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.
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
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.
Removal Rate [%]
Biological oxygen demand (BOD)
Pollutant removal rates of 17 oxidation ditch plants in the
City of Casa Grande, Arizona, USA (1999)
Total suspended solids (TSS)
Total Nitrogen (TN)
Wastewater treatment plant in the Town of Edgartown, Massachusetts,
Design and Operational Parameters of Oxidation Ditches
0.20 – 0.35 m/s
RAS recycle ratio
75 – 150%
1500 – 5000 mg/L
SRT for nitrification
160 g/1000L - 4x104 g/1000L
Typical design BOD loading rate
240 g/1000L per day
0.2-0.85 kg TSS / kg BOD
•1.1-1.5 kg O2 / kg BOD
•4.57 kg O2 / kg TKN
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 . 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.
The AquaNES Project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 689450
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