Current through Register Vol. 41, No. 3, September 23, 2024
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.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
