Product Evaluation * A Comparison of Methods to Determine the Particle Size Distribution of Solids in Stormwater Samples Introduction There currently exist a multitude of techniques to determine particle size distributions. However, in assessing the particle size distribution (PSD) of solids in a stormwater sample, the number of suitable techniques is considerably reduced due to the limited quantity of solids in a typical stormwater sample. The electrical sensing zone method (ESZ), which utilizes the Coulter Principle, and laser diffraction (LAS) method are capable of yielding particle size information of a stormwater sample. However, these high precision techniques do have limitations and error susceptibility. A comparison of the two methods is important to identify the method best suited for application to stormwater samples. Consistent use of one commercially available method is critical for both manufacturers of best management practices (BMPs) and regulators evaluating BMP performance. To contribute to this important and evolving discussion, identical water samples were analyzed using a laser diffraction instrument and an ESZ instrument. Three of the four sample sets were simulated stormwater samples containing mg/l of suspended solids concentration (SSC) of varying mineral and organic composition. The remaining sample set consisted of actual stormwater. For this document a sample set is defined as a sample and its duplicate. A hydrometer and sieve method of assessing PSD was applied to the SSC source materials to provide an additional reference for comparison. The goal of comparing the results was to identify differences attributed to the method of analysis. Laser diffraction and ESZ methods yielded similar PSD results for two of the four sample sets. However, results from the other two samples sets including the true stormwater sample showed large differences which are most likely due to varying sample handling techniques or bulk sample splitting that resulted in unequal sample generation. To utilize PSDs in the assessment of individual BMP performance, in addition to comparisons of different BMPs, consistent sample handling techniques and standard operating procedures are of equal importance as identifying a specific analysis method. Based on numerous guidance documents established for BMP submissions for approval, regulators have acknowledged that PSD is a significant variable in BMP performance. An assessment of influent PSD is a necessary component in reports to the Technology Assessment Protocol Ecology (TAPE) established by the Washington State Department of Ecology (WADOE, 4). The New Jersey Department of Environmental Protection requires influent PSD data in accordance with the Technology Acceptance and Reciprocity Partnership (TARP) Tier II Stormwater Protocol (NJDEP, 6). Studies have associated prevalence of contaminants with different particle size ranges. Results from a study by Sansalone, et. al. (4) suggest that of the particulate bound contaminants, significant amounts of metals, and to a lesser extent nutrients, are associated with particles greater than 75 µm. Whereas Vase and Chiew (4) found the greatest portion of particulate bound total phosphorus and total nitrogen were associated with the particle size range from 11 to 15 µm. * This document will be presented at StormCon 6, Denver, CO, on July 25, 6. 6 CONTECH Stormwater Solutions contechstormwater.com PE-G5 4/5/6 Calvert, de Ridder, Lenhart 1 of 6
Particle sizing through laser diffraction is an indirect measurement based on the detection of scattered light due to the presence of suspended particles in a laser beam path. Laser diffraction instruments translate light scatter to particle size based on either the Fraunhofer or Mie theories. The Fraunhofer theory involves the light diffraction around particles whereas the Mie theory additionally incorporates light refraction through particles and light adsorption or reflection by particles. Both theories assume spherical particles. The Fraunhofer theory is accurate for multi-species minerals above µm. The Mie theory yields better fine particle size accuracy but is sensitive to the refractive index of particles. Regardless of theory, irregularly shaped particles are subject to measurement error. The longer the optical path length of a laser diffraction instrument, the more dilute the solids concentration the instrument is capable of analyzing. A benefit of laser diffraction analysis is its quick rate of analysis. ESZ instruments are true particle counters. A solution containing an electrolyte is passed through an aperture which is bridged by an electrical current. As the particle travels through the aperture, a voltage spike occurs which is proportional to the particle volume. Particles from about 2% to 5% of the aperture diameter can be detected. The particles are registered in a certain size class or channel of which there can be up to 256 for one aperture. Error potentials arise upon considering the simultaneous passage of particles registering as one particle also known as coincidence. This can be addressed by application of a statistical correction based on established probability density functions of coincidence occurrence relative to orifice diameter. ESZ is also less accurate in measuring porous materials as compared to solid materials. Contrasted against the laser diffraction method, ESZ has higher precision. Laser diffraction instruments are capable of examining a wide range of particle sizes. For ESZ instruments to achieve a similar range, multiple aperture use is typically required. Both methods tend to be less accurate at the boundaries of the analyzed particle range. Results from both methods are provided in percent volume measures relative to an equivalent spherical diameter. The percent volume is proportional to the percent mass if a constant density is assumed. Methods Three of the four sample sets were simulated stormwater samples comprised of deionized water and various solids. The SCS samples contained Sil-Co-Sil 6 (SCS 6), a finely ground silica material manufactured by US Silica Company. WADOE specifies use of SCS 6 for laboratory testing of BMPs for solids removal capability (4). The SSW1 samples were comprised of approximately 5% SCS 6 by mass and 5% fine leaf compost material. It was noted that the compost material did contain solids of mineral origin and was, therefore, not entirely organic. The SSW2 samples contained soil extracted from a soil pit. The final sample set, CompSW, is a composite of stormwater samples collected through use of automated samplers at the inlet and outlet of stormwater BMPs. Characteristics of the samples are displayed in Table 1. Table 1. Sumary of samples characteristics. ND signifies that TVS was not detected at or above the reporting limit of mg/l. Sample SSC TVS Solids Source Set (mg/l) (mg/l) SCS Sil-Co-Sil 6 5 ND SSW1 5% Sil-Co-Sil 6, 5% compost 114 15. SSW2 soil pit 111 ND CompSW actual stormwater samples 38.8 37. 2 of 6
The bulk samples for SCS, SSW1, and SSW2 samples were split using a 14-L churn splitter. The CompSW samples were split using an 8-L churn splitter. From each bulk sample, four ml samples were collected for particle size analysis; 25 ml for total volatile solids (TVS) analysis by North Creek Analytical, Inc., located in Beaverton, OR, according to EPA 1.4; 25 ml for in-house SSC analysis using the ASTM D3977B method; and 5 ml for inhouse wet sieving to determine the proportional mass split at 25 µm. One sample plus a duplicate were sent to each of the two laboratories conducting the particle size analysis using the laser diffraction and ESZ methods. Samples provided to these laboratories were analyzed within seven days of sample collection. Sample handling instructions were to analyze the samples as they were received without sample splitting or dispersant application other than manual sample agitation. Results of ESZ and laser diffraction particle size analysis were scaled based on the sieving acquired fraction of solids mass below 25 µm. The ESZ method was applied by Particle Technology Labs, Ltd. (PTL), located in Downers Grove, IL, according to their internal standard operating procedure which is a detailed version of the Elzone 112 instrument instructions. Three apertures were used to cover a wide span of particle diameter ranges. The count from each aperture goes through a data blending process, using software associated with the equipment, which combines counts from overlapping particle diameters resulting from the use of three apertures. Statistically based extrapolation is applied to the fine particle diameters. Laser diffraction analysis was conducted at Sequoia Scientific, Inc. (SSI), located in Bellevue, WA, using the LISST according to the American Water Works Association (AWWA) Standard Method 25D. The LISST functions based on the Mie theory. Software associated with the instrument produced results based on an average of readings per sample. Separate preparation of soils was required for hydrometer and sieve PSD analysis of material used in SCS, SSW1, and SSW2 samples. Not enough sample mass was present in the CompSW sample to conduct a hydrometer and sieve analysis. Testing was performed in the CONTECH Stormwater Solutions Inc. laboratory in Portland, OR using wet sieving modification of a standard hydrometer and sieve method (Gee and Bauder, 1986; Stormwater3, 5). Results and Discussion Particle size results are shown graphically in Figure 1 relative to USDA defined sand, silt, and clay size ranges. In examining the graphs shown in Figure 1, the particle size results for sample set SSW1 and SSW2 appear to be very similar. Differences are evident in the silt and clay ranges of SCS samples in addition to the silt and sand ranges of the CompSW samples. There is disagreement as to the cause of dissimilar ESZ and laser diffraction particle size results in the silt and clay range for the SCS samples. In comparing visual observations made by the instrument operators, the SCS samples received for analysis by laser diffraction appeared to have a notable concentration of fines unlike those samples received for ESZ analysis. However, no deviation from the standard churn splitting procedure occurred which would substantiate a difference in samples. If the laser diffraction results were affected by a mismatch between the refractive index of the solids verse that specified by the instrument, a similar difference, although to a lesser extent, would be expected between ESZ and laser diffraction results for the related sample, SSW1. 3 of 6
