Particulates in Water - American Chemical Society

14 Use of Particle Size Distribution Measurements for Selection and Control of Solid/Liquid

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Separation Processes M I C H A E L C . K A V A N A U G H , C A R O L H . T A T E , A L B E R T R. T R U S S E L L , R. R H O D E S T R U S S E L L , and G O R D O N T R E W E E K James M . Montgomery, Consulting Engineers, Inc., Walnut Creek, C A 94596, and Pasadena, C A 91101 Particle size distribution data for aqueous particulates larger than 1 μm in several freshwater and wastewater sys tems are shown to be modeled accurately with a two -parameter power-law function d N / d l = Al , the exponent βproviding an estimate of particulate contributions by size to the total particulate number, surface area, volume and mass concentration, and light-scattering extinction coeffi cient. The power-law exponent for several particulate sys tems computed from particle size distribution data deter mined by a variety of particle counters ranges from 1.8 to 4.5. A methodology for selection of solids/liquid separation processes in water treatment applications based on the use of size distribution data is demonstrated. Applications of data on total particle count, and various statistical parame ters of the particle size distribution are shown for evaluation of pilot plant studies, and process control of particulate separation processes. When the power-law exponent is greater than three, submicron particles, which are not measured by particle counters, will control the magnitude of total count, surface area, and light-scattering extinction coefficient. In this case, measurements of both the particle size distribution and the particulate light-scattering charac teristics are recommended. -β

n p h e major objective of water treatment facilities is to provide a finished product at a reasonable cost, meeting specified standards irrespective of the source water quality. Achieving this objective requires engineered 0-8412-0499-3/80/33-189-305$06.00/0 © 1980 American Chemical Society

Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.


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PARTICULATES I N W A T E R

facilities capable of removing materials ranging i n size from those i n true solution to coarse suspensions. The particulate fraction of these constituents is defined here to include both colloidal materials (1 n m 1 /xm) and coarse suspensions (greater than 1 /xm), a range covering approximately seven size decades. Because many undesirable inorganic and organic constituents are associated with this particulate fraction, system design for particulate removal assumes a key role i n facilities plan ning for potable water treatment, wastewater treatment, industrial water and wastewater treatment, and water reuse systems (30). Process design of solids/liquid separation processes consists of four principal elements: the selection of technically feasible process alterna tives, the selection of design criteria for each process, selection of the optimum process combinations, and selection of process control strate gies. Knowledge of the physical, chemical, and biological properties of the particulates as a function of size would provide a rational basis for selection of process alternatives. However, such data are rarely available for fresh water systems ( 7 ) , and determination of this information is likely to be prohibitively expensive as evidenced by the level of effort required to characterize particulates i n the world's oceans (4,10,11). Thus until recently, process selection for solids/liquid separation prob lems has been based on easily measured characteristics of the particulate fraction including turbidity (principally 90° scattered light) and gravi metric determinations "(total suspended solids). Recent developments i n the technology of aqueous particulate size distribution ( P S D ) measurements, however, provide the process engineer with rapid methods for determining the number densities and size distri bution of particulates with at least one dimension greater than about 1 /xm. F o r this size fraction, such data provide a direct measure of par ticulate counts based on a specified statistical length, and an estimate of the total mass or surface area i n the particulate fraction above 1 /xm. It also appears that particulate counting instruments may be more sensitive monitoring devices of particulate removal processes than the traditional turbidity instruments. Thus, particle size distribution measurements could provide a more accurate basis for decisions i n the selection, design, and operation of solids/liquid separation processes. This chapter w i l l present a number of examples to support this contention.

