United States
                      Environmental Protection
                      Office of Water
                      Washington, D.C.
September 2000
Waste water
Technology Fact  Sheet
Oxidation  Ditches

An oxidation ditch is a modified activated sludge
biological treatment process that utilizes long solids
retention times (SRTs) to remove biodegradable
organics. Oxidation ditches are typically complete
mix systems, but they can be modified to approach
plug flow conditions. (Note: as conditions approach
plug flow, diffused air must be used to  provide
enough mixing.  The system will also no longer
operate as an oxidation ditch).  Typical oxidation
ditch treatment systems consist of a single or multi-
channel  configuration within  a ring, oval,  or
horseshoe-shaped basin.  As a result, oxidation
ditches are  called  "racetrack  type" reactors.
Horizontally or vertically mounted aerators provide
                     circulation,  oxygen transfer,  and aeration in the

                     Preliminary treatment, such as bar screens and grit
                     removal,  normally precedes the oxidation ditch.
                     Primary settling prior to  an  oxidation ditch is
                     sometimes practiced, but  is  not typical in  this
                     design.   Tertiary filters  may be required  after
                     clarification,   depending  on  the   effluent
                     requirements.    Disinfection  is  required   and
                     reaeration may be necessary prior to final discharge.
                     Flow to the oxidation ditch is aerated and mixed
                     with return  sludge from a secondary clarifier. A
                     typical process flow diagram for an activated sludge
                     plant using an oxidation ditch is shown in Figure 1.
                      Oxidation Ditch
                                         Return Activated Sludge
                                                                      To Disinfection
                                              Sludge Pumps
     From Primary Treatment
   Source: Parsons Engineering Science, Inc., 2000.


Surface  aerators,  such as  brush  rotors, disc
aerators,  draft  tube aerators,  or  fine  bubble
diffusers are used to circulate the mixed liquor.
The mixing process entrains oxygen into the mixed
liquor to foster microbial growth and the motive
velocity ensures contact of microorganisms with the
incoming  wastewater.   The  aeration   sharply
increases the dissolved oxygen (DO) concentration
but decreases as biomass uptake  oxygen as the
mixed liquor travels through the ditch.  Solids are
maintained  in  suspension  as the  mixed liquor
circulates around the ditch.  If design SRTs are
selected   for  nitrification,  a  high degree  of
nitrification will occur. Oxidation ditch effluent is
usually settled in a separate secondary clarifier. An
anaerobic tank may be added prior to the ditch to
enhance biological phosphorus removal.

An oxidation ditch may also be operated to achieve
partial denitrification. One of the most  common
design modifications for enhanced nitrogen removal
is known as the Modified Ludzack-Ettinger (MLE)
process. In this process, illustrated in Figure 2,  an
anoxic tank  is added upstream of the ditch along
with mixed  liquor recirculation from the aerobic
zone  to  the tank to achieve higher levels of
denitrification.  In the aerobic basin, autotrophic
bacteria (nitrifiers) convert ammonia-nitrogen to
nitrite-nitrogen and then to nitrate-nitrogen. In the
anoxic zone, heterotrophic bacteria convert nitrate-
nitrogen to nitrogen  gas which is released to the
atmosphere.  Some mixed liquor from the aerobic
basin  is recirculated to the anoxic zone to provide
a mixed liquor with a high-concentration of nitrate-
nitrogen to the anoxic zone.

Several manufacturers have developed modifications
to the oxidation ditch design to remove nutrients in
conditions cycled or phased between the anoxic and
aerobic states. While the mechanics of operation
differ  by  manufacturer, in general, the process
consists of two separate aeration basins, the first
anoxic and the second  aerobic.   Wastewater and
return activated sludge  (RAS) are introduced into
the first reactor  which  operates  under anoxic
conditions. Mixed liquor then flows into the second
reactor operating under aerobic conditions. The
process is  then  reversed  and the second reactor
begins to operate  under  anoxic conditions.


The oxidation ditch process is a fully demonstrated
secondary   wastewater  treatment   technology,
applicable in any situation where activated sludge
treatment  (conventional or extended aeration) is
appropriate.  Oxidation ditches are  applicable in
plants that  require nitrification because the basins
can be  sized using an appropriate SRT to achieve
nitrification  at   the   mixed   liquor   minimum
temperature.  This technology is very effective in
small  installations, small communities, and isolated
institutions, because it requires more land than
conventional treatment plants.

