A process is provided for treating waste water biologically and clarifying the biologically treated water; considerable economic advantage is obtained by conducting both the biological treatment and the clarification of biologically treated water in a single vessel rather than in separate vessels; it is further found possible to treat in this system water containing much higher concentration of waste; in the process the supply of oxygen to the biological reaction zone is carefully monitored to meet the biological oxygen demand and avoid the occurrence of undissolved oxygen in the form of gas bubbles in the biological reaction zone or the clarification zone; there is further provided improvements in oxygen dissolving devices rendering them especially suitable for the two zone treatment.