Environ. Sci. Technol. 1984, 18, 947-952
New Method of Wet Density and Settling Velocity Determination for Wastewater Effluent Erdogan Ozturgut
Ozturgut Oceanographics, 3006 N.E. 194th Street, Seattle, Washington
98155
J. Wllllam Lavelle
NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
A method has been developed to directly measure the wet density/size spectra of low density but settleable wastewater effluent particles. In this method particles settle to their isopycnic levels in a linear density stratified column. Then samples are drawn, and particle size distributions are determined by Coulter Counter. The wet density/size distribution of particles in a wastewater sample was measured in this manner in the 1.0-1.4 g/cm3 density range and 1.0-64-pm size range. Measurement of size and density outside these ranges permitted construction of a composite settling velocity distribution. We believe these measurements and computed settling velocities are independent of concentration because the density/size spectra of particles can be determined at concentrations low enough to preclude significant flocculation. Introduction
Many coastal communities, including major municipalities like Los Angeles, San Francisco, Seattle, Boston, and Miami discharge treated wastewater effluent directly into the marine environment through outfalls. The volume of wastewater discharged into the ocean is extremely large. During 1980 and 1981 the flow from the five largest ocean discharges in southern California averaged 1.1 X lo8 gal/day (4.1 X lo9 L/day) and contained 610 metric tons of suspended solids (1). The suspended solids discharged from municipal outfalls are the subject of considerable concern since these particles often contain pollutants or have adsorbed onto their surfaces potential contaminants. Knowledge of particle size, wet density, and settling velocity of these particles is essential for accurate prediction of their fate and effect on the marine environment. The particle size distributions determine their potential for ingestion by filter-feeding organisms and their specific surface area for pollutant adsorption and desorption processes and for attachment by bacteria and are important to particle size dynamics, i.e., aggregation and disaggregation. The size and wet density distribution permite the computation of settling velocity distributions and mass flux. The wet density distribution of effluent particles also specifies the fractions of particles that will remain afloat, will be neutrally buoyant, or will raft on density interfaces. The particle settling velocity determines the spatial and temporal extent of the wastefield, the length of time that a particle is available for interaction with the biosphere and for exchanging contaminants with the receiving water, and the fraction that will fall out at various distances from the discharge point (e.g., see ref 2 and 3). The available settling velocity estimates ( 4 4 9 , all made with Southern California discharge, are summarized by Faisst (4). He shows median settling velocities of effluent varying over 3 orders of magnitude. It is not surprising then that estimates of the mass of effluent solid reaching the local sediments within 16 km of one outfall vary by 0013-936X/84/0918-0947$01.50/0
981 15
a factor of 10 (9). Until the present time, no information has been available on the wet density/size spectra of wastewater particles. Furthermore, conventional methods of settling velocity determination of effluent particles provide estimates of questionable accuracy (10). The reason for this inaccuracy is that flocculation (aggregation, coagulation) occurs in the settling column during the experiments (4-6, 10). For many years, it has been known that fine particles flocculate in a weak electrolyte (e.g., see ref 11and 12) and flocs settle at a more rapid rate than their constituent particles. Particle agglomeration mediated by particle organic coating is also recognized (e.g., ref 13). Flocculation depends both on the concentration and on the size and settling velocity distribution of particles; it also depends on the shear, turbulence, and, for small particles, the molecular motion of the fluid. Observations of concentration-dependent settling using large concentrations of particles which settle rapidly is often interpreted as evidence of floc formation. Morel and Schiff (10) argue that the conventional settling velocity experiments are actually coagulation kinetic experiments. Measurements on wastewater samples heretofore have not been possible at concentrations low enough to preclude flocculation. The primary objective of this study was to examine the feasibility of an alternative way of estimating the settling velocity spectra of wastewater effluent. A secondary objective was to characterize the size and wet density spectra of effluent. Using techniques new to this problem, we directly measured the wet density/size spectra of particles from the METRO wastewater treatment plant in Seattle, WA. From these results, the settling velocity distribution of the particles has been computed. These procedures permit measurement of wet density (pa) and size (d) spectra of particles in the density range 1.0-1.4 g/cm3 and diameters of 1.0-64 pm at concentrations of several milligrams per liter only; at these very low concentration levels, flocculation during the measurements is not likely to be significant. Materials and Methods
