2122
© IWA Publishing 2014 Water Science & Technology
|
69.10
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2014
Performance evaluation and a sizing method for hydrodynamic separators treating urban stormwater runoff D. H. Lee, K. S. Min and J.-H. Kang
ABSTRACT This study reports on 6 years of performance monitoring of stormwater hydrodynamic separators in Korean urban catchments. One hundred and thirty-seven storm events were monitored in four hydrodynamic separators of two different types from 2006 to 2012. Mean values of the event average removal efficiencies of total suspended solids (TSS) for the four hydrodynamic separators were 43.69, 8.54, 42.84, and 14.35% with corresponding mean values of the event average surface overflow rates of 28.62, 40.07, 16.02, and 38.81 m/h, respectively. The low TSS removal efficiency was due to the high instantaneous surface overflow rates frequently occurring throughout a storm event and the abundance of fine particle fractions in the inflow (median particle diameter A-1 > B-2, A-2. This observation implies that in general a smaller mean surface overflow rate might result in a greater pollutant removal efficiency. However, little statistical difference was found in pollutant removal efficiencies between Type A and B treatment devices (t-statistic ¼ 0.273, df ¼ 53, and p ¼ 0.768). According to the data for metals in Type A-1 (not shown in Table 1), mean removal efficiencies were 31.47, 16.54, 16.23, 27.86, 6.17, 14.05 and 25.37% for As, Cd, Cr, Cu,
Figure 2 shows the solid concentrations in different size ranges in the composite samples of inflow and outflow of the Type B-1 device for the four monitored storm events (October 22, October 27, November 11, and November 26) in 2012. The TSS removal efficiencies for the storm events October 22, October 27, November 11 and November 26 were 21.72, 4.71, 26.54, and 77.04%, respectively. Corresponding event average surface overflow rates for the four storm events were 34.16, 24.38, 15.70 and 0.35 m/h, respectively. As expected, larger particles were removed more effectively, while particles smaller than 75 μm were rarely removed. Figure 2 implies that TSS removal efficiency can be improved when removal of smaller particles is enhanced by operating the device at a very low range of surface overflow rate. This is generally infeasible because a typical hydrodynamic separator is operated at an extremely wide range of surface overflow rate, depending on the storm size and the time-varying flow rate within a storm event. Therefore a hydrodynamic separator should be more useful as a pretreatment system, targeting relatively large particles.
RESULTS AND DISCUSSION
Performance simulation of the hydrodynamic separators As shown in Figure 2, the field monitoring data indicated that low pollutant removal efficiency of the hydrodynamic separators might be due to the abundance of fine particles in the inflow, which can be rarely settled. A series of simulations were conducted to further investigate the impact of PSD on the performance of the hydrodynamic separators. PSD data were collected from the literature to determine a representative functional form of PSD in urban runoff. Three two-parameter distribution functions including lognormal, gamma and Weibull functions were fitted with the
2127
Figure 2
D. H. Lee et al.
|
|
Performance evaluation and sizing method for hydrodynamic separators
Water Science & Technology
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69.10
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2014
Solid concentrations in different size ranges in the composite samples of inflow and outflow of the Type B-1 device for the four monitored storm events in 2012 (ETSS ¼ TSS removal efficiency, and vof ¼ event average surface overflow rate).
