Major emphasis must be laid on guaranteeing high standards in drinking water quality. This can only be achieved when the drinking water treatment plant operates optimally. In this paper a treatment plant, consisting of ozonation with low dosage of ozone, pellet softening, pH correction and activated carbon filtration, treating lake water with high content of natural organic matter is analysed. Models are developed to describe the processes under changing conditions. For integrated modelling interaction of the treatment steps is considered to take place through the water stream. Water quality parameters are influenced by preceding treatment steps and they determine the performance of the following treatment steps. Assuming that disinfection is sufficient in the treatment plant and suspended and colloidal solids are sufficiently removed in preceding steps, the main controlled parameters for a treatment plant are chemical stability (saturation index SI), biological stability (assimilable organic carbon AOC), disinfection by-products (bromate) and organic micro-pollutants (pesticides). It is concluded that with good use of the by-pass of a pellet softening reactor and with optimised operation of the fluidised bed, the dosage of chemicals can be minimised, resulting in reduction of costs, environmental emissions and maintenance. Ozonation has a key role in the bio-stability and the toxicity of the drinking water. When ozone is only applied for oxidation (low dosages) some organic micro-pollutants are reduced and the run-time of the activated carbon filters is prolonged. However, ozonation decreases the bio-stability of the drinking water and enhances bromate formation. Therefore, the quality benefits related to oxidation of organic micro-pollutants should be weighted against the formation of bromate and AOC. The increase of AOC concentrations should be compensated by longer contact times in the biological filters.

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