The oxidation process originated in the Netherlands,
                                     Mixed Liquor Recirculation
                                     Anoxic       Aerobic

                                         Return Activated Sludge
                   Primary Sludge
                                              0.5Q - 1Q
                                                                Waste Activated
    Source: Parsons Engineering Science, Inc., 1999


with  the  first  full  scale  plant  installed  in
Voorschoten,  Holland,  in 1954.    There  are
currently more than 9,200 municipal oxidation ditch
installations in  the United States (WEF,  1998).
Nitrification to less than 1 mg/L ammonia nitrogen
consistently occurs when ditches are designed and
operated for nitrogen removal.



The main advantage  of the oxidation ditch is the
ability to achieve removal performance objectives
with low operational requirements and operation
and maintenance costs. Some specific advantages
of oxidation ditches include:

•     An   added  measure of  reliability  and
      performance over other biological processes
      owing  to a   constant  water  level  and
      continuous discharge which lowers the weir
      overflow  rate  and eliminates the periodic
      effluent surge common to other biological
      processes, such as SBRs.

•     Long hydraulic retention time and  complete
      mixing minimize the impact of a shock load
      or hydraulic surge.

•     Produces  less sludge than other biological
      treatment  processes  owing  to  extended
      biological activity during the activated sludge

      Energy efficient operations result in reduced
      energy costs compared with other biological
      treatment processes.


      Effluent suspended solids concentrations are
      relatively  high   compared  to   other
      modifications of the activated sludge process.

      Requires  a  larger  land area than  other
      activated sludge treatment options. This can
      prove  costly,  limiting  the   feasibility  of
      oxidation ditches in urban, suburban, or other
      areas  where  land  acquisition costs  are
      relatively high.


Oxidation ditches are commonly constructed using
reinforced concrete, although gunite, asphalt, butyl
rubber, and clay have also been used. Impervious
materials, are usually used to prevent erosion.

Design Parameters

Screened wastewater enters the ditch, is aerated, and
circulates at about 0.25 to 0.35 m/s (0.8 to 1.2 ft/s)
to maintain the solids in suspension  (Metcalf &
Eddy, 1991).  The RAS recycle ratio is from 75 to
150 percent, and the mixed liqour suspended solids
(MLSS) concentration ranges from 1,500 to 5,000
mg/L (0.01  to 0.04 Ibs/gal) (Metcalf & Eddy, 1991).
The oxygen transfer efficiency of oxidation ditches
ranges from 2.5 to 3.5 Ib./Hp-hour (Baker Process,

The  design criteria are  affected  by the influent
wastewater parameters and  the required effluent
characteristics, including the decision or requirement
to  achieve  nitrification, denitrification,   and/or
biological  phosphorus removal.  Specific  design
parameters  for oxidation ditches include:

Solids Retention Time  (SRT):  Oxidation ditch
volume is sized based on the required SRT to meet
effluent quality requirements. The SRT is selected
as a function of nitrification requirements and  the
minimum mixed liquor temperature. Design SRT
values vary from 4 to 48 or more days. Typical
SRTs required for nitrification range from 12 to 24

BOD Loading: BOD loading rates vary from less
than 160,000 mg/1000 liters (10 Ib./lOOO  ft3) to
more than 4xl07 mg/1000 liters (50 Ib./lOOO ft3). A
BOD loading rate of 240,000 mg/1000 liters per  day
(15 Ib./lOOO ft3/day) is commonly used as a design
loading rate. However, the BOD loading rate is  not
typically  used  to  determine  whether  or   not
nitrification occurs.

Hydraulic  Retention Time: While rarely used as a
basis for oxidation ditch design, hydraulic Retention
Times (HRTs) within the oxidation ditch range from

6  to 30 hours for most municipal wastewater
treatment plants.


As   fully-demonstrated  secondary   treatment
processes, oxidation  ditch processes are readily
adaptable for nitrification and denitrification.   As
part  of an Evaluation of Oxidation Ditches for
Nutrient Removal (EPA, 1991), performance data
were collected from 17 oxidation ditch plants.  The
average design flow for these plants varied between
378  to 45,425 m3/day (0.1 to 12 MOD).   The
average performance of these plants, summarized in
Table 1, indicates that oxidation  ditches achieve
BOD, suspended  solids, and  ammonia nitrogen
removal of  greater than  90 percent.  Likewise,
Rittmann and Langeland (1985) reported nitrogen
removals of greater than 90 percent from oxidation
ditch processes.

