Examining the differences between total suspended solids and suspended sediment concentrate
What is Total Suspended Solids (TSS)? What is TSS with respect to storm water? Simply put, TSS is the generic name given to represent sediment in storm water runoff. For most storm water practitioners, this catchall phrase applies to a wide range of solids found in runoff.
Strictly defined, TSS is simply an analytical method, such as EPA 160.2, used to determine the concentration of suspended solids (as opposed to dissolved) within a water sample. This method was originally developed in the 1970’s to test effluent quality from wastewater treatment plants.
A sample, also known as an aliquot, is drawn from a container and poured through a glass fiber filter that retains the solids (excluding non-representative particulates such as macro-organic debris). The solids retained on the filter are then dried and weighed. The mass of the sediment in the aliquot is then extrapolated to the total sample volume to get a calculated concentration.
In storm water management, TSS is the benchmark pollutant used by the majority of the regulatory community for establishing quantitative pollutant removal goals. The most common regulatory goal is the removal of 80% of influent TSS. Regulations have focused on sediment removal for several reasons. Sediment is the most significant pollutant, by mass, found in our nation’s waterways. Additionally, many studies have shown that pollutants, such as heavy metals, adsorb onto sediment and therefore effective sediment removal translates to synergistic pollutant removal.
The regulatory community finds itself in a quandary over interpreting the intent of using the word TSS to represent pollutant sediments because, in most cases, TSS was never defined in the legislation. For example, in New Jersey, the Department of Environmental Protection (DEP) interpreted TSS as the results obtained by the analytical method of the same name for particles smaller than 1000 microns. This interpretation was arrived at because language in the legislation explicitly used the word TSS and is most strictly defined as an analytical method. The 1000 micron upper limit was apparently selected arbitrarily due to rational need, i.e. a cobble could not be considered a TSS particle.
The reason the regulatory community is seeking to define TSS is due in part to the imprecise nature of the TSS analytical method and the investigation of more appropriate methods to determine sediment concentration. One such method is ASTM D 3977-97, also known as the Suspended Sediment Concentration (SSC) method, in which the whole sample, including all sediments and not just an aliquot, is strained through a glass fiber filter and weighed, giving the total sediment mass, which is then divided by the sample volume. There is no extrapolating of aliquot data to the whole sample.
Essentially, the SSC method could be called the Whole Sample TSS method because the whole sample is processed without the need for extrapolation, while the traditional TSS method could, representatively, be called the Partial Sample TSS method.
An analogy of how the use of the partial sample TSS method can be misleading is as follows: Imagine a piggy bank full of loose pocket change composed of half dollars, quarters, nickels, dimes and pennies. You will be paid in bills based upon the selection of one of the following methods for establishing the piggy bank’s value:
Method 1: A sample is drawn from the piggy bank, but the sample may only represent the smaller coins, such as the dimes and pennies. This sample is then extrapolated to represent the total value found in the piggy bank; or Method 2: The whole piggy bank is analyzed such that all the coins are counted to get the total value.
It is obvious that if the piggy bank has a heterogeneous mix of coins, Method 1 would not give the correct total value. The same logic can be applied to a container of dirty storm water. In most cases, aliquots used in the Partial Sample TSS method are drawn using a pipette, which can exclude coarse sediment. Only the Whole Sample TSS method will include all particles and therefore be more representative of the total concentration.
Advantages to SSC
Two other questions can be raised with respect to the precision and accuracy of the Partial Sample TSS method (EPA 160.2); the following are excerpts from the method: “Precision data are not available at this time (Section 9.1),” and “Accuracy data on actual samples cannot be obtained (Section 9.2).”
The reason for this lack of guidance could be attributed to the process by which the method suggests drawing aliquots. Under this method, shaking the sample vigorously and then pipetting out a portion of the solution would yield a viable aliquot. But how fast does the sample need to be shaken? Could not all of the heavy solids reside at the bottom of the sample container? What would results look like if the aliquot were drawn from the surface of the sample?
One argument of why TSS should be used in place of SSC as the measure of sediment concentrations in storm water is that it will almost always return a lower value. This is interpreted as an inherent safety factor built into the TSS method. However, the safety cannot be predefined because the difference in concentration determined by the two methods is a function of the size of the sediment being measured. Essentially, if the majority of the particles are small (<62 microns), then the results of a TSS method will be very close to the results of the SSC method. However, when the coarse fraction (<62 microns) begins to represent greater than 25% of the total sediment mass, then the results of the two methods can be very different. This means the safety factor cannot be defined and may not exist.
Obviously, many factors in the Partial Sample TSS method could result in lack of data consistency, even between duplicate samples. In the article, “Limitations of Current Solids Measurements in Stormwater Runoff,” published in the July/August 2005 issue of Stormwater magazine, the author includes the results of duplicate runoff samples taken at multiple roadway locations and processed by three different labs.
Each lab used the same method for analyzing TSS, yet when we look at the average of the coefficients of variation (COV), a statistical method used to compare standard deviation against the mean of a data set, we get a value of 42%, which indicates there was a very high degree of deviation from the mean concentration (see Table 1). This example highlights the inherent difficulties associated with obtaining reliable results from the Partial Sample TSS method.
Conversely, Whole Sample TSS samples processed in accordance with ASTM D 3977-97 method B, in which known concentrations were sent to nine laboratories, resulted in the COV values found in Table 2.
Comparing the results from the two tables, we can see that when concentrations were in excess of approximately 100mg/L, the COV for the Partial Sample TSS method and the Whole Sample TSS method were 42% and 5.8%, respectively. This means the Whole Sample TSS method has a much higher rate of reproducibility because there is less deviation found in duplicate samples. Given the differences between the two methods and the inherent inadequacies of the Partial Sample TSS Method, a whole sample method for determining sediment concentrations seems to be most appropriate.
Development and science
Our collective goal is to stop the impairment of our nation’s waterways. This is going to be a function of planning, conservation, smart development and good science. The knowledge base for storm water management is broadening every day. New methods need to be evaluated, and as better processes are identified, they need to be adopted. This follows the logic that we cannot solve today’s problems with our present knowledge. The Whole Sample TSS method is a more accurate and reliable test for determining sediment concentrations. In keeping with our collective goal, it is our duty to use the best resources available.