xvEPA
                          United States
                          Environmental Protection
                          Agency
                       Office of Water
                       Washington, D.C.
EPA 832-F-99-065
September 1999
Wastewater
Technology Fact Sheet
Fine  Bubble  Aeration
  DESCRIPTION

  In  wastewater treatment  processes,  aeration
  introduces air into a liquid, providing an aerobic
  environment for microbial degradation of organic
  matter.  The purpose of aeration is two-fold: 1) to
  supply the  required oxygen to the metabolizing
  microorganisms and 2) to provide mixing so that
  the microorganisms come into intimate contact with
  the dissolved and suspended organic matter.

  The  two most  common aeration systems are
  subsurface and mechanical. In a subsurface system,
  air is introduced by diffusers or other devices
  submerged in the wastewater. A mechanical system
  agitates the wastewater by various means (e.g.,
 propellers, blades, or brushes) to introduce air from
 the atmosphere.

 Fine pore diffusion is a subsurface form of aeration
 in which air is introduced in the form of very small
 bubbles.  Since the energy crisis in the early 1970s,
 there has been increased interest in  fine pore
 diffusion of air as a competitive system due to its
 high oxygen transfer efficiency (OTE).   Smaller
 bubbles result in more bubble surface area per unit
 volume and greater OTE.

 APPLICABILITY

 Saukville Wastewater Treatment Plant,
 Saukville, Wisconsin

 The Saukville Wastewater Treatment Plant (S WTP)
 uses  fine bubble  aeration to increase treatment
 efficiency and oxygen transfer, as well as reduce
power costs. Initially, the plant used stainless-steel
coarse bubble diffusers requiring two centrifugal
                     blowers.  Although these diffusers required little
                     maintenance, oxygen transfer was inadequate to
                     meet process needs, and power requirements were
                     excessive.

                     In an attempt to reduce power costs and increase
                     treatment plant efficiency, the plant was retrofitted
                     with a combination of coarse bubble  and fine
                     bubble ceramic diffusers in 1985.  One of the two
                     treatment cells was retrofitted with fine bubble
                     ceramic  diffusion, and the second retained its
                     coarse bubble  diffusion system.   This  reduced
                     power requirements by eliminating the need for one
                     blower.

                     After start-up, it was noticed that the mixed liquor
                     dissolved oxygen (DO) levels were not as high in
                     the fine bubble cell as in the coarse bubble cell, due
                     to uneven air distribution between cells. To correct
                     this,  a positive displacement blower was added to
                     the fine bubble cell.   To reduce  fouling of the
                     ceramic diffusers, an in-place gas-cleaning system
                     was installed to inject anhydrous hydrochloric acid
                     into  the  air stream while the system was in
                     operation.

                    In 1990, the plant  switched to  fine  bubble
                    membrane diffusers, which were interchangeable
                    with  the  ceramic diffusers.   The ceramic and
                    membrane efficiencies  were comparable, so few
                    adjustments in air rate were needed.  The  SWTP
                    was awarded the U.S.  Environmental Protection
                    Agency Award of Excellence in 1991 based on
                    overall energy savings  and optimized operations
                    and maintenance practices.

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 Renton Wastewater Treatment Plant, Renton,
 Washington

 The Renton Wastewater Treatment Plant serves the
 urban and suburban areas east, south, and north of
 Lake Washington, just  east of Seattle.  Rising
 power costs created a need for modification of its
 coarse bubble aeration system.

 Perforated membrane fine bubble diffusers were
 selected for an in-plant  study in 1982.   These
 diffusers were placed hi the first two passes of one
 aeration tank, while the other tanks retained their
 coarse bubble units. DO was compared in the two
 systems, and it was determined that the perforated
 membrane diffusers required 30 to 40% less air
 than  coarse bubble  diffusers  to  maintain  a
 comparable  mixed  liquor  DO.   Total  energy
 consumption decreased from 390 to 355 kW/1,000
 m3 after installation of the fine bubble diffusers.

 Ridgewood Wastewater Treatment Plant,
 Ridgewood, New Jersey

 The  Ridgewood  Wastewater  Treatment  Plant
 (RWTP)  was  retrofitted  from  coarse bubble
 diffusion to a fine pore diffusion aeration system in
 1983. Fine pore aeration would allow the use of
 one  blower and  maintain  the  same oxygen
 utilization rate as provided by the coarse bubble
 system. Oxygen transfer studies were performed on
 the fine pore ceramic dome diffusers  in order to
 compare results with the coarse bubble diffusers.

