Virginia Administrative Code › Title 9 - ENVIRONMENT › Agency 25 - STATE WATER CONTROL BOARD › Chapter 790 - VIRGINIA POLLUTANT DISCHARGE ELIMINATION SYSTEM (VPDES) PERMIT REGULATION › Part III - Manual of Practice for Sewerage Systems and Treatment Works › Article 6 - Biological Treatment › Section 9VAC25-790-700 - Oxidation ditches

Current through Register Vol. 40, No. 24, July 15, 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.