Stormwater Magazine

Editor’s Comments: Grains of Sand

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Credit: iStock/bukkerka
Of all the things you need to do this month, spending some time with one particular article in this issue should be on your list. It’s the summary, on page 40, of ASCE’s new certification guidelines for stormwater BMPs.

The need for reliable, repeatable, and comparable third-party verification of the performance of stormwater treatment devices has been acknowledged for a very long time. As the article notes, several certification and verification programs have been developed. Some are regional; others, like EPA’s Environmental Technology Verification program, are no longer active. The challenges in creating such a program are complex: It must accurately reflect local conditions such as soil type and particle size; it must be scalable; it must be acceptable to and provide useful information for local decision-makers who are spending their often scarce funds on devices that will remove pollutants from runoff and improve local water quality.

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The American Society of Civil Engineers (ASCE) took all of these factors into account when developing its guidelines. The author of the article, Gordon England, served as vice chair of the ASCE committee that developed the testing and certification standards, and he describes the committee’s goals and what the various areas guidelines address. If you use manufactured stormwater treatment devices in any part of your program—and even if you’ve relied on a different testing protocol than the one described here—you need to be familiar with what ASCE has developed.

One point in the article stood out to me, as it likely will to many of you: the ASCE guidelines recommend “that the parameter of suspended sediments concentration (SSC) be used to more accurately measure sediment concentrations, rather than the parameter of TSS [total suspended solids].”

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This might seem to be a minor point, and yet it’s a debate that goes back years. As long ago as 2001, the US Geological Survey was recommending that SSC replace TSS in the evaluation of stormwater BMPs. Measurement of TSS, as USGS pointed out then, was originally used for analyzing wastewater rather than stormwater. For the larger particles—particularly sand—found in stormwater, SSC offers greater accuracy. I quoted the USGS statement back then and I’ll repeat it here: “Using the TSS analytical method to determine concentrations of suspended material in open channel-flow can result in unacceptably large errors and is fundamentally unreliable…. TSS data can result in errors in load computations of several orders of magnitude.” The change is hard to make, though, especially because so many devices have been rated based on TSS measurements, and there is no handy conversion from one measurement to the other; even now, many protocols cite a percentage of TSS removal as a key measure of BMP performance.

Whether SSC eventually replaces TSS as the common standard or whether we keep on in the direction we’ve been going with TSS, it’s important that the stormwater communities—manufacturers, regulators, and ultimately those who select and use the products—agree so that we’re all measuring the same thing and working toward the same water-quality standards. What are your thoughts on the accuracy of TSS measurements? Join the discussion at SW_bug_web

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