 The results showed that the coarse bubble diffuser
 had an average standard oxygen transfer efficiency
 (SOTE) under field conditions of 4.8% with an
 average alpha (a) of 0.55. In contrast,  with two
 tanks in  operation,  the fine pore system had an
 average SOTE under field conditions of about 9.5%
 and an average a of 0.4 during normal daytime
 high-load operation. Alpha is defined as the ratio
 of KLa (volumetric mass transfer coefficient) of a
 clean diffuser in process water to the KLa of the
 same diffuser in clean water.

 Two methods of cleaning were used at the RWTP:
anhydrous hydrochloric acid brushing and water
hosing. Installation of the fine pore aeration system
achieved  the oxygen utilization desired, reduced
 power consumption by approximately 28%, and
 resulted in a significant improvement in effluent
 quality with respect to nitrification.

 ADVANTAGES AND DKSADVANTAGES

 Some advantages and disadvantages of various fine
 pore diffusers are listed below:

 Advantages

        Exhibit high OTEs.

        Exhibit high aeration efficiencies (mass
        oxygen transferred per unit power per unit
        tune).

        Can satisfy high oxygen demands.

       Are easily adaptable to existing basins for
       plant upgrades.

       Result in lower volatile organic compound
       emissions  than  nonporous  diffusers or
       mechanical aeration devices.

 Disadvantages

       Fine  pore diffusers  are  susceptible to
       chemical or biological fouling, which  may
       impair transfer efficiency and generate high
       head loss. As a result, they require routine
       cleaning. (Although not totally without cost,
       cleaning does not need to be expensive or
       troublesome.)

       Fine pore diffusers may be susceptible to
       chemical  attack  (especially   perforated
       membranes).   Therefore,  care  must be
       exercised  in  the   proper  selection  of
       materials for a given wastewater.

      Because of the high efficiencies of fine pore
       diffusers at low  airflow  rates,  airflow
       distribution is critical to their performance
       and selection of proper  airflow control
       orifices is important.

      Because of the high efficiencies of fine pore
       diffusers, required airflow in an aeration

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        basin (normally at the effluent end) may be
        dictated by mixing - not oxygen transfer.

       Aeration basin design must incorporate a
        means  to  easily dewater  the tank  for
        cleaning.    In small  systems where no
        redundancy of  aeration tanks exists, an
        in-situ, nonprocess-interruptive method of
        cleaning must be considered.

 DESIGN CRITERIA

 Diffusers

 In the past, various diffusion devices have been
 classified based on their OTE as either fine bubble
 or coarse bubble.  Since it  is difficult to clearly
 demarcate  or define between fine and  coarse
 bubbles, diffused  aeration  systems  have  been
 classified based on the physical characteristics of
the equipment.  Diffused aeration systems can be
classified into three categories:

       Porous (fine bubble) diffusers: Fine pore
       diffusers are mounted or screwed into the
       diffuser header pipe (air manifold) that may
       run along the length or width of the tank or
       on a short manifold mounted on a movable
       pipe (lift pipe).  These diffusers come in
       various shapes and sizes,  such as discs,
       tubes, domes, and plates.

      Nonporous (coarse bubble) diffusers: The
      common types  of nonporous diffusers are
      fixed orifices (perforated piping, spargers,
      and  slotted  tubes); valved  orifices;  and
      static tubes.   The bubble size of these
      diffusers is larger than the porous diffusers,
      thus lowering the OTE.

      Other diffusion devices: These include jet
      aerators (which discharge a mixture of air
      and liquid through a nozzle near the tank
      bottom); aspirators (mounted at the basin
      surface to supply a mixture of air  and
      water); and U tubes (where compressed air
      is discharged into the  down leg of a deep
      vertical shaft).
  Types of fine pore diffusers

  Fine pore diffusers (discs, tubes, domes, and plates)
  are usually made from either ceramics, plastics, or
  flexible perforated membranes.  Although many
  materials can be used to make fine pore diffusers,
  only  these few  are being  used  due  to cost
  considerations, specific characteristics, market size,
  and other factors.

  Ceramic media diffusers have been in use for many
  years and have essentially become the standard for
  comparison since, in the past, they were the primary
  media in the fine pore aeration market. .Ceramic,
  plastic, and flexible materials are resistant to the
  chemicals used in wastewater treatment. Discussed
  below are common types of fine bubble diffusers.
  However,  recent advances in technology have.
  resulted in modifications to these types, which are
  shown in Figure 1.
                  Orifice
              Baseplate
                ' .Retainer
                  Ring
           Diffuser Holder-
                               Membrane
   Air Distribution Pipe
  a) Membrane Disc Diffuser
              Diffuser Bott
            Washer & Gasket
Ceramic Dome Diffuse

  Dome Gasket
 b) Dome Diffuser Assembly
Source: SANITAIRE brand products, reprinted with
permission by the Water Pollution Control Corporation
Brown Deer, Wisconsin, 1999.

