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 254nm, 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 28mg/L ozone dose and PCO experiments using 250W lamp (emission range: 315-400nm; emission peak: 14.7W/cm(2) at 340nm) and varying TiO2 dose from 1 to 3g/L were carried out. Ozonation (t(1/2)=71.6min) was not found as effective as PCO for organic matter degradation at all TiO2 doses, and 3g/L of TiO2 was found to be the most effective photocatalyst loading (t(1/2)=41.8min). 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, Cl-2 disinfection of treated wastewater samples played an improving role in toxicity of PCO-treated samples. In particular, in 30min 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 Cl-2 disinfection in 3g/L of TiO2 treated sample.