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Economic and environmental analysis of using constructed riparian wetlands to support urbanized municipal wastewater treatment
Ecological Engineering 44 (2012) 249–258
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Economic and environmental analysis of using constructed riparian wetlands to support urbanized municipal wastewater treatment Chia-Ji Teng a , Shao-Yuan Leu b,∗ , Chun-Han Ko c,∗∗ , Chihhao Fan d , Yiong-Shing Sheu e , Hui-Yu Hu f a
Department of Safety, Health and Environmental Engineering, Tungnan University, New Taipei City 22202, Taiwan Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA c School of Forest and Resources Conservation, National Taiwan University, Taipei 10617, Taiwan d Department of Safety, Health and Environmental Engineering, Mingchi University of Technology, New Taipei City 24301, Taiwan e Department of Water Quality Protection, Environmental Protection Administration, Executive Yuan, Taipei 10042, Taiwan f MWH Americas Inc. Taiwan Branch Office, Taipei 10695, Taiwan b
a r t i c l e
i n f o
Article history: Received 16 November 2011 Received in revised form 7 March 2012 Accepted 26 March 2012 Available online 2 May 2012 Keywords: Wetland Wastewater Economic Reaction kinetics Modeling
a b s t r a c t This paper evaluated the economic and environmental benefits of using riparian constructed wetlands (CWs) for municipal wastewater treatment in an urban watershed. The monitoring data were collected from seven CWs in Tan-Shui River basin of the metropolitan Taipei, and were compared with a centralized wastewater treatment plant in the same watershed. Operation parameters such as site conditions, effluent quality, construction and operation/maintenance (O&M) costs were analyzed and used to calculate the first order reaction kinetics (kv20 ) of three pollutants, i.e. TSS, BOD, and NH4 –N. The reaction kinetics and costs vary significantly among the CWs and treatment plant. The overall treatment performances of BOD and NH4 –N generally confirmed with the references, with variation only among the different sites, but the removal of TSS was less desirable for all CWs. The total costs of the CWs were between 0.425 and 3.621 USD per kg total BOD removed, and the costs of the centralized wastewater treatment plant was approximately 1.186 USD per kg total BOD removed. The wetlands provide reasonable pollutants removal and show additional benefits on education and recreation while the sanitary sewer system and new treatment plants are under construction. The experiment results confirmed with the references and the methodology can be used to developing water quality management plans for urbanized watersheds in subtropical areas. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Constructed wetlands (CWs) are an economic and ecological alternative for wastewater treatment. This on-site wastewater treatment process has been used to treat different types of wastewaters, i.e. domestic wastewaters (Brix et al., 2007; Öövel et al., 2007), animals or agriculture wastewaters (Cronk, 1996; Harrington and McInnes, 2009), urban runoffs (Scholz, 2006; Kao et al., 2001), and industrial wastewaters (Chen et al., 2006; Di Luca et al., 2011), and can effectively remove many contaminants, i.e. nutrients (Kadlec and Knight, 1996; Lin et al., 2009; Kadlec, 2009), organic chemicals (Larue et al., 2010), and metals (Scholz, 2006; Maine et al., 2007). In addition, some CWs are habitats of many
∗ Corresponding author. Tel.: +1 310 526 1534; fax: +1 310 206 2222. ∗∗ Corresponding author. Tel.: +886 2 3366 4615; fax: +886 2 2365 4520. E-mail addresses: [email protected] (S.-Y. Leu), [email protected] (C.-H. Ko). 0925-8574/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2012.03.009
wild lives and can provide additional benefits on research, restoration, and recreation (Knight, 1992; Zedler and Leach, 1998). U.S. EPA (2000) suggests that CWs are more suitable than activated sludge processes (ASPs) for wastewater treatment in small communities with wastewater flow less than 3800 m3 /day and when the land is available. The key challenges of the CWs are the treatment efficiencies and operation stability. The treatment performances of the CWs are affected by many factors, i.e. hydraulic loading rates (HRL) and ambient temperature (Kadlec and Knight, 1996), vegetation species and properties (Vymazal, 2011), and weather or damages caused by nature disasters (Fan et al., 2009; Ko et al., 2010). It is essential to obtain complete understandings of the benefits and limitations of the CWs before incorporating these processes in a municipal wastewater treatment system. The goals of this paper were to develop a methodology to quantify the economic and environmental benefits of the CWs for municipal wastewater treatment. The experimental results were
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collected from seven free water surface (FWS) wetlands and a centralized wastewater treatment plant (WWTP) in the Tan-Shui River basin of the metropolitan Taipei. In Taiwan, sanitary sewage systems are still in developing and riparian wetlands have been used as a mitigation plan to reduce pollution and remediate the polluted rivers (Kao et al., 2001). Since 2004, 14 riparian CWs have been constructed in the Tan-Shui River basin and the associated tributaries. The wetlands served well for the purposes of wastewater purification, urban recreation, environmental education, and nature habitats restoration. The remediation program is successful and improvements in water quality have been documented in the receiving waters (Cheng et al., 2011). The monitoring results of the wetlands and the centralized WWTP provide an excellent opportunity for comparison and future planning. In this study, operation conditions and water quality data were analyzed and the first-order reaction kinetics (k value) of different contaminants was calculated. Capital costs and the operation and maintenance (O&M) costs were recorded to quantify the 20-year annuity of each wastewater treatment facility. Recommendations were provided to the future development of wastewater treatment programs. The methodology and case examples can serve as a reference to developing wastewater management plan in watersheds with similar background. 2. Materials and methods 2.1. Sites background The locations of the studied wetlands and WWTP are shown in Fig. 1. The studied CWs (Site-1 to Site-7) are all located upstream to the centralized WWTP (Site-C) along the Tan-Shui River. The shaded areas in Fig. 1 show the served sewage basins of the studied wastewater treatment facilities. The current total wastewater flow in Taipei City is approximately 1,400,000 m3 /day. The area served by the WWTP covers more than 30% of the sewer lines in Taipei city, and is approximately 3 times the area of all the CWs in the
New Taipei City. In addition, Site-2 to Site-4 and Site-6 to Site-7 received wastewaters from the same sewage basins. The designed conditions of the studied wastewater treatment facilities were presented in Table 1. The CWs were all FWS wetlands constructed between 2004 and 2010. The designed treatment capacities of the CWs are between 4000 and 30,000 m3 /day, and the total capacity of the CWs is 81,500 m3 /day. The designed hydraulic retention times (HRTs) of the CWs are between 3.22 and 5.69 days, and the areas of the wetlands are between 4.9 and 80 ha. The water surface areas are approximately between 3.4 and 19.3 ha. The areas of sewage basins and populations served were also shown in Table 1 for the wastewater treatment facilities. The CWs are all composed of 3–5 cells, and the cells are classified based upon the functionalities, i.e. primary clarifiers, vegetated zones, open water surface zones, and stabilization ponds. This paper discusses only the treatment performances among the CWs, but does not attempt to investigate the effects of changing configurations and/or functionalities of the unit cells in one CW. Fan et al. (2009) and Ko et al. (2010, 2011) investigated two of the studied CWs (i.e. Site-2 and Site-3) in greater detail. The centralized wastewater treatment plant (Site-C) was constructed in 2003, and the designed capacity of the plant was 500,000 m3 /day. This wastewater flow was approximately 6 times the total flows of the studied CWs. The plant provided secondary treatment using ASPs, and the solids retention times (SRTs) of the WWTP was approximately 5–15 days. The mixed liquor suspended solids concentration (MLSS) was approximately 3000–3500 mg/L and the dissolved oxygen concentration (DO) was approximately 1.5–3.0 mg/L. The depth of the aeration tanks was approximately 10 m. The secondary processes of the WWTP provided nearly full nitrification and partial denitrification of the domestic wastewater. Nitrate removal was limited because of the relatively small anoxic zones in the aeration tanks. The HRT of the treatment processes is approximately 10.8 h, and the total area of the treatment plant is approximately 4.6 ha. All the major unit processes of the WWTP (i.e. the aeration tanks) are built underground for odor control and
Fig. 1. Tain-Shui river watershed and locations of the studied wastewater treatment facilities (symbols: 䊉 = constructed wetlands; and = centralized sewer treatment plant). The shaded areas show the service areas of each facility and the detailed background information of the facilities was presented in the text and Table 1.
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258
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Table 1 Designed conditions of the studied wastewater treatment systems. Sitea
Name
Year built
Number of cell
Flow rate (m3 /day)
Hydraulic retention time (day)
Total area (ha)
Water surface area (ha)
Sewer service area (ha)
Population Served (P.E.)
1
Hua-Jiang
2008
5
9000
4.70
13.0
9.2
196
34,655
2 3 4
Hsin-Hai Bridge (I) Hsin-Hai Bridge (II) Hsin-Hai Bridge (III)
2004 2006 2009
4 4 3
6000 4000 5000
5.69 4.61 5.39
10.9 4.9 6.5
7.2 3.4 4.8
415
19,920
5
Fu-Zhou
2010
5
4.96
80.0
28.1
6 7
Da-Niao-Pi Chen-Lin
2006 2009
4 6
11,000 16,500
3.22 4.17
13.1 26.5
8.5 19.3
Di-Hua
2003
–
500,000
0.45
7.8
C a
30,000
–
2,720 5.1 12,240
355,660 60,700 ∼1,600,000
Note: Site-1 through Site-7 are constructed wetlands, and Site-C is the centralized wastewater treatment plant.
