Abstract: Even discharge standard grade A is achieved in wastewater treatment plants (WWTPs), the effluent still contains abundant nitrogen pollution, which can cause deterioration of quality of sensitive receiving waters. Thus, development of tertiary denitrification technique to advanced nitrogen removal is required. In this study, four liquid carbon sources, including sodium acetate, glucose, methanol and ethanol were used as external carbon sources to explore the feasibility of using the sequencing batch reactor (SBR) suspended sludge system to further remove low-level nitrogen from secondary effluent. The experimental results show that when the nitrogen volume loading rate was 0.03 kg/(m³·d), all of four denitrifying-sequencing batch reactor (DN-SBR) systems can reach a NO_x^--N removal efficiency of more than 97.80% with effluent NO_x^--N less than 1 mg/L. The COD/ρ(N) required for the four systems were 5, 12, 6, and 7, corresponding to a carbon source required of 7.35, 12.00, 4.00, and 3.37 g/g NO_x^--N, respectively. The required dose of ethanol was the lowest and the glucose was the highest for complete denitrification. Determined by in-situ measurement method, the average nitrogen removal rates of the four systems were 0.72, 0.19, 0.32 and 0.73 kg/(m³·d), respectively. The ethanol and acetate system presented higher reaction rate and the glucose system was the lowest. There were great differences between the compositions of microbial community in these reactors added with different carbon sources. After an operational period of 78 d, the diversity of microbial communities in acetate- and glucose systems decreased. Firmicutes phylum microorganisms dominated in the reactor with acetate, accounting for 94% of the total microorganisms. The dominated phylums in the glucose-reactor were Actinobacteria (45%) and Patescibacteria (44%). In contrast, the diversity of microbial communities in the methanol and ethanol systems increased compared to that of seed sludge.