Aesthetic pollutant loadings in upstream combined sewers.

An improvement to the standard of discharges from combined sewer overflows is necessary to meet the challenges set by the EC Urban Waste Water Treatment Directive (1991). To prevent aesthetic pollutants being discharged from a CSO, it is first necessary to gain an understanding of the quantity and temporal distribution of solids that enter the structure. This knowledge can then be used to predict the quantity of solids that are presented to a CSO under storm conditions. A field study was undertaken to determine the quantity and temporal distribution of solids entering a combined sewerage system and their transportation through the network in dry and wet weather. Flow monitoring and aesthetic pollutant sampling was undertaken in dry and wet weather at three catchments that contained different population types. The aesthetic pollutants sampled were characterised and tested in a laboratory to determine their settling rate.Analysis of the sampled data has indicated that aesthetic pollutants are stored in upstream sewers and private drainage connections during the dry weather period prior to a storm occurring. These pollutants and those within the flow in dry weather are transported out of the sewer at the start of the storm as part of a first foul flush. Sampling showed that over 80% of the solids by mass were faeces and toilet tissue, with a comparatively small number of sanitary products in the remaining percentage. Different solids are stored at different rates and this is clearly linked to the antecedent dry weather period and the quantity of solids that enter the system. The quantity of solids entering is also linked to the population type.A solid input and transportation model has been developed to predict the quantity of solids entering and moving through an upstream sewer system in dry and wet weather. The accuracy of the model is good particularly in the context of the variability of the solids that enter the system. The model predicts the quantity, rate and temporal distribution of six types of solids that could enter an upstream CSO and could aid in the development of more cost effective CSO solutions.