2018-050 Water Storage Tank DrainingDate: 5/4/2018 Report No. 2018-050
INFORMAL STAFF REPORT
TO MAYOR AND CITY COUNCIL
SUBJECT:
Water Storage Tank Draining by the Water Production Division of the City of Denton.
BACKGROUND:
Water storage tanks are used as an integral part of the water supply, transmission and distribution
system network. The most common tanks are either ground or elevated storage tanks (“ESTs”,
often referred to as “water towers”).
Ground storage tanks are used to store treated water at one of the city’s two water treatment
plants prior to pumping the water into the distribution systems. ESTs are located within the
water transmission and distribution system, and are used to store pumped water during low water
demand times of the day for and then supply this stored water during when demands peak later
that day. ESTs also help maintain water pressure within the distribution system and are used to
maintain emergency reserves for unplanned events such as fighting fires, loss of pumping due to
power outages, or major losses of water due to main breaks.
The city currently has two water treatment plants and four ground storage tanks with a total
storage capacity of 12 million gallons. Within the water transmission and distribution system
network, the city has ESTs within the Central Middle Pressure Zone totaling 7 million gallons of
storage capacity (Riney Road, McKenna Park, and Roselawn). The City also has two ESTs that
serve the Northwest Upper Pressure Zone and the Southwest Upper Pressure Zone. These two
tanks (referred to as Northwest the Southwest) add an additional 4 million gallons of elevated
storage to these two service areas. The total storage capacity in all of these tanks combined is 23
million gallons. This compares to the average daily water demand for the city of approximately
21 million gallons per day and the combined maximum water treatment plant capacity of 48
million gallons per day.
DISCUSSION:
On occasion, water storage tanks are drained to perform various maintenance functions
throughout the year. The reasons these tanks may be drained and taken out of service vary and
can include routine inspections, maintenance and repairs, or to address water quality concerns
related to water age, nitrification, or insufficient disinfection concentrations.
The Texas Commission on Environmental Quality (TCEQ) requires all public water systems to
conduct annual inspections of all water storage tanks. This involves exterior visual condition
inspections which are performed by city staff. Utilities also conduct “pull down” inspections on
a less frequent basis to inspect the interior tank coating systems. A typical pull down inspection
requires the tank to be drained, washed out, manned entry with visual inspections, and tank
disinfection prior to refilling. Some water systems may use water tank contractors that deploy
robotic inspection devices, or even underwater divers to inspect the interior of the tank while it is
still in service. While these types of inspection do not require tanks to be drained, the
inspections do require specialized contractors that have the appropriate equipment and expertise.
Date: 5/4/2018 Report No. 2018-050
The City of Denton currently uses Water Department staff to conduct the annual external visual
inspections, and to perform internal inspections of tanks on 3 to 5 year intervals.
Storage tanks may also be drained to conduct more extensive condition assessments of the tank’s
interior coating systems. These inspections are infrequent, and are conducted by tank inspection
and coating system experts. Results are used by Water Production staff to prepare specifications
and bid information for removing and replacing the interior coating system. These activities are
generally conducted every 15 to 20 years. As a part of process, the tank will be drained to
complete recoating work and then drained again in a couple of years to inspect for defects and
facilitate warranty repairs. This sequence can result produce situations where tanks are drained
on multiple occasions (inspection, recoating, re-inspection, and warranty repairs if needed).
In addition to regulatory inspections and interior coating maintenance tasks, there can be
situations where water storage tanks are drained for water age and water quality reasons. This
may be done to meet regulatory compliance requirements or to simply ensure water customers
have the highest quality and safest drinking water possible. The majority of these situations are
associated with longer water age and higher water temperatures. Issues can include nitrification,
loss of disinfection residual and, if left unmanaged, bacterial regrowth. There are many methods
to manage these potential water quality concerns and the simplest and most effective method is
to operate pumping systems to keep water moving into and out of water storage tanks on a daily
basis. To accomplish this, water system operators at the treatment plants have standard operating
protocols for finished water pumping cycles that include maximizing water circulation into and
out of the tanks. Tank design features such as mechanical mixers within the tank and having
separate inlets and outlets for the tank can also be used. Disinfection residual loss due to water
age can be resolved by using disinfection boosting facilities at the storage tank to manage
disinfection residual levels. Over the years, the City of Denton has installed some of these
features in their newer facilities has retrofitted many of the older facilities. These actions have
greatly reduced the need to drain tanks or take tanks out of service for managing water quality
reasons.
Over the past two years, the Water Production Division staff has drained water storage tanks on
10 occasions. This included three events related to tank coatings and regulatory inspections, two
events related to installation of tank mixers or tank disinfection reboost systems, and five events
related to water quality maintenance. The five water quality maintenance events occurred
within the Northwest, McKenna Park, and Southwest tanks. The Northwest EST and the
Southwest EST have been more prone to these types of problems since water in these tanks has
already been stored and pumped out of the Central Pressure Planes into the Upper Pressures
Planes. Water quality events typically occur in late summer and early fall when water
temperatures are elevated but water system demand can quickly drop, leading to increasing water
age within the system. To address these concerns, staff has recently installed a disinfection
reboost system for the Southwest EST and plans to install a similar system at the Northwest EST
in the near future.
Date: 5/4/2018 Report No. 2018-050
CONCLUSION:
There are multiple reasons why a water storage tank may need to be drained. Even though
draining of a tank represents a large volume of water, the net loss of water from these activities
represents approximately 0.2% of the total water pumped into the system each year. Staff has
implemented various strategies to minimize this loss of water and continues to explore new
methods to minimize operational water losses.
ATTACHMENT(S):
None
STAFF CONTACT:
Tim Fisher, P.E.
Director of Water Utilities
(940) 349-7190
Tim.Fisher@cityofdenton.com