to reduce the footprint. The ground level of the WWTP consists of the administrative buildings and a few public service facilities (i.e. sporting fields, parking lots, and a swimming pool) to serve the local communities. 2.2. Water quality measurement and analysis The influent and effluent water samples were collected once per month for all the studied sites. Since 2008, at least 8–36 samples were collected from each sampling site and the monitoring periods varied with the dates of complete construction; a total of 24 samples were collected from the WWTP. The concentrations of TSS, BOD, and ammonia (NH4 –N) were measured for all samples, according to the American Public Health Association (APHA) standard methods (APHA, 2005). Total nitrogen (TN) and total phosphorous (TP) were measured for the water samples collected from Site-2, Site-3, and Site-6 by using the same standard methods (APHA, 2005). The reaction kinetics of the pollutants was calculated based upon the previous studies of Kadlec (1997, 2009). The area-based reaction coefficients (k, m/yr) were calculated by using a first-order steady state tank-in-series (TIS) model. Kadlec (2009) suggested that this model is more accurate than the conventional plug-flow model to simulating CWs with multiple cells. The relationships between k and measurement data are expressed as follows: 1 C − C∗ = P Ci − C ∗ (1 + k/Pq)
(1)
where C is the effluent concentration of pollutant (mg/L); C* is the background concentration of pollutant (mg/L); and Ci is the influent concentration of pollutant (mg/L); P is the number of reactors (cells) in series; q is the hydraulic loading rate (m/yr), which is calculated as: q = 365 ×
Qm A
(2)
where Qm is the measured flow rate (m3 /day); and A is the area of water surface (m2 ); To calculate k Eq. (1) were modified as: k = Pq
C − C ∗ −1/P Ci − C ∗
−1
kv20 = k20 ×
1 Qd = k20 × D A
(5)
where D is the average water depth of the wetland basin (m); is the designed hydraulic retention time (day); Qd is the designed wastewater flow rate (m3 /day). Several empirical constant parameters were used to calculate k20 : the background concentrations C* of TSS, BOD, and NH4 –N were assumed to be 5 mg/L, 5 mg/L, and 0 mg/L, respectively; and the temperature coefficients were 1.000, 0.985, 1.049 for the three pollutants, respectively. The BOD target was defined to be secondary (Kadlec, 2009), and the uses of C* were introduced to account for the residual of pollutants which directly bypass the CWs (Kadlec and Knight, 1996; Kadlec, 1997). Kadlec (2000) discussed the untreated “short-circuiting” flow and its effects to C* in greater detail. 2.3. Cost analysis To obtain the total costs of the studied sites, all the capital costs were converted to present values and break into the 20-year annuity (w) by the following equation: w=
20P(1 + r)r 20(1 + r) − 1
(6)
where P is the present value of the capital costs; and r is the interest rate. The present values were calculated based upon the historical inflation rates published by National Statistics, Taiwan, Republic of China (http://eng.stat.gov.tw/), and the interest rate was assumed to be 0.05. The total costs were the summation of the annuity of the costs and the O&M costs. The cost analysis did not include the land values since all the riparian wetlands were public properties owned by the metropolitan Taipei. 3. Results and discussion
(3)
The reaction kinetics was converted to the standardized condition at 20 ◦ C (k20 , m/yr) using the following equation: k20 = k 20−T
To compare the treatment performance of the CWs and the centralized WWTP, the “per-area-based” reaction coefficient k20 was converted to the “per-volume-based” coefficient (kv20 , 1/yr) by dividing k20 to the average depth of the wetlands. The average depth was calculated by the following function of hydraulic retention time, designed flow, and water surface area:
(4)
where is the temperature coefficient of the pollutants; T is the measured water temperature.
3.1. Water quality analysis The average flow rates, water temperatures, DOs, and the water quality data are shown in Table 2. The flow rates of the CWs and WWTP are controlled by influent pumps and are relatively constant throughout the monitoring periods (the ranges of the flow rates were not shown in this study). The numbers following the average values are the measured ranges of the experiment results
n/a 0.7 ± 0.6 0.9 ± 0.7 n/a n/a 0.3 ± 0.3 n/a 1.0 ± 0.7 n/a 1.7 ± 0.9 1.8 ± 0.9 n/a n/a 1.3 ± 0.9 n/a 3.1 ± 1.3
In Out In
n/a 10.2 ± 9.4 9.5 ± 8.9 n/a n/a 4.1 ± 3.3 n/a 3.3 ± 1.5 1.7 6.9 7.5 3.0 0.2 1.7 2.6 2.7 13.1 7.7 7.2 4.2 10.9 6.0 3.5 5.3 ± ± ± ± ± ± ± ± 17.3 17.0 16.9 10.9 14.9 9.7 5.6 14.9
In In In
14.3 15.6 8.1 16.1 6.6 10.0 11.5 9.2 23.8 23.8 18.5 9.5 27.8 29.2 14.8 11.7 3.8 ± 1.7 5.5 ± 5.3 4.1 ± 2.7 2.4 1.6 7.2 ± 3.3 5.6 ± 2.3 2.2 ± 0.7 25.6 ± 5.3 27.3 ± 3.6 26.5 ± 3.9 >25 >25 27.2 ± 3.9 >25 24.6 ± 2.6 4300 5730 4010 4500 3300 11,000 7000 417,000 1 2 3 4 5 6 7 C
DO (mg/L) Flow rate (m3 /day) Site
Temperature (◦ C)
65.4 120.8 159.6 113.6 96.9 150.4 31.7 104.0
± ± ± ± ± ± ± ±
46.7 107.8 148.9 81.6 72.1 142.1 20.5 45.2
Out
± ± ± ± ± ± ± ±
19.1 22.2 16.4 6.7 10.0 27.8 7.2 5.7
39.9 38.7 38.5 40.1 41.0 24.6 30.9 95.8
± ± ± ± ± ± ± ±
BOD (mg/L) TSS (mg/L)
24.9 26.7 25.5 19.4 36.5 19.4 17.0 44.5
Out
± ± ± ± ± ± ± ±
7.1 13.0 5.9 8.2 1.6 8.1 8.4 3.4
NH4 –N (mg/L)
Out
± ± ± ± ± ± ± ±
1.7 6.8 7.5 2.6 0.2 1.7 1.6 1.8
TN (mg/L)
n/a 19.9 ± 7.8 20.5 ± 9.5 n/a n/a 13.9 ± 6.1 n/a 23.2 ± 6.7
TP (mg/L)
Out
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 Table 2 Experiment results of water quality analysis of the wastewater treatment facilities. The first number in each column shows the average value and the second number is the range of the measurements. (Abbreviations and nomenclatures: In = influent concentrations; Out = effluent concentrations; DO = dissolved oxygen concentration; TSS = total suspended solids; BOD = biochemical oxygen demand; TN = total nitrogen; and TP = total phosphorous.).
252
(i.e. minimum to maximum). The water temperature of the CWs ranges from 19.8 to 32.9 ◦ C, and the average is 26.7 ◦ C; DOs vary dramatically and are between 0.1 and 10.1 mg/L. The average influent TSSs and BODs of all studied CWs and WWTP were lower than the typical low strength wastewaters (i.e. 390 mg/L TSS, and 110 mg/L BOD, respectively; Metcalf and Eddy Inc., 2003). The sewer lines in the metropolitan Taipei were sometimes installed downstream to septic tanks that were originally constructed with the buildings. Effluent TSSs and BODs of the septic tanks, or the influent concentrations of the wetlands, were therefore lower than the typical average wastewaters after primary treatments. NH4 –N, however, was within the ranges of typical low to medium strength domestic wastewaters because most septic tanks were anaerobic and cannot remove NH4 –N. The influent BODs of the WWTP were higher than the CWs because the WWTP received pure domestic wastewater but the influents of the CWs were composed of domestic wastewater, stormwater runoffs, and some groundwater. The higher variations of the influent concentrations for all pollutants in the CWs than the WWTP were most likely due to the influences of the storm water combined sewer and mixing of groundwater. The effluent concentrations of TSS, BOD, NH4 -H, and the available TN and TP data were also presented in Table 2. Nutrient removal is recommended but currently not regulated in the metropolitan Taipei. TN and TP were only measured for research purpose in the selected CWs (i.e. Site-2, Site-3, and Site-6). Nitrate concentrations were calculated by subtracting NH4 –N from TN, and the average nitrate–nitrogen retention were 89.3, 146.0, 107.9 gN m−2 yr−1 for Site-2, Site-3, and Site-6, respectively. Mitsch et al. (2005) measured the water quality data from 12 wetlands or 50 wetland-years and showed that the nitrate–nitrogen retention rate of the wetlands in the Mississippi River basin was approximately 29 g-Nm−2 yr−1 . The higher nitrate–nitrogen retention in the studied CWs of the metropolitan Taipei than the Mississippi River basin is possibly due to higher water temperature and shorter operation time after construction. U.S. EPA, 2000 suggested that removal performances of nutrients can be over-stated when the CWs are newly constructed. Longer monitoring time and more data from different sites may be required to clarify this issue. Effluent TSSs, BODs, and NH4 –N of the CWs increased with the influent loadings of the pollutants, and the relationships were presented in Fig. 2(a) through Fig. 2(c). The measurement results of the CWs were plotted in a similar fashion as the reference (Kadlec, 2009), and the dashed lines show the empirical data obtained from the same reference for comparison (not the regressions of the measurements). The effluent concentrations generally confirmed with the empirical model for the studied contaminants, but the treatment performances are different among the studied CWs. Effluent TSSs were higher than the reference values for almost all the studied CWs except for Site-4. This consistently higher TSS confirmed with previous observations in the receiving waters (Cheng et al., 2011), and may be due to the disturbances of extreme weather conditions during the testing period; i.e. typhoons, floods, and sandstorms. A long term investigation is needed and can be useful to identify the factors to causing this higher effluent TSS, but it is beyond the scope of this study to discuss this effect in greater detail. For most of the studying period the effluent TSSs were lower than 30 mg/L and met the water quality standards. Fig. 2(b) shows that the average BOD removal of the CWs agrees with the empirical data, suggesting that the excess TSSs are mostly composed of inorganic compounds and did not contribute to BODs. Effluent BODs of four CWs were lower than the empirical values at Site-2, Site-3, Site-4, and Site-6, implying good removal performances of the treatment systems. Effluent BODs of Site-1, Site-5, and Site-7; and NH4 –N of Site-7, however, were higher than
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 100
100
(b) BOD
(a) TSS
(c) NH 4-N
10
6 3
1
7
Effluent BOD (mg/L)
2
4
10
2 4
1 7 6
10
2
Effluent NH 4 -N (mg/L)
5
Effluent TSS (mg/L)
253
3 5
4
7
1 6
1
5
0.1
1
1 0.1
1
10
Kadlec, 2009 This Study
Kadlec, 2009 This Study
Kadlec, 2009 This Study
1
100
3
10
10
100
100
1000
Inlet TKN loading (g/m2/yr)
Inlet BOD loading (kg/ha/day)
Inlet TSS loading (g/m2/day)
Fig. 2. Treatment performances of the constructed wetlands. Notice that different units are used for the inlet pollutant loads (mass/area/time). The models and configurations of the figures are based upon Kadlec (2009).
from literature (Kadlec and Knight, 1996; Kadlec, 2009) for comparison. The empirical reaction coefficients k20 of TSS, BOD, and NH4 –N were 1000 m/yr, 41 m/yr, and 14.7 m/yr, respectively. Reaction coefficients of TSS are lower than the reference values for all the studied wetlands, and the results of BOD and NH4 –N generally confirmed with the references. The average reaction coefficients of
1
10000
(1/yr)
the reference values. A possible explanation of the higher effluent BODs and NH4 –N is the short operation time of the CWs. Site-1, Site-5, and Site-7 are relatively newer CWs which have only been operated for 3, 1, and 2 years prior to this study, respectively; and the influent flow rates of these sites were all lower than the designed flow rates. The designed and measured flow rates of the studied CWs were shown in Fig. 3. Site-5 was designed to treat 30,000 m3 /day but only received approximately 3300 m3 /day; the wastewater flows accounted to only 11% of the designed capacity. Sites-3 and Site-6 both performed good removal for BODs and NH4 –N; and have received reasonable wastewater flow close to designed capacities for more than 5 years.
