1.  Field of the Invention
This invention relates to wastewater treatment.  The invention especially relates to methods and apparatuses for clarifying mixed liquor in activated sludge processes that are conducted in partial or non-barriered oxidation ditches of racetrack or loop channel configuration. 
2.  Review of the Prior Art
Oxidation ditches for continuous aerobic treatment of liquid wastewaters have been used since the early 1950's.  They were developed in Hollaand by Dr. Ir A. Pasveer, as a variation of the activated sludge process, and were patented in Dutch Pat.  No.  87,500 and British pat.  No.  796,438. 
Barrier oxidation ditches have been operating since 1977, primarily for treating municipal and poultry processing wastewaters.  Barrier oxidation ditches are described in U.S. Pat.  No.  4,260,486 of John H. Reid, which is fully incorporated herein by reference.  A barrier oxidation ditch comprises an endless channel, a barrier disposed athwart the channel, a vertically mounted pump having an impeller within a draft tube which is vertically mounted at the upstream side of the barrier, a J-shaped discharge duct which is flow-connected to the draft tube and is mounted below the bottom of the channel and below the barrier to provide a discharge on the downstream side thereof, and an aeration means which includes a sparge disposed beneath the impeller, and, if needed, diffusers which are removably mounted so that they introduce additional diffused air at about the bottom of the discharge duct.  This barrier oxidation ditch, which pumps all of the circulating mixed liquor past the barrier, is herein described as a total barrier oxidation ditch. 
It has further been ascertained that the barrier oxidation ditch of U.S. Pat.  No.  4,260,486 creates a differential hydraulic head that is readily measurable when the flow is being pumped through one or more draft tubes and discharge ducts, thereby indicating that there exists a dammed-up momentum in the mixed liquor which is approaching the barrier.  It is important to release the energy contained in the dammed-up momentum. 
An improved barrier oxidation ditch is disclosed in U.S. Pat.  No.  4,278,547 of John H. Reid; it is also incorporated herein by reference.  This ditch comprises a barrier having adjustably sized openings and/or gateways therethrough for conserving the momentum of the mixed liquor, a pump means for pumping most of the liquor past the barrier, and an aeration means for aerating the pumped liquor and for selectively aerating the induced-flow liquor passing through the openings so that there is no backmixing of aerated liquor and relatively little heterogeneous aeration when the aerated liquor is blended, downstream of the barrier, with the unaerated induced-flow liquor.  This barrier oxidation ditch, which pumps a portion of the circulating mixed liquor past the barrier and provides openings for the remainder to move through the barrier, is herein described as a partial barrier oxidation ditch. 
One of the major benefits of the barrier oxidation ditch of U.S. Pat.  No.  4,260,486 is that the sparge in the downdraft tube provides for introducing diffused air to the mixed liquor at a shallow depth, thereby forming an air-liquor mixture, and then for pumping this mixture downwardly with its impeller into the discharge duct to a sub-channel oxygen-transfer depth that is below the channel bottom.  This oxygen-transfer depth increases the driving force for transferring oxygen molecules across the films at the gas-liquid interfaces of the air bubbles.  Other additional benefits of great practical importance are: (1) the energy required for downwardly pumping the air-liquor mixture is considerably less than the energy required for downwardly pumping the liquor alone plus the energy required for separately compressing air to the hydraulic pressure existing at the oxygen-transfer depth; and (2) a very high level of turbulence is provided in the oxygen-transfer zone, measured by brake horsepower/1000 ft.sup.3. 
For any aeration system used in transferring oxygen to a particular wastewater, sewage, or mixed liquor, its oxygen transfer efficiency is a function of five major parameters: bubble size, bubble retention time, driving force across the air-liquid interface for the dissolved oxygen, hydrostatic pressure, and degree of turbulence in the oxygen-transfer zone.  However, the adjustable gateways through the barrier of U.S. Pat.  No.  4,278,547 allow the induced-flow portion of the mixed liquor to pass through the barrier and be selectively aerated at a depth above the channel bottom instead of at the sub-channel depth that is available within a discharge duct, thereby losing at least some of the advantages of retention time, driving force, hydrostatic pressure, and turbulence. 
U.S. Pat.  No.  4,455,232 of John H. Reid accordingly discloses a barriered circulator/aerator in the endless channel of a barrier oxidation ditch which provides a directly pumped flow of mixed liquor into a central liquor inlet zone and an induced flow of mixed liquor into a surrounding liquor inlet zone at the inlet of a deep oxygen contact duct which passes beneath the barrier to the discharge channel on the downstream side thereof.  It further provides mixing of diffused air with the directly pumped flow and/or the induced flow and then moving the combined air-liquor flows into the deepest portion of the contact duct where point-source pressurized aeration of both flows occurs.  Eddy jet diffusers are preferably used for aerating the induced flow.  Oxygen transfer efficiencies are obtained that are 1.6 to 2.2 times as great per brake horsepower per hour as that attainable by 100% pumping of the mixed liquor in a total barrier oxidation ditch, as disclosed in U.S. Pat.  No.  4,260,486. 
This improved barrier oxidation ditch, however, compels 100% of the flow, both pumped and induced, to change direction 90.degree.  while moving downwardly and then to change direction 135.degree.  while moving beneath the barrier and upwardly.  As is well known in hydraulic theory, such 225.degree.  of direction changing causes significant energy consumption.  It would be desirable to provide a means for passing the liquor from the intake portion of the endless channel to the discharge portion of the channel with minimum directional change. 
When the air-liquor mixture has reached the lowest portion of the discharge duct, there is also very little time available for oxygen transfer from bubble to liquor across the films of the liquor-gas interface before the liquor/air mixture begins to rise.  Yet, it is at this point in passage from the intake channel that transfer efficiency is highest because of maximum hydrostatic pressure.  Another factor of importance is that the microorganisms are in oxygen-starved condition and avidly utilize the oxygen as fast as it transfers across the liquor-air films into the bulk liquor, so that the bulk liquor cannot become saturated if the MLSS content is reasonably high.  An unusually large proportion of the oxygen in the bubbles is accordingly able to transfer across the films into the bulk liquor.  It would accordingly be desirable to prolong the bubble retention time at the maximum depth, even though lengthening the discharge duct may tend to waste the available land area.