The Impact of Natural Variation of OH Radical Demand of Drinking Water Sources on the Optimum Operation of the UV/H2O2 Process

Life cycle energy use and greenhouse gas emissions for a novel algal-osmosis membrane system versus conventional advanced potable water reuse processes: Part I

This study applied a life cycle assessment (LCA) methodology for a comparative environmental analysis between an innovative algae resource recovery and near zero-liquid discharge potable reuse system (i.e., the main system) versus a conventional potable reuse system (i.e., the benchmark system) through energy use and greenhouse gas (GHG) emissions. The objective of this study is to demonstrate that pilot-scale data coupled with LCA would provide valuable information for system optimization, integration, and improvements for the design of environmentally sustainable full-scale systems. This study also provides decision-makers valuable information regarding the energy demand and environmental impact of this innovative main system compared to a typical tried-and-true system for potable water reuse. The main system consists of a novel algal-based wastewater treatment coupled with a dual forward osmosis and seawater reverse osmosis (Algal FO-SWRO) membranes system for potable water recovery and hydrothermal liquefaction (HTL) to recover biofuels and valuable nutrients from the harvested algal biomass. The benchmark system refers to the current industry standard technologies for potable water reuse and waste management including a secondary biological treatment, microfiltration (MF), brackish water reverse osmosis (BWRO), ultraviolet/advanced oxidation process (UV-AOP), and granular activated carbon (GAC), as well as anaerobic digestion for sludge treatment. Respective energy and GHG emissions of both systems were normalized and compared considering 1 m³ of water recovered. Based on an overall water recovery of 76% designed for the benchmark system, the energy consumption totaled 4.83 kWh/m³, and the system was estimated to generate 2.42 kg of CO₂ equivalent/m³ with most of the emissions coming from the biological treatment. The main system, based on an overall water recovery of 88%, was estimated to consume 4.76 kWh/m³ and emit 1.49 kg of CO₂ eq/m³. The main system has high environmental resilience and can recover bioenergy and nutrients from wastewater with zero waste disposal. With the application of energy recovery devices for the HTL and the SWRO, increase in water recovery of the FO membrane, and replacement of the SWRO membrane with BWRO, the main system provides an energy-competitive and environmentally positive alternative with an energy demand of 2.57 kWh/m³ and low GHG emissions of 0.94 kg CO₂ eq/m³.

Multiple Roles of Dissolved Organic Matter in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) can degrade a wide range of trace organic contaminants (TrOCs) to improve the quality of potable water or discharged wastewater effluents. Their effectiveness is impacted, however, by the dissolved organic matter (DOM) that is ubiquitous in all water sources. During the application of an AOP, DOM can scavenge radicals and/or block light penetration, therefore impacting their effectiveness toward contaminant transformation. The multiple ways in which different types or sources of DOM can impact oxidative water purification processes are critically reviewed. DOM can inhibit the degradation of TrOCs, but it can also enhance the formation and reactivity of useful radicals for contaminants elimination and alter the transformation pathways of contaminants. An in-depth analysis highlights the inhibitory effect of DOM on the degradation efficiency of TrOCs based on DOM's structure and optical properties and its reactivity toward oxidants as well as the synergistic contribution of DOM to the transformation of TrOCs from the analysis of DOM's redox properties and DOM's transient intermediates. AOPs can alter DOM structure properties as well as and influence types, mechanisms, and extent of oxidation byproducts formation. Research needs are proposed to advance practical understanding of how DOM can be exploited to improve oxidative water purification.

Micropollutant degradation by UV/H2O2 in drinking water: Facilitated prediction through combination of model simulation and portable measurement

Ultraviolet-based advanced oxidation processes (UV-AOPs) are highly effective for micropollutant degradation. However, it is considerably time and labor consuming to evaluate the practical performance of UV-AOPs. This study developed a novel method, through combination of model simulation with portable measurement (MS-PM), to facilitate prediction of the photon fluence-based rate constant of micropollutant degradation (k'_p,MP) by UV/H₂O₂, a commercially available UV-AOP. Model simulation was performed with photochemical, hydroxyl radical (HO^•) concentration steady-state approximation, and quantitative structure-activity relationship models; and portable measurement was conducted on a mini-fluidic photoreaction system to quantify the HO^• scavenging capacity (HRSC) of a water matrix. The method was established and further verified experimentally in seven test waters by taking sulfamethazine (SMN) as a model micropollutant. A lower k'_p,SMN was predicted in a water matrix with a higher HRSC, for example, 57.5 and 347.8 m² einstein^-1 in the raw water (HRSC = 5.91 × 10⁵ s^-1) and sand-filtered effluent (HRSC = 5.25 × 10⁴ s^-1) of a drinking water treatment plant at an H₂O₂ dose of 25 mg L^-1, respectively. The predicted values agreed generally well with the experimental ones. The MS-PM method has advantages of high efficiency and convenience, low cost, and acceptable accuracy, which will significantly facilitate the design and field assessment of UV-AOPs for micropollutant removal from drinking water.

Organic pollutant degradation by UV/peroxydisulfate process: Impacts of UV light source and phosphate buffer

In recent years, ultraviolet (UV) based advanced oxidation processes have been extensively studied for degradation of refractory organic pollutants in water and wastewater, and selection of an appropriate UV light source is an important issue. In this study, bench-scale tests were conducted on a mini-fluidic photoreaction system (MFPS) to determine the degradation kinetics of methylene blue (MB) by UV/peroxydisulfate (UV/PDS) process equipped with a low-pressure UV (LPUV), vacuum UV (VUV)/LPUV, or medium-pressure UV (MPUV) mercury vapor lamp. Results indicate that MB degradation by UV/PDS with various light sources all followed the pseudo-first order kinetics, and the photon fluence-based rate constant (k_p,λ^') had a descending order of: VUV/LPUV/PDS ≫ MPUV/PDS > LPUV/PDS. Moreover, it is noted that phosphate buffer (PB) notably inhibited MB degradation: the k_p,LPUV^', k_p,VUV/LPUV^' and k_p,MPUV^' decreased by 35.0%, 44.9% and 37.5% with the PB concentration increasing from 0 to 20 mM, respectively. The maximal decrease in k_p,VUV/LPUV^' was ascribed to a strong competition of PB for VUV photons. Thereafter, pilot-scale tests were conducted to evaluate the practical performance of UV/PDS in terms of the electrical energy consumption per order (E_EO). It was found again that the VUV/LPUV lamp was the optimal light source in UV/PDS for organic pollutant degradation. This study helps optimize the UV/PDS process for its practical application to water and wastewater treatment.

Model Predictive Control Strategy for the Degradation of Pharmaceutically Active Compounds by UV/H2O2 Oxidation Process

Hydroxyl radical (•OH) scavenging demand can be an indicator that represents the water quality characteristics of raw water. It is one of the key parameters predicting UV/H2O2 system performance and affects the operating parameters. Based on the •OH scavenging demand, we developed a model predictive control strategy to meet the target compound removal efficiency and energy consumption simultaneously. Selected pharmaceutically active compounds (PhACs) were classified into three groups depending on the UV direct photolysis and susceptibility to •OH. Group 1 for photo-susceptible PhACs (acetaminophen, amoxicillin, diclofenac, iopromide, ketoprofen, and sulfamethoxazole); group 2 for PhACs susceptible to both direct photolysis and •OH oxidation (bisphenol A, carbamazepine, ibuprofen, naproxen, ciprofloxacin, and tetracycline); and group 3 for photo-resistant PhACs (atenolol, atrazine, caffeine, and nitrobenzene). The results of modeling to achieve 90% removal of PhACs at N and B plants were as follows. For group 2, the optimized operating parameter ranges were as follow (N plant: UV 510–702 mJ cm−2, H2O2 2.96–3.80 mg L−1, EED 1088–1302 kWh m−3; B plant: UV dose 1179–1397 mJ cm−2, H2O2 dose 3.56–7.44 mg L−1, EED 1712–2085 kWh m−3). It was confirmed that the optimal operating conditions and EED values changed according to the •OH scavenging demand.