3
4
5
6
7
C
3
4
5
6
7
C
(a) TSS
1000 100 10 1 0.1 10000
kVBOD (1/yr)
The percent removal, area-based reaction coefficients (k20 ), and the volume-based reaction coefficients (kv20 ) of TSS, BOD, and NH4 –N are presented in Table 3. No significant relationships were observed between removal performances and reaction kinetics, but the pattern of the reaction kinetics in the CWs was consistent for the three pollutants. For example, Site-3, Site-4, and Site-6 have higher reaction rates for all three pollutants; Site-5 and Site-7 have the lowest reaction rates among the CWs for all pollutants. The results were plotted with the references data in Fig. 4(a) through Fig. 4(c) for TSS, BOD, and NH4 –N, respectively. The error bars in Fig. 4 represent the standard deviations of the samples, and the solid lines across Site-1 through Site-7 show the reference values obtained
k
V TSS
3.2. Reaction kinetics
2
(b) BOD
1000 100 10 1 0.1
35000
Designed Flow
10000
Treated Flow
kVNH4 (1/yr)
Flow Rate (m3/day)
30000 25000 20000 15000
(c) NH4-N
1000 100 10 1
10000
0.1
5000
1 0
2
Wastewater Treatment Facility 1
2
3
4
5
6
7
Site Fig. 3. Comparison of designed and measured flow rates of the studied constructed wetlands. The recent flow rates of Site-1, Site-5, Site-6, and Site-7 are lower than the designed flows.
Fig. 4. Reaction kinetics of (a) TSS; (b) BOD; and (c) NH4 –N of the constructed wetlands (Site-1 through Site-7) and the wastewater treatment plant (Site-C). The symbols show the average reaction coefficients kv20 and the error bars show the standard deviation of measurements. The solid lines across Site-1 to Site-7 show the references kv20 .
254
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258
Table 3 Percent removals and the first-order reaction kinetics of three contaminants in the studied wastewater treatment systems (Note: k20 = standardized 20 ◦ C area-based reaction kinetics; and kv20 = standardized 20 ◦ C volumetric-based reaction kinetics). Site
TSS
1 2 3 4 5 6 7 C
BOD
NH4 –N
Removal (%)
k20 (m/yr)
kv20 (1/yr)
Removal (%)
k20 (m/yr)
kv20 (1/yr)
Removal (%)
k20 (m/yr)
kv20 (1/yr)
68.1 63.5 65.7 87.4 59.1 74.4 34.9 87.8
81.7 79.2 121.4 175.0 6.4 135.9 17.9 –
177.7 166.5 221.8 311.7 12.2 326.0 50.1 7481
59.2 63.2 75.4 57.5 84.3 56.6 58.4 89.1
30.1 59.7 180.1 41.2 17.2 121.7 3.5 –
65.4 125.5 329.1 73.3 32.6 292.0 9.7 9052
88.6 62.7 54.6 79.6 98.7 83.9 57.5 82.2
42.2 55.7 58.6 259.8 101.7 169.1 22.1 –
86.9 122.2 59.9 462.7 192.2 405.7 62.0 4150
BOD removal of Site-1, Site-5, and Site-7 are slightly lower than the references, which confirmed with the higher effluent BODs in Fig. 2. All the studied CWs performed great nitrogen removal during the testing period. The average reaction coefficients of NH4 –N are higher than references for all CWs, but in some occasions the reaction kinetics did not meet the model values. Site-4 shows the highest nitrogen removal rate among all the studied CWs, and Site-2, Site-5, and Site-6 also perform high efficiencies of ammonia removal at all time. The results of TN and TP from the limited sources (Site-2, Site-3, and Site-6) are very similar to the results of NH4 –N removal (results not shown), whereas longer time of operation and more measurements are required to obtain a more conclusive assay of the fate of nutrients in the CWs. The reaction kinetics kv20 of the WWTP was also presented in Fig. 4 and Table 3. The kv20 was calculated by assuming that the WWTP is a steady state single batch reactor. The number provided a basic idea of the treatment efficiency for comparison even though this impression oversimplified the ASP model, in which Monod kinetics (Metcalf and Eddy, 2003) or other complex structural model was used (Leu et al., 2010). The reaction kinetics of the WWTP for all studied contaminants was one or two orders of magnitude higher than the CWs. The centralized WWTP performed greater wastes degrading capability than the CWs in a small footprint.
the operation of each CW under modified loadings. As discussed previously, the newly constructed CWs (i.e. Site-1, Site-5, Site-6, and especially Site-7) were not receiving enough flows to match the designed capacities, and the total treated flow of all CWs was only 45% of the designed flow. Since the CWs were designed to share the wastewater loadings, it was expected to increase the wastewater flow to the designed capacities. The following attempt and discussion applied the empirical models and calculated reaction kinetics to predict the possible performances of the CWs. Kadlec (2000) showed that the reaction coefficient is linearly correlated with the hydraulic loading rates of the CWs. This relationship was applied in this study to predict the potential treatment performances of the studied CWs. The relationships between HRL and the reaction kinetics (k20 ) of TSS, BOD, and NH4 –N of our study were shown in Fig. 5(a) through Fig. 5(c), respectively. The dashed lines in Fig. 5 showed the linear regressions of the measurement results. The functions of each pollutant (using linear fits regression) as well as the coefficients of determinations (R2 ) were provided in the figures. Among the three pollutants, TSS and BOD showed stronger relationships than NH4 –N: the R2 of TSS and BOD were 0.73 and 0.67, respectively; and the R2 was close to zero for NH4 –N. Unlike TSS and BOD, NH4 –N is a nutrient of the aquatic microorganisms and vegetations of the CWs, therefore the degradation rate of NH4 –N are not a constant and unpredictable. The relationships between k20 and HRL were used to calculate the treatment performances of the studied CWs at increased flows. New k20 were calculated for TSS and BOD based upon the linear regressions with the increasing HRL, and a constant k20 was used for NH4 –N. The new reaction kinetics was used in Eq. (3) to calculate the effluent concentrations and the treatment loads, and the simulation results were shown in Table 4. The two scenarios
3.3. Recommendations to future operation The validated model parameters were used to simulate the impacts of different management scenarios and predict the potentials of the CWs. A sensitivity analysis was performed to optimize
200
200
b) k
(a)
.67
(c)
3.664X HLR; R 0 Average 101.3
120
80
40
k NH -N (m/yr)
160
k BOD (m/yr)
k TSS (m/yr)
160
300 3.791X HLR-29.884; R
120
80
200
100
40
0
0 0
10
20
30
40
Hydraulic Loading Rate (m/yr)
50
0
10
20
30
40
Hydraulic Loading Rate (m/yr)
50
0
10
20
30
40
Hydraulic Loading Rate (m/yr)
Fig. 5. Relationship between reaction kinetics and hydraulic loading rates of three pollutants: (a) TSS; (b) BOD; and (c) NH4 –N.
50
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 Table 4 Simulated treatment performances with increased loads. Parameter (Unit)
Reaction kinetics (k20 , m/yr) TSS BOD NH4 –N
Change (%)
Current
Future
16.6 36,600 10.2
37.0 81,500 4.6
89.6 65.6 101.3
135.1 74.9 101.3
+50.7 +14.2 0
8.1 11.9 3.4
13.4 11.0 1.7
+65.4 −7.5 −50.4
Effluent concentration (mg/L) TSS BOD NH4 –N Total removal (kg/day) TSS BOD NH4 –N
3.4. Monetary costs and environmental values of the constructed wetlands
Scenario
HLR (m/yr) Flow (m3 /day) HRT (day)
2672 836 330
255
8809 1997 974
+123 +123 −55
+230 +140 +195
compared in Table 4 are the current and future operations of the studied CWs with increased flow rates. The influent concentrations of the future operation were averages of the influents from the stabilized Sites (i.e. Site-2, Site-3, Site-4, and Site-6). The average HLR increased significantly by 123% and HRT decreased from 10.2 days to 4.6 days. With the increased flows and reaction kinetics, the total removal of the three pollutants increased dramatically by 230%, 140%, and 195% for TSS, BOD, and NH4 –N, respectively, even though the effluent concentrations did not change significantly (i.e. from 8.1 to 13.4 for TSS; 11.9 to 11.0 for BOD; and 3.4 to 1.7 for NH4 –N). The simulation strategy and model parameters used in the previous section were based only on the experimental results of this study. The increased flow rates and HLRs of the designed scenarios are the upper limit of the observed values. Since our results fit fairly well with the empirical results of Kadlec (2009) (Fig. 2), who tested many CWs that received at much higher maximum pollutant loadings (approximately 50–100 times for TSSs and BODs), our model may be further extended to simulate the performances of the studied CWs under over-loaded conditions, i.e. during flooding seasons or typhoons. In addition, the model can be further verified and applied with the water quality management plans. Based on the current plan, the total influent wastewater flows of the CWs will gradually increase with the increasing sewer basins coverage, and then decrease dramatically when the new centralized wastewater treatment plants start working online.
