• TECHNICAL ASSESSMENT


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A DIGITAL CONTROL SYSTEM FOR
OPTIMAL OXYGEN TRANSFER
EFFICIENCY
Arnold Schwarzenegger
Governor
APPENDIX B:
TECHNOLOGY ASSESSMENT REPORT
PIER FINAL PROJECT REPORT
Prepared For:
California Energy Commission
Public Interest Energy Research Program
Prepared By: January 2010
Southern California Edison CEC-500-2009-076-APB
University of California, Los Angeles
Utility Technology Associates
Prepared By:
Southern California Edison
Lory Larson
Los Angeles, California 95616
Prepared For:
Public Interest Energy Research (PIER) Program
California Energy Commission
Paul Roggensack
Contract Manager
Michael Lozano
Program Area Lead
Industrial/Agricultural/Water End-Use Energy
Efficiency
Virginia Lew
Office Manager
Energy Efficiency Research Office
Thom Kelly, Ph.D.
Deputy Director
ENERGY RESEARCH & DEVELOPMENT
DIVISION
Melissa Jones
Executive Director
DISCLAIMER
This report was prepared as the result of work sponsored by the California Energy Commission. It does not necessarily represent
the views of the Energy Commission, its employees or the State of California. The Energy Commission, the State of California, its
employees, contractors and subcontractors make no warrant, express or implied, and assume no legal liability for the information
in this report; nor does any party represent that the uses of this information will not infringe upon privately owned rights. This
report has not been approved or disapproved by the California Energy Commission nor has the California Energy Commission
passed upon the accuracy or adequacy of the information in this report.
TECHNICAL ASSESSMENT
OF
THE OFF-GAS TECHNOLOGY
FOR
AERATION PERFORMANCE MONITORING
Submitted to:
Southern California Edison
Customer Service Department
Design & Engineering Services Group
6042B N. Irwindale Avenue
Irwindale, CA 91702
Prepared by:
Utility Technology Associates
14988 El Soneto Drive
Whittier, CA 90605
June 2006
TABLE OF CONTENTS
Page No.
1. Executive Summary …………………………………………………… 3
2. Background …………………………………………………………………... 5
2.1 What is wastewater aeration? …………………………………….. 5
2.2 Why is aeration important to wastewater industry? ……………… 5
2.3 How does aeration work? ………………………………………… 5
2.4 What is the energy implication of aeration? ……………………… 6
3 Problems Associated with Aeration ………………………………………….. 7
3.1 Operating concerns ………………………………………………. 7
3.2 Monitoring concerns ……………………………………………... 7
4 Project Objectives ……………………………………………………………. 9
5 Aeration Technology Overview ……………………………………………… 10
5.1 Description of an activated sludge system ……………………….. 10
5.2 Types of aeration processes ………………………………………. 10
5.3 Method of aeration ……………………………………………….. 12
5.4 Types of fine bubble diffusers for sub-surface aeration ………… 13
5.5 Process design considerations …………………………………… 14
5.6 Factors that influence oxygen transfer efficiency ……………….. 15
5.7 Aeration system maintenance and cleaning ……………………… 17
6 Technology Available for Aeration Control/ Monitoring ……………………. 18
6.1 Traditional practices …………………………………………… 18
6.2 More recent point source DO monitoring ……………………… 19
6.3 Improved on-line DO monitoring technologies ……………….. 20
6.4 Two advanced competing technologies ………………………... 21
7 Analysis of OTE Technology ………………………………………………… 23
7.1 Factors that affect estimation of OTE field testing ………………. 23
7.2 Advantages and limitations of the off-gas technology …………… 24
7.3 Advantages and limitations of the ORP technology ……………… 24
7.4 Advantages and limitations of the Respirometer technology …….. 25
7.5 Comparison of the off-gas to the two advanced technologies ……. 25
7.6 Potential applications of the off-gas technology to other industries 26
8 Potential Benefits of the Off-gas technology ………………………………… 27
9 Summary Conclusions ……………………………………………………….. 28
10 Recommendations ……………………………………………………………. 29
11 References ……………………………………………………………………. 31
2
1.0
EXECUTIVE SUMMARY
Over 60 percent of all wastewater treatment plants in the U.S. use the activated sludge
process as secondary treatment system. About 50 to 85 percent of the total energy
consumed in a biological wastewater treatment plant is in aeration. The activated sludge
process, the most common process, is performed in large aeration basins to provide air
for microorganisms, through biodegradation, to remove nutrients and pollutants.
