003) have applied the in-stream wetlands (ISW) in order to restore the nitrogencontaminated stream owing to the NPS. Water Practice & Technology Vol 1 No 2 IWA Publishing 2006 doi: /WPT

2 The two main types of CW are surface and subsurface flow (Kadlec and Knight, 1996; Mitsch and Gosselink, 2000). In the surface flow constructed wetland (SFCW), the nearsurface layer of water is aerobic while the deeper water regions and the substrate are usually kept under anaerobic condition. The bed is filled with porous media such as soil, sand, rock and gravel in the subsurface flow constructed wetland (SSFCW). In this system, water flows horizontally or vertically through the porous media. The majority of the saturated bed containing the porous media is kept under anaerobic condition. Nitrogen reduction in CW is mainly occurred via nitrification/denitrification reaction (Gersberg et al., 1983; Reddy et al., 1989; Brix, 1993). Therefore, in terms of eliminating nitrogen, the co-existence of aerobic and anaerobic zones in CW would be necessary to enhance the treatability. Recently, the many multistage wetland systems combining SFCW and SSFCW in series are used to remove highly contaminated nitrogen in streams and rivers (Jing and Lin, 2004). These systems require a smaller footprint and show a higher treatability comparing with a single application of SFCW and SSFCW (Brix, 1993). To increase the efficiency of nitrogen removal, therefore, it would be necessary to change the pattern of surface and subsurface flow inside the CW system. All wetland soils have some capacity to sorb phosphorus, but their capacity is quite variable and limited (Kadlec and Knight, 1996). Phosphorus reduction in CW is frequently represented by a stable decreasing gradient of phosphorus from inflow to outflow (Kadlec, 1999). Sedimentation and sorption are two important physical processes for particulates and soluble phosphorus removal in wetland (Kadlec and Knight, 1996). This storage may be quickly exhausted in many SFCW, in contrast, a variety of media in SSFCW may accumulate the larger amount of phosphorus via sorption. To increase the sorption capacity, for instance, the media containing iron- and aluminum-rich materials, limestone and expanded clay composites in subsurface layer are employed. In this study, the CW system for the restoration of polluted stream water has been newly developed. Firstly, it is composed of two wetlands connected in series by inserting the flow shifter in the middle of this wetland system. Since the flow shifter, which converts the direction of surface and subsurface flows between two wetlands simultaneously, the treatability of organics and TN has enhanced. Secondly, the yellow-soil media is placed at the bottom layer. And it supports a better flow patterns within the lower layer comparing with other conventional wetland soil because it can provide higher void fraction and/or porosity. Subsequently, it allows the larger influx into the subsurface from the surface layer via the shifter. Also, it has a sorption capacity of phosphate ions. The main objectives of this study were: (1) to compare the performances of a newly designed CW system with and without the flow shifter, (2) to evaluate the role of a flow shifter in terms of increasing removal efficiencies and HLR, and (3) to determine the adsorption characteristics of the yellow-soil media. Methods Description of constructed wetlands In this study, the first (Run 1) and second (Run 2) operating period was March to August 2003, and October 2003 to January 2005, respectively. This wetland system was placed in a 2