Role of Antioxidant Moieties in the Quenching of a Purine Radical by Dissolved Organic Matter

Dissolved organic matter (DOM) has been known to inhibit the degradation of trace organic contaminants (TrOCs) in advanced oxidation processes but quantitative understanding is lacking. Adenine (ADN) was selected as a model TrOC due to the wide occurrence of purine groups in TrOCs and the well-documented transient spectra of its intermediate radicals. ADN degradation in the presence of DOM during UV/peroxydisulfate treatment was quantified using steady-state photochemical experiments, time-resolved spectroscopy, and kinetic modeling. The inhibitory effects of DOM were found to include competing for photons, scavenging SO₄^•- and HO^•, and also converting intermediate ADN radicals (ADN(-H)^•) back into ADN. Half of the ADN(-H)^• were reduced back to ADN in the presence of about 0.2 mg_C L^-1 of DOM. The quenching rate constants of ADN(-H)^• by the 10 tested DOM isolates were in the range of (0.39-1.18) × 10⁷ M_C^-1 s^-1. They showed a positive linear relationship with the total antioxidant capacity of DOM. The laser flash photolysis results of the low-molecular-weight analogues of redox-active moieties further supported the dominant role of antioxidant moieties in DOM in the quenching of ADN(-H)^•. The diverse roles of DOM should be considered in predicting the abatement of TrOCs in advanced oxidation processes.

Machine Learning-Assisted QSAR Models on Contaminant Reactivity Toward Four Oxidants: Combining Small Data Sets and Knowledge Transfer

To develop predictive models for the reactivity of organic contaminants toward four oxidants─SO₄^•-, HClO, O₃, and ClO₂─all with small sample sizes, we proposed two approaches: combining small data sets and transferring knowledge between them. We first merged these data sets and developed a unified model using machine learning (ML), which showed better predictive performance than the individual models for HClO (RMSE_test: 2.1 to 2.04), O₃ (2.06 to 1.94), ClO₂ (1.77 to 1.49), and SO₄^•- (0.75 to 0.70) because the model "corrected" the wrongly learned effects of several atom groups. We further developed knowledge transfer models for three pairs of the data sets and observed different predictive performances: improved for O₃ (RMSE_test: 2.06 to 2.01)/HClO (2.10 to 1.98), mixed for O₃ (2.06 to 2.01)/ClO₂ (1.77 to 1.95), and unchanged for ClO₂ (1.77 to 1.77)/HClO (2.1 to 2.1). The effectiveness of the latter approach depended on whether there was consistent knowledge shared between the data sets and on the performance of the individual models. We also compared our approaches with multitask learning and image-based transfer learning and found that our approaches consistently improved the predictive performance for all data sets while the other two did not. This study demonstrated the effectiveness of combining small, similar data sets and transferring knowledge between them to improve ML model performance.

Comparative evaluation of 2-isopropyl-3-methoxypyrazine, 2-isobutyl-3-methoxypyrazine, and 2,4,6-trichloroanisole degradation by ultraviolet/chlorine and ultraviolet/hydrogen peroxide processes