One of the wastewater industry’s major challenges is the monitoring of how well the
secondary treatment process performs in breaking down waste. There are several ways to
monitor this secondary treatment process. The most informative method is to calculate
the oxygen transfer efficiency (OTE) using data collected by an instrument that measures
oxygen in the off-gas emitted from the basin. The OTE data is also used to determine
when the basin’s diffuser system requires cleaning, repair, or replacement. Currently, the
commercially available OTE instruments including the off-gas monitoring equipment are
large, heavy, and fragile, requiring a crew of four to six to operate. In addition, other
tests are needed to measure factors for the conversion of OTE to standard conditions.
These measurements are time, labor, and equipment intensive and require special staff
expertise. The only option for obtaining this OTE data is to outsource the tests to
expensive consultants rather than doing it in-house. Because of the expense and
difficulty of OTE measurements, most wastewater treatment plants rarely take them.
Project Objectives and Tasks Performed
This overall project has the objective of designing and demonstrating a new portable,
fully-automated OTE monitoring technology for the purpose of evaluating and
optimizing oxygen uptake, thereby reducing energy demand. In addition, the project will
develop practical tools for the wastewater industry to facilitate the routine “in-house”
monitoring of wastewater performance and with the intent to reduce energy use, improve
operating efficiency, and conserve energy by 25 to 40% at treatment plants throughout
California. Technology Assessment is one of the seven tasks proposed for this study. The
main objective of the technology assessment task is to perform an independent
assessment of the off-gas technology, and provide an objective critique of the usefulness
of the technology for the wastewater industry. This report is not written as a technical
publication for the scientific community. The general audiences are not considered
wastewater treatment savvy. It is assumed that they know little about wastewater
aeration. For those reasons, the report provides an introduction to the importance of
aeration and the type of aeration systems in wastewater treatment to make the reading
easier to understand and follow. Specific tasks performed in this report include:
• Introduction to wastewater aeration
• Concern of aeration to energy consumption
• Overview of the aeration processes
• Technologies for aeration control/monitoring
• Analysis of the off-gas technology
• Summary conclusions and recommendations
3
Aeration Technology Overview
Aeration is the heart of an activated sludge process in wastewater treatment. Since energy
use in aeration contributes over 50% of total usage, it is important to understand the
principles and processes governing sludge aeration as well as the types of processes,
methods of aeration, types of diffusers, factors influencing oxygen transfer efficiency and
the importance of aeration maintenance and cleaning.
Problem Statement
To minimize potential aeration problems which ultimately lead to excessive energy use, it
is important to address both operational and monitoring concerns. Operational concerns
relate primarily to diffuser fouling which is a function of cleaning and maintenance
frequency of nozzles. Monitoring concerns relate to accuracy and ease of monitoring to
optimize performance with least aeration energy. Current monitoring technologies are
either too bulky, difficult to perform, costly, and/or labor intensive.
Technologies Available for Aeration Control/Monitoring
Several technologies were found to be applicable to aeration monitoring. Traditional
practices were to measure pH and DO of the wastewater, sometimes complemented with
BOD/COD grab sample analyses. More recent measurements use DO probes which are
fuel cells with a semi-permeable membrane placed ahead of the electrode. With this
method, one cannot determine the operating efficiency nor calculate energy consumption
or requirements. In addition, the readings drift frequently making them unreliable and/or
inaccurate. The more recent online DO monitoring technologies include oxidation
reduction potential (ORP), respirometer, inert gas tracer, non-steady state H2O2 method,
and off-gas technologies. All these technologies have been used with some degree of
success. Advantages and limitations of these technologies are discussed and analyzed.
