Mainstream partial nitritation/anammox with integrated fixed-film activated sludge: Combined aeration and floc retention time control strategies limit nitrate production

Abstract Implementation of mainstream partial nitritation/anammox (PN/A) can lead to more sustainable and cost-effective sewage treatment. For mainstream PN/A reactor, an integrated fixed-film activated sludge (IFAS) was operated (26°C). The effects of floccular aerobic sludge retention time (AerSRTfloc), a novel aeration strategy, and N-loading rate were tested to optimize the operational strategy. The best performance was observed with a low, but sufficient AerSRTfloc (∼7d) and continuous aeration with two alternating dissolved oxygen setpoints: 10 minutes at 0.07-0.13 mg O2 L-1 and 5 minutes at 0.27-0.43 mg O2 L-1. Nitrogen removal rates were 122±23 mg N L-1 d-1, and removal efficiencies 73±13%. These conditions enabled flocs to act as nitrite sources while the carriers were nitrite sinks, with low abundance of nitrite oxidizing bacteria. The operational strategies in the source-sink framework can serve as a guideline for successful operation of mainstream PN/A reactors.