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Seasonal size distributions of suspended solids in a stormwater management pond
~
Pergamon
PIT: S0273-1223(99)00016-5
War. Sci Tech. Vol. 39, No.2, pp, 127-134,1999. . C J999IAWQ Published by Elsevier Science LId Printedin Greal Britain,All rightsreserved 0273-1223199 S19'00 + 0-00
SEASONAL SIZE DISTRIBUTIONS OF SUSPENDED SOLIDS IN A STORMWATER
MANAGEMENT POND B. G. Krishnappan*, 1. Marsalek*, W. E. Watt** and B. C. Anderson** • NationalWaterResearch Institute, Burlington. ON. L7R 4A6, Canada •• Queen'sUniversity, Kingston, ON, K7L3N6, Canada
ABSTRACT Three seasonal surveys of suspended solids were carried out in an on-stream storrnwater management pond, by means of a submersible laser particle size analyser. Size distributions were measured at up to 17 points in the pond. and water samples collected at the same locations were analysed for primary particles aggregated in floes . Observed suspended solids were mostly composed of floes, with maximum sizes ranging from 30 to 212 ~m for winter and summer surveys, respectively. Using a relat ionsh ip defi ning the floc dens ity as a funct ion of floc size and Stokes' equation for settling, an empirical relationship expressing the floc fall velocity as a function of floc size was produced. This relationship indicates that naturally formed floes in the size range from S to IS um would settle faster than both smaller primary particles of higher density. and somewhat larger floes of lower dens ity. which are however susceptible to break up by turbulence. @ 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Floc size; floc density; seasonal floc properties; stormwater ponds; suspended solids settling. INTRODUCTION PolIutant removal in stormwater management ponds is accomplished by physical, chemical and biological processes, including settling, adsorption by active sediments, and interception and uptake by aquatic plants, algae and fauna (Lawrence et al., 1996). Among these processes, most attention focused on settling, which was characterised by widely varying estimates (from -30 to 99%) of removal of suspended solids in ponds (Brown and Schueler, 1997). Even though some of this variation may be caused by uncertainties in the reported data, it is obvious that the design of ponds for removal of suspended solids requires more attention and good understanding of the non-quiescent settling typical for field conditions. With respect to solids settling in stormwater ponds, a dynamic settling equation (Fair and Gayer, 1954) was recommended by several authors and agencies in the folIowing form (Driscoll, 1989; Ellis, 1989; MOEE, 1994): (1)
R= 1-[1 + (lin) evs/(Q/A))"n 127
128
B. G. KRISHNAPPAN et al.
=
=
=
=
where R fraction of initial solids removed (R*lOO % removal), Vs settling velocity, Q/A inflow divided by the surface area of basin (also referred to as the surface loading rate), n = a turbulence or shortcircuiting parameter which varies from I to 00 (n I corresponds to very poor settling conditions, n 00 corresponds to ideal conditions).
=
=
However, it should be recognised that eq. (1) was originally developed for steady flow and mixing in ideal settling basins, subject to many simplifying assumptions (a rectangular settling basin with an uniformly distributed, steady inflow along the upstream edge, uniform distributions of suspended solids in the lateral direction, and particles entering the sludge zone remaining permanently removed). The applicability of this concept to stormwater ponds with unsteady flow, thermal or chemical density stratification, wind generated turbulence, natural flocculation and floc break up is hardly appropriate. The settling of suspended solids in ponds is a complex process encompassing such sub-processes as particle transport by advection and turbulent diffusion, with concomitant particle aggregation (flocculation) and disaggregation (by turbulence), and the resulting sediment deposition, or scouring (Chocat, 1997; Lau and Krishnappan, 1997). To advance the understanding of specific aspects of dynamic settling in ponds, a field study of suspended solids was undertaken in the Kingston pond. The primary purpose of this paper is to examine the seasonally varying characteristics of suspended solids in a stormwater pond, with focus on both particle sizes and density. The originality of the approach taken consists in the measurement of suspended aggregate sizes in situ, by means of a custom built laser instrument, and comparing them to the sizes of primary particles, which represent building blocks of these aggregates. A good understanding of the nature of the suspended solids in ponds is a prerequisite for developing realistic models of stormwater settling in ponds. N
!
Perking Lot
Innow 1\
II
".----"""'-........
"'_/
II
II
1\
I
Dry Pond Wet Pond
oL
25m !
Figure I. Kingston stormwatermanagement pond.