Potential Benefits of the Off-gas Technology
The intent of this project is to develop a simple and low cost monitoring system for the
off-gas technology to be used at all wastewater treatment plants that have activated
sludge treatment processes. Proper and frequent monitoring of the off-gas from aeration
will provide accurate information on treatment performance and mitigate occurrences for
over-aeration. By so doing, significant energy savings can be derived. It is estimated,
within the SCE service territory, a maximum potential of energy savings in the amount of
71.18 million kWh annually is achievable.
Summary Conclusions and Recommendations
This report reviewed several monitoring technologies for DO and oxygen transfer
efficiency measurement. It was concluded that the off-gas technology, as currently
evaluated by the UCLA researchers, is an excellent monitoring technology, well suited
for evaluating process water performance. Because of its current bulky configuration and
weight constraints as well as the costs associated with procuring and performing the
actual monitoring, the technology is not commonly used by wastewater treatment plant
operators. It is recommended that UCLA’s on-going research for reducing size, weight,
and costs of the off-gas equipment be continued, leading to commercial deployment. In
addition, two additional research areas for improving aeration performance also need to
be addressed. They are: blower control and SRT/DO optimization.
4
2.0
BACKGROUND
The wastewater treatment industry is one of the most energy intensive industries in
California, consuming over 3 percent of the state’s total energy. Recently, the industry is
faced with serious problems of excessive energy usage including high energy costs,
supply shortage, and inefficient/outdated monitoring equipment. The wastewater
industry in California wastes millions of dollars in energy costs each year from excessive
aeration.
2.1 What is Wastewater Aeration?
Wastewater aeration is the process aimed at injecting air into the wastewater to promote
aerobic biodegradation of waste material. It is an integral part of most biological
wastewater treatment systems. Unlike chemical treatment which uses chemicals to react
and stabilize solids in the waste stream, biological treatment uses microorganisms that
occur naturally in wastewater to degrade and stabilize wastewater contaminants. In
municipal wastewater treatment, aeration is part of the secondary treatment process,
preceded usually by pretreatment using grid chambers for large objects and grit removal
and primary treatment with sedimentation basins for suspended solids removal. The most
common processes where aeration is employed are the activated sludge process, trickling
filters, and aerated lagoons.
Activated sludge process is the most common option in secondary treatment. It is based
on aerating a biological tank, which promotes microbial growth in the wastewater. The
microbes feed on the organic material, form aggregated flocs, which then can easily settle
out. Afters settling in a separate settling tank, bacteria forming the "activated sludge" are
continually recirculated back to the aeration basin to increase the rate of organic
decomposition. Microorganisms adsorb dissolved and suspended matter, thus converting
nonsettleable solids into settleable solids. This usually corresponds to approximately 85%
removal of the biological oxygen demand (BOD) and total suspended solids (TSS) in the
wastewater.
2.2 Why is Aeration Important to Wastewater Treatment?
Aeration is the most critical component of treatment system using activated sludge
process. A well designed aeration system has a direct impact on the level of wastewater
treatment it achieves. An ample and evenly distributed oxygen supply system in an
aeration basin is the key to rapid, economically-viable, and effective wastewater
treatment.
