Virginia Law

A. An oxidation ditch process typically employs an extended aeration type of activated sludge process with a single channel or multiple interconnected concentric channels used as an aeration basin with a detention volume of 18 hours or more at the design flow rate. However, they may utilize some batch type operational principles.

B. Design. The geometry of the channels can vary; however, the oval is the most common configuration. Design requirements involving the use of duplicate oxidation ditches within the flow range of 40,000 gpd to 200,000 gpd shall be determined by the reliability class of the treatment works (Class I, II or III), the nutrient removal requirements, and the use of conventional dual final clarifiers. For design flows up to 100,000 gpd, a single oxidation ditch should be sufficient for secondary treatment of discharges to Class I reliability waters, if provided with external duplex clarifiers. In Class II and Class III waters, a single oxidation ditch may be acceptable for secondary treatment of flows up to 200,000 gpd. However, for treatment works permitted with effluent limits less than secondary or nutrient removal requirements, duplicate reactors and clarifiers shall be provided. In other cases, the treatment works size and location may allow for an exception for specific designs.

1. The multiple concentric channel basin can have any number of interconnected channels. This channel design scheme provides some process flexibility, since with minor modifications it can be changed to other activated sludge process modes. Typically, the outer channel (if multiple channels are present) receives unsettled raw sewage with a loading of 15 pounds per 1,000 cubic feet of volume or less. Shallow channels are usually four to six feet deep with 45° sloping walls. Deep channels have vertical side walls and are normally 10 to 12 feet deep.

2. The channels are characteristically lined to prevent erosion and leakage. Ditch lining should be constructed of reinforced concrete, asphalt or plastic liners. Shallow channels with sloped sidewalls are often constructed of concrete poured against earth backing and reinforced with welded wire mesh. Deep vertical wall channels require reinforced concrete walls.

3. Oxidation ditches may also be operated in alternating modes through on/off operation of aeration/mixing devices with intermittent changes in flow rates or direction. Influent wastewater can be diverted through one or more multiple reactors in which different operational phases (anoxic, aerobic, etc.) may occur. Effluent clarification may be accomplished within the reactor or within a separate clarifier. Automatically controlled weirs regulate flow direction and alternating operation of aeration/mixing equipment controls the operating mode. As with either standard continuous flow, or batch-type processes, the design duration of each operating phase is critical to performance reliability.

C. Aeration. Since oxidation ditches are considered a variation of the extended aeration modification of the activated sludge process, the requirements set forth in this chapter are applicable except as follows:

1. The mixing system shall be capable of maintaining a minimum velocity throughout the oxidation ditch cross-section of 1.0 fps at maximum design depth and solids concentration. For designs utilizing in-channel suspended solids removal the mixing system shall provide for all necessary variations in flow velocity to achieve adequate separation of suspended solids. Calculations and certified performance data for the mixing system shall be submitted to substantiate the adequacy of the proposed design.

2. Designs based on anoxic operation shall provide mixing and aeration system capacity for aerobic operation with adequate turndown capability to operate in the anoxic mode. Flexibility to allow for operation in the anoxic mode should be considered for all designs.

3. Designs should provide for variation in the oxygen supply independent of the mixing function.

4. The outlet from the oxidation ditch shall be separated from the inlet in such a manner as to prevent discharging of partially treated effluent.

5. Intra-channel clarifiers may be utilized if conventional settling rates are maintained and sludge handling, treatment and management provisions are satisfactorily addressed.