Anaerobic Zone...
Anaerobic zone detention time is set at the minimum necessary to ensure the development and maintenance of the A/0 system biomass, without concern for maximizing phosphorus release. For municipal waste, this is typically between 30 and 60 minutes based on the design-average flow of influent. As the wastewater passes through the anaerobic zone, most of the influent BOD is absorbed into the biomass. BOD, both sorbed and unsorbed, is then oxidized in the oxic zone.