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Mitigating the Impacts of Elevated Water
Storage Tanks on Water Quality
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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Copyright © Stantec Inc. All Rights Reserved. This document may be
freely redistributed in its entirety provided that this copyright notice is not
removed. This document may not be sold for profit or used in commercial
documents without the written permission of the copyright holder. This
document is for information purposes only and does not constitute the
rendering of professional advice or recommendations by Stantec Inc. All
information herein reflects the writer’s professional judgment given the
information available at the time of preparation.
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
Table of Contents
Introduction............................................................................................... 1
Barrhaven Water Pressure Zone.............................................................. 1
Benefits Of Elevated Storage ................................................................... 2
Elevated Tank Design – Improve Internal Mixing..................................... 4
Elevated Tank Operations – Tank Replenishment................................... 5
Summary ................................................................................................ 10
Acknowledgments .................................................................................. 10
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
Introduction
From an operational perspective, elevated water storage tanks are
generally considered extremely beneficial to water distribution systems.
These “floating” storage tanks reduce peak pumping rates, stabilize
pressures, and increase reliability. However, elevated storage can be
significantly more costly than in-ground storage (depending on pumping
needs) and, if not designed and operated properly, can have a
detrimental impact on water quality. As water quality is becoming an
increasingly critical factor in water supply and distribution systems, it is
crucial that potentially negative water quality impacts from elevated
storage be recognized and eliminated.
Water quality in the distribution system has been addressed at both the
design and operational stages at the Barrhaven Elevated Water Storage
Tank, recently constructed in the City of Ottawa. The goal is to ensure
that the overall travel time from the treatment plant to the customer does
not increase significantly with the implementation of an elevated tank.
This helps ensure that adequate chlorine residuals are maintained at all
points in the system. The tank design features include a dual-riser pipe
system with check valves to allow for complete mixing of new water with
old, thereby eliminating the possibility of stagnation from “dead” areas
within the tank bowl. Operational guidelines have also been established
to help maintain maximum turnover of water in the tank and to control the
replenishment of water in the lengthy feed pipe connecting the tank to
the distribution system. Since the feed pipe to this particular tank is over
2700m in length (from the last connection to the distribution system) and
is 610mm in diameter, there is a significant volume of water in the pipe at
any given time. An analysis was undertaken to confirm that the water
volume in both the tank and feed pipe could be effectively replenished
with fresh chlorinated water and thus maintain appropriate chlorine
residual levels at all times.
This paper will review the design, discuss operational procedures, and
explain how water quality issues were addressed for the Barrhaven
Elevated Water Storage Tank.
Barrhaven Water Pressure Zone
The Barrhaven Pressure Zone is one of eleven (11) major pressure
zones in the City of Ottawa’s central water system. It is fed from the
Britannia Water Purification Plant through approximately 18km of
1525mm and 1220mm diameter water main. The Barrhaven Pump
Station increases the hydraulic grade line (HGL) to feed this zone at
appropriate pressures. The Barrhaven Elevated Tank is located at the
western extremity of the pressure zone, away from existing development,
as shown in Figure 1. A single 610mm water main and a combination of
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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305mm and 406mm water mains feed the tank from the pump station
through the developed area.
Figure 1: Barrhaven Tank – General Location
The current serviced population in the Barrhaven Pressure Zone is just
over 25,000 persons. The maximum day demand for this zone, which
consists primarily of low and medium residential development, is
approximately 19ML/d and the peak hour demand is close to 40ML/d. By
2021, the population of this area is expected to be close to 90,000
persons, an increase of 360%.
Benefits Of Elevated Storage
Water storage is an extremely important element in a water distribution
system. The principal advantages of distribution storage is that it can
moderate the demands placed on the major supply sources, production
works as well as major transmission mains. As a result, the sizes and/or
capacities of each of these distribution system elements may be
reduced. Additionally, distribution storage normally results in stabilized
system pressures. Reserve supplies of water in distribution storage also
provide a redundant source of water during emergencies, such as fires;
water main breaks, and pump station power outages. The major
purposes of storage are summarized as:
•
Operational equalization (balancing storage)
•
Fire flow provision
•
Emergency needs
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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There are two types of distribution storage. These are in-ground (or atgrade) reservoirs and elevated storage tanks. Generally, elevated
storage tanks have been limited to a volume of approximately 9.0ML,
whereas in-ground reservoirs are not limited in volume. In-ground
reservoirs may utilize either a direct pumping system (which requires a
pumping system to pump water from the reservoir to system pressures)
or an indirect pumping system (water “floats” on the system and flows by
gravity from the tank to system pressures). Elevated storage tanks are
virtually always indirect systems and float at the connected system HGL.
