Beyond an Applicable Rate in Low-Strength Wastewater Treatment by Anammox: Motivated Labor at an Extremely Short Hydraulic Retention Time

The application of anammox technology in low-strength wastewater treatment is still challenging due to unstable nitrite (NO2 ––N) generation. Partial denitrification (PD) of nitrate (NO3 ––N) reduction ending with NO2 ––N provides a promising solution. However, little is known about the feasibility of accelerating nitrogen removal toward the practical application of anammox combined with heterotrophic denitrification. In this work, an ultrafast, highly stable, and impressive nitrogen removal performance was demonstrated in the PD coupling with an anammox (PD/A) system. With a low-strength influent [50 mg/L each of ammonia (NH4 +–N) and NO3 ––N] at a low chemical oxygen demand/NO3 ––N ratio of 2.2, the hydraulic retention time could be shortened from 16.0 to 1.0 h. Remarkable nitrogen removal rates of 1.28 kg N/(m3 d) and excellent total nitrogen removal efficiency of 94.1% were achieved, far exceeding the applicable capacity for mainstream treatment. Stimulated enzymatic reaction activity of anammox was obtained due to the fast NO2 ––N jump followed by a famine condition with limited organic carbon utilization. This high-rate PD/A system exhibited efficient renewal of bacteria with a short sludge retention time. The 16S rRNA sequencing unraveled the rapid growth of the genus Thauera, possibly responsible for the incomplete reduction of NO3 ––N to NO2 ––N and a decreasing abundance of anammox bacteria. This provides new insights into the practical application of the PD/A process in the energy-efficient treatment of low-strength wastewater with less land occupancy and desirable effluent quality.