The wet densities of wastewater effluent particles are determined by measuring their buoyant density. Effluent particles in suspension are introduced into a linear-density stratified column. The medium used in preparation of the column is an inorganic salt solution that is nonreactive with the particles and has a low viscosity and neutral pH. The criteria for selection of the density gradient medium and details of the linear-density gradient preparation are the subject of another paper (14). Figure 1 shows the experimental scheme for preparation of the linear-density stratified column; Figure 2 shows the density stratification in the settling column during this experiment, extending to p = 1.4 g/cm3. This density limit can be increased. A 24-h composite wastewater effluent sample for this study was obtained from the Municipality of Metropolitan
0 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 12, 1984 947
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Seattle (METRO), a primary sewage treatment plant at West Point, Seattle, WA (courtesy of R. D. Tomlinson of METRO). Sodium azide was added to the sample to retard bacterial action. A total of 500 mL of the sample was diluted with 500 mL of 3.5% by weight sodium chloride solution ( p = 1.02 g/cm3) and mixed gently, and then the particles settled in a container 12 cm high. After 77 h the top 10 cm of the solution was removed; in this paper, this portion of the sample will be referred to as the supernatant fraction. The material in the lower 2 cm of the container (28 mg of solids in 150 mL of fluid) consisting of one-sixth of the entire sample and particles that settled to that depth during 77 h was introduced gradually (over a period of several hours, in units of 25 mL) with a wide bore (3 mm) pipet into the top of the density stratified column (6.25 cm inside diameter, 86 cm height). A t any point in the settling column concentrations were kept low by the manner of introduction so that flocculation within the column would be minimized. This manner of introduction contrasts sharply with the conventional method of uniformly mixing the particles over the entire sedimentation column. After introduction of the sample into the column, approximately 171 h was allowed for the particles to settle. 948
This settling period was not sufficient for all of the particles to reach their equilibrium levels. Subsequent computations considering the decrease in the particle density differential with depth in the settling column indicated that some particles with d < 4 pm, depending on their densities, might not have reached their isopycnic levels. However, since these particles (d < 4 pm) constituted less than 7% by volume of the particles introduced, we have assumed for analysis purposes that equilibrium had been reached. To avoid this in future studies, a shorter column or longer settling time can be used, and the effluent sample can be better fractionated before introduction into the column. After 171 h, fluid samples were withdrawn slowly with a wide bore siphon starting from the top and at 2.5-cm intervals down to within 1 cm of the bottom. Each sample was stirred gently, and the fluid density (equal to the density of the particles in it) was determined by a hydrometer (graduations every 0.002 g/cm3). Particle size and number concentration were measured with a Model TA I1 Coulter Counter, which measures the volume of electrolyte displaced by a particle as it passes through a small aperture. Procedures for calibration, instrument operation, and sample handling followed those outline by Kranck and Milligan (15). Results from the two highest and two lowest channels were discarded to suppress noise. Both 50- and 200-pm aperture tubes were used, which allowed spherical particle diameters of 1.0-16 and 5-64 pm to be determined. Stirrer speed was kept low to minimize particle breakup (e.g., see ref 16 and 17); when necessary, particle concentrations were diluted to avoid problems of coincident passage of several particles through the aperture.