   FIGURE 1  SCHEMATIC OF VARIOUS
        FINE BUBBLE DIFFUSERS

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  3Disc diffusers

  Disc  diffusers are relatively flat and range from
  approximately  18 to 24 cm in diameter with
  thicknesses of 13 to 19 mm.  Materials for discs
  include ceramics, porous plastics, and perforated
  membranes.  Therefore, thicknesses vary, as do
  construction features.   Currently, manufacturers
  provide plenums or base plates that will accept all
  materials.

  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.   More  commonly, the disc  is
  attached to the holder via a screw-on retainer ring.
  Disc diffiisers are designed to have an airflow range
  of 0.25 to 1.5 L/s 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 difruser has a media portion up to 200
  cm long. The thickness of the difruser varies, but
 the outside diameter is approximately 6.4 to 7.6 cm.
 The various components of a tube diffuser are made
 of stainless steel or durable plastic. Threaded rods
 are used with ceramic or porous plastic. The rod is
 threaded into the feed end of the holder with a
 hexagonal nut secured  on the rod  to hold the
" assembly in place. Air flows through tube diffusers
 in the range of 1 to 5 L/s.

 Dome diffusers

 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 horizontal or flat
 surface. The dome diffuser is mounted on either a
 poly vinyl chloride or a steel saddle-type base plate.
The airflow rate for dome diffusers is usually 0.5
L/s with a range of 0.25 to 1 L/s.

Plate diffusers

Plate   diffusers   are  flat   and  rectangular,
approximately 30 cm2 in area, and 2.5 to  3.8 cm
  thick.  They are normally made from ceramic or
  membrane materials.  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. Plate
  diffusers have  largely  been replaced by porous
  discs, domes, and tubes in new installations.

  PERFORMANCE

  The performance of diffused aeration systems under
  normal operating conditions is directly related  to
  the following parameters:

        Fouling.

        Wastewater characteristics.

        Process type and flow regime.

        Loading conditions.

        Basin geometry.

        Diffuser type, size,  shape,  density, and
        airflow rate.

        Mixed   liquor dissolved  oxygen (DO)
        control and air supply flexibility.

       Mechanical integrity of the system.

       Operator expertise.

      The quality of the preventive operation and
       maintenance (O&M) program.

 The wastewater characteristics that establish the
 oxygen demand placed  on  a fine pore aeration
 system are the  influent flow  rate,  biochemical
 oxygen  demand  load,  and  ammonia-nitrogen
 (NH3-N) load.

 Fouling is generally classified as one of two types:
Type I and Type II.  Characteristics of Type I
fouling are clogging of the diffuser pores, either by
airborne particulates clogging the air side, or metal
hydroxides and carbonates clogging the liquid side.
Type II is characterized by a biofilm layer forming

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  and growing on the surface of the diffuser.  In
  practice, it can be difficult to distinguish between
  the two types because they occur together, with one
  or the other dominating.

  Historically, the rate of fouling was measured by
  monitoring the rise in backpressure. However, this
  proved  to be a crude and qualitative  method
  because significant fouling can occur without much
  increase in backpressure, but with great reductions
  in OTE.

  The presence of constituents such as surfactants,
  dissolved solids, and suspended solids can affect
  bubble shape and size and result in diminished
  oxygen transfer capability.   In general, ceramic
  domes and discs yield slightly higher clean water
  transfer  efficiencies than typical porous plastic
  tubes or flexible sheath tubes in a grid placement.
  Other key parameters that have an effect on the
  performance  characteristics  of a fine pore media
  diffuser are permeability, uniformity, dynamic wet
 pressure, and strength.

 Effective long-term  process control depends  on
 appropriate selection and integration of the solids'
 retention time, the food-to-microorganisms loading,
 and the  wastewater flow regime.   Short-term,
 day-to-day variables at the disposal of the operator
 include control of diffuser airflow rate and mixed
 liquor  DO  concentration.   It is essential  to
 understand how each of these parameters affects
 aeration efficiency in order  to develop optimum
 short- and long-term operating procedures.

 OPERATION AND MAINTENANCE

 The main operational  objective is  to  achieve
 acceptable effluent quality while maximizing the
 aeration efficiency. It is essential that diffusers be
 kept  clean  through  cost-effective  preventive
 maintenance procedures. Preventive maintenance
 can virtually eliminate  air-side (blower filtration
 system)  paniculate  fouling  of fixed  fine pore
 diffusers.

