A protocol for the use of particle counting in stormwater suspended solids analysis

Abstract

Particle count technology that has been used for more than two decades in potable water testing is finding increasing application in stormwater as well. Particle count is a two-dimensional measurement of particle number by size, therefore it can potentially produce accurate and precise information on water quality, system performance and stormwater treatment efficiencies compared to conventional TSS, SSC and turbidity measurement methods. In this study, a protocol for using PAMAS S4031, a particle counter based on light obscuration has been developed. Most of the modern particle counter instruments currently being adapted for liquid analysis are designed with the narrow spectrum analytical requirement in mind (for example,counting blood cells and fuel spray droplets size) , so it should be noted that these instruments have limitations. In this paper, the limits and capabilities of available instrument technology will be presented in detail to make the best use for analysing suspended particles in stormwater. A silica powder LANG LANG SILICA 60G, obtained from highly pure sand with known particle size distribution (PSD) in term of percentage of mass and uniform density of 2.66 gm/cm3 was used for the verification of the particle counter. Initially two different methods, namely sieve analysis and settling column, were used to confirm the given PSD of silica. Wet and dry sieving were performed to fraction particles in the range between 45μm to 250 μm. Settling column analysis was used to fraction particles larger than 2μm whereas the pipette method was used for smaller particles between 2 μm to 62.5 μm. The PSD obtained from these methods were compared with the given PSD. Based on particle density information, particle counts were calculated mathematically for given PSD and were compared with particle count obtained from PAMAS particle counter. It was found that although the measurement range was between 1 to 100μm, the instrument was most effective in measuring particles smaller than 10 μm only. The protocol will thus mainly focus on fine particles < 10μm. This study comprises of series of monitoring trials, conducted at different concentrations, flow rates, instrumental set ups etc. The protocol is expected to be fast and easy to use and can be used to evaluate stormwater quality and treatment performance. It can also assist in the selection of treatment systems on a more consistent basis.