Particle size distributions in combined sewer overflows in a high-intensity urban catchment in Shanghai, China

Abstract

To determine the dissolved-phase chemicals and solid-phase particulate matter (PM) in a combined sewer overflow (CSO), the event mean concentrations of typical particulate-band and dissolved-band contaminants were analyzed. Particulate-band chemical oxygen demand and total phosphorous formed the majority of particles, while only around 50% of the total nitrogen was found in the particulate-band. Particle size distributions (PSDs) for PM in 49 CSO samples collected during seven rainfall events, and 18 run-off samples collected during two rainfall events, were analyzed. Variation in PSDs in both CSO and dry weather flow samples occurred with sample storage time. Mean values of particle size in CSO samples were not significantly different from median values for particles within the size range from 0.41 to 3,080 μm, at a 95% confidence level. Values for D50 and D90 for PSDs of PM in both CSO and run-off samples were higher than for previous studies. It was caused by high flow rates during the operation of storm water lifting pumps. The first flush of particles in different size ranges was quantitatively evaluated using PSDs and total suspended solids in each sample. Given the prevalence of CSOs in China and other countries, knowledge of the PSD, and its variation with time, facilitates both the design of CSO treatment procedures and optimization of wastewater treatment plants; it also provides important input for modeling water treatment operations.