Wastewater Technology Fact Sheet: Oxidation Ditches

1 United StatesEnvironmental ProtectionAgencyOffice of WaterWashington, 832-F-00-013 September 2000 WastewaterTechnology Fact SheetOxidation DitchesDESCRIPTIONAn Oxidation ditch is a modified activated sludgebiological treatment process that utilizes long solidsretention times (SRTs) to remove biodegradableorganics. Oxidation Ditches are typically completemix systems, but they can be modified to approachplug flow conditions. (Note: as conditions approachplug flow, diffused air must be used to provideenough mixing. The system will also no longeroperate as an Oxidation ditch). Typical oxidationditch treatment systems consist of a single or multi-channel configuration within a ring, oval, orhorseshoe-shaped basin.

2 As a result, oxidationditches are called racetrack type or vertically mounted aerators providecirculation, oxygen transfer, and aeration in treatment , such as bar screens and gritremoval, normally precedes the Oxidation settling prior to an Oxidation ditch issometimes practiced, but is not typical in thisdesign. Tertiary filters may be required afterclarification, depending on the effluentrequirements. Disinfection is required andreaeration may be necessary prior to final to the Oxidation ditch is aerated and mixedwith return sludge from a secondary clarifier. Atypical process flow diagram for an activated sludgeplant using an Oxidation ditch is shown in Figure DisinfectionClarifierAeratorHopperOxidat ion DitchReturn Activated Sludge (RAS)Sludge PumpsFrom Primary TreatmentTo DisinfectionClarifierAeratorHopperOxidat ion DitchReturn Activated Sludge (RAS)Sludge PumpsFrom Primary TreatmentSource: Parsons Engineering Science, Inc.

3 , 1 TYPICAL Oxidation DITCH ACTIVATED SLUDGE SYSTEMS urface aerators, such as brush rotors, discaerators, draft tube aerators, or fine bubblediffusers are used to circulate the mixed mixing process entrains oxygen into the mixedliquor to foster microbial growth and the motivevelocity ensures contact of microorganisms with theincoming Wastewater . The aeration sharplyincreases the dissolved oxygen (DO) concentrationbut decreases as biomass uptake oxygen as themixed liquor travels through the ditch. Solids aremaintained in suspension as the mixed liquorcirculates around the ditch. If design SRTs areselected for nitrification, a high degree ofnitrification will occur.

4 Oxidation ditch effluent isusually settled in a separate secondary clarifier. Ananaerobic tank may be added prior to the ditch toenhance biological phosphorus Oxidation ditch may also be operated to achievepartial denitrification. One of the most commondesign modifications for enhanced nitrogen removalis known as the Modified Ludzack-Ettinger (MLE)process. In this process, illustrated in Figure 2, ananoxic tank is added upstream of the ditch alongwith mixed liquor recirculation from the aerobiczone to the tank to achieve higher levels ofdenitrification. In the aerobic basin, autotrophicbacteria (nitrifiers) convert ammonia-nitrogen tonitrite-nitrogen and then to nitrate-nitrogen.

5 In theanoxic zone, heterotrophic bacteria convert nitrate-nitrogen to nitrogen gas which is released to theatmosphere. Some mixed liquor from the aerobicbasin is recirculated to the anoxic zone to providea mixed liquor with a high-concentration of nitrate-nitrogen to the anoxic zone. Several manufacturers have developed modificationsto the Oxidation ditch design to remove nutrients inconditions cycled or phased between the anoxic andaerobic states. While the mechanics of operationdiffer by manufacturer, in general, the processconsists of two separate aeration basins, the firstanoxic and the second aerobic. Wastewater andreturn activated sludge (RAS) are introduced intothe first reactor which operates under anoxicconditions.

