The importance of PSD when sizing stormwater BMPs CONCRETE PIPE When sizing a stormwater quality device, some municipalities ignore a crucial piece of data that has a significant impact on stopping pollutants from reaching natural waterways – Particle Size Distribution (PSD). A defined PSD identifies all sediment particle sizes found in a stormwater runoff sample, including their diameter, content and concentration. It’s a blueprint of what is barreling down the pavement during each rainfall. However, when sizing stormwater BMPs, PSD is often neglected. Instead, BMPs are often sized by their ability to remove an unspecified portion of When sizing a stormwater quality device, some municipalities ignore a crucial piece of data that has a significant impact on Total Suspended reaching natural waterways as the dry mass of solids retained on a 1 µm filter. stopping pollutants from Solids (TSS) – defined— Particle Size Distribution (PSD). Particle Size Distribution A defined regulating authorities particle sizes found insimply removing sample, including an acceptedcontent and Many PSD identifies all sediment are content with a stormwater runoff 80% of TSS – their diameter, concentration. for a blueprint of whatBut thefrom the pavement during each rainfall. However, when sizing stormwater BMPs, standard It’s water quality. is lifted question is, 80% of what? The answer is 80% of whatever PSD is often neglected. Instead, BMPs are often sized by of particle size. an unspecified portion of Total Suspended solids are trapped by this filter, regardless their ability to remove Solids (TSS) — defined as the dry mass of solids retained on a 1 mm filter. Specifiers authorities are content with simply removing 80% of TSS — an accepted standard for water quality. But Many regulatingthen often seek any BMP that meets this 80% removal rate, though exactly what isthe being treated and The answer is 80% to pass is anyone’s guess. Some regardless of particle to question is, 80% of what? what is allowed of whatever solids are trapped by this filter, municipalities trysize.look Specifiers then often seek any stormwater treatmentremoval rate, though exactly what is being treated and what is allowed ineffective in sizing a BMP that meets this 80% system. to pass is anyone’s guess. Some municipalities try to look beyond TSS by treating a mean particle size (d-50) of a sediment sample, but even this can be ineffective in sizing a stormwater treatment system. If a complete PSD is not used, inadequate systems may be implemented, capturing larger If a complete PSD is not used, inadequate systems may be implemented, capturing larger coarser particles but letting smaller coarser particles And yet smaller particles are potentially the most harmful to natural water resources. particles pass untreated. but letting smaller particles pass untreated. And yet smaller particles are beyond TSS by treating a mean particle size (d-50) of a sediment sample, but even this can be potentially the most harmful to natural water resources. Smaller Particles, Bigger Environmental impact Smaller particles, bigger environmental Impact Sediment can be into four into four categories: gravel, sands, silts and clays – their relative in the Sediment can be divideddivided categories: gravel, sands, silts and clays — their relative size difference can be seensize image below. can be seen in the image below. difference GRAVEL 256,000 μm 2,000 μm SAND 50 μm SILT 2 μm CLAY 0 μm A diagram of relative particle size in microns. PAGE 6 LAFARGE PIPE eMANUAL