1. Introduction
In urban cities, wastewater treatment plants (WWTPs) are essential to remediate water and give it a second life. For this reason, sewer systems are utilized globally to collect sewage, contribute to pollution control, and improve human health. However, their technology is not strong enough to remove all the pathogens from the water, so many of them are released into the environment. Although this is done in low concentrations, it is still enough to cause infections [
1]. Nowadays, disinfection is an essential and significant process for water treatment to protect the environment and human beings from pathogenic infections [
2]. There are a wide variety of techniques and methods, such as UV, chlorination, radiation, or coagulation. However, some of these technologies have drawbacks because, during the disinfection process, some carcinogenic disinfection by-products are released and some pathogens are resistant [
3,
4].
In recent days, advanced oxidation processes (AOPs), which are mainly based on the formation of highly oxidative species (mainly HO
• or SO
4•−), have attracted great interest as disinfection treatments because they have proved to be effective for bacteria inactivation [
5,
6]. Furthermore, they are environmentally friendly and can non-selectively destroy most organic and organometallic contaminants until their complete mineralization; that is, their conversion into CO
2, H
2O, and inorganic species [
7,
8,
9]. Furthermore, the inactivation of microorganisms’ pathogens is achieved through the membrane, proteins, lipids, enzymes, DNA, and RNA damage [
4].
In this study, the attention focuses on two AOPs, the Fenton process (FP) based on the traditional Fenton reaction where hydroxyl radicals are produced, and Fenton-like processes (FLP) in which the generation of free radicals as sulfate radicals are proposed as an alternative to hydroxyl radicals [
10]. The FP is based on the hydrogen peroxide decomposition with the presence of Fe
+2 to the formation of radicals through equation 1. The organic matter could be oxidized by hydrogen abstraction or by hydroxyl addition of those hydroxyl radicals [
11]. Apart from the oxidant and the catalyst, there is another variable in the process: the pH of the reaction, which will influence the process. Low pH (around 3) is necessary to accomplish the treatment.
In recent years, several concentration ranges of oxidants have been tested for FP; however, a high concentration (>20 mM) of H
2O
2 is necessary in order to react with the cell membrane directly to increase its permeability and damage all the macromolecules [
4,
12]. In addition, most of the studies were performed using simple matrices (distilled water), without the presence of compounds present in the real wastewater [
13,
14] which can reduce the efficiency of the disinfection interfering as a scavenger to the radicals formed. Thus, there is a need to increase the knowledge related to the disinfection under conditions similar to real applications.
On the other hand, in the FLP sulfate, radicals can be generated by activating persulfate or peroxymonosulfate (PMS) using UV, heat, transition metals, and an alkaline medium (Equations (2) and (3)) [
15]. The FLP has shown a notable efficacy in applications such as water treatment, where sulfate radicals mainly react via electron transfer with the pollutants [
16,
17]. Furthermore, hydroxyl radicals can be generated during this treatment (Equation (4)), and no pH adjustment is necessary because the presence of sulfate radicals reduces the pH in the solution (Equation (5)) [
18]. The targeted persulfate or PMS concentration usually varies between 0 and 10 mM [
19], and the selection of the targeted concentration depends on the composition of wastewater. Thus, although elevated disinfection can be achieved by a low concentration of 0.1 mM using distilled water as the water matrix [
20], a concentration of 10 mM is necessary when using complexing matrices, as reported by Rodriguez-Chueca et al. [
21] in the disinfection of winery wastewater.
The inactivation processes by sulfate radicals display potential over traditional disinfection methods that form dangerous disinfection by-products [
19]. Nevertheless, different knowledge gaps were presently identified in the inactivation of pathogenic microorganisms by sulfate radicals. Thus, further investigation into the influence of operational factors, such as the dosage of disinfectants, catalyst effect, and treatment time for a comprehensive evaluation of the technology, is needed.
Based on the aforementioned approaches to disinfection research, this study focuses on the optimization using simulated wastewater of the different variables involving the processes: precursor concentration (PMS and H2O2), catalyst concentration (Fe+2), and pH in the Fenton process. To achieve this, the central composite design (CCD) is used for designing the steps of the study and response surface methodology (RSM) is used for the modelling and optimization of the disinfection by the FP and FLP of E. coli in simulated wastewater. After that, the optimized conditions are applied to real wastewater matrices.
4. Conclusions
In this study, RSM was utilized to model and optimize the disinfection of E. coli process by the FP and the FLP. The obtained models using simulated wastewater were significant and fitted well with the experimental data according to the high values of the coefficient of determination. The quadratic function relationship between the disinfection and the three influencing factors for the FP in E. coli disinfection determined that the studied factors were significant, and their influence was followed [H2O2] > [Fe+2] > pH. For the FLP, only [PMS] was reported as significant in the studied space, despite a significant response being obtained by the studied factors, PMS, and Fe dosages. Finally, under optimal conditions, the disinfection efficiency was validated in real wastewater for a WWTP, which demonstrates the viability for the use of these AOP for microbial removal. The present study enhances the knowledge about the use of these technologies, scarcely studied in real matrices, and additionally demonstrates that the optimization of the dosage of oxidants achieved encouraged results similar to the use of combined processes. Therefore, the present study facilitates the practical application of the AOP process in WWTPs.