The behaviour of gross and visible solids was studied at two combined sewer overflow sites using the WRc Gross Solids Sampler. At one of the sites the daily variation of gross solids during dry weather was determined. There was found to be close correspondence between the variations of gross and suspended solids in the sewage. A relationship was developed between the load of gross solids and that of suspended solids at the observation point in dry weather. This relationship is presented for use with other predictive methods. A chart is presented which differentiates with a high degree of reliability the gross solids production of two different types of catchments, one being a collector sewer catchment, the other being a trunk. The rate of gross solids production has been found to be a critical factor in differentiating between the catchments. A consistent and further differentiation in the accumulation of gross solids is presented on the basis of antecedent dry period, greater than 24h allowing considerably greater accumulations than shorter dry periods.
