Water Treatment - The Drinking Water - page 241

• Disinfectant time —The time required for water to move from one
point of disinfectant application (or the previous point of residual
disinfectant measurement) to a point before or at the point where the
residual disinfectant is measured.
• Disinfectant contact time (T in C×T × T calculation) —The time (in minutes)
required for water to move from the point of disinfectant applica-
tion or the previous point of disinfection residual measurement to
a point before or at the point where residual disinfectant concentra-
tion ( C ) is measured. Where only one C is measured, T is the time (in
minutes) required for water to move from the point of disinfectant
application to a point before or at where residual disinfectant con-
centration ( C ) is measured. Where more than one C is measured, T
is defined as follows:
• For the irst measurement of C , the time (in minutes) required for
water to move from the first or only point of disinfectant applica-
tion to a point before or at the point where the first C is measured
• For subsequent measurements of C , the time in minutes that
water takes to move from the previous C measurement point
to the C measurement point for which the particular T is being
calculated
• Disinfection byproduct —A compound formed by the reaction of a dis-
infectant such as chlorine with organic material in the water supply.
• Presence or absence of coliforms —Presence of coliform bacteria in
water is an indication that the water may be contaminated by patho-
genic organisms. Absence of coliform bacteria is considered to be
sufficient evidence that pathogens are absent—if the source is good,
a chlorine residual level is maintained and the supply has a good
h i s t or y.
• Sterilization —The destruction of all microorganisms. Sterilizing pota-
ble water requires the application of a much higher dose of chemi-
cal disinfectants, which would greatly increase operating costs and
would create taste problems for the consumer. Excessive application
of disinfectants also generates excessive levels of unwanted disinfec-
tion byproducts. For these reasons, current treatment practices are
used for turbidity removal and subsequent disinfection to the extent
necessary to eliminate known disease-causing organisms sufficient
to protect public health.
Note: Sterilization should not be confused with disinfection.
• Waterborne disease —Caused by pathogenic organisms in water.