Water Research 37 (2003) 2365–2371 Wastewater disinfection with PAA and UV combined treatment: a pilot plant study Cecilia Caretti*, Claudio Lubello Dipartimento di Ingegneria Civile, Universit " a degli Studi di Firenze, via S.Marta 3, Firenze 50139, Italy Received 22 April 2002; accepted 20 December 2002 Abstract 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/100 ml 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. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Wastewater; Disinfection; Peracetic acid; Ultraviolet irradiation; Advanced oxidation processes; Agricultural reuse 1. Introduction The advanced oxidation processes (AOPs) are based on two main steps—the generation of free radicals and the oxidation reactions of the polluting molecules caused by these radicals: AOP- d OH  ! þpollutants CO 2 þ H 2 O þ inorganic ions: In literature, the principal applications of these processes refer to the oxidation of organic compounds, dissolved inorganic compounds and other pollutants that are toxic and/or refractory to biological treatments [1–3,22]. Few references were found, however, regarding their use for wastewater disinfection [4,5]. The hydroxyl radical is primarily responsible for the efficiency of the AOPs. Its high oxidation potential in fact guarantees a rapid degradation of the polluting molecules. In both natural and drinking water, the average life-span of the d OH radical is very short (about 10 ms); however, sufficient for the radical to perform its oxidizing action and promote the formation of a chain reaction capable of producing other free radicals [6,7]. Since the disinfection process of PAA seems to take place according to radical type reactions, we have tried to research if and how the UV could accelerate such reactions. *Corresponding author. Tel.: +39-0554796435; fax: +39- 055495333. E-mail address: cecilia@dicea.unifi.it (C. Caretti). 0043-1354/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00025-3