The compositions of the O&M costs for the studied CWs and WWTP were shown in Fig. 6(a and b), respectively. The larger pie in Fig. 6(b) represents the higher O&M costs per volume wastewater treated for the WWTP (0.064 USD/m3 ) comparing to the CWs (0.033 USD/m3 ). The costs of electricity were significant for the two systems, approximately accounted to 27% of the total O&M costs. The costs of regular operation of instruments and reparation of the hardware, including all the consumables and labor costs, account to the majority part (49%) of O&M costs for the CWs. Sludge handling and re-vegetation account to 8 and 7% of O&M costs, respectively. The administration costs of the wetlands include the costs for water quality monitoring reports, presentation and training, ecological survey and investigation, and aerial photography. Chemicals used in the disinfection and clarification processes, and tap water are required only in the centralized WWTP, accounting to 14 and 12% of the O&M costs, respectively. The costs for wastes hauling (i.e. sludge disposal) account to 25% of the O&M costs in the centralized plant, which is considerably higher than the costs for sludge handling in CWs. The calculated costs of wastewater treatment using the CWs and WWTP were shown in Table 5. The costs were all converted to the 2011 U.S. dollar equivalents per unit volume/mass pollutant treated. The total costs of the present CWs were between 0.425 and 3.621 USD per kg total BOD treated, and the cost of the WWTP was 1.186 USD per total BOD treated. The results were plotted in Fig. 7 to visualize the differences between the current costs and predicted costs for all the wastewater treatment facilities. Fig. 7(a and b) shows the total costs per m3 wastewater treated for the present and future operations, respectively; and the capital costs and O&M costs were plotted in different colors. The current cost of Site-5 is much higher than the other CWs and the WWTP, because currently the influent flow rate of Site-5 is remarkably lower than the designed flow rates. If all the CWs are fully loaded, the total costs of the CWs can be reduced to approximately half of the costs of the WWTP. The costs of different CWs varied significantly: Site1, Site-3, and Site-4 cost approximately 1.5–2 times the cost of the other CWs. The costs were mostly site-specific and varied with the locations: some of the CWs were built on existing wetlands and the others were constructed on remote open water surfaces, which increase the costs of construction. Similar comparisons were also made in Fig. 7(c and d) for the total costs per kg total BOD show removed for present and future operations, respectively. The total BODs were calculated by BOD the oxygen demand of nitrification; and the oxygen demand of NH4 –N
Table 5 Cost analysis of the studied wastewater treatment systems. Site
Year built
1 2 3 4 5 6 7 C
2008 2004 2006 2009 2010 2006 2009 2003
a b c
Capital cost (USDa )
Capital cost – 20 years annuity (USD/yr)
Previous value
Present value
1,986,000 434,000 689,000 1,061,000 4,037,000 898,000 2,465,000 216,667,000
1,957,000 392,000 644,000 1,036,000 3,984,000 839,000 2,303,000 192,429,000
Currency presented in U.S. dollars. Operation and maintenances. Including the summation of the capital costs and O&M costs.
157,100 31,400 51,600 83,100 319,700 67,300 184,800 15,441,000
Present O&Mb cost (USD/yr)
56,400 27,900 54,000 70,700 111,300 98,700 62,700 808,900
Present total costc (USD/yr)
Cost per volume sewer treated (USD/m3 )
Cost per mass BOD removed (USD/kg)
213,500 59,300 105,600 153,800 431,000 166,000 247,500 16,249,900
0.136 0.028 0.072 0.093 0.355 0.059 0.097 0.165
1.465 0.425 1.015 1.598 3.621 1.197 2.991 1.186
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C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258
a
0.40
O&M Capital
Cost (USD/m3)
0.35 0.30 0.25 0.20 0.15 0.10
b
0.16
Cost (USD/m3)
Fig. 6. Composition of the operation and maintenance costs: (a) constructed wetlands and (b) centralized wastewater treatment plant.
0.12
O&M Capital
0.14 0.10 0.08 0.06 0.04 0.02
0.05
0.00
0.00 1
2
3
4
5
6
7
1
C
2
3
Site
d
4.0
O&M Capital
3.5
5
6
7
C
5
6
7
C
Site
Cost (USD/kg total BOD)
Cost (USD/kg total BOD)
c
4
3.0 2.5 2.0 1.5 1.0 0.5 0.0
1.6
O&M Capital
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
1
2
3
4
Site
5
6
7
C
1
2
3
4
Site
Fig. 7. (a) Current; and (b) predicted total costs per volume wastewater treated of the constructed wetlands and the wastewater treatment plant; and (c) current; and (b) predicted total costs per kg total BOD removed for the wastewater treatment facilities. The total BODs were the summation of BODs and the oxygen demand of nitrification (assume mass ratio of O2 :NH4 –N is 4.33:1).
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 Table 6 Water environment education activities and visitor count of the studied constructed wetlands in 2009. Activity Schools field trips – total 157 classes Local tours for water environment education Training of wetland narrators and volunteers Wetland photo contests – 3 times Concerts at the wetlands Wetland movie theater Total visitor count
Visitor count 17,770 15,000 37,000 1000 200 200 71,170
257
of the CWs were between 0.425 and 3.621 USD per kg total BOD removed, and the costs of the centralized wastewater treatment plant was approximately 1.186 USD per total BOD removed. The CWs also provided non-market benefits equivalent to approximately 130,000 USD in 2009. The CWs serve reasonably well as an integrating wastewater treatment system in addition to their environmental values while the centralized sewer system or other advanced systems are under construction.
Acknowledgments was assumed to be 4.33 kgO2 per kg NH4 –N. The total costs of centralized WWTP to treat the same amount of BOD and NH4 –N are comparable to the costs of the CWs, implying the great potential of using CWs to support a constructing overall wastewater treatment system, especially for nitrogen removal. In addition to the construction and operation costs, CWs obtain many additional benefits, i.e. the ecological, educational, and recreational functions (Marble, 1984) to the local communities. These “non-market” benefits, often related to consumer’s willingnessto-pay, are often evaluated by applying environmental survey with designed questionnaires (Yang et al., 2008), or by calculating the ecosystem services values of various indexes (Chen et al., 2009). This study provides an example to calculate the values of an additional benefit of the CWs. In-depth investigation of the environmental values, however, requires large amount of data and it is currently beyond the scope of this study to perform the analysis in greater details. Woodward and Wui (2001) showed that the economic values of wetland services can be calculated by four methods: (a) the net factor income (NFI) method; (b) the replacement cost; (c) travel cost; and (d) contingent valuation. The example provided here was based upon the travel cost approach, which quantified the consumer’s willingness-to-pay by calculating the total transportation fee spent to visit the target wetlands. Table 6 shows the documented activities and visitor count of the studied CWs during 2009. The listed activities are all reported by public sources, i.e. schools, the city, and environmental groups; personal activities from private organization or individual visitors were not reported. The reported total visitor count in 2009 was 71,170 P.E., and the real amount of visitors should be higher than this number. The average transportation fee to the CWs (Site-1) was 1.83 ± 0.91 USD per person (round trip) based upon the information provided by Taipei Metro (http://www.trtc.com.tw/). The approximate non-market benefits produced by the CWs in 2009 were therefore 130,000 USD. The environmental values provided by the CWs accounted to approximately 27% of the O&M costs in additional to the benefits of water quality improvement. 4. Conclusion This study investigated the environmental and economical impacts of seven riparian CWs and one centralized WWTP for municipal wastewater treatment in Tan-Shui River of the metropolitan Taipei. The treatment performances of the CWs confirmed with the references, but the variation among the CWs were significant. Influent flow rates were ones of the most essential operating parameters to affect the performance of the CWs. The treatment efficiency of the centralized WWTP was approximately one to two orders of magnitude better than the CWs, and the total costs per wastewater treated were approximately 2 times the costs of CWs. If the total costs were converted to USD per unit mass total oxygen demand (oxygen demand of BOD plus NH4 –N), the costs of WWTP were comparable to the CWs. The total costs
The authors thank C.-L. Yang (San Ying Enterprises Co.); J. Chen, Wu, S. (MWH Taiwan); and Apollo Technology Co., Ltd. for sampling, technical mapping, and water quality analysis.
References APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed. American Public Health Association, Washington, D.C. Brix, H., Schierup, H.H., Arias, C.A., 2007. Twenty years experience with constructed wetland systems in Denmark: what did we learn? Water Sci. Technol. 56, 63–68. Chen, T.Y., Kao, C.M., Yeh, T.Y., Chien, H.Y., Chao, A.C., 2006. Application of a constructed wetland for industrial wastewater treatment: a pilot-scale study. Chemosphere 64, 497–502. Chen, Z.M., Chen, G.Q., Chen, b., Zhou, J.B., Yang, Z.F., Zhou, Y., 2009. Net ecosystem services value of wetland: environmental economic account. Commun. Nonlinear Sci. Numer. Simul. 14, 2837–2843. Cheng, B.-Y., Liu, T.-C., Shyu, G.-S., Chang, T.-K., Fang, W.-T., 2011. Analysis of trends in water quality: constructed wetlands in metropolitan Taipei. Water Sci. Technol. 64, 2143–2150. Cronk, J.K., 1996. Constructed wetlands to treat wastewater from dairy and swine operations: a review. Ecosyst. Environ. 58, 97–114. Di Luca, G.A., Maine, M.A., Mrfarrege, M.M., Hadad, H.R., Sanchez, G.C., Bonetto, C.A., 2011. Metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment. Ecol. Eng. 37, 1267–1275. Fan, C.H., Chang, F.-C., Ko, C.-H., Sheu, Y.-S., Teng, C.-J., Chang, T.-C., 2009. Urban pollutant removal by a constructed riparian wetland before typhoon damage and after reconstruction. Ecol. Eng. 35, 424–435. Harrington, R., McInnes, R., 2009. Integrated constructed wetlands (ICW) for livestock wastewater management. Bioresour. Technol. 100, 5498–5505. Larue, C., Korboulewsky, N., Wang, R., Mévy, J.P., 2010. Depollution potential of three macrophytes: exudated, wall-bound and intracellular peroxidase activities plus intracellular phenol concentrations. Bioresour. Technol. 101, 7951–7957. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. CRC Press, Boca Raton, Florida, p. 893. Kadlec, R.H., 1997. Deterministic and stochastic aspects of constructed wetland performance and design. Water Sci. Technol. 35, 149–156. Kadlec, R.H., 2000. The inadequacy of first-order treatment wetland models. Ecol. Eng. 15, 105–119. Kadlec, R.H., 2009. Comparison of free water and horizontal subsurface treatment wetlands. Ecol. Eng. 35, 159–174. Kao, C.M., Wang, J.Y., Lee, H.Y., Wen, C.K., 2001. Application of a constructed wetland for non-point source pollution control. Water Sci. Technol. 44, 585–590. Knight, R.L., 1992. Ancillary benefits and potential problems with the use of wetlands for nonpoint source pollution control. Ecol. Eng. 1, 97–113. Ko, C.-H., Chang, F.-C., Lee, T.-M., Chen, P.-Y., Chen, H.-H., Hsieh, H.-L., Guan, C.-Y., 2010. Impact of flood damage on pollutant removal efficiencies of a subtropical urban constructed wetland. Sci. Total Environ. 408, 4328–4333. Ko, C.-H., Lee, T.-M., Chang, F.-C., Liao, S.-P., 2011. The correlations between system treatment efficiencies and aboveground emergent macrophyte nutrient removal for the Hsin-Hai Bridge phase II constructed wetland. Bioresour. Technol. 102, 5431–5437. Leu, S.-Y., Libra, J., Stenstrom, M.K., 2010. Monitoring off-gas O2 /CO2 to predict nitrification performance in activated sludge processes. Water Res. 44, 3434–3444. Lin, Y.-F., Jing, S.-R., Lee, D.-Y., Wang, T.-W., 2009. Nutrient removal from aquaculture wastewater using a constructed wetlands system. Aquaculture 209, 169–184. Maine, M.A., Sune, N., Hadad, H., Sanchez, G., Bonetto, C., 2007. Removal efficiency of a constructed wetland for wastewater treatment according to vegetation dominance. Chemosphere 68, 1105–1113. Marble, A.D., 1984. A method for assessing wetland characteristics and values. Landscape Plann. 11, 1–17. Metcalf and Eddy Inc., 2003. Wastewater Engineering: Treatment, Disposal, Reuse, 4th Ed. McGraw-Hill, New York, N.Y. Mitsch, W.J., John Jr., W.D., Li, Z., Robert, R.L., 2005. Nitrate–nitrogen retention by wetlands in the Mississippi River basin. Ecol. Eng. 24, 267–278. Öövel, M., Tooming, A., Mauring, T., Mander, Ü., 2007. Schoolhouse wastewater purification in a LWA-filled hybrid constructed wetland in Estonia. Ecol. Eng. 29, 17–26.
