Formate enhances partial-denitrification and energy production by anammox bacteria in simultaneous anammox-denitrification system

Simultaneous anammox-denitrification (SAD) has great potential for achieving deep nitrogen removal in wastewater treatment, but it generally requires external organics to serve as electron donors, e.g., acetate, causing problems such as overgrowth of heterotrophs. In this study, formate as the external carbon source achieved desired nitrogen removal (>95 %, TNeff < 15 mg/L) at a relatively low dosage (COD/NO3--N = 1.5) with high EPS production and anammox activity in the SAD system. Based on selective inhibition and C-13/N-15 isotope labelling tests, anammox bacteria became the major contributors to partial-denitrification (PD, nitrate reduction to nitrite) (54.04 +/- 4.12 % vs 43.80 +/- 5.73 % at COD/NO3--N = 1.5). Besides, more energy (ATP and NADH were 35.67 % and 30.45 %) and less sludge (growth yield was 62.16 %) were produced after the addition of formate compared to acetate. Formate enriched Candidatus Brocadia and the methylotrophic denitrifying bacteria Desulfobacillus and ELB16-189 which likely exchanged necessities such as methionine, molybdopterin cofactor (MOCO), folate and 4 '-phospho-pantetheine (PSH-4'-P). Compared with acetate, formate limited the overgrowth of heterotrophic bacteria and ensured stability of SAD community. This study not only provided options for novel carbon sources for SAD, but also deepened the understanding of the mechanism of C1 compound utilization by different bacteria in the anammox community.