Oxic Zone...
Various laboratory, pilot-plant, and full-scale studies have shown that an A/0 system will remove BOD when oxic detention time is less than the minimum time needed to fully oxidize the BOD stored within the A/0 biomass. This biomass will function acceptably for BOD removal while maintaining a reservoir of stored, unoxidized BOD within the sludge. This unoxidized BOD is removed from the system through the normal wasting of sludge from the system. Since oxygen requirements are directly related to the amount of BOD oxidized, operating the A/0 system with- out fully oxidizing the BOD in the oxic section allows for a net reduction in the amount of oxygen that must be transferred into the mixed liquor. The net result is a reduction in oxygen or horse- power used for the same amount of BOD removal compared to a conventional activated-sludge system. Oxygen savings of 15 percent or more can be achieved for A/0 systems compared to conventional activated-sludge systems. The detention time of the oxic zone is determined by the rate of biological oxidation of stored BOD, the rate of sorption and oxidation of unstored BOD, the degree of oxidation desired, and mass transfer characteristics of the aeration equipment.