Unraveling the role of formate in improving nitrogen removal via coupled partial denitrification-anammox

The addition of traditional carbon sources (e.g., acetate) could favor heterotrophic overgrowth in partial denitrification coupled with anammox (PD–A) systems, thus hindering the performance and stability of this novel wastewater nitrogen removal technology. Therefore, it is necessary to develop an effective, environmentally friendly, and inexpensive alternative. This study demonstrated the potential of formate to enhance the performance and community stability of PD–A under mainstream conditions. In a laboratory-scale biofilm reactor, formate addition (COD/NO₃⁻–N = 1.75) improved nitrogen removal efficiency (from 72.1 ± 3.5% to 81.7 ± 2.7%), EPS content (from 106.3 ± 8.1 to 163.0 ± 15.5 mg/gVSS) and increased anammox bacteria growth (predominantly Candidatus Brocadia, from 29.5 ± 0.7% to 34.5 ± 5.4%) while maintaining stable heterotrophs dominated by methylotrophic Desulfobacillus. FISH-NanoSIMS revealed a formate uptake using Ca. Brocadia and Desulfobacillus, with Ca. Brocadia being the major contributor to partial nitrate reduction to nitrite. Desulfobacillus can synthesize diverse hydrophobic amino acids and provide key nutrients for Ca. Brocadia. To achieve comparable nitrogen removal, the cost of the formate-driven PD–A process should be 11.2% lower than that of acetate. These results greatly enrich our understanding of C1 metabolism represented by formate in anammox communities and its application in the context of coupling partial denitrification-anammox toward enhanced nitrogen removal in global wastewater treatment systems.