New insights into differential salinity tolerance between autotrophic and heterotrophic partial nitrification

Autotrophic (R1) and heterotrophic (R2) partial nitrification showed considerable differences under salinity free conditions, however, their response to salinity stress remains elusive. In this study, R2 (87.29%) outperformed the R1 (79.56%) by achieving higher ammonia removal efficiency (ARE) at 0 g/L NaCl. A subsequent increase in salinity load gradually diminished the comparative nitrification advantages held by R2. Moreover, ARE of R1 surpassed R2 at 45 g/L NaCl (about 70.00%), and ARE disparity between two reactors widened with an elevation in load. Biofilm morphology analysis revealed that R1 biofilm was disrupted at a faster rate by salinity than R2. Successive peeling of & beta;-polysaccharide and protein layer resulted in loose biofilm structures in R1. These loose biofilms structures and coping mechanisms, such as increased polysaccharide production in the tightly bound extracellular polymeric substances (TB-EPS), could improve the efficiency of R1's ability to transmit nutrients under salinity stress. In contrast, R2 retained higher total EPS and its biofilm was covered by proteins even at 70 g/L NaCl. Whereas, a change in load (50-70 g/L NaCl) resulted in a substantial loss of important AOB like Nitrosococcus accounted for its inferior ARE during this period, indicating that the higher amount of EPS had limited protective roles under high salinity stress. In a nut shell, the results of this study demonstrated the significance of controlling organic inflow while treating high saline wastewater.