W Bauwens

Evaluation of uncertainty propagation into river water quality predictions to guide future monitoring campaigns

To evaluate the future state of river water in view of actual pollution loading or different management options, water quality models are a useful tool. However, the uncertainty on the model predictions is sometimes too high to draw proper conclusions. Because of the complexity of process based river water quality models, it is best to investigate this problem according to the origin of the uncertainty. If the uncertainty stems from input data or parameter uncertainty, more reliable results are obtained by performing specific measurement campaigns. The aim of the research reported in this paper is to guide these measurement campaigns based on an uncertainty analysis. The practical case study is the river Dender in Flanders, Belgium. First an overview of different techniques that give valuable information for the reduction of input and parameter uncertainty is given. A global sensitivity analysis shows the importance of the different uncertainty sources. Further an analysis of the uncertainty bands is performed to find differences in uncertainty between certain periods or locations. This shows that the link between periods with high uncertainty and specific circumstances (climatological, eco-regional, etc.) can help in gathering data for the calibration of submodels (e.g. diffuse pollution vs. point pollution).

Integrated modelling: Comparison of state variables, processes and parameters in sewer and wastewater treatment plant models

All the parts of an urban drainage system, i.e. the sewer system, the waste water treatment plant (WWTP) and the river, should be integrated into one single model to assess the performance of the overall system and for the development of design and control strategies assisting in its sustainable and cost effective management. Existing models for the individual components of the system have to be merged in order to develop the integrated tool. One of the problems arising from this methodology is the incompatibility of state variables, processes and parameters used in the different modelling approaches. Optimisation of an urban drainage system and of the waste water treatment process in particular requires a good knowledge of the waste water composition. As important transformations take place between the emission from the household and the arrival at the treatment facility, sewer models should include these transformations in the sewer system. At present, however, research is still needed in order to increase our knowledge concerning these in-sewer processes. A comparison of the state variables, processes and parameters has been carried out in both sewer models (SMs) and activated sludge models (ASMs). An ASM approach is used for the description of reactions in sewer models. However, a difference is found in the expression for organic material (expressed in terms of BOD) and heterotrophic biomass is absent as a state variable, resulting in differences in processes and parameters. Reconciliation of both the models seems worthwhile and a preliminary solution is suggested in this paper.

Modelling Water Quality In European Rivers

An in-stream water quality model, QUESTOR (Quality Evaluation and Simulation Tool for River-systems), has been applied to rivers in Belgium, Finland and the United Kingdom as part of a study of the effects of climate change on the quality of freshwater resources in Europe. QUESTOR provides a flexible modelling framework that enables the assessment of alternative model structures based on different process representations. Results are presented from the application of a basic form of the model that includes a limited set of processes. The results for present-day simulations are good on all three catchments although the catchments have very different characteristics. However, the requirement for a more sophisticated model for simulations under possible future climatic conditions is identified.