Tennessee Water Wastewater Utility

Partnership

Aeration Efficiency
and Optimization
Lee E. Ferrell, P.E., BCEE, CEM,
LEED Green Assoc.

Make the most of

SM
your energy
Aeration in Water Wastewater

2
 WWW Challenges: Drivers/Trends

●Demand for WWW ●Reduced financial resources

●Age of infrastructure ●Energy efficiency awareness
●Legislative compliance ●Energy use

Typical wastewater process

0.5% 0.3% 0.1% energy usage breakdown
1.4% Aeration
3.2% Collection
3.9%
Anaerobic Digestion
8.1%
Lighting and Buildings

Belt Press
14.2% 54.1%
Clarifiers

Grit
14.3%
RAS

Chlorination
Pumping = 87% Gravity Thickening
Process = 91.9%
3
 Water
energy use
71% of consumed electricity is used to
turn motors

65% of this energy is used for fluid

applications

Wastewater
energy use
Aeration System Overview and
Fundamentals

5
Overview
● The blower system:
Components End-use
– Blower – Water treatment
– Motors, engines – Wastewater treatment
– Piping – Water distribution
– Valves and fittings – Power generation
– Controls and – Irrigation
instruments
– Heat exchangers
– Tanks
– Others

6
Overview, continued
Electric utility System Approach
feeder ● Component optimization involves
segregating components and analyzing in
Transformer isolation
● System optimization involves studying
how the group functions as one as well as
Motor
breaker/starter how changing one component can help
the efficiency of another
Adjustable
speed drive
(electrical)

Served
Motor Coupling Blower Air System
Process(es)

7
Dresser Roots Wastewater Aeration Controls

Control Systems for Total Value

 Dissolved Oxygen (DO) Control to Minimize Air Flow
 Blower Control to Optimize Efficiency
 Direct Flow Control and Most Open Valve Control
to Minimize Pressure
 Blower Protection to Maintain Your Investment

8
DO Control Fundamentals
It’s all about the bubbles!

9
Aeration Energy
● Mechanical Aerators
● VFD Reduces Energy
● Challenge:
● Limit due to suspension
of solids
● Install submersed mixers
● Low Aerator speed
saves energy
● Mixer energy may be
less than savings
● Not needed at high
aeration

10
 Jet or Diffused
Air
● Energy reduction via:
● Basin/Zone air control
● Blower control

● Challenge
● Air used to mix water

● Blower efficiency only as

good as air distribution

11
DO Control Fundamentals

 Air is supplied to aeration basins in suspended growth diffused

air processes to provided oxygen needed to maintain biological
activity in the aeration basins.

 Oxygen required is basically proportional to organic loading –

both BOD5 reduction and Nitrification

 Air also provides mixing to keep the bacteria suspended and

aids in flocculation

12
DO Control Fundamentals

Aeration Control System Objectives:

 Satisfy the Oxygen Demand of the Treatment Process

 Achieve Process Requirements at the lowest

possible cost

13
DO Control Fundamentals
 DO (Dissolved Oxygen) concentration is an indirect
indicator of proper air flow to the process

 “Normal” DO concentration means the process is not

oxygen limited
 If you have very low or zero DO you cannot have adequate
process performance
 You can have high DO and not have adequate process
performance
 Most operators set DO concentration too high
 Typical 2.0 BOD – can be as low as 1.0
 Typical 3.0 Nitrification – can be as low as 1.0
 If BNR use as low as possible to avoid “oxygen poisoning

14
DO Control Fundamentals

 Low DO can cause undesirable organisms to develop

 High DO can cause poor settling, undesirable organisms to

develop

 Excess DO does always not result in more biological

activity

 Bugs don’t work twice as hard at 4.0 ppm DO than they do

at 2.0 ppm DO

 High DO just wastes power

15
DO Control Fundamentals

16
DO Control Fundamentals
● O2 for BOD5 Reduction:

LbO 2
Re quire dO2_f or_B OD = 1.1
Lb B OD

● O2 for Nitrification (NH3 to NO3):

LbO 2
Requ iredO2 _fo r_ Nitrifi ca tio n= 4.6
Lb NH
3
In BNR systems denitrification typically recovers 25% of O2 used for
nitrification

17
DO Control Fundamentals

Total Air Flow Required:

0.335 mgd
S C FM = ppmBO Dremoved 1.1 ppmNH 3converted 4.6
O TE
● mgd = Wastewater flow rate, million gallons per day

● SCFM = Air Flow Rate, Standard Cubic Feet per Minute (68°F, 14.7 psia, 36% RH)

● OTE = Actual Oxygen Transfer Efficiency, Site Conditions

OTE is not a constant!

