The conventional practice for an anoxic denitrification basin has been to minimize oxygen input on the basis that it is detrimental to the process. For existing secondary treatment systems, allotting 25-35% of the aeration volume for an unaerated anoxic zone will significantly reduce plant capacity. Further, one group has held that bulking control is best achieved by eliminating all forms of oxygen from the initial contact or biological selector zones. The Phoenix 91st Avenue WWTP was designed with nitrate recycle to aerated selector zones and the anoxic zones were provided with a dense array of fine bubble diffusers. The prototype NdeN process was able to maintain the 1.31 m3/s secondary capacity with aerated anoxic zone receiving 20-25% of the total airflow. Net sludge yields were 30-50% higher than anticipated due to primary clarifier solids losses at higher flows which reduced SRTT to ≤ 5 days. At 5.0-5.5 day SRTT, effluent averaged 8.3 mg/L TN, 1.75 mg/L NH4N and 5.7 mg/L NO3N. Nitrobacter N oxidation rates were unexplainably lower than the Nitrosomonas N oxidation rates causing effluent NO2N.

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