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Maine Water Utilities Association Conference No. 490-April 12, 2007

Technical Program: Tanks, Tanks and More Tanks

Serkan Mahmutoglu P.E.

Earth Tech, Inc.

“STORAGE TANK OPERATION

AND

WATER AGE”

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• Introduction

• Water Age

• Tank Operation

• Questions

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Introduction

• What is water age in a storage tank?

“The average duration of water in the storage tank between the time it enters and leaves the tank.”

Theoretical Water Age

=

Tank Volume

Flow Rate

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Water Age

• Why is water age important?

Inadequate mixing and long detention times can cause poor water quality. disinfectant residual bacterial regrowth growth of disinfectant byproducts

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Water Age

• What controls water age?

Mixing

Volume Exchange (i.e. Turnover)

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Mixing

• Typically water storage tanks rely on the kinetic energy of the tank’s inflow to provide the mixing energy (Needs to be sufficiently turbulent (Re > 3,000) and fully developed)

Design/Construction: Natgun Corporation

CFD: Earth Tech Inc.

1999

Cleveland, OHPhoto courtesy of Natgun Corporation

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Volume Exchange

• Ratio of stored volume vs. flow rate (i.e. demand)

Vol

ume

(MG

)

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Water Age - Tips

• “Excessive storage capacity should be avoided to prevent potential water quality deterioration problems.” (10 State Standards)

• Even with ideal mixing, one may still need increasing turnover (i.e. volume exchange).

• Want to promote mixed flow rather than plug flow conditions.

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Why Mixed Flow Conditions

From “Water Quality Modeling of Distribution System Storage Facilities”, AWWA

Plug

Mixed

Chlorine Residual

High

Low

Inflow

Inflow

Outflow

Outflow

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Example 1:

• Total Storage Tank Volume

5.3 MG

• Daily Demand

Average 0.2 MGD Peak 1.3 MGD

Theoretical

Water Age

26 days (average)

4 days (peak)

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Step 1 – Water Age (CSTR)

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Step 2 – Select Best Operation

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Change in Water Age

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Step 3 – Is It Well Mixed?

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Step 3a – Use CFD to Design

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CFD Verified Solution

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Example 2:

• Tank 1

16.6 MG

• Tank 2

12.7 MG

• Daily Demand

Average 37 MGD Peak 60-80 MGD

Theoretical

Water Age

<1 day (average)

<0.5 days (peak)

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Step 1 – CSTR Model

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Step 2a – CFD Model

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Technical Approach - Simple

• Use CSTR (i.e. well-mixed) model to calculate water age

• Determine volume exchange needed to achieve well-mixed conditions

V/V>9.03d/V1/3 (Rossman and Grayman, 2004)

• Consider impact of stratification |T|=CQ2/H2d3 (Rossman and Grayman, 2004)

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Technical Approach - Complex

Step 1Use CSTR Water Age Model to

Calculate Tank Turnover (assumes well mixed conditions)

Step 2Use CSTR Water Age Model to

Adjust Operation to Achieve Targeted Tank Turnover

(assumes well mixed conditions)

Step 3Check Well

Mixed Assumption

(e.g. use CFD)

Step 4Adjust Water Age Model and Implement Model Recommended Operation Plan

Yes

Step 3aUse CFD to Design

a Well Mixed System

No

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Take Home Message

• Storage vs. Demand

• Operational Needs vs. Storage

• Water Quality (How old is your water?)

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Questions & Answers

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Thank You