Oxidation Ditch - Activated Sludge Treatment Process
Oxidation ditches are most often used to treat municipal wastewater but I do come across this activated sludge treatment process configuration in the industrial wastewater environment quite frequently. I’m always surprised when I do because oxidation ditches are intended for use in treating “low-strength” wastewaters and most industrial wastewater systems tend toward being more highly loaded, or of greater strength, than a typical municipal wastewater. Part of the reason for the popularity of oxidation ditches is that they are considered to require minimal operator input. I don’t agree with this because an oxidation ditch is just like any other aeration tank where wastewater enters and combines with return activated sludge. An oxidation ditch is neither easier nor more difficult to operate than any other activated sludge process, requiring the same level of operator attention as a conventional activated sludge process.
A simple schematic of a 3-pass oxidation ditch is portrayed in Figure 1. Oxidation ditches can be simple, oval, one-pass systems, two-pass, or three-pass. I’ve never seen more than three passes.
Figure 1: Oxidation Ditch Schematic
Advantages of Oxidation Ditches
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:
Disadvantages of Oxidation Ditches
In the Metcalf & Eddy handbook (Wastewater Engineering: Treatment and Reuse. 4th ed.), table 8-16 on page 747 (reproduced at Activated Sludge Table), the oxidation ditch is described as being a “plug flow” reactor. In looking at the flow pattern in Figure 1, above, and in Figure 2, below, you can see how this might support the designation of an oxidation ditch as a plug flow reactor. But my experience with oxidation ditches also supports the designation that oxidation ditches function as “complete mix” reactors. I am not without support in this view. The text below is from a Siemens brochure for their Orbal (oxidation ditch) system and the photograph in Figure 2 was scanned from that 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.
Figure 2: Oxidation Ditch Photo from Siemens Water Technologies
A modified version of Metcalf & Eddy’s table is shown in Tables 1 and 2. In table 1 the food-to-mass ratio is based on 5-day biochemical oxygen demand (BOD) values as is the organic loading per thousand cubic feet of aeration tank volume. In table 2 the units have been modified using chemical oxygen demand (COD) in place of BOD because COD is the more commonly used measure of the organic strength of the wastewater in an industrial environment. The conversion from BOD to COD was done simply by multiplying the BOD values by a typical COD/BOD ratio of 2.13. It should be noted that the COD/BOD ratio is always a “site- and sample-specific” value and the ratio used (2.13) may not be applicable to your wastewater system. The value of 2.13 should only be used when no site-specific values are available.
Table 1: Metcalf & Eddy Design Parameters (BOD basis)
Table 2: Metcalf & Eddy Design Parameters (COD basis)
There are several things to notice about the design parameters for oxidation ditches in tables 1 and 2 when compared to a conventional plug flow or complete mix aeration system. For one, the lower end of the sludge retention time (SRT) for an oxidation ditch is the upper end of a conventional 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 and a much lower F:M ratio that results in their operation being similar to a lightly-loaded extended aeration system.
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.