Energy Conservation by Optimizing
Aeration Systems
By
Tom Jenkins, P.E.
Dresser Roots, Inc.
Vikram M. Pattarkine, Ph.D.
Brinjac Engineering, Inc
Michael K. Stenstrom, Ph.D., P.E.
C & EE Dept, UCLA
Outline
• Overview
• Types of Aeration Systems, Terminology
and Relative Efficiency
• Operational Issues – Cleaning and
Avoiding Power Loss through Fouling and
Scaling
• Blower Overview and Optimization
• Conclusion
Types of Aeration Systems
• Mechanical or surface aerators
– High speed – 900 to 1200 RPM, no gear
boxes, easy to install, high heat loss, spray
issues, low efficiency
– Low speed – gear boxes to reduce RPM to 30
to 60, long lead time to install, high heat loss,
spray issues, medium efficiency
Types of Aeration Systems
• Diffused or subsurface aerators
– Coarse bubble – ¼ to ½ inch orifices, low
efficiency, low maintenance, low to ultra-low
efficiency
– Fine bubble or fine pore – millimeter to submillimeter orifices or porous media, highest
efficiency, significant cleaning and
maintenance issues
Types of Aeration Systems
• Combined systems
– Jets, turbines, and aspirating devices –
generally two prime movers such as a blower
and a motor-gearbox, low efficiency, generally
not used for new applications unless there are
special concerns or needs
Terminology
• Efficiency
– Standard oxygen transfer efficiency (SOTE) (percent
oxygen transferred)
– Standard oxygen transfer rate (SOTR) (mass
transferred per unit time)
– Standard aeration efficiency (SAE) (mass transferred
per unit time per unit power)
• All “standard” terminologies defined for clean
water such as tap water (secondary process
effluent is never suitable for clean water testing)
Terminology
• Process Conditions (OTE, OTR, AE)
– Adjustment formulas based upon driving force,
temperature, barometric pressure, water quality,
saturation concentration, etc.
– Driving force and water quality the most significant
– Driving force = (DOS – DO)/DOS
– Water quality – alpha factor, 0 to 1 !
– Total correction can result in process water transfer of
only 30 to 80% of clean water transfer
ASCE/EWRI Standards
• Clean Water Oxygen Transfer Standard
– 1984, 1991 and 2006.
• Process Water Testing Guidelines
– 1996
These two documents are quite useful in
defining aeration performance, and create
a “level playing” field to evaluate aeration
systems and facilitate low bid or life-cycle
purchase evaluations – use them!
Energy Approximations (wire power)
Aerator
Type
SAE
lbO2/hp-h (kgO2/kW-h)
Low SRT AE
at 2 mg/L DO
High SRT AE
At 2 mg/L DO
High
Speed
1.5–2.2 (0.9–1.3)
0.7–1.4 (0.4-0.8)
Low
Speed
2.5–3.5 (1.5–2.1)
1.2-2.5 (0.7–1.5)
Turbine
2-3 (1.2-1.8)
0.6-0.9
(0.4-0.6)
0.9-1.4
(0.6-0.8)
Coarse
Bubble
1-2.5 (0.6 –1.5)
0.5 – 1.2
(0.3-0.7)
0.6–1.6
(0.4-0.9)
Fine
Pore
6–8 (3.6–4.8)
1.2-1.6
3.3-4.4
(0.7–1.0)
(2–2.6)
Approximations – use only as a guideline – transfer
efficiency will depend on site specific conditions
Most Common Systems Today
• Municipal Treatment Plants – fine pore
systems:
– Discs, ceramic, plastic and membranes
– Tubes, membranes
– Panels and strips
• Municipal HPO-AS Systems
– Low speed mechanical
– Some new impeller designs to improve
efficiency
Most Common Systems Today
• Lagoons, ditches, industrial systems
sometimes are best designed with
alternative aeration systems due to
extremely high oxygen uptake rates, odd
geometries, heat loss considerations,
requirements for wet installation or wet
maintenance
Fine Pore Aeration Systems
• Why fine “pore” and not fine “bubble” ???
– Fine bubbles can be created by turbines and
other mechanical devices.
– Fine pore systems create bubbles by passing
air through pores or orifices
• Generally the best design choice for
energy conservation, but there are issues
and problems to avoid
Some Example Systems
• Ceramic domes – legacy system
• Ceramic discs – popular today
• Membrane discs – maybe most popular at
present
• Membrane tubes – popular today
• Membrane panels and strips – popular
today, and among the most energy
efficient
Ceramic Domes
Ceramic Discs
Membrane and Plastic Discs
Tubes
EPDM
PVC
Ceramic
EPDM
Plastic
Panels and Strips
Efficiency Varies
• Key to overall transfer efficiency is the air flow
per unit area of diffuser surface and the number
of diffusers used
• More diffusers and more area creates
efficiencies that are at the upper part of the fine
pore range
• Few diffusers and high flow per diffusers will
provide only low efficiency, at the low end of the
range or even approximating lower efficiency
devices
Fouling and Scaling
• Fine pore diffusers invariably undergo fouling,
scaling and material changes that reduce
efficiency
• Some type of routine maintenance program is
always required: otherwise, efficiency declines to
values that may be so low that they don’t justify
the capital investment
• Also, back pressure may built which may
prevent plant operation
Our Database of Full-Scale Results
• More than 20 years of observations of ~ 35
plants
• Ceramic discs, ceramic domes, membrane
discs, membrane and plastic tubes, panels
and strips
• New (< 1 month), Used (< 24 months) and
Old (> 24 months) and cleaned
• Cleaning – tank top hosing, brushing, acid
washing
Our Database of Full-Scale Results
• Process operation matters
• Conventional, low MCRT or sludge age –
lowest efficiency
• Long MCRT, nitrifying, good efficiency
• Long MCRT, nitrifying, denitrifying, best
efficiency
• Flow per unit area of diffuser and tank
surface is more important to defining
performance that the generic diffuser type or
material
Efficiency per process type
1.6
5.4
NEW
CLEANED
NEW &
CLEANED
NEW
USED
CLEANED
USED
USED
USED
SOTE / (%/ft)
(%/ft)
SOTE/Z
4.6
USED
NEW
1.2
OLD
4.60%/m
NEW
USED
<24 mo.
