Stormwater Management, Inc.

A Comparison of Methods to Determine the Particle Size

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Courtesy of Stormwater Management, Inc.

There currently exist a multitude of techniques to determine particle size distributions. However, in assessing the particle size distribution (PSD) of solids in a stormwater sample, the number of suitable techniques is considerably reduced due to the limited quantity of solids in a typical stormwater sample. The electrical sensing zone method (ESZ), which utilizes the Coulter Principle, and laser diffraction (LAS) method are capable of yielding particle size information of a stormwater sample. However, these high precision techniques do have limitations and error susceptibility. A comparison of the two methods is important to identify the method best suited for application to stormwater samples. Consistent use of one commercially available method is critical for both manufacturers of best management practices (BMPs) and regulators evaluating BMP performance.

 To contribute to this important and evolving discussion, identical water samples were analyzed using a laser diffraction instrument and an ESZ instrument. Three of the four sample sets were simulated stormwater samples containing 100 mg/L of suspended solids concentration (SSC) of varying mineral and organic composition. The remaining sample set consisted of actual stormwater. For this document a sample set is defined as a sample and its duplicate. A hydrometer and sieve method of assessing PSD was applied to the SSC source materials to provide an additional reference for comparison. The goal of comparing the results was to identify differences attributed to the method of analysis.

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