SCS SSW1 SSW2 CompSW 9 8 5 4 9 8 5 4 9 8 5 4 9 8 5 4 CLAY SILT SAND ESZ LAS H&S sieve.1 1 Particle Size (um) Figure 1. Results from particle size distribution analyses. Size segmentation is based on USDA definition of sand (> 5 um), silt (< 5 um, > 2 um), and clay (< 2 um). The round symbols represent typical sieve data as provided by US Silica Co. A volume based % finer pertains to curves corresponding to ESZ and laser diffraction. A mass based % finer pertains to the curves corresponding to the hydrometer and sieve. 4 of 6
The substantial difference in the CompSW results may be due to various factors related to the differing processes upon which the ESZ and laser diffraction instruments measure particle size. However, it is likely that different sample handling techniques by the labs caused the greatest impact in the difference of particle size distributions. SSI gently swirled the samples whereas PTL vigorously shook the samples prior to analysis. The CompSW samples were more sensitive to shaken versus stirred approaches to sample handling due to its high organic content. The TVS is 95% of the total solids in CompSW. The high amount of natural organic material contributes to solids agglomeration. Gentle agitation may not have broken up the agglomerates. Differences in sample handling have also posed a hurdle to past examinations of different particle sizing techniques and suspended solids analysis (SCCWRP, 1994; NIST, 2). The definition of what is different changes in comparing the results presented as curves Figure 1 versus the points plotted on the soil texture triangle shown in Figure 2. A difference between the ESZ and laser diffraction results for SCS, an approximately 14% higher silt content according to laser diffraction, does not place the solids in different textural classes. However, a 7% difference in sand content can span up to three textural classes as shown by the ESZ and laser diffraction results of SSW2. 8 9 Electrical Sensing Zone Laser Diffraction Hydrometer and Sieve Source: Brady, N. C., & Weil, R. R. (1999). The Nature and Properties of Soil (12th ed.). Upper Saddle River, NJ: Prentice-Hall. % Clay 4 5 Loamy Sand Sand 9 8 Sandy Clay Sandy Clay Loam Sandy Loam SSW1 Clay Clay Loam Loam SSW2 5 4 4 Silty Clay 5 Silty Clay Loam SCS Silt Loam % Silt Silt 8 9 % Sand Figure 2. Soil texture triangle displaying the results of particle size analyses. PSDs of the CompSW set are shown as the four the points not contained within a dark outlined segment. Based on the results presented, it can be stated that comparable results of particle size distributions of stormwater samples can be obtained using either the ESZ or laser diffraction methods if the sample contains a small amount of organic material. Further examination is 5 of 6
required to determine the effect of high organic content on the results provided by the two methods given consistent sample handling procedures. It should be emphasized that variations in particle density are not considered by the laser diffraction and ESZ methods. Particle density is an important variable in determining particle fall velocities. As solids settling is an essential treatment mechanism of stormwater BMPs, variable particle density should be incorporated in the analysis of size distributions. Gravimetric methods, of which differing particle densities are a factor, should be examined for application to determining the particle size distribution of solids in stormwater samples. Conclusion Variant sample handling contributed to the greatest difference in particle size results between application of the ESZ and laser diffraction methods. Therefore, it is not only important for regulating bodies to define one method of determining particle size distributions, but implementation of a common sample handling procedure is also critical. Acknowledgements Special thanks to Chuck Pottsmith with Sequoia Scientific, Inc. and Richard Karuhn and William Kopesky with Particle Technology Labs, Ltd. for their assistance. References Ferraris, C.F., V.A. Hackley, A.I Aviles, and C.E. Buchanan. 2. Analysis of the ASTM Round Robin Test on Particle Size Distribution of Portland Cement: Phase I. Report No. NISTIR 6883. National Institute of Standards and Technology. Gee, G.W. and J.W. Bauder. 1986. In A. Klute (Ed.), Methods of Soil Analysis: Part 1 Physical and Mineralogical Methods (2 nd ed.). pp. 383-411. Madison, Wisconsin: American Society of Agronomy, Soil Science Society of America. New Jersey Department of Environmental Protection (NJDEP). 6. New Jersey Tier II Stormwater Test Requirements Amendments to Tarp Tier II Protocol. Trenton, New Jersey: Author. Sansalone, J.J, H. Lin, and J. Ma. 4. Granulometric-Based Distributions of Metals and Phosphorus for Particulate Matter Transported in Stormwater. In StormCon 4 Conference Workshop Proceedings. Forester Communications. Southern California Coastal Water Research Project (SCCWRP). 1994. Sediment Grain Size: Interlaboratory Intercalibration Experiment. Report No. 276. Westminster, California: Author. Stormwater3. 5. Standard Operating Procedure: Hydrometer and Sieve Wet Method Particle Size Analysis. Report No. PE-SP11. Portland, Oregon: Author. Vaze, J. and F.H.S. Chiew. 4. Nutrient Loads Associated with Different Sediment Sizes in Urban Stormwater and Surface Pollutants. Journal of Environmental Engineering. pp. 391-396. Washington State Department of Ecology (WADOE). 4. Guidance for Evaluating Emerging Stormwater Treatment Technologies: Technology Assessment Protocol Ecology. Publication no. 2--37. Olympia, Washington: Author. 6 of 6