2-Isopropyl-3-methoxypyrazine (IPMP), 2-Isobutyl-3-methoxypyrazine (IBMP), and 2,4,6-Trichloroanisole (TCA) are the primary emerging taste and odor (T&O) compounds in water systems with low thresholds (ng L^-1). The selected T&O compounds are known to be difficult to remove using conventional water treatment processes. In this study, we compared the removal characteristics of the three T&O compounds using UV/Cl₂ and UV/H₂O₂. The removal rates of the three compounds by direct photolysis at 254 nm were less than 10%, even at a high UV dose (approximately 1000 mJ cm^-2). Under conditions of an oxidant injection volume of 5 mg L^-1 and UV dose of 1000 mJ cm^-2, the degradation rate of the target compounds in the UV/H₂O₂ process exceeded that of the UV/Cl₂ process. Moreover, the results revealed that pH has a significant impact on the removal of the T&O compounds during the UV/Cl₂ process. The IPMP, IBMP, and TCA were found to be more reactive with hydroxyl radicals than reactive chlorine species (RCS). A predictive tool was developed to determine the optimal operating condition using the generalized reduced gradient (GRG) nonlinear solver. In the UV/H₂O₂ process, the EED value for 90% removing rate was 0.156 kWh m^-3 for the IPMP, 0.135 kWh m^-3 for the IBMP, and 0.154 kWh m^-3 for the TCA, respectively. In case of the UV/Cl₂, the EED value for 50% removing rate was 0.174 kWh m^-3 for the IPMP, 0.138 kWh m^-3 for the IBMP, and 0.169 kWh m^-3 for the TCA, respectively.

Occurrence of unknown reactive species in UV/H2O2 system leading to false interpretation of hydroxyl radical probe reactions

The UV/H₂O₂ process is a benchmark advanced oxidation process (AOP) that in situ generates highly reactive and nonselective hydroxyl radical (^•OH) to oxidatively destroy a wide range of organic compounds. Accurately quantifying the concentration of short-lived ^•OH is essential to predict process performance, optimize the operation parameters, and compare with other process options. The ^•OH concentration is typically measured using organic probe molecules that react with ^•OH but not with other oxidants. In the extremely well-characterized UV/H₂O₂ system in which ^•OH is proven to be the dominant oxidant, using photolysis-resistant probes such as benzoic acid and its derivatives is a widely agreed and practiced norm. We herein report that certain ^•OH probe compounds can be degraded in UV/H₂O₂ system by unknown reactive species that has not been reported in the past. Several common organic probes, particularly p-substituted benzoic acid compounds (i.e., p-hydroxybenzoic acid, p-chlorobenzoic acid, and p-phthalic acid), were found to be vulnerable to attack by the unknown reactive species, leading to false quantification of ^•OH concentration under high radical scavenging conditions. Lines of evidence obtained from a series of ^•OH scavenging experiments performed under various conditions (i.e., different concentrations of H₂O₂, ^•OH probe compounds, and dissolved oxygen) point toward excited state H₂O₂. The results from this study suggest the importance of using appropriate ^•OH probe compounds in mechanistic studies and needs for considering the unidentified role of excited state of H₂O₂ on the UV/H₂O₂ process and related AOPs.

Hydroxyl radical scavenging factor measurement using a fluorescence excitation-emission matrix and parallel factor analysis in ultraviolet advanced oxidation processes

The performance of the UV/H₂O₂ advanced oxidation process (AOP) is dependent on water quality parameters, including the UV absorbance coefficient at 254 nm and hydroxyl radical (•OH) water background demand (scavenging factor, s^-1). The •OH scavenging factor represents the •OH scavenging rate of the background substances in the water matrix, and it is known to be one of the key parameters to predict the performance of the UV/H₂O₂ process. The •OH scavenging factor has been determined experimentally by using a probe compound such as pCBA and rhodamine B. The experimental method has been validated to accurately predict the micropollutants removal in the UV/H₂O₂ process, but there is a need for an easier and simple method of determining the OH scavenging factor. We evaluated the alternative method to analyze the •OH scavenging factor using fluorescence excitation-emission matrix and parallel factor analysis (F-EEM/PARAFAC). The correlation between •OH scavenging factor and the spectroscopic characteristics and structure of different organic matter types was evaluated. Organic matter was characterized using a fluorescence excitation-emission matrix, parallel factor analysis, and liquid chromatography-organic carbon detection. Second-order reaction rates of humic acid sodium salt, sodium alginate, Suwannee River humic acid and bovine serum albumin were calculated as 1.30 × 10⁸ M^-1 s^-1, 1.39 × 10⁸ M^-1 s^-1, 1.03 × 10⁸ M^-1 s^-1, and 3.17 × 10⁷ M^-1 s^-1, respectively. Results of PARAFAC analysis, the ratio of humic and fulvic fluorescence component 2 to terrestrial humic-like fluorescence component 1 (C2/C1), and •OH scavenging factor showed high linearity. A predictive model, which combines with the F-EEM/PARAFAC method, predicted the optimal UV and H₂O₂ dose to achieve target compound removal.

