vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA832-F-00-013
September 2000
Waste water
Technology Fact Sheet
Oxidation Ditches
DESCRIPTION
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
ditch.
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
(RAS)
To Disinfection
Sludge Pumps
From Primary Treatment
Source: Parsons Engineering Science, Inc., 2000.
FIGURE 1 TYPICAL OXIDATION DITCH ACTIVATED SLUDGE SYSTEM
-------
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.
APPLICABILITY
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
Sludge
Source: Parsons Engineering Science, Inc., 1999
FIGURE 2 THE MODIFIED LUDZACK-ETTINGER PROCESS
-------
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.
ADVANTAGES AND DISADVANTAGES
Advantages
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
process.
Energy efficient operations result in reduced
energy costs compared with other biological
treatment processes.
Disadvantages
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.
DESIGN CRITERIA
Construction
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,
1999).
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
days.
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.
PERFORMANCE
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.
TABLE 1 PERFORMANCE OF CASA
GRANDE, AZWWTP
BOD
TSS
Total N
Average
Monthly
Influent
(mg/L)
226
207
35.4
Average
Monthly
Effluent
(mg/L)
8.86
5.23
1.99
Percent
Removal (%)
96
97
94
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.
TABLE 2 PERFORMANCE OF
EDGARTOWN, MA WWTP
BOD
TSS
Total N
Average
Monthly
Influent
(mg/L)
238
202
27.1
Average
Monthly
Effluent
(mg/L)
3.14
5.14
2.33
Percent
Removal
(%)
99
97
90
Source: Town of Edgartown, 1999.
OPERATION AND MAINTENANCE
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).
COSTS
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).
REFERENCES
Other Related Fact Sheets
Other EPA Fact Sheets can be found at the
following web address:
2x)y^
1.
2.
3.
5.
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,
1(2).
Metcalf and Eddy, Inc., 1991. Wastewater
Engineering: Treatment, Disposal, Reuse.
ord
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.
ADDITIONAL INFORMATION
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
Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Ave., NW
Washington, D.C. 20460
MTB
Excelence fh tompfance through optfhial tethnltal solutfons
MUNICIPAL TECHNOLOGY BRANCH
------- |