literature PSD data. Table 2 summarizes the results. In this study, particle fractionization was conducted using composite samples for four storm events in Type B-1, but was not used for the data fitting, because sufficiently fine particles were not sieved to accurately estimate the median particle diameter (d50) that was considered to be less than 75 μm. The value of d50 in urban runoff showed a large variability among different studies, as well as within a single study, and was as low as 7.8 μm (Goncalves & Van Seters ). In this study, the majority (>76%) of particles by dry weight were less than 75 μm. The Weibull function generally resulted in the smallest RMSE values between measured and simulated cumulative PSD curves of the three distribution functions. The Weibull function approximation for PSD in urban runoff is also supported by Selbig & Fienen (). The scale parameter (λ) of the fitted Weibull function widely varied ranging from 12.8 to 999 while the shape parameter (κ) was comparatively less variable, ranging from 0.37 to 2.37 (the coefficients of variation for λ and κ were 0.95 and 0.46, respectively). Figure 3(a) compares the measured and simulated event TSS removal efficiencies as functions of the event average surface overflow rate for all the 137 monitored storms from the four hydrodynamic separators. PSDs following Weibull functions with different λ values were applied to the discrete settling mechanism obeying Stoke’s law with a
constant particle density of 2.65 to calculate settling efficiency. Application of the discrete settling mechanism in a hydrodynamic separator is supported by previous studies (Woodward-Clyde Group, Inc. ; Wilson et al. ). Simulated values of λ ranged from 9.3 to 116, corresponding to a range of d50 from 6 to 75 μm. The shape parameter κ was fixed in the simulation, as it was considered less variable based on the data fitting results previously performed. As shown, Stoke’s law coupled with a Weibull function for PSD reasonably well simulated the non-linearly decreasing trend of TSS removal efficiency with the increased event average surface overflow rate. The measured data were scattered around the simulated curves with different λ values compared to the simulated curve of TSS removal efficiency, reflecting the variable conditions of PSD for different storm events. Design application Determination of the design surface overflow rate The developed curves of TSS removal efficiency (Figure 3(a)) can be used to determine the size (i.e., active volume) of a hydrodynamic separator. Event average runoff rate and PSD may be first used to obtain event average surface overflow rate at a required level of TSS removal efficiency,
2128 D. H. Lee et al.
|
|
Two-parameter distribution functions fitted with PSD data in stormwater runoff samples from paved areas, found in the literature Fitted distribution function Log-normal
Gamma
Weibull
Pavement type na
Obs. db50 (μm)
μ
σ
Est. dc50 (μm)
RMSEd
k
θ
Est. dc50 (μm)
RMSEd
λ
κ
Est. dc50(μm)
RMSEd
Asphalt
4(0)
< 75
–
–
–
–
–
–
–
–
–
–
–
–
Highway
Asphalt
7(3)
< 20–124.8 3.58–4.67 1.02–1.26 36.2–106.8 0.018–0.062 0.95–1.01 58.6–155.9 39.6–108.6 0.003–0.032 60.8–156.4
0.79–0.10
38.2–108.3 0.003–0.032
Parking lot
Asphalt
4(4)e
7.8–16.4
1.85–2.68 1.43–1.97 6.4–14.6
0.91–1.19
8.5–17.3
Highway
Concrete
6(6)
50.0–204.5
3.77–5.17 1.22–1.81 43.5–175.9 0.030–0.080 0.78–0.94 72.3–254.3 48.1–167.7 0.044–0.072 80.0–261.9
0.66–0.82
47.7–163.7 0.045–0.061
Highway
Asphalt
21(17) 6.0–590.0
2.07–6.34 0.93–2.46 7.9–566.4
0.37–2.38
6.9–572.5
Residential,
n/a
3(3)e
69.8–198.3
4.16–5.23 0.98–1.77 64.0–186.4 0.053–0.086 0.68–0.97 92.5–257.1 60.4–170.1 0.025–0.038 106.9–347.2 0.613–0.883 61.0–181.4 0.025–0.038
Concrete
8(8)
29–300
3.28–5.99 1.57–2.94 26.5–398.2 0.028–0.118 0.70–0.94 46.5–429.1 31.3–252.4 0.040–0.070 48.5–999.0
Reference
Location
Land-use
This study
Yongin, Korea Highway
(Type B-1) Furumai et al. Winterthur, () Goncalves & Van Seters
Switzerland Ontario,
0.063–0.085 0.93–1.07 12.5–24.5
8.4–17.3
0.016–0.030 12.8–24.2
0.016–0.034
Canada
() Li et al. () Los Angeles, CA, USA Horwatich et al. () Selbig & Bannerman
Milwaukee,
Sansalone
0.030–0.143 16.8–827.6
0.028–0.129
WI, USA Madison, WI, USA
commercial
() Kim &
0.023–0.184 0.56–2.11 10.4–824.9 6.8–534.0
Performance evaluation and sizing method for hydrodynamic separators
Table 2
streets Baton Rouge,
Highway
0.433–0.879 31.9–429.3 0.038–0.065
LA, USA
() a
Number of data sets. Values in the brackets are the number of data points used in the data fitting (data fitting was performed only for the PSD data in which both particles greater and less than d50 were fractionized into more than two
b
Observed median diameter.
c
Median diameter estimated from data fitting for a given PSD function. d RMSE between observed and estimated data points for a given cumulative PSD function. e
average of multiple observations.
