1. Introduction
Municipal wastewater (MWW) is the second limitless source of water [
1]. MWW mostly contains water (99.9%) with relatively small concentrations of suspended and dissolved organic and inorganic solids [
2]. MWWs usually contain around 5% to 10% settleable suspended solids. They also contain approximately 1000 parts per million of dissolved and colloidal solids, most of which are organic in nature and usually difficult to remove with biological treatment [
3]. The following organic compounds are present in MWW: carbohydrates, synthetic detergents, fats, proteins, lignin, soaps and their decomposition products, various natural and synthetic organic chemicals from the process industries [
2]. Some of these compounds pose serious problems in biological treatment systems due to their resistance to biodegradation and/or toxic effects on microbial processes [
4,
5]. Municipal wastewater also comprises different inorganic substances from domestic and industrial sources, including several potentially toxic elements such as arsenic, cadmium, lead, mercury, etc. In addition, pathogenic viruses, bacteria, protozoa and helminths may be present in raw municipal wastewater and survive in the environment for long periods [
6].
Recently, the effluents from municipal wastewater treatment plants (WWTPs) have been identified as a major source of emerging micropollutants, such as hormones, pharmaceuticals, and personal care products [
7]. Despite their low concentration (from a few ng/L to several μg/L), they are resistant to biodegradation, since conventional WWTPs cannot provide a high rate of removal of micropollutants [
8]. Strategies for removing these compounds from MWW are currently being discussed [
9].
Advanced Oxidation Processes (AOPs) have been widely applied for the treatment of municipal wastewater [
4]. AOPs using highly reactive hydroxyl radical (OH∙) as the main oxidant were first proposed for the treatment of drinking water in the 1980s. Then AOPs were widely used to treat various types of wastewater since strong oxidants can easily degrade persistent organic contaminants and remove inorganic pollutants from wastewater [
10].
In recent years, a new type of oxidizing agent has attracted the interest of researchers. S
2O
82− is a strong oxidizing agent with a standard oxidation potential (E
o) of 2.01 V [
11]. It can form more powerful sulfate radicals (SO
4••−, E
o = 2.6 V) after activation by heat, ultraviolet (UV) irradiation (Equation (1)), transition metals (Equation (2)), or elevated pH, and further initiate sulfate radical-based AOPs [
12,
13]. The mechanisms of activation of persulfate with elevated pH are still unclear [
14]. The temperature for the thermal activation of persulfate ranges from 35 to 130 °C [
14].
As seen in Equations (1) and (2), the metal activation method gives only a 50% yield of sulfate radicals, which is obtained by heating or UV-activated persulfate method with the same molar concentration of persulfate. Consequently, the metal activation method is theoretically ineffective. The most commonly used metals include ferrous (Fe (II)) and ferric (Fe (III)) ions [
15].
Sulfate radicals, like hydroxyl radicals, are highly reactive species with a short lifetime, although both radicals have different reaction patterns. Hydroxyl radicals have a tendency to attach to C=C bonds or remove H from C–H bonds during their reactions with organic substances [
16], while sulfate radicals tend to remove electrons from organic molecules, which are consequently converted into organic radical cations [
10]. In addition, hydroxyl radicals can be generated from sulfate radicals via Equations (3)–(5) [
17,
18,
19].
Moreover, Equation (5) demonstrates that more hydroxyl radicals can be produced from sulfate radicals at alkaline conditions [
10]. Unlike hydrogen peroxide (H
2O
2), persulfate can also oxidize some organic substances directly, without the participation of radical species [
20].
Potassium persulfate (KPS) (K
2S
2O
8) has good stability at room temperature, it is inexpensive, and solid at ambient temperatures, making it easy to store and transport [
21]. The technical benefits of persulfate-AOP over H
2O
2-AOP include: (i) lower storage and transportation costs as a result of the availability of persulfate salts, (ii) higher achievable yields of radical formation, (iii) less dependence of the treatment efficiency on operating parameters, such as pH, initial peroxide loading, background constituents, and (iv) a wider range of available persulfate activation methods [
20].
Velo-Gala et al. [
19] studied the effectiveness of oxidation processes based on the application of UV radiation, UV/H
2O
2, and UV/K
2S
2O
8, for the degradation of sodium diatrizoate in the aqueous medium. The UV/K
2S
2O
8 process was found to be more efficient than the UV/H
2O
2 system, with higher rate constants [
19].
Yang et al. [
22] investigated sulfate-based oxidation technologies for the defluorination of aqueous perfluorooctanesulfonate (PFOS). The defluorination efficiency of PFOS with different treatments corresponded to the following order: HT (hydrothermal)/K
2S
2O
8 > UV/K
2S
2O
8 > Fe
2+/K
2S
2O
8 > US (ultrasound)/K
2S
2O
8. The increase in the persulfate amount had a positive effect on the defluorination of PFOS [
22].
Dbira et al. [
23] found that the photo-Fenton process was more efficient for the tannic acid degradation in aqueous solution than the UV/persulfate system, concluding that hydroxyl radicals were stronger oxidizing agents than sulfate radicals.
Since iron-based heterogeneous catalysts activated with persulfate have received a lot of attention as a potentially advanced and sustainable water treatment system, Pervez et al. [
24] employed a novel Fe
3O
4 impregnated graphene oxide (Fe3O4@GO)-activated persulfate (Fe
3O
4@GO+K
2S
2O
8) system for the efficient degradation of dye pollutants in real wastewater treatment.
Persulfate-based oxidation technologies were recently classified as realistic for full-scale application based on energy consumption values [
25]. However, most of works have been conducted using synthetic wastewater or aqueous solutions [
19,
22,
23], although real wastewaters include several inorganic (chlorides, bicarbonates, carbonates) and organic constituents that may have a scavenging effect on oxidative radicals [
26]. Therefore, it is essential to examine the efficiency of the process in the case of real wastewaters.
The present study focused on the effectiveness of AOPs based on the application of iron salts, potassium persulfate, and UV radiation to treat real municipal wastewater. There are no published studies applying Fe2+/S2O82−/UV process to treat wastewaters. Response surface methodology (RSM) and artificial neural network (ANN) were used to optimize the photo-Fenton-like treatment of the MWW.
4. Conclusions
Response surface methodology and artificial neural network have been utilized for the optimization of the photochemical treatment of municipal wastewater using the photo-Fenton-like process. According to the RSM, the optimum conditions for the highest TOC removal (100% both experimentally and statistically) were achieved at the reaction time of 106.06 min, pH of 7.7, persulfate concentration of 30 mM, and K2S2O8/Fe2+ molar ratio of 7.5. According to the ANN results, the optimum conditions for the highest TOC removal (100% experimentally and 98.7 theoretically) were obtained at the reaction time of 104.93 min, pH of 7.7, persulfate concentration of 30 mM, and K2S2O8/Fe2+ molar ratio of 9.57. Both RSM and ANN models inaccurately predicted optimum conditions for TC and TN removal. Thus, the conditions with the highest removal of TC (71.31%) and TN (60.13%) were observed at the reaction time of 140 min (for TC) and 100 min (for TN), pH of 5.35 (for both TC and TN), persulfate concentration of 30 mM (for TC) and 10 mM (for TN), K2S2O8/Fe2+ molar ratio of 10 (for TC) and 12.5 (for TN). ANOVA was able to indicate the significant parameters for the photo-Fenton-like process. Factors such as reaction time, pH, K2S2O8, interactions between reaction time and pH, pH and K2S2O8, and K2S2O8 and K2S2O8/Fe2+ molar ