Sewage treatment process with phosphorus removal

In a sewage treatment plant and process having a liquid side in which nitrogen removal is effected, a method of operating the liquid side of the treatment process so as additionally to remove phosphorus, comprising:

• (a) maintaining an aerobic/anoxic zone receiving plant influent wastewater,
• (b) maintaining dissolved oxygen concentration in the aerobic/anoxic zone in a fluctuating range,
• (c) with dissolved oxygen concentration toward the lower end of said range and tending to promote denitrification, monitoring the status of bacteria in the aerobic/anoxic zone and detecting when bacteria near the centers of flocs turn anaerobic,
• (d) allowing the anaerobic condition to develop in the flocs to a preselected extent and thereby causing phosphorus to be released into the liquid, then adding dissolved oxygen to the aerobic/anoxic zone to effect recovery of at least some of the bacteria from the anaerobic condition and causing uptake of phosphorus, as well as effecting nitrification,
• (e) then limiting dissolved oxygen to the aerobic/anoxic zone to an extent to allow the bacteria to consume the dissolved oxygen,
• (f) repeating steps (c) through (e), and
• (g) moving effluent from the aerobic/anoxic zone to an aerobic zone in which dissolved oxygen concentration is maintained in a higher range sufficient to enhance biological phosphorus uptake,
• whereby nitrification, denitrification, phosphorus release and some biological phosphorus uptake occur in the aerobic/anoxic zone, and further nitrification and biological phosphorus uptake are enhanced in the aerobic zone.