Two zone process for biological treatment of waste water

A process of treating waste water containing biodegradeable waste to provide a clarified liquid effluent and a disposable sludge, in which waste water is continuously passed through a single treating enclosure open to the atmosphere containing waste-degrading microorganisms, 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 removed;

• in which there is initially established a charge including (a) in a lower part of the enclosure a biological reaction zone containing mixed liquor containing said microorganisms and in which a biological reaction to degrade the waste is conducted, (b) in an upper part of the enclosure a clarification zone in which clarified liquid rises and overflows, and (c) between the reaction and clarification zones a transition zone in which the liquid of the mixed liquor rises and the solids settle;
• and continuously there is withdrawn from the biological reaction zone a recycle stream of mixed liquor from the reaction zone and the stream conducted through an oxygen-dissolving device disposed outside the reaction zone and influent waste water and oxygen are added to it;
• the thus supplemented stream is injected into a lower part of the reaction zone remote from the vicinity of withdrawal, the waste water is conducted 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, oxygen is added 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 maintaining 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;
• the overall flow rate of said recycle stream is controlled to a substantially constant rate several times that of the incoming waste water effective to provide (d) for dissolving the oxygen which is added to the recycle stream, (e) 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, the flow of said supplemented recycle stream entering the reaction zone is distributed to reach a substantial area of a lower part thereof, (f) 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, (g) 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 said separate reaction and clarification zones intervened by said transition zone;
• the concentration of dissolved oxygen in the reaction zone is continuously monitored to determine variations thereof resulting from variations in the flow rate and concentration therein of waste;
• the rate of addition of the oxygen to the recycle stream is periodically adjusted 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;
• the effluent is continuously withdrawn from the clarification zone to keep pace with the influent waste water;
• and the excess sludge is continuously removed from the reaction zone and carbon dioxide from the mixed liquor, comprising the steps of,
• continuously injecting along the bottom of the biological reaction zone said supplemented recycle stream in a horizontal shallow inflow having a width substantially greater than its depth, and 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, and in which,
• the depth of the charge is from about 8 feet to about 100 feet,
• the depth of the clarification zone is at least about 2 feet,
• the distance between the inflow and the outflow is from about 6 feet to about 200 feet,
• the initial depth of the inflow is within the range from about 6 inches to about 6 feet,
• the depth of the biological zone is at least about 2 feet,
• the calculated average linear velocity of the inflow at the vicinity of the injection is within the range from about 1 to about 35 feet per minute,
• the average horizontal velocity in the reaction zone is within the range from about 1/2 to about 20 feet per minute,
• and the recirculation rate is within the range from about 1 to about 15 times the average waste water influent flow rate.