Micropollutants as internal probe compounds to assess UV fluence and hydroxyl radical exposure in UV/H2O2 treatment

Robin Wünsch,Carina Mayer,Julia Plattner,Fabienne Eugster,Richard Wülser,Jens Gebhardt,Uwe Hübner,Silvio Canonica,Thomas Wintgens,Urs Von Gunten

doi:10.1016/j.watres.2021.116940

Abstract

Organic micropollutants (MPs) are increasingly detected in water resources, which can be a concern for human health and the aquatic environment. Ultraviolet (UV) radiation based advanced oxidation processes (AOP) such as low-pressure mercury vapor arc lamp UV/H2O2 can be applied to abate these MPs. During UV/H2O2 treatment, MPs are abated primarily by photolysis and reactions with hydroxyl radicals (•OH), which are produced in situ from H2O2 photolysis. Here, a model is presented that calculates the applied UV fluence (Hcalc) and the •OH exposure (CT•OH,calc) from the abatement of two selected MPs, which act as internal probe compounds. Quantification of the UV fluence and hydroxyl radical exposure was generally accurate when a UV susceptible and a UV resistant probe compound were selected, and both were abated at least by 50 %, e.g., iopamidol and 5-methyl-1H-benzotriazole. Based on these key parameters a model was developed to predict the abatement of other MPs. The prediction of abatement was verified in various waters (sand filtrates of rivers Rhine and Wiese, and a tertiary wastewater effluent) and at different scales (laboratory experiments, pilot plant). The accuracy to predict the abatement of other MPs was typically within ±20 % of the respective measured abatement. The model was further assessed for its ability to estimate unknown rate constants for direct photolysis (kUV,MP) and reactions with •OH (k•OH,MP). In most cases, the estimated rate constants agreed well with published values, considering the uncertainty of kinetic data determined in laboratory experiments. A sensitivity analysis revealed that in typical water treatment applications, the precision of kinetic parameters (kUV,MP for UV susceptible and k•OH,MP for UV resistant probe compounds) have the strongest impact on the model's accuracy.

Highlights

A novel modeling tool based on the abatement of micropollutants acting as internal probe compounds was developed to determine the applied UV fluence and the hydroxyl radical exposure during laboratory- and pilot-scale UV/H2O2 treatment
This modeling approach has the advantage that water matrix parameters affecting the abatement of micropollutants, as well as hydrodynamics and non-ideal characteristics of the UV-reactor are implicitly considered by the model
If the model largely underpredicts the abatement of a substance based on photolysis and reactions with OH, and the rate constants of the predicted substance were obtained from careful measurements in the laboratory, results might indicate significant contributions of other radical species to its abatement

Summary

Introduction

To mitigate the impact of organic micropollutants (MPs), advanced oxidation processes (AOPs) are a treatment option in utilities for drinking water production (Kruithof et al, 2007; Scheideler et al, 2011; Stefan, 2018; von Gunten, 2018) or (in)direct potable water reuse (Roback et al, 2019; Stefan, 2018; Trussell et al, 2019). Despite the many options for radical generation, AOPs deployed in full-scale drinking water treatment are currently either ozone-based (O3, O3/H2O2) or ultraviolet (UV) radiation based (UV/H2O2, UV/Cl2) (Miklos et al, 2018b). Other considerations can lead to the selection of a UV-based AOP, if e.g., the targeted MPs are more efficiently abated by direct photolysis, as in the case of N-nitrosodimethylamine (NDMA) or most X-ray contrast media (Scheideler et al, 2011)

Methods

Results

Conclusion