Partial denitrification-anammox granular sludge domesticated from high-strength anammox granules and the high-efficiency performance in treating low-nitrogen wastewater

Partial denitrification (PD) is an effective approach to supply nitrite for the anaerobic ammonium oxidation (anammox) reaction, and the PD-anammox process can achieve nearly 100 % nitrogen removal when denitratation, denitritation and anammox processes are well balanced. However, most of the previous studies initiated the PD-anammox process by a mixed inoculation of denitrification and anammox sludges, leading to low anammox bacteria abundance and a risk of unstable operation in the long-term. To address this, this study used high-strength anammox granules as seed sludge for rapid PD-anammox process start-up. During the continuous operation, nitrogen removal efficiencies of 97.6 ± 1.6 % and 91.0 ± 2.0 % were attained under influent conditions of 159.4 ± 3.4 mg N/L and 78.1 ± 3.5 mg N/L, respectively. In addition, the nitrite conversion rate of the sludge reached 5.34 ± 0.37 g N/g VSS/d during the operation, and the anammox activity was stable at 0.27–0.41 g N/g VSS/d, indicating a synergistic PD-anammox relationship. Furthermore, PD bacteria were efficiently enriched in the reactor within a short period of time, with Thauera accounting for 7.6 % of the relative abundance. Notably, the abundances of dominating anammox bacteria, Ca. Brocadia and Ca. Kuenenia were 16.6 % and 4.9 %, respectively, which were among the highest of the reported PD-anammox processes. High anammox microbial abundance is conducive to ensuring the dominance and stability of the anammox reaction, enabling efficient nitrite conversion and nitrogen removal throughout the continuous operation. Finally, nitrogen removal pathways based on mass balance were proposed to clarify the fate of different nitrogen compounds in the PD-anammox system.