2.3 How does Aeration Work?
Aeration provides oxygen to bacteria for treating and stabilizing the wastewater. Oxygen
is needed by the bacteria to allow biodegradation reactions to proceed. The supplied
5
oxygen is utilized by bacteria in the wastewater to break down the organic matter
containing carbon to form carbon dioxide and water. Without the presence of sufficient
oxygen, bacteria are not able to biodegrade the incoming organic matter in a reasonable
time. In the absence of dissolved oxygen, degradation must occur under septic conditions
which are slow, odorous, and yield incomplete conversions of pollutants. Under septic
conditions, some of the carbon will react with hydrogen and sulfur to form hydrogen
sulfide and methane. Other carbon will be converted to organic acids that create low pH
conditions in the basin and make the water more difficult to treat and promote odor
formation. Biodegradation of organic matter in the absence of oxygen is a very slow
biological process.
2.4 What are the Energy Implications of Aeration?
Activated sludge aeration systems consume 50-90% of the energy used at most
wastewater treatment plants, according to a recent survey by the U.S. EPA. A significant
component of operation and maintenance costs, aeration systems have an equal effect on
plant performance and capacity. A typical size 10 MGD activated sludge wastewater
plant uses approximately 2,500 kWh/MG of electricity (CEC published data). Assume an
average of 65 percent of the plant energy use is related to aeration, this equates to1,625
kWh/MG; or for a 10 MGD plant, this would correspond to 5.9 million kWh/year for
aeration alone. Improper aeration can easily increase energy usage by as much as 50
percent.
6
3.0
PROBLEM STATEMENT
Over 60 percent of all wastewater treatment plants in the U.S. use the activated sludge
process as their secondary treatment system. As discussed in Section 2.4 of this report, at
least 50 percent of the energy consumed in a wastewater treatment plant is in aeration.
The activated sludge process is performed in large aeration basins and provides air for the
microorganisms to remove nutrients and pollutants through biodegradation. The
development of fine-pore diffuser (small bubble) aeration made the operating costs of
these plants much lower than they were with earlier coarse-bubble (large bubble)
aeration. Unfortunately, porous ceramic disk and dome diffusers, and to a lesser extent,
membrane diffusers, used in most fine-pore systems, are highly vulnerable to fouling by
bacterial-slime growth and hard-water scale deposition. Cessation of airflow through the
diffuser, due to either a power failure or intentional system shutoff, allows water pressure
at the bottom of the tank to drive foulants onto the surface of the diffusers. This greatly
reduces oxygen transfer efficiency (OTE) in a few minutes. Fouling of diffusers reduces
OTE and significantly increases the amount of air, and hence energy and cost, needed to
run the process.
3.1 Operation Concern - Diffuser Fouling
There are three main reasons for selecting fine-pore diffusers: a) the OTE of fine-pore
systems is typically twice as high as when coarse bubbles are used; b) air flowrate, and
hence energy consumption for aeration, is inversely proportional to OTE, because the
amount of oxygen actually transferred must at least be equal to the oxygen consumed by
the process; c) the power used by the blowers for the aeration tanks ranges from 50% to
80% of the total energy consumption of an activated sludge plant. Most of this energy is
consumed by the process’s blower system in the aeration process. This pumped-in air
contains the oxygen that microorganisms need to breakdown waste in the water. Diffuser
fouling for fine-pore aeration is the most significant operational concern in wastewater
aeration. When the fine-pore diffusers are fouled, their efficiency is reduced to the same
levels of plants that use coarse-pore diffusers. Diffuser fouling diminishes potential
energy savings derived by switching from older aeration coarse bubble to fine-pore
diffusers.
3.2 Monitoring Concerns
In addition to diffuser fouling, the other major energy-wasting concern is the monitoring
of how well the secondary treatment process performs in breaking down the waste.
