The oxidation ditch wastewater treatment process is a modified form of activated sludge treatment process (extended aeration type), which is economical, highly efficient involving simple waste treatment principles and is comparable in performance with the oxidation pond.

The process involves a single unit treatment in an endless channel equipped with a special type of rotor, which serves the dual purpose of oxidation of wastewater and circulation. The oxidation ditch design comprises of a trapezoidal cross-section of relatively shallow depth (0.9 to 1.5 m) forming a continuous circuit.

There is normally no primary tank and the raw sewage passes directly through a bar screen to the ditch. The oxidation ditch forms the aeration basin where the raw sewage is mixed with the active organisms in the ditch. The cage rotor entrains the necessary oxygen into the liquid and keeps the contents of the ditch mixed and moving. The oxidation ditch design entails a physical-cum biological process in which a small portion of the organic matter undergoes direct chemical oxidation, while the bulk of the organic matter is stabilized by the biochemical activities of the microorganisms previously formed in the system.

The mixed liquor in the ditch flows to the clarifier for separation. The clarified liquid passes over the effluent weir for disposal into the receiving stream, while the settled sludge from the bottom of the clarifier is removed by pumping and returned to the ditch for further treatment.

The oxidation ditch design is standardized to normally employ a detention period up to 24 hours and is designed to carry mixed liquor suspended solid concentration of 3000 to 8000 mg/l with a minimum circulation speed of 25 cm per sec. in the ditch. The plant can be operated either intermittently or continuously. These ditches have been found to treat wastes having BOD as high as 8000 mg/l, quite efficiently. These are being used at a number of industrial plants in Canada and Australia. In some places, oxidation ditch process has been used to treat domestic as well as industrial wastes.