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Scholz, M., 2006. Wetland Systems to Control Urban Runoff. Elsevier, Amsterdam, p. 360. EPA, U.S., 2000. Manual for Constructed Wetlands Treatment of Municipal Wastewaters EPA/625/R-99/010. National Risk Management Research Laboratory, Office of Research and Development, Cincinnati, OH, p. 166. Vymazal, J., 2011. Plants used in constructed wetlands with horizontal subsurface flow: a review. Hydrobiologia 674, 133–156.
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Economic and environmental analysis of using constructed riparian wetlands to support urbanized municipal wastewater treatment Chia-Ji Teng a , Shao-Yuan Leu b,∗ , Chun-Han Ko c,∗∗ , Chihhao Fan d , Yiong-Shing Sheu e , Hui-Yu Hu f a
Department of Safety, Health and Environmental Engineering, Tungnan University, New Taipei City 22202, Taiwan Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA c School of Forest and Resources Conservation, National Taiwan University, Taipei 10617, Taiwan d Department of Safety, Health and Environmental Engineering, Mingchi University of Technology, New Taipei City 24301, Taiwan e Department of Water Quality Protection, Environmental Protection Administration, Executive Yuan, Taipei 10042, Taiwan f MWH Americas Inc. Taiwan Branch Office, Taipei 10695, Taiwan b
a r t i c l e
i n f o
Article history: Received 16 November 2011 Received in revised form 7 March 2012 Accepted 26 March 2012 Available online 2 May 2012 Keywords: Wetland Wastewater Economic Reaction kinetics Modeling
a b s t r a c t This paper evaluated the economic and environmental benefits of using riparian constructed wetlands (CWs) for municipal wastewater treatment in an urban watershed. The monitoring data were collected from seven CWs in Tan-Shui River basin of the metropolitan Taipei, and were compared with a centralized wastewater treatment plant in the same watershed. Operation parameters such as site conditions, effluent quality, construction and operation/maintenance (O&M) costs were analyzed and used to calculate the first order reaction kinetics (kv20 ) of three pollutants, i.e. TSS, BOD, and NH4 –N. The reaction kinetics and costs vary significantly among the CWs and treatment plant. The overall treatment performances of BOD and NH4 –N generally confirmed with the references, with variation only among the different sites, but the removal of TSS was less desirable for all CWs. The total costs of the CWs were between 0.425 and 3.621 USD per kg total BOD removed, and the costs of the centralized wastewater treatment plant was approximately 1.186 USD per kg total BOD removed. The wetlands provide reasonable pollutants removal and show additional benefits on education and recreation while the sanitary sewer system and new treatment plants are under construction. The experiment results confirmed with the references and the methodology can be used to developing water quality management plans for urbanized watersheds in subtropical areas. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Constructed wetlands (CWs) are an economic and ecological alternative for wastewater treatment. This on-site wastewater treatment process has been used to treat different types of wastewaters, i.e. domestic wastewaters (Brix et al., 2007; Öövel et al., 2007), animals or agriculture wastewaters (Cronk, 1996; Harrington and McInnes, 2009), urban runoffs (Scholz, 2006; Kao et al., 2001), and industrial wastewaters (Chen et al., 2006; Di Luca et al., 2011), and can effectively remove many contaminants, i.e. nutrients (Kadlec and Knight, 1996; Lin et al., 2009; Kadlec, 2009), organic chemicals (Larue et al., 2010), and metals (Scholz, 2006; Maine et al., 2007). In addition, some CWs are habitats of many
∗ Corresponding author. Tel.: +1 310 526 1534; fax: +1 310 206 2222. ∗∗ Corresponding author. Tel.: +886 2 3366 4615; fax: +886 2 2365 4520. E-mail addresses: [email protected] (S.-Y. Leu), [email protected] (C.-H. Ko). 0925-8574/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2012.03.009
wild lives and can provide additional benefits on research, restoration, and recreation (Knight, 1992; Zedler and Leach, 1998). U.S. EPA (2000) suggests that CWs are more suitable than activated sludge processes (ASPs) for wastewater treatment in small communities with wastewater flow less than 3800 m3 /day and when the land is available. The key challenges of the CWs are the treatment efficiencies and operation stability. The treatment performances of the CWs are affected by many factors, i.e. hydraulic loading rates (HRL) and ambient temperature (Kadlec and Knight, 1996), vegetation species and properties (Vymazal, 2011), and weather or damages caused by nature disasters (Fan et al., 2009; Ko et al., 2010). It is essential to obtain complete understandings of the benefits and limitations of the CWs before incorporating these processes in a municipal wastewater treatment system. The goals of this paper were to develop a methodology to quantify the economic and environmental benefits of the CWs for municipal wastewater treatment. The experimental results were
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collected from seven free water surface (FWS) wetlands and a centralized wastewater treatment plant (WWTP) in the Tan-Shui River basin of the metropolitan Taipei. In Taiwan, sanitary sewage systems are still in developing and riparian wetlands have been used as a mitigation plan to reduce pollution and remediate the polluted rivers (Kao et al., 2001). Since 2004, 14 riparian CWs have been constructed in the Tan-Shui River basin and the associated tributaries. The wetlands served well for the purposes of wastewater purification, urban recreation, environmental education, and nature habitats restoration. The remediation program is successful and improvements in water quality have been documented in the receiving waters (Cheng et al., 2011). The monitoring results of the wetlands and the centralized WWTP provide an excellent opportunity for comparison and future planning. In this study, operation conditions and water quality data were analyzed and the first-order reaction kinetics (k value) of different contaminants was calculated. Capital costs and the operation and maintenance (O&M) costs were recorded to quantify the 20-year annuity of each wastewater treatment facility. Recommendations were provided to the future development of wastewater treatment programs. The methodology and case examples can serve as a reference to developing wastewater management plan in watersheds with similar background. 2. Materials and methods 2.1. Sites background The locations of the studied wetlands and WWTP are shown in Fig. 1. The studied CWs (Site-1 to Site-7) are all located upstream to the centralized WWTP (Site-C) along the Tan-Shui River. The shaded areas in Fig. 1 show the served sewage basins of the studied wastewater treatment facilities. The current total wastewater flow in Taipei City is approximately 1,400,000 m3 /day. The area served by the WWTP covers more than 30% of the sewer lines in Taipei city, and is approximately 3 times the area of all the CWs in the
New Taipei City. In addition, Site-2 to Site-4 and Site-6 to Site-7 received wastewaters from the same sewage basins. The designed conditions of the studied wastewater treatment facilities were presented in Table 1. The CWs were all FWS wetlands constructed between 2004 and 2010. The designed treatment capacities of the CWs are between 4000 and 30,000 m3 /day, and the total capacity of the CWs is 81,500 m3 /day. The designed hydraulic retention times (HRTs) of the CWs are between 3.22 and 5.69 days, and the areas of the wetlands are between 4.9 and 80 ha. The water surface areas are approximately between 3.4 and 19.3 ha. The areas of sewage basins and populations served were also shown in Table 1 for the wastewater treatment facilities. The CWs are all composed of 3–5 cells, and the cells are classified based upon the functionalities, i.e. primary clarifiers, vegetated zones, open water surface zones, and stabilization ponds. This paper discusses only the treatment performances among the CWs, but does not attempt to investigate the effects of changing configurations and/or functionalities of the unit cells in one CW. Fan et al. (2009) and Ko et al. (2010, 2011) investigated two of the studied CWs (i.e. Site-2 and Site-3) in greater detail. The centralized wastewater treatment plant (Site-C) was constructed in 2003, and the designed capacity of the plant was 500,000 m3 /day. This wastewater flow was approximately 6 times the total flows of the studied CWs. The plant provided secondary treatment using ASPs, and the solids retention times (SRTs) of the WWTP was approximately 5–15 days. The mixed liquor suspended solids concentration (MLSS) was approximately 3000–3500 mg/L and the dissolved oxygen concentration (DO) was approximately 1.5–3.0 mg/L. The depth of the aeration tanks was approximately 10 m. The secondary processes of the WWTP provided nearly full nitrification and partial denitrification of the domestic wastewater. Nitrate removal was limited because of the relatively small anoxic zones in the aeration tanks. The HRT of the treatment processes is approximately 10.8 h, and the total area of the treatment plant is approximately 4.6 ha. All the major unit processes of the WWTP (i.e. the aeration tanks) are built underground for odor control and
Fig. 1. Tain-Shui river watershed and locations of the studied wastewater treatment facilities (symbols: 䊉 = constructed wetlands; and = centralized sewer treatment plant). The shaded areas show the service areas of each facility and the detailed background information of the facilities was presented in the text and Table 1.
