Comparison of TSS and SSC Methods of Analyzing Suspended Solids Concentrations

Transcription

1 Comparison of TSS and SSC Methods of Analyzing Suspended Solids Concentrations Introduction Sediment is one of the most widely occurring pollutants in stormwater, making suspended solids an important measure of water quality. Over the years an industry standard of 80% suspended solids removal has developed and is now enforced by numerous regulatory agencies across the country. Many of these agencies also use suspended solids data as a surrogate for trace-element pollutants found in stormwater runoff. There are two separate procedures for measuring suspended solids, the TSS (total suspended solids) method and the SSC (suspended-sediment concentration) method. Historically the TSS method was a widely used and accepted method and is today the required method for numerous state agencies throughout the United States. However, the USGS, a keystone agency involved in water quality and stormwater management, has found that the TSS method tends to produce data that is negatively biased when compared to the SSC method, which can result in large, unacceptable errors. In November 2000, the SSC method was established as the USGS standard for determining suspended solids in surface water samples (USGS, 2000). The difference between the two methods is rather simple. Both methods involve measuring the dry weight of sediment in a sample of water to calculate a suspended solids concentration (typically reported in mg/l). The key difference between the two methods is that for the TSS method only a subsample of the original sample is used for analysis, whereas for the SSC method the whole original sample is used. Since results from the two methods differ, the ability to convert data between the two is a growing concern. To accurately complete a data conversion a significant relationship between the data must be established. Table 1. Summary of TSS and SSC data for all five contaminants. Contaminant Range Method n Mean Standard (% sand) (mg/l) Deviation SigC (0) TSS SSC SCS-106 (19) TSS SSC TC (35) TSS SSC LSN (47) TSS SSC OK-110 (100) TSS SSC Relative Difference Between TSS and SSC (%) Procedure To compare the TSS and SSC methods of testing suspended solids, separate stock solutions, each containing one of five different contaminants, were created and then tested using both methods. These contaminants include Sil-Co-Sil 106 (SCS 106), OK-110, Sigmacell Cellulose type 20 (SigC) and sediment samples collected from the cartridge bays of SMI stormwater facilities located at Lake Stevens North (LSN), and Thurston County Health and Social Services (TC). The location of origin and particle size distributions for all five contaminants can be found in Table CONTECH Stormwater Solutions contechstormwater.com PE-E022 6/12/07 G. Tellessen, S. de Ridder 1

2 Table 2. Location of origin and sand/silt/clay fractions for contaminants. Contaminant Origin Particle Size Distribution Location Type % Sand % Silt % Clay SCS 106 US Silica Co. manufactured OK-110 US Silica Co. manufactured SigC Sigma Chemical Co. manufactured 0 ND ND TC Olympia, WA natural LSN Lake Stevens, WA natural Stock solutions for SCS, OK-110, TC and LSN were created individually in the SMI lab following the same general procedure and using the same equipment. A stock solution was created in a 14-L churn sample splitter (Bel-Art Products) with a target concentration of 50 mg/l. The required amount of concentrate was added along with DI water to create the 14-L stock solution. The churn splitter was then used to produce ten 250-mL samples for TSS analysis and ten 250mL samples for SSC analysis on an alternating basis, beginning with a TSS sample. All samples were dispensed into 250-mL (8-oz) HDPE bottles and handled in accordance with standard handling techniques. Samples were promptly refrigerated and sent to North Creek Analytical (NCA) in Beaverton, Oregon for analysis. NCA performed TSS analysis according to EPA method 160.2, which uses only a sub-sample of the original 250-mL sample for analysis. SSC analysis was performed according to the ASTM method D3977, which is essentially a whole-sample variation of the EPA method 160.2, where the entire 250- ml was tested and sample-splitting, along with the associated bias, was completely eliminated (SMI, 2002). Data for SigC came directly from NCA. SigC is used by NCA as a lab control for all suspended solids tests performed. Results The mean, standard deviation, and range for each of the five data sets are shown in Table 1. Excluding SigC, which contains 0% Sand, all sets show lower means for TSS results when compared to the SSC results for the same contaminant. For each contaminant, the relative percent difference (RPD) between the two methods is also calculated and can be seen in Table 1. RPD is calculated using equation 1: x y RPD = ( x + y) / 2 where, x equals the mean of SSC data and y equals the mean of TSS data. Discussion Looking at the TSS and SSC data from this experiment, it is apparent that a difference in results obtained does exist between the two analytical methods. A practical evaluation of the data confirms this difference. However, a relationship between the percent sand and RPD of a given sample also exists. Regression analysis of percent sand and the relative difference between the two methods, shown in Figure 1, gives us a coefficient of determination (r 2 value) of This value shows the statistically determined relationship between x and y with an r 2 value of 1 showing a perfect relationship. An r 2 value of suggests that a valid relationship does exist, which in turn, allows us to use this relationship as a conversion factor between the two methods. Results from this experiment show RPDs between the two methods that range from 0% to 54%. Overall, the RPD increases as the amount of sand in a given contaminant increases. For example, if US Silica Company, P.O. Box 187, Berkley Springs, WV 25411; ; Sigma Chemical Company, 6050 Spruce St., St. Louis, MO 63103; North Creek Analytical, 9405 SW Nimbus Ave, Beaverton, Oregon 97008;

3 the amount of sand increases from 50% to 60% the RPD will increase from 29.1% to 34.3%. This comparison can be seen in Figure 1. By using this percent sand and RPD relationship along with equation 1, we are able to translate data between TSS and SSC. For example, a sample was collected from the influent of a stormwater facility and was analyzed by a lab using the SSC method. The lab reported the total suspended solids concentration in the sample to be 100 mg/l. A PSD shows that the sample contains 57% sand. Using the relationship defined in Figure 1 a RPD of 32.7% can be determined. Using Equation 1 the SSC data can easily be converted in TSS data as follows: 100mg / L TSS = (100mg / L + TSS) / 2 TSS = 71.9mg / L 60 Relative Differance (%) Percent Sand (%) 3

4 factors, is helpful when information is shared between companies and regulatory agencies that use different analytical methods of determining total suspended solids concentrations. References Stormwater Management Inc. (SMI). (2002). Influence of analytical method, data summarization method, and particle size on total suspended solids (TSS) removal efficiency (Report No. PD ). Portland, Oregon: Author. U.S. Geological Survey (USGS). (2000). Collection and Use of Total Suspended Solids Data (Office of Water Quality Technical Memorandum No ). Reston, VA: Author. 4

5 Revision Summary PE-E022 Document re-branded and document number updated to reflect current numbering system. PD Additional material added to the conclusions sections. PD Original.