Abstract • Sludge fermentation liquid addition resulted in a high NAR of 97.4%. • Extra NH4+-N from SFL was removed by anammox in anoxic phase. • Nitrogen removal efficiency of 92.51% was achieved in municipal wastewater. • The novel system could efficiently treat low COD/N municipal wastewater. Biological nitrogen removal of wastewater with low COD/N ratio could be enhanced by the addition of wasted sludge fermentation liquid (SFL), but the performance is usually limited by the introducing ammonium. In this study, the process of using SFL was successfully improved by involving anammox process. Real municipal wastewater with a low C/N ratio of 2.8–3.4 was treated in a sequencing batch reactor (SBR). The SBR was operated under anaerobic-aerobic-anoxic (AOA) mode and excess SFL was added into the anoxic phase. Stable short-cut nitrification was achieved after 46d and then anammox sludge was inoculated. In the stable period, effluent total inorganic nitrogen (TIN) was less than 4.3 mg/L with removal efficiency of 92.3%. Further analysis suggests that anammox bacteria, mainly affiliated with Candidatus_Kuenenia, successfully reduced the external ammonia from the SFL and contributed approximately 28%–43% to TIN removal. Overall, this study suggests anammox could be combined with SFL addition, resulting in a stable enhanced nitrogen biological removal.