United States
                        Environmental Protection
                       Office of Water
                       Washington, D.C.
  EPA 832-F-99-063
  September 1999
Technology Fact Sheet
Ozone  Disinfection

 Disinfection is considered to be the primary
 mechanism  for the inactivation/destruction of
 pathogenic organisms to prevent the spread of
 waterborne diseases to downstream users and the
 environment. It is important mat wastewater be
 adequately treated prior to disinfection in order for
 any disinfectant to be effective. Table 1 lists some
 common  microorganisms  found  in domestic
 wastewater and the diseases associated with them.

 Ozone is produced when oxygen (O2) molecules are
 dissociated by an energy source into oxygen atoms
 and subsequently collide with an oxygen molecule
 to form an unstable gas, ozone (O3), which is used
 to disinfect wastewater. Most wastewater treatment
 plants generate  ozone by imposing a high voltage
 alternating current  (6 to 20 kilovolts) across a
 dielectric  discharge  gap  that   contains  an
 oxygen-bearing gas.  Ozone is generated onsite
 because it is unstable and decomposes to elemental
 oxygen in a short amount of time after generation.

 Ozone is a very strong oxidant and virucide. The
mechanisms of disinfection using ozone include:

      Direct oxidation/destruction of the cell wall
      with leakage of cellular constituents outside
      of the cell.

      Reactions with radical by-products of ozone

      Damage  to the constituents of the nucleic
      acids (purines and pyrimidines).
                           Breakage of carbon-nitrogen bonds leading
                            to depolymerization.

                          TABLE 1 INFECTIOUS AGENTS
                             DOMESTIC WASTEWATER
 Disease Caused
                      Escherichia coli
                      Leptospira (spp.)
                      Salmonella typhi
                      Salmonella (=2,100 serotypes)
                      Shigetta (4 spp.)

                      Vibrio cholerae
                      Cryptosporidium pan/urn
                      Entamoeba histolytica

                      Giardia lamblia
                      Ascaris lumbricoides
                      T. solium
                      Trichuris trichiura
                      Enteroviruses (72 types, e.g.,
                      polio, echo, and coxsackie
                      Hepatitis A virus
                      Norwaik agent

 Typhoid fever
 Shigellosis (bacillary

 Amebiasis (amoebic


 Gastroenteritis, heart
 Infectious hepatitis
                                            Source: Adapted from Crites and Tchobanoglous, 1998.

 When ozone decomposes in water, the free radicals
 hydrogen peroxy (HO2) and hydroxyl (OH) that are
 formed have great oxidizing capacity and play an
 active role in the  disinfection process.   It is
 generally believed that the bacteria are destroyed
 because of protoplasmic oxidation resulting in cell
 wall disintegration (cell lysis).

 The effectiveness of disinfection depends on the
 susceptibility of the target organisms, the contact
 time, and the concentration of the ozone. A line
 diagram of the ozonation process is  shown in
 Figure 1. The components of an ozone disinfection
 system   include  feed-gas  preparation,  ozone
 generation,  ozone   contacting,   and  ozone

 Air or pure oxygen is used as the feed-gas source
 and is passed to the ozone generator at a set flow
 rate. The energy source for production is generated
 by  electrical discharge in a  gas  that  contains
 oxygen. Ozone generators are typically classified

      The control mechanism (either a voltage or
      frequency unit).

      The cooling mechanism (either water, air,
      or water plus oil).
       The physical arrangement of the dielectrics
        (either vertical or horizontal).

       The name of the inventor.

 However,  generators manufactured  by different
 companies have unique characteristics but also have
 some common configurations.

 The  electrical discharge  method  is  the most
 common energy source  used to produce  ozone.
 Extremely dry air or pure oxygen is exposed to a
 controlled, uniform high-voltage discharge at a high
 or low frequency.  The dew point of the feed gas
 must be -60C (-76 F) or lower. The gas  stream
 generated from air will contain about 0.5 to 3.0%
 ozone by weight, whereas pure oxygen will form
 approximately two to four times that concentration.

