Simultaneous partial nitritation and anammox at low temperature with granular sludge

• Proof of concept: autotrophic N-removal with granules at main stream conditions.
• Effective suppression of nitratation at DO up to 2.5 mg-O2 L−1 with NOB presence.
• Nitrogen removal rates of 0.4 g-NTot L−1 d−1 with high removal efficiencies.
• Decreased anammox activity and process efficiency after prolonged operation at 10 °C.

Autotrophic nitrogen removal in the main stream appears as a prerequisite for the implementation of energy autarchic wastewater treatment plants. To investigate autotrophic nitrogen removal a lab-scale gas-lift sequencing batch reactor with granular sludge was operated for more than 500 days. The reactor was operated at temperatures between 20 and 10 °C on autotrophic medium with ammonium (60 and 160 mg-N L−1) as only nitrogen compound at an HRT of 0.23–0.3 d. The dissolved oxygen (DO) concentration was shown to be an effective control parameter for the suppression of the undesired nitratation process. DO control guaranteed the effective suppression of the nitratation both at 20 and 15 °C, allowing nitrogen removal rates of 0.4 g-NTot L−1 d−1 at nitrogen removal efficiencies of 85-75%. Prolonged operation at 10 °C caused a slow but unrestrainable decrease in anammox activity and process efficiency. This study represents a proof of concept for the application of the autotrophic nitrogen removal in a single reactor with granular sludge at main stream conditions.

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