Sewer solids separation by sedimentation - the problem of modelling, validation and transferability
In the last years a lot of research work has been done to develop models for sedimentation process in hydrodynamic separators (e.g. LUYCKX, G. et al., 1998 ), settling chambers (e.g. VAES et al., 1999), inline retention sewers (FREHMAN, 2004) and wetlands (WONG et al., 1999). But in Germany it is not accepted to use these models for the submission for a stromwater discharge license. The authorities have the opinion that these models are not validated for this use. So the aim of this study was to examine if these worries are reasonable or if the models are ready for use.
For this survey a check-list was developed which contains all requirements for the development of a valid mathematical model. It is included, that the development of a model must consist of two parts: A study at a material model to create the structure of the mathematical model and a study at a full-scale facility for the final validation of model parameters. The study at the material model must base on a complete relevance list, all influencing variables must be varied independently and model laws (like Froude’s law) must be taken into account. For the study at the full scale facility it is essential, that not only computed and measured output polutographs for single events are compared. These polutographs are not only influenced by the sedimentation process but also by the input concentration and especially by the mixing behavior. Instead of that, variables must be developed, which quantify specific the particle removal by sedimentation. These variables must be compared for computing and measuring, and the difference of the values must be acceptable for a number of events for the same set of model parameters.
It was remarkable, that none of the models fulfilled the whole check-list positively. Some authors did a good job at the material model but none of them was able to show a validation at the full-scale-study. Some showed only polutographs for one event others were not able to find a dependence between hydraulic influencing parameters and the efficiency of the sedimentation process.
The key findings of this study are:
Currently, we do not have models for the sedimentation process in stormwater treatment facilities which are validated at full-scale-facilities for a number of events.
There is a need of a guideline developed by the scientific community that gives hints for the validation of mathematical models by measurement at material modells and full-scale facilities.
References:
FREHMANN, T. (2003): Untersuchung der Wirksamkeit von Stauraumkanälen mit unten liegender Entlastung zur Regenwasserbehandlung. Forum Siedlungswasserwirtschaft und Abfallwirtschaft der Universität Essen, Heft 21, Diss., in German
LUYCKX, G.; VAES, G.; BERLAMONT, J. (2002) Surface load as predominant factor for CSO efficiency. In: Proc. 9th ICUD, Portland, USA
WONG, T.H.F, ROEDDER, A., GEIGER, W.F. (1999) Predicting the Performance of a Constructed Combined Sewer Overflow Wetland. In: Proc. 8ICUSD, Sydney, Australia
VAES, G.; LUYCKX, G.; DE COCK, W.; BERLAMONT, J. (1999) A conceptual model for the impact assessment of storage sedimentation basins. In: Proc. 8th ICUSD, Sydney, Australien, S. 1415 - 1422