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258
251
Table 1 Designed conditions of the studied wastewater treatment systems. Sitea
Name
Year built
Number of cell
Flow rate (m3 /day)
Hydraulic retention time (day)
Total area (ha)
Water surface area (ha)
Sewer service area (ha)
Population Served (P.E.)
1
Hua-Jiang
2008
5
9000
4.70
13.0
9.2
196
34,655
2 3 4
Hsin-Hai Bridge (I) Hsin-Hai Bridge (II) Hsin-Hai Bridge (III)
2004 2006 2009
4 4 3
6000 4000 5000
5.69 4.61 5.39
10.9 4.9 6.5
7.2 3.4 4.8
415
19,920
5
Fu-Zhou
2010
5
4.96
80.0
28.1
6 7
Da-Niao-Pi Chen-Lin
2006 2009
4 6
11,000 16,500
3.22 4.17
13.1 26.5
8.5 19.3
Di-Hua
2003
–
500,000
0.45
7.8
C a
30,000
–
2,720 5.1 12,240
355,660 60,700 ∼1,600,000
Note: Site-1 through Site-7 are constructed wetlands, and Site-C is the centralized wastewater treatment plant.
to reduce the footprint. The ground level of the WWTP consists of the administrative buildings and a few public service facilities (i.e. sporting fields, parking lots, and a swimming pool) to serve the local communities. 2.2. Water quality measurement and analysis The influent and effluent water samples were collected once per month for all the studied sites. Since 2008, at least 8–36 samples were collected from each sampling site and the monitoring periods varied with the dates of complete construction; a total of 24 samples were collected from the WWTP. The concentrations of TSS, BOD, and ammonia (NH4 –N) were measured for all samples, according to the American Public Health Association (APHA) standard methods (APHA, 2005). Total nitrogen (TN) and total phosphorous (TP) were measured for the water samples collected from Site-2, Site-3, and Site-6 by using the same standard methods (APHA, 2005). The reaction kinetics of the pollutants was calculated based upon the previous studies of Kadlec (1997, 2009). The area-based reaction coefficients (k, m/yr) were calculated by using a first-order steady state tank-in-series (TIS) model. Kadlec (2009) suggested that this model is more accurate than the conventional plug-flow model to simulating CWs with multiple cells. The relationships between k and measurement data are expressed as follows: 1 C − C∗ = P Ci − C ∗ (1 + k/Pq)
(1)
where C is the effluent concentration of pollutant (mg/L); C* is the background concentration of pollutant (mg/L); and Ci is the influent concentration of pollutant (mg/L); P is the number of reactors (cells) in series; q is the hydraulic loading rate (m/yr), which is calculated as: q = 365 ×
Qm A
(2)
where Qm is the measured flow rate (m3 /day); and A is the area of water surface (m2 ); To calculate k Eq. (1) were modified as: k = Pq
C − C ∗ −1/P Ci − C ∗
−1
kv20 = k20 ×
1 Qd = k20 × D A
(5)
where D is the average water depth of the wetland basin (m); is the designed hydraulic retention time (day); Qd is the designed wastewater flow rate (m3 /day). Several empirical constant parameters were used to calculate k20 : the background concentrations C* of TSS, BOD, and NH4 –N were assumed to be 5 mg/L, 5 mg/L, and 0 mg/L, respectively; and the temperature coefficients were 1.000, 0.985, 1.049 for the three pollutants, respectively. The BOD target was defined to be secondary (Kadlec, 2009), and the uses of C* were introduced to account for the residual of pollutants which directly bypass the CWs (Kadlec and Knight, 1996; Kadlec, 1997). Kadlec (2000) discussed the untreated “short-circuiting” flow and its effects to C* in greater detail. 2.3. Cost analysis To obtain the total costs of the studied sites, all the capital costs were converted to present values and break into the 20-year annuity (w) by the following equation: w=
20P(1 + r)r 20(1 + r) − 1
(6)
where P is the present value of the capital costs; and r is the interest rate. The present values were calculated based upon the historical inflation rates published by National Statistics, Taiwan, Republic of China (http://eng.stat.gov.tw/), and the interest rate was assumed to be 0.05. The total costs were the summation of the annuity of the costs and the O&M costs. The cost analysis did not include the land values since all the riparian wetlands were public properties owned by the metropolitan Taipei. 3. Results and discussion
(3)
The reaction kinetics was converted to the standardized condition at 20 ◦ C (k20 , m/yr) using the following equation: k20 = k 20−T
To compare the treatment performance of the CWs and the centralized WWTP, the “per-area-based” reaction coefficient k20 was converted to the “per-volume-based” coefficient (kv20 , 1/yr) by dividing k20 to the average depth of the wetlands. The average depth was calculated by the following function of hydraulic retention time, designed flow, and water surface area:
(4)
where is the temperature coefficient of the pollutants; T is the measured water temperature.
3.1. Water quality analysis The average flow rates, water temperatures, DOs, and the water quality data are shown in Table 2. The flow rates of the CWs and WWTP are controlled by influent pumps and are relatively constant throughout the monitoring periods (the ranges of the flow rates were not shown in this study). The numbers following the average values are the measured ranges of the experiment results
n/a 0.7 ± 0.6 0.9 ± 0.7 n/a n/a 0.3 ± 0.3 n/a 1.0 ± 0.7 n/a 1.7 ± 0.9 1.8 ± 0.9 n/a n/a 1.3 ± 0.9 n/a 3.1 ± 1.3
In Out In
n/a 10.2 ± 9.4 9.5 ± 8.9 n/a n/a 4.1 ± 3.3 n/a 3.3 ± 1.5 1.7 6.9 7.5 3.0 0.2 1.7 2.6 2.7 13.1 7.7 7.2 4.2 10.9 6.0 3.5 5.3 ± ± ± ± ± ± ± ± 17.3 17.0 16.9 10.9 14.9 9.7 5.6 14.9
In In In
14.3 15.6 8.1 16.1 6.6 10.0 11.5 9.2 23.8 23.8 18.5 9.5 27.8 29.2 14.8 11.7 3.8 ± 1.7 5.5 ± 5.3 4.1 ± 2.7 2.4 1.6 7.2 ± 3.3 5.6 ± 2.3 2.2 ± 0.7 25.6 ± 5.3 27.3 ± 3.6 26.5 ± 3.9 >25 >25 27.2 ± 3.9 >25 24.6 ± 2.6 4300 5730 4010 4500 3300 11,000 7000 417,000 1 2 3 4 5 6 7 C
DO (mg/L) Flow rate (m3 /day) Site
Temperature (◦ C)
65.4 120.8 159.6 113.6 96.9 150.4 31.7 104.0
± ± ± ± ± ± ± ±
46.7 107.8 148.9 81.6 72.1 142.1 20.5 45.2
Out
± ± ± ± ± ± ± ±
19.1 22.2 16.4 6.7 10.0 27.8 7.2 5.7
39.9 38.7 38.5 40.1 41.0 24.6 30.9 95.8
± ± ± ± ± ± ± ±
BOD (mg/L) TSS (mg/L)
24.9 26.7 25.5 19.4 36.5 19.4 17.0 44.5
Out
± ± ± ± ± ± ± ±
7.1 13.0 5.9 8.2 1.6 8.1 8.4 3.4
NH4 –N (mg/L)
Out
± ± ± ± ± ± ± ±
1.7 6.8 7.5 2.6 0.2 1.7 1.6 1.8
TN (mg/L)
n/a 19.9 ± 7.8 20.5 ± 9.5 n/a n/a 13.9 ± 6.1 n/a 23.2 ± 6.7
TP (mg/L)
Out
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 Table 2 Experiment results of water quality analysis of the wastewater treatment facilities. The first number in each column shows the average value and the second number is the range of the measurements. (Abbreviations and nomenclatures: In = influent concentrations; Out = effluent concentrations; DO = dissolved oxygen concentration; TSS = total suspended solids; BOD = biochemical oxygen demand; TN = total nitrogen; and TP = total phosphorous.).