The following section discusses the performance of
two recently designed oxidation ditch facilities.

            GRANDE, AZWWTP

Total N
Removal (%)
Source: City of Casa Grande, AZ, 1999.

Casa Grande Water Reclamation Facility

The  City  of  Casa  Grande,  Arizona,  Water
Reclamation Facility began operation in February
1996. The system was designed to treat 15,142
m3/day (4.0 MGD)  and  uses an  anoxic zone
preceeding the aerobic zone of each train to
provide   denitrification.  With  influent  design
parameters of 270 mg/L BOD (0.002 Ibs/gal BOD),
300 mg/L TSS (0.003 Ibs/gal TSS),  and 45 mg/L
TKN (3.8xlQ-4  Ibs/gal  TKN),  the  plant  has
consistently achieved effluent obj ectives of 10 mg/L
BOD (8.34xlQ-5  Ibs/gal  BOD),  15 mg/L TSS
(1.2xlO'4  Ibs/gal   TSS),   1.0  mg/L  ammonia
(8.34xlO"6 Ibs/gal ammonia), and 5.0 mg/L nitrate-
nitrogen (4.2xlO"5 Ibs/gal nitrate-nitrogen). Table 1
summarizes the plant's performance between July
1997 and July 1999.

Edgartown, Massachusetts WWTP

The Edgartown, Massachusetts WWTP, located on
the island of Martha's Vineyard, is designed to treat
757 nrVday (0.20 MGD) in the winter months and
2,839 nrVday (0.75 MGD) in the summer.  Two
Carrousel® denitIR basins are installed and the plant
has achieved performance objectives since opening.
Table 2 summarizes average monthly influent and
effluent data.


Total N
 Source: Town of Edgartown, 1999.


Oxidation   ditches   require  relatively   little
maintenance compared to other secondary treatment
processes.   No  chemicals are required  in  most
applications, but metal salts can be added to enhance
phosphorus removal.

Residuals Generated

Primary sludge is  produced  if primary clarifiers
precede the oxidation ditch.  Sludge production for
the oxidation ditch process ranges from 0.2 to 0.85
kg TSS per kg (0.2 to 0.85 Ib. TSS per Ib). BOD
applied (Sherwood Logan and Associates, 1999).
Typical sludge production is 0.65 kg TSS per kg of
BOD (0.65 Ib TSS per Ib. of BOD).  This is less than
conventional activated sludge facilities because of
long SRTs.

Operating Parameters

The oxygen coefficient for BOD removal varies with
temperature and SRT. Typical oxygen requirements

range from 1.1 to 1.5  kg of O2 per kg of BOD
removed (1.1  to  1.5 Ibs of O2 per Ib.  of BOD
removed) and 4.57 kg of O2 per kilogram of TKN
oxidized (4.57 Ibs of O2 per Ib. of TKN oxidized)
(EPA,  1991;  Baker Process, 1999).   Oxygen
transfer efficiency ranges from2.5 to 3.5 Ib./Hp-
hour (Baker Process, 1999).


The  basin volume  and footprint  required for
oxidation ditch plants have traditionally been very
large compared with other secondary treatment
processes. Larger footprints result in higher capital
costs,  especially  in urbanized locations  where
available land is very expensive. Vertical reactors,
in which process flow travels downward through
the  reactor,  are  generally more expensive than
traditional horizontal reactors.  However, because
they require  less  land  than  more  conventional
horizontal reactors, they can significantly reduce
overall capital  costs where land costs are high.

The  cost of  an  oxidation  ditch  plant  varies
depending  on treatment  capacity  size,  design
effluent limitations, land cost, local construction
costs, and other site specific factors.  Construction
capital costs for ten plants were evaluated by EPA
in 1991, with construct on costs ranging from $0.52
to $3.17/liter per  day ($1.96  to  $12.00/gpd)
treated. These costs have been updated with the
ENR construction cost index (ENR = 5916).