3 container that could only admit the sunlight through the proof covered with glass. This was located in Kyungan stream at the central western part of Korea. As shown in Figure 1, the performance of the one-stage wetland (CW1) during the first period (Run 1) has been evaluated. The dimension of the system is 8 m (L) 1 m (W) 0.7 m (H) including the outlet zone. During the second period (Run 2), it has been modified as the two-stage wetland (CW1 and CW2) by inserting a new CW2 and two flow shifters in the middle of the system. Thus, the dimension of the extended system is 20 m (L) 1 m (W) 0.7 m (H). The flow shifter converts the two flow directions as shown in Figure 2 C. The mean diameters of sand, crushed stone and yellow-soil media were 1.0, 32 and 80 mm, respectively. 1. Primary sedimentation 2. Wire mesh gabion 3. Water layer 4. Reeds 5. Flow shifter 6. Sand bed 7. Crushed stone bed 8. Yellow-soil media 9. Roses 10. Outlet zone Figure 1 Schematic diagram of the newly designed constructed wetland system A B C An upper layer in CW2 An upper layer in CW1 A lower layer in CW2 A lower layer in CW1 Figure 2 View of two flow shifters (A); inside view (B) and two flow paths in the flow shifter (C) Operational conditions and influent characteristics The influent is continuously pumped from the adjacent stream into the wetland basins and finally discharges back into the stream. At Run 2 (Run 1), the daily treated amount of stream water, total wetland volume (V T ), surface area, and HLR were 44 m 3 /d (15), m 3 (5.46), 19.8 m 2 (7.8), and 222 cm/d (192), respectively. The tracer test was performed in order to determine the hydraulic residence time (HRT) and void volume of wetland (V P ) by adding the lithium chloride of g. In the test, the 2 porosity (ε), dimensionless variance ( σ θ ) and dispersion number (D/uL) were found to be , and 0.172, respectively. Therefore, HRT, V P and actual velocity for Run 1 3

4 were calculated to be 5.80 hr, 3.62 m 3 and 32.3 m/d, and 5.02 hr, 9.20 m 3 and 94.7 m/d for Run 2, respectively. During the operating period, pollutant loading rates of BOD 5, COD, SS, TN, and TP ranged , 153-2,713, 38-1,472, and kg/ha/day, with an average of 306, 547, 331, 402 and 17.7 kg/ha/day, respectively. BOD 5 /COD ratio of the influent ranged with an average of which indicates that the polluted stream water contains very low biodegradable organic portion. On average, COD/TN and PO P/TP ratio in the influent was and 0.828, respectively. About 26% of TN presented as oxidized nitrogen. The ph value and water temperature were measured in the range of and 5-26, respectively. Batch test using the yellow-soil media Adsorption equilibrium isotherm experiments were conducted using the yellow-soil media. Before calcination at 900, yellow-soil was mixed with a certain amount of straw which could provide very fine pores inside the media. The different amounts of this media (5, 25, 50, 75 and 100 g dry matter) were added into glass bottles. The initial ph and NH + 4 -N concentration were and 20 mg/l (C 0 = const.), respectively. As a nitrogen source, NH 4 Cl was used. And the bottles were sealed with screw caps and continuously agitated in a shaking incubator (130 rpm) at 25 for 2 day. Another isotherm experiments were performed on this media with initial concentration of 50 mg PO P/L (C 0 = const.) using KH 2 PO 4 as phosphorus source for 3 days by following the above-mentioned experimental procedure. Analytical methods for water quality The following parameters were analyzed for influent, effluent and samples form upper and lower layers: biochemical oxygen demand (BOD 5 ), chemical oxygen demand (COD), suspended solids (SS), total nitrogen (TN) and total phosphorus (TP). These analyses were carried out according to Standard Methods for the Examination of Water and Wastewater, 20th edition (1998). Total Kjeldahl nitrogen and ammonia nitrogen were determined by a Kjeltec Auto 1035/38 Sampler system (Tecator). Ion Chromatography (DX-120) was used to measure nitrite, nitrate and phosphate. Results and discussion Constructed wetland efficiency In this section, the results from the operation of Run 1 for 180 days and Run 2 for 470 days are illustrated. On average, water temperature was 5.3 in winter, 14.7 in spring and autumn, and 24.6 in summer during the operating period. The effluent concentration of BOD 5 was found to be less than 5 mg/l, and its removal efficiency ranged from 75 to 81 % for Run 1 and Run 2, except for an adaptation period and winter. The average removal efficiency of BOD 5 at Run 1 was similar to that at Run 2. In and out concentrations of COD, SS, TN and TP, and its removal efficiencies at Run 1 and Run 2 are shown in Table 1. During the spring and summer, the removal efficiencies of COD and TN were 29.9±2.5 and 10.9±0.1 % at Run 1, and 57.9±5.5 and 40.6±7.4 % for Run 2, respectively. And the mass removal rate of COD and TN were 127 and 35 kg/ha/day at Run 1 and 309 and 177 kg/ha/day at Run 2, respectively. Its removal capacity of COD and TN at Run 2 with the 4

