Ozone Application during Coagulation of Wastewater: Effect on Dissolved Organic Matter

Abstract

The aim of this work was to determine the effect of ozone on dissolved organic matter (DOM) during wastewater coagulation using alum. Adding ozone to the coagulation treatment (O3ECT) enhanced the quality of the final effluent in comparison to conventional coagulation treatment (CT). Final effluents were analyzed by Fourier transform infrared Spectroscopy (FTIR) and UV-Vis spectroscopy. The hydrophilic polar fraction was separated from the hydrophobic fraction using fractionation resins XAD-8 and XAD-4. Wastewater hydrophilic DOM was characterized by molecular weight distribution (MWD), and, BOD5, TOC and DOC parameters. Results show that FTIR spectra for both effluents were very similar in the frequency range 7000 to 1400 cm−1; however, some differences occurred in the O3ECT effluent in the range 1100 to 700 cm−1, which indicated that amines and aromatics groups were affected. In the same way, the UV-Vis spectra showed that there was an increase in absorbance within the wavelength range 285 to 300 nm for the O3ECT effluent (when compared with CT) while the absorbance decreased in the range 210 to 455 nm, particularly at 380 nm. With regards to the hydrophilic polar compounds of DOM, the most abundant fraction in raw wastewater was found in the 3–10 kDa range. After CT, the effluents fraction concentrations decreased in all cases, but for O3ECT the 10–30 kDa range exhibited a marked increase while the smaller (<3 kDa, 3–10 kDa) and larger fractions (>30 kDa) decreased. The ozone application during coagulation slightly increased TOC and DOC percentage removals, 84.4% to 89.1% and 80% to 82.5%, respectively. This suggests that the small dose of ozone transferred (1.47 mg/L) caused only minor changes in DOM. Finally, the O3ECT effluent exhibits 10 mg/L more of biological oxygen demand (BOD5) than a single CT.