The practice of manipulating activated sludge reaction environments to obtain maximum nitrogen removal has been optimized using cyclic activated sludge technology. In its simplest form, the sequences of fill aeration, settle and decant are consecutively and continuously operated in a compartmented variable volume reactor in which an initial reaction volume performs the function of a biological selector using biomass from a final reaction volume. The technology incorporates the principles of biological accumulation-regeneration processing in which a variable volume plug-flow reaction environment precedes a variable volume complete-mix reaction environment, both of which are typically in continuous fluid communication. This configuration and mode of operation allows removal performance in domestic wastewater treatment applications to meet less than 5 mgL−1 total nitrogen discharge limits. Low nitrogen concentrations are obtained using modified aeration sequences only in which the bulk phase sequenced dissolved oxygen concentration is typically less than 2 mgL−1 in a ramped profile beginning at zero. By that means a simultaneous or co-current nitrification-denitrification mechanism is obtained. Cycle manipulation required to maintain high nitrogen removal performance during less than design loadings is addressed. In-basin monitoring illustrates nitrogen removal performance obtained in domestic wastewater applications. This mode of operation is also important to the efficacy of biological phosphorus removal where available readily degradable soluble substrate is near to limiting.

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