AERATED ANOXIC OXIDATION-DENITRIFICATION PROCESS

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, a group has held that bulking sludge control is best achieved by eliminating all forms of oxygen, including nitrates, from the initial contact or biological selector zones. The 6.75 m(3)/s Phoenix 91st Avenue wastewater-treatment plant (WWTP) was designed with nitrate recycle to the selector zones for bulking sludge control and the anoxic zones were provided with fine-bubble diffusers to increase oxidation capacity while simultaneously removing the recycled nitrates. This paper provides the process, design rationale, plant-layout features, plant modifications to existing structures, and the ope-rating results for the facility. The successful performance of the 1.3 m(3)/s prototype plant resulted in the initiation of the design for conversion of the other four plants, totaling 5.3 m(3)/s capacity, to the aerated anoxic design.