Currently, of conventional disinfectants used in water treatment, only ozone a nd chlorine dioxide are recognized to have the ability to inactivate Cryptosporidium, an opportunistic pathogen. San Facisco ahs been conducting pilot studies since August 1992 on their 300 mgd Hetch Hetchy source. One of the elements of the study has been developing ozone dose criterial for Giardia and Cryptosporidium inactivation for the case where the source remains unfiltered. WHile the Ct products for Giardia and Cryptosporidium vary by an order of magnitude, the oone dose requirements vary by a factor of 1.6 to 4.0 depdending on pH, water quality, and contact time. The implication for ozone system design is that loger contact times are more critical when targeting Cryptosporidium inactivation.
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