Partial nitrification under various concentrations of NaCl (0-40 g/l) at a constant operational condition was investigated in a submerged biofilter reactor. The highest NO2-N/NOx-N ratio (0.76) was achieved at the NLR of 830 g NH4-N/m(3) day with salt free wastewater. Small increase the salt content led to higher activities and the NH4-N removal efficiency increased from 92% to 95% at 1 g/l NaCl concentration. Over this concentration, each NaCl addition provoked the NH4-N oxidation and a sharp increase of inhibition was observed. The total oxidized NH4-N was achieved at the nitrogen loading rate (NLR) and surface loading rate (SLR) of 0.754 kg/m(3) day and 3.23 g/m(2) day, respectively without salt in the feed wastewater and it was decreased to 0.322 kg/m(3) day and 1.38 g/m(2) day at the salinity of 40 g/l in the PNBR. (C) 2012 Elsevier Ltd. All rights reserved.