5 flow shifter was 2-4 times higher than that at Run 1 without the shifter. The major contribution for the increment of the treatability in this system is the insertion of the flow shifter which helps treat both surface and subsurface flows by converting the flow pattern. In terms of the seasonal variations, the removal efficiencies of COD and TN were highly dependant on temperature. The higher efficiency is associated with the rise in temperature because the microbial activity in constructed wetlands is being enhanced under warmer conditions. Similar results were reported by Thut (1989) and Trautmann et al. (1989). The removal efficiency of SS was less affected by temperature. The mean concentration of SS in the effluent was less than 4.0 mg/l regardless of the influent loading. And, the mean removal efficiency was kept higher than 60 % throughout the whole period of operation. At Run 1, the mass removal rate of SS was ranged from 140 to 292 kg/ha/day with an average of 216 kg/ha/day. The removal capacity has increased by 25% at Run 2 with the shifter: the mass removal rate and its mean value were found to be 69 to 453 and 269 kg/ha/day, respectively. Even though HRT at Run 2 was shorter than at Run 1, a higher TP removal capacity was observed at Run 2. At Run 1 and 2, the daily averaged mass removal rate was 4.61 and 6.14 kg/ha, respectively. Consequently, the removal capacity for COD, TN, SS and TP has been enhanced at Run 2 by inserting the flow shifter. The reason is that the stream water at both of surface and subsurface layer is treated in the system since the flow direction in CW 1 and 2 is converted via the shifter. Additionally, the yellow-soil media plays a supporting role in enhancing the treatability owing to its characteristics: allowing the larger influx at the subsurface layer and adsorbing both of ammonia nitrogen and phosphate ions. Table 1 Summary of the performances during the experimental period Run 1 Run 2 Spring (3/003-5/2003) Summer (6/2003-8/2003) Autumn (10/ /2003) Winter (12/2003-2/2004) Summer (6/2004-8/2004) Autumn (9/ /2004) Winter (12/2004-1/2005) In 28.6 ( ) 16.6 ( ) 18.5 ( ) 29.5 ( ) 27.3 ( ) 21.5 ( ) COD SS TN TP Out R.E. (%) In 20.8 ( ) ( ) 11.2 ( ) ( ) 11.6 ( ) ( ) 18.2 ( ) ( ) Spring (3/2004-5/2004) ( ) 52.4 ( ) 7.9 ( ) ( ) ( ) ( ) ( ) 27.3 ( ) 19.2 ( ) ( ) Out R.E. (%) ( ) ( ) 1.9 ( ) 1.8 ( ) In 20.3 ( ) 12.2 ( ) 4.1 ( ) ( ) 5.9 ( ) ( ) 2.9 ( ) 87.3 Out 18.0 ( ) 10.9 ( ) R.E. (%) ( ) ( ) In 1.14 ( ) 0.76 ( ) 0.88 ( ) ( ) ( ) ( ) ( ) ( ) 1.37 ( ) 0.61 ( ) ( ) ( ) ( ) ( ) 66.3 ( ) ( ) 15.9 ( ) Out 0.89 ( ) 0.53 ( ) 0.74 ( ) 0.98 ( ) 0.73 ( ) 0.35 ( ) 0.32 ( ) 0.26 ( ) R.E. (%) Regression equation Table 2 shows the observed relationship between mass loading rate and effluent concentrations of COD, SS, TN and TP at Run 2. Using the equation shown below, the effluent concentration has predicted based on the influent concentration and HLR. 5