There are several ways to monitor this secondary treatment process. One informative
method is to calculate OTE using data collected by the off-gas technology, an instrument
that measures oxygen in the gas emitted from the basin. The OTE data is used to
determine when the basin’s diffuser system requires cleaning, repair, or replacement. At
present, the commercially available off-gas instruments are large, heavy, and fragile,
requiring a crew of four to six to operate. In addition, other experiments are needed to
7
measure factors for the conversion of OTE to standard conditions. These measurements
are time, labor, and equipment intensive and require special staff expertise. Currently,
the only option to obtain this OTE data is to outsource the tests to expensive consultants
rather than monitoring by treatment plant staff in-house. Because of the expense and
difficulty of OTE measurements, most plants rarely take them. Without this vital
information, the wastewater industry is unable to determine when efficiencies have
dropped to a level where it becomes cost-effective to clean, repair, or replace the diffuser
systems. For example, at the City of Los Angeles’s Terminal Island Treatment Plant
(TITP), dirty diffusers can cause an efficiency drop of over 50%, corresponding to at
least $197,100/yr wasted, essentially due to the limitations of present day off-gas testing
equipment.
A further problem with secondary wastewater treatment is that plants lack the
information necessary to optimize aeration in response to process change conditions. The
basic theory is that, in activated sludge systems, when air flux decreases OTE increases.
This is the key relationship in aeration control because it determines the change of
airflow needed to respond to a change in oxygen demand. Qualitatively, this relationship
is understood: increasing air fluxes increase the average diameters of bubbles, thereby
reducing the surface/volume ratio and increasing the speed of the bubbles, both of which
work against oxygen transfer. However, a thorough quantitative description of this
relationship under changing process conditions has yet to be given, mainly because of the
lack of a user-friendly device for measuring OTE.
8
4.0
TECHNOLOGY ASSESSMENT OBJECTIVES
This is a planned research program intended to benefit the wastewater treatment industry
in California, and ultimately the nation with the overall objective of designing and
demonstrating a new portable, fully-automated off-gas technology for the purpose of
evaluating and optimizing oxygen uptake, thereby reducing energy demand. In addition
this project will also provide practical tools for the wastewater industry to reduce energy
use, improve operating efficiency, reduce time, labor, and cost for OTE monitoring, and
conserve energy by 25 to 40% at treatment plants throughout California. Technology
Assessment is one of the seven tasks proposed for this study. This task presents the
following information:
• Introduction to wastewater aeration
• Concern of wastewater aeration and energy consumption
• Overview of the aeration processes
• Technologies for aeration control/monitoring
• Analysis of the off-gas technology
• Summary conclusions and recommendations
9
5.0
AERATION TECHNOLOGY OVERVIEW
Aeration is the heart of an activated sludge process in wastewater treatment. According to
the U.S. Environmental Protection Agency, activated sludge aeration systems consume
50 to 80 percent of the energy used at most wastewater treatment plants. It is important,
therefore, to understand the fundamental principles governing activated sludge aeration
as well as the types of aeration processes, methods of maintenance, and aeration
control/monitoring required.
5.1 Description of an Activated Sludge System
In an activated sludge system, wastewater is aerated in a tank (reactor) where bacteria
are encouraged to grow with a supply of: oxygen (air), food (biological oxygen demand,
BOD), proper temperature, and adequate contact time. As bacteria consume BOD, they
grow and multiply. After sufficient contact time, treated wastewater flows into secondary
clarifiers (sedimentation basins) where bacterial cells settle and are removed from
clarifiers as sludge. Part of the sludge is recycled back to the activated sludge tank to
maintain bacteria population. The remainder sludge is wasted. Flow characteristics of the
two most commonly used reactor types are the complete mix or continuous flow stirred-
tank reactor (CSTR) and the plug flow reactor (PFR). In CSTR operation, complete
mixing occurs when particles entering the tank are completely dispersed throughout the
tank. In plug flow, fluid particles pass through the tank and are discharged in the same
sequence in which they enter. In PFR operation, fluid particles pass through the tank and
are discharged in the same sequence in which they enter. The particles retain their
identity and remain in the tank for a time equal to the theoretical detention time.
Longitudinal dispersion is absent in the PFR operation.
5.2 Types of Activated Sludge (A/S) Processes
The activated sludge process is very flexible and can be adapted to a variety of treatment
schemes depending on the type of biological wastes requiring treatment. In general, the
following six types of A/S processes are most commonly encountered: conventional,
complete-mix, step-feed, modified-aeration, contact-stabilization, and extended-aeration.
