Secondary effluent disinfection by combined peracetic acid and ultraviolet radiation

The combination of peracetic acid (PAA) and ultraviolet radiation for the disinfection of secondary effluent is an interesting alternative to achieve high bacterial inactivation or to ensure the preservation of the effluent microbiological quality after discharge in the case of wastewater reuse. In the present work the process was assessed at the laboratory scale by monitoring fecal coliforms and Escherichia coli inactivation: the occurrence of synergistic mechanisms in disinfection efficiency and the enhancement in bacterial inactivation by the combined process were studied. Several combinations of PAA doses (1, 2 mgPAA/L), PAA contact times (6, 12 minutes) and UV doses (5-70 mJ/cm2) were assessed. Furthermore, the ability of PAA (1, 2 mgPAA/L) to ensure adequate coverage downstream (5, 24 h) from a low dose (up to 45 mJ/cm2) UV disinfection process was evaluated to comply with the Italian legislation on agricultural reuse (10 CFUE.coli/100 mL). No synergistic effects on disinfection efficiency were determined by the combined process, displaying only additive effects. Significant improvements in microbial inactivation compared to the UV disinfection were obtained only over 2 mg/L PAA dose and 12 minutes PAA contact time. PAA had an effective inhibitory action for 24 hours against bacterial regrowth and photo-reactivation after UV disinfection