【正文】 and its DBPs ? UV radiation – low pressure mercury lamp: low intensity。 monochromatic at 254 nm – medium pressure mercury lamp: higher intensity。 polychromatic 220280 nm) – reacts primarily with nucleic acids: pyrimidine dimers and other alterations Disinfection Kiics ? Disinfection is a kiic process ? Increased inactivation with increased exposure or contact time. – Chick39。s Law: disinfection is a firstorder reaction. (NOT!) – Multihithit or concave up kiics: initial slow rate。 multiple targets to be “hit” – Concave down or retardant kiics: initial fast rate。 decreases over time ? Different susceptibilities of microbes to inactivation。 heterogeneous population ? Decline of of disinfectant concentration over time ? CT Concept: Disinfection can be expressed at the product of disinfectant concentration X contact time – Applies best when disinfection kiics are first order ? Disinfectant concentration and contact time have an equal effect on CT products ? Applies less well when either time ofrconcentration is more important. Contact Time Multihit First Order Retardant DISINFECTION AND MICROBIAL INACTIVATION KINETICS Log Survivors Factors Influencing Disinfection of Microbes ? Microbe type: disinfection resistance from least to most: vegetative bacteria ?viruses ? protozoan cysts, spores and eggs ? Type of disinfectant: order of efficacy against Giardia from best to worst – O3 ? ClO2 ? iodine/free chlorine ? chloramines – BUT, order of effectiveness varies with type of microbe ? Microbial aggregation: – protects microbes from inactivation – microbes within aggregates can not be readily reached by the disinfectant Effects of Water Quality on Disinfection ? Particulates: protect microbes from inactivation – microbes shielded or embedded in particles ? Dissolved anics: protects – consumes or absorbs (UV radiation) disinfectant – coats microbes ? Inanic pounds and ions: effects vary with disinfectant ? pH: effects depend on disinfectant. – Free chlorine more biocidal at low pH where HOCl predominates. – Chlorine dioxide more microbiocidal at high pH ? Reactor design, mixing and hydraulic conditions。 better activity in plug flow than in batchmixed reactors. Inactivation of Cryptosporidium Oocysts in Water by Chemical Disinfectants Disinfectant CT99 (mgmin/L) Reference Free Chlorine 7,200+ Korich et al., 1990 Monochloramine 7,200+ Korich et al., 1990 Chlorine Dioxide 78 Korich et al., 1990 Mixed oxidants 120 Venczel et al., 1997 Ozone ~318 Finch et al., 1994 Korich et al., 1990 Owens et al., 1994 C. parvum oocysts inactivated by low doses of UV radiation: 10 mJoules/cm2 Disinfection: A Key Barrier Against Microbes in Water ? Free chlorine still the most monly used disinfectant ? Maintaining disinfectant residual during treated water storage and distribution is essential. – A problem for O3 and ClO2, which do not remain in water for very long. – A secondary disinfectant must be used to provide a stable residual ? UV radiation is a promising disinfectant because it inactivates Cryptosporidium at low doses – UV may have to be used with a chemical disinfectant to protect the water with a residual through distribution and storage