Size distributions of particle mass, chemical oxygen demand (COD) and phosphorus at four municipal waste water treatment plants were investigated by fractionation of samples through 0.1-8.0 μm polycarbonate filters and 32-125 μm sieves, respectively. Particle size analysis of primary and secondary effluents at each plant showed significantly different size distribution functions. These distribution functions remained constant with respect to time under dry weather conditions so they can be considered as characteristic patterns for each sample location. Advanced treatment by deep bed filtration can be improved by taking into consideration the distribution pattern in the secondary effluents. In raw sewage and primary effluents 45-90% of COD and 35-80% of phosphorus were associated with suspended solids. In the secondary effluents particulate COD drops to 14-45% but particulate phosphorus increases to 55-80%. Conclusions are drawn for a better design of advanced contaminant reduction in connection with suspended solids removal processes.

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