Recommended Design Criteria for Oxidation Ditches and / or Systems...
General Considerations...
Since the "oxidation ditch" is the more commonly used system, the design criteria will be promulgated on that basis.

1. Applicability : Oxidation systems may be used where the treatment of wastewater is amenable to aerobic biological treatment and the plant design capacities generally do not exceed 1.0 mgd. The oxidation ditch is a form of an aeration basin where the wastewater is mixed with return sludge. The oxidation ditch is essentially a modification of a completely mixed activated sludge system used to treat wastewater from small communities. This system can be classified as an extended aeration process and is considered to be a low loading rate system. This type of treatment facility can remove 90 percent or more of the influent BOD₅. Oxygen requirements will generally depend on the maximum diurnal organic loading, degree of treatment, and suspended solids concentration to be maintained in the aerated channel mixed liquor suspended solids (MLSS).

2. Aeration Unit Selections : For an oxidation ditch or alternate system to function satisfactorily, the velocity gradients and dissolved oxygen throughout the entire area of the ditch or channel should be relatively constant. This can be accomplished by various types of mechanical aerators and channel arrangements in such systems as the Orbal, Carrousel, or the more commonly known oxidation ditch. For selection purposes, each system will be briefly described. The oxidation ditch system combines the processes of oxidation and sedimentation by means of mechanical aeration and final clarification. The system involves a closed ring-shaped ditch in which untreated wastewater is introduced into the ditch where it is mechanically aerated by a paddle wheel or brush aeration rotor. The rotor is mounted on a revolving horizontal shaft that is partly submerged in the wastewater for oxygenation and circulation of the ditch contents. The ditch effluent is clarified and the settled sludge is returned to maintain a desirable MLSS concentration. The MLSS concentration generally ranges from 3,000 mg/l to 5,000 mg/l; however, this is dependent upon the surface area provided for sedimentation, the rate of sludge return, and the aeration process. The Carrousel system involves the same principles as the oxidation ditch, but utilizes vertical shaft, surface-type mechanical aerators for aeration and to impart a spiral flow through the channels. The aerators, installed adjacent to each other, operate in adjacent parallel channels with a concentric 180 degrees bend at the remote end. Each channel contains a baffle wall and a turning vane to redirect the flow direction to the next aeration unit and to form a closed circuit. The organic loading limit in this ditch system utilizing vertical aeration units is approximately 30 pounds BOD₅ applied daily per 1,000 cubic feet. Within the immediate area of aeration units, the depth of the channel is 9 to 10 feet greater than the diameter of the aerator and freeboard is approximately 4 feet. Another system somewhat similar to the oxidation ditch is the Orbal system. This system consists of several concentric ovaloid channels which are inter-connected for stage aeration of wastewater and mixed liquor and for sludge stabilization. Mechanical aeration consists of rotors mounted with large perforated disks rotating on horizontal shafts that have about 40 percent of disks diameter submerged in the liquid to provide the aeration and liquid movement in the channels.

