Municipal Water Treatment

When Microorganisms Are Resistant to Chlorine

During the summer of 1993, Milwaukee, Wisconsin made headlines with one of the worst drinking water catastrophes in U. S. history. Heavy rains washed an excessive amount of runoff into Lake Michigan. While the reason is still unknown the runoff contained excessive amounts of the protozoa Cryptosporidium parvum. The runoff was drawn into Milwaukee's offshore intake pipe and processed by the city's water treatment plant. The Cryptosporidium was unaffected by the chlorination and continued on to the homes of the city residents. This quiet contamination went undetected for the next two weeks. The outbreak that followed left an estimated 400,000 residents with intestinal illness, 4,000 hospitalized and 50 deaths.

This tragic event led Milwaukee Water Works to replace chlorine with ozone as the primary disinfectant in 1998. The city operators are currently achieving 99% disinfection effectiveness against Cryptosporidium and have become leading experts in the science of Cryptosporidium inactivation.

A 2002 Milwaukee Water Works report stated that Cryptosporidium oocysts are present in 65% - 97% of surface waters tested throughout the country. Furthermore, oocysts have been detected in filtered tap water in 27% - 54% of the communities evaluated in their study. With chlorine and UV proven ineffective against hardened microorganisms on an industrial scale, the clear choice is ozone.

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