A retrofitted activated-sludge plant with sequential nitritation and anammox obtains dischargeable effluent

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Abstract
New Activated Sludge (NAS) represents a hybrid, floc-based nitrogen removal process, based on the control of solids retention times (SRT) and dissolved oxygen (DO) levels. The aim of this study was to examine the performance of a full-scale NAS plant, which treated anaerobically digested industrial wastewater. The batch-fed partial nitritation step oxidized nitrogen to nitrite (45-47%) and some nitrate (13-15%). Serial anammox, denitrification and nitrification compartments were followed by a final settler. In the anammox step, 77% of the nitrogen was removed, with an estimated contribution of 71% by the genus Kuenenia, which constituted 3.1% of the biomass. Overall, a nitrogen removal efficiency of 95% was obtained, yielding a dischargeable effluent. The performance of this novel and cost-effective technology demonstrates the feasibility of retrofitting existing systems based on conventional activated sludge.

Introduction
For wastewaters with an ammonium level below 5 g N L −1 and a relatively low ratio of biochemical oxygen demand to nitrogen (typically ≤ 2.5), nitrogen removal by partial nitritation and anoxic ammonium oxidation (anammox) is economically the preferred treatment (Mulder, 2003). Equilibrating the stoichiometries of aerobic and anoxic ammoniumoxidizing bacteria (AerAOB and AnAOB), yields the overall reaction for this process (eq. 1).

NH4 + + 0.792 O2 + 0.080 HCO3 −
    → 0.435 N2 + 0.111 NO3 − + 1.029 H+ + 0.052 CH1.4O0.4N0.2 + 0.028 CH2O0.5N0.15 + 1.460 H2O (eq. 1)

The aim of this study was to examine the performance of a novel, floc-based partial nitritation and anammox process. The characterized full-scale nitrogen removal process discharges effluent to surface water and is preceded by anaerobic digestion and struvite precipitation (Anphos ), jointly representing the WWTP of a potato-processing factory. Previously, the nitrogen removal plant was operated as a conventional activated-sludge nitrification/denitrification system. However, by choosing appropriate DO setpoints and SRT, the system was retrofitted to a hybrid nitrogen removal process, consisting of partialnitritation (2370 m3), anammox (1650 m3), denitrification (1600 m3) and nitrification (2300m3) (Fig. 1). This novel process was designated New Activated Sludge (NAS), removing nitrogen without external carbon addition nor pH or temperature control.

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