UV/H₂O₂ process stability and pilot-scale validation for trace organic chemical removal from wastewater treatment plant effluents

Highlights

• Comparison of UV/H₂O₂ efficiency at pilot- and laboratory scale.

• Removal assessment of 15 trace organic chemicals (TOrCs).

• Ambient TOrC concentrations in municipal wastewater effluent (no spiking).

• Scavenger capacity is dominated by nitrite concentration fluctuations.

• Mechanistic modeling of OH-radical exposure by water matrix and process parameters.

Abstract

This study investigated the removal of 15 trace organic chemicals (TOrCs) occurring at ambient concentrations from municipal wastewater treatment plant effluent by advanced oxidation using UV/H₂O₂ at pilot-scale. Pseudo first-order rate constants (k_obs) for photolytic as well as combined oxidative and photolytic degradation observed at pilot-scale were validated with results from a bench-scale collimated beam device. No significant difference was determined between pilot- and lab-scale performance. During continuous pilot-scale operation at constant UV fluence of 800 mJ/cm² and H₂O₂ dosage of 10 mg/L, the removal of various TOrCs was investigated. The average observed removal for photo-susceptible (k_UV>10⁻³ cm²/mJ; like diclofenac, iopromide and sulfamethoxazole), moderately photo-susceptible (10⁻⁴<k_UV<10⁻³ cm²/mJ; like climbazole, tramadol, sotalol, citalopram, benzotriazole, venlafaxine and metoprolol), and most photo-resistant (k_UV<10⁻⁴ cm²/mJ; like primidone, carbamazepine and gabapentin) compounds was 90%, 49% and 37% including outliers, respectively. The poorly reactive compound TCEP was not significantly eliminated during pilot-scale experiments. Additionally, based on removal kinetics of photo-resistant TOrCs, continuous pilot-scale operation revealed high variations of OH-radical exposure determined from removal kinetics of photo-resistant TOrCs, primarily due to nitrite concentration fluctuations in the feed water. Furthermore, a correlation between OH-radical exposure and scavenging capacity could be determined and verified by mechanistic modeling using UV fluence, H₂O₂ dosage, and standard water quality parameters (i.e., DOC, NO₃⁻, NO₂⁻ and HCO₃⁻) as model input data. This correlation revealed the possibility of OH-radical exposure prediction by water matrix parameters and proved its applicability for pilot-scale operations.

Introduction

In recent years, trace organic chemicals (TOrC) such as pharmaceutical residues, personal care products, emerging pesticides, and industrial chemicals have been detected and extensively investigated in the aquatic environment and in all parts of the water cycle (Lim, 2008; Blum et al., 2017; Hofman-Caris et al., 2017; Yang et al., 2017). Besides urban and agricultural run-off, wastewater treatment plant (WWTP) effluents are considered to be the most significant TOrC emitters to the aqueous environment (Gros et al., 2010; Luo et al., 2014; Dong et al., 2015). Although concentrations hardly exceed μg/L concentrations, persistent substances remain in WWTP effluents being discharged in surface waters, since conventional physical and biological wastewater treatment can only partially remove these substances (Lim, 2008; Zhang et al., 2008; Luo et al., 2014). For the removal of these compounds from WWTP effluents, advanced oxidation might be a promising treatment approach. Advanced oxidation processes (AOP) are generally defined as processes that intentionally form highly reactive radicals in situ (Comninellis et al., 2008; Yang et al., 2014). Specifically, OH-radicals produced are known for their rapid and non-selective oxidation of organic water contaminants with second-order reaction rate constants in the range of 10⁸ - 10¹⁰ M⁻¹s⁻¹. In current practice, utilization of AOPs is mostly limited to highly treated wastewater effluents including reverse osmosis treatment and advanced drinking water treatment with high UV transmittance.

Among UV-based AOPs, UV/H₂O₂, where H₂O₂ is directly activated by UV light to form two OH-radicals, is a commonly applied AOP in water reuse and advanced drinking water treatment for contaminant as well as taste and odor removal. TOrC removal during low pressure UV/H₂O₂ is achieved by two major reaction pathways, direct photolysis by UV-C irradiation at 254 nm and oxidation by hydroxyl radicals formed in situ. Since generated OH-radicals react unselectively with all water constituents, transformation of target compounds in wastewater is competing with oxidation of other organic and inorganic compounds. The occurrence of so-called radical scavengers, which terminate radical chain reactions, can significantly reduce oxidation efficiency in AOPs (Keen et al., 2012). Furthermore, changes in UV-transmittance directly affect the activation of H₂O₂. Therefore, UV/H₂O₂ effectiveness is highly susceptible to water matrix changes. The influence of scavenging on UV/H₂O₂ has been investigated thoroughly (Liao et al., 2001; Rosenfeldt and Linden, 2004) and a good overview of scavengers and their reactivity with OH-radicals is given by Wols and Hofman-Caris (2012). Kinetic models can be adopted to estimate the influence of radical scavengers on the degradation performance and represent a useful tool to predict TOrC degradation by UV/H₂O₂ in different water matrices as proposed by Bolton and Stefan (2002) and Wols et al. (2013).

UV/H₂O₂ is a thoroughly examined AOP (Pereira et al., 2007) for all kinds of water applications, with most studies performed at laboratory scale (Yuan et al., 2011). Only a limited number of studies, however, have been carried out specifically on wastewater effluent at the laboratory (Rosario-Ortiz et al., 2010; Keen and Linden, 2013; Yu et al., 2015) or pilot scale (Audenaert et al., 2011; Köhler et al., 2012; De La Cruz et al., 2013; Lester et al., 2014; Merel et al., 2015; Cedat et al., 2016). Some pilot-scale studies on UV/H₂O₂ investigated its viability for TOrC removal (Sarathy et al., 2011; Wang et al., 2015; Cedat et al., 2016; Chu et al., 2016; Gerrity et al., 2016; Miralles-Cuevas et al., 2016). However, to the best of our knowledge, verifying pilot-scale studies using data from standardized lab-scale systems combined with mechanistic modeling efforts are lacking in the peer-reviewed literature. Furthermore, little is known about UV/H₂O₂ applicability for municipal wastewater effluents with respect to OH-radical scavenging across changing water qualities (Gerrity et al., 2016; Lee et al., 2016).

In this study, we characterize the applicability of UV/H₂O₂ for advanced municipal wastewater treatment using lab- and pilot-scale set-ups. Piloting results are directly compared to results conducted using a lab-scale collimated beam device (CBD) to verify the up-scaling effort. A total of 15 TOrCs occurring at ambient concentrations and representing a range of different photolytic and OH-radical reactivities were measured. To assess the viability of UV/H₂O₂ for continuous treatment of tertiary effluents, a comprehensive investigation of OH-radical scavenging caused by different water quality parameters, such as dissolved organic carbon (DOC), nitrite, nitrate and alkalinity, was performed during continuous operation of a pilot-scale UV/H₂O₂ system. Finally, the experimental results were compared to mechanistic modeling estimations.