Aeration is often provided during the fill react period to complete the aerobic reactions. However, aerated react alternated with mixed react will provide alternating periods of aerobic and anoxic, or even anaerobic, conditions. This is a normal procedure for nitrification and denitrification. During periods of aeration, nitrate concentration increases as organic nitrogen and ammonia are converted to nitrites and nitrates. The mixed react results in anoxic conditions needed for denitrification and the conversion of nitrates to nitrogen gas. Anaerobic conditions are necessary if some waste constituents are degraded only anaerobically, or partially degraded anaerobically followed by a complete degradation under aerobic conditions.

For mixed wastes, the easily degraded constituents are removed first, and the more difficult to degrade constituents are removed later during extended periods of aeration. Long periods of aerated react, after removal of soluble substrates, may be necessary to condition the biomass, to remove internal storage products, or to aerobically digest the biomass. Aerated react may also be stopped soon after the soluble substrate is removed. This saves energy and maximizes sludge production, which is desirable when separate anaerobic sludge digestion is used to stabilize these waste solids and to produce methane, an energy-rich and useful by-product.

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