1. A BETTER TOMORROW made possible 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”

2. A BETTER TOMORROW made possible Introduction Water Age Tank Operation Questions

3. A BETTER TOMORROW made possible 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

4. A BETTER TOMORROW made possible 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

5. A BETTER TOMORROW made possible Water Age What controls water age?  Mixing  Volume Exchange (i.e. Turnover)

6. A BETTER TOMORROW made possible 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, OH Photo courtesy of Natgun Corporation

7. A BETTER TOMORROW made possible Volume Exchange Ratio of stored volume vs. flow rate (i.e. demand) Volume (MG)

8. A BETTER TOMORROW made possible 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.

9. A BETTER TOMORROW made possible Why Mixed Flow Conditions From “Water Quality Modeling of Distribution System Storage Facilities”, AWWA Plug Mixed Chlorine Residual High Low Inflow Outflow

10. A BETTER TOMORROW made possible 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)

11. A BETTER TOMORROW made possible Step 1 – Water Age (CSTR)

12. A BETTER TOMORROW made possible Step 2 – Select Best Operation

13. A BETTER TOMORROW made possible Change in Water Age

14. A BETTER TOMORROW made possible Step 3 – Is It Well Mixed?

15. A BETTER TOMORROW made possible Step 3a – Use CFD to Design

16. A BETTER TOMORROW made possible CFD Verified Solution

17. A BETTER TOMORROW made possible 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)

18. A BETTER TOMORROW made possible Step 1 – CSTR Model

19. A BETTER TOMORROW made possible Step 2a – CFD Model

20. A BETTER TOMORROW made possible 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/V 1/3 (Rossman and Grayman, 2004) Consider impact of stratification  |  T|=CQ 2 /H 2 d 3 (Rossman and Grayman, 2004)

21. A BETTER TOMORROW made possible Technical Approach - Complex Step 1 Use CSTR Water Age Model to Calculate Tank Turnover (assumes well mixed conditions) Step 2 Use CSTR Water Age Model to Adjust Operation to Achieve Targeted Tank Turnover (assumes well mixed conditions) Step 3 Check Well Mixed Assumption (e.g. use CFD) Step 4 Adjust Water Age Model and Implement Model Recommended Operation Plan Yes Step 3a Use CFD to Design a Well Mixed System No

22. A BETTER TOMORROW made possible Take Home Message Storage vs. Demand Operational Needs vs. Storage Water Quality (How old is your water?)

23. A BETTER TOMORROW made possible Questions & Answers

24. A BETTER TOMORROW made possible Thank You