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OXIDATION AND DISINFECTION 10.27

to degrade, so storage time should be limited. Because degradation is a strong function
of concentration, the chemical will degrade more in absolute terms on day 1 than on day
10 or 20.
In that the rate of degradation is a function of temperature and concentration, storage
time should be limited. Often storage is limited to less than 28 days and temperatures to
70 ° F (21 ° C) or less. The chemical may be delivered warm, that is, at temperatures up
to 85 ° F (30 ° C), which must be considered in the system design. Alternative approaches
to sodium hypochlorite management include installation of the storage tanks in a climate-
controlled room to maintain the chemical at the desired temperature, provision for addi-
tional storage capacity to allow for dilution of the chemical upon delivery, or installation
of immersion coolers in the storage tanks connected to a chilled water source to cool the
chemical. The first and second approaches are far more common than the third.
Materials that are used for bulk storage of sodium hypochlorite include FRP (fiber-
glass-reinforced plastic), polyethylene (polyethylene resins vary, so specific selections
must be specified with a tank supplier), or lined carbon steel. FRP tanks are the most com-
mon selection. They are superior in construction as compared to polyethylene and less
expensive than lined carbon steel. Sodium hypochlorite tanks should be vented, and pro-
vision should be made for sampling the contents.
Schedule 80 PVC and CPVC are the most commonly used materials used for sodium
hypochlorite piping. Historically, many PVC and CPVC piping systems have failed due
to leaks at the solvent welded connections. Special attention must be paid to the glues
used to put the PVC and CPVC pipe together. Glues generally available are not suitable
for sodium hypochlorite service. The glues consist of solvent, native material (PVC or
CPVC), and additives. Sodium hypochlorite attacks the additives and results in leaks at
the solvent welded connections. As a result, only glues that consist solely of solvent and
the native material can be used.
Special attention must be paid to the valves used in sodium hypochlorite systems. Com-
mon choices include ball and diaphragm valves. Ball valves are unsuitable for sodium
hypochlorite service for two reasons: Chemical can be trapped in the interstitial volumes
of the valves, resulting in exploding valves, and the presence of sodium hydroxide results
in valve sticking. Special attention must also be paid to the specifics of the diaphragm
valves selected. Sodium hypochlorite is especially aggressive to elastomers, and as such,
common elastomers such as EPDM or Viton cannot be used in direct contact with sodium
hypochlorite. Rather a two-piece diaphragm consisting of a Teflon diaphragm contacting
the chemical with an elastomeric backing (usually EPDM) should be used. The Teflon
and elastomeric backing must be independent of the diaphragm and cannot be directly
bonded to each other.

Hypochlorite Feed
Most small to medium-size water systems feed hypochlorite with positive displacement
diaphragm metering pumps. Because of the effervescing experienced in sodium hypochlo-
rite systems due to the degradation characteristics, metering pumps are subject to air bind-
ing. When metering pumps air-bind, the pumps stall and are incapable of pumping chem-
ical, which results in interruption of chemical feed (and hence lack of chlorination and
potential plant shutdown). As such, special provisions should be followed to limit pump
air binding:
° The metering pumps should be located as close to the storage tanks as possible. The
suction lines should be oversized.
• The pumps should be located well below the level of the liquid in the storage tank.