The facj!jty studied The Kingston pond (Fig. I) is an on-stream stormwater storage facility consisting of two cells; a wet cell, with an approximate surface area of 0.5 ha and a depth of 1.2 m, and a dry cell of similar surface area. The dry cell becomes flooded during periods of runoff when the water level in the pond increases by 0.2 m above the normal level corresponding to baseflow conditions. The pond was constructed in 1982 to mitigate the impacts of runoff from a newly built shopping plaza (impervious area = 12.6 ha) on Little Cataraqui Creek, which flows through the pond and ultimately drains into Lake Ontario. Upstream from the pond, the creek drains an urbanising catchment with a drainage area of 4.5 km 2. Continuing development of the creek catchment has increased its streamflow and reduced the pond effectiveness in flow control. Furthermore, the pond storage volume has been reduced by ongoing sedimentation. By 1996, about 0.25 m of sediment accumulated on the pond bottom, originating mostly from the urbanizing catchment rather than the shopping
Seasonal sizedistributions of suspended solids
129
plaza (Marsalek, 1997). A more detailed description of this facility and its performance in pollutant removal can be found in VanBuren et al. (1997). The Kingston pond has served as a research site for investigating fundamental processes used in stormwater management facilities for enhancing water quality . With reference to sediment, the earlier work of interest dealt with solids removal in the pond (Van Buren et al., 1997), physical and chemical characteristics of bottom sediment and suspended solids in the pond (Marsalek et al., 1997), and the rate of sediment deposition on the pond bottom (Marsalek, 1997). METHODS The size distribution of suspended solids in the pond was measured by a new Submersible Laser Particle Size Analyser (SLPSA), which was developed at the National Water Research Institute, Burlington, Ontario, Canada, and fully described elsewhere (Krishnappan et al., 1992). This instrument measures the size distribution of sediment floes directly in situ, and overcomes the problem of floc disruption that is ~ncountered in traditional approaches based on the collection of sediment samples and subsequent analysis 10 the laboratory. The SLPSA operates on the principle of laser diffraction, and consists of a 2mW laser, a receiving lens, a detector plate, an electronic interface (all housed in a watertight canister), and a microcomputer. The distribution of diffracted light of a laser beam passing through the water sensing volume is used to calculate the size distribution of the particles in the sensing volume by means of Fraunhofer diffraction theory (Wiener, 1984). The length of the laser beam passing through the water column is adjustable in this instrument to keep the n~mber of particles in the measuring volume at an optimum level required for producing the optimum diffraction pattern. This is achieved with the aid of the obscuration reading of the instrument, where the obscuration is defined as a measure of the fraction of the light energy that is lost due to the presence of particles. An optimum value of obscuration will ensure that measurements are not made either with too many or too few particles. An excessive number of particles in the measuring volume can lead to multiple scattering of the laser beam and an insufficient number of particles in the measuring volume would produce a scattering signal that is not strong enough to yield a statistically significant size distribution of particles. For the adjustable length of the laser beam, which varies from a few millimetres to 300 rom, suspended solids concentrations from about 5 to 500 mg-L'! can be measured fairly accurately. The particulate size measured by the SLPSA is a measure of the diameter of an equivalent spherical particle. The use of the instrument for field particles which may be of irregular and non-spherical shapes can be justified, because the size dist ribution given by the instrument is derived from a large number (1500) of "sweeps" of light energy distributions. As the particles are constantly moving through the measurement volume, it is highly probable that the particles are exposed to the laser beam in different orientations and hence the size distribution given by the instrument reflects a characteristic average size of the particles in the measurement volume. These characteristic sizes are referred to herein as particle diameters. Three seasonal field surveys of suspended solids in the pond were undertaken reflecting fall (Nov. 26, 1996), winter (Feb. 4, 1997) and summer (Sept. 16, 1997) conditions. In open water surveys, the SLPSA was deployed from an inflatable rubber boat , lowered into water so that the sensing volume was 0.5 m below the surfece, and activated for 60 s, during which the instrument made 1500 sweeps of light energy distributions. Such readings were corrected by subtracting background light energy (i.e., in water without suspended solids), which was determined as the SLPSA reading in the filtered pond water. From corrected light energy, an average size distribution of suspended solids was computed, examined on screen, and stored for later analysis. In the winter survey, the instrument was lowered through holes drilled in the ice cover. To examine spatial distributions of suspended solids properties, measurements were made at up to 17 points referenced in a grid with 15 m x 15 m cells. On a number of occasions, the instrument touched and disturbed the bottom sediment; those measurements had to be discarded. At the time of in-situ measurements, water and sediment samples were also collected at the same locations for size analysis of primary particles by a
130
B. G. KRISHNAPPAN et al.
laboratory version of the laser instrument with the same operating principle to avoid instrument bias. Before undertaking this analysis, sediment samples were thoroughly dispersed by ultrasound to break up floes. RESULTS AND DISCUSSION Weather data (including air and water temperatures, and wind speeds and directions) and basic characteristics of pond conditions during the three surveys are summarised in Table I. Table I. Field conditions during suspended solids surveys Date
Tair (C)
T water (C)
Wind speed (m-s")
Wind direction
26/11196 04/02/97 16109/97
-3.1 -1.6 18.6
4.0 2.4 16.7
2.11 1.77 1.00
282 262 1242
Pond inflow/ outflow (L·s·l) 29/35 n/a 19/24
SS (mg-L")
13.4 14.5 28.2
The median sizes of suspended solids (SS) measured during the three surveys are shown in Table 2. For brevity, and recognising the low spatial variability of the measured data, only mean values of all pond readings and the assoeiated coefficients of variation (C v) are listed. Table 2. Summary of measured D so(floe median diameter)
No. of readings Mean 0'0 (J.Ul1)
Cv
November 1996 Floes primary part. 16 17
February 1997 floes primary part.
5
IS
8.11
3.58
6.62
3.90
17.49
6.16
0.03
0.12
0.15
0.07
0.17
0.18
September 1997 floes primary part. 16 17
The results in Table 2 indicate that suspended solids in pond water represent floes (particle aggregates) of different sizes (050) observed in individual surveys. The smallest size was found in winter (under ice), just 6.62 um, with a relatively small coefficient of variation (0.15) . The next larger size corresponded to the fall survey, 0 50 8.11 urn, and finally, the summer survey produced a mean 050 of 17.5 urn . With reference to ambient conditions, the most significant difference among these surveys was pond water temperature (4, 2.5 and 16.7°C. respectively), with flows through the pond being comparable. about 20-35 Los·I, and low variation in wind conditions. During the first and third surveys, pond outflow exceeded the inflow, and the pond was draining . For the ice covered pond, flow data were not available, but low baseflow (without runoff) could be expected during that part of the year « 20 Los·I). Variation in wind characteristics among the three surveys. with potential implications for turbulence in the pond, was low. On Nov. 26. 1996, wind veloeities were 2.11 rn-sl : on Feb. 4.1997, the flow field in the ice covered pond was not affected by wind at all; and, on Sept. 16, 1997, the mean wind speed was just 1.00 mos·l.
=
Floeculation of suspended particulates is enhanced by microbiological activities, which produce polymeric substances contributing to particle bonding. Recognising that the level of such activities increases with water temperature and supply of nutrients, more floeculation can be expected during the summer, as confirmed by the Sept. 1997 pond survey producing the largest floes. Water temperatures in the fall and winter surveys differed just by 1.6°C, and the difference in the corresponding floe 0so's (6.62 and 8.11 J.UD) was not statistically significant. 050 values are also presented for primary particles, which represent building blocks of floes. As expected, these values were significantly smaller than those of floes, and ranged from 3.6 to 6.2 um , Additional data on primary particles in the pond were available from an earlier work (Marsalek et at•• 1997), in which
Seasonal sizedistributions of suspended solids
131
surficial bottom sediment was analysed by sieving and sedigraph analysis (the latter procedure derives particle sizes from their settling velocities) and indicated 050 3.17 J.1m (C; 0.14). Statistical analysis of all four mean values (i.e., 3.58, 3.90 and 6.16 J.1m listed in Table 2, and 3.17 J.1m reported earlier) indicated that the differences among the three values below 4 J.1m were not statistically significant at the 95% level of confidence, but the mean of 6.16 J.1m (Sept. 1997) was significantly larger.
