Relevant impact of irradiance (vs. dose) and evolution of pH and mineral nitrogen compounds during natural water disinfection by photo-Fenton in a solar CPC reactor

The inactivation of total coliforms/E. coli (10(4) CFU/mL) and Salmonella spp. (10(5) CFU/mL) in well water naturally containing dissolved (Fe2+/3+) and solid iron forms (e.g. iron oxides) was carried out by photo Fenton treatment (Fe-2+,Fe-3+/H2O2/hv). In a preliminary run under simulated solar radiation, beyond 4 mg/L of added H2O2, the enteric bacteria were totally inactivated after 90 min. Thereafter, 25 L of well water were treated in the compound parabolic collector (CPC) under direct solar radiation. Three irradiation periods (i) 8 am to 2 pm (8-14h), (ii) 10 am to 4 pm (10-16 h) and (iii) 12 pm to 6 pm (12-18 h) were evaluated for assessing the impact of different solar irradiances (W m(-2)) on the enteric bacterial inactivation rates. Both studied strains were totally inactivated under sole exposure to solar radiation in the CPC when the experiments were conducted from 8 to 14h or 10 to 16 h. However, Salmonella spp. strains regrowth was noticed after the 24 h dark storage in all the samples previously treated with bare solar radiation. As the treated water contained Fe, the photo-Fenton disinfection at field scale in the CPC was carried out with the addition of H2O2 (10 mg/L). Significant enhancement of the enteric bacteria inactivation rate was therefore recorded comparatively to the one obtained under bare solar treatment. No regrowth was observed in water treated by photo-Fenton disinfection one week after the treatment. The comparative evaluation of photo-Fenton disinfection rate as a function of different irradiation periods was based on the monitoring of the effective disinfection time (EDT), or required time to acquire the total inactivation of targeted bacteria in defined conditions. Therefore, significant impact of the irradiance on the process was noticed. High average irradiance (Al) of 35W m(-2) led to the total inactivation of Salmonella spp. in an EDT of 45 min and a dose of 26 Wh m(-2); while low irradiance of 20W m(-2) required-an-EDT of 180 min for a-dose-60 Wh m(-2). Thus, the experiments revealed that lower irradiance level leads to higher doses to achieve the bacterial disinfection. pH, as well as nitrite and nitrate concentration, increased during the photo-disinfection processes, while depletion was recorded for aqueous ammonia concentration. (c) 2013 Published by Elsevier B.V.