5.2.1 Conventional
The conventional A/S process consists of an aeration tank, a secondary clarifier, and a
sludge recycle line. Sludge wasting is accomplished from the recycled or mixed liquor
line. Both influent settled sewage and recycled sludge enter the tank at the head end and
are aerated for a period of about 6 hours. During this period, adsorption, flocculation, and
oxidation of organic matter take place. The mixed liquor is settled in the clarifier, and
sludge is returned at a rate of 25 to 50% of influent flowrate.
5.2.2 Complete-Mix
The complete-mix process represents the hydraulic regime of a mechanically stirred
reactor. The influent settled sewage and return sludge flow are introduced at several
points in the aeration tank from the central channel. The mixed liquor is aerated as it
10
passes from the central channel to effluent channels at both sides of the aeration tank. The
aeration tank effluent is collected and settled in the A/S clarifier.
5.2.3. Step-Feed
The step-feed or step aeration process is a modification of the A/S process in which the
settled sewage is introduced at several points in the aeration tank to equalize the organic
loading and reduce peak oxygen demand. The aeration tank is subdivided into four or
more parallel channels through the use of baffles. Each channel comprises a separate
step, and the several steps are linked together in series. Return activated sludge enters the
first step of the aeration tank along with a portion of the settled sewage. The multiple-
point introduction of feed maintains an activated sludge with high absorptive properties,
so that the soluble organics are removed within a relatively short contact period. Higher
BOD loadings are therefore possible per 1,000 cu ft of aeration-tank volume.
5.2.4 Modified-Aeration
The flow diagram for the modified-aeration process is identical with that of the
conventional or tapered-aeration process. The modified-aeration process, however, uses
shorter aeration times, usually 1.5 to 3 hr, and a high food to micoroorganism (F/M) ratio.
BOD removal is only in the 60 to 75% range and is, therefore, not suitable where high-
quality effluent is desired.
5.2.5 Contact-Stabilization
Contact stabilization differs from the above processes in that an additional basin/tank or
stabilization basin is provided for the return sludge from the clarifier to further
breakdown the residual BOD from the contact basin. In this process, BOD removal is
postulated to take place in two stages. The first is the absorptive phase which requires 20
to 40 min. During this phase most of the colloidal, finely suspended, and dissolved
organics are absorbed in the activated sludge. The second phase, oxidation, then
continues, and the absorbed organics are metabolically assimilated. The settled sewage is
mixed with return activated sludge and aerated in a contact tank for 30 to 90 min. During
this period, the absorbed organics are utilized for energy and production of new cells. A
portion of the return sludge is wasted prior to recycle, to maintain a constant MLVSS
(mixed liquor volatile suspended solid) concentration in the tank. The Kraus Process is a
variation of this process in which the anaerobically digested sludge and digester
supernatant are added to the return sludge to improve the settling of the floc.
5.2.6 Extended-Aeration
The extended-aeration process operates in the endogenous respiration phase of the
growth curve, which necessitates a relatively low organic loading and long aeration time.
It is generally applied to small treatment plants of less than 1 mgd capacity. This process
is used extensively for prefabricated package plants that provide treatment for housing
subdivisions, isolated institutions, small communities, schools, etc. Aerobic digestion of
the excess solids, followed by dewatering on open sand beds, usually follows separate
sludge wasting. Primary sedimentation is omitted from the process to simplify the sludge
treatment and disposal. Oxidation ditch is a variation of this process.
11
A typical range of design parameters for the various activated sludge processes discussed
above is shown in Table 5.1.
