Two zone apparatus for biological treatment of waste water

An apparatus for treating waste water containing biodegradeable waste to provide a clarified liquid effluent and a disposable sludge including a single treating enclosure open to the atmosphere for containing waste-degrading microorganisms and through which waste water is continuously passed, and to which oxygen is added to sustain the microorganisms and from which the clarified effluent is continuously overflowed and from which excess sludge and gases are continually removed, in which a lower part of the enclosure constitutes a biological reaction zone for containing mixed liquor containing said microorganisms and in which a biological reaction to degrade the waste is conducted, an upper part of the enclosure constitutes a clarification zone in which clarified liquid rises and overflows, and there is between the reaction and clarification zones a transition zone in which the liquid of the mixed liquor rises and the solids settle,

• an oxygen-dissolving device disposed outside the reaction zone,
• means for continuously withdrawing a recycle stream of mixed liquor from the reaction zone and conducting the stream through said oxygen-dissolving device,
• means for continuously adding influent waste water into the recycle stream at a variable rate within a range related to the depth and surface area of the enclosure to provide a residence time within the reaction zone effective for the biodegradation of the waste and for the formation and settling of biological floc,
• means including a source of oxygen for continuously adding oxygen to said recycle stream at a rate to provide an oxygen concentration within a controlled range below the saturation level of oxygen in the liquid effective to meet the oxygen demand of the organisms and to maintain it in contact with the liquid in a contact zone of said stream for a time and under a pressure such that the oxygen is dissolved in the liquid, and means for passing the thusly supplemented stream into a lower part of the reaction zone of the enclosure remote from the vicinity of withdrawal.
• means for continuously controlling the overall flow rate of said recycle stream to a substantially constant rate several times that of the incoming waste water effective to provide for dissolving the oxygen which is added to the recycle stream, and an amount of dilution of the recycle stream entering the reaction zone effective to prevent the oxygen coming out of solution at an upper part of the reaction zone,
• means for continuously distributing the flow of said recycle stream entering the reaction zone to reach a substantial are of a lower part thereof to provide a wide spread direct flow through the reaction zone, from the vicinity of injection to the vicinity of withdrawal, whereby there is controlled agitation effective to keep the solids dispersed, and good access of the organisms to the biodegradeable waste, and to provide at an intermediate level of the enclosure, an upward velocity of the mixed liquor less than the settling rate of the solids, whereby there is maintained in the enclosure separate reaction and clarification zones intervened by said transition zone,
• means for continuously monitoring the concentration of dissolved oxygen in the reaction zone to determine variations thereof resulting from variations in the flow rate and concentration therein of waste including a probe located within said reaction zone,
• means including a dissolved oxygen analyzer and controller responsive to the probe, for periodically adjusting the rate of addition of the oxygen to the recycle stream in response to variations in the oxygen concentration in the reaction zone to maintain said concentration within said controlled range and at a level where there is substantially avoided effervescence that would lead to gas bubbles rising into the clarification zone,
• means for continuously withdrawing said effluent from the clarification zone to keep pace with the influent waste water,
• means for continually removing excess sludge from the reaction zone and carbon dioxide from the mixed liquor,
• and means for directing the supplemented recycle stream in the form of a horizontal shallow inflow having a width substantially greater than its depth, and means for withdrawing mixed liquor from near the bottom of the reaction zone at a vicinity remote from the inflow in an outflow having a substantially greater width than its depth thereby to provide between the inflow and the outflow a horizontally flowing undercurrent having an extensive uninterrupted interface with an overlying relatively quiescent upwardly flowing body of mixed liquor.