time, which will increase the number of organisms
to biogrowths. A strong biocide is usually needed to
destroyed, it will also increase trihalomethane formation.
prevent extensive marine growths from fouling intakes,
When an open intake requires optional disinfection as
piping and storage facilities. An overview of disinfection
shown in figure 5-1 and chlorination is shown by
alternatives can be found in figure 5-3. Chlorination is
pretreatment investigation to bring the trihalomethane
not advisable in the pretreatment process when organic
content above 0.1 milligrams per liter, then no form of
contents are high.
chlorine shall be used as a raw water disinfectant. The
a. Ultraviolet irradiation. One effective disinfection
legal limit (per National Interim Drinking Water
technique is ultraviolet irradiation.
Regulations) for trihalomethanes in potable water
Raw wafers flow through a chamber where they are
effluents for communities and installations with an
exposed to ultraviolet lamps. The resulting disinfection
effective population of 10,000 or more is 0.1 milligrams
prevents bacterial destruction of cellulose acetate-based
membrane systems and eliminates biofouling of
c. Dechlorination. When chlorine is used as a
polyaromatic-based membrane systems.
disinfectant, complete dechlorination of pretreated
polyaromatic fiber membranes are very susceptible to
waters must occur before desalination in polyaromatic
chemical oxidant attack, ultraviolet disinfection is the
Dechlorination can be
best disinfection technique for these systems.
accomplished chemically through sulfite compound
b. Oxidants. The addition of chemical oxidants,
addition or passage through granular-activated carbon.
such as chlorine, bromine, iodine, or ozone, can provide
Complete dechlorination and destruction of the chlorine
biological disinfection before membrane processes. The
residual by reducing compounds will ensure that
use of oxidants must be monitored carefully to keep the
chemicals do not attack these sensitive membrane
chlorine below 1.0 milligrams per liter of free chlorine
systems. When a chlorinated water source must be
residual that would even damage cellulose acetate
used and complete dechlorination below 1.0 milligrams
membrane systems. For cellulose acetate membrane
per liter as free chlorine is not possible, use
systems, a carefully monitored oxidant addition is the
best technique for disinfection of biologically active feed
economically practical to between 0.2 and 1.0
waters. When biological and other organic materials are
milligrams per liter as free chlorine, a cellulose acetate-
chlorinated, the resulting chlorine oxidation generates
based membrane system may be used.
halogenated carbon compounds, such as the
trihalomethane class of compounds.
placement of chlorine disinfection upstream of
coagulation of filtration will increase chlorine contact