18
DO Control Fundamentals
Oxygen Demand Varies in Time from Diurnal
Variations

Typical Diurnal Flow Variation

140
120
Flow, % ADF

100
80
60
40
20
0
12:00 AM 08:00 AM 04:00 PM 12:00 AM
04:00 AM 12:00 PM 08:00 PM
Tim e o f Da y

Ratio of Peak to Minimum Flow is Typically 2:1

19
OTE and DO Control

 When load increases at a constant air flow DO

concentration drops

 Concentration does not drop to zero because OTE

changes and may compensate for increased loading

 OTE is NOT a constant!

20
OTE and DO Control
OTE Varies with Air Flow per Diffuser
OTE Variation with Flow
DO ppm
0.50 1.00 1.50 2.00 2.50 3.00 3.50

14.0%

12.0%
Oxygen Transfer
Efficiency (OTE)

10.0%

8.0%

6.0%

4.0%
0.00 1.00 2.00 3.00 4.00 5.00
SCFM per Diffuser

21
OTE and DO Control
OTE Varies with DO Concentration:
OTE Variation with DO
SCFM per Diffuser

1.00 2.00 3.00 4.00

14.0%
Oxygen Transfer
Efficiency (OTE)

12.0%

10.0%

8.0%

6.0%

4.0%
0.00 1.00 2.00 3.00 4.00
DO Concentration, ppm

22
OTE and DO Control

At steady state the Oxygen Transfer Rate (OTR)

demanded by the process is equal to the OTR provided
by the aeration system. When this is not true, the
process is not steady state and the DO concentration
changes until a new equilibrium is established at new
steady state conditions.

OTE SCFM air %O2 OTR OUR TankVolume

23
OTE and DO Control
Response of DO to 20% Load Increase
Starting at 3.0 ppm DO
Oxygen Transfer Rate Variation with Air Flow
Response to 20% Organic Load Incre ase
6.00
1.0 2.0 3.0 4.0 5.0 SCFM/diffuser
5.00
4.00
1 3
DO, ppm

3.00
2.00
2
1.00

0.00
20.0 40.0 60.0 80.0 100.0 120.0
Oxygen Transfer Rate (OTR), kg/hr
1) Initial operation at 50 kg/hr OTR, 2 SCFM per diffuser, 3.0 ppm DO
2) 20% load increase to 60 kg/hr OTR, 2 SCFM per diffuser, DO drops to 1.3 ppm
3) Operation at 60 kg/hr OTR, air flow increases to 2.5 SCFM per diffuser,
restore 3.0 ppm DO
25% flow change required to
correct 20% load change!
24
DO Control
DO Control Operation:

 Establish Targets (Setpoints)

 Response to Deviations from Targets:

 If DO > Setpoint reduce oxygen supply
 If DO < Setpoint increase oxygen supply

 Control basin and blower air flow

25
DO Control

● Automatic DO Control will save 25%

to 40% of Aeration System Energy
Compared to Manual Control

26
DO Control
Savings Proportional to Driving Force of O2

Increased System Complexity Must Be Justified by

Increased Savings
 Group Basin DO Control

 Individual Basin DO and Air Flow Control with Most-Open-Valve

(MOV)

 Individual Zone DO and Flow Control for Each Basin

27
Basic Control System

28
Blower Control Concepts

Once the Optimum Aeration System Air Flow Has Been

Determined It Is Necessary to Provide the Correct Flow
From the Blowers

 The Purpose of Blower Control is to Provide the

Correct Air Flow

 Process Requires Controlled Mass Flow Rate

 Control Technique Varies With Type of Blower

29
Blower Control Concepts

Positive Displacement (PD)

