United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-00-013

September 2000

Wastewater

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 multichannel 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.

To Disinfection

Clarifier

Aerator

Hopper

Oxidation Ditch

Return Activated Sludge 

(RAS) Sludge Pumps

From Primary Treatment

To Disinfection

Clarifier

Aerator

Hopper

Oxidation Ditch

Return Activated Sludge 

(RAS) 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 nitratenitrogen 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 nitratenitrogen 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,

Primary Sludge 0.5Q – 1 Q

Waste Activated 

S ludge

M ixed Liquor Recirculation

Secondary 

C larifier

Anoxic Aerobic

Return Activated Sludge

Primary 

Settling

3Q – 4 Q

Q Q

Primary Sludge 0.5Q – 1 Q

Waste Activated 

S ludge

M ixed Liquor Recirculation

Secondary 

C larifier

Anoxic Aerobic

Return Activated Sludge

Primary 

Settling

3Q – 4 Q

Q Q

Primary Sludge 0.5Q – 1 Q

Waste Activated 

S ludge

M ixed Liquor Recirculation

Secondary 

C larifier

Anoxic Aerobic

Return Activated Sludge

Primary 

Settling

3Q – 4 Q

Q Q

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:

C 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.

C Long hydraulic retention time and complete

mixing minimize the impact of a shock load

or hydraulic surge. 

C Produces less sludge than other biological

treatment processes owing to extended

biological activity during the activated sludge

process.

C Energy efficient operations result in reduced

energy costs compared with other biological

treatment processes.

Disadvantages

C Effluent suspended solids concentrations are

relatively high compared to other

modifications of the activated sludge process.

C 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 lbs/gal) (Metcalf & Eddy, 1991).

The oxygen transfer efficiency of oxidation ditches

ranges from 2.5 to 3.5 lb./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 lb./1000 ft3) to

more than 4x107 mg/1000 liters (50 lb./1000 ft3). A

BOD loading rate of 240,000 mg/1000 liters per day

(15 lb./1000 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 m

3

/day (0.1 to 12 MGD). 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. 

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

m

3

/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 lbs/gal BOD),

300 mg/L TSS (0.003 lbs/gal TSS), and 45 mg/L

TKN (3.8x10-4 lbs/gal TKN), the plant has

consistently achieved effluent objectives of 10 mg/L

BOD (8.34x10-5 lbs/gal BOD), 15 mg/L TSS

(1.2x10-4 lbs/gal TSS), 1.0 mg/L ammonia

(8.34x10-6 lbs/gal ammonia), and 5.0 mg/L nitratenitrogen (4.2x10-5 lbs/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 m3/day (0.20 MGD) in the winter months and

2,839 m3/day (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.

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 lb. TSS per lb). BOD

applied (Sherwood Logan and Associates, 1999).

Typical sludge production is 0.65 kg TSS per kg of

BOD (0.65 lb TSS per lb. 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

TABLE 2 PERFORMANCE OF

EDGARTOWN, MA WWTP

Average

Monthly

Influent

(mg/L)

Average

Monthly

Effluent

(mg/L)

Percent

Removal

(%)

BOD 238 3.14 99

TSS 202 5.14 97

Total N 27.1 2.33 90

Source: Town of Edgartown, 1999.

TABLE 1 PERFORMANCE OF CASA

GRANDE, AZ WWTP

Average

Monthly

Influent

(mg/L)

Average

Monthly

Effluent

(mg/L)

Percent

Removal (%)

BOD 226 8.86 96

TSS 207 5.23 97

Total N 35.4 1.99 94

Source: City of Casa Grande, AZ, 1999.
range from 1.1 to 1.5 kg of O2

 per kg of BOD

removed (1.1 to 1.5 lbs of O2 per lb. of BOD

removed) and 4.57 kg of O2 per kilogram of TKN

oxidized (4.57 lbs of O2 per lb. of TKN oxidized)

(EPA, 1991; Baker Process, 1999). Oxygen

transfer efficiency ranges from2.5 to 3.5 lb./Hphour (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 construction 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 MGD

to 6.8 MGD) 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:

http://www.epa.gov/owmitnet/mtbfact.htm

1. Baker Process, 1999. Personal

communication with Betty-Ann Custis,

Senior Process Engineer, Memorandum to

Parsons Engineering Science, Inc.

2. City of Casa Grande, Arizona, 1999.

Facsimile from Jerry Anglin to Parsons

Engineering Science, Inc.

3. Ettlilch, William F., March 1978. A

Comparison of Oxidation Ditch Plants to

Competing Processes for Secondary and

Advanced Treatment of Municipal Wastes.

4. Ellington, Jimmy, 1999. Plant

Superintendent, Tar River Water

Reclamation Facility. Personal conversation

with Parsons Engineering Science, Inc.

5. Kruger, Inc. 1996. A2O &ATAD Processes

provide Effective Wastewater, Biosoilds

Treatment for Titusville, Fla. Fluentlines,

1 (2). 

6. Metcalf and Eddy, Inc., 1991. Wastewater

Engineering: Treatment, Disposal, Reuse.

3

rd edition. New York: McGraw Hill.

7. Sherwood Logan and Associates, Inc., 1999.

Personal communication with Robert

Fairweather. Faxsimile transmitted to

Parsons Engineering Science, Inc.
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

1200 Pennsylvania Ave., NW

Washington, D.C. 20460

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. Office 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

110 W. 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.