Environ. Sci. Technol., Vol. 18, No. 12, 1984
The concentration of solids in the entire sample before dilution, retained on a 0.2-pm Nucleopore filter, was 80.0 mg/L. The concentrations, of suspended solids in the supernatant and settled fractions were 24.9 and 55.1 mg/L, respectively. The suspended solids in the supernatant fraction represent either particles with settling velocities less than 3.6 X cm/s (settling 10 cm in 77 h) or buoyant particles with wet densities less than or equal to the density of the ambient fluid (1.01 g/cm3). Wet sieving of the entire sample with a 64-pm mesh sieve showed that larger particles had a mass of 6.8 mg/L, 8.5% of the entire sample by weight. Density separation of this material (d > 64 pm), indicated that 10.3% by weight were flotables (pa < 1.02 g/cm3), 6.6% had 1.2 < pa < 1.4 g/cm3, and 83.1% had pa > 1.4 g/cm3. Particles with d > 64 pm and pa > 1.4 g/cm3 would have settling velocities in excess of 1 X lo-' cm/s and would be deposited in the immediate vicinity of the discharge point. The remainder of this study is concerned only with those effluent particles sized by Coulter Counter (d < 64 pm). The size analysis of the supernatant and of the settled fractions and that of the entire sample (Figure 3) are based on a minimum of four Coulter measurements of each sample, after editing spurious data. The supernatant distribution is skewed toward fine particles, whereas the settleable fraction distribution is dominated by a peak at larger diameters. The settled fraction which had been introduced into the density stratified column was sampled, and particle size distributions were determined (Figure 4A-C). A common feature of the density/size spectra is the presence of a mode at the 16-32-pm diameter range, characteristic of the density undifferentiated sample (Figure 3). The total contribution of the particles comprising this mode is best
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Figure 3. Particle volume distribution of the supernatant (-), settleable fraction (.e.), and entire sample (---) of the wastewater effluent,
illustrated by a contour map of percent distribution (Figure 4D). It is noted that nearly 60% of the total volume of particles with pa < 1.4 g/cm3 is contributed by these particles. The particles at low density and small diameters are supernatant particles that are not completely screened by our separation process. The size distribution of particles with pa 2 1.4 g/cm3 is not shown because it is similar to the original size spectrum of the settleable material (figure 3), except that there are fewer particles with d < 4 pm. Total particle volumes and volume fractions were calculated for each density level (Table I). The volume fractions of supernatant and light density (1.02 < pa < 1.4 g/cm3) and heavy density particles (pa 3 1.4 g/cm3) were 0.27,0.33, and 0.40, respectively.
The validity of size distributions for particles that may be subject to breakage during the size-counting process must still be established. The effect of high shear on particles near the sensing zone of the Coulter Counter (18) and subsequent particle breakage during analysis has been reported for artificially aggregated clays (16,17)and for some natural flocs (16). Kranck and Milligan (15) on the other hand, reported no significant particle breakage of natural suspended matter during Coulter analysis. We conducted a test on effluent particles to examine this question. The size distribution of an effluent sample diluted 1:lOO with salt water was determined by using both 50- and 200-pm aperture tubes. The sample, after passage through each aperture, was recovered after the count and recounted. During the count, particularly with the smaller aperture, large particles that could not pass through but which blocked the aperture had to be brushed back into the sample. The resulting particle size distributions of the initial count and of the recovered samples are similar (Figure 5A) except for a slight increase in the number of particles with 1.6 < d < 3.2 pm. If there had been substantial particle breakage, the shape of the size distribution of the recovered sample would be significantly different. Calculations for both distributions demonstrate volume conservation. We have therefore concluded that the size distributions presented above have not been significantly affected by particle breakage during Coulter analysis. The strength of the particles, if agglomerates (13),was investigated by subjecting the initial sample after 1:lOO dilution in salt water to insonification for 20 min. Figure 5B shows the comparison of size distribution of particles before and after the ultrasonic treatment. The results indicate that particles contributing to the mode at 20 pm
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Figure 4. (A-C) Size spectra of wastewater effluent particles with their wet densities indicated by the different symbols. (D) Percent volume distribution of the particles in the density range from 1.0 to 1.38 g/cm3 and the size range from 1.0 to 64 pm, The contour levels are in percent, and the dots show the data points used.
