Abstract: The sludge-based sulfur-driven autotrophic denitrification （SADN） process generally has the disadvantage of a long startup period,which to some extent affects application of this treatment technology.In this study,an UASB reactor is employed,the main objective is to investigate the start-up time of a sludge-based SADN reactor with low height-to-diameter rate by varying the control conditions such as flow rate,NO₃^-N loading,and reflux ratio.The UASB reactor is inoculated with anoxic sludge obtained from a municipal wastewater treatment plant as the seed sludge,using artificially formulated NO₃^--N wastewater as the experimental water and S₂O₃^2- as the electron donors.The results show that when the water temperature is between 15.8 and 20.7°C,using an UASB with a low height-to-diameter of 10 can achieve successful start-up within 41 days under the parameters of a low NO₃^--N loading（2±0.03 mg-N/g-VSS/h）,a low flow rate （2 L/h）,and an ultra-high reflux ratio（1000%～2059%）.