Quantification of the electron donating capacity and UV absorbance of dissolved organic matter during ozonation of secondary wastewater effluent by an assay and an automated analyzer

Nicolas Walpen,Joanna Houska,Elisabeth Salhi,Michael Sander,Urs Von Gunten

doi:10.1016/j.watres.2020.116235

Abstract

Ozonation of secondary wastewater treatment plant effluent for the abatement of organic micropollutants requires an accurate process control, which can be based on monitoring ozone-induced changes in dissolved organic matter (DOM). This study presents a novel automated analytical system for monitoring changes in the electron donating capacity (EDC) and UV absorbance of DOM during ozonation. In a first step, a quantitative photometric EDC assay was developed based on electron-transfer reactions from phenolic moieties in DOM to an added chemical oxidant, the radical cation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS·+). The assay is highly sensitive (limit of quantification ∼0.5 mgDOC·L−1) and EDC values of model DOM isolates determined by this assay were in good agreement with values determined previously by mediated electrochemical oxidation (slope = 1.01 ± 0.07, R2 = 0.98). In a second step, the photometric EDC measurement method was transferred onto an automated fluidic system coupled to a photometer (EDC analyzer). The EDC analyzer was then used to monitor changes in EDC and UV absorbance of secondary wastewater effluent treated with ozone. While both parameters exhibited a dose-dependent decrease, a more pronounced decrease in EDC as compared to UV absorbance was observed at specific ozone doses up to 0.4 mgO3·gDOC−1. The concentration of 17α-ethinylestradiol, a phenolic micropollutant with a high ozone reactivity, decreased proportionally to the EDC decrease. In contrast, abatement of less ozone-reactive micropollutants and bromate formation started only after a pronounced initial decrease in EDC. The on-line EDC analyzer presented herein will enable a comprehensive assessment of the combination of EDC and UV absorbance as control parameters for full-scale ozonation.

Highlights

Ozonation of secondary wastewater treatment plant (WWTP) effluents combined with biological post-treatment is a widely recognized strategy for abating a broad range of organic micropollutants of environmental concern (Eggen et al, 2014; Oulton et al, Abbreviations: aλ, absorption coefficent at the wavelength λ; ABTS, 2,2 -azino-bis(3-ethylbenzothiazoline-6-sulphonate); ABTS·+, radical cation of ABTS; Dissolved organic carbon (DOC), dissolved organic carbon; DOM, dissolved organic matter; EDC, electron donating capacity; EH, reduction potential reported against the standard hydrogen electrode; ε(λ), molar absorption coefficient at the wavelength λ; LOQ, limit of quantification; MEO, mediated electrochemical oxidation; SUVAλ, specific UV absorbance at the wavelength λ; wastewater treatment plants (WWTPs), wastewater treatment plant
Bromate concentrations sharply increased to >10 μg·L–1 for relative decreases in EDC and UV absorbance down to 34% and 44%, respectively (Fig. S14, Supplementary Information (SI)). These results demonstrate that the EDC analyzer, which was designed for continual, long-term process monitoring, achieves a similar analytical performance and predictive power for micropollutant abatement as a previously presented laboratory method based on size-exclusion chromatography (Chon et al, 2015)
We developed and comprehensively validated (i) a photometric assay and (ii) an automated analyzer for the quantification of changes in EDC and UV absorbance of DOM during ozonation of secondary WWTP effluents

Summary

Introduction

Ozonation of secondary wastewater treatment plant (WWTP) effluents combined with biological post-treatment is a widely recognized strategy for abating a broad range of organic micropollutants of environmental concern (Eggen et al, 2014; Oulton et al., Abbreviations: aλ, absorption coefficent (cm−1) at the wavelength λ; ABTS, 2,2 -azino-bis(3-ethylbenzothiazoline-6-sulphonate); ABTS·+, radical cation of ABTS; DOC, dissolved organic carbon; DOM, dissolved organic matter; EDC, electron donating capacity; EH, reduction potential reported against the standard hydrogen electrode; ε(λ), molar absorption coefficient at the wavelength λ; LOQ, limit of quantification; MEO, mediated electrochemical oxidation; SUVAλ, specific UV absorbance at the wavelength λ; WWTP, wastewater treatment plant.2010; Patel et al, 2019; Prasse et al, 2015; von Gunten, 2018). The treatment objectives specified by the Swiss Federal Office for the Environment demand an average abatement of 80% for a suite of indicator substances from raw wastewater to WWTP effluent, which requires the application of minimum specific ozone doses (Bourgin et al, 2018). Water quality requirements limit the upper value for ozone exposures (i.e., ∫cO3 dt) due to the formation of undesired oxidation by-products which are not degraded during biological post-treatment (Hollender et al, 2009; Zimmermann et al, 2011). Energy demand for ozone production incentivizes lower ozone doses (Katsoyiannis et al, 2011) To balance these treatment objectives and water quality requirements, an accurate, real-time determination of the optimal ozone dose based on robust control parameters is critical

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