Recent information obtained from manufacturers on
facilities ranging 3,785 to 25,740 m3/day (1.0 MOD
to 6.8  MOD)  indicates that construction capital
costs of oxidation ditch plants range from $0.66 to
$1.10/liter per day ($2.50 to $4.00 per gpd).  For
example,  the  Blue  Heron  Water  Reclamation
Facility in Titusville, Florida- a 15,142 m3/day (4.0
MGD) oxidation ditch and sludge handling facility
which began operation in 1996, was constructed for
about $0.80/liter per day ($3.00 per gpd) (Kruger,
1996). The facility features a multi-stage biological
nutrient  removal  process  and  a  sophisticated
Supervisory Control and Data Acquisition System
(SCADA) control system.

Oxidation  ditches  offer   significantly  lower
operation and maintenance  costs than other
secondary treatment processes.  Compared to other
treatment  technologies, energy requirements are
low, operator attention is minimal, and chemical
addition is not usually required.  For example the
Tar  River Wastewater Reclamation  Facility  in
Louisburg, North Carolina has documented energy
savings of 40 percent compared with conventional
activated  sludge plants (Ellington, 1999).  The
oxidation  ditch has also eliminated chemical costs
and plant staff are available for other facility needs
(Ellington, 1999).


Other Related Fact Sheets

Other  EPA Fact  Sheets  can  be found at the
following web address:
       Baker   Process,   1999.   Personal
       communication  with Betty-Ann  Custis,
       Senior Process Engineer,  Memorandum to
       Parsons Engineering Science, Inc.

       City  of  Casa  Grande,  Arizona,  1999.
       Facsimile  from  Jerry Anglin  to Parsons
       Engineering Science,  Inc.

       Ettlilch,  William  F., March  1978.   A
       Comparison of Oxidation Ditch  Plants to
       Competing Processes for Secondary  and
       Advanced Treatment of Municipal Wastes.

       Ellington,    Jimmy,    1999.    Plant
       Superintendent,  Tar  River   Water
       Reclamation Facility.  Personal conversation
       with Parsons Engineering  Science, Inc.

       Kruger, Inc. 1996. A2O &ATAD Processes
       provide Effective  Wastewater,  Biosoilds
       Treatment for Titusville,  Fla.  Fluentlines,

       Metcalf and Eddy, Inc., 1991.  Wastewater
       Engineering:  Treatment,  Disposal, Reuse.
          edition. New York:  McGraw Hill.
       Sherwood Logan and Associates, Inc., 1999.
       Personal  communication  with    Robert
       Fairweather.    Faxsimile  transmitted  to
       Parsons Engineering Science, Inc.

8.      Town of Edgartown, Massachusetts, 1999.
       Facsimile  from Mike Eldridge to Parsons
       Engineering Science, Inc

9.      U.S.  Environmental  Protection Agency,
       February 1980. Innovative and Alternative
       Technology Assessment Manual. Offi ce of
       Water Program Operations, Washington,
       D.C.   and  Office  of  Research  and
       Development,  Cincinnati,  Ohio.

10.    U.S.  Environmental  Protection Agency,
       Municipal  Environmental   Research
       Laboratory, September 1991.  Office  of
       Research  and Development,  Cincinnati,
       Ohio,   EPA-600/2-78-051. Prepared by
       HydroQual,  Inc.    Preliminary  Draft
       Evaluation  of  Oxidation  Ditches for
       Nutrient Removal.

11.    Water  Environment  Federation,  1998.
       Design of Municipal Wastewater Treatment
       Plants, 4th edition, Manual of Practice No.
       8: Vol 2,  Water Environment Federation:
       Alexandria, Virginia.


City of Findlay, Ohio
Jim Paul, Supervisor - Water Pollution Control
1201 South River Road
Findlay, OH 45840

Edgartown Wastewater Department
Michael Eldredge, Chief Operator
P.O. Box 1068
Edgartown, MA 02539
Casa Grande WWTP
Jerry Anglin, Chief Operator
1194 West Koartsen
Casa Grande, AZ 85222

Tar River Wastewater Reclamation Facility
Jimmy Ellington, Superintendent
HOW. Nash St.
Louisburg, NC 27549

National Small Flows Clearing House
at West Virginia University
P.O. Box 6064
Morgantown, WV 26506

The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by  the  U.S. Environmental Protection
                                                         For more information contact:
                                                         Municipal Technology Branch
                                                         U.S. EPA
                                                         Mail Code 4204
                                                         1200 Pennsylvania Ave., NW
                                                         Washington, D.C. 20460
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