Slow sand filtration is a treatment process that contributes importantly to drinking water safety. In water practice, however, the concentrations of microorganisms are so low that one cannot demonstrate the level of a filter’s effectiveness. This will therefore be studied in an experimental column set-up, in which doses of high concentrations of organisms will be added to the water.
The results will be used to validate a model which can predict the effectiveness of slow sand filtration under different operational conditions. The safety of drinking water can thus be continuously secured.
Securing safe drinking water
Water utilities that make use of surface water must conduct an analysis of microbial safety of drinking water (Dutch initials: AMVD). The analysis, which uses a quantitative microbial risk analysis (QMRA), must show that the infection risk does not exceed the threshold value of 1 infection per 10,000 people per year. Slow sand filtration (SSF) makes an important contribution to the removal of pathogenic microorganisms in this regard. The extent of the removal of the pathogens will be determined through experiments and, subsequently, translated via a process model to the operational context, in which the relevant process conditions can vary.
Testing sand filters for all pathogens
In 2016 at the De Punt treatment plant, it was shown that Cryptosporidium was very effectively removed. This was not anticipated, because earlier tests had actually shown relatively poor removal levels of bacteria and viruses. There is therefore a need, for the purposes of the AMVB, for a new assessment of the removal effectiveness of bacteria and viruses under better controlled conditions than those of the earlier tests.
Validation of the slow sand filtration process model
The results of tests of the effectiveness under different conditions can be used to validate a model for slow sand filtration. This would enable predictions for the removal of pathogens under different operational conditions. This knowledge will make it possible to manage the process in such a way that safe water is always produced. A reliable, validated SSF model is also of interest to water utilities Dunea and Waternet, which also make use of SSF.