Network operation without safety chlorination in Zürich

In 1993 the Zürich Water Supply started a full scale test of distributing drinking water without adding any chemicals at the outlet of the plants for network protection. The introduction of this test was planned over several years, during which the applied dose of disinfectants was gradually reduced. No problems occurred in the distribution network during the disinfectant-free operation period. The median of the heterotrophic plate count (HPC) increased from 0-4 to 0-7 CFU/mL in the treated water and from 0-3 to 0-5 CFU/mL at the sampling points in the network. Occasional increases in HPC to 100-300 CFU/mL which also occurred during periods of network protection, were removed by flushing with water at a velocity of 0.5 m/s.