Mixed fill begins the biological reactions by bringing influent organics into contact with the biomass. Mixing without aeration reduces the energy needed for aeration, because some organics are biologically degraded using residual oxygen or alternative electron acceptors, such as nitrate-nitrogen. If nitrate-nitrogen is the electron acceptor, a desirable denitrification reaction occurs. The period when alternative electron acceptors are present and oxygen is absent is called anoxic. Anaerobic conditions develop after all electron acceptors are consumed, and fermentation reactions may occur. Thus, in a single reactor, aerobic, anoxic and anaerobic treatment conditions exist by correctly varying mixing and aeration policies during fill.
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