 Constant flow at constant speed
 Pressure varies with system requirements
 Use VFDs (Variable Frequency Drives) to modulate air flow
 Power consumption directly proportional to flow and pressure

30
Blower Control Concepts
57

BHP @ Constatnt Pressure

52
47
42
37 Performance
32 Design
27
22
17
12
1000 2000 3000 4000 5000
BLOWER SPEED (RPM)

1212

1012
812
ICFM

Performance
612
Design
412

212
12
1000 2000 3000 4000 5000
BLOWER SPEED (RPM)

31
Blower Control Concepts
Multistage Centrifugal
 Variable flow at Approximately Constant Pressure
 Usually controlled by inlet throttling to modulate flow
 VFDs to modulate air flow will improve efficiency and
turndown (with appropriate curves)

32
Blower Control Concepts
110
AVERAGE TEMP 61 F NO CONTROL
INLET THROTTLED HIGH TEMP 100 F
100

AVERAGE TEMP
90 NO CONTROL

% POWER
80

70

60

AVERAGE TEMP
REDUCED SPEED 57.3 Hz
50

40
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

120
AVERAGE TEMP AVERAGE TEMP 61 F
INLET THROTTLED NO CONTROL
110

100
% GAUGE PRESSURE

AVERAGE TEMP
REDUCED SPEED 57.3 Hz
90

80

SYSTEM CURVE
70
NO CONTROL
HIGH TEMP 100 F
60

50
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

33
Blower Control Concepts
Single Stage Centrifugal
 Variable flow
 Pressure varies with load
 High efficiency
 Most common > 500 hp
 Inlet Guide Vanes and/or Variable Discharge Diffusers to
modulate flow and improve turndown

34
Blower Control Concepts - IGV
110
MAX IGV OPENING
100

90

% POWER
80
REDUCED IGV OPENING

70

60

50

40
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

120

MAX IGV OPENING

110
REDUCED IGV OPENING
100
% GAUGE PRESSURE

FRICTION LOSSES

90

80
STATIC PRESSURE

70

60

50
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

35
Blower Control Concepts – Variable
DDV 110
MAX DDV OPENING
100

90

% POWER
80

70

MIN DDV OPENING

60

50

40
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

120

MIN DDV OPENING MAX DDV OPENING

110

100
% GAUGE PRESSURE

FRICTION LOSSES

90

80
STATIC PRESSURE

70

60

50
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

36
Blower Control Concepts
Dual Vane Operation Maintains Efficie ncy
120

MAX IGV and DDV OPENING

110
BEP

X
% GAUGE PRESSURE

100

90

80

70

60
MIN IGV at MAX DDV OPENING
MIN IGV and DDV POSITION

50
0 10 20 30 40 50 60 70 80 90 100 110
% MASS FLOW RATE

37
Blower Control Concepts

Evaluate Total Blower Performance

 Equipment Cost

 Installation Cost

 Maintenance Cost

 ENERGY COST

38
Blower Energy Evaluation
 Use realistic inlet conditions

 Average temperature and pressure

 Include inlet losses

 Use expected range of operating air flow

 The Blower With the Highest Design Point Efficiency May Not
Provide Lowest Power Consumption

 Include control system characteristics in evaluation

 Determine if energy payback justifies higher initial investment

39
 Manual Diffused Air Control Scheme
● Advantages
● Economical Construction
Aeration Aeration
● Disadvantages
DO DO
● Energy Intensive
● Efficiency = Continual adjustment
Basin Basin

● Solution
● Motor Starters/RVSS

[
Blower
Aeration

[
DO Blower
Basin

[
Blower
40
 Average Diffused Air Control Scheme
● Average Basin DO controls blowers
Aeration Aeration
● Advantages
● Less operator intensive than manual DO DO
● Economical construction compared Basin Basin
to automatic distribution