The indirect pumping system (with floating or elevated storage) has
several advantages over a direct pumping system in a water distribution
system:
•
Lowering peak pumping rates
•
Stabilizing pressure variations as demands fluctuate
•
Maintains constant, reliable water supply and pressures
•
Increased operational flexibility, efficiency and convenience
•
Balancing and leveling pump operations
•
Reduces need for wide range of pump sizes
•
Decreasing power costs – particularly for “time-of-day” energy
pricing
•
Immediate emergency response (main break, power failure)
•
Immediate fire flow and pressure response
•
Surge relief – dampens extreme low and high low pressures
associated with hydraulic transients
•
Ensures constant system pressure and helps prevent
contamination from possible inflow
Based on the above, it is thus evident that distribution system storage is
extremely beneficial to a water supply system. In particular, storage with
indirect pumping (i.e. floating storage) provides clear advantages to the
operation of the system and should thus be considered whenever and
wherever practical and feasible.
The major disadvantage with any distribution storage is the possibility of
increased travel time from the treatment plant (chlorination point) “to the
tap”. Ensuring adequate replenishment of water in storage facilities is a
key factor in minimizing the potential chorine decay in the treated water.
Two factors may be considered to improve this – ensuring good mixing
of fresh water with stored water in the tank and ensuring that the system
is operated to maximize the replenishment of fresh water in the tank. It is
noted that in-tank mixing is preferred for smaller tank volumes – “plug”
flow may be preferred to ensure adequate water turnover in larger tanks.
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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As such, a review of the potential impact of the Barrhaven Elevated
Water Storage Tank on water quality has been undertaken to determine
any special measures or operating procedures should be followed after
the tank is commissioned.
Elevated Tank Design – Improve Internal Mixing
To improve mixing of fresh water with “old” water inside the tank bowl, a
two-riser pipe system with check valves has been incorporated in the
design. This is shown schematically in Figure 2. The check valves
ensure the tank fills through only one of the risers and is drawn down
through the second riser pipe. The top of the “fill” riser pipe is angled to
promote circulation in the tank and minimize the opportunity for “shortcircuiting” of flow in the bowl.
The fill riser pipe will extend into the bowl of the tank to approximately
300mm below the minimum level of the normal operating range. The
draw riser pipe will be located at the base of the bowl. A series of check
valves will be used to ensure that the proper riser pipe is used for either
filling or drawing. Each riser pipe will have a diameter of 600mm and will
be insulated. A 400mm diameter overflow pipe will also be located in the
structure to ensure that the maximum water level is never exceeded.
Valves located in the control room will consist of a check valve for the fill
riser pipe, a check valve for the draw riser pipe, a main shutoff valve and
shutoff valve to form a drain connection between the overflow and the
draw riser pipe. This drain connection will allow for the tank to be
emptied via the overflow pipe outlet. A chemical feed nozzle will also be
included on the drainpipe to allow for dechlorination during the emptying
of the tank. It is also proposed to add two additional valves and couplings
on the fill riser pipe and the draw riser pipe to allow for a future recirculation pump, should one be required.
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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Figure 2 Elevated Tank – Internal Riser Design
Elevated Tank Operations – Tank Replenishment
The proposed tank has a total volume of 6.8ML or 6800m3. A range of
normal operating volumes has been considered, varying from 20% to
60% of tank usage on a typical day (40% is generally considered normal
for the City of Ottawa).
