If you’re new to stormwater management or a professional with 30 years of experience, you are probably aware that 80% TSS removal efficiency is the key metric for getting designs approved, meeting regulation and showing that your design is a success. But the lack of an influent sediment particle size standard means that 80% ratings may not mean what you think it does. Check out the break down of these measurements and how the lack of a particulate definition means you may not be comparing apples to apples in your stormwater device review.
The Importance of Particle Size Distribution (PSD) When Defining 80% TSS Removal Efficiency
Stormwater management manuals often include design guidelines for manufactured treatment devices (MTDs) that include technologies such as hydrodynamic separation and filtration. Local jurisdictions may approve an MTD provided that testing demonstrates at least 80% TSS removal efficiency. However, this performance criterion lacks specification for an influent sediment particle size distribution (PSD) for either laboratory or field-testing programs. The critical role that test sediment PSD plays in MTD performance testing is essential, and those gradations affect TSS removal efficiency for both annual and per storm event sizing.
A laboratory hydrodynamic separator (HDS) performance curve is compared to a series of performance curves calculated using the Peclet Number for the same tested device based on median (d50) values ranging from 45 to 125 microns. These performance curves can also be used to size an HDS based on both an annual and per storm event basis.
A number of consequences result from excluding a PSD specification for MTDs. While undersized MTDs may exhibit diminished performance for TSS removal efficiency, they can also cause concern for functionality and flow-through conveyance for undersized piping. Undersized facilities may also experience increased maintenance frequency and associated costs due to decreased storage capacity. While oversizing may provide a conservative treatment approach, it (a) increases a system’s footprint including any bypass structures, (b) may limit options for limited space and retrofit installations, and (c) increases project costs. To learn more, check out our presentation on SlideShare. You may also be interested in reading: How Manufactured Treatment Devices (MTDs) Tackle Different Soil Types.
Tell us what you think about the TSS Removal Efficiency Standard in the comments!
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