● Disadvantages
● Rough control = Energy Intensive
● “Goldilocks effect”
M

[
● Solution Blower
Aeration
● Motor Starters/RVSS M

[
● PLC for blower control DO Blower
Basin M

[
Blower
41
 Basin Control Schemes
● Each Basin Controls DO
● Flow to each basin measured
● Blowers adjusted to meet total Aeration Aeration
demand
DO DO

● Advantages Basin Basin

● More accurate control
) ( ) (
● Manual observation eliminated

M
● Disadvantages

M
● Inter-basin DO variation ) ( M

[
Blower
Aeration
● Solution M

[
● Motor Starters / RVSS Blower
DO
● PLC for Basin and Blower Control Basin M

[
Blower
42
 Multi Zone Control Scheme
● Each zone controls its DO
M M

) (

) (
DO DO
● Advantages Aeration Aeration
● Greatest energy savings M M

) (

) (
● No/Low operator intervention DO DO
● Minimize inter-basin DO Basin Basin
M M
variation

) (

) (
DO DO

● Disadvantages
● Most expensive to construct
● Energy Savings / Process
Stability Tradeoff M
M

[
) (
DO Blower
● Solution Aeration
M
● Motor starters/RVSS M

[
) (
● PLC DO Blower
●Basin valve control Basin M
M

[
●Blower inlet valve control DO ) ( Blower
43
● Inlet Throttling
● Traditional method
● Inlet Valve controls air volume
● Fixed speed
●Fixed curve
●Surge is at fixed
power/amperage
● Surge done through:
●Current meter
●Power meter

44
 VFD Based Blower Control

● Better energy savings then inlet throttling valve

● But with limits
● Must design differently

45
 P.D.
Blowers
under
different
rules

46
 Inlet Throttling
● Curve sized for 60 Hertz VFD
● Throttled to meet
demand
● Surge Control via power
or current meters ●Curve sized for 80%
operating time
●Full Speed
● ~ 10% of the time
● Accept Power &
Pressure rise
●Surge control
algorithm with flow 47
Blower Energy Evaluation

Typical Diurnal Flow Air Estimated Air

Variation
150%
% of Average

100%

50%

0%
12:00 AM 04:00 AM 08:00 AM 12:00 PM 04:00 PM 08:00 PM 12:00 AM
Time of Day

48
Flow Control Basics

 DO concentration depends on air flow, NOT on

Pressure

 Blowers create air flow, not pressure

 The system creates pressure through resistance to air

flow

49
Flow Control Basics

What is operating point?

50
Flow Control Basics
The System Curve identifies the relationship between flow
and back pressure (resistance to flow)

51
Flow Control Basics
Combining the System Curve and the Blower Curve
defines the actual operating air flow

52
Pressure Control Basics

DO is not a function of system pressure. DO control

requires control of air flow only.

Blower control means modulating air flow rates.

Pressure control is designed to minimize interference

between basins and to coordinate blower output and
basin air flow demand.

Use of direct flow control and MOV logic to reduce

pressure and power

53
Pressure Control
Excess Pressure Simply Wastes Power

54
Pressure Control
Pressure control has a number of problems:
Problems with Pressure Control:
 Instability
 Basin and blower control loops “fight” each other
 Difficulties operating at extremes of blower capacity
 Interactions with multiple basins

 Tuning difficulties and re-tuning requirements

 Inherent PID tuning complications
 One blower vs. two blowers running
 Night vs. day operation

 Wasted power – 10% or more

55
Direct Flow Control

56
Direct Flow Control
 Basin Air Flow Control Saves Power Because “Identical”
Basins Don’t Perform Identically
 Variations Due to Influent Channels
 Variations Due to RAS
 Variations Due to Air Piping and Manual Valve Adjustments

 Direct Flow Control Approach

 Summation of basin flow requirements = total blower flow
 Δ flow, not absolute setpoint
 Modulate blowers into safe operating range

57
Direct Flow Control

 Eliminates need for pressure control to save power and energy

cost

 Totalize changes in air flow for tanks and modulate blowers air
flow accordingly

 Integrate air flow control at BOTH ends of air piping

58
Most-Open-Valve Control Concepts
 Excess blower discharge pressure wastes power

 Operators tend to set pressure setpoint too high –

often 1 to 1.5 psig above requirement

 Optimum energy use is achieved when the pressure in the

header is just enough to overcome static pressure plus friction
loss through the worst case diffuser
header