Water Science & Technology
size bins).
| 69.10
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Figure 3
D. H. Lee et al.
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|
Performance evaluation and sizing method for hydrodynamic separators
Water Science & Technology
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69.10
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2014
(a) Observed and simulated event TSS removal efficiency with respect to event average surface overflow rate. (b) Relationship between event average and event peak surface overflow rates.
Figure 4
|
(a) Probability of occurrence of the maximum 1 h rainfall intensity during 2008–2012 (5 years). (b) Simulated fraction of total runoff volume entering the facility as a function of design peak flow rate during 2008–2012.
as it is very difficult to implement time-varying characteristics of runoff flow and PSD in determining the size of a hydrodynamic separator. Then, event average surface overflow is converted to the event peak surface flow rate using the ratio between the event peak and average surface overflow rate. This can be obtained from field observations, as shown in Figure 3(b). Figure 3(b) shows the relationship between peak surface overflow rates and average surface overflow rates observed for the 137 monitored storms from the four devices. The event peak surface overflow was approximately four times greater than the event average
surface overflow in the study catchments. Once the event peak surface overflow rate is determined, the active volume of a treatment device can also be determined with additional consideration for the device-specific structural configuration. Consideration of water quality flow It is typical to bypass the excess water, if the inflow rate to a treatment device exceeds the maximum allowable flow rate. This is termed ‘water quality flow’ (WQF). The device size
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D. H. Lee et al.
|
Performance evaluation and sizing method for hydrodynamic separators
and construction cost can be considerably reduced without significantly compromising the water quality goal by choosing an appropriate WQF. A similar approach to the method of determining a design rainfall depth for a detention type BMP can be used for this purpose. Figure 4(a) shows the probability of occurrence for the maximum 1 h rainfall intensity (I1 h,max) developed using the past 5 years’ precipitation data in the catchment for Type B-1. From Figure 4(a), a 1 h rainfall intensity can be chosen as a design value at a given probability of occurrence (F) and a WQF can be calculated by the Rational Method. For example, values of I1 h,max at F ¼ 60, 70, 80, and 90% are 4, 6, 10 and 16 mm/h, respectively. The corresponding WQFs using a runoff coefficient of 0.95 and a catchment area of 1.27 ha are 0.003, 0.005, 0.008 and 0.013 m3/s, respectively. A design WQF can be chosen so that a required volume fraction of total annual runoff treated by the treatment device is achieved. Figure 4(b) shows the volume fraction of total annual runoff with respect to WQF simulated by an urban runoff model, USEPA’s SWMM in the catchment for Type B-1. As shown in Figure 4(b), 60, 75, 86 and 90% of the total annual runoff volume can be treated by the treatment device at F ¼ 60, 70, 80, and 90%, respectively. The size of a hydrodynamic separator can be reduced by one-sixth by choosing I1 h,max at F ¼ 80%, instead of F ¼ 100%, according to the calculation.
CONCLUSIONS In this study, little statistical difference was observed in the pollutant removal efficiency between the two different types of hydrodynamic separators. Wet-sieve analyses for composite runoff samples from one of the study catchments showed that the majority of the particles were in a fraction less than 75 μm, resulting in low TSS removal efficiency. Moreover, the average removal efficiencies of all measured pollutants were below ∼50% with episodic negative efficiencies, implying that a hydrodynamic separator should be more useful as a pre-treatment device for the downstream processes in a treatment train, and should be properly maintained to avoid sediment washout from the device. The settling efficiency of smaller particles should be enhanced by decreasing the surface overflow rate of the hydrodynamic separator to increase removal efficiencies of TSS and associated pollutants. Three different probability functions were fitted to numerous PSD data found in the literature and the Weibull function was considered the best function representing the
Water Science & Technology
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PSD in urban runoff. The hypothetical PSD function (Weibull type PSD) was applied to the discrete settling mechanism to simulate TSS removal efficiency as a function of surface overflow rate and PSD. The calculated TSS removal efficiency simulated well the non-linearly decreasing tendency of TSS removal efficiency with the increased surface overflow rate, as observed in the monitoring data from the four hydrodynamic separators. The developed curves of TSS removal efficiency as a function of PSD and surface overflow rate should be useful in determining the water quality flow and active volume of a hydrodynamic separator given the historical information on the rainfall data and catchment characteristics. Simulations using an urban runoff model (USEPA’s SWMM) showed that approximately 85% of total annual runoff can be captured by a hydrodynamic separator with a WQF corresponding to 80% probability of occurrence of the 1 h maximum rainfall intensity.
ACKNOWLEDGEMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (#2011-0009144).
REFERENCES CONTECH CDS Guide: Operation, Design, Performance and Maintenance. CONTECH Construction Products Inc., West Chester, OH, USA. Furumai, H., Balmer, H. & Boller, M. Dynamic behaviour of suspended pollutants and particle size distribution in highway runoff. Water Science and Technology 46 (11–12), 413–418. Goncalves, C. & Van Seters, T. Characterization of Particle Size Distributions of Runoff from High Impervious Urban Catchments in the Greater Toronto Area. Toronto and Region Conservation Authority, Downsview, Ontario, Canada. Horwatich, J. A., Bannerman, R. T. & Robert, P. HighwayRunoff Quality, and Treatment Efficiencies of a Hydrodynamic-Settling Device and a Stormwater-Filtration Device in Milwaukee, U.S. Geological Survey Scientific Investigations Report 2010-5160, Reston, VA, USA. KGIC (Korean Government Interagency Consortium) The 2nd Master Plan for Managing Non-point Source Pollution, Report 11-1480000-001222-01, KGIC, Seoul, Korea. Kim, J.-Y. & Sansalone, J. J. Event-based size distributions of particulate matter transported during urban rainfall-runoff events. Water Research 42 (10–11), 2756–2768.
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Lee, J., Bang, K., Choi, J., Ketchum Jr, L. H. & Cho, Y. The vortex concentrator for suspended solids treatment. Water Science and Technology 47 (9), 335–341. Li, Y., Lau, S.-L., Kayhanian, M. & Stenstrom, M. K. Particle size distribution in highway runoff. ASCE, Journal of Environmental Engineering 131 (9), 1267–1276. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal, and Reuse. 4th edn, McGraw-Hill, New York, USA. Moheseni, O., Kieffer, J. & Koehler, J. A tool for the performance assessment of hydrodynamic separators. Proceedings of World Environmental and Water Resources Congress 2009, May 17–21, Kansas City, Missouri, USA. Pathapati, S.-S. & Sansalone, J. J. CFD modeling of a stormwater hydrodynamic separator. ASCE, Journal of Environmental Engineering 135 (4), 191–202. Selbig, W. R. & Bannerman, R. T. Characterizing the size distribution of particles in urban stormwater by use of fixedpoint sample-collection methods, U.S. Geological Survey Open-File Report 2011-1052, Reston, VA, USA.
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Selbig, W. R. & Fienen, M. N. Regression modeling of particle size distribution in urban storm water: Advancements through improved sample collection methods. ASCE, Journal of Environmental Engineering 138 (12), 1186–1193. Tran, D. & Kang, J.-H. Optimal design of a hydrodynamic separator for treating runoff from roadways. Journal of Environmental Management 116, 1–9. Wilson, M. A., Mohseni, O., Gulliver, J. S., Hozalski, R. M. & Stefan, H. G. Assessment of hydrodynamic separators for storm-water treatment. ASCE, Journal of Environmental Engineering 135 (5), 383–392. Woodward-Clyde Group, Inc. Santa Monica Bay Area Municipal Stormwater/Urban Runoff Pilot ProjectEvaluation of Potential Catchbasin Retrofits. Project Report, Santa Monica Cities Consortium, Santa Monica, CA, USA. Yu, S. L. & Stopinski, M. D. Testing of Ultra-Urban Stormwater Best Management Practices. VTRC 01-R7. Virginia Transportation Research Council, Charlottesville, Virginia, USA.
