The design of conventional settlement devices used for primary treatment at wastewater treatment works has traditionally been based on generalised loading criteria. These generalised loadings do not necessarily take into account the nature and settling characteristics of the suspended material in influent wastewater streams. The paper describes the practical use of wastewater characterisation, in the form of settling velocity distributions, for the design of primary sedimentation devices, such as the hydrodynamic separators used in the Swirl-FloTM process.
Results of settling velocity distributions for diverse wastewater sources (including municipal and industrial sources) are presented together with comparisons between steady-state model predictions with observed performance for waste streams at different sites. The observed differences in settling characteristics for the wastewater streams investigated, together with the implications these have for design and performance assessment, are used as basis for advocating the need to incorporate wastewater settling characteristics in the design process for wastewater sedimentation devices.
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.
A research programme at Aston University, the aim of which was to investigate the distribution of chemicals within the settling velocity grading curve of suspended sewage solids, has recently been completed. The pollutants investigated were COD, TKN, Phosphorous and a suite of 7 metals. Tables summarising their distributions, and the range of values observed, are presented in the paper. Although the 15 sites studied were divided into large, medium and small classes, no evidence was found to indicate that catchment size influences the distribution of chemicals significantly. However, the TKN and P loadings tended to be higher for small catchments. This is probably due to a greater proportion of their area being rural, than in the medium and large catchments. It is pointed out that when differing units are used to present the results, different conclusions can be drawn regarding the settling velocity fraction which carries the heaviest pollutant load.
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