AERATED ANOXIC BIOLOGICAL NDEN PROCESS

The conventional practice for an anoxic denitrification basin has been to minimize oxygen input on the basis that if is detrimental to the p rocess. For existing secondary treatment systems, allotting 25-35% of the aeration volume for an unaerated anoxic zone will significantly re duce plant capacity. Further one group has held that bulking control i s best achieved by eliminating all forms of oxygen from the initial co ntact or biological selector zones. The Phoenix 91st Avenue WWTP was d esigned with nitrate recycle to aerated selector zones and the anoxic zones were provided with a dense array of fine bubble diffusers. The p rototype NdeN process was able to maintain the 1.31 m(3)/s secondary c apacity 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 SRT(T) to less than or equal to 5 days. At 5.0-5.5 day SRT(T), effluent averaged 8.3 mg/L TN, 1.75 mg/L NH4N and 5.7 mg/L NO3N. Nitrobacter N oxidation rat es were unexplainably lower than the Nitrosomonas N oxidation rates ca using effluent NO2N.