6 C e = a C i b q c where C e is the estimated effluent concentration, C i is the influent concentration, q is HLR (cm/d) and a, b, c are the regression coefficients. Those values of R 2 determined by the regression of experimental data were somewhat similar to the values from North American Wetland Treatment System (NADB). The site-specific factors influencing the treatability of pollutants were not included in the above equation so that the values were relatively low. In the case of TP, the value of R 2 for the regression has found to be which is slightly higher than that described in NADB. The removal of COD, TN and SS is quite dependent on some factors, especially the temperature and resuspension so that the values are low. However, the removal of TP is less sensitive to those two factors. Table 2 Regression equations at Run 2 (calculated C e at q=222 cm/d) Equation Input range Output range R 2 COD C e = C i q < C i < < C e < SS C e = C i q < C i < < C e < TN C e = C i q < C i < < C e < TP C e = C i q < C i < < C e < Pollutants profiles in constructed wetlands In order to inspect the decreasing trends of COD, SS, ammonia and oxidized nitrogen and TP with ph variations more closely at Run 2, the samples were taken from the upper and lower layers at uniform intervals of approximately 1 m. As shown in Figure 3 A-1 and A-2, COD was gradually decreased along the length of the CW system due to the oxidation at the upper and consumption at the lower layer by denitrification process. The SS reduction was mainly occurred mainly in lower layers at the front of each wetland (Figure 3 B-1 and B-2) by filtration and sedimentation. As shown in Figure 4 A-1 and A-2, the ammonia reduction was occurred in the upper layers at each wetland via nitrification and more reduction was observed at CW1. Another reduction was occurred at the lower layer of CW2, whereas not observed in CW1. It might be due to the adsorption of ammonia nitrogen onto the yellow-soil media. However, it could be disturbed in CW1 by the accumulation of SS onto the media which could limit the adsorption capacity. In fact, more amounts of SS were reduced at CW1 than CW2. In both CW1 and CW2, the oxidized nitrogen concentration was gradually increased in the upper layers due to the nitrification process as shown in Figure 4 B-1. On the other hand, the inlet oxidized nitrogen was decreased in the lower layers at CW1 due to the denitrification process as shown Figure 4 B-2. Likewise, the remaining ammonia and oxidized nitrogen which flowed into the CW2 via the shifter were removed in the same manner. Also, the change in ph showed that nitrogen reduction occurred via nitrification and denitrification in the system (Figure 5 A-1 and A-2). 6

7 More reduction of TP was occurred in the lower layer than the upper at each wetland as shown in Figure 5 B-1 and B-2. Unlike the removal of SS, the concentration of TP was gradually decreased along the length at each wetland. The removal of particulate and soluble phosphorus in the system would be related to the filtration, precipitation, adsorption, uptake by plants, and so on. The experimental evidence suggests that the reducing trends of each pollutant have been clearly distinguished at each zone inside of two wetlands which are interconnected by the flow shifter. As discussed early, the shifter plays an essential role to treat pollutants in both surface and subsurface flow. Removal capacity of yellow-soil media Adsorption equilibrium isotherm experiments were performed in order to elucidate the reduction of ammonium and phosphorus in the lower layers filled with the yellow-soil media. As shown in Table 3, the adsorption capacities of the media for ammonium and phosphate ion were better fitted to Freundlich isotherm than Langmuir. In the case of phosphorus, its reduction could be achieved via the adsorption and/or precipitation. Since yellow-soil contains various minerals such as calcium, iron, aluminium, magnesium, and so on, the phosphate ion should be reduced in the form of precipitates such as calcium phosphate owing to the extraction of those minerals from the media. According to the experimental data (not shown here), certain amounts of minerals extracted from the media were observed. The longterm performance monitoring or column test should be done to find out the maximum removal capacity of the media regarding ammonium and phosphate ions. Figure 3 Changes of COD (A-1 and A-2) and SS (B-1 and B-2) ratios in the upper and lower layers at Run 2 (C i: inlet concentration and C L: concentrations along the length of the wetland) 7

