Wastewater disinfection with PAA and UV combined treatment: a pilot plant study.

This study is part of a larger research project on Advanced Treatments for wastewater reuse in agriculture. Because of Italy's strict microbiological limits on unrestricted wastewater reuse in agriculture (2 MPN/100ml Total Coliforms), a very high degree of disinfection is necessary. The objective of this study is to proceed in validating, with a pilot plant experimentation, previous laboratory results on the disinfection efficacy of the synergic combined treatment between ultraviolet irradiation (UV) and peracetic acid (PAA). The research has been carried out through a 5 month on-site experimental study in a pilot plant, considering four different solutions: PAA addition, UV irradiation, addition of PAA upstream the UV device (PAA+UV) and addition of PAA downstream the UV device (UV+PAA). In the investigated experimental conditions (2-8 ppm of PAA with 10-30 min contact time; 100-300 mJ/cm(2) UV), it has been impossible to meet the microbiological limits through an exclusive use of UV irradiation or PAA. The disinfection efficacy enhances by using the UV+PAA treatment, but a much higher efficacy gain occurs by using the PAA+UV treatment. In this latter case, the higher efficiency is recognized as being brought about by the formation of free radicals due to the photolysis of the PAA when in presence of the UV rays. A preliminary cost analysis has been carried out in order to highlight the more economically advantageous solution which guarantees compliance to the strict limits.