3. Pretreatment : Generally, no primary clarifiers are used in this process. Bar screens and comminutors are required for the protection of the mechanical equipment such as the rotors and pumps. A grit chamber will be required unless the design engineer can demonstrate through field data that grit removal is not necessary. The screened debris shall not be returned to the plant process.
Design Considerations...
1. Oxidation Ditch : ( a ) The volume of the oxidation ditch shall be based on a maximum loading of 15 pounds of BOD₅ applied daily per 1,000 cubic feet of channel or on retention time; whichever requires the greater volume. ( b ) A hydraulic retention time or aeration period of 24 hours is recommended based on an average daily wastewater flow rate exclusive of recirculation. The retention time may be reduced to a minimum of 16 hours where the raw influent BOD₅ is less than 200 mg/l or when adequate rotor aeration and efficient suspended solids separation in the final clarifier are provided. In the cold climates, the temperature drop in the system must be considered when selecting long retention times. Liquid temperatures below 40 degrees F will reduce the bacterial activity in the channel and increase the clarifier effluent suspended solids concentration. ( c ) The channel configuration shall be such that it forms a closed ring-shaped circuit which will not create eddies or dead areas. It is recommended that the channel cross-section be trapezoidal rather than rectangular in shape. ( d ) The channel should be designed to be operated between 3 and 5 feet of depth. Inlet and outlet control devices should be provided for adjusting the liquid levels and to allow the oxidation ditch to be operated intermittently or continuously. Inlets should be submerged and directed downstream from the outlet or overflow device to minimize short-circuiting. ( e ) A minimum of 1 foot of freeboard shall be provided at the maximum liquid depth. It is recommended that the freeboard be increased within the immediate area (10 feet or more on each side) of the aeration units. The top of the channel walls shall be at least 6 inches above the surrounding terrain or 100-year floodplain; whichever is higher. ( f ) 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. ( g ) A minimum flow through velocity of 1 foot per second shall be provided throughout the entire cross- sectional area of the channel. The travel time between the aerator(s) should not exceed 3 to 4 minutes. ( h ) The channel shall be constructed of concrete, except when package plants are utilized for small installations; then other impervious, durable, and corrosion resistant materials may be considered. ( i ) Oxidation ditches not designed with grit chambers should have provisions in the channel design for a sludge collection hopper or basin to facilitate draining and the removal of grit and sludge buildups. The collection point should be located downstream between the first aeration unit and the first bend. ( j ) The raw wastewater inlet to the channel and the return activated sludge flow, when provided, should be immediately upstream from the rotor(s). 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. ( k ) 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.

2. Rotor Aerators : ( a ) 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 a dissolved oxygen (DO) content of 2 mg/l except between the raw wastewater inlet and the immediate downstream rotor. The DO content in the ditch should never be less than 1.0 mg/l. The combined rotor installation shall supply required oxygen demand at peak organic and/or hydraulic load conditions which should be at least 1.8 pounds oxygen per pound of BOD₅ applied to the system. If only one rotor installation is provided, the rotor unit shall be designed such that it can be readily maintained and/or spare parts and/or spare unit shall be provided on the site. ( b ) The rotor(s) should be located such that a long straight section is provided downstream of the rotor. The rotor(s) should direct the flow toward the long straight section of the channel. ( c ) The rotor mechanism should be capable of being easily adjusted for varying the liquid level and for the depth of immersion. A 3 to 12 inch adjustment variability for rotor immersion should be provided for flexibility, oxygenation, and circulation.

3. Sedimentation and Sludge Recycling : Refer to Chapter VIII for applicable requirements other than : Extended aeration systems shall have provisions for holding and/or disposal of excess sludge that must be wasted periodically. Sand-bed drying is a permissible method of handling the wasted sludge, although other techniques may be permitted.
Miscellaneous Considerations...
1. Access, Safety, and Security : Convenient walkways and accessways should be provided for maintenance of the channel and equipment. All oxidation ditches shall be equipped with appropriate safety features for the protection of operators. Walkways shall not be placed above rotor installations or close enough to be affected by spray, ice, or hinder removal of the rotor(s). The oxidation ditch shall be enclosed with non- climbable chain-link fence.

2. Housing : Consideration should be given to protecting, to the greatest extent possible, the rotor unit(s) from excessive ice buildup during cold weather operation. It is recommended that removable covers constructed of durable and corrosion-resistant materials be provided rather than enclosed building structures. Enclosed structures shall be well lighted and ventilated. In addition, electrical heaters should be considered for vertically mounted mechanical aerator motors and oil-filled gear cases.

3. Electrical Equipment and Controls : Electrical equipment and controls shall meet the requirements of the National Electrical Code.

4. Treatment Plant Operator(s) : Treatment plant operator(s) should be supplied with a complete set of operation procedures, including maintenance schedules, tools, sampling equipment, and spare parts as may be necessary.
Deviations from Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.