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Water Lecture 3 A. Disinfection B. Water Safety Plans - page 9 / 47

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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 not be readily reached by the disinfectant

Particulates: protects from inactivation; shielded/embedded in particles

Dissolved organics: protects

consumes or absorbs (UV radiation) disinfectant; coats microbes

Inorganic compounds 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.

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