The water safety and potability of stored water can be
maintained with periodic testing and chlorination. The
residual levels of free chlorine should be kept at between
0.2 to 1.5 ppm. However, this is often difficult since chlorine
chemicals decompose and volatilize, losing their strength
over time and with changing environmental conditions. Water
storage tanks that hold a known initial volume of water may
be initially chlorinated to insure water potability. However,
over time, as water is being used, the addition of progressively
smaller amounts of chlorine chemicals may be needed to
maintain safe levels of residual chlorine.
Water stored in overflow storage tanks, used for example
in low-yield wells, is seldom chlorinated as the water
volumes vary daily and turnover rates are variable. Note,
however, that periodic cleaning and disinfection of overflow
tanks and supporting equipment (pumps, pipe systems)
is necessary and recommended to prevent the growth of
opportunistic pathogens such as the bacteria Legionella sp. and
Mycobacterium. These bacteria are commonly found in water
environments and tend survive in biofilms. More recently
they have been found to grow at rapid rates in distribution
systems or storage tanks that are not regularly disinfected or
properly maintained (Rock un-published data).
Once water is chlorinated, following the steps shown
above, the level of residual free chlorine may be tested using a