Autotrophic denitrification with elemental sulfur (S0) as an electron donor is a clean and cost-effective solution for nitrogen removal from contaminated waters poor in organics and has gained increasing interest in the last decades. In contrast, less attention has been driven towards autotrophic denitritation and the effect of nitrogen oxide (N-NOx- ) loading and feeding strategy on the process. This study investigates the denitrification and denitritation performances of two membrane bioreactors fed with chemically synthesized S0 (S0chem) and biogenic S0 (S0bio) under different N-NOx- loading rates (50 and 100 mg N-NOx- L-1 d-1) and feeding strategies (nitrate or nitrite only and both nitrate and nitrite) and their influence on the specific nitrogen uptake rate (SNUR) of S0-oxidizing denitrifiers. N-NOx- removal was faster with S0bio compared to S0chem at all tested N-NOxloading rates and feeding strategies, confirming S0bio as an advantageous electron donor for autotrophic NO3- and NO2- reductions. Influent NO2- levels as high as 100 mg N-NO2- L-1 did not inhibit autotrophic denitrification and denitritation. On the contrary, batch activity tests showed that NO2- feeding resulted in higher SNUR values compared to feeding with both NO3- and NO2- and with NO3- only, confirming NO2- as a more thermodynamically favorable electron acceptor than NO3- . The impact of different operational conditions on membrane filtration performance was also evaluated, showing that S0bio was the main responsible for pressure increase and membrane fouling during bioreactor operation.