CLEANED
USED USED
4.30%/m
NEW
OLD
NEW
USED
NEW
OLD
3.8
OLD
USED
0.8
>24 mo.
3.75%/m
OLD
3.0
USED
USED
MCRT (d) 2 4 6
MCRT (d) 13 15 17 19 21
MCRT (d) 10
14
18
22
0.4
CONVENTIONAL
Conventional
N-ONLY
Nitrifying only
NDN
Nitrification/Denitrification
SOTE/Z
(%/m)
SOTE /  (%/m)
NEW
Transfer Efficiency
• Here-to-fore, transfer efficiency measurements
required an expert using an off-gas analyzer, a
few days of time, and thousands of dollars in
fees
• A real-time off-gas oxygen transfer efficiency
analyzer has been developed by the UCLASouthern California Edison Team, with California
Energy Commission Funding, and the design is
in the public domain
• It is described in detail during the technical
sessions
Economics of Fouling
power / initial power
2.4
40
30
2
30
20
40
20
1.6
40
30
20
30
30
20
power waste / cleaning cost
1.2
10
10
20
0.8
20
40
10
0.4
40
30
40
30
20
20
10
10
30
30
0
0
4
8
12
16
months in operation
20
24
28
Summary
• Fine pore systems generally, but not always
offer the best energy conservation
• Fine pore systems require a dedication to
maintenance; otherwise, select different
alternatives
• Reputable manufactures have valuable
experience with piping and assembly – Listen to
them!
• The consultant or process engineer must define
the efficiency – Require this information from
them!
Blowers
• All fine pore diffuser systems, coarse
bubble systems and most combined
aeration systems require blowers – they
are an indispensable part of the system
• The next section describes blower types
and guides for selection
Energy Conservation:
Most Common Blower Types
Positive
Displacement (PD)
Constant flow at constant
speed
Pressure varies with load
Most common <200 hp
Energy Conservation:
Most Common Blower Types
Multistage
Centrifugal
Variable flow
Approx. Constant Pressure
Most common 100 < hp > 750
Energy Conservation:
Most Common Blower Types
Single Stage
Centrifugal
Variable flow
Pressure varies with load
High efficiency
Most common > 500 hp
Energy Conservation:
Uncommon Blower Types
Regenerative
Very High Speed
Centrifugal
Characteristics similar to
PD
Limited to small flows and
low pressures
New proprietary technology
High efficiency
Limited size range
Energy Conservation:
Blower System Design Considerations
Provide lots of turndown capability
Use multiple smaller blowers
Select blowers for current requirements
Evaluate energy over range of actual
near term operating condition
Energy Conservation:
Blower System Design Considerations
Minimize system pressure
Most Open Valve Control for automatic
controls
Keep diffuser drop leg valves open for manual
control
Minimize diffuser pressure drop – orifice size,
diffuser configuration, clean diffusers
Energy Conservation:
Blower System Design Considerations
Use automatic DO control
20% to 50% energy reduction
Newer technology IS reliable
Energy Conservation:
Blower System Design Considerations
Use efficient blower control
Use technology appropriate to
blower system
Integrate with basin controls
Energy Conservation:
Blower Upgrades / Revamps
Collect Actual Operating Data on Process:
Dissolved Oxygen (DO) Concentration
Air Flow Rates
Dissolved Oxygen (DO) Concentration
System Pressure
Minimum, Maximum, and Average for Typical Operation
Compare to Design Conditions
Energy Conservation:
Blower Upgrades / Revamps
Collect Actual Operating Data on Blowers:
Number of Units Operating
Blower Power (kW) or Amps
Minimum, Maximum, and Average for Typical
Operation
Frequency of Manual Adjustments
Compare to Design Conditions
Energy Conservation:
Blower Upgrades / Revamps
All Blower Types
Provide proper maintenance – filters, seals, diffuser
cleaning
Change to energy efficient motors
Add smaller blowers to achieve turndown
Combine air use for other functions (Post-Aeration,
Channel Aeration, etc.)
Update Controls
Energy Conservation:
Blower Upgrades / Revamps
PD Blowers
Change sheaves to optimize capacity
Multistage Centrifugal Blowers
Change impellers to match actual
conditions
Single Stage Centrifugal Blowers
Change impellers to match actual conditions
Add Inlet Guide Vanes and/or Variable
Discharge Diffuser Vanes
Energy Conservation:
Control System Techniques
All Blower Types
Automatic DO Control to match air rates to process demand
Use MOV Control to minimize pressure
Automatic starting and stopping of blowers
Parallel control instead of cascade control
Design Control System for Reasonable Payback – 2 to 5
years
Include Process Improvement in Evaluation
Energy Conservation:
Control System Techniques
PD Blowers
Use VFDs (Variable Frequency Drives) to modulate air flow
Multistage Centrifugal Blowers
VFDs to modulate air flow (with appropriate curves)
Automatically controlled inlet throttling to modulate flow and
improve turndown
Single Stage Centrifugal Blowers
Inlet Guide Vanes and Variable Discharge Diffusers to
modulate flow and improve turndown