=
=
Besides the mean 050 values combined for all pond stations, the distribution in space was also of interest. Comparative analysis of data from individual grid points, with reference to the point location with respect to the main flow streamline through the pond, indicated that spatial variation in primary particle sizes was random and too small to warrant further examination. November 1996
80 ~ 70-
CI Floc size distribution • Primary particle size distribution
.2 60~ 50 , ~ 4030:~ 20-10 ~
il
o.
o --
3.5 6.95 10 13.919.929.839.851.368.691.7 123 164 212
100
February 1997
80 -
40 20 -
o _- __llJ. 11- II
.D.
3.5 6.95 10 13.919.929.839.851.368.691.7 123 164 212
~
September 1997
60 -
i~:l i 1~ ~ -
I, ,[J,ll fJ
aJ
_111.1]_
0.., _ .
3.5 6.95 10 13.919.929.839.851.368.691.7 123 164 212 Geometric mean size In microns Figure2. Comparison of flocand primary particle sizedistributions at Station no.9.
Fun size distributions of suspended solids were also examined and examples of such distributions for all three surveys, and both floes and primary particles, are given in Fig. 2 (sampling station 9). These distributions indicate some similarity with respect to primary particle distributions being skewed towards small sizes, with 50 to 85% of all particles characterised by a geometric mean of 3.5 J.1m. The percentages of flocculated particles in this size category were less than 40% in all cases. The highest skew of the distributions was noted for the winter survey and confirms the low intensity of natural flocculation in cold
132
B.G.KJUS~APP~etal.
water. Summer floes were as large as 212 um (because of scaling, the small percentage value recorded in this size category is not visible in Fig. 2), as opposedto maximum floc sizes of 40 um in fall and winter,and these large floes also incorporated the largest primary particles. The summer floc sizes compared well with those reportedfor this facility earlier by Watt and Marsalek(1994). While the suspended solids size distributions are valuable for assessing the contaminant adsorption characteristics and the sediment-biota interactions, for assessing the settling characteristics of suspended solids in the pond, additional information on suspended solids density is needed. For a given particle size and density, and low particle Reynolds numbers (laminar flow conditions), the particle fall velocity, V, can be calculatedfrom Stokes' equation (Metcalfand Eddy, 1991): V
=g (Pr - P ) dr I 18 P.
(2)
where g is the acceleration due to gravity, Pr is the density of floc, p is the density of fluid, and ~ is the dynamic viscosity. In general, floc density Pr is smaller than the density of parent sediment, which is typically taken as Ps =2.65 gocm· 3. Some earlier studies of density of flocculating sediment (Lagvankar and Gemmel, 1968; Gibbs, 1985) had shown that the density of floes decreased as the size of floes increased. Lau and Krishnappan (1997) established a functional relationship between floc density and its size, for the Kingstonpond sediment, using simultaneous measurements of size distributions by two different instruments; the earlier described laser particlesize analyser and a water elutriation apparatus developed by Wallingand Woodward(1993). The elutriation apparatus comprised four connected cylindrical settling chambers arranged in series. The chamber diameter increased in flow direction, with each successive chamber having the diameter twice that of the precedingchamber. Water was drawn into the first chambernear the bottom, and dischargedfrom the top via a tube connected to the bottom of the next chamber, and so on, until it left the apparatus. The flow through this system was forced by a peristaltic pump, located downstream from the last settling chamber, and therefore not affecting the structure of floes in the apparatus. In this application (Lau and Krishnappan, 1997), pond water with suspendedsediment was drawn into the apparatus from a circular flume, rotatingat a speed adequatefor maintaining suspendedsolids in suspension. With each successive chamber doubling in diameter, the upward velocity of water decreased fourfold from one chamber to the next, and accordingto Stokes' Law, the particles which settle in one chamber should be twice the diameter of those settling in the next larger chamber, and so on. Thus, the elutriation apparatus fractionated the sediment into different size classes and providedan estimation of size distribution based on the settling velocity rather than the physical size of particles. By comparing the physical size distribution measured by the Laser Particle Size Analyser with the settling size distribution from the elutriation apparatus, a density distribution function was established by Lau and Krishnappan (1997) for the Kingston pond sedimentas: Pr - Pw =Ps exp (-bd r C)
(3)
where, Pr, Pw and P5 are densities of the sediment floc, water and the parent material (the last one measured in water) in gocm-3, respectively, dr is the floc diameter (~m), and the empirical constants b and c were determined as b = 0.02 and c = 1.85. Thus, for a selected floc size (diameter), it is possible to calculate the corresponding density from eq, (3), and after substituting the floc diameter and density into eq. (2), the floc fall velocity can be determined for laminarflow conditions. The results of such calculations are presented in Fig. 3 for pond water temperature of 25°C, and offer a conceptual insight into floc settling. As the floc increases in size from 1 to about 9 um, its settling velocity is increasing. For floes larger than 9 urn, the effects of rapidly reducing floc density outweigh the effect of increasing floc diameter, and the fall velocity diminishes quickly. Thus, floes within some range of sizes, say from 5 to 15 um, settle faster than both smaller but denser particles (resembling
Seasonalsize distributionsof suspendedsolids
133
more primary particles) and larger but less dense aggregates. The fall velocities shown in Fig. 3 could be applied to the individual floc size classes (e.g., as shown in Fig. 2) to obtain a distribution of fall velocities for the whole distribution of particulate sizes, within the limitations of Stokes' equation. Fall velocities in Fig. 3 indicate that large flocs (Df > 50 11m) would hardly settle and this was reported earlier by Watt and Marsalek (1994), who observed large floes, ranging in size from 100 to 250 um, which remained in suspension in the Kingston pond. Obviously. these naturally formed floes have much lower density than those formed by chemical aids and rapid settling. -. . -
.