TABLE 5.1 TYPICAL DESIGN PARAMETERS FOR
ACTIVATED SLUDGE PROCESSES
Process Өc (d) Ө (h) F/M Qr/Q X (mg/L)
Conventional 5 - 15 4-8 0.2 - 0.4 0.25 - 5 1,500 – 3,000
Complete-mix 5 – 15 3–5 0.2 – 0.6 0.25 – 1 3,000 – 6,000
Step-aeration 5 – 15 3–5 0.2 – 0.4 0.25 – 0.75 2,000 – 3,500
Modified-aeration 0.2 – 0.5 1.5 - 3 1.5 – 5.0 0.05 – 0.15 200 – 500
Contact- 5 – 15 0.5 – 1 0.2 – 0.6 0.25 – 1 1,000 – 3,000
stabilization 3-6 4,000 – 6,000
Extended-aeration 20 - 30 18 - 36 0.05 - 0.15 0.75 – 1.5 3,000 – 6,000
Legend: Өc mean cell retention time or solids retention time (SRT) in days
Өh hydraulic retention time in hours (V/Q)
F/M food to microorganism ratio (lbs BOD/lb of MLVSS-day)
Qr/Q sludge recirculation ratio
X mixed liquor volatile suspended solids concentration (MLVSS in mg/L)
5.3 Methods of Aeration
The equipment used to deliver oxygen to the aeration system is typically provided by
surface mechanical type aerators or subsurface diffused aeration systems. Some common
types of mechanical surface aeration equipment include low speed mechanical blade
aerators, direct drive surface propeller aerators, and brush type surface aerators.
Subsurface diffused aeration systems include a low pressure, high volume air compressor
(blower), air piping system, and diffusers that break the air into bubbles as they are
dispersed through the aeration tank. The most commonly used blowers are positive
displacement type blowers and centrifugal blowers (single and multi-stage).
The diffusion of air can be accomplished with several types of diffusers. Typical clean
water oxygen transfer rates are shown in Table 5.2.
TABLE 5.2 TYPICAL CLEAN WATER OXYGEN TRANSFER RATES (EPA
1984)
Diffuser Type and Placement Oxygen Transfer Rate
Lb O2/hp-hr
Coarse Bubble Diffusers 2.0
Fine Bubble Diffusers 6.5
Surface Mechanical Aerators 3.0
Submerged Turbine Aerators 2.0
Jet Aerators 2.8
12
To more accurately compare aeration system equipment, the relative rate of oxygen
transfer (alpha value) in wastewater compared to clean water must be established.
Typical alphas (α) are shown in Table 5.3.
TABLE 5.3 TYPICAL ALPHA (α) VALUES
Aeration System Typical Alpha (α)
Coarse Bubble Diffusers 0.8
Fine Bubble Diffusers 0.4-0.8
Jet Aeration 0.75
Surface Mechanical Aerators 0.8-1.0
Submerged Turbines 0.85
The value for oxygen saturation in wastewater compared to clean water is known as the
beta value (β). For municipal wastewater, 0.95 to 1.0 is typically used (EPA 1989).
Another important function of the aeration equipment is to provide adequate mixing in
the tanks to prevent solids from settling. This is an important aspect which is often
overlooked when aeration systems are evaluated for energy saving opportunities. Table
5.4 shows typical minimum mixing values for aeration tanks (EPA 1989).
TABLE 5.4 TYPICAL AERATION TANK MIXING REQUIREMENTS
Type of Aeration System Mixing Requirements
Coarse Bubble Diffusers 20 to 30 scfm/1000 cu.ft.
Fine Bubble Diffusers 7 to 10 scfm/1000 cu.ft.
Mechanical Surface Aeration 0.6 to1.15 hp/1000 cu.ft.
