Removal of pathogenic indicator microorganisms during partial nitrification: the role of free nitrous acid

Digested wastewater contains pathogenic microorganisms and high ammonia concentrations, which can pose a potential risk to public health. Effective removal of pathogens and nitrogen is crucial for the post-treatment of digested wastewater. Partial nitrification-anammox is an energy-saving nitrogen removal process. Free nitrous acid (FNA), an intermediate product of partial nitrification, has the potential to inactivate microorganisms. However, the efficiency and mechanisms of FNA-related inactivation in pathogens during partial nitrification remains unclear. In this study, Enterococcus and Escherichia coli (E. coli) were selected to investigate the efficiency and mechanisms of FNA-related inactivation in partial nitrification process. The results revealed that 83% ± 13% and 59% ± 27% of E. coli and Enterococcus were removed, respectively, in partial nitrification process at FNA concentrations of 0.023–0.028 mg/L. When the concentration of FNA increased from 0 to 0.5 mg/L, the inactivation efficiencies of E. coli and Enterococcus increased from 0 to 99.9% and 89.9%, respectively. Enterococcus exhibited a higher resistance to FNA attack compared to E. coli. 3D-laser scanning microscopy (3D-LSM) and scanning electron microscopy (SEM) revealed that FNA exposure caused the surface collapse of E. coli and Enterococcus, as well as visible pore formation on the surface of E. coli cells. 4′,6-Diamidino-2-phenylindole dihydrochloride n-hydrate (DAPI)/propidium iodide (PI) and biomolecule leakage confirmed that inactivation of E. coli and Enterococcus occurred due to breakdown of cell walls and cell membranes. These findings indicate that partial nitrification process can be used for the removal of residual pathogenic microorganisms.