 Filtration equipment maintenance entails cleaning
 and  changing filter media.   Calibration and/or
 zeroing of meters is necessary as part of preventive
maintenance  because accurate airflow and DO
  measurements are a critical part  of monitoring
  aeration systems.

  Preventive maintenance is  needed  to  keep  an
  aeration system operating at the required level of
  performance and to decrease the need for corrective
  maintenance. In addition, preventive maintenance
  will reduce the number of interruptions in the air
  supply, thus preventing solids from entering the air
  distribution system.

  The cleaning methods  used to restore diffuser
  efficiency are either process interruptive (aeration
  basin out of service) or process noninterruptive
  (access to  basin not needed).   Diffusers can be
  cleaned by removing them from the basin (ex-situ)
  or onsite inside the basin (in-situ). Some cleaning
  techniques used are acid washing, alkaline washing,
  gas injection, high-pressure water jetting, and air
  bumping.

  When placing an empty aeration basin into service,
 all recommended operational steps for start-up and
 shutdown should be followed. If a basin is put into
 service during cold weather, care must be exercised
 to prevent any damage from buoyant forces exerted
 by ice trying to float. Aeration basins must not be
 drained during freezing weather unless absolutely
 necessary because  ice and frost can cause serious
 damage.  In the event that an aeration basin should
 stand idle for more than 2 weeks,  it  should be
 drained and cleaned thoroughly.

 COSTS

 The aeration system consumes  approximately 50 to
 65%  of the net  power  .demand for  a  typical
 activated sludge   wastewater  treatment  plant.
 Therefore, the designer is responsible for selecting
 a system that will  meet the mixing and  oxygen
 requirements for the process  at  the lowest  cost
 possible.  Once the requirements for aeration are
 determined, comparative costs for different types of
 aeration systems can be estimated and the final
 equipment configuration selected to best match the
requirements of the job.

Construction cost items mainly consist of aeration
basins,  air piping  and headers  as  appropriate,
aeration devices and their supports,  air cleaning

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 equipment, blowers, and buildings to house these
 items.   O&M  costs are primarily for power,
 cleaning, and replacement.

 Operational costs are determined in part by the OTE
 of the fine bubble aeration system being used, as
 well  as  the  characteristics  of  the  influent
 wastewater.  Aerator cleaning costs depend on the
 aerator type; how easily the aerators can be
 removed, cleaned, or replaced; and the plant's O&M
 procedures.

 REFERENCES

 1.     Egan-Benck, K.;  G. McCarty; andW.
       Winkler.  1993.  "Choosing Diffusers."
       Water Environment and Technology, vol. 5.
       no. 2. pp. 54-59.

 2.     McCarthy,   J.   1982.   "Technology
       Assessment of Fine Bubble Aerators." U.S.
       Environmental  Protection  Agency (EPA)
       Wastewater Research Division. Municipal
       Environmental  Research  Laboratory.
       Cincinnati, Ohio.

 3.     U.S. EPA, 1995. Technological Assessment
       of   Fine   Bubble  Aerators.
       EPA-600/2-82-003.

4.     Metcalf & Eddy, Inc. 1991. Wastewater
       Engineering: Treatment,  Disposal,  and
       Reuse,  3d  ed.  The   McGraw-Hill
       Companies. New York, New York.

5.      U.S. EPA, 1985. Summary Report: Fine
      Pore (Fine Bubble) Aeration Systems. EPA
       Water Engineering Research Laboratory.
      Cincinnati, Ohio. EPA/625/8-85/010.

6.     U. S.  EPA,  1989. Design  Manual: Fine,
      Pore Aeration Systems.  EPA Center  for
      Environmental   Research   Information.
      Cincinnati, Ohio. EPA/625/1-89/023.
 ADDITIONAL INFORMATION

 Mike McGee
 President
 EP Aeration
 2615 Meadow Street
 San Luis Obispo, CA 93401*

 Gordon F. Pearson
 Vice President
 International Ecological Systems & Services, IESS
 P.O. Box 21240
 B-l Oak Park Plaza
 Hilton Head Isl, SC 29925

 Leroy C. Reid, Jr., Ph.D
 1273 Annapolis Drive
 Anchorage AK 99508-4307
                       1 I         '          ' '
 The  mention of trade  names  or  commercial
products  does not  constitute  endorsement  or
recommendation for use by the U.S. Environmental
Protection Agency.
      For more information contact:

      Municipal Technology Branch
      U.S. EPA
      Mail Code 4204
      401 M St., S.W.
      Washington, D.C., 20460
                                                      IMTB
                                                     Excellence In compliance through optimal technical solutions
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