1 JCI 1 1 Energy can be neither created nor destroyed, but only converted from one form to another. First Law of Thermodynamics Karl Friedrich Mohr In a closed system the mass of the system must remain constant over time. Law of Conservation of Mass Antoine Lavoisier

2 CEE 426 Wastewater Treatment Plant Design October 23, :05 pm Room 1213 Engineering Hall Thomas E. Jenkins President JenTech Inc N. Elm Tree Road Milwaukee, WI

3 Calculating Air Rates Types of Aerators and Processes Blower Selection Aeration and Energy 3

4 Aeration is the Most Significant Energy Use in a WWTP Aeration and Energy 4

5 5 Rates vary with: Time of Day On-peak, Off-Peak Day of Week Size of Costumer Demand (kW) and Consumption (kWh) The First Goal of an ECM (Energy Conservation Measure) is to Reduce Cost Energy Consumption ≠ Energy Cost

6 6 BOD Removal Oxygen Required for metabolism Amount of O 2 is proportional to Organic Load Utilization Approximately 1.1 lb. O 2 /lb BOD 5 Types of Processes

7 7 Nitrification NH 3  NO 3 Oxygen Required to convert Ammonia to Nitrate Amount of O 2 is proportional to Ammonia Concentration Utilization 4.6 lb. O 2 /lb NH 3 Converted Hydraulic Residence Time and SRT (Solids Retention Time) Determine if Nitrification Will Occur Types of Processes

8 8 Denitrification NO 3  N 2 and O 2 Anoxic Reaction Carbon Source Required Approximately 25% of O 2 Used for Nitrification Can be Recovered Only if raw wastewater is used as carbon source Types of Processes

9 9 Mechanical Fine Pore Coarse Bubble Jet Aeration All work by creating contact between air and the mixed liquor. All require the transfer of mass across the liquid/gas boundary. Types of Aerators

10 Horizontal Mechanical Aerator (Brush Aerator) Mechanical Aerators 10

11 Vertical Shaft and Aspirating Mechanical Aerator Mechanical Aerators 11

12 Calculating O 2 Transfer Typical aerator SOTR 2.0 to 4.5 lbO 2 /hp-hr

13 Determining Power Required for Mechanical Aerators: Calculating Air Rates and O 2 Transfer Note that hp USED is a function of design! Mixing Limits: 1 ft/sec velocity 0.5 hp/1,000 ft 3

14 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 Mechanical Aerator Energy

15 Mechanical Aerator Energy Lower speed = lower power

16 Mechanical Aerator Energy Lower speed = lower oxygen transfer

17 Mechanical Aerator Energy Net result is lower efficiency Does It Matter?

18 Coarse Bubble Diffused Aeration

19 Fine Pore (Fine Bubble) Diffused Aeration

20 Calculating OTE and O 2 Transfer

21 Calculating Air Flow Rates

22 Proportional to Submergence Deeper is Better Also Requires Higher Blower Power Proportional to Air Flow per Diffuser Flux Rate (1 to 10 SCFM/ft 2 Typical) Lower is better Proportional to Actual DO Concentration Lower is Better DO Control is Significant Diffused Aeration Energy

23 Calculating Air Rates and OTE Typical OTE f ≈ 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

24 OTE Increases with Higher Submergence OTE Decreases with Higher Air Flow per Diffuser Diffused Aeration 7.6m = 25’ 6.1m = 20’ 4.6m = 15’ 3.0m = 9.8’ 1.5m = 5’ 1m 3 /hr = 0.58 CFM 5m 3 /hr = 2.94 CFM 9m 3 /hr = 5.30 CFM

25 Excess DO means significantly more aeration power. Diffused Aeration

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

27 Energy use varies with time of day OTE, α, Organic Load, Hydraulic Load, All Vary with Time of Day Aeration and Energy 27

28 Efficiency Control Turndown Design Parameters Blower Selection Considerations

29 Efficiency Varies with Blower Type Range is 60% to 80% Aeration and Energy 29

30 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 Aeration Blower Power 30

31 Aeration Blower Power 31

32 Aeration Blower Power Ignoring Relative Humidity

33 Aeration Blower Power

34 Aeration Blower Power Blower System Q max :Q min = 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

35 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 Aeration Blower Power 35

36 Process Requirements Come First! Systems should be justified by payback Aeration and Energy 36

37 Over Life of Equipment: Energy Cost is more Significant than Equipment, Installation, or Maintenance Costs Aeration and Energy 37

nds/Pages/envirex.aspx n-of-Energy-Conservation-Measures-for-Wastewater- Treatment-Facilities.pdf Internet References

39 Aeration and Energy Questions? 39