6 Mixed liquor then flows into the secondreactor operating under aerobic conditions. Theprocess is then reversed and the second reactorbegins to operate under anoxic Oxidation ditch process is a fully demonstratedsecondary Wastewater treatment Technology ,applicable in any situation where activated sludgetreatment (conventional or extended aeration) isappropriate. Oxidation Ditches are applicable inplants that require nitrification because the basinscan be sized using an appropriate SRT to achievenitrification at the mixed liquor minimumtemperature. This Technology is very effective insmall installations, small communities, and isolatedinstitutions, because it requires more land thanconventional treatment Oxidation process originated in the Netherlands,Primary 1 QWaste Activated SludgeMixed Liquor RecirculationSecondary ClarifierAnoxicAerobicReturn Activated SludgePrimary Settling3Q 4 QQQP rimary 1 QWaste Activated SludgeMixed Liquor RecirculationSecondary ClarifierAnoxicAerobicReturn Activated SludgePrimary Settling3Q 4 QQQP rimary 1 QWaste Activated SludgeMixed Liquor RecirculationSecondary ClarifierAnoxicAerobicReturn Activated SludgePrimary Settling3Q 4 QQQS ource.

7 Parsons Engineering Science, Inc., 1999 FIGURE 2 THE MODIFIED LUDZACK-ETTINGER PROCESS with the first full scale plant installed inVoorschoten, Holland, in 1954. There arecurrently more than 9,200 municipal Oxidation ditchinstallations in the United States (WEF, 1998).Nitrification to less than 1 mg/L ammonia nitrogenconsistently occurs when Ditches are designed andoperated for nitrogen removal. ADVANTAGES AND DISADVANTAGESA dvantagesThe main advantage of the Oxidation ditch is theability to achieve removal performance objectiveswith low operational requirements and operationand maintenance costs. Some specific advantagesof Oxidation Ditches include:CAn added measure of reliability andperformance over other biological processesowing to a constant water level andcontinuous discharge which lowers the weiroverflow rate and eliminates the periodiceffluent surge common to other biologicalprocesses, such as hydraulic retention time and completemixing minimize the impact of a shock loador hydraulic surge.

8 CProduces less sludge than other biologicaltreatment processes owing to extendedbiological activity during the activated efficient operations result in reducedenergy costs compared with other biologicaltreatment suspended solids concentrations arerelatively high compared to othermodifications of the activated sludge a larger land area than otheractivated sludge treatment options. This canprove costly, limiting the feasibility ofoxidation Ditches in urban, suburban, or otherareas where land acquisition costs arerelatively CRITERIAC onstructionOxidation Ditches are commonly constructed usingreinforced concrete, although gunite, asphalt, butylrubber, and clay have also been used.

9 Imperviousmaterials, are usually used to prevent ParametersScreened Wastewater enters the ditch, is aerated, andcirculates at about to m/s ( to ft/s)to maintain the solids in suspension (Metcalf &Eddy, 1991). The RAS recycle ratio is from 75 to150 percent, and the mixed liqour suspended solids(MLSS) concentration ranges from 1,500 to 5,000mg/L ( to lbs/gal) (Metcalf & Eddy, 1991).The oxygen transfer efficiency of Oxidation ditchesranges from to (Baker Process,1999).The design criteria are affected by the influentwastewater parameters and the required effluentcharacteristics, including the decision or requirementto achieve nitrification, denitrification, and/orbiological phosphorus removal.

10 Specific designparameters for Oxidation Ditches include:Solids Retention Time (SRT): Oxidation ditchvolume is sized based on the required SRT to meeteffluent quality requirements. The SRT is selectedas a function of nitrification requirements and theminimum mixed liquor temperature. Design SRTvalues vary from 4 to 48 or more days. TypicalSRTs required for nitrification range from 12 to Loading: BOD loading rates vary from lessthan 160,000 mg/1000 liters (10 ft3) tomore than 4x107 mg/1000 liters (50 ft3). ABOD loading rate of 240,000 mg/1000 liters per day(15 ft3/day) is commonly used as a designloading rate. However, the BOD loading rate is nottypically used to determine whether or notnitrification Retention Time: While rarely used as abasis for Oxidation ditch design, hydraulic RetentionTimes (HRTs) within the Oxidation ditch range from6 to 30 hours for most municipal wastewatertreatment fully-demonstrated secondary treatmentprocesses, Oxidation ditch processes are readilyadaptable for nitrification and denitrification.