Unlocking Mallard Creek’s Hidden Aeration Capacity

1. Charlotte Mecklenburg Utilities Department (CMUD) Unlocking Mallard Creek’s Hidden Aeration Capacity NC AWWA-WEA, November 11 | 2013 George Anipsitakis, Darrell Dewitt, Peter Schuler

2. Project Highlights • Inability to operate enough blowers due to high pressures. • Difficulty meeting DO. • Blowers and piping undersized. • Several options to correct this, but most very costly and not feasible. • Blower upgrades, a success story! • Cost $170,000. • Provided 30+% more air. • No disruption in plant operations.

3. Background Unlocking Mallard Creek’s Hidden Aeration Capacity

4. Mallard Creek WRF • 12 MGD • 3 aeration trains • Tanks with different depths • Design DO = 1 and 2 mg/L at peak and average flows • Total Air Demand at 12 MGD: 22,300 scfm • Train 1: 35% • Train 2: 40% • Train 3: 25% • Blowers • Design: 21,000 scfm @ 6.9 psig

5. What was the problem? No more than 3 blowers operating at a time High system pressures >7.4 psig result in blower surging Air demand nearly not met with DO = 0.3 – 0.5 mg/L in summer Limited air flow distribution No redundancy Overloading motors

6. What caused the problem? Unlocking Mallard Creek’s Hidden Aeration Capacity

8. Air Piping System Pressures at Design Conditions (5 blowers, 12 MGD)

9. Air Piping System Pressures at Current Conditions (4 blowers, 75% capacity)

10. Air Supply – Blowers • Defining points from left to right • Surge: 1900 scfm @ 7.1 psig • Design: 4200 scfm @ 6.9 psig • Trip: 4750 scfm @ 6.2 psig or 200 HP • Operating pressures for supply system 6.2 to 7.1 psig • Operating pressure for demand system around 7.5 – 8.0 psig

11. What is going on? • No overlap between supply and demand pressures at 9 MGD • System can only supply air for 6 MGD equivalent capacity. • Blowers and air piping undersized • Multiple bottlenecks

13. What could be done? Unlocking Mallard Creek’s Hidden Aeration Capacity

15. What is being done?

16. Old vs. Upgraded Blower

17. Results Unlocking Mallard Creek’s Hidden Aeration Capacity

18. Air System Improvements

19. Air System Improvements

20. Actual Construction Costs

21. Old vs. Upgraded Blower

22. Old vs. Upgraded Blower • Old • Actual pressure range: 7.2 to 7.7 psig • Three (3) old blowers: 12,000 scfm at 7.2 psig • New • Actual pressure range: 7.5 to 8.6 psig • Four (4) new blowers: 15,900 scfm at 7.5 psig • 33% increase in air supplied

23. Conclusions Unlocking Mallard Creek’s Hidden Aeration Capacity

24. A Success Story Before After • High system pressures resulted in blower surging • Air demand nearly not met • Limited air flow distribution • No automation • Overloading motors • No redundancy • Upgraded blowers matching system pressures • 30+% increase in air supply • Good air flow distribution • Automation • Overloading motors • No redundancy

25. Take Home Message • Multistage Centrifugal Blowers • Everybody has them • Not everybody can afford or should get single-stage turbo blowers • Newer impeller designs for more efficiency • Add stages for more pressure or flow • Add motorized inlet valves to vary air volume • Economical to upgrade • Good Practice • Specify NOL motors throughout curve • Size air piping for appropriate velocities • Size control valves for throttling • Provide redundancy

26. Questions Unlocking Mallard Creek’s Hidden Aeration Capacity

28. Typical Control Strategy • Loop 1: Throttle dropleg valves at AB based on DO demand • DO setpoint not met, need more air, valves open, P drops • DO setpoint exceeded, too much air, valves close, P rises • Loop 2: Blowers ON/OFF based on P • At 3 operating blowers, demand increases, P drops, 6400 scfm, 7.8 psi, add a 4th blower • At 4 operating blowers, demand decreases, P increases, 8600 scfm, 8.6 psi, drop the 4th blower • From 3 to 4 blowers gain is 2200 scfm or 34%