Measurement of Particle Size Distributions Measurement of aqueous particle size distributions presents a num ber of challenges to the analyst because of the heterogeneous characteris tics of particulates. Shape, density, refractive index, and other physical properties are usually nonuniform throughout the six- to seven-decade



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size range of the particulate fraction. N o single measurement technique is able to measure the particle size distribution over this wide size range. These measurements are further complicated by changes i n the distribu tion attributable to particulate aggregation or breakup during sampling, sample preparation, and in the sensing zone of the counting instrument. Many techniques suffer from precision and accuracy problems, and the statistical reliability of the data must be evaluated continually. The principal techniques for particle size distribution measurements in natural waters are compared in Table I with respect to measurable size ranges and detection limits, sample handling, and ease of measure ment. Additional details on these techniques are found i n commercial literature or in texts on particle size measurements (for example, Ref. 1). The electron microscope is the only instrument capable of measuring the size of particles in the colloidal range (1 n m - 1 /xm). Both the trans mission and scanning electron microscopes require sample evaporation and coating of the particulates with carbon or gold. Thus, sample prepa ration can alter the original particle size distribution, unless freeze-drying techniques are used. In addition, size data must be determined manu ally, so these measurements may require up to 16 hr to complete. As a result, electron microscopy is used principally to measure the concentra tion of trace inorganic particulates in water, such as asbestos fibers (6). Particle size distribution determination with the optical microscope is also a time-consuming technique, requiring up to 8 hr, depending on sample-handling requirements and the spread of the particulate size distribution. Automatic image analyzers for particle size distribution measurements of aqueous particulates have had limited success because of sample preparation problems, inaccurate counting resulting from mul tiple counts, and the high cost of equipment (19). Because of the small sample volumes scanned at any given magnification, detection limits are low. Despite these limitations, the optical microscope remains the essen tial technique for instrument calibration and determination of particulate shape factors. F o r nonfibrous particulates with one dimension greater than 1 /xm, the electrical sensing zone method (Coulter Principle) can reduce the total time required for particle size distribution determinations to less than 1 hr, depending on the size range of the distribution. In this tech nique, particulates suspended i n a conducting solution (approximately 0.9% N a C l or its equivalent) are pumped through a small orifice through which a current is flowing. E a c h particle produces a pulse proportional to particle volume attributable to the change i n electrical resistance i n the orifice passage. The appropriate software provides a tabulated or graphical output of the particle size distribution. The particle size dis tribution of suspensions with a dynamic size ratio of less than 20 can be measured in less than 5 min.


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Table I.

Techniques for Measuring

Size Limits


Measuring Principle

Equivalent Size Measured

Minimum (pm)

R

a

Electron microscopy

statistical length

0.001

50-200

Optical microscopy

statistical length

0.3

40

Electrical sensing zone volume diameter method (Coulter principle)

1

20

Light-scattering low-angle, forward-scattering laser light source

cross section diameter

1

10-50

Light obscuration

cross section diameter

1

50-60

R — ratio of maximum to minimum size for single sensing element, or single magnification. Speed depends on spread of distribution, number of size intervals required to characterize suspension; assumes P S D with range from 2 to 100 /xm; includes sample preparation and instrument time. a

b

This technique was developed originally for counting blood cells which are homogeneous i n shape and of a narrow size range. Particle size distribution measurements of heterogeneous particulate suspensions with this technique are limited because of particle clogging of the sensor orifice and particle breakup. The recommended range for each orifice is approximately 2-40% of the orifice diameter ( I ) . Because most natural particulate suspensions contain particles with sizes up to 50 /xm or more, two to three orifices must be used with appropriate sample handling to avoid clogging and breakup in the smaller (30 and 70 /xm) orifices. Frac tionation of the distribution by serial filtration can be used, but the accuracy of this procedure is questionable (8). In contrast to the previously mentioned techniques, some optical sizing methods offer the possibility of on-line process monitoring and control. Instruments designed on the basis of light scattering and light obscuration, using various light sources, analyze from 5 to 1000 m L of liquid sample, and thus exhibit improved detection limits compared to the electrical sensing zone method. Instruments using the light obscura tion principle employ various sensors, each with a size measurement ratio (maximum to minimum size) of about 60. Thus the particle size distri bution of a typical aqueous suspension with a size range from 2 to 100/xm could be determined using only one sensor. This reduces the time required for a single measurement to less than 10 min. W h e n par ticulate densities exceed about 5000 m L " , samples must be diluted to 1


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Solid/Liquid

Aqueous Particle Size Distributions Typical Volume per Scan (mL)


1000X-10" 100X-10"

2

0.05-2 10-1000

(PSD)

Sample Handling

5

,

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Separation Process

Time Required for Typical PSD in Natural Waters*

evaporation, carbon coating

8-16 hr

concentration may be required

4-8 hr

sample must contain 0.9% salt solution; scalping may be necessary

45 min 15 min

dilution to reduce coincidence errors

I

10 min 5-100 Depends on magnification, sample preparation, size range of interest.