First received 24 October 2013; accepted in revised form 24 February 2014. Available online 13 March 2014
Copyright of Water Science & Technology is the property of IWA Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
© IWA Publishing 2014 Water Science & Technology
|
69.10
|
2014
Performance evaluation and a sizing method for hydrodynamic separators treating urban stormwater runoff D. H. Lee, K. S. Min and J.-H. Kang
ABSTRACT This study reports on 6 years of performance monitoring of stormwater hydrodynamic separators in Korean urban catchments. One hundred and thirty-seven storm events were monitored in four hydrodynamic separators of two different types from 2006 to 2012. Mean values of the event average removal efficiencies of total suspended solids (TSS) for the four hydrodynamic separators were 43.69, 8.54, 42.84, and 14.35% with corresponding mean values of the event average surface overflow rates of 28.62, 40.07, 16.02, and 38.81 m/h, respectively. The low TSS removal efficiency was due to the high instantaneous surface overflow rates frequently occurring throughout a storm event and the abundance of fine particle fractions in the inflow (median particle diameter A-1 > B-2, A-2. This observation implies that in general a smaller mean surface overflow rate might result in a greater pollutant removal efficiency. However, little statistical difference was found in pollutant removal efficiencies between Type A and B treatment devices (t-statistic ¼ 0.273, df ¼ 53, and p ¼ 0.768). According to the data for metals in Type A-1 (not shown in Table 1), mean removal efficiencies were 31.47, 16.54, 16.23, 27.86, 6.17, 14.05 and 25.37% for As, Cd, Cr, Cu,
Figure 2 shows the solid concentrations in different size ranges in the composite samples of inflow and outflow of the Type B-1 device for the four monitored storm events (October 22, October 27, November 11, and November 26) in 2012. The TSS removal efficiencies for the storm events October 22, October 27, November 11 and November 26 were 21.72, 4.71, 26.54, and 77.04%, respectively. Corresponding event average surface overflow rates for the four storm events were 34.16, 24.38, 15.70 and 0.35 m/h, respectively. As expected, larger particles were removed more effectively, while particles smaller than 75 μm were rarely removed. Figure 2 implies that TSS removal efficiency can be improved when removal of smaller particles is enhanced by operating the device at a very low range of surface overflow rate. This is generally infeasible because a typical hydrodynamic separator is operated at an extremely wide range of surface overflow rate, depending on the storm size and the time-varying flow rate within a storm event. Therefore a hydrodynamic separator should be more useful as a pretreatment system, targeting relatively large particles.
RESULTS AND DISCUSSION
Performance simulation of the hydrodynamic separators As shown in Figure 2, the field monitoring data indicated that low pollutant removal efficiency of the hydrodynamic separators might be due to the abundance of fine particles in the inflow, which can be rarely settled. A series of simulations were conducted to further investigate the impact of PSD on the performance of the hydrodynamic separators. PSD data were collected from the literature to determine a representative functional form of PSD in urban runoff. Three two-parameter distribution functions including lognormal, gamma and Weibull functions were fitted with the
2127
Figure 2
D. H. Lee et al.
|
|
Performance evaluation and sizing method for hydrodynamic separators
Water Science & Technology
|
69.10
|
2014
Solid concentrations in different size ranges in the composite samples of inflow and outflow of the Type B-1 device for the four monitored storm events in 2012 (ETSS ¼ TSS removal efficiency, and vof ¼ event average surface overflow rate).
literature PSD data. Table 2 summarizes the results. In this study, particle fractionization was conducted using composite samples for four storm events in Type B-1, but was not used for the data fitting, because sufficiently fine particles were not sieved to accurately estimate the median particle diameter (d50) that was considered to be less than 75 μm. The value of d50 in urban runoff showed a large variability among different studies, as well as within a single study, and was as low as 7.8 μm (Goncalves & Van Seters ). In this study, the majority (>76%) of particles by dry weight were less than 75 μm. The Weibull function generally resulted in the smallest RMSE values between measured and simulated cumulative PSD curves of the three distribution functions. The Weibull function approximation for PSD in urban runoff is also supported by Selbig & Fienen (). The scale parameter (λ) of the fitted Weibull function widely varied ranging from 12.8 to 999 while the shape parameter (κ) was comparatively less variable, ranging from 0.37 to 2.37 (the coefficients of variation for λ and κ were 0.95 and 0.46, respectively). Figure 3(a) compares the measured and simulated event TSS removal efficiencies as functions of the event average surface overflow rate for all the 137 monitored storms from the four hydrodynamic separators. PSDs following Weibull functions with different λ values were applied to the discrete settling mechanism obeying Stoke’s law with a
constant particle density of 2.65 to calculate settling efficiency. Application of the discrete settling mechanism in a hydrodynamic separator is supported by previous studies (Woodward-Clyde Group, Inc. ; Wilson et al. ). Simulated values of λ ranged from 9.3 to 116, corresponding to a range of d50 from 6 to 75 μm. The shape parameter κ was fixed in the simulation, as it was considered less variable based on the data fitting results previously performed. As shown, Stoke’s law coupled with a Weibull function for PSD reasonably well simulated the non-linearly decreasing trend of TSS removal efficiency with the increased event average surface overflow rate. The measured data were scattered around the simulated curves with different λ values compared to the simulated curve of TSS removal efficiency, reflecting the variable conditions of PSD for different storm events. Design application Determination of the design surface overflow rate The developed curves of TSS removal efficiency (Figure 3(a)) can be used to determine the size (i.e., active volume) of a hydrodynamic separator. Event average runoff rate and PSD may be first used to obtain event average surface overflow rate at a required level of TSS removal efficiency,