 After generation, ozone is fed into a down-flow
 contact  chamber containing the wastewater to be
 disinfected. The main purpose of the contactor is to
 transfer ozone from the gas bubble into the bulk
 liquid while providing sufficient  contact time for
 disinfection. The commonly used contactor types
 diffused bubble (co-current and counter-current) are
positive  pressure  injection,  negative pressure
 (Venturi), mechanically agitated, and packed tower.
Because ozone  is consumed quickly, it must  be
contacted uniformly in a near plug flow contactor.
Feed Gas Preparation
Oxygen Production
Oxygen Storage
Air/Oxygen Treatment

Ozone Destruction

v-'^UIlc OcilcIculOn



Ozone Con
IdCl U3Sin *
Wastewater In

  The off-gases from the contact chamber must be
  treated to destroy  any remaining  ozone before
  release into  the atmosphere.   Therefore,  it is
  essential to maintain an optimal ozone dosage for
  better efficiency. When pure oxygen is used as the
  feed-gas, the off-gases from the contact chamber
  can be recycled to generate ozone or for reuse in the
  aeration tank. The ozone off-gases that are not used
  are sent  to the ozone destruction unit  or are

  The  key  process control parameters are dose,
  mixing, and contact time.  An ozone disinfection
  system strives for the maximum solubility of ozone
  in wastewater,  as disinfection depends on the
 transfer of ozone to the wastewater. The amount of
 ozone that will dissolve in wastewater at a constant
 temperature is a  function of the partial pressure of
 the gaseous ozone above the water  or in the gas
 feed stream.

 It is critical that all ozone disinfection systems be
 pilot tested and calibrated prior to installation to
 ensure they meet  discharge permit requirements for
 their particular sites.


 Ozone disinfection is generally  used at medium to
 large sized plants after at least secondary treatment.
 In addition to disinfection, another common use for
 ozone in wastewater treatment is odor control.

 Ozone disinfection is the least used method in the
 U.S. although  this technology has  been  widely
 accepted in Europe for decades. Ozone treatment
 has  the  ability  to  achieve   higher  levels  of
 disinfection than  either chlorine or UV, however,
 the  capital  costs  as  well  as   maintenance
 expenditures are  not  competitive with available
 alternatives.    Ozone  is therefore   used  only
 sparingly,  primarily  in  special  cases  where
 alternatives are not effective.


      The ozonation process utilizes a short contact
      time (approximately 10 to 30 minutes).

       There are no harmful residuals that need to
       be removed after ozonation because ozone
       decomposes rapidly.

       After ozonation, there is  no regrowth of
       microorganisms, except for those protected
       by the particulates in the wastewater stream.

       Ozone is generated onsite, and thus, there
       are fewer safety problems associated with
       shipping and handling.

       Ozonation elevates the dissolved oxygen
       (DO) concentration of the effluent.  The
       increase in DO can eliminate the need for
       reaeration and also raise the level of DO in
       the receiving stream.
       Low dosage may not effectively inactivate
       some viruses, spores, and cysts.

       Ozonation is a more complex technology
       than  is  chlorine  or  UV  disinfection.
       requiring  complicated  equipment  and
       efficient contacting systems.

       Ozone is very reactive and corrosive, thus
       requiring corrosion-resistant material such
       as stainless steel.

       Ozonation is not economical for wastewater
       with high levels of suspended solids (SS).
       biochemical   oxygen   demand  (BOD).
       chemical oxygen demand, or total organic

       Ozone is extremely irritating and possibly
      toxic, so off-gases from the contactor must
      be destroyed to prevent worker exposure.

      The cost of treatment can be relatively high
      in capital and in power intensiveness.
     Ozone is more effective than chlorine  in
     destroying viruses and bacteria.

                                                   OPERATION AND MAINTENANCE
  Belmont and Southport Wastewater
  Treatment Plants in Indianapolis, Indiana

  In 1985, the City of Indianapolis, Indiana, operated
  two-125 million gallons per day (mgd) advanced
  wastewater  treatment  plants  at  Belmont  and
  Southport using ozone  disinfection.   The rated
  capacity of the oxygen-fed ozone generators was
  6,380 pounds per day, which was used to meet
  geometric mean weekly and monthly disinfection
  permit limits for fecal coliforms of 400 and 200 per
  100 mL, respectively.