 Most-Open-Valve Control is a technique for power minimization,

not a blower control or DO control technique

59
Most-Open-Valve Control Concepts

Impact of excess pressure on blower power:

 PD blowers: very significant

 Inlet throttled multi-stage centrifugals: minor

 Inlet guide vane controlled single-stage or multi-stage

centrifugals: significant

 VFD controlled multi-stage centrifugals: very significant

60
Most-Open-Valve Control Concepts

 Most-Open-Valve (MOV) implies that one basin air flow control

valve is at maximum position to minimize system pressure

 MOV logic is independent of blower control logic

 Older style pressure control based systems typically use a

changing pressure setpoint based on valve positions

61
Energy Efficiency in Pumps and Blowers
• Load Characteristics
Water Wastewater Load Characteristics

Variable Constant Constant

Torque Torque Power

Typical Centrifugal Pumps Positive No applications

and Blowers Displacement
Applications Pumps, Blowers,
Mixers, and
Chemical Feed
Pumps

Energy Substantial Lowest Potential No Potential

Potential – Largest
Savings of all VFD
Potential applications

The Main
Target ( first The Next
priority) Step ( second
priority)
62
VFD Benefits with Blowers

63
VFD Benefits with Pumps
• Physical Laws for Centrifugal Loads
Its Pure Physics: Due to the laws that govern centrifugal pumps,
the flow of water decreases directly with pump speed

Affinity laws of centrifugal loads :

Flow = f (motor speed)

Pressure = f (motor speed)2

Power = f (motor speed)3

64
VFD Benefits with Pumps
• Physical Laws for Centrifugal Loads
A motor running at 80% of full speed requires 51% of
the electricity of a motor running at full speed.

65
VFD Benefits with Pumps
• Physical Laws for Centrifugal Loads
A motor running at 50% of full speed requires 12.5% of
the electricity of a motor running at full speed.

66
Aeration Efficiency and Optimization

67
Efficiency of Blower Systems
• Equations for efficiency

Blower Energy Usa ge Blower Efficiency Measuremen t

0.011503 * (Q * pi * X ) kW kW
kW MGD Q
( Motor * Blower * Drive )
(Q * pi * X )
Q Blower inlet volu metric flowrate, ICFM Motor * Blower * Drive
(0.011503 * k W)
pi Blower inlet pressure, psia
η efficiency (Q * pi * X )
Wire to air
X Blower adiabatic factor (0.011503 * k W)
0.283
pd
X 1
pi
pd Blower discharge pressure, psia
pi Blower inlet pressure, psia

68
Blower Energy Optimization is Complex
• Simple Guidelines for reducing
energy usage
Match blower airflow to the process
requirements
Minimize system discharge pressure and
inlet losses
Provide flexibility and adequate turndown
for loading variations

69
VFD Benefits with Blowers
• Other Benefits
In addition to Energy Saving, using a VFD has many other
advantages:
• Less mechanical stress on motor and system
• Less mechanical devices - Less Maintenance
• Process regulation with PID regulators, load management
functions
• Reduce noise, resonance avoidance
• Performance and flexibility, range settings, above base
operations
• Easier installation and settings, drive mechanics
• Can be controlled with Automation, Communication networks

70
Process Energy Optimization
Automation is the Key
• Develop consistent and
appropriate milestone and
deliverable expectations
• Standardize program schedule
tracking requirements
• Establish key energy
management performance
metrics
• Produce meaningful reports
that allow for clear and concise
decision-making
• Install additional monitoring
equipment as needed
71
Benchmarking and Key
Performance Indicators

72
Possible Benchmarking KPIs
• kWh/MG
• kWh/OTR
• kWh/lb BOD Removed
• kWh/TSS Removed

73
Thank you for your time

Lee E Ferrell, P.E., BCEE

Schneider Electric Water Wastewater Competency Center
864-784-1002
lee.ferrell@us.schneider-electric.com
http://www.schneider-electric.us/solutions/water-wastewater/

74
Appendix

75