At StormCon ’06 in Denver in July, a panel of researchers and scientists discussed stormwater BMP trends, and they revisited the question of TSS versus SSC as a performance standard for measuring the effectiveness of best management practices. Although the question of which measurement to use has been discussed before, notably when the US Geological Survey proposed replacing TSS with SSC six years ago, the panelists’ comments summed up some of the key points of the ongoing—and, for those who design, specify, or test BMPs, critical—debate.
The percentage of total suspended solids (TSS) that a particular stormwater-quality device or technique is able to remove from stormwater runoff has been the commonly used performance standard, and it’s the means by which many people purchasing manufactured BMPs compare products. A goal of at least 80% TSS removal is typical. Suspended sediment concentration (SSC) is more representative of the entire gradation of particles in a sample, but it’s generally a more difficult and expensive test to perform. The main problem with TSS is that it generally misses the heavier, rapidly settling particles—those greater than 100 microns or so like sand and other coarse solids—that are often found in stormwater, giving only a partial picture of what’s in the water.
As panelist John Sansalone of the University of Florida pointed out, the TSS standard was adopted when stormwater was still a young, evolving science; as the science develops, he said, we should apply our new knowledge. Gordon England, also on the panel, noted that at the time TSS was adopted, sediment was the primary pollutant of concern for most states (and still is for many); however, if you’re dealing with a TMDL for nutrients or some other pollutant, the TSS standard isn’t very useful. It’s not necessarily appropriate—no matter how easy it would seem to make product comparisons—to have one simple, universal standard to measure performance of all BMPs.
Panelist Gary Minton pointed out that if SSC becomes the new industry standard, the performance goal for BMPs should be considerably higher than 80% removal. He suggested looking at the removal rate from the point of view of “maximum extent practicable” and basing the goal on the capabilities of the technologies available.
Interestingly, many people assume that a large number of organic and inorganic pollutants sorb to sediment particles and are removed along with sediment–making the amount of TSS removed from stormwater runoff a fairly good indicator of the amount of other pollutants that are being removed as well. The panel’s moderator, Masoud Kayhanian of the Center for Environmental and Water Resources Engineering at the University of California—Davis, said that in his experience there is not as much correlation as often believed between TSS and other pollutants.
Nikos Singelis of the USEPA’s Office of Wastewater Management was in the audience for the discussion and pointed out that nothing in the regulations mandates 80% TSS removal. He agreed with several of the panelists that an 80% (or any percent) removal rate seems like a flawed measurement and noted that EPA is instead looking at pollutant concentrations in effluent, rather than percent removal, as a better standard—a point on which most of the panelists agreed.
The issue won’t be resolved any time soon, with so many products, specifications, and state and municipal guidelines invested in the TSS standard. Still, it’s useful to hear the points reiterated and to be reminded that the measurements we’re using are open to interpretation.
