An improved barrier oxidation ditch is provided which has an inclined duct for the induced flow, whereby energy in the dammed-up momentum is more efficiently released. This induced-flow duct is inclined at 30. degree. to 60 to the horizontal, preferably 45, and is disposed upstream of, downstream of, or alongside of the circular/aerator in the intake channel. The induced-flow duct is preferably rectangular in cross-section and may have its downstream wall formed by the barrier if it is downstream of the circulator/aerator. It is connected at its lower end to the discharge duct to initiate a combined-flow portion thereof. An in-channel clarifier, disposed in the discharge channel so that its upstream wall is the barrier and its solid bottom is the ceiling of the combined-flow portion of the discharge duct, provides prolonged bubble retention time at maximum hydraulic pressure, whereby the efficiency of oxygen transfer is increased.
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