~
..
3000
0.1
._- 2500 ~~
-. 2000
E
.5-
~
'i01 .... ~
1500 ~ III
0.01
8
1000
~
i
500
~
0
0.001 10
100
Particle dlamet,r D, (11m) ":"-Prim : pa rticla lall velocitY. t· 25-'0 _._Floc fall velocity. T· 2S C _Floc density . -- -" .-
. -:. --::: ~. :
- r-z sz-
Figure 3. Densitiesand Stokes'fall velocities of stormwaterparticulateof varioussizes.
The results shown in Fig. 3 are not intended for general applications to settling ponds. because the relationship between the floc density and its size is site specific. Furthermore, turbulence occurring in field installations (but not accounted for in Fig. 3) will affect both flocculation and floc break up. The primary purpose of presenting the data in Fig. 3 is to demonstrate the effect of the size-density relationship of the cohesive. suspended particles on the settling velocities and to show that large size floes may remain in suspension in actual ponds. as observed earlier by Watt and Marsalek (1994) at this facility. Further work is needed to develop models for dynamic settling in ponds. Lau and Krishnappan (1997) reported that large floes are fragile and easily broken up by turbulence. Thus, the life cycle of floes in a stormwater pond starts with aggregation of primary particles into floes, followed by the floc growth in size with a concomitant decrease in density. and if exposed to turbulence, floc break up and reduction in size. The process of floc break up by turbulence is not reproduced in settling columns, which mimic quiescent settling (Metcalf and Eddy. 1991). CONCLUSIONS Seasonal surveys of sizes of suspended solids in a stormwater pond indicate the complex nature of such materials. Suspended solids contained mostly floes of various sizes and densities. The floc median diameter. D50• varied from 6.6 to 17.s Jim. depending on ambient water temperature. Observations from three seasonal pond surveys, combined with laboratory elutriation experiments reported earlier, offered some insight into floc density, which decreased with floc size. The highest floc settling velocities were calculated for the range of sizes between 5 and 15 Jim. Larger floes would settle more slowly, but may be broken up by flow turbulence into smaller fragments, which settle readily. Smaller floes were formed by the relatively smaIl primary particles typical for baseflow conditions. but larger summer floes contained coarser primary
134
B. G. KRISHNAPPAN et al.
particles. Further research on floc dynamics is needed to advance understanding of the operation of stormwater ponds. REfERENCES Brown, W. and Schueler, T. (1997). National pollutant removal performance database for stormwater BMPs. Center for WatershedProtection,Silver Spring,MD., August. Chocat, B. (Coordinator) (1997). Encyclopedi« de l'hydrologie urbainet de l'assainissement, LavoisierlEC & DOC, Paris. Driscoll,E. D. (1989). Long term performance of waterquality ponds. In: Design of Urban RunoffQuality Control. L. A. Roesner and B. Urbonas (Eds), Proc. of Eng. Foundation Conference, Potosi, Missouri, July 10-15, 1988. ASCE. New York, pp. 145·163. Ellis. B. J. (1989). The development of environmental criteria for urban detention pond design in the U.K. In: Design of Urban Runoff Quality Control, L. A. Roesner and B. Urbonas (Eds), Proc. of Eng. Foundation Conference. Potosi, Missouri, July 10-15. 1988,ASCE, New York, pp. 14-28. Fair, G. M. and Geyer. J. C. (1954). WaterSupplyand Wasle·warer Disposal. John Wiley & Sons. New York. Gibbs. R. J. (1985). Estuarine flocs: their size, settling velocity and density. J. Geophys. Res; 90(C2). 3249·3251. Krishnappan, B. G., Madsen. N.• Stephens, R. and Ongley, E. D. (1992). Field instrument for measuring silt distribution of suspendedsediment in rivers. Proceedings 8th Congress of the IAHR. Asia and Pacific Divn., Central Water and Power ResearchStn.• Pune, India, pp. F-7I to F-81 . Lagvankar, A. L. and Gemmel,R. S. (1968). A size-densityrelationshipfor floes. J. Am. Warer Works Assoc.• 60(9).1040-1046. Lau, Y. Land Krishnappan, B. G. (1997). Measurement of size distributions of settlingfloes. NWRI Publication No. 97-223, NationalWater ResearchInstitute.Environment Canada.CCIW, Burlington. Ontario,Canada. Lawrence.A. I., Marsalek, J•• Ellis. J. B. and Urbonas, B. (1996). Stonnwater detention and BMPs. J. HydrauL Res; 34(6), 799814. Marsalek, J•• Wan. W. E.• Anderson, B. C. and Jaskot. C. (1997). Physical and chemical characteristics of sediments from a stormwatermanagementpond. WaterQual. Res. J. Canada, 32(1). 89·100. Marsalek, P. M. (1997). Special characteristics of an on-stream stormwaterpond: winter regime and accumulation of sediment and associatedcontaminants. M.Sc. thesis,Queen's University, Kingston. Ontario,Canada. Melcalfand Eddy. (1991). Wastewater engineering. Third edition,McGrawHill Inc.• New York. MOEE (Minislly of Environmentand Energy). (1994). Stormwatermanagement practices: planningand design manual. MOEE, Toronto, Ontario, Canada. Van Buren, M. A., Walt. W. E. and Marsalek, J. (1997). Removal of urban stormwater constituents by an on-stream pond. J. Environ. Plan. Mgmt., 40(1), 5-18. Walling, D. E. and Woodward.J. C. (1993). Use of a field based water e1utriation system for monitoring the in-situ particle size characteristicsof fluvial suspendedsediment. WaterRes., 27(9), 1413-1421. Watt, W. E. and Marsalek,J. (1994). Comprehensive stormwaterpond monitoring. Wat. Sci. Tech.; 29(1-2). 337-345. Wiener. B. B. (1984). Particle and droplet sizing using Fraunhoferdiffraction. In: Modern Methods of Particle Siz.e Analysis, H. G. Barth (Ed). John Wiley and Sons, New York, NY, pp. 135-172.
