Two sulfur-oxidizing membrane bioreactors (SMBRs) performing autotrophic denitrification at different HRTs (6-26 h), one supplemented with biogenic elemental sulfur (S-bio(0)) and the other with chemically-synthesized elemental sulfur (S-chem(0)), were compared in terms of nitrate reduction rates, impact on membrane filtration and microbial community composition. Complete denitrification with higher rates (up to 286 mg N-NO3-/L d) was observed in the SMBR supplemented with S-bio(0) (SMBRbio), while nitrate was never completely reduced in the SMBR fed with S-chem(0) (SMBRchem). Trans membrane pressure was higher for SMBRbio due to smaller particle size and colloidal properties of S-bio(0). Microbial communities in the two SMBRs were similar and dominated by Proteobacteria, with Pleomorphomonas and Thermomonas being the most abundant genera in both bioreactors. This study reveals that S-bio(0) can be effectively used for nitrate removal in autotrophic denitrifying MBRs and results in higher nitrate reduction rates compared to S-chem(0).