Storage TanksImpact on Water Quality and

Assessing Performance

Presentation 3April 2015

Overview Tank functions & designs Tank operating parameters impacting

water quality (chlorine residual) Sampling at tanks and in their vicinity Tank case histories Assessing Performance Approaches to improve water quality Case study Summary

Physical Characteristics Storage tanks can vary by the following:

Size – diameter, height, volume

Shape – cylindrical (d>h), standpipe (h>d), rectangular, ellipsoidal, pedestal, etc.

Elevation – underground, ground, elevated

Inlet/Outlet – configuration (common or “flow-through”), diameter, location

Other – baffling, pillars, mixing systems (static or mechanical)

Function of Storage Tanks

Equalize water supply, so pumping doesn’t have to equal demand: Fire flows Main breaks Off peak pumping

Maintain system pressure in specific range

Important to consider tank functions when assessing potential operational changes

Ground Tank

VentHatch(Closed)

Overflow

Inlet/Outlet

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Underground Tank

Vent/OverflowHatch

(Closed)

Inlet

Outlet

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Elevated Tank

Vent

Overflow

Access Hatch

Inlet/Outlet

Monitor

OperatingRange

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Factors Affecting Chlorine Residual in Tanks

Water age (tank turnover)

Tank Mixing

Fill period chlorine residual (chlorine in)

Chlorine demand in the tank Bulk water Tank surface Sediment

Temperature Higher temp = lower residual

Water Age in Tanks

Water age determined by average tank volume and volume

added per day.

Water age will be the same throughout a tank only if it is

well mixed.

Stagnant zones:

Stratification

Poor mixing

Short draw/fill cycle

Operating Range?

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Tank Mixing

VentHatch(Closed)

Overflow

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Short Circuiting

VentHatch(Closed)

Overflow

InletOutlet

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Inlet/Outlet

Monitor

Last in / First Out

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Tank Sampling

VentHatch(Closed)

Overflow

Inlet

Outlet

Courtesy of Jeff Swertfeger

Greater Cincinnati Water Works

Some “Problem Tanks” We Have Met

Selected Case Histories

Case Study A

Case Study B

30

32

34

36

38

5/13/2008 12:00 PM 5/13/2008 6:00 PM 5/14/2008 12:00 AM 5/14/2008 6:00 AM 5/14/2008 12:00 PM 5/14/2008 6:00 PM

Date & Time

Ap

pro

xim

ate

Tan

k L

evel

(ft

)

0.00

0.30

0.60

0.90

1.20

Ch

lori

ne

Res

idu

al (

mg

/L)

Tank Level Cl Residual

Case Study B

30

32

34

36

38

40

5/13/2008 12:00 PM 5/13/2008 6:00 PM 5/14/2008 12:00 AM 5/14/2008 6:00 AM 5/14/2008 12:00 PM 5/14/2008 6:00 PM

Date & Time

Ap

pro

xim

ate

Tan

k L

evel

(ft

)

0

30

60

90

120

DB

P C

on

cen

trat

ion

(p

pb

)

Tank Level TTHM HAA5 THM Plus

Case Study C

Bayer Tank9/23/2008 - 9/27/2008

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

9/23

/200

8 0:

00

9/24

/200

8 0:

00

9/25

/200

8 0:

00

9/26

/200

8 0:

00

9/27

/200

8 0:

00

Date

Fre

e C

hlo

rin

e

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51

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56

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58

59

60

Pre

ssu

re

Free Cl2 mg/L

Free Cl2 Grab mg/L

Pressure psi

Cl Probe Calibration

Case Study C

Case Study D

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17

19

21

23

25

27

9/26/09 0:00 9/27/09 0:00 9/28/09 0:00 9/29/09 0:00 9/30/09 0:00

Tan

k L

evel

(ft

)

0

0.1

0.2

0.3

0.4

0.5

0.6

Fre

e C

hlo

rin

e (m

g/l)

Tank Level Free Chlorine

Case Study D

Case Study E

Case Study E

Case Study E

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

4/1/2009 4/11/2009 4/21/2009 5/1/2009 5/11/2009 5/21/2009 5/31/2009 6/10/2009 6/20/2009 6/30/2009

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60

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70

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80

85

90Chlorine Temperature

Case Study F

Case Study F

Assessing Storage Tank Performance

Objective: To develop an accurate “picture” of storage tank performance related to water quality

Water Quality Data

Operational Data

Physical

Character-

istics

Tank Parameters Impacting Water Quality

Water quality in storage tanks is significantly influenced by both:

Turnover Time (Water Age) – Average length of time that water

resides in a tank.

Optimization Guideline: Turnover time should be less than

3-5 days.

Each tank should be evaluated individually and given its own

goal.

Tank Mixing – A function of the momentum of the inlet flow

during a fill cycle.

Tank level (or pressure) data and physical characteristics are

needed to calculate these parameters.

What Impacts Turnover Time?

Can be influenced by: Tank volume

Location/elevation

Consumer demands

Pump schedules (i.e., duration, frequency, and rate of fill-and-draw cycles)

Operating range:

Minimum emergency storage capacity

System pressure requirements

Storage Tanks in Series

WTP

2.0 Days 3.0 Days 4.0 Days

3.5 Days

4.5 Days

3.5 Days

2.0 + 4.5 + 3.0 + 3.5 + 3.5 + 4.0 = 20.5 Days!

What Impacts Mixing? Can be influenced by:

Tank shape

Temperature differentials: Inflow > Ambient = Turnover

Inflow < Ambient = Stratification

Momentum of inlet flow (function of flow rate and diameter

of inlet/outlet)

Inlet/Outlet – configuration (common or

“flow through”), diameter, and location

Fill cycle duration

Engineering controls – baffling, static, or mechanical mixers

Temperature Effects on Tank Mixing Example

Courtesy of Jeff SwertfegerGreater Cincinnati Water Works

Water Quality Monitoring

Common parameters used for tank assessment:

Free chlorine

Temperature

pH

DBPs (optional)

Water Quality Monitoring

Grab Sampling (In-Tank or DS): Quick and easy assessment of water quality

Must consider whether tank is filling/draining

May not capture variability of data

Less expensive, but requires more time and labor

Online Monitoring: Convenient and continuous assessment of water quality

Continuous readings capture variability of water quality throughout tank cycle

More expensive, but requires less time and labor

Online vs. Grab Sampling

30

32

34

36

38

5/13/2008 12:00 PM 5/13/2008 6:00 PM 5/14/2008 12:00 AM 5/14/2008 6:00 AM 5/14/2008 12:00 PM 5/14/2008 6:00 PM

Date & Time

Ap

pro

xim

ate

Tan

k L

evel

(ft

)

0.00

0.30

0.60

0.90

1.20

Ch

lori

ne

Res

idu

al (

mg

/L)

Tank Level Cl Residual

Summary

Understanding how tanks operate is a critical aspect of assessing their performance. Both adequate turnover and mixing are necessary.

Storage tanks are unique, and their performance should be assessed individually.

Consider secondary impacts and understand the extent of a problem when making operational changes.

Water quality data is the best indicator of tank performance.