Investigating Peroxymonosulfate-UV Advanced Oxidation Process Efficiency for Treated Municipal Wastewater Disinfection and Effluent Quality Recovery

Introduction
Eliminating or deactivating the microbial pathogens in wastewater treatment plants effluent has been of particular interest to experts in order to reach the standard quality required. In the meantime, it seems obvious – due to a lack of water resources – that reusing the treated wastewater is a necessity, along with employing and developing modern, efficient solutions. In this regard, the disinfected effluent of wastewater treatment plants has raised interest in various applications containing agriculture, green space irrigation, aquifer feeding, industry, and other municipal activities. In the recent couple of decades, it can be seen that an ever-increasing attention has been paid to the use of ultraviolet (UV) radiation as an efficient method in wastewater treatment plants. However, the results of investigations and existing experiences indicate the negative impact of parameters such as turbidity, hardness, and suspended solids in wastewater on account of the diffusion of radiant energy, as well as obstructing the influence of direct radiation on microorganisms and, in turn, the performance of the UV radiation method in a disinfection process; thus, the application of modern, efficient technologies is requisite for the performance improvement of this process under different operating conditions. Due to their capability of removing toxic, resistant and non-biodegradable compounds, Advanced Oxidation Processes (AOPs) may be adopted as a suitable approach to reach the abovementioned goal. The ultraviolet radiation in conjunction with the effective radicals is considered efficient UV-based AOPs in urban wastewater treatment plants. Not only do these methods lead to reducing the probability of forming toxic and hazardous by-products, they also produce radicals that are highly reactive and react with organic compounds in a non-selective manner. AOPs can eliminate a vast variety of biological and chemical substances, as well as significantly decrease organic and inorganic pollutants by producing highly reactive free radicals, e.g., hydroxyls (OH˚) and sulfate radicals (SO4-˚). Suitable for deactivating microorganisms, these radicals decompose an extensive spectrum of resistant compounds as well. Hydroxyl-based advanced oxidation processes (HR-AOPs) have been comprehensively studied. It should be noted that the sulfate radical has higher reactivity and oxidation potential compared to the oxidizing agent of the hydroxyl radical. As a substitute for HR-AOPs, Sulfate-based Advanced Oxidation Processes (SR-AOPs) have been the focus of many researchers in recent years. This process encompasses the use of chemical oxidants such as Peroxymonosulfate (PMS), which is environmentally friendly and easily activated. The research results in recent years unanimously reveal the high efficiency of AOPs in substantial eliminations of the microbial community within the specimens tested. In the current research, the efficacy of a UV/PMS AOP, which is able to produce both sulfate and hydroxyl radicals, was investigated so as to remove the total amount of coliforms from the effluent of municipal wastewater treatment plants. In order to identify the relative advantages of this method, the obtained results were compared to those of persulfate-UV, hydrogen peroxide-UV, and UV alone methods. In addition, the effect of removing the parameters related to the quality characteristics of wastewater treatment plant’s effluent, comprising turbidity, chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total nitrogen (TN), electrical conductivity (EC), total dissolved solids (TDS) and total suspended solids (TSS), was investigated under optimal operating conditions of laboratory reactors.
Materials and Methods
In this work, the treated wastewater effluent samples were collected daily near the endpoint before chlorination from an activated sludge municipal WWTP located in northeast Tehran. All the experiments were performed by a 250-cc cylindrical reactor and a UV-C lamp model UV-6W. To measure the effluent's quality parameter, we used methods from the standard methods for the examination of wastewater.
Discussion of Results
In this section, the processes of UV/PMS, UV/PS, UV/H2O2, and UV alone have been investigated on the removal of the total coliform, the results achieved as Figures 1 to 3. It was determined that increasing the production of hydroxyl and sulfate radicals during the experiments leads to an increase in the removal of total coliform. We have evaluated the optimum operational conditions for each process. In this way, 400 MPN in 100 ml was considered the allowable limit for the total coliforms to successfully disinfection according to the wastewater reuse standards for agriculture purposes. Regarding the importance of reducing the qualitative parameters of effluent, in this study, the most important parameters such as COD, BOD5, TSS, TDS, TN, TP, EC, and turbidity have been studied under optimum operational conditions of each disinfection method.
In comparison, the UV-radiation alone showed the limited capability to achieve coliforms reduction requirements. As shown in Figure 1, the hydroxyl and sulfate radicals are both produced. The highest removal efficiency is by a PMS dosage of 0.09 mmol/L in a reaction time of 30 min. It should be noted, that the optimum operational conditions for the reactor to achieve the allowable limit of maximum 400 coliforms in 100 ml in a PMS dosage of 0.06 mmol/L, can be obtained by achieving log (MPN) = 2.45 (e.g., 285 MPN/100ml) in a reaction time of 20 minutes, whereas in UV/PS and UV/H2O2 the optimum operating conditions were in the dosage of 3 and 0.35 mmol/L at the time of 30 and 25 minutes, respectively. Therefore, The UV/PMS process showed the best capability to achieve coliforms reduction requirements and efficient disinfection of wastewater treatment plants effluent.
As the results are shown in Table 1, the UV/PMS is the most efficient method to earn the best reduction of effluents' quality parameters. In comparison between these four processes under the optimum operational conditions, these results are achievable that the UV/PMS process had a considerable amount in the removal of turbidity and TSS. Also, it showed an appropriate efficiency in other parameters like COD and BOD5. But this method is still incapable of removing existing TN and TP concentrations.
Conclusion
Wastewater treatment processes with the aim of reuse and reclamation have been investigated and improved during the last decades. Also, efficient disinfection of wastewater effluent before discharge is counted as the essential requirement of reuse. In this regard, employing novel disinfection processes is vital and followed by researchers. In this study, a new approach for improving the UV-based disinfection process for wastewater effluent was investigated. The peroxymonosulfate-ultraviolet was considered as the primary method, and because the obtained results indicate the capability of this method, it was compared with persulfate-ultraviolet, hydrogen peroxide-ultraviolet and conventional UV-radiation methods. In terms of disinfection process improvement, the results demonstrate that employing the peroxymonosulfate-ultraviolet process is suitable for achieving a more reliable solution accordingly. It was also found that this method has the ability to effectively reduce the effluent's quality parameters, in addition to disinfection. It should be noted that using the peroxymonosulfate-ultraviolet method in wastewater treatment plants requires complementary studies regarding possible by-products of the process in a semi-industrial scale which are recommended to be considered in future research.