Daily Balancing/Operating
Volume Used
3
Volume Remaining
5440 m3
20%
1360 m
40%
2720 m3
4080 m3
60%
4080 m3
2720 m3
The Basic Day demands for the present and future are used as these
represent the minimum expected consumption during low demand
periods:
2001
2011
BSDY Demand 7,000 m3/day
2021
16,000 m3/day 20,000 m3/day
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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The volumes of different pipe sections have also been calculated as part
of this exercise. The major pipe sections are delineated as follows:
Tank Connecting Pipe (from last water connection to tank)
790 m3
610mm from tank to Jockvale/Cedarview (2700m)
Existing Pipes (from tank to Barrhaven PS)
610mm from tank to Jockvale/Cedarview (2700m)
790 m3
406mm from Jockvale/Cedarview to Barrhaven PS (5250m)
680 m3
1470 m3
TOTAL
Future Pipes (from tank to Barrhaven PS)
610mm from tank to Jockvale/Cedarview (2700m)
790 m3
610mm from Jockvale/Cedarview to
Strandherd/Greenbank (4100m)
1200 m3
762mm from Strandherd/Greenbank to Barrhaven PS (1560m)
TOTAL
710 m3
2700 m3
Existing and Future Pipes (both routes from tank to Barrhaven PS)
610mm from tank to Jockvale/Cedarview (2700m)
790 m3
406mm from Jockvale/Cedarview to Barrhaven PS (5250m)
680 m3
610mm from Jockvale/Cedarview to
Strandherd/Greenbank (4100m)
1200 m3
762mm from Strandherd/Greenbank to Barrhaven PS (1560m) 710 m3
TOTAL
3380 m3
The first level of analysis considers the daily consumption in the pressure
zone and the volume in the pipe sections only. This is used to determine
if the entire volume of water in the pipes is replenished with “fresh” water
every day based solely on the consumption in the pressure zone without
using storage. The 2001 Basic Day demand of 7000m3 is sufficient to
ensure turnover of the water in each of the existing pipe sections and the
future major pipe sections. Thus, full replenishment of the water in the
pipes is expected on a daily basis.
The second level of analysis considers the daily consumption in the
pressure zone and the volume used in the tank on a daily basis. This is
used to determine if the entire operating volume of the tank can be used
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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every day for consumption only in the pressure zone without using
pumping. The 2001 Basic Day demand (7000m3/day) is greater than the
expected daily operating volumes of the tank under all circumstances.
Thus, the full operating volume of water from the tank can be consumed
in the system regardless of how the tank is operated.
The third level considers the amount of water remaining in storage (for
different operating conditions) and the volume of water in the pipe
section between the tank and the last water connection. In this case, the
“last” connection is considered to be that at the intersection of Jockvale
and Cedarview, which is considered conservative since there is a feed
off this pipe to the developed lands northwest of Fallowfield and
Cedarview. This assessment is used to determine approximately how
much volume in the tank is refilled with fresh water on a daily basis, as
opposed to “old” water (“old” water being the un-used volume in the tank
plus the amount in the final pipe section between the tank and the last
connection, which equals the total amount of “old” water in the tank after
refilling).
Daily Balancing Use
Tank (daily)
Fresh Water Volume
Fresh Water In
20%
1360 – 790 = 570 m3
8%
40%
2720 – 790 = 1930 m3
28 %
60%
4080 – 790 = 3290 m3
48 %
To determine the potential impact on water quality given varying
amounts of “fresh” water in the tank resulting from varying the amount of
storage used on a daily basis, curves were developed to show the net
chlorine residual after a given period of time. These curves, which are
based on a chlorine decay rate of 0.1mg/L per day, are shown in Figure
3 and suggest the following:
•
If the storage is operated at 20% balancing per day, the net
chlorine residual in the tank will continuously decay to eventual
depletion.
•
If the storage is operated at 40% balancing per day, the net
chlorine residual in the tank will level off after about 7 days and
will have depleted by about 0.25 mg/L.
•
If the storage is operated at 60% balancing per day, the net
chlorine residual in the tank will level off after about 5 days and
will have depleted by about 0.11 mg/L.
•
The rate and magnitude of the depletion is constant, regardless
of the incoming “fresh” water chlorine concentration. For
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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example, if “fresh” water enters the tank at a concentration of
0.75mg/L, the long-term chlorine concentration in the tank, if
operated at 40% balancing, will be approximately 0.5mg/L.