Pergamon
PIT: S0273-1223(99)00016-5
War. Sci Tech. Vol. 39, No.2, pp, 127-134,1999. . C J999IAWQ Published by Elsevier Science LId Printedin Greal Britain,All rightsreserved 0273-1223199 S19'00 + 0-00
SEASONAL SIZE DISTRIBUTIONS OF SUSPENDED SOLIDS IN A STORMWATER
MANAGEMENT POND B. G. Krishnappan*, 1. Marsalek*, W. E. Watt** and B. C. Anderson** • NationalWaterResearch Institute, Burlington. ON. L7R 4A6, Canada •• Queen'sUniversity, Kingston, ON, K7L3N6, Canada
ABSTRACT Three seasonal surveys of suspended solids were carried out in an on-stream storrnwater management pond, by means of a submersible laser particle size analyser. Size distributions were measured at up to 17 points in the pond. and water samples collected at the same locations were analysed for primary particles aggregated in floes . Observed suspended solids were mostly composed of floes, with maximum sizes ranging from 30 to 212 ~m for winter and summer surveys, respectively. Using a relat ionsh ip defi ning the floc dens ity as a funct ion of floc size and Stokes' equation for settling, an empirical relationship expressing the floc fall velocity as a function of floc size was produced. This relationship indicates that naturally formed floes in the size range from S to IS um would settle faster than both smaller primary particles of higher density. and somewhat larger floes of lower dens ity. which are however susceptible to break up by turbulence. @ 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Floc size; floc density; seasonal floc properties; stormwater ponds; suspended solids settling. INTRODUCTION PolIutant removal in stormwater management ponds is accomplished by physical, chemical and biological processes, including settling, adsorption by active sediments, and interception and uptake by aquatic plants, algae and fauna (Lawrence et al., 1996). Among these processes, most attention focused on settling, which was characterised by widely varying estimates (from -30 to 99%) of removal of suspended solids in ponds (Brown and Schueler, 1997). Even though some of this variation may be caused by uncertainties in the reported data, it is obvious that the design of ponds for removal of suspended solids requires more attention and good understanding of the non-quiescent settling typical for field conditions. With respect to solids settling in stormwater ponds, a dynamic settling equation (Fair and Gayer, 1954) was recommended by several authors and agencies in the folIowing form (Driscoll, 1989; Ellis, 1989; MOEE, 1994): (1)
R= 1-[1 + (lin) evs/(Q/A))"n 127
128
B. G. KRISHNAPPAN et al.
=
=
=
=
where R fraction of initial solids removed (R*lOO % removal), Vs settling velocity, Q/A inflow divided by the surface area of basin (also referred to as the surface loading rate), n = a turbulence or shortcircuiting parameter which varies from I to 00 (n I corresponds to very poor settling conditions, n 00 corresponds to ideal conditions).
=
=
However, it should be recognised that eq. (1) was originally developed for steady flow and mixing in ideal settling basins, subject to many simplifying assumptions (a rectangular settling basin with an uniformly distributed, steady inflow along the upstream edge, uniform distributions of suspended solids in the lateral direction, and particles entering the sludge zone remaining permanently removed). The applicability of this concept to stormwater ponds with unsteady flow, thermal or chemical density stratification, wind generated turbulence, natural flocculation and floc break up is hardly appropriate. The settling of suspended solids in ponds is a complex process encompassing such sub-processes as particle transport by advection and turbulent diffusion, with concomitant particle aggregation (flocculation) and disaggregation (by turbulence), and the resulting sediment deposition, or scouring (Chocat, 1997; Lau and Krishnappan, 1997). To advance the understanding of specific aspects of dynamic settling in ponds, a field study of suspended solids was undertaken in the Kingston pond. The primary purpose of this paper is to examine the seasonally varying characteristics of suspended solids in a stormwater pond, with focus on both particle sizes and density. The originality of the approach taken consists in the measurement of suspended aggregate sizes in situ, by means of a custom built laser instrument, and comparing them to the sizes of primary particles, which represent building blocks of these aggregates. A good understanding of the nature of the suspended solids in ponds is a prerequisite for developing realistic models of stormwater settling in ponds. N
!
Perking Lot
Innow 1\
II
".----"""'-........
"'_/
II
II
1\
I
Dry Pond Wet Pond
oL
25m !
Figure I. Kingston stormwatermanagement pond.
