Aerated fill begins the aerobic reactions that are later completed during react. Aerated fill reduces cycle time because the aerobic reactions occur during the fill period. In some cases, a biodegradable influent constituent may be present in concentrations that are toxic to the organisms. When that condition exists, aeration during fill begins early to limit concentration of that constituent. For example, if the wastewater constituent is toxic at 10 mg/L, and present in the influent at a concentration of 30 mg/L, aeration should begin prior to the reactor becoming three-quarters full, if the SBR liquid volume at the end of draw was one-half the volume at the end of fill. This assumes toxic constituent degradation is at a rate greater than or equal to its rate of addition. If the rate of degradation is low, then aeration should begin earlier, or a higher volume of treated effluent should be held in the SBR for more dilution.
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