Oxidation Ditches

Concept & Field of Application

An oxidation ditch is a modified activated sludge process using long solid retention times to remove biodegradable organics (US EPA, 2000). A typical oxidation ditch treatment system consists of an oval shaped channel configuration (Figure 1). Mounted aerators provide circulation of the wastewater and mixed liquor in the channel as well as oxygen transfer to the biomass in the ditch and the shallow liquid depth (0.9 to 1.5 m) helps to prevent anaerobic conditions from occurring at the bottom of the ditch. The circulation process entrains oxygen into the mixed liquor to foster microbial growth, maintains the solids in suspension and ensures the contact of microorganisms with the incoming wastewater. After treatment, the wastewater is pumped to a clarifier where the sludge and the water are allowed to separate. It should be noted that the configuration can be adapted to provide biological nutrients removal by including anoxic zones. Figure 1 shows the schematic description of that process as well as an example of an oxidation ditch.

Figure 1: Typical process flow diagram of the oxidation ditch process (left). Oxidation ditch overview (right). (Source: The Wastewater Treatments: Oxidation Ditch, 2017)

The oxidation ditch is able to achieve removal objectives with more efficient use of energy, lower operational requirements (no chemicals required) and less sludge production compared to conventional activated sludge facilities (due to the long solids retention time - SRT). However the effluent TSS concentrations are relatively high and the required land area is relatively large compared to other modifications of the activated sludge process.

The longer the SRT within the ditch, the greater the amount of organic matter which is consumed by the aerobic bacteria. The volume of the oxidation ditch depends on the required SRT. With an appropriately chosen SRT, an oxidation ditch can also achieve nitrification. Oxidation ditch processes are therefore readily adaptable for nutrient removal. Specific design parameters for oxidation ditches include: SRT, BOD loading and hydraulic retention time (HRT).

Preliminary treatment (screening and grit removal) usually precedes an oxidation ditch.

Performance - Pollutant Removal Efficiency

As all conventional aerobic biological systems, oxidation ditches are very efficient at removing organic matter from wastewater, and, depending on the configuration, they can also be adapted to remove nutrients and in particular nitrogen (nitrification/denitrification with aerobic and anoxic zones in the channel). However, phosphorus removal may be limited due to a low biodegradable COD to phosphorus ratio in the incoming wastewater. An average pollutant removal efficiency of 17 oxidation ditch plants and a wastewater treatment plant in the USA is presented in Table 1.

Table 1. Pollutant removal rates of 17 oxidation ditch plants and a wastewater treatment plant. (Source: Shammas et al., 2009)

Pollutant

Removal Rate [%]

Source

Biological oxygen demand (BOD)

96 

Pollutant removal rates of 17 oxidation ditch plants in the City of Casa Grande, Arizona, USA (1999)

Total suspended solids (TSS)

97

Total Nitrogen (TN)

94

Pollutant

Removal Rate [%]

Source

Biological oxygen demand (BOD)

99

Wastewater treatment plant in the Town of Edgartown, Massachusetts, USA (1999)

Total suspended solids (TSS)

97

Total Nitrogen (TN)

90

Table 2. Design and Operational Parameters of Oxidation Ditches (Source: Shammas et al., 2009)

Design and Operational Parameters of Oxidation Ditches

Circulation

0.20 – 0.35 m/s

RAS recycle ratio

75 – 150%

MLSS

1500 – 5000 mg/L

SRT

4-48 days

SRT for nitrification

12-24 days

BOD loading

160 g/1000L - 4x104 g/1000L

Typical design BOD loading rate

240 g/1000L per day

HRT

6-30 hours

Sludge production

0.2-0.85 kg TSS / kg BOD

Oxygen requirements

•1.1-1.5 kg O2 / kg BOD

•4.57 kg O2 / kg TKN

Role in cNES

Various synergetic effects are expected from the process combination and are so far confirmed by preliminary results. NF as post-treatment after Bank Filtration (BF) is a reliable measure to remove biopolymers, a primary membrane foulant, and ensures a high level of membrane filtration performance due to biofouling prevention. The long-term stability was already demonstrated for SWNF of strictly anoxic groundwater [1]. In this study the concept is adapted to BF and anoxic to suboxic conditions. CapNF can remove sulphate and specific OMP’s (e.g. EDTA) in one treatment step.

Operation at single wells for decentralized partial removal of selected inorganic and organic compounds has the advantage that the overall treated water volume can be lowered.

References
  • Shammas , N. K. , Wang, L. K. , Pereira, N. C. , & Hung, Y. (2009). Biological Treatment Processes. In Shammas , N. K. , Wang, L. K. , Pereira, N. C. , Hung, Y. (eds) , The Handbook of Environmental  Engineering. Towona NJ: The Humana Press.
  • The Wastewater Treatments: Oxidation Ditch. (2017).  Retrieved from http://www.thewatertreatments.com/wastewater-sewage-treatment/oxidation-ditch-sewage-treatment/
  • US EPA, United States Environmental Protection Agency. (2000). Wastewater Technology Fact Sheet Oxidation Ditches. Washington, D.C.: Municipal Technology Branch.
  • Tchobanoglous, G. , Burton, F. L. , & Stensel, H. D. (2003). Wastewater Engineering – Treatment and Reuse. New York: Mc Graw-Hill.
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