1

J

avoid coincidence errors. Thus the principal application of these tech niques w i l l be monitoring the effluent quality from solids/liquid separa tion processes rather than influent water quality (3,32). Size Distributions in Natural Waters Compared to the extensive data describing the ocean particulate (10, 11), size distribution data on particulates i n fresh water systems and wastewaters are relatively scarce. Particle size distribution data for sev eral low ionic strength solutions are shown i n Figure 1 with the water source, particulate counting method, and references as noted. The size frequency distribution of the four heterogeneous suspensions shown can be modeled by a two-parameter power-law distribution function (2) given by the expression

§

3, the total surface area concentration of the particulate fraction resides predominately in the fine-size fractions below 10 /xm. Control of the concentration of an adsorbed contaminant would thus require removal of fine-sized particles by appropriate solid/liquid separation processes. If, however, /3 < 3, removal of coarser particulates (1 > 10 /xm) may satisfy the effluent standards. The selection of solids/liquid separation processes w i l l thus depend, in part, on the shape of the size distribution function, as reflected by the value of Process Selection W h e n the required particulate removal efficiency has been deter mined for each size fraction i n the distribution, the technical feasibility of available solids/liquid separation processes must be evaluated. Such a screening procedure w i l l provide the framework for determining the extent of analytical modeling or pilot plant studies needed to develop process design criteria. The solids/liquid separation processes used in water and wastewater treatment exploit the physical-chemical properties of the particulates to achieve rapid and therefore economical separation from the treated water. Microscreening devices remove most particulates with at least one dimension larger than the minimum opening in the



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315

Separation Process

screen. The size and density of particulates determine whether gravity separation by sedimentation or flotation is an economical process alterna tive. The efficiency of granular-media filtration depends on the size, density, and surface-chemical properties (stability) of the particulates (25). W h e n these alternatives are ineffective, modification of the surface properties and/or size distribution can be achieved using coagulation/ flocculation. A suggested screening method for evaluating alternative separation processes is illustrated in Figure 4. The three variables plotted are the total number concentration iV , the mass concentration M, and the num ber volume mean size, Z , which are related by the expression T

NV

M = C P

3

I

N V

3

iY

(6)

T

where p is the average particulate density, selected as 1020 k g m " for 3

COLLOIDAL

3, particulates i n the colloidal range (10 n m - 2 txm) control the magnitude of the extinction coefficient and are thus responsible for the decrease in the transmitted light. Because present counters cannot detect particles smaller than about 1-2 /xm, light-scattering measurements would be a more sensitive indicator of changes in the particle size distribution when p > 3. H o w ever, when p < 3 the particle counters would detect the particulate fractions dominating the magnitude of the extinction coefficient. A comparison of the sensitivity of a light-blockage particle counter ( H I A C Model 320) and a nephelometric instrument was carried out during recent investigations conducted by the authors (16) on the appli cability of granular-media filtration for treatment of the Columbia River at Kennewick, Washington. A variety of influent turbidities was used to simulate fluctuations in the turbidity levels in the river. In one test series, an influent turbidity of 40 T U was prepared from inorganic material found in the river sediments. Suspensions were fed to the dual-media granular filter at a flow rate of 15 m/hr with coagulants of 5 m g / L alum and 1 m g / L cationic polymer. F r o m the results shown in Figure 9, effluent particulate counts began to increase several hours before the effluent turbidity increased. The filter effluent history for five size classes between 2.5 and 100 /xm is shown in Figure 10. As can be seen, particulates between 2.5 and 40 /xm ap peared in the filter effluent prior to the breakthrough of the larger ( > 40 /xm) particulates. Analysis of the effluent particle size distribution before and after the turbidity breakthrough shows a shift in the power-law exponent from approximately 2.5 to 3.5, with least-squares regression coefficients greater than 0.95 for the power-law fit. As shown in Figure 8, with p < 3 particulates larger than 5 /xm dominate the extinction coefficient and thus control the amount of trans mitted light. F o r particles i n this size range, light scattered at 90° to the incident beam is negligible compared to forward scattered light (20). Thus, the light-blockage instrument, which senses particulates by the change in transmitted light, would be expected to exhibit greater sensi tivity than nephelometric measurements when ft < 3 and large particles ( > 5 /xm) dominate the extinction coefficient. These results have important implications for process control of solids/liquid separation processes. Because many suspensions in natural waters have particle size distributions with p > 3 (see Table I I ) , pres ently available particle counters w i l l not detect particulates that dominate the surface area concentration. Light-scattering devices may be needed to monitor the influent raw water supplies. However, if p values are less than 3 in effluent streams from solids/liquid separation processes, particle counters could be used for process monitoring and ultimately, process control.