5.4 Types of Fine Bubble Diffusers for Subsurface Aeration
In a subsurface system, air is introduced by diffusers or other devices submerged in the
wastewater. Fine pore diffusion is a subsurface form of aeration in which air is
introduced in very small bubbles. There are 5 basic types of diffusers: discs, tubes,
domes, panels, and plates. These diffusers can be made from a variety of materials
including ceramics, plastics, or flexible perforated membranes. Ceramic diffusers have
been manufactured in dome, plate, tube and disc shapes, however the most commonly
marketed version today is the disc. The discs are typically 9” (3.54 cm) in diameter,
although they are made in sizes from 7” (2.76 cm) to 20” (7.87 cm) diameter. The
thickness of the media is from 0.75” to 7.5”. Membranes are available in various
materials and thicknesses, from to 0.0197” to 0.0315” (0.5-0.8 mm). Membrane materials
include Ethylene Propylene Dimer (EPDM) which is the most common type, as well as
Urethane and Silicone. The main ingredient in an EPDM membrane, which is the most
13
commonly used type, is EPDM; however, natural rubber, carbon black, ash, organic
additives, peroxides and plasticizers are also used in varying proportions.
Disc Diffusers: These are relatively flat and range from 7” to 9.4” (18 to 24 cm) in
diameter with thicknesses of 0.5” to 0.75” (13 to 19 mm). Materials for discs include
ceramics, porous plastics, and perforated membranes. The disc is mounted on a plastic
saddle-type base plate, and either a center bolt or a peripheral clamping ring is used to
secure the media and the holder together. Airflow is typically in the range of 0.25 to 1.5
liters/second per diffuser.
Tube/Flexible Sheath Diffusers: A typical tube diffuser is either a rigid ceramic or plastic
hollow cylinder (tube) or a flexible membrane secured by end plates in the shape of a
tube. A tube diffuser has a media portion up to 200 cm long and the outside diameter is
approximately 6.4 to 7.6 cm. Tube diffusers are made of either stainless steel or durable
plastic. Threaded rods are used with ceramic or plastic. Air flows through the tube
diffusers in the range of 1 to 5 liters/second.
Dome Diffusers: Dome diffusers are made from ceramics or porous plastics. Dome
diffusers are typically circular, 18 cm in diameter and 3.8 cm high. The media is about 15
mm thick on the edges and 19 mm on the flat surface. The dome diffuser is mounted on
either a polyvinyl chloride or a steel saddle-type base plate. The airflow rate for dome
diffusers is 0.25 to 1.0 L/s with a typical value of 0.5 L/s.
Panels: These types of diffusers consist of a flat box of custom size with a large sheet of
punched polymeric membrane, such as the membrane used for tubes, closing the top of
the box. Air is distributed inside the box and released from the top membranes. In
general, these diffusers have an air flowrate of the same range of tubes, on a L/m2 basis.
Typically, higher tank floor coverage is reached when employing panels over smaller
fine-pore diffuser models.
Plate Diffusers: Plate diffusers, made usually from ceramic material, are flat and
rectangular, approximately 30 cm2 in area, and 2.5 to 3.8 cm thick. Installation involves
either grouting the plates into recesses in the floor, cementing them into prefabricated
holders, or clamping them into metal holders. Air plenums run under the plates and
supply air from headers. In new installations, plate diffusers have largely been replaced
by porous discs, domes and tubes.
5.5 Factors that Influence Oxygen Transfer Efficiency
There are numerous parameters in wastewater treatment that can impact the rate and
efficiency of oxygen transfer. A number of these are either inherent in the physical plant
design (dimension of reactor, retention time, hydraulic flow rate, etc.) or intrinsic to the
wastewater (salinity, pH, temperature, hardness, etc.) and are beyond the plant operator’s
control. What we are discussing below are a few notable facts that are known to influence
the oxygen transfer efficiency.
5.5.1 Bubble Size
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Bubble size affects the oxygen transfer efficiency. Smaller bubbles have more surface
area per unit volume. This provides more area through which oxygen can diffuse and
thereby increases overall transfer efficiency. Since fine bubbles provide larger total
surface area, they create more friction and rise slower than coarse bubbles. The
combination of more transfer area and a greater contact time enhances transfer efficiency.
By inference


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