252
(i.e. minimum to maximum). The water temperature of the CWs ranges from 19.8 to 32.9 ◦ C, and the average is 26.7 ◦ C; DOs vary dramatically and are between 0.1 and 10.1 mg/L. The average influent TSSs and BODs of all studied CWs and WWTP were lower than the typical low strength wastewaters (i.e. 390 mg/L TSS, and 110 mg/L BOD, respectively; Metcalf and Eddy Inc., 2003). The sewer lines in the metropolitan Taipei were sometimes installed downstream to septic tanks that were originally constructed with the buildings. Effluent TSSs and BODs of the septic tanks, or the influent concentrations of the wetlands, were therefore lower than the typical average wastewaters after primary treatments. NH4 –N, however, was within the ranges of typical low to medium strength domestic wastewaters because most septic tanks were anaerobic and cannot remove NH4 –N. The influent BODs of the WWTP were higher than the CWs because the WWTP received pure domestic wastewater but the influents of the CWs were composed of domestic wastewater, stormwater runoffs, and some groundwater. The higher variations of the influent concentrations for all pollutants in the CWs than the WWTP were most likely due to the influences of the storm water combined sewer and mixing of groundwater. The effluent concentrations of TSS, BOD, NH4 -H, and the available TN and TP data were also presented in Table 2. Nutrient removal is recommended but currently not regulated in the metropolitan Taipei. TN and TP were only measured for research purpose in the selected CWs (i.e. Site-2, Site-3, and Site-6). Nitrate concentrations were calculated by subtracting NH4 –N from TN, and the average nitrate–nitrogen retention were 89.3, 146.0, 107.9 gN m−2 yr−1 for Site-2, Site-3, and Site-6, respectively. Mitsch et al. (2005) measured the water quality data from 12 wetlands or 50 wetland-years and showed that the nitrate–nitrogen retention rate of the wetlands in the Mississippi River basin was approximately 29 g-Nm−2 yr−1 . The higher nitrate–nitrogen retention in the studied CWs of the metropolitan Taipei than the Mississippi River basin is possibly due to higher water temperature and shorter operation time after construction. U.S. EPA, 2000 suggested that removal performances of nutrients can be over-stated when the CWs are newly constructed. Longer monitoring time and more data from different sites may be required to clarify this issue. Effluent TSSs, BODs, and NH4 –N of the CWs increased with the influent loadings of the pollutants, and the relationships were presented in Fig. 2(a) through Fig. 2(c). The measurement results of the CWs were plotted in a similar fashion as the reference (Kadlec, 2009), and the dashed lines show the empirical data obtained from the same reference for comparison (not the regressions of the measurements). The effluent concentrations generally confirmed with the empirical model for the studied contaminants, but the treatment performances are different among the studied CWs. Effluent TSSs were higher than the reference values for almost all the studied CWs except for Site-4. This consistently higher TSS confirmed with previous observations in the receiving waters (Cheng et al., 2011), and may be due to the disturbances of extreme weather conditions during the testing period; i.e. typhoons, floods, and sandstorms. A long term investigation is needed and can be useful to identify the factors to causing this higher effluent TSS, but it is beyond the scope of this study to discuss this effect in greater detail. For most of the studying period the effluent TSSs were lower than 30 mg/L and met the water quality standards. Fig. 2(b) shows that the average BOD removal of the CWs agrees with the empirical data, suggesting that the excess TSSs are mostly composed of inorganic compounds and did not contribute to BODs. Effluent BODs of four CWs were lower than the empirical values at Site-2, Site-3, Site-4, and Site-6, implying good removal performances of the treatment systems. Effluent BODs of Site-1, Site-5, and Site-7; and NH4 –N of Site-7, however, were higher than
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 100
100
(b) BOD
(a) TSS
(c) NH 4-N
10
6 3
1
7
Effluent BOD (mg/L)
2
4
10
2 4
1 7 6
10
2
Effluent NH 4 -N (mg/L)
5
Effluent TSS (mg/L)
253
3 5
4
7
1 6
1
5
0.1
1
1 0.1
1
10
Kadlec, 2009 This Study
Kadlec, 2009 This Study
Kadlec, 2009 This Study
1
100
3
10
10
100
100
1000
Inlet TKN loading (g/m2/yr)
Inlet BOD loading (kg/ha/day)
Inlet TSS loading (g/m2/day)
Fig. 2. Treatment performances of the constructed wetlands. Notice that different units are used for the inlet pollutant loads (mass/area/time). The models and configurations of the figures are based upon Kadlec (2009).
from literature (Kadlec and Knight, 1996; Kadlec, 2009) for comparison. The empirical reaction coefficients k20 of TSS, BOD, and NH4 –N were 1000 m/yr, 41 m/yr, and 14.7 m/yr, respectively. Reaction coefficients of TSS are lower than the reference values for all the studied wetlands, and the results of BOD and NH4 –N generally confirmed with the references. The average reaction coefficients of
1
10000
(1/yr)
the reference values. A possible explanation of the higher effluent BODs and NH4 –N is the short operation time of the CWs. Site-1, Site-5, and Site-7 are relatively newer CWs which have only been operated for 3, 1, and 2 years prior to this study, respectively; and the influent flow rates of these sites were all lower than the designed flow rates. The designed and measured flow rates of the studied CWs were shown in Fig. 3. Site-5 was designed to treat 30,000 m3 /day but only received approximately 3300 m3 /day; the wastewater flows accounted to only 11% of the designed capacity. Sites-3 and Site-6 both performed good removal for BODs and NH4 –N; and have received reasonable wastewater flow close to designed capacities for more than 5 years.
3
4
5
6
7
C
3
4
5
6
7
C
(a) TSS
1000 100 10 1 0.1 10000
kVBOD (1/yr)
The percent removal, area-based reaction coefficients (k20 ), and the volume-based reaction coefficients (kv20 ) of TSS, BOD, and NH4 –N are presented in Table 3. No significant relationships were observed between removal performances and reaction kinetics, but the pattern of the reaction kinetics in the CWs was consistent for the three pollutants. For example, Site-3, Site-4, and Site-6 have higher reaction rates for all three pollutants; Site-5 and Site-7 have the lowest reaction rates among the CWs for all pollutants. The results were plotted with the references data in Fig. 4(a) through Fig. 4(c) for TSS, BOD, and NH4 –N, respectively. The error bars in Fig. 4 represent the standard deviations of the samples, and the solid lines across Site-1 through Site-7 show the reference values obtained
k
V TSS
3.2. Reaction kinetics
2
(b) BOD
1000 100 10 1 0.1
35000
Designed Flow
10000
Treated Flow
kVNH4 (1/yr)
Flow Rate (m3/day)
30000 25000 20000 15000
(c) NH4-N
1000 100 10 1
10000
0.1
5000
1 0
2
Wastewater Treatment Facility 1
2
3
4
5
6
7
Site Fig. 3. Comparison of designed and measured flow rates of the studied constructed wetlands. The recent flow rates of Site-1, Site-5, Site-6, and Site-7 are lower than the designed flows.
Fig. 4. Reaction kinetics of (a) TSS; (b) BOD; and (c) NH4 –N of the constructed wetlands (Site-1 through Site-7) and the wastewater treatment plant (Site-C). The symbols show the average reaction coefficients kv20 and the error bars show the standard deviation of measurements. The solid lines across Site-1 to Site-7 show the references kv20 .
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C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258
Table 3 Percent removals and the first-order reaction kinetics of three contaminants in the studied wastewater treatment systems (Note: k20 = standardized 20 ◦ C area-based reaction kinetics; and kv20 = standardized 20 ◦ C volumetric-based reaction kinetics). Site
TSS
1 2 3 4 5 6 7 C
BOD
NH4 –N
Removal (%)
k20 (m/yr)
kv20 (1/yr)
Removal (%)
k20 (m/yr)
kv20 (1/yr)
Removal (%)
k20 (m/yr)
kv20 (1/yr)
68.1 63.5 65.7 87.4 59.1 74.4 34.9 87.8
81.7 79.2 121.4 175.0 6.4 135.9 17.9 –
177.7 166.5 221.8 311.7 12.2 326.0 50.1 7481
59.2 63.2 75.4 57.5 84.3 56.6 58.4 89.1
30.1 59.7 180.1 41.2 17.2 121.7 3.5 –
65.4 125.5 329.1 73.3 32.6 292.0 9.7 9052
88.6 62.7 54.6 79.6 98.7 83.9 57.5 82.2
42.2 55.7 58.6 259.8 101.7 169.1 22.1 –
86.9 122.2 59.9 462.7 192.2 405.7 62.0 4150
BOD removal of Site-1, Site-5, and Site-7 are slightly lower than the references, which confirmed with the higher effluent BODs in Fig. 2. All the studied CWs performed great nitrogen removal during the testing period. The average reaction coefficients of NH4 –N are higher than references for all CWs, but in some occasions the reaction kinetics did not meet the model values. Site-4 shows the highest nitrogen removal rate among all the studied CWs, and Site-2, Site-5, and Site-6 also perform high efficiencies of ammonia removal at all time. The results of TN and TP from the limited sources (Site-2, Site-3, and Site-6) are very similar to the results of NH4 –N removal (results not shown), whereas longer time of operation and more measurements are required to obtain a more conclusive assay of the fate of nutrients in the CWs. The reaction kinetics kv20 of the WWTP was also presented in Fig. 4 and Table 3. The kv20 was calculated by assuming that the WWTP is a steady state single batch reactor. The number provided a basic idea of the treatment efficiency for comparison even though this impression oversimplified the ASP model, in which Monod kinetics (Metcalf and Eddy, 2003) or other complex structural model was used (Leu et al., 2010). The reaction kinetics of the WWTP for all studied contaminants was one or two orders of magnitude higher than the CWs. The centralized WWTP performed greater wastes degrading capability than the CWs in a small footprint.
the operation of each CW under modified loadings. As discussed previously, the newly constructed CWs (i.e. Site-1, Site-5, Site-6, and especially Site-7) were not receiving enough flows to match the designed capacities, and the total treated flow of all CWs was only 45% of the designed flow. Since the CWs were designed to share the wastewater loadings, it was expected to increase the wastewater flow to the designed capacities. The following attempt and discussion applied the empirical models and calculated reaction kinetics to predict the possible performances of the CWs. Kadlec (2000) showed that the reaction coefficient is linearly correlated with the hydraulic loading rates of the CWs. This relationship was applied in this study to predict the potential treatment performances of the studied CWs. The relationships between HRL and the reaction kinetics (k20 ) of TSS, BOD, and NH4 –N of our study were shown in Fig. 5(a) through Fig. 5(c), respectively. The dashed lines in Fig. 5 showed the linear regressions of the measurement results. The functions of each pollutant (using linear fits regression) as well as the coefficients of determinations (R2 ) were provided in the figures. Among the three pollutants, TSS and BOD showed stronger relationships than NH4 –N: the R2 of TSS and BOD were 0.73 and 0.67, respectively; and the R2 was close to zero for NH4 –N. Unlike TSS and BOD, NH4 –N is a nutrient of the aquatic microorganisms and vegetations of the CWs, therefore the degradation rate of NH4 –N are not a constant and unpredictable. The relationships between k20 and HRL were used to calculate the treatment performances of the studied CWs at increased flows. New k20 were calculated for TSS and BOD based upon the linear regressions with the increasing HRL, and a constant k20 was used for NH4 –N. The new reaction kinetics was used in Eq. (3) to calculate the effluent concentrations and the treatment loads, and the simulation results were shown in Table 4. The two scenarios
3.3. Recommendations to future operation The validated model parameters were used to simulate the impacts of different management scenarios and predict the potentials of the CWs. A sensitivity analysis was performed to optimize
200
200
b) k
(a)
.67
(c)
3.664X HLR; R 0 Average 101.3
120
80
40
k NH -N (m/yr)
160
k BOD (m/yr)
k TSS (m/yr)
160
300 3.791X HLR-29.884; R
120
80
200
100
40
0
0 0
10
20
30
40
Hydraulic Loading Rate (m/yr)
50
0
10
20
30
40
Hydraulic Loading Rate (m/yr)
50
0
10
20
30
40
Hydraulic Loading Rate (m/yr)
Fig. 5. Relationship between reaction kinetics and hydraulic loading rates of three pollutants: (a) TSS; (b) BOD; and (c) NH4 –N.
