introduCtion Particle Size Distribution Removing silt particles from runoff ensures that the majority of the pollutants, such as hydrocarbons and heavy metals that adhere to fine particles, are not discharged into our natural water courses. The table below lists the particle size distribution used to define the annual TSS removal. The two most common particle size distributions used for Stormceptor sizings in Western Canada are the “recipes” for OK110 and F-95 sands, which are highlighted below: F-95 Particle Size (mm) 53 75 106 150 212 300 Percentage (%) 3 15 42 30 9 1 Specific Gravity 2.65 2.65 2.65 2.65 2.65 2.65 Clays and silts have the most surface area by mass, allowing pollutants (heavy metals, hydrocarbons, nutrients, etc.) to easily attach to them. In fact, smaller particles (less than 60 microns) contain up to 80% of a sample’s contaminant load, making them the most important particles to capture. Examining highway stormwater runoff, The Center for Research in Water Resources (CRWR) at the University of Texas at Austin stated in a 2005 report, “The concentration of metal, zinc for example, increased as the particle size decreased.” Smaller particles also carried higher concentrations of copper, phosphorus and nitrogen. “Larger particles in stormwater runoff settle out, but smaller particles remain suspended in stormwater runoff and travel greater distances,” the CRWR also noted. These findings, along with the CRWR’s research led it to conclude: “Treatment systems must be able to effectively remove fine particles in runoff to significantly reduce pollutant loads.” ConCrete pipe stormwater treatment teChnologies box seCtions OK110 Particle Size (mm) 1 53 75 88 106 125 150 Percentage (%) 0 3 15 25 40.8 15 1 Specific Gravity 2.65 2.65 2.65 2.65 2.65 2.65 2.65 manhole and CatCh basin material LAFARGE PIPE eMANUAL PAGE 7