•
To maintain a reasonable chlorine residual in the tank in the long
term, it must be operated with more than 20% of the total volume
being used on a daily basis. As expected, the more water that is
moved through the tank in a single day, the higher the chlorine
residual in the tank.
Net Chlorine Residual
@ 0.1mg/L Decay per Day
1.0
0.9
Concentration (mg/L)
0.8
0.7
0.6
20% Balancing
40% Balancing
60% Balancing
0.5
0.4
0.3
0.2
0.1
31
29
27
25
23
21
19
17
15
13
11
9
7
5
3
1
0.0
Days
Figure 3: Net Chlorine Residual – Constant Decay
Figure 4 indicates the residual chlorine level if it is assumed that that
chlorine decays at a constant 10% per day (provided for comparative
purposes only). At 40% balancing storage operations, the long-term
chlorine residual is expected to drop by 20% under this scenario.
The results presented herein were derived from a conservative analysis
of the Barrhaven Tank and associated distribution system. The purpose
of this high level analysis is to provide some insight as to how water
quality is generally affected by the tank size based on various modes of
operation.
The analysis assumed there was complete mixing in the tank and that
the physical characteristics of the water remained unchanged, such as
pH and temperature. A constant chlorine decay rate of 0.1 mg/L per day
was used. The analysis also assumed plug flow condition in the pipes,
and no water demand to the west of the tank. It should also be noted that
the analysis does not reflect the water quality at the customer’s tap
because of further mixing of the water from the tank in the water
distribution system before it reaches the customer. Therefore it is
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
Page 8 of 10
expected that the pipes will be maintained at higher chlorine residuals
than that in the tank.
Figure 4 indicates the residual chlorine level if it is assumed that that
chlorine decays at a constant 10% per day (provided for comparative
purposes only). At 40% balancing storage operations, the long-term
chlorine residual is expected to drop by 20% under this scenario.
The results presented herein were derived from a conservative analysis
of the Barrhaven Tank and associated distribution system. The purpose
of this high level analysis is to provide some insight as to how water
quality is generally affected by the tank size based on various modes of
operation.
The analysis assumed there was complete mixing in the tank and that
the physical characteristics of the water remained unchanged, such as
pH and temperature. A constant chlorine decay rate of 0.1 mg/L per day
was used. The analysis also assumed plug flow condition in the pipes,
and no water demand to the west of the tank. It should also be noted that
the analysis does not reflect the water quality at the customer’s tap
because of further mixing of the water from the tank in the water
distribution system before it reaches the customer. Therefore it is
expected that the pipes will be maintained at higher chlorine residuals
than that in the tank.
Net Chlorine Residual
@ 10% Decay per Day
1.0
0.9
Concentration (mg/L)
0.8
0.7
0.6
20% Balancing
40% Balancing
60% Balancing
0.5
0.4
0.3
0.2
0.1
31
29
27
25
23
21
19
17
15
13
9
11
7
5
3
1
0.0
Days
Figure 4 : Net Chlorine residual – Recent Decay
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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Summary
Elevated water storage tanks can be extremely beneficial to water
distribution systems operations. These “floating” storage tanks reduce
peak pumping rates, stabilize pressures, and increase reliability. It is,
however, extremely important that potentially negative water quality
impacts from storage be recognized and eliminated.
To help minimize potential water quality degradation, simple tank design
features should be considered along with specific operational
procedures. Recommended tank design features, which will help
promote complete mixing of fresh water with “old” water in the tank and
eliminate the possibility of stagnation from “dead” areas within the tank
bowl, include:
•
Dual-riser pipe system with check valves,
•
Angled inflow riser stem.
•
Operational procedures should consider water quality, and
should include:
•
Maximize fresh water turnover (operate tank to lowest level
possible),
•
Consider tank volume as well as supply piping in water quality
analysis.
Acknowledgments
John D. Krug, P.Eng., Stantec Consulting Ltd.
Ziad Ghadban, P.Eng., City of Ottawa
Mitigating The Impacts of Elevated water Storage
Tanks on Water Quality
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