Integration of ultrafiltration to conventional drinking water treatment for a better particle removal — efficiency and costs?

Abstract

The conventional drinking water treatment plant at Lake Constance consisting of microstraining, ozonation, rapid sand filtration and chlorination, produces drinking water for almost 3,500,000 people. Although compliance with the drinking water regulation was guaranteed at every time, discussions about a possible breakthrough of parasites and their insufficient inactivation during disinfection resulted in thinking about a further improvement of particle removal of the existing water treatment process. Therefore a pilot-scale ultrafiltration plant was operated for two years with sand filtered and microstrained surface water. While the conventional water treatment process achieved a removal of particles (1–100 μm) around 90%, UF improved particle removal by more than 99% with total particle counts < 1 particle per mL in the ultrafiltrate. The most important operational parameter achieved during the pilot experiment was permeability. Its mean value of 1.25 L/hm 2kPa at 5 to 6°C was marginally influenced by the concentration of chlorine for the periodic back-washing. Although particle removal was very effective, calculations concerning investment costs prove UF as very expensive compared to costs of alternative techniques like direct filtration. However, considerations are made at BWV to use membrane filtration for recycling filter backwash water.