By embedding immobilized biofiller, 100% denitrification efficiency was achieved with nitrate or nitrite as electron acceptors utilizing volatile fatty acids (VFAs) from domestic wastewater after hydrolysis acidification. The consumption patterns of VFAs by functional bacteria and differences in nitrogen metabolic gene expression were thoroughly analyzed. Total consumption of acetic and propionic acids with >
95% VFAs utilization was achieved utilizing nitrate, whereas the consumption of butyric and valeric acids was enhanced utilizing nitrite. Denitrification-related genes were all upregulated, particularly nosZ, indicating systemic N