Novel Visible Light-Driven Photocatalytic Chlorine Activation Process for Carbamazepine Degradation in Drinking Water

Photolysis of free chlorine (HOCl/ClO^-) is an advanced oxidation process (AOP) to produce hydroxyl (HO^•) and other radicals for refractory micropollutant degradation. However, HOCl/ClO^- is only conducive to activation and production of radicals by ultraviolet (UV) light. For the first time, we show the use of visible light (>400 nm) to produce HO^• and ClO^•, through use of graphitic carbon nitride (g-C₃N₄) and photogenerated h_vb⁺, e_cb^-, and O₂^•- in the presence of HOCl/ClO^-, which was termed visible light g-C₃N₄-enabled chlorine AOP (VgC-AOP). The VgC-AOP increased the pseudo first-order degradation rate constant of a model micropollutant, carbamazepine, by 16 and 7 times higher than that without g-C₃N₄ and HOCl/ClO^-, respectively, and remained active over multiple use cycles. Effects of water quality [pH, alkalinity, Cu(II), and natural organic matter (NOM)] and the operational conditions (g-C₃N₄ and HOCl/ClO^- concentrations, irradiation wavelength, and dose) were investigated. Of particular significance is its superior performance in the presence of NOM, which absorbs less light at visible light wavelengths and scavenges less surface-bonded reactive species, compared against UV/TiO₂ or UV/chlorine AOPs. The VgC-AOP is practically relevant, feasible, and easily implementable and it expands the potential types of light sources (e.g., LEDs and solar light).

Degradation of contaminants of emerging concern by UV/H2O2 for water reuse: Kinetics, mechanisms, and cytotoxicity analysis

Advanced oxidation using UV and hydrogen peroxide (UV/H₂O₂) has been widely applied to degrade contaminants of emerging concern (CECs) in wastewater for water reuse. This study investigated the degradation kinetics of mixed CECs by UV/H₂O₂ under variable H₂O₂ doses, including bisphenol A, estrone, diclofenac, ibuprofen, and triclosan. Reverse osmosis (RO) treated water samples from Orange County Water District's Groundwater Replenishment System (GWRS) potable reuse project were collected on different dates and utilized as reaction matrices with spiked additions of chemicals (CECs and H₂O₂) to assess the application of UV/H₂O₂. Possible degradation pathways of selected CECs were proposed based on high resolution mass spectrometry identification of transformation products (TPs). Toxicity assessments included cytotoxicity, aryl hydrocarbon receptor-binding activity, and estrogen receptor-binding activity, in order to evaluate potential environmental impacts resulting from CEC degradation by UV/H₂O₂. Cytotoxicity and estrogenic activity were significantly reduced during the degradation of mixed CECs in Milli-Q water by UV/H₂O₂ with high UV fluence (3200 mJ cm^-2). However, in GWRS RO-treated water samples collected in April 2017, the cytotoxicity and estrogen activity of spiked CEC-mixture after UV/H₂O₂ treatment were not significantly eliminated; this might be due to the high concentration of target CEC and their TPs, which was possibly affected by the varied quality of the secondary treatment influent at this facility such as sewer-shed and wastewater discharges. This study aimed to provide insight on the impacts of post-UV/H₂O₂ CECs and TPs on human and ecological health at cellular level.