1. CEE 426 October 25 2010 12:05 PM UW Madison Room 1209 Engineering Hall
Aeration and Energy
Thomas E. Jenkins, P.E.

2. Aeration and Energy
“An engineer is someone who can do for $5,000 what any damn fool can do for $50,000.
Anonymous

3. Aeration and Energy
Calculating Air Rates
Types of Aerators and Processes
Basin Geometry
Blower and Pump Selection

4. Aeration and Energy
Aeration is the Most Significant Use

5. Oxygen Transfer Efficiency (OTE %)
Calculating Air Rates
BOD
Nitrification NH3  NO3
Denitrification
Mixing Limitations
Oxygen Transfer Efficiency (OTE %)
Oxygen Transfer Rate (OTR lbO2/hp-hr)
α and β
Diurnal Variations and Equalization
Basin Depth

6. Types of Processes
BOD Removal
Oxygen Required for metabolism
Amount of O2 is proportional to Organic Load
Utilization Approximately 1.1 lb. O2/lb BOD5

7. Types of Processes
Nitrification NH3  NO3
Oxygen Required to convert Ammonia to Nitrate
Amount of O2 is proportional to Ammonia Concentration
Utilization 4.6 lb. O2/lb NH3 Converted
Hydraulic Residence Time and SRT (Solids Retention Time) Determine if Nitrification Will Occur

8. Denitrification NO3  N2 and O2 Anoxic Reaction Carbon Source Required
Types of Processes
Denitrification NO3  N2 and O2
Anoxic Reaction
Carbon Source Required
Approximately 25% of O2 Used for Nitrification Can be Recovered
Only if raw wastewater is used as carbon source
If methanol or other carbon source is provided then no oxygen recovery

9. Calculating Air Rates and O2 Transfer
Basic Calculations
Rectangular
Round

10. Calculating Air Rates and O2 Transfer
Basic Calculations
For BOD or NH3 or NO3

11. Calculating Air Rates and O2 Transfer
Basic Calculations

12. Types of Aerators
Mechanical
Fine Pore
Coarse Bubble
Jet Aeration

13. Mechanical Aerators
Horizontal Mechanical Aerator (Brush Aerator)

14. Mechanical Aerators
Vertical Shaft and Aspirating Mechanical Aerator

15. Calculating O2 Transfer
Typical aerator SOTR 2.0 to 4.5 lbO2/hp-hr

16. Calculating Air Rates and O2 Transfer
Mechanical Aerators

17. Calculating Air Rates and O2 Transfer
Mechanical Aerators
Mixing Limits:
1 ft/sec velocity
0.5 hp/1,000 ft3

18. Mechanical Aerator Energy
Proportional to Immersion
Reduced Immersion < OTR and < hp
Variable Weir Level Control
Proportional to Speed
Lower Speed < OTR and < hp
Variable Frequency Drives, Two Speed Motors
Proportional to Actual DO Concentration
Lower is Better
DO Control Significant

19. Diffused Aeration
Coarse Bubble

20. Diffused Aeration
Fine Pore (Fine Bubble)

21. Calculating OTE and O2 Transfer

22. Calculating Air Flow Rates

23. Calculating Air Flow Rates

24. Diffused Aeration Energy
Proportional to Submergence
Deeper is Better
Also Requires Higher Blower Power
Proportional to Air Flow per Diffuser
Flux Rate (1 to 10 SCFM/ft2 Typical)
Lower is better
Proportional to Actual DO Concentration
Lower is Better
DO Control is Significant

25. Calculating Air Rates and OTE
Typical OTEf ≈ 50% SOTE
SOTE Fine Pore ≈ 2% per foot submergence
SOTE Coarse Bubble ≈ ¾ % per foot submergence
Mixing Limits:
Fine Pore: 0.12 to 0.08 CFM/sq ft
Coarse Bubble: 20 CFM/1,000 cu ft

26. 1m3/hr = 0.58 CFM 5m3/hr = 2.94 CFM 9m3/hr = 5.30 CFM
Diffused Aeration
OTE Increases with Higher Submergence
OTE Decreases with Higher Air Flow per Diffuser
7.6m = 25’ m = 20’ m = 15’ m = 9.8’ m = 5’
1m3/hr = 0.58 CFM 5m3/hr = 2.94 CFM 9m3/hr = 5.30 CFM

27. Excess DO means significantly more aeration power.
Diffused Aeration
Excess DO means significantly more aeration power.