The facj!jty studied The Kingston pond (Fig. I) is an on-stream stormwater storage facility consisting of two cells; a wet cell, with an approximate surface area of 0.5 ha and a depth of 1.2 m, and a dry cell of similar surface area. The dry cell becomes flooded during periods of runoff when the water level in the pond increases by 0.2 m above the normal level corresponding to baseflow conditions. The pond was constructed in 1982 to mitigate the impacts of runoff from a newly built shopping plaza (impervious area = 12.6 ha) on Little Cataraqui Creek, which flows through the pond and ultimately drains into Lake Ontario. Upstream from the pond, the creek drains an urbanising catchment with a drainage area of 4.5 km 2. Continuing development of the creek catchment has increased its streamflow and reduced the pond effectiveness in flow control. Furthermore, the pond storage volume has been reduced by ongoing sedimentation. By 1996, about 0.25 m of sediment accumulated on the pond bottom, originating mostly from the urbanizing catchment rather than the shopping
Seasonal sizedistributions of suspended solids
129
plaza (Marsalek, 1997). A more detailed description of this facility and its performance in pollutant removal can be found in VanBuren et al. (1997). The Kingston pond has served as a research site for investigating fundamental processes used in stormwater management facilities for enhancing water quality . With reference to sediment, the earlier work of interest dealt with solids removal in the pond (Van Buren et al., 1997), physical and chemical characteristics of bottom sediment and suspended solids in the pond (Marsalek et al., 1997), and the rate of sediment deposition on the pond bottom (Marsalek, 1997). METHODS The size distribution of suspended solids in the pond was measured by a new Submersible Laser Particle Size Analyser (SLPSA), which was developed at the National Water Research Institute, Burlington, Ontario, Canada, and fully described elsewhere (Krishnappan et al., 1992). This instrument measures the size distribution of sediment floes directly in situ, and overcomes the problem of floc disruption that is ~ncountered in traditional approaches based on the collection of sediment samples and subsequent analysis 10 the laboratory. The SLPSA operates on the principle of laser diffraction, and consists of a 2mW laser, a receiving lens, a detector plate, an electronic interface (all housed in a watertight canister), and a microcomputer. The distribution of diffracted light of a laser beam passing through the water sensing volume is used to calculate the size distribution of the particles in the sensing volume by means of Fraunhofer diffraction theory (Wiener, 1984). The length of the laser beam passing through the water column is adjustable in this instrument to keep the n~mber of particles in the measuring volume at an optimum level required for producing the optimum diffraction pattern. This is achieved with the aid of the obscuration reading of the instrument, where the obscuration is defined as a measure of the fraction of the light energy that is lost due to the presence of particles. An optimum value of obscuration will ensure that measurements are not made either with too many or too few particles. An excessive number of particles in the measuring volume can lead to multiple scattering of the laser beam and an insufficient number of particles in the measuring volume would produce a scattering signal that is not strong enough to yield a statistically significant size distribution of particles. For the adjustable length of the laser beam, which varies from a few millimetres to 300 rom, suspended solids concentrations from about 5 to 500 mg-L'! can be measured fairly accurately. The particulate size measured by the SLPSA is a measure of the diameter of an equivalent spherical particle. The use of the instrument for field particles which may be of irregular and non-spherical shapes can be justified, because the size dist ribution given by the instrument is derived from a large number (1500) of "sweeps" of light energy distributions. As the particles are constantly moving through the measurement volume, it is highly probable that the particles are exposed to the laser beam in different orientations and hence the size distribution given by the instrument reflects a characteristic average size of the particles in the measurement volume. These characteristic sizes are referred to herein as particle diameters. Three seasonal field surveys of suspended solids in the pond were undertaken reflecting fall (Nov. 26, 1996), winter (Feb. 4, 1997) and summer (Sept. 16, 1997) conditions. In open water surveys, the SLPSA was deployed from an inflatable rubber boat , lowered into water so that the sensing volume was 0.5 m below the surfece, and activated for 60 s, during which the instrument made 1500 sweeps of light energy distributions. Such readings were corrected by subtracting background light energy (i.e., in water without suspended solids), which was determined as the SLPSA reading in the filtered pond water. From corrected light energy, an average size distribution of suspended solids was computed, examined on screen, and stored for later analysis. In the winter survey, the instrument was lowered through holes drilled in the ice cover. To examine spatial distributions of suspended solids properties, measurements were made at up to 17 points referenced in a grid with 15 m x 15 m cells. On a number of occasions, the instrument touched and disturbed the bottom sediment; those measurements had to be discarded. At the time of in-situ measurements, water and sediment samples were also collected at the same locations for size analysis of primary particles by a
130
B. G. KRISHNAPPAN et al.
laboratory version of the laser instrument with the same operating principle to avoid instrument bias. Before undertaking this analysis, sediment samples were thoroughly dispersed by ultrasound to break up floes. RESULTS AND DISCUSSION Weather data (including air and water temperatures, and wind speeds and directions) and basic characteristics of pond conditions during the three surveys are summarised in Table I. Table I. Field conditions during suspended solids surveys Date
Tair (C)
T water (C)
Wind speed (m-s")
Wind direction
26/11196 04/02/97 16109/97
-3.1 -1.6 18.6
4.0 2.4 16.7
2.11 1.77 1.00
282 262 1242
Pond inflow/ outflow (L·s·l) 29/35 n/a 19/24
SS (mg-L")
13.4 14.5 28.2
The median sizes of suspended solids (SS) measured during the three surveys are shown in Table 2. For brevity, and recognising the low spatial variability of the measured data, only mean values of all pond readings and the assoeiated coefficients of variation (C v) are listed. Table 2. Summary of measured D so(floe median diameter)
No. of readings Mean 0'0 (J.Ul1)
Cv
November 1996 Floes primary part. 16 17
February 1997 floes primary part.
5
IS
8.11
3.58
6.62
3.90
17.49
6.16
0.03
0.12
0.15
0.07
0.17
0.18
September 1997 floes primary part. 16 17
The results in Table 2 indicate that suspended solids in pond water represent floes (particle aggregates) of different sizes (050) observed in individual surveys. The smallest size was found in winter (under ice), just 6.62 um, with a relatively small coefficient of variation (0.15) . The next larger size corresponded to the fall survey, 0 50 8.11 urn, and finally, the summer survey produced a mean 050 of 17.5 urn . With reference to ambient conditions, the most significant difference among these surveys was pond water temperature (4, 2.5 and 16.7°C. respectively), with flows through the pond being comparable. about 20-35 Los·I, and low variation in wind conditions. During the first and third surveys, pond outflow exceeded the inflow, and the pond was draining . For the ice covered pond, flow data were not available, but low baseflow (without runoff) could be expected during that part of the year « 20 Los·I). Variation in wind characteristics among the three surveys. with potential implications for turbulence in the pond, was low. On Nov. 26. 1996, wind veloeities were 2.11 rn-sl : on Feb. 4.1997, the flow field in the ice covered pond was not affected by wind at all; and, on Sept. 16, 1997, the mean wind speed was just 1.00 mos·l.
=
Floeculation of suspended particulates is enhanced by microbiological activities, which produce polymeric substances contributing to particle bonding. Recognising that the level of such activities increases with water temperature and supply of nutrients, more floeculation can be expected during the summer, as confirmed by the Sept. 1997 pond survey producing the largest floes. Water temperatures in the fall and winter surveys differed just by 1.6°C, and the difference in the corresponding floe 0so's (6.62 and 8.11 J.UD) was not statistically significant. 050 values are also presented for primary particles, which represent building blocks of floes. As expected, these values were significantly smaller than those of floes, and ranged from 3.6 to 6.2 um , Additional data on primary particles in the pond were available from an earlier work (Marsalek et at•• 1997), in which
Seasonal sizedistributions of suspended solids
131
surficial bottom sediment was analysed by sieving and sedigraph analysis (the latter procedure derives particle sizes from their settling velocities) and indicated 050 3.17 J.1m (C; 0.14). Statistical analysis of all four mean values (i.e., 3.58, 3.90 and 6.16 J.1m listed in Table 2, and 3.17 J.1m reported earlier) indicated that the differences among the three values below 4 J.1m were not statistically significant at the 95% level of confidence, but the mean of 6.16 J.1m (Sept. 1997) was significantly larger.
