At present in Switzerland drinking water is obtained from karstic spring water in more than 35 plants – often in only one processing stage. Most plants are located in the Valais, in the foothills of the Alps in central Switzerland, in the Jura or in the area around the Säntis mountain. They have an output of less than 1000 m3/day. Apart from pre-filtration at 200 μ they can be operated without any other pre-treatment, however some natural water contains such high levels of organic material after thunderstorms or when the snow melts that pre-treatment becomes necessary. Initial results from a research project show that by determining a fouling index, which is underpinned by LC-OCD analysis, useful information on the feasibility of ultra-filtration as well as the best type of pre-treatment with flocking agents or powdered coal can be obtained.