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By Erika N. Henderson, Director of Research, Pittsburg Tank & Tower
Can Mixing Systems Help
Prevent Aging Surface Water?
14
NCRWA.COM |
Spring 2014
feature
n the past, many water operators thought daily turnover and fluctuation
of volume was enough to mix the water in a storage tank. Today,
research and tests are showing more effort may be needed to effectively
mix storage tank water and improve water quality. Several water tank
features can contribute to how effective water is mixed and the rate at
which it ages.
A tank’s design, daily use, and location in the system should all be
considered when deciding the best action for improving water quality.
Tanks with high height to diameter ratio, such as standpipes, are more
susceptible to aging surface water because greater water variations can
develop among the many layers. As the distance between surface and
bottom water increases, more effort is required for the bottom layers to
reach the distant top layers. Same thing applies when inlet and outlet
pipes are both placed in the bottom ring of a tank: the new incoming
water is also the first to exit the tank while surface water is left to
continue aging. However, if the inlet pipe is extended to allow over-the-
top filling, then surface water can be pushed to the bottom and mixed
with incoming water.
“Water quality is a significant concern to water distribution system
managers. Disinfection by-product (DBP) formation is largely
dependent on reaction time, and it can continue for several days within
the distribution system. At the same time, disinfectant residual must
be maintained throughout the most remote components of the system
to ensure pathogen-free water. Managing the residence time of water
within storage tanks is one practice available to minimize water age
within the distribution system. Water system managers and engineers
should consider the need for circulation of water and residence
time management within storage tanks during the design phase.”
AWWA M42
Although, water quality and circulation are often discussed during the
design phase, they are continuing concerns for water operators. Great
responsibility comes with being a water operator and many pride
themselves on providing excellent water quality. A popular method for
improving water quality is to install a mixing system. Mixing systems
can help prevent aging surface water that often leads to stratification,
accumulation of disinfectant by-products, water quality loss and
ice formation.
Stratification
Stratification occurs when water has become separated into layers.
Layers are arranged according to density, and density differences are
created when variations of water temperature or pH exist. The warmer,
less dense, older water sits above the cooler, denser, new water coming
in creating layers. The incoming water stays near the bottom and is also
the first to exit the tank if an over-the-top fill has not been installed. The
top layer of older water is left to age even further. As water ages, the
quality deteriorates and bacterial growth increases. The bacteria may
not cause sample problems in the warmer months, but may show up
when the older, surface water begins to cool and sink to the bottom in
the cooler months.
Water temperature can be checked every five feet during an inspection
to help determine whether stratification is a problem. If stratification is
an issue, then a mixing system may be needed. Mixing systems should
be designed according to the tank’s unique dimensions and needs.
Most are designed to take the denser, newer water from the bottom
and mixing it with the less dense, warmer surface water. Mixing helps
maintain consistent water temperatures and pH levels throughout
the tank thus preventing stratification. However, mixing must occur
regularly to prevent reoccurring stratification, sample inconsistences
and inaccurate disinfection treatment.
