[Effects of electron donors on denitrification efficiency and microbial community dynamics]

The small-molecule compounds produced by the acidogenic fermentation products of organic solid waste can be used as carbon sources for denitrification. However, the microbial denitrification processes, which are driven by numerous electron donors, as well as the microbial response mechanisms, remain unclear. In this study, the activated sludge denitrification system was operated in a long-term semi-continuous reactor with acetate, ethanol, lactate, and glucose as electron donors. The link between denitrification efficiency and microbial community structure under different carbon sources was established by comprehensive 16S rRNA high-throughput sequencing and microbial co-occurrence network analysis. The findings showed that denitrification efficiency was the highest for acetate, followed by ethanol, lactic acid, and glucose. Although the microbial community structures were generally similar among different systems, the abundance distribution of the main denitrifying bacteria varied significantly. Thauera was predominant in the acetate and ethanol groups; Denitratisoma and Ellin6067 were abundant in the glucose group; Zoogloea was dominant in the lactic acid group. Network analysis demonstrated that the types of electron donors affected microbial interaction patterns and key species functions, resulting in differences in denitrification efficiency. The acetate group formed a tightly coupled network, with Hyphomicrobium functioning as an important genus for efficient nitrogen removal. Despite the numerous connections in the ethanol group, the limited interaction between modules reduced the denitrification rate. The lactate group had a high degree of modularization, and it relied on Sulfuritalea to coordinate sulfur metabolism to keep nitrogen removal stable. Ellin6067 promoted nitrification in the glucose group, intensifying competition between ammonia-oxidizing and denitrifying bacteria and resulting in the lowest nitrogen removal efficiency. We investigated the denitrification characteristics and microbial responses under various carbon sources from the perspective of microbial ecological mechanisms, providing a theoretical foundation and practical guidance for the targeted use of carbon sources from organic solid waste, as well as low-carbon optimization of wastewater treatment.