Enhancing Competitiveness of Anammox Bacteria with Domestic Wastewater as Electron Donor for Nitrate-Preferential Denitrification: Experimental Evidence and Metagenomic Mechanism

The novel partial denitrification coupling with the anammox (PD/A) process is emerging as a promising alternative for cost-efficient nitrogen removal from wastewater. However, it encounters challenges of competition with heterotrophic denitrification for NO2–-N and inhibition of complex organic carbon involved in wastewater. This work demonstrated the efficient and stable nitrogen removal from nitrate-containing wastewater (NO3–-N of 20–400 mg/L) with organic carbon from domestic wastewater (NH4+-N of 60.8 ± 5.6 mg/L, COD of 195.6 ± 58.4 mg/L) by a single-stage PD/A process. The efficient NO2–-N generation was achieved by utilizing organic carbon in domestic wastewater. With the feed volume ratio of nitrate-containing wastewater to domestic wastewater (VNO3/Vdom) decreasing from 2.4/0.6 to 0.5/2.5, the removal efficiency of NH4+-N and NO3–-N achieved 84.2% and 98.3%, respectively. Anammox bacteria were found to strongly compete over denitrifying bacteria with the contribution of the anammox pathway to total nitrogen removal as high as 91.6%. Enhanced competitiveness of anammox bacteria for substrate NO2–-N was achieved with increasing utilization of the carbon source from domestic wastewater. Metagenomic analysis revealed that the electrons produced by NADH were more preferentially transported to the NO3–-N reductase rather than the NO2–-N reductase. Therefore, denitrifying bacteria Thauera as a main holder of Nar genes and anammox bacteria Candidatus Brocadia jointly removed NO3–-N and NH4+-N. Sludge granulation with increasing domestic wastewater also facilitated the efficient retention of anammox bacteria. Overall, this study highlighted the efficient heterotrophic–autotrophic cooperation by in situ utilization of the complex carbon source from domestic wastewater, providing valuable technical support to advance the PD/A process toward practical application.