The scarcity of rapidly biodegradable organics, which serve as essential electron donors for the partial denitrification (PD) process, significantly hinders the combined application of PD coupled with anammox (PDA) in municipal wastewater treatment plants. This study innovatively applied, for the first time, a step-feed strategy combined with the use of sludge fermentation liquid (SFL) as an external carbon source in an integrated fixed-film activated sludge (IFAS) system, successfully driving full nitrification and PDA to achieve advanced nitrogen removal from low C/N real municipal wastewater. Moreover, the associated nitrogen removal mechanism of this system was systematically analyzed. By employing second-step SFL feed as a supplementary carbon source, the nitrogen removal efficiency reached 92.26 ± 2.77 % and the effluent total inorganic nitrogen was 6.43 ± 2.23 mg/L, with anammox contributing approximately 70 % to total inorganic nitrogen removal. 16S rRNA gene sequencing and fluorescence in situ hybridization analysis unveiled the extensive cooperation and synergistic interactions among anammox bacteria, denitrifying bacteria, and nitrifying bacteria, with Candidatus Brocadia being highly enriched in biofilms with a relative abundance of 2.21 %. Metagenomic sequencing confirmed that the relative abundance of the narGHI gene was greater than that of the nirS gene, providing stable nitrite accumulation conditions for the anammox process. Overall, this study proposes an innovative synergistic treatment scheme that utilizes a step-feed full nitrification-PDA process driven by SFL to achieve advanced nitrogen removal in municipal wastewater treatment plants. This approach is characterized by low energy consumption, low operational costs and a high nitrogen removal efficiency.