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Cage Rotor

Cage Rotor

One or more mechanical surface aerators with horizontal axis, generally known as cage rotors are attached to the side of the ditch. Cage rotors generally resemble a large circular brush.

The cage rotor is mounted on a revolving horizontal shaft that is partly submerged in the wastewater for oxygenation and circulation of the ditch contents. The aerators slowly rotate to facilitate the introduction of oxygen to the wastewater. Raw wastewater is delivered to the ditch where it is slowly mixed by the cage rotor


Generally, no primary clarifier is used in this process. Bar screens, grit removal and comminutors are required in the primary treatment for the protection of the mechanical equipment such as the rotors and pumps. A grit chamber will also be required if grit present in the wastewater is appreciable.

Advantages and Disadvantages

Advantages of oxidation ditch are:

  • BOD removal is more than 90%
  • They require moderate amount of skill to operate and maintain,
  • They work satisfactorily under all weather conditions,
  • They are suitable for small communities


  • The system can be noisy and can also produce odors if not operated properly.

Design Criteria of Oxidation Ditch

The design criteria for oxidation ditch are listed below:

Oxidation ditch

  • Volume: The volume of the oxidation ditch shall be based on a maximum loading of 25 kg of BOD5 applied daily per 100 m3 volume of channel.
  • Detention time: A hydraulic retention time or aeration period of 24 hours of average flow (exclusive of recirculation) is recommended. The retention time may be reduced to a minimum of 16 hours where the influent BOD5 is less than 200 mg/1.
  • Channel configuration: The channel configuration shall be in the form of a closed ring-shaped circuit which will not create eddies or dead areas. The channel cross-section shall be trapezoidal in shape with liquid depth between 0.9 and 1.50 m.
  • Inlet: Inlet with control device should be provided for adjusting the liquid levels and to allow the oxidation ditch to be operated intermittently or continuously. The raw wastewater  inlet to the channel and the return activated sludge flow, should be immediately upstream from the rotor(s). Inlets should be submerged and directed downstream from the outlet or overflow device to minimize short-circuiting.
  • Outlet : The outlet should be located at least one-third (1/3) the length of the channel upstream from the raw wastewater inlet where one rotor is provided or just upstream of the rotor on the opposite end from the raw wastewater inlet where two rotors are provided. The outlet from the channel shall be of the overflow type to allow foam and floatables to be removed from the ditch and separated in the clarifier.
  • Free board: A minimum of 0.30 m of freeboard shall be provided at the maximum liquid depth. The freeboard may be increased within the immediate area (3.0 m or more on each side) of the aeration units. The top of the channel walls shall be at least 150 mm above the surrounding terrain or floodplain; whichever is higher.
  • Ends of channel: The ends of the channel should consist of 180 degrees or well-rounded bends to prevent eddying and stagnant zones. The median strip should be such that the radius of curvature will not severely increase the frictional resistance to retard the liquid flow. Baffling and turning vanes may be utilized to prevent settling and provide more uniform velocities.
  • Flow through velocity: A minimum flow through velocity of 0.3 m/s shall be provided throughout the entire cross- sectional area of the channel. The travel time between the aerators should not exceed 3 to 4 minutes.

Cage Rotors

  • Number of rotors: A minimum of two complete rotor installations are desirable such that each rotor installation is designed to meet the oxygen demand with average design conditions. Under average design conditions, it is desirable that the rotor installations maintain dissolved oxygen (DO) content of 2 mg/ L at all sections except between the raw wastewater inlet and the immediate downstream rotor.
  • Oxygen required: The combined rotor installations shall supply required oxygen demand at peak organic and/or hydraulic load conditions, at least 1.8 kg oxygen per kg of BOD5 applied to the system.
  • The rotor mechanism should be capable of being easily adjusted for varying the liquid level and for the depth of immersion of 75 to 300 mm.
  • Oxygenation capacity of rotor : The oxygenation capacity of cage rotor of 70 cm diameter at 75 rpm speed at 16 cm immersion depth shall be 2.5 to 3.0 kg of oxygen / hour/ m – length.
  • Power : The aeration power requirement for generally varies from 1 to 1.5 kW per metre length of rotor. The power requirement is generally 13 to 20 kWh/person/year.

Secondary Clarifier and Sludge Recycling

  • Secondary clarifier for settlement of solids shall be provided.

Extended aeration systems shall have provisions for holding and/or disposal of excess sludge that must be wasted periodically.

Access and Safety Measures

All oxidation ditches shall be equipped with appropriate safety features for the protection of operators.

Convenient walkways and access ways should be 5rovided for maintenance of the channel and equipment.

Walkways shall not be placed above rotor installations or close enough to be affected by spray, or hinder removal of the rotor(s).

The oxidation ditch area shall be enclosed with non- climbable chain-link fence.

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