A comparative evaluation of ozonation and heterogeneous photocatalytic oxidation processes for reuse of secondary treated urban wastewater

Abstract

This study aimed to optimize two oxidation processes for wastewater reuse of an urban wastewater treatment plant (UWWTP) effluent intended for human consumption. Ozonation and TiO₂ photocatalytic oxidation (PCO) processes were compared for their effectiveness in terms of organic matter degradation (measured as UV absorbance at 254 nm, UV₂₅₄), disinfection by products formation (measured as trihalomethanes formation potential, THMFP), and ecotoxicity (evaluated by Daphnia magna and Lepidium sativum), on secondary treated wastewater collected from a UWWTP in the province of Salerno (southern Italy). Accordingly, ozonation experiments using 28 mg/L ozone dose and PCO experiments using 250 W lamp (emission range: 315–400 nm; emission peak: 14.7 μW/cm² at 340 nm) and varying TiO₂ dose from 1 to 3 g/L were carried out. Ozonation (t_1/2 = 71.6 min) was not found as effective as PCO for organic matter degradation at all TiO₂ doses, and 3 g/L of TiO₂ was found to be the most effective photocatalyst loading (t_1/2 = 41.8 min). Moreover, PCO-treated samples resulted in a lower THMFP compared to ozonation process. While ozonation did not increase toxicity to D. magna severely (10% of immobilization), PCO-treated samples exhibited varying toxicity end-points possibly due to formed oxidation intermediate products. However, Cl₂ disinfection of treated wastewater samples played an improving role in toxicity of PCO-treated samples. In particular, in 30 min oxidized samples, toxicity to D. magna was decreased up to 50% (from 40 to 20% of immobilization) while germination index of L. sativum, a phytotoxicity test used for irrigation quality testing purpose, improved at 30% after Cl₂ disinfection in 3 g/L of TiO₂ treated sample.

Keywords:

• Advanced oxidation processes
• Disinfection by products
• Ecotoxicity
• Ozonation
• Photocatalysis
• Phyto-toxicity
• Trihalomethanes
• Wastewater reuse

Acknowledgements

The authors would like to acknowledge MIUR (Ministry of University and Research), Italy, for supporting this study (Controllo dei sottoprodotti della disinfezione nelle acque potabili e nei reflui destinati al riutilizzo). Contributions by Dr Claudio Della Rocca, Dott. sa Mafalda Inglese, and Mr Paolo Napodano are greatly appreciated.