Responses of endogenous partial denitrification process to acetate and propionate as carbon sources: Nitrite accumulation performance, microbial community dynamic changes, and metagenomic insights

Endogenous partial denitrification (EPD) offered a promising pathway for supplying nitrite to anammox, and it also enabled energy-efficient and cost-effective nitrogen removal. However, information about the impact of different carbon sources on the EPD system was limited, and the metabolic mechanisms remained unclear. This study operated the EPD system for 180 days with various acetate and propionate ratios over eight phases. The nitrate-to-nitrite transformation ratio (NTR) decreased from 81.7 % to 0.4 % as the acetate/propionate (Ac/Pr) ratio shifted from 3:0 to 0:3, but the NTR returned to 86.1 % after propionate was replaced with acetate. Typical cycles indicated that PHB (126.8 and 133.9 mg COD/g VSS, respectively) was mainly stored, facilitating a higher NTR (87.8 % and 67.7 %, respectively) on days 58 and 180 in the presence of acetate. In contrast, on day 158 in the presence of propionate, PHV (84.8 mg COD/g VSS) was predominantly stored, resulting in negligible nitrite accumulation (0.2 mg N/L). Metagenomic analysis revealed that the microbial community structure did not significantly change, and the (narGHI+napAB)/nirKS ratio consistently exceeded 7:2, despite variations in the carbon source. Compared with acetate, propionate as carbon source reduced the abundance of genes encoding NADH-producing enzymes (e.g., mdh), likely owing to a shift in PHAs synthesis and degradation pathways. Consequently, limited NADH affected electron distribution and transfer rates, thereby decreasing the nitrate reduction rate and causing nitrite produced by narGHI and napAB to be immediately reduced by nirKS. This study provided new insights and guidance for EPD systems to manage the conditions of carbon deficiency or complex carbon sources.