The relationship between settling velocity grading and the characteristics of the contributing catchment

A method for measuring the settling velocity gradings of both storm and dry weather flow (DWF) sewage was investigated. This method was then used experimentally to determine whether a relationship exists between the settling velocity grading of a sewage as it enters a treatment works and the characteristics of the contributing catchment. The problems associated with the collection of data on the characteristics of the contributing catchments are discussed. These include the availability of up to date information within the water companies and the time consuming nature of collecting full sets of information for a particular wastewater treatment works. An S-shaped curve was fitted to all the settling velocity gradings produced during the research. Relationships relating the curve constants to the catchment characteristics were derived using multiple regression techniques. The methodology was proved, but the model was not sensitive enough to be used successfully as a predictive tool without further analysis involving either more sites and/or a more careful selection of the test sites.