  Disinfection  was  required at both Indianapolis
  treatment plants from April 1 through October 31,
  1985. Equipment performance characteristics were
  evaluated during the  1985 disinfection season and
  consequently,  disinfection  performance  was
  optimized during the  1986 season. The capital cost
  of both ozone systems represented about 8% of the
  plants' total construction cost. The ozone system's
  Operation  and   Maintenance  (O&M)   cost
  represented about 1.9% and 3.7% of the total plant
  O&M costs at the Belmont and Southport plants,

 In  1989, a disciplined process monitoring and
 control program was initiated. Records indicated a
 significant effect on process performance due to
 changes in wastewater flow, contactor influent fecal
 coliform concentration, and ozone demand.

 Previously,  ozone  demand  information   was
 unknown.   Several studies  were conducted  to
 enable better  control of  the ozone  disinfection
 process. These included the recent installation of a
 pilot-scale ozone contactor to allow the plant staff
 to measure ozone demand on a daily basis. Also,
 tracer  tests were conducted to measure contactor
 short-circuiting potential.  Results demonstrated a
 noticeable benefit of adding additional  baffles.
 Results also indicated operating  strategies that
 could maximize fecal  coliform removal, such as
 reducing the number of contactors in service at low
and moderate flow conditions.
  Ozone generation uses  a significant amount of
  electrical power. Thus, constant attention must be
  given to  the  system to  ensure  that power is
  optimized for controlled disinfection performance.

  There must be  no  leaking  connections  in  or
  surrounding the ozone generator.   The operator
  must on a regular basis  monitor the appropriate
  subunits to ensure that they are not overheated.
  Therefore,  the  operator  must check  for  leaks
  routinely,  since  a very  small  leak  can  cause
  unacceptable ambient ozone concentrations. The
  ozone monitoring equipment must be tested and
  calibrated  as recommended  by the  equipment

  Like  oxygen, ozone  has  limited solubility and
  decomposes more rapidly in water than in air. This
  factor, along with ozone reactivity, requires that the
  ozone contactor be well covered and that the ozone
  diffuses into the  wastewater  as effectively  as

 Ozone in gaseous form is explosive once it reaches
 a concentration of 240 g/m3. Since most ozonation
 systems   never   exceed  a  gaseous   ozone
 concentration of 50 to  200 g/m3, this is generally
 not a problem.  However, ozone in gaseous form
 will remain hazardous  for a significant amount of
 time  thus, extreme caution   is needed  when
 operating the ozone gas systems.

 It  is   important   that  the  ozone  generator,
 distribution,   contacting,   off-gas,  and   ozone
 destructor inlet piping be purged before opening the
 various systems or subsystems.  When entering the
 ozone  contactor, personnel must recognize the
 potential for oxygen deficiencies or trapped ozone
 gas in spite of best efforts to purge the system.  The
 operator  should  be aware of all  emergency
 operating procedures required if a problem occurs.
All safety  equipment  should  be available  for
operators to use  in  case of an emergency.  Key
O&M parameters include:

      Clean feed gas with a dew point of -60 C
       (-76 F), or lower, must be delivered to the
       ozone generator. If the supply gas is moist,

               the rection of the ozone  and the
               moisture will yield a very corrosive
               condensate on  the  inside  of the
               ozonator.    The  output  of  the
               generator could be lowered by the
               formation of nitrogen oxides (such
               as nitric acid).

       Maintain the required  flow of generator
        coolant (air, water, or other liquid).

       Lubricate the  compressor  or blower  in
        accordance  with  the  manufacturer's
        specifications.  Ensure that all compressor
        sealing gaskets are in good condition.

       Operate the ozone  generator within its
        design parameters.  Regularly inspect and
        clean the ozonator, air supply, and dielectric
        assemblies, and monitor the temperature of
        the ozone generator.

       Monitor the ozone gas-feed and distribution
        system to ensure that the necessary volume
        conies  into  sufficient  contact  with the

       Maintain ambient levels of ozone below the
        limits of applicable safety regulations.


