An improved biomass waste treatment process wherein an aerobic biological reaction takes place in a vertical chamber having a first portion defining a first zone and a second portion defining a second zone below said first zone, said process comprising feeding a first biomass influent having a first biomass concentration to the first zone; feeding a first oxygen-containing gaseous stream to the first zone to effect mixing of the first oxygen-containing gaseous stream with the first biomass influent to effect circulation in unidirectional upward flow by gas lift and bioxidation to provide a first biomass effluent having a second biomass concentration; feeding first biomass effluent in plug flow mode to the second zone; feeding a second oxygen-containing gaseous stream to the second zone to effect mixing of the second oxygen-containing gaseous stream with the first biomass effluent without re-circulation to the first zone to effect bioxidation and provide a second biomass effluent and removing second biomass effluent from the second zone.
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