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Oxidation Ditches

Ideal for Biological Nutrient Removal and Stormwater Management
An oxidation ditch is a modified activated sludge biological treatment process that utilizes long solids retention times (SRTs) to remove biodegradable organics. Oxidation ditches are typically complete mix systems, but they can be modified to approach plug flow conditions.

 Oxidation Ditches and Dedicated Treatment Zones

The Orbal® system incorporates a trademarked concentric loop design that creates dedicated zones for specific treatment processes. These treatment zones operate in series which is essential to the simultaneous nitrification-denitrification (SND) process. The volume split of the concentric loop design naturally applies the correct volume-to-oxygen input ratio required in the design of an SND plant.

By design, the aeration discs placed in the outer channel supply approximately 50% of the systems's total oxygen requirement. The design ensures a constant oxygen deficit condition throughout this channel with the DO set-point below zero.

The oxygen deficit environment of the outer channel delivers an overall denitrification performance rate of 80% without internal recycle. The majority of the system’s nitrification takes place in the outer channel where the anoxic condition also channels where the anoxic condition also drives denitrification. This is the definition of a TrueSND™ biological treatment system.

The mixed liquor then flows hydraulically to the middle channel where DO conditions swing depending on daily load variation. Finally, the mixed liquor flows to the inner channel where the DO is designed to operation at 2.0 mg/l. Final nitrification is completed in the inner channel (or oxygenation zone) in surplus oxygen conditions.

 Technology Options

​Evoqua oxidation ditch technology has been proven to deliver results in over 800 installations. In addition to the Orbal configuration, Vertical Loop Reactors (VLR) and Verticel configurations are available.


VLR® Systems from Evoqua utilize a vertical loop based design with looped reactors in series that allows DO stratification. As such, it is suited for simultaneous nitrification/denitrification, biological phosphorus removal, and storm water treatment. The VLR system tanks are similar to oxidation ditches that have been flipped on their sides.


The VertiCel® System utilizes tanks in series design and consists of an aerated-anoxic tank followed by an aerobic tank. The VertiCel® system is superior to other aeration technologies because it saves energy and space, while producing an unparalleled effluent quality. It is ideally suited for wastewater treatment plants with process flows above two million gallons per day.