Oxidation Ditch

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

Simplified 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:

  • 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

  • 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 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

Siemens Orbal (oxidation ditch) system

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)

M&E oxidation ditch BOD parameters

Table 2: Metcalf & Eddy Design Parameters (COD basis)

M&E oxidation ditch COD parameters

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.