2128 D. H. Lee et al.
|
|
Two-parameter distribution functions fitted with PSD data in stormwater runoff samples from paved areas, found in the literature Fitted distribution function Log-normal
Gamma
Weibull
Pavement type na
Obs. db50 (μm)
μ
σ
Est. dc50 (μm)
RMSEd
k
θ
Est. dc50 (μm)
RMSEd
λ
κ
Est. dc50(μm)
RMSEd
Asphalt
4(0)
< 75
–
–
–
–
–
–
–
–
–
–
–
–
Highway
Asphalt
7(3)
< 20–124.8 3.58–4.67 1.02–1.26 36.2–106.8 0.018–0.062 0.95–1.01 58.6–155.9 39.6–108.6 0.003–0.032 60.8–156.4
0.79–0.10
38.2–108.3 0.003–0.032
Parking lot
Asphalt
4(4)e
7.8–16.4
1.85–2.68 1.43–1.97 6.4–14.6
0.91–1.19
8.5–17.3
Highway
Concrete
6(6)
50.0–204.5
3.77–5.17 1.22–1.81 43.5–175.9 0.030–0.080 0.78–0.94 72.3–254.3 48.1–167.7 0.044–0.072 80.0–261.9
0.66–0.82
47.7–163.7 0.045–0.061
Highway
Asphalt
21(17) 6.0–590.0
2.07–6.34 0.93–2.46 7.9–566.4
0.37–2.38
6.9–572.5
Residential,
n/a
3(3)e
69.8–198.3
4.16–5.23 0.98–1.77 64.0–186.4 0.053–0.086 0.68–0.97 92.5–257.1 60.4–170.1 0.025–0.038 106.9–347.2 0.613–0.883 61.0–181.4 0.025–0.038
Concrete
8(8)
29–300
3.28–5.99 1.57–2.94 26.5–398.2 0.028–0.118 0.70–0.94 46.5–429.1 31.3–252.4 0.040–0.070 48.5–999.0
Reference
Location
Land-use
This study
Yongin, Korea Highway
(Type B-1) Furumai et al. Winterthur, () Goncalves & Van Seters
Switzerland Ontario,
0.063–0.085 0.93–1.07 12.5–24.5
8.4–17.3
0.016–0.030 12.8–24.2
0.016–0.034
Canada
() Li et al. () Los Angeles, CA, USA Horwatich et al. () Selbig & Bannerman
Milwaukee,
Sansalone
0.030–0.143 16.8–827.6
0.028–0.129
WI, USA Madison, WI, USA
commercial
() Kim &
0.023–0.184 0.56–2.11 10.4–824.9 6.8–534.0
Performance evaluation and sizing method for hydrodynamic separators
Table 2
streets Baton Rouge,
Highway
0.433–0.879 31.9–429.3 0.038–0.065
LA, USA
() a
Number of data sets. Values in the brackets are the number of data points used in the data fitting (data fitting was performed only for the PSD data in which both particles greater and less than d50 were fractionized into more than two
b
Observed median diameter.
c
Median diameter estimated from data fitting for a given PSD function. d RMSE between observed and estimated data points for a given cumulative PSD function. e
average of multiple observations.
Water Science & Technology
size bins).
| 69.10
| 2014
2129
Figure 3
D. H. Lee et al.
|
|
Performance evaluation and sizing method for hydrodynamic separators
Water Science & Technology
|
69.10
|
2014
(a) Observed and simulated event TSS removal efficiency with respect to event average surface overflow rate. (b) Relationship between event average and event peak surface overflow rates.