=
=
Besides the mean 050 values combined for all pond stations, the distribution in space was also of interest. Comparative analysis of data from individual grid points, with reference to the point location with respect to the main flow streamline through the pond, indicated that spatial variation in primary particle sizes was random and too small to warrant further examination. November 1996
80 ~ 70-
CI Floc size distribution • Primary particle size distribution
.2 60~ 50 , ~ 4030:~ 20-10 ~
il
o.
o --
3.5 6.95 10 13.919.929.839.851.368.691.7 123 164 212
100
February 1997
80 -
40 20 -
o _- __llJ. 11- II
.D.
3.5 6.95 10 13.919.929.839.851.368.691.7 123 164 212
~
September 1997
60 -
i~:l i 1~ ~ -
I, ,[J,ll fJ
aJ
_111.1]_
0.., _ .
3.5 6.95 10 13.919.929.839.851.368.691.7 123 164 212 Geometric mean size In microns Figure2. Comparison of flocand primary particle sizedistributions at Station no.9.
Fun size distributions of suspended solids were also examined and examples of such distributions for all three surveys, and both floes and primary particles, are given in Fig. 2 (sampling station 9). These distributions indicate some similarity with respect to primary particle distributions being skewed towards small sizes, with 50 to 85% of all particles characterised by a geometric mean of 3.5 J.1m. The percentages of flocculated particles in this size category were less than 40% in all cases. The highest skew of the distributions was noted for the winter survey and confirms the low intensity of natural flocculation in cold
132
B.G.KJUS~APP~etal.
water. Summer floes were as large as 212 um (because of scaling, the small percentage value recorded in this size category is not visible in Fig. 2), as opposedto maximum floc sizes of 40 um in fall and winter,and these large floes also incorporated the largest primary particles. The summer floc sizes compared well with those reportedfor this facility earlier by Watt and Marsalek(1994). While the suspended solids size distributions are valuable for assessing the contaminant adsorption characteristics and the sediment-biota interactions, for assessing the settling characteristics of suspended solids in the pond, additional information on suspended solids density is needed. For a given particle size and density, and low particle Reynolds numbers (laminar flow conditions), the particle fall velocity, V, can be calculatedfrom Stokes' equation (Metcalfand Eddy, 1991): V
=g (Pr - P ) dr I 18 P.
(2)
where g is the acceleration due to gravity, Pr is the density of floc, p is the density of fluid, and ~ is the dynamic viscosity. In general, floc density Pr is smaller than the density of parent sediment, which is typically taken as Ps =2.65 gocm· 3. Some earlier studies of density of flocculating sediment (Lagvankar and Gemmel, 1968; Gibbs, 1985) had shown that the density of floes decreased as the size of floes increased. Lau and Krishnappan (1997) established a functional relationship between floc density and its size, for the Kingstonpond sediment, using simultaneous measurements of size distributions by two different instruments; the earlier described laser particlesize analyser and a water elutriation apparatus developed by Wallingand Woodward(1993). The elutriation apparatus comprised four connected cylindrical settling chambers arranged in series. The chamber diameter increased in flow direction, with each successive chamber having the diameter twice that of the precedingchamber. Water was drawn into the first chambernear the bottom, and dischargedfrom the top via a tube connected to the bottom of the next chamber, and so on, until it left the apparatus. The flow through this system was forced by a peristaltic pump, located downstream from the last settling chamber, and therefore not affecting the structure of floes in the apparatus. In this application (Lau and Krishnappan, 1997), pond water with suspendedsediment was drawn into the apparatus from a circular flume, rotatingat a speed adequatefor maintaining suspendedsolids in suspension. With each successive chamber doubling in diameter, the upward velocity of water decreased fourfold from one chamber to the next, and accordingto Stokes' Law, the particles which settle in one chamber should be twice the diameter of those settling in the next larger chamber, and so on. Thus, the elutriation apparatus fractionated the sediment into different size classes and providedan estimation of size distribution based on the settling velocity rather than the physical size of particles. By comparing the physical size distribution measured by the Laser Particle Size Analyser with the settling size distribution from the elutriation apparatus, a density distribution function was established by Lau and Krishnappan (1997) for the Kingston pond sedimentas: Pr - Pw =Ps exp (-bd r C)
(3)
where, Pr, Pw and P5 are densities of the sediment floc, water and the parent material (the last one measured in water) in gocm-3, respectively, dr is the floc diameter (~m), and the empirical constants b and c were determined as b = 0.02 and c = 1.85. Thus, for a selected floc size (diameter), it is possible to calculate the corresponding density from eq, (3), and after substituting the floc diameter and density into eq. (2), the floc fall velocity can be determined for laminarflow conditions. The results of such calculations are presented in Fig. 3 for pond water temperature of 25°C, and offer a conceptual insight into floc settling. As the floc increases in size from 1 to about 9 um, its settling velocity is increasing. For floes larger than 9 urn, the effects of rapidly reducing floc density outweigh the effect of increasing floc diameter, and the fall velocity diminishes quickly. Thus, floes within some range of sizes, say from 5 to 15 um, settle faster than both smaller but denser particles (resembling
Seasonalsize distributionsof suspendedsolids
133
more primary particles) and larger but less dense aggregates. The fall velocities shown in Fig. 3 could be applied to the individual floc size classes (e.g., as shown in Fig. 2) to obtain a distribution of fall velocities for the whole distribution of particulate sizes, within the limitations of Stokes' equation. Fall velocities in Fig. 3 indicate that large flocs (Df > 50 11m) would hardly settle and this was reported earlier by Watt and Marsalek (1994), who observed large floes, ranging in size from 100 to 250 um, which remained in suspension in the Kingston pond. Obviously. these naturally formed floes have much lower density than those formed by chemical aids and rapid settling. -. . -
.
