fa تخصصي Special مقاله اصیل Original Article <div style="text-align: justify;">&nbsp;Background: Increasing nitrate concentrations in groundwater resources is considered a common environmental and public health problem worldwide. In this research, an autotrophic up-flow bioreactor with pumice as media was used to study the effects of the sulfur-to-nitrogen (S/N) ratio and empty bed contact time (EBCT) on nitrate removal efficiency and byproducts.<br> Methods: Experiments were carried out in a 3.47 L up-flow, fixed-bed reactor with 3 sampling ports. To evaluate the overall impact of S/N ratio and EBCT on the performance of the bioreactor, several phases with different S/N ratios and EBCTs were applied.<br> Results: At a constant S/N ratio of 3.85 g/g, as EBCT decreased from 24 hours to 2 hours, the nitrate removal efficiency decreased from 98% to 64%. On the other hand, at the desired EBCT of 4 hr, as S/N ratio decreased from 3.85 to 1.51 g/g, nitrate removal efficiency was reduced from 85% to 32%. Changing the EBCT and S/N ratio also affected the effluent nitrite and sulfate concentrations as byproducts. At the S/N ratio of 3.85 g/g<br> and EBCT of 24 hours, effluent nitrite and sulfate concentrations were 0.1 mg NO2--N/L and 463 mg SO4 2-/L, respectively. Decreasing the S/N ratio to 1.51 g/g and the EBCT to 4 hours caused drastic changes in effluent nitrite and sulfate concentrations.<br> Conclusion: The results indicated that the autotrophic denitrification with thiosulfate as electron donor and pumice as media was feasible and applicable for nitrate contaminated groundwater.</div> Autotrophic Denitrification, Pumice, Nitrate, Nitrite, Thiosulfate 1 7 http://ehemj.com/browse.php?a_code=A-10-1-100&slc_lang=fa&sid=1 Masoud Tourang 10031947532846003580 10031947532846003580 No Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, Iran Behnoush Aminzadeh bamin@ut.ac.ir 10031947532846003581 10031947532846003581 Yes Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, Iran Ali Torabian 10031947532846003582 10031947532846003582 No Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, Iran