Figure 4
|
(a) Probability of occurrence of the maximum 1 h rainfall intensity during 2008–2012 (5 years). (b) Simulated fraction of total runoff volume entering the facility as a function of design peak flow rate during 2008–2012.
as it is very difficult to implement time-varying characteristics of runoff flow and PSD in determining the size of a hydrodynamic separator. Then, event average surface overflow is converted to the event peak surface flow rate using the ratio between the event peak and average surface overflow rate. This can be obtained from field observations, as shown in Figure 3(b). Figure 3(b) shows the relationship between peak surface overflow rates and average surface overflow rates observed for the 137 monitored storms from the four devices. The event peak surface overflow was approximately four times greater than the event average
surface overflow in the study catchments. Once the event peak surface overflow rate is determined, the active volume of a treatment device can also be determined with additional consideration for the device-specific structural configuration. Consideration of water quality flow It is typical to bypass the excess water, if the inflow rate to a treatment device exceeds the maximum allowable flow rate. This is termed ‘water quality flow’ (WQF). The device size
2130
D. H. Lee et al.
|
Performance evaluation and sizing method for hydrodynamic separators
and construction cost can be considerably reduced without significantly compromising the water quality goal by choosing an appropriate WQF. A similar approach to the method of determining a design rainfall depth for a detention type BMP can be used for this purpose. Figure 4(a) shows the probability of occurrence for the maximum 1 h rainfall intensity (I1 h,max) developed using the past 5 years’ precipitation data in the catchment for Type B-1. From Figure 4(a), a 1 h rainfall intensity can be chosen as a design value at a given probability of occurrence (F) and a WQF can be calculated by the Rational Method. For example, values of I1 h,max at F ¼ 60, 70, 80, and 90% are 4, 6, 10 and 16 mm/h, respectively. The corresponding WQFs using a runoff coefficient of 0.95 and a catchment area of 1.27 ha are 0.003, 0.005, 0.008 and 0.013 m3/s, respectively. A design WQF can be chosen so that a required volume fraction of total annual runoff treated by the treatment device is achieved. Figure 4(b) shows the volume fraction of total annual runoff with respect to WQF simulated by an urban runoff model, USEPA’s SWMM in the catchment for Type B-1. As shown in Figure 4(b), 60, 75, 86 and 90% of the total annual runoff volume can be treated by the treatment device at F ¼ 60, 70, 80, and 90%, respectively. The size of a hydrodynamic separator can be reduced by one-sixth by choosing I1 h,max at F ¼ 80%, instead of F ¼ 100%, according to the calculation.
CONCLUSIONS In this study, little statistical difference was observed in the pollutant removal efficiency between the two different types of hydrodynamic separators. Wet-sieve analyses for composite runoff samples from one of the study catchments showed that the majority of the particles were in a fraction less than 75 μm, resulting in low TSS removal efficiency. Moreover, the average removal efficiencies of all measured pollutants were below ∼50% with episodic negative efficiencies, implying that a hydrodynamic separator should be more useful as a pre-treatment device for the downstream processes in a treatment train, and should be properly maintained to avoid sediment washout from the device. The settling efficiency of smaller particles should be enhanced by decreasing the surface overflow rate of the hydrodynamic separator to increase removal efficiencies of TSS and associated pollutants. Three different probability functions were fitted to numerous PSD data found in the literature and the Weibull function was considered the best function representing the
Water Science & Technology
|
69.10
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PSD in urban runoff. The hypothetical PSD function (Weibull type PSD) was applied to the discrete settling mechanism to simulate TSS removal efficiency as a function of surface overflow rate and PSD. The calculated TSS removal efficiency simulated well the non-linearly decreasing tendency of TSS removal efficiency with the increased surface overflow rate, as observed in the monitoring data from the four hydrodynamic separators. The developed curves of TSS removal efficiency as a function of PSD and surface overflow rate should be useful in determining the water quality flow and active volume of a hydrodynamic separator given the historical information on the rainfall data and catchment characteristics. Simulations using an urban runoff model (USEPA’s SWMM) showed that approximately 85% of total annual runoff can be captured by a hydrodynamic separator with a WQF corresponding to 80% probability of occurrence of the 1 h maximum rainfall intensity.
ACKNOWLEDGEMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (#2011-0009144).
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First received 24 October 2013; accepted in revised form 24 February 2014. Available online 13 March 2014
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