Environ. Sci. Technol., Vol. 18, No. 12, 1984
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Table I. Fractions of Wastewater Particles by Density"
fraction
density, g/cm3
supernatant settled light density
51.01 1.02-1.03 1.03-1.05 1.05-1.07 1.07-1.10 1.10-1.12 1.12-1.15 1.15-1.17 1.17-1.20 1.20-1.24 1.24-1.30 1.30-1.35 1.35-1.38 1.4 g/cm3) which constituted 40% by volume of the solids in the sample. This is likely to be inorganic terrigenous material originating in part as urban surface runoff. Typical material and densities in street surface solids at other treatment plants (22) include (in g/cm3) cement (2.7-3.0),brick (1.4-2.2),clay (1.8-2.6), silica (2.1-2.2),glass (2.4-2.8),and metals (Cr, Cu, Fe, etc., 7-9.0). According to one estimate for Chicago discharge (23)85% of the solids with p > 1.4 g/cm3 were rock and glass ( p = 2.6 ghm3). It seems, therefore, reasonable to expect that most of the particles in the heavy fraction will have densities between 1.4 and 2.65 g/cm3. A third uncertainty comes from the meaning of particle volume for an aggregated particle. Treweek and Morgan (24)show that the Coulter Counter does not measure pore volume but only the volume of solids. Since pore volume should be included in determining settling velocity, the equivalent spherical diameter given by the Coulter Counter may be an underestimation: the higher the porosity, the larger the error in estimation. However, we think that this instrumental limitation will not seriously bias these results. This conclusion is based on the data in Figure 5B, which showed tightly bound agglomerates (low porosity) and volume conservation after insonification. For the data shown in Figure 4A-C, we computed the settling velocity distribution using the measured particle mean density for each increment and particle size distribution (Figure 6). For the particles with pa 2 1.4g/cm3 and d 64 pm, we computed two settling velocity spectra using the measured size distribution and densities of 1.4 and 2.65 g/cm3 (Figure 7). At a given diameter, pa of 1.4
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SETTLING VELOCITY (cm/s) Figure 8. Composite settling velocity distribution of the wastewater effluent sample. The distribution is prepared by incorporatlng the appropriate contributions of the supernatant, the light fraction (less than 1.4 g/cm3), and the heavy fraction (greater than 1.4 g/cm3) assuming densities for the latter of 1.4 (0)and 2.65 g/cm3 (0).
and that of 2.65 g/cm8 result in a 4-fold difference in settling velocities. Particles in the supernatant, as stated earlier, had settling velocities less than 3.6 X cm/s. Figure 8 shows the cumulative distribution of settling velocity for the entire sample. This composite distribution is the sum of the supernatant fraction, the material with 1.02 < pa < 1.4 g/cm3, and the material with 1.4 < pa < 2.65 g/cm3. Particles not sized by the Coulter Counter (8.5% by weight) are not part of this distribution. In this figure, the initial point at 4 X cm/s and -31% represents the sum of the contributions of the supernatant, 27.1% and of the denser material, -4%. The largest settling velocity inferred is 1 X 10-1cm/s. Solid particles with settling velocities greater than (4-5)X cm/s are not intended for release because solids in the influent are allowed to settle 2.7-3.0 m in the primary sedimentation tanks for 11/2-2h prior to discharge (25).
Conclusions A new method to measure wet densitylsize spectra of wastewater particles in the density range 1.0-1.4 g/cm3 and the size range 1-64 pm has been presented. To our knowledge, no density data other than bulk density have been available for wastewater effluent. This information is important in assessing what fractions of the discharge Environ. Sci. Technol., Vol. 18, No. 12, 1984 951
will raft on ocean density surfaces (e.g., see ref 26). Together the wet densityfsize spectra have been used to calculate a settling velocity spectrum for the fraction of the discharge with diameters less than 64 pm (91.5% sieved weight). In contrast to conventional methods, the method we describe permits working at particle concentrations of a few milligrams per liter. In our experiments it was found that the mass concentration of particles employed was considerably more than needed. Because of the sensitivity of the Coulter Counter, we estimate that initial concentrations of 1-2 mg/L can be analyzed. Furthermore, the concentrating procedure, i.e., the separation of settleable and supernatant, might also be avoided. Working at low concentrations (1)reduces the probability of significant flocculation which makes conventional techniques unsuitable and (2) permits analysis of effluent samples after discharge near an outfall. Comparison of the wet densityfsize spectral characteristics of the effluent, before and after discharge, will provide data to establish the significance of flocculation (agglomeration) in the natural environment where concentrations and fluid shear are quickly reduced away from the diffuser. Measurements of settling characteristics of effluent after discharge have never been made. Much variability in wet densityfsize and settling spectra can be expected for different outfalls, as discharge characteristics will vary from site to site, as well as over time at one site. Seasonal and even daily variations will depend on the character of the influent (e.g., episodic storm runoff) and the treatment plant procedures. Understanding this variability will be important to accurately quantify the dispersal of effluent in marine waters.
Acknowledgments This research was conducted at Ozturgut Oceanographics laboratory without any outside funding.