28. Diffused Aeration

29. Aeration and Energy
Process Mechanism for DO Control

30. Types of Processes
Miscellaneous Aeration Processes
Aerobic Digestion
Sludge Holding
Post-Aeration
Channel Aeration and Equalization

31. Basin Geometry Variations
Complete Mix
Plug Flow
Most Common
Tapered Aeration can Minimize Energy
Can be Achieved with Baffles
SBRs (Sequencing Biological Reactors)
MBRs (Membrane Bioreactors)
IFAS (Integrated Fixed Film Activated Sludge)
Lagoons
Usually Shallow

32. Basin Geometry Variations
SBRs (Sequencing Biological Reactors)
MIX FILL REACT FILL REACT
SETTLE DECANT/SLUDGE WASTE

33. Basin Geometry Variations
IFAS (Integrated Fixed Film Activated Sludge)

34. Aeration and Energy
Energy use varies with time of day
OTE, α, Organic Load, Hydraulic Load, All Vary with Time of Day

35. Aeration and Energy
Equalization Can Minimize Power

36. Blower Selection Considerations
Efficiency
Control
Turndown
Design Parameters

37. Aeration and Energy
Efficiency Varies with Blower Type
Range is 60% to 80%

38. Aeration Blower Power
Blower Power is a Function of Pressure and Flow Rate
Efficiency Varies Flow Rate for a Given Blower
Efficiency Drops as Flow is Decreased
Good Turndown (>50%) is Desirable

39. Aeration Blower Power

40. Aeration Blower Power
Ignoring Relative Humidity

41. Aeration Blower Power

42. Blower System Qmax:Qmin = 8:1 Specify Blower System for Turndown
Aeration Blower Power
Blower System Qmax:Qmin = 8:1
Specify Blower System for Turndown
Four with Design Flow at 33% Max Q
Two with Design Flow at 50% Max Q Plus Two with Design Flow at 25% Max Q
50% Turndown Each Blower

43. Aeration Blower Power
Control Method Significantly Impacts Energy

44. Aeration Blower Power
Control Method Significantly Impacts Energy

45. Aeration Blower Power
Design Specifications Identify Worst Case Operating Conditions
For Discharge Pressure
Max Temperature
Max Relative Humidity
Max System Flow Rate and ΔP
For Motor Power
For Throttled Centrifugal bhp at Min Temperature
For Variable Speed Centrifutal bhp at Max Temperature
Evaluation Conditions do not Equal Design Conditions

46. Pump Selection Considerations
RAS, WAS, Recycle
Efficiency
Control
Design Parameters

47. Pump Selection Considerations
For Aeration most Pumps Low Static Head

48. Pump Selection Considerations
Try to Operate at BEP (Best Efficiency Point)
Efficiency Varies, Typical 75% to 85%

49. Pump Power

50. Pump Design Parameters
Max Q, Min Q
TDH = Total Dynamic Head
Inlet Losses and Lift
Discharge Static Head
Discharge Friction Losses
Suction Head (Prevent Cavitation)
Motor hp
Control Influences Power
VFD more Efficient than Throttling

51. Aeration and Energy
Process Requirements Come First!
Systems should be justified by payback

52. Aeration and Energy
Over Life of Equipment Energy Cost is more Significant than Equipment, Installation, or Maintenance

53. Internet References http://www.sanitaire.com/3117803.asp

54. Aeration and Energy
Questions?