~
..
3000
0.1
._- 2500 ~~
-. 2000
E
.5-
~
'i01 .... ~
1500 ~ III
0.01
8
1000
~
i
500
~
0
0.001 10
100
Particle dlamet,r D, (11m) ":"-Prim : pa rticla lall velocitY. t· 25-'0 _._Floc fall velocity. T· 2S C _Floc density . -- -" .-
. -:. --::: ~. :
- r-z sz-
Figure 3. Densitiesand Stokes'fall velocities of stormwaterparticulateof varioussizes.
The results shown in Fig. 3 are not intended for general applications to settling ponds. because the relationship between the floc density and its size is site specific. Furthermore, turbulence occurring in field installations (but not accounted for in Fig. 3) will affect both flocculation and floc break up. The primary purpose of presenting the data in Fig. 3 is to demonstrate the effect of the size-density relationship of the cohesive. suspended particles on the settling velocities and to show that large size floes may remain in suspension in actual ponds. as observed earlier by Watt and Marsalek (1994) at this facility. Further work is needed to develop models for dynamic settling in ponds. Lau and Krishnappan (1997) reported that large floes are fragile and easily broken up by turbulence. Thus, the life cycle of floes in a stormwater pond starts with aggregation of primary particles into floes, followed by the floc growth in size with a concomitant decrease in density. and if exposed to turbulence, floc break up and reduction in size. The process of floc break up by turbulence is not reproduced in settling columns, which mimic quiescent settling (Metcalf and Eddy. 1991). CONCLUSIONS Seasonal surveys of sizes of suspended solids in a stormwater pond indicate the complex nature of such materials. Suspended solids contained mostly floes of various sizes and densities. The floc median diameter. D50• varied from 6.6 to 17.s Jim. depending on ambient water temperature. Observations from three seasonal pond surveys, combined with laboratory elutriation experiments reported earlier, offered some insight into floc density, which decreased with floc size. The highest floc settling velocities were calculated for the range of sizes between 5 and 15 Jim. Larger floes would settle more slowly, but may be broken up by flow turbulence into smaller fragments, which settle readily. Smaller floes were formed by the relatively smaIl primary particles typical for baseflow conditions. but larger summer floes contained coarser primary
134
B. G. KRISHNAPPAN et al.
particles. Further research on floc dynamics is needed to advance understanding of the operation of stormwater ponds. REfERENCES Brown, W. and Schueler, T. (1997). National pollutant removal performance database for stormwater BMPs. Center for WatershedProtection,Silver Spring,MD., August. Chocat, B. (Coordinator) (1997). Encyclopedi« de l'hydrologie urbainet de l'assainissement, LavoisierlEC & DOC, Paris. Driscoll,E. D. (1989). Long term performance of waterquality ponds. In: Design of Urban RunoffQuality Control. L. A. Roesner and B. Urbonas (Eds), Proc. of Eng. Foundation Conference, Potosi, Missouri, July 10-15, 1988. ASCE. New York, pp. 145·163. Ellis. B. J. (1989). The development of environmental criteria for urban detention pond design in the U.K. In: Design of Urban Runoff Quality Control, L. A. Roesner and B. Urbonas (Eds), Proc. of Eng. Foundation Conference. Potosi, Missouri, July 10-15. 1988,ASCE, New York, pp. 14-28. Fair, G. M. and Geyer. J. C. (1954). WaterSupplyand Wasle·warer Disposal. John Wiley & Sons. New York. Gibbs. R. J. (1985). Estuarine flocs: their size, settling velocity and density. J. Geophys. Res; 90(C2). 3249·3251. Krishnappan, B. G., Madsen. N.• Stephens, R. and Ongley, E. D. (1992). Field instrument for measuring silt distribution of suspendedsediment in rivers. Proceedings 8th Congress of the IAHR. Asia and Pacific Divn., Central Water and Power ResearchStn.• Pune, India, pp. F-7I to F-81 . Lagvankar, A. L. and Gemmel,R. S. (1968). A size-densityrelationshipfor floes. J. Am. Warer Works Assoc.• 60(9).1040-1046. Lau, Y. Land Krishnappan, B. G. (1997). Measurement of size distributions of settlingfloes. NWRI Publication No. 97-223, NationalWater ResearchInstitute.Environment Canada.CCIW, Burlington. Ontario,Canada. Lawrence.A. I., Marsalek, J•• Ellis. J. B. and Urbonas, B. (1996). Stonnwater detention and BMPs. J. HydrauL Res; 34(6), 799814. Marsalek, J•• Wan. W. E.• Anderson, B. C. and Jaskot. C. (1997). Physical and chemical characteristics of sediments from a stormwatermanagementpond. WaterQual. Res. J. Canada, 32(1). 89·100. Marsalek, P. M. (1997). Special characteristics of an on-stream stormwaterpond: winter regime and accumulation of sediment and associatedcontaminants. M.Sc. thesis,Queen's University, Kingston. Ontario,Canada. Melcalfand Eddy. (1991). Wastewater engineering. Third edition,McGrawHill Inc.• New York. MOEE (Minislly of Environmentand Energy). (1994). Stormwatermanagement practices: planningand design manual. MOEE, Toronto, Ontario, Canada. Van Buren, M. A., Walt. W. E. and Marsalek, J. (1997). Removal of urban stormwater constituents by an on-stream pond. J. Environ. Plan. Mgmt., 40(1), 5-18. Walling, D. E. and Woodward.J. C. (1993). Use of a field based water e1utriation system for monitoring the in-situ particle size characteristicsof fluvial suspendedsediment. WaterRes., 27(9), 1413-1421. Watt, W. E. and Marsalek,J. (1994). Comprehensive stormwaterpond monitoring. Wat. Sci. Tech.; 29(1-2). 337-345. Wiener. B. B. (1984). Particle and droplet sizing using Fraunhoferdiffraction. In: Modern Methods of Particle Siz.e Analysis, H. G. Barth (Ed). John Wiley and Sons, New York, NY, pp. 135-172.