8 Figure 4 Changes of ammonia (A-1 and A-2) and oxidized nitrogen (B-1 and B-2) ratios in the upper and lower layers at Run 2 (C i: inlet concentration and C L: concentrations along the length of the wetland) Figure 5 Changes of ph (A-1 and A-2) and TP (B-1 and B-2) ratios in the upper and lower layers at Run 2 (C i: inlet concentration and C L: concentrations along the length of the wetland) Table 3 Coefficients of Freundlich and Langmuir isotherms for the yellow-soil media Freundlich Langmuir R 2 K n R 2 a b Ammonium Phosphate

9 Conclusions In this study, the performance of a new CW system for the restoration of polluted stream water has been monitored for 16 months. In this system, the flow shifter has interconnected two wetlands at the middle and successfully converted the direction of surface and subsurface flow of each. The main conclusions from this study are:! According to the experimental results, the role of the shifter has shown to be significantly effective in increasing pollutants removal efficiencies and HLR.! With the shifter, the yellow-soil media plays a supporting role in enhancing the treatability owing to its characteristics: allowing the larger influx at the subsurface layer and adsorbing both of ammonia nitrogen and phosphate ions.! The majority of nitrogen compounds are eliminated via nitrification and denitrification and some of ammonia is subjected to adsorption onto the media.! Unlike the removal of SS, the concentration of TP was gradually decreased along the length in lower layers at each wetland. The reduction of particulate and soluble phosphorus is mainly due to the filtration, precipitation and adsorption. References Brix, H. (1993). Wastewater treatment in constructed wetlands: system design, removal processes, treatment performance. In: Constructed Wetlands for Water Quality Improvement, G.A. Moshiri (ed.), Lewis Publishers, Boca Raton, Florida, pp Gersberg, R.M., Elkins, B.V. and Goldman, C.R. (1983). Nitrogen removal in artificial wetlands. Water Res., 17(9), Hunt, P.G., Stone, K.C., Humenik, F.J., Matheny, T.A. and Johnson, M.H. (1999). In-stream wetland mitigation of nitrogen contamination in a USA coastal plain stream. Journal of Environmental Quality, 28(1), Jing, S.R. and Lin, Y.F. (2004). Seasonal effect on ammonia nitrogen removal by constructed wetlands treating polluted river water in southern Taiwan. Environmental Pollution, 127(2), Kadlec, R.H. (1999). The limits of phosphorus removal in wetlands. Wetlands Ecology and Management, 7(3), Kadlec R.H. and Hey D.L. (1994). Constructed wetlands for river water quality improvement. Wat. Sci. Tech., 29(4), Kadlec, R.H. and Knight R.L. (1996). Treatment Wetlands. Lewis Publishers, Boca Raton, Florida. Mitsch, W.J. and Gosselink J.G. (2000). Wetlands. 3rd edn, John Wiley & Sons, Inc., New York. Reddy, K.R., Patrick, W.H.Jr. and Lindau, C.W. (1989). Nitrification-denitrification at the plant rootsediment interface in wetlands. Limnol Oceanogr., 34(6), Standard Methods for the Examination of Water and Wastewater (1998). 20th edn, American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC, USA. Stone, K.C., Hunt, P.G., Novak, J.M. and Johnson, M.H. (2003). In-stream wetland design for non-point source pollution abatement. Transactions of ASAE, 19(2), Thut, R.N. (1989). Utilization of artificial marshes for treatment of pulp mill effluents. In: Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural, D.A. Hammer (ed.), Lewis Publishers, Chelsea, Michigan, pp

10 Trautmann, N.M., Martin, J.H., Porter, K.S. and Hawk, K.C. (1989) Use of artificial wetlands for treatment of municipal solid waste landfill leachate. In: Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural, D.A. Hammer (ed.), Lewis Publishers, Chelsea, Michigan, pp USEPA (2002). National Water Quality Inventory; 2000 Report, EPA-841-R , Office of Water, Washington DC. 10