Critical Review
In this review, I will discuss the article “Removal of Micropollutants by an Electrochemically driven UV/chlorine (E-UV/Cl2) Processfor Decentralized Water Treatment.” The objective of this article was to evaluate the feasibility and efficiency of an E-UV/Cl2 that combines UV irradiation with in situ electrochemical production of Cl2 for micropollutants removal under conditions simulating decentralized water treatment (Zhang et al., 2020). During the E-UV/CL2 process, the initial chloride in water is electrochemically oxidized to free chlorine, and then the UV photolysis of free chlorine in the water samples leads to formation of hydroxyl radicals and reactive chlorine species (RCS), which can oxidize UV- and/or chlorine-resistant micropollutants. In this study, four micropollutants were spiked into a sampled surface water. Then, the performance of E-UV/Cl2 was compared with difference processes, which include UV-irradiation, electrolysis, and conventional UV/Cl2 process, by conducting a series of experiments. This study found out that the E-UV/Cl2 process can eliminate all micropollutants spiked in a surface water sample within 5 minutes.The authors concluded that the E-UV/Cl2 process could effectively abated micropollutants in short treatment time with acceptable energy consumption and chlorinated-by product formation.
The objectives of this article were clearly addressed and supported by the experimental data and results obtained in this study. I think that this study demonstrates a feasible and cost-efficient way to removal micropollutants in decentralized water treatment systems, so it is significant to advancing the field of environmental engineering. The water sources, such as surface water,greywater, and municipal wastewater effluent, in decentralized water treatment often contain micropollutants, which can cause human health impacts and soil/groundwater contamination (Turner et al., 2019).Conventional advanced oxidation process (e.g. UV/Cl2, O3/H2O2and UV/H2O2) has been widely used to remove micropollutants in full-scaled centralized water system, but it requires transport and storage of chlorine stock solution; therefore, it was not suitable to apply in decentralized system (Barazesh et al., 2015). In contrast, E-UV/Cl2 could avoid chemical management and improve treatment performancewith acceptable cost making it more attractive for decentralized system (Chaplin, 2019). Because decentralized water treatment offers many benefitsfor rural communities or undeveloped countries, I believe that this study provides a promisingoption to relieve some of the environmental challenges.
Although this article demonstrates that E-UV/Cl2 provides a competitive technology for micropollutants abatement, I think that the authors couldimprove the quality of their work by providing a better correlation between chloride concentration and treatment performance.The surface water sample used in this study was from a lake in China, and the initial chloride concentrationof water samples was5.5 mg/L, which was considered as too low to facilitate theE-UV/Cl2 process by the authors. The authors added NaCl into the water samples to adjust chloride concentrations to 30-300 mg/L for all the experiment (Guo et al., 2018). I think that the authors could point out the importance of adding NaCl into water that has low chloride concentration since the performance of E-UV/Cl2 process is highly depending on the chloride concentration in water.Without sufficient amount of chloride, the process could not form enough hydroxyl radicals and RCS for micropollutants abatement. Decentralized water treatment systems could be used to treat various water sources, and some water sources (e.g. surface water and groundwater) do not always have a high chloride concentration as desired for E-UV/Cl2 process. Therefore, I think that it is important to figure out the performance of E-UV/Cl2 processunder low chloride concentration condition. The authors compare the micropollutant (carbamazepine) abatement at 30, 100, 300 mg/L chloride concentrations, and show the micropollutant removal efficiency decreases with decreasingchloride concentrations. I recommend that the authors could repeat the experiment to find out the performance of E-UV/Cl2 processat 5.5 mg/L chloride concentration, which is the original concentration in water samples, and this could be a reference chloride level in this study.If the removal efficiency is not satisfying at 5.5 mg/L chloride concentration, the authors could consider testing different conditions to explore the minimum chloride concentration for E-UV/Cl2 process to achieve the desired abatement efficiency. Even thougha higher chloride concentration could shorten the micropollutants abatement time and enhance energy efficiency during the E-UV/Cl2 process, I think that it is important to control the chloride concentration because it can leads to formation of undesirable disinfection by-products, such as chlorate, trihalomethanes and haloacetic acids(Bagastyo, 2012). Therefore, it is also significant to determine the optimal amount of NaCl needed to add into the water to minimize the formation of by-products if treating water that contains a low chloride concentration in decentralized water systems.
Another point that I think the authors strengthen this article is to explain the selection of micropollutants used in the experiment in depth. The authors spiked a surface water with four micropollutants, including metoprolol, carbamazepine, ciprofloxacin, and trimethoprim, that have varying UV and chlorine reactivities.The reason of picking these four organic pollutants was very briefly explained in this article. From my point of view as a reader, I feel a little bit unclear about why these four micropollutants are suitable to represent the E-UV/Cl2 process’s performance on micropollutants abatement. Hundreds of micropollutants are commonly found in wastewater effluent and surface water, and they are usually from pharmaceuticals, personal care products and pesticides (Das, et al., 2017). I recommend the authors consider giving a more thorough explanation on the selection of concerned micropollutants used in this study and how do they broadly cover the variety of micropollutants in general on the section of Materials and Methods. Moreover, the authors could improve the credibility of this study by repeating this experiment on other micropollutants that are resistance to thecurrently commonly used treatment processes in the decentralized water treatment process, if possible. I believe that it is significant to reveal the selection of micropollutants in order to sufficiently demonstrate the performance of the E-UV/Cl2 process for all of the common occurring micropollutants abatement not just restricting to certain micropollutants.
Lastly, the authors could make some minor revisions on improving the consistency throughout the article. In section 3.2.2, the authors refer to a reactive chlorine specie,ClO2‾•, for several times, but I presume that the authors are actually referring to Cl2‾• instead of ClO2‾•based on the reference to the context. Moreover, the authors reported that all spiked micropollutants were eliminated to below their detection limits within 5 min of the E-UV/Cl2 process in the beginning of the article. But in the later section, the authors noted that about 90% of micropollutants was abated by the E-UV/Cl2 process in 5 minutes. It is important to using the precise wording/phrasing and report consistent results because it can help the readers understand the article better.
From my opinion, this article is worthwhile for publication because it shows the performance of electrochemical driven UV/Cl2 process on micropollutant abatement with strong arguments and results, and the overall experimental set up and discussions are precise and reliable. However,further discussion on relationship between chloride concentration and E-UV/Cl2 process performance and explanation on selection of the four micropollutants are recommended to advance the significance of this research.