Literature Cited (1) Schaffer, H. A. “Characteristics of Municipal Wastewaters”. Southern California Coastal Research Project, Los Angeles, CA, 1983, 1981-1982 Biennial Report, p p 11-16. (2) Koh, R. C. Y. Environ. Sci. Technol. 1982, 16, 757-783. (3) Hendricks, T. J. “Numerical Model of Sediment Quality Near an Ocean Outfall”. Southern California Coastal Research Project Authority, Los Angeles, CA, 1983, fiial report to NOAA. (4) Faisst, W. K. In “Particulates in Water”; Kavanaugh, M. C.; Leckie, J. O., Eds.; American Chemical Society: Washington, DC, 1981; Adv. Chem. Ser. No. 189, pp 259-282.
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(5) Morel, F. M. M.; Westall, J. C.; O’Melia, C. R.; Morgan, J. J. Environ. Sci. Technol. 1975, 9, 756-761. (6) Herring, J. R. In “Particulates in Water”; Kavanaugh, M.
C.; Leckie, J. O., Eds.; American Chemical Society: Washington, DC, 1981; Adv. Chem. Ser. No. 189, pp 283-304. (7) Brooks, N. H. “Settling Analyses of Sewage Effluents”. Los Angeles, CA, 1956, memorandum to Hyperion Engineers.
(8) Myers, E. P. Ph.D. Dissertation, California Institute of Technology, Pasadena, CA, 1974. (9) NACOA “The Role of the Ocean in a Waste Management Strategy”; National Advisory Committee on Oceans and Atmosphere: Washington, DC, 1981. (10) Morel, F. M. M.; Schiff, S. L. In “Ocean Disposal of Municipal Wastewater: The Impact on Estuary and Coastal Waters”; Myers, E., Ed.; M.I.T. Sea Grant Program: Cambridge, MA, 1983; p p 249-421. (11) Whitehouse, N. G.; Jeffrey, L. M.; Debrecht, J. D. In “Clay and Clay Minerals”; Swineford, A,, Ed.; Pergamon Press: New York, 1960; pp 1-79. (12) Krone, R. B. “A Study of Rheologic Properties of Estuarine Sediments”. University of California, Berkeley, 1963, Hydraulic Engineering Laboratory Report 63-8, (13) Zabawa, C. F. Science (Washington,D.C), 1978,202,49-51. (14) Ozturgut, E.; Lavelle, J. W., unpublished results. (15) Kranck, K.; Milligan, T. “The Use of the Coulter Counter in Studies of Particle Size Distributions in Aquatic Environments”; Bedford Institute of Oceanography Report Series: Dartmouth, Nova Scotia, 1979. (16) Gibbs, R. J. J . Sediment. Petrol. 1982, 52 (2), 657-660. (17) Hunt, J. R. In “Particulates in Water”; Kavanaugh, M. C.; Leckie, J. O., Eds.; American Chemical Society: Washington, DC, 1981; Adv. Chem. Ser. No. 189, pp 243-257. (18) Hannah, S. A.; Cohen, J. M.; Robeck, G. G. J.-Am. Water Works Assoc. 1967,59, 843-858. (19) McCave, I. N.; Jarvis, J. Sedimentology 1973,20,305-315. (20) Batchelor, G. K. “An Introduction to Fluid Dynamics”; Cambridge University Press: Cambridge, England, 1967. (21) Lerman, A. “Geochemical Processes: Water and Sediment Environments”; Wiley: New York, 1979. (22) Chemical Rubber Company “Handbook of Environmental Control: Wastewater Treatment and Disposal”; CRC Press: Cleveland, OH, 1974; Vol. IV. (23) Klemetaon, S. L.; Keefer, T. N.; Simons, R. K. “Movement and Effects of Combined Sewer Overflow Sediments in Receiving Waters”; U.S. Environment Protection Agency: Cincinnati, OH, 1980; EPA 600/2-80-126. (24) Treweek, G. P.; Morgan, J. J. J. Water Pollut. Control Fed. 1979, 51, 1859-1877. (25) Uchida, B., METRO, Seattle, WA, personal communication, 1983. (26) Ozturgut, E.; Lavelle, J. W. Mar. Environ. Res. 1984, 12, 127-142.
Received for review January 11, 1984. Revised manuscript received May 17, 1984. Accepted July 25, 1984.
