By: Ben Farrell - Imbrium Stormwater Design Engineer
Pollutant removal in stormwater exhibits a distribution of settling velocities, which is a reflection of the complex mixture of transported total suspended solids (TSS). The TSS mixture creates a settling distribution due to the different particle sizes, shapes, temperature, and specific gravities.
Solids can be further categorized by type: organic, inorganic, biological, and chemical. The specific gravities, shape, and settling velocities of each type of solid commonly transported in stormwater runoff differs significantly. The specific gravity of most inorganic solids (5 to 100 microns), ranges from 1.5 to 2.65, with sand (>50 microns), 2.45 to 2.65. Some clays particles (<2 microns) and other organics have a lower specific gravity because of entrained water, on the order of 1.5. In comparison, common laboratory test sediment used to comprise a PSD (particle size distribution) used for oil grit separator (OGS) testing often has a single specific gravity of 2.65, which differs from the real world.
The figure above shows an example of the variability in settling velocities as a function of size and specific gravity. The figure indicates there is significant deviation in settling velocity as the particle size decreases.
To better explain the graph the following example demonstrates the effect that the specific gravity parameter has on OGS treatment chamber size. The table on the left indicates the PSD required for testing per the new Canadian Environmental Technology Verification (ETV) Procedure for Laboratory Testing of Oil-Grit Separators (CA ETV PSD). The PSD table on the right specifies a modified (experimental) version of the PSD where the highlighted specific gravity has been revised to more accurately reflect observed field research as noted above.
In effort to evaluate the specific gravity effects the modified CA ETV PSD values have on the required OGS unit size, the PCSWMM Stormceptor sizing program has been used. All supplemental design parameters have been held constant.
The Stormceptor sizing results indicate that a 16,490 liter treatment chamber is recommended to capture 70% TSS from the site with the lab test CA ETV PSD, while a 20,940 liter treatment chamber is recommended to capture 70% TSS from the site with the Modified ETV PSD. The extra volume of 4,450 liters required to achieve the same performance is a direct result of the additional settling time needed for capture of the lower specific gravity particles. In the case of the Stormceptor – that is the difference of using a STC 4000 versus a STC 5000.
This result confirms that specifying a detailed Particle Size Distribution (PSD) parameter is critical for designing stormwater quality treatment systems such as OGS units that rely on the process of gravity settling, which is a critical deciding factor for selecting the size of an OGS treatment system. There is no absolute correct PSD or specific gravity, as this parameter is dependent on the site location, characteristics, loading, materials handling practices, rainfall and regulatory requirements. It is recommended that municipalities and Engineers select a broad range (i.e. 1 to 1,000 micron PSD, with inclusion of a realistic detailed specific gravity), before designing, sizing and specifying an OGS. This ensures the OGS design is performance based, more accurate to reality and can be compared on an apples-and-apples basis.