 The cost of ozone disinfection systems is dependent
 on the manufacturer, the site, the capacity of the
 plant, and the characteristics of the wastewater to be
 disinfected. Ozonation costs are generally high in
 comparison with other disinfection techniques.

 Table 2 shows a typical cost estimate (low to
 medium) for ozone  disinfection  system used to
 disinfect one mgd of wastewater. The costs are
 based on the wastewater having passed through
 both primary and secondary treatment processes of
 a properly designed system (the  BOD content does
 not exceed 30 milligrams per liter [mg/L] and the
 SS content is less than 30 mg/L).  In general, costs
 are largely influenced by site-specific factors, and
thus, the estimates that follow  are typical values
and can vary from site to site.
  Capital Costs

    Oxygen feed gas and compressor
    Contact vessel (500 gpm)
  Destruct unit:

    Small (around 30 cfm)

    Large (around 120)
    Non-component costs
  Annual O&M Costs

    Other (filter replacements,
  compressor oil, spare dielecrtic, etc.)








90 kW

 gpm = gallons per minute
 cfm = cubic feet per minute
 Source: Champion Technology, 1998.
Because the concentration of ozone generated from
either  air  or oxygen  is  so low,  the transfer
efficiency to the liquid phase is a critical economic
consideration.    For this reason,  the contact
chambers used are usually very deep and covered.

The overall cost  of an  ozonation system is also
largely  determined  by  the  capital  and  O&M
expenses.  The annual operating costs for  ozone
disinfection  include power   consumption,  and
supplies, miscellaneous  equipment  repairs,  and
staffing requirements.

Another consideration for the cost  is that each
ozonation system  is site specific, depending  on the
plant's  effluent limitations.  Chemical suppliers
should be contacted for specific cost information.

  Crites, R. and G. Tchobanoglous.  1998.
  Small  and  Decentralized  Wastewater
  Management Systems.  The McGraw-Hill
  Companies. New York, New York.

  Martin, E. J. and  E.  T.  Martin.  1991.
  Technologies  for  Small  Water  and
  Wastewater   Systems.  Environmental
  Engineering Series. VanNostrand Reinhold
  (now acquired by John Wiley & Sons, Inc.).
  New York, New York. pp. 209-213.

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

  Rakness, K. L.; K. M. Corsaro; G. Hale;
  and  B. D. Blank. 1993.  "Wastewater
  Disinfection with Ozone: Process Control
  and Operating Results." Ozone: Science and
  Engineering, vol. 15. no. 6. pp. 497-514.

 Rakness,  K.  L.; R. C. Renner;  D. B.
 Vomehm;   and  J.   R.  Thaxton.   1988.
 "Start-Up and Operation of the Indianapolis
 Ozone Disinfection Wastewater Systems."
 Ozone: Science and Engineering, vol. 10.
 no. 3. pp. 215-240.

 Rudd, T. and L. M. Hopkinson. December
 1989.  "Comparison  of  Disinfection
 Techniques  for  Sewage   and  Sewage
 Effluents." Journal of International Water
 and Environmental Management, vol. 3. pp

 Task  Force  on Wastewater Disinfection.
 1986. Wastewater Disinfection. Manual of
 Practice  No.  FD-10.  Water Pollution
 Control Federation. Alexandria, Virginia.

 U.S.  Environmental  Protection  Agency
(EPA). 1986. Design Manual: Municipal
Wastewater Disinfection. EPA Office of
                                                       Research and Development.  Cincinnati,
                                                       Ohio. EPA/625/1-86/021.

                                                9.     Water Environment  Federation  (WEF).
                                                       1996. Operation of Municipal Wastewater
                                                       Treatment Plants. Manual of Practice No.
                                                       11.  5th  ed.  vol. 2. WEF.  Alexandria,

                                                10.    Rasmussen,  Karen (Frost & Sullivan).
                                                       1998.    Pollution Engineering   Online.
                                                       "Market Forecast: Wastewater Treatment
                                                       Equipment Markets."  WEFTEC, Orlando,

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