Oxidation Ditch
- Rick Fuller
- Aug 30
- 5 min read
Updated: Sep 2
Oxidation ditches are most often used to treat municipal wastewater, but I do come across this activated sludge configuration in the industrial wastewater environment as well (e.g., sugar and petrochemical plants). Oxidation ditches are intended for use in treating “low strength” wastewater and most industrial wastewater systems tend toward being much more highly loaded (higher influent BOD/COD) than a typical municipal wastewater treatment plant. Oxidation ditches can be simple, oval, one-pass systems, two-pass, or three-pass. I’ve never seen more than three passes.
Part of the reason for the popularity of oxidation ditches is that they are considered to require minimal operator input. I don’t really agree with this point-of-view because an oxidation ditch is just like any other bioreactor where return activated sludge (RAS) and waste activated sludge (WAS) flow rates need to be adjusted, mixed liquor suspended solids (MLSS) concentrations and oxygen levels maintained, etc. In my opinion, an oxidation ditch is neither easier nor more difficult to operate than any other activated sludge process, requiring the same level of operator attention and skill as a conventional plug flow or complete mix activated sludge system, for example.
For any wastewater operator responsible for the operation of a bioreactor, any type of bioreactor, there are only three control points available as shown in the schematic below.
Advantages of Oxidation Ditches, In Theory
The main advantage of the oxidation ditch is the ability to achieve removal performance objectives with low operational requirements and low operation and maintenance costs. Some specific advantages of oxidation ditches include:
Oxidation ditches are well-suited for treating typical domestic waste, have moderate energy requirements, and work effectively under most types of weather.
Oxidation ditches provide an inexpensive wastewater treatment option with both low operation and maintenance costs and operational needs.
Oxidation ditches provide 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 sequencing batch reactors (SBRs).
Oxidation ditches operate with a long hydraulic retention time and complete mixing minimizes the impact of shock loads or hydraulic surges.
Oxidation ditches produce less waste activated sludge than other biological treatment processes because they operate under conditions similar to extended aeration systems with long sludge ages.
Energy efficient operations result in reduced energy costs compared with other biological treatment processes.
Oxidation ditch systems can be used with or without clarifiers, which affects flexibility and cost.
Oxidation ditch systems consistently provide high quality effluent in terms of TSS, BOD, and ammonia levels.
Disadvantages of Oxidation Ditches, In Theory
Oxidation ditch systems produce effluent suspended solids concentrations that are relatively high compared to other modifications of the activated sludge process.
Oxidation ditches can be noisy due to mixer/aeration equipment, and tend to produce odors when not operated correctly.
Biological treatment using an oxidation ditch is unable to treat highly toxic waste streams.
Oxidation ditches require more 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.
Oxidation ditch systems have less flexibility should regulations for effluent requirements change.
In the Metcalf & Eddy (M&E) handbook Wastewater Engineering: Treatment and Reuse. 4th ed., Table 8-16 on page 747 (reproduced in the table below), the oxidation ditch is described as being a “plug flow” reactor. In looking at the flow pattern in the Oxidation Ditch Schematic above you can see how the flow supports the designation of an oxidation ditch as a plug flow reactor. But my experience with oxidation ditches also supports the designation that oxidation ditches can function as “complete mix” reactors. I am not without support for this opinion. The text below is from an old Siemens brochure, from 2013, for their Orbal Oxidation Ditch system and the photograph above, with the caption "Orbal Oxidation Ditch", at the start of this blog post, was scanned from this informative brochure, which you can download from the link below. If you need to know more about oxidation ditches I highly recommend you download this brochure and the EPA Oxidation Ditch Factsheet further down this post.
Quote From Siemens Orbal Oxidation Ditch Brochure
"The Orbal® multichannel oxidation ditch from Siemens Water Technologies is well-suited for conventional activated sludge, advanced secondary sludge treatment, simultaneous nitrification-denitrification, enhanced nutrient removal, and storm water treatment. It is a complete mix, looped reactor system."
Metcalf & Eddy Activated Sludge Design Parameters
In the table below from M&E, I have added two columns of data, shown in red. I did this because many industrial wastewater systems use COD, rather than BOD, to determine the organic concentration at various locations in their wastewater plant. Keep in mind I used a COD/BOD ratio of 2.1. This gives me a starting point to calculate loading rates.
M&E Design Parameters for Commonly Used Activated Sludge Processes
There are several things to notice about the design parameters for oxidation ditches in the M&E table above when compared to a conventional plug flow or complete mix aeration system. For one, the lower end of the sludge or solids retention time (SRT) for an oxidation ditch is the upper end of a conventional plug flow system at 15 days. This is also true of the mixed liquor suspended solids (MLSS) concentration where the typical upper limit in a conventional activated sludge process has the MLSS at around 3,000 mg/L which is the lower end for an oxidation ditch. Oxidation ditches also have a much greater hydraulic detention time or hydraulic residence time (HRT) and a much lower food-to-mass (F:M) ratio that results in their operation being similar to a lightly loaded extended aeration system.
You can download a six page PDF document on oxidation ditches from the US EPA, below.
Regarding the designation of plug flow or complete mix, there is one more description of oxidation ditches from the United States Environmental Protection Agency, taken from their excellent, short, to-the-point, paper on oxidation ditches entitled Wastewater Technology Fact Sheet: Oxidation Ditches.
"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."
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