Spring 2014 |
NCRWA.COM
15
feature
Disinfectant By-Products
Disinfectant decay occurs when chemicals used for disinfection react
with other organic material, organisms, and surfaces. These reactions
create disinfectant by-products known as Trihalomethanes (TTHMs)
and halo-acetic acids (HAAs). By-products are present in almost all
disinfected water supplies, but the key is keeping these levels as low as
possible. Mixing systems can help keep these levels low by effectively
blending all water with disinfectants increasing the contact time.
Ice Formation
Most compounds become denser as they change from a liquid to a solid
because the molecules become more tightly packed. But water is most
dense at 39.2°F (4° C), and becomes less dense at lower temperatures
causing ice to float. Water in a tank is the same and according to NFPA
25, “the temperature of water tanks shall not be less than 40°F (4.4°C).”
When temperatures fall below freezing, ice can form on the surface.
The ice will continue to expand and get thicker as the water temperature
remains below freezing. Ice can cause serious damage to a tank: tank
coatings are often damaged, pipes can become clogged, leaks and
structural damage can result, and even tank failures have occurred from
ice forming in water tanks.
When a tank freezes, one or more of the following
conditions may result and cause leakage:
• Inside overflow or other piping breaks occur.
• Ladders or other attachments to the container are pulled out by ice,
making a hole at the point of attachment.
• Ice pressure can expand and place hoop stress loads on the steel and
seams, which may cause the tank wall to yield or burst.
• Leaks due to corrosion become apparent.” AWWA M42
Mixing systems can provide regular water circulation to help maintain a
consistent water temperature above freezing and prevent the water from
separating into layers where dangerous ice can form on the surface and
sides of tanks.
Maintaining Water Quality
All tanks should be regularly monitored for mixing efficiency, but tanks
with low filling cycles, high volume, or at the end of a water system
should be monitored more often. These types of tanks may require
mixing system upgrades and may also require more frequent inspections
and cleanings to maintain water quality and reduce aging surface water.
Mixing systems can help prevent aging surface water, but they must
be designed and installed properly to be effective. A reputable tank
company with knowledge and experience in these types of upgrades
should be contacted to perform the work. For further information on
water tanks and mixing systems please contact Don Johnston at 270-
748-1343 or
-Erika N. Henderson has a bachelor’s degree in International Relations
from University of Southern Indiana. She has been published in
numerous water publications, and may be contacted at 270-826-
9000 ext. 228 or
to answer any questions
related to tanks.
References
American Water Works Association, 2013. Steel Water Storage Tanks. Manual M42 Revised
Edition, Chapter 5, Water Quality Issues.
American Water Works Association, 2013. Steel Water Storage Tanks. Manual M42 Revised
Edition, Chapter 10, Results of Freezing.
National Fire Protection Association, 2011. Standard for the Inspection, Testing, and
Maintenance of Water-Based Fire Protection Systems. NFPA 25 2011 Edition, Section
9.2.4.1.
