NOVELTY - A bio-tank oxygen replenishment system, comprises a tank (96) for holding wastewater; a mixer for oxygenating the wastewater; a probe for measuring oxygen concentration of the wastewater in the tank; and a controller (102) for regulating oxygen replenishment rate of the wastewater, and sets a circulation pump speed based on real-time oxygen concentration information provided by the probe.    USE - As bio-tank oxygen replenishment system for mixing oxygen and biological species to perform aerobic degradations for treating contaminated liquid, such as wastewater, industrial wastewater; and for pre-treating wastewater, if needed, to remove large particles, fat, grease, and physically emulsified oils.    ADVANTAGE - The system pre-treats contaminated wastewater in a dissolved air flotation system to remove suspended solid waste, including large particles such as fats, grease, and physically emulsified oils. The purified wastewater containing dissolved waste is oxygenated and further decontaminated in a bio-reactor tank. The biological species in the bio-reactor tank consume oxygen and perform bodily functions that converts dissolved waste into easily removable carbon dioxide and suspend solids. The carbon dioxide and suspended solids are more easily separated from water than dissolved solids. Oxygen consumed by the biological species is replenished by pumping the wastewater though the liquid-oxygen mixer.    DETAILED DESCRIPTION - A bio-tank oxygen replenishment system, comprises a tank (96) for holding wastewater; a mixer for oxygenating the wastewater, and including a hydro-cyclone head inlet, a wastewater inlet (92) and a gas inlet; an accelerator head in the mixer for spinning the wastewater into a vortex in a down tube, where the vortex includes a central evacuated area; a probe for measuring oxygen concentration of the wastewater in the tank; an oxygenated head space in the mixer having essentially pure oxygen; a sensor for measuring the size of the head space; a port for changing the size of the head space in response to input from the sensor; a baffle for diverting oxygenated wastewater from a down tube outlet, to the head space; a conduit for transferring oxygen from the head space for remixing with the wastewater; a circulation pump for transferring wastewater from the tank to the mixer; and a controller (102) for regulating the oxygen replenishment rate of the wastewater, and sets the circulation pump speed based on real-time oxygen concentration information provided by the probe. The mixer is fluidly coupled to a holding tank. The holding tank is fluidly coupled to a dissolved air flotation system and a corresponding liquid-solid mixer (20). The hydro-cyclone head inlet is generally circular, multi-circular, or has an aspect ratio of 24:1, 10:1, 6:1, 2.6:1 or 1:1. The maximum dissolvable oxygen concentration in the wastewater is distinct for each hydro-cyclone head inlet and corresponding mixer pressure.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the bio-reactor tank and system for mixing oxygen and biological species to perform aerobic degradations.    Liquid-oxygen mixer (20)    Bio-reactor tank (22)    Wastewater inlet (92)    Wastewater holding tank (96)    Controller. (102)