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FILTER

325

R U N T I M E IMINSI

Figure 9. Effluent history with respect to particulate volume, number count, and turbidity. Pilot studies of direct filtration, Columbia River water; influent turbidity = 40 TU; influent particulate volume = 6 X 10' l/L; flow rate = 15 m/hr.

4

TIME

Figure 10.

[HRS]

Effluent history of four particulate size classes compared to turbidity (experimental conditions as in Figure 9)


326



Summary Reduction of aqueous particulates is a major objective of water and wastewater treatment plants to meet aesthetic, health, and toxicity-based standards. Selection and design of particulate separation processes to meet these standards traditionally have been based on easily measurable collective parameters characterizing the particulate fraction, such as 90° scattered light (turbidity) or total suspended solids. Such measurements, though useful for some applications, correlate poorly with particle num ber counts or parameters of the particulate size distribution, data that could significantly improve the reliability of process selection and process monitoring. Previous methods for determining the particulate size distri bution were time-consuming, and not suitable for on-line measurements. However, particle counters now available based on light-scattering or light-blockage principles may provide rapid and accurate measurements of the particle size distribution for many particulate systems in natural waters with at least one dimension greater than 1 /xm. Particulate size data reported for numerous particulate systems in natural waters can be modeled accurately with a two-parameter power law, given by dN/dl = AZ~ . The exponent of the power law, has been shown to be a useful estimator of the relative contribution of particulate size classes to the total number, surface area, mass and volume concen tration, and extinction coefficient of the particulate fraction. Reported values of /? range from 1.8 to 4.5 in low ionic-strength solutions. 3

F o r most particulates above 1 /xm in natural waters and wastewaters, the power-law coefficient appears to be greater than 3. Therefore, ade quate removal of the particulate fraction by sedimentation or flotation requires a reduction in ft by, for example, coagulation/flocculation, which shifts the major portion of particulate surface area and mass into size classes above about 30/xm. If granular-media filtration is used as the particulate separation process, only particulate destabilization may be necessary to achieve desired removals. The efficiency of solids/liquid separation processes for reduction of trace contaminants (such as heavy metals) and toxic organic compounds associated with the particulate fraction could be estimated if the chemi cal composition of the particulates as a function of size were known. However, such data are scarce and of questionable accuracy. As a first approximation, the distribution of an adsorbed constituent between various size classes in the particulate fraction can be estimated from a knowledge of the power-law coefficient. This combined with perform ance models of solids/liquid separation processes should provide an improved basis for process selection to meet increasingly stringent stand ards for water and wastewater treatment.