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C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 Table 4 Simulated treatment performances with increased loads. Parameter (Unit)
Reaction kinetics (k20 , m/yr) TSS BOD NH4 –N
Change (%)
Current
Future
16.6 36,600 10.2
37.0 81,500 4.6
89.6 65.6 101.3
135.1 74.9 101.3
+50.7 +14.2 0
8.1 11.9 3.4
13.4 11.0 1.7
+65.4 −7.5 −50.4
Effluent concentration (mg/L) TSS BOD NH4 –N Total removal (kg/day) TSS BOD NH4 –N
3.4. Monetary costs and environmental values of the constructed wetlands
Scenario
HLR (m/yr) Flow (m3 /day) HRT (day)
2672 836 330
255
8809 1997 974
+123 +123 −55
+230 +140 +195
compared in Table 4 are the current and future operations of the studied CWs with increased flow rates. The influent concentrations of the future operation were averages of the influents from the stabilized Sites (i.e. Site-2, Site-3, Site-4, and Site-6). The average HLR increased significantly by 123% and HRT decreased from 10.2 days to 4.6 days. With the increased flows and reaction kinetics, the total removal of the three pollutants increased dramatically by 230%, 140%, and 195% for TSS, BOD, and NH4 –N, respectively, even though the effluent concentrations did not change significantly (i.e. from 8.1 to 13.4 for TSS; 11.9 to 11.0 for BOD; and 3.4 to 1.7 for NH4 –N). The simulation strategy and model parameters used in the previous section were based only on the experimental results of this study. The increased flow rates and HLRs of the designed scenarios are the upper limit of the observed values. Since our results fit fairly well with the empirical results of Kadlec (2009) (Fig. 2), who tested many CWs that received at much higher maximum pollutant loadings (approximately 50–100 times for TSSs and BODs), our model may be further extended to simulate the performances of the studied CWs under over-loaded conditions, i.e. during flooding seasons or typhoons. In addition, the model can be further verified and applied with the water quality management plans. Based on the current plan, the total influent wastewater flows of the CWs will gradually increase with the increasing sewer basins coverage, and then decrease dramatically when the new centralized wastewater treatment plants start working online.
The compositions of the O&M costs for the studied CWs and WWTP were shown in Fig. 6(a and b), respectively. The larger pie in Fig. 6(b) represents the higher O&M costs per volume wastewater treated for the WWTP (0.064 USD/m3 ) comparing to the CWs (0.033 USD/m3 ). The costs of electricity were significant for the two systems, approximately accounted to 27% of the total O&M costs. The costs of regular operation of instruments and reparation of the hardware, including all the consumables and labor costs, account to the majority part (49%) of O&M costs for the CWs. Sludge handling and re-vegetation account to 8 and 7% of O&M costs, respectively. The administration costs of the wetlands include the costs for water quality monitoring reports, presentation and training, ecological survey and investigation, and aerial photography. Chemicals used in the disinfection and clarification processes, and tap water are required only in the centralized WWTP, accounting to 14 and 12% of the O&M costs, respectively. The costs for wastes hauling (i.e. sludge disposal) account to 25% of the O&M costs in the centralized plant, which is considerably higher than the costs for sludge handling in CWs. The calculated costs of wastewater treatment using the CWs and WWTP were shown in Table 5. The costs were all converted to the 2011 U.S. dollar equivalents per unit volume/mass pollutant treated. The total costs of the present CWs were between 0.425 and 3.621 USD per kg total BOD treated, and the cost of the WWTP was 1.186 USD per total BOD treated. The results were plotted in Fig. 7 to visualize the differences between the current costs and predicted costs for all the wastewater treatment facilities. Fig. 7(a and b) shows the total costs per m3 wastewater treated for the present and future operations, respectively; and the capital costs and O&M costs were plotted in different colors. The current cost of Site-5 is much higher than the other CWs and the WWTP, because currently the influent flow rate of Site-5 is remarkably lower than the designed flow rates. If all the CWs are fully loaded, the total costs of the CWs can be reduced to approximately half of the costs of the WWTP. The costs of different CWs varied significantly: Site1, Site-3, and Site-4 cost approximately 1.5–2 times the cost of the other CWs. The costs were mostly site-specific and varied with the locations: some of the CWs were built on existing wetlands and the others were constructed on remote open water surfaces, which increase the costs of construction. Similar comparisons were also made in Fig. 7(c and d) for the total costs per kg total BOD show removed for present and future operations, respectively. The total BODs were calculated by BOD the oxygen demand of nitrification; and the oxygen demand of NH4 –N
Table 5 Cost analysis of the studied wastewater treatment systems. Site
Year built
1 2 3 4 5 6 7 C
2008 2004 2006 2009 2010 2006 2009 2003
a b c
Capital cost (USDa )
Capital cost – 20 years annuity (USD/yr)
Previous value
Present value
1,986,000 434,000 689,000 1,061,000 4,037,000 898,000 2,465,000 216,667,000
1,957,000 392,000 644,000 1,036,000 3,984,000 839,000 2,303,000 192,429,000
Currency presented in U.S. dollars. Operation and maintenances. Including the summation of the capital costs and O&M costs.
157,100 31,400 51,600 83,100 319,700 67,300 184,800 15,441,000
Present O&Mb cost (USD/yr)
56,400 27,900 54,000 70,700 111,300 98,700 62,700 808,900
Present total costc (USD/yr)
Cost per volume sewer treated (USD/m3 )
Cost per mass BOD removed (USD/kg)
213,500 59,300 105,600 153,800 431,000 166,000 247,500 16,249,900
0.136 0.028 0.072 0.093 0.355 0.059 0.097 0.165
1.465 0.425 1.015 1.598 3.621 1.197 2.991 1.186
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C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258
a
0.40
O&M Capital
Cost (USD/m3)
0.35 0.30 0.25 0.20 0.15 0.10
b
0.16
Cost (USD/m3)
Fig. 6. Composition of the operation and maintenance costs: (a) constructed wetlands and (b) centralized wastewater treatment plant.
0.12
O&M Capital
0.14 0.10 0.08 0.06 0.04 0.02
0.05
0.00
0.00 1
2
3
4
5
6
7
1
C
2
3
Site
d
4.0
O&M Capital
3.5
5
6
7
C
5
6
7
C
Site
Cost (USD/kg total BOD)
Cost (USD/kg total BOD)
c
4
3.0 2.5 2.0 1.5 1.0 0.5 0.0
1.6
O&M Capital
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
1
2
3
4
Site
5
6
7
C
1
2
3
4
Site
Fig. 7. (a) Current; and (b) predicted total costs per volume wastewater treated of the constructed wetlands and the wastewater treatment plant; and (c) current; and (b) predicted total costs per kg total BOD removed for the wastewater treatment facilities. The total BODs were the summation of BODs and the oxygen demand of nitrification (assume mass ratio of O2 :NH4 –N is 4.33:1).
C.-J. Teng et al. / Ecological Engineering 44 (2012) 249–258 Table 6 Water environment education activities and visitor count of the studied constructed wetlands in 2009. Activity Schools field trips – total 157 classes Local tours for water environment education Training of wetland narrators and volunteers Wetland photo contests – 3 times Concerts at the wetlands Wetland movie theater Total visitor count
Visitor count 17,770 15,000 37,000 1000 200 200 71,170
257
of the CWs were between 0.425 and 3.621 USD per kg total BOD removed, and the costs of the centralized wastewater treatment plant was approximately 1.186 USD per total BOD removed. The CWs also provided non-market benefits equivalent to approximately 130,000 USD in 2009. The CWs serve reasonably well as an integrating wastewater treatment system in addition to their environmental values while the centralized sewer system or other advanced systems are under construction.
Acknowledgments was assumed to be 4.33 kgO2 per kg NH4 –N. The total costs of centralized WWTP to treat the same amount of BOD and NH4 –N are comparable to the costs of the CWs, implying the great potential of using CWs to support a constructing overall wastewater treatment system, especially for nitrogen removal. In addition to the construction and operation costs, CWs obtain many additional benefits, i.e. the ecological, educational, and recreational functions (Marble, 1984) to the local communities. These “non-market” benefits, often related to consumer’s willingnessto-pay, are often evaluated by applying environmental survey with designed questionnaires (Yang et al., 2008), or by calculating the ecosystem services values of various indexes (Chen et al., 2009). This study provides an example to calculate the values of an additional benefit of the CWs. In-depth investigation of the environmental values, however, requires large amount of data and it is currently beyond the scope of this study to perform the analysis in greater details. Woodward and Wui (2001) showed that the economic values of wetland services can be calculated by four methods: (a) the net factor income (NFI) method; (b) the replacement cost; (c) travel cost; and (d) contingent valuation. The example provided here was based upon the travel cost approach, which quantified the consumer’s willingness-to-pay by calculating the total transportation fee spent to visit the target wetlands. Table 6 shows the documented activities and visitor count of the studied CWs during 2009. The listed activities are all reported by public sources, i.e. schools, the city, and environmental groups; personal activities from private organization or individual visitors were not reported. The reported total visitor count in 2009 was 71,170 P.E., and the real amount of visitors should be higher than this number. The average transportation fee to the CWs (Site-1) was 1.83 ± 0.91 USD per person (round trip) based upon the information provided by Taipei Metro (http://www.trtc.com.tw/). The approximate non-market benefits produced by the CWs in 2009 were therefore 130,000 USD. The environmental values provided by the CWs accounted to approximately 27% of the O&M costs in additional to the benefits of water quality improvement. 4. Conclusion This study investigated the environmental and economical impacts of seven riparian CWs and one centralized WWTP for municipal wastewater treatment in Tan-Shui River of the metropolitan Taipei. The treatment performances of the CWs confirmed with the references, but the variation among the CWs were significant. Influent flow rates were ones of the most essential operating parameters to affect the performance of the CWs. The treatment efficiency of the centralized WWTP was approximately one to two orders of magnitude better than the CWs, and the total costs per wastewater treated were approximately 2 times the costs of CWs. If the total costs were converted to USD per unit mass total oxygen demand (oxygen demand of BOD plus NH4 –N), the costs of WWTP were comparable to the CWs. The total costs
The authors thank C.-L. Yang (San Ying Enterprises Co.); J. Chen, Wu, S. (MWH Taiwan); and Apollo Technology Co., Ltd. for sampling, technical mapping, and water quality analysis.
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