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Particle counters also show promise for process control and per formance evaluation, particularly i n low-turbidity waters. Applications include more accurate evaluation of pilot plant studies and more sensitive control of particulate separation processes. However, when the powerlaw coefficient, is greater than 3, submicron particles, which escape detection with available counters, may control the magnitude of the total surface area concentration and the light-scattering properties of the par ticulate. It is likely that accurate process control and monitoring of solids/liquid separation processes w i l l require both turbidimetric a n d particle size distribution measurements. Particle counting appears most promising as a feedback control sensor. Literature Cited 1. Allen, T. "Particle Size Measurement," 2nd ed.; Chapman and Hall: London, 1975. 2. Bader, H. J. Geophys. Res. 1970, 75, 2822. 3. Beard, J. D.; Tanaka, T. S. J. Am. Water Works Assoc. 1977, 69, 533-538. 4. Bishop, J. K. B.; Ketten, D. R. Deep-Sea Res. 1977, 24, 511-548. 5. Black, A. P.; Hannah, S. A. J. Am. Water Works Assoc. 1965, 57, 901-916. 6. "Direct Filtration of Lake Superior Water for Asbestiform Fiber Removal," EPA Report, No. 670/2-75-050a, June 1975. 7. Committee on Safe Drinking Water of the National Research Council, "Drinking Water and Health"; National Academy of Sciences: Washington, D.C., 1977. 8. Faisst, W. K. Report No. 13, Environmental Quality Laboratory, California Institute of Technology, Pasadena, CA, June 1976. 9. Friedlander, S. K. "Smoke, Dust, and Haze"; Wiley-Interscience: New York, 1977. 10. Geochemical Ocean Sections Program—Collected Papers, Earth Planet. Sci. Lett. 1976, 32(2), 217-473. 11. "Suspended Solids in Water"; Gibbs, R. J., Ed.; Plenum: New York, 1974. 12. Harris, J. E. Deep-Sea Res. 1977, 24, 1055-1061. 13. Hulbert, H. M. Katz, S. Chem. Eng. Sci. 1964, 19, 555-574. 14. Hutchinson, G. E. "A Treatise on Limnology"; Wiley-Interscience: New York, 1966; Vol. II. 15. James M. Montgomery, Consulting Engineers, Inc., Pasadena, CA, Water Quality Studies, Vol. IV, June 1977, report submitted to the Los Angeles Department of Water and Power. 16. James M. Montgomery, Consulting Engineers, Inc., 1978, unpublished data. 17. Kavanaugh, M. C.; Toregas, G.; Chung, M.; Pearson, E. A. Prog. Water Technol. 1978, 10(5/6), 197-215. 18. Kavanaugh, M. C.; Vagenknecht, A. Gas, Wasser, Abwasser 1975, 55, 554-559. 19. Kavanaugh, M.; Zimmermann, U.; Vagenknecht, A. Schweiz. Z. Hydrol. 1977, 39(1), 86-98. 20. Kerker, M. "The Scattering of Light and Other Electromagnetic Radiation"; Academic Press: New York, 1969. 21. Lai, D. Science 1977, 198(4321), 997-1009. 22. Lai, D . Lerman, A. J. Geophys. Res. 1975, 80, 423-430. 23. Lerman, A; Carder, K. L.; Betzer, P. R. Earth Planet. Sci. Lett. 1977, 37, 61-70. ;

;


328 24. 25. 26. 27.


28. 29. 30. 31. 32. 33.


O'Melia, C. R.; Ali, W. Prog. Water Technol. 1978, 10, 123-137. O'Melia, C. R.; Stumm, W. J. Am. Water Works Assoc. 1967, 59, 1393. Parker, D. S., Ph.D. thesis, University of California, Berkeley, CA, 1970. "Proceedings of Turbidity Workshop, National Oceanographic Instrumentation Center"; U.S. Department of Commerce: Washington, D.C., May 1974. Sharp, J. H. Limnol. Oceanogr. 1973, 18, 441-447. Sheldon, R. W.; Prakash, A.; Sutcliffe, W. H . Limnol. Oceanogr. 1972, 17, 327. Stumm, W. Environ. Sci. Technol. 1977, 11, 1066-1070. Swift, D. L.; Friedlander, S. K. J. Colloid Sci. 1964, 19, 621. Tate, C. H.; Trussell, R. R. J. Am. Water Works Assoc. 1978, 70, 691-698. Weber, W. "Physicochemical Treatment Processes for Water Quality Control"; Wiley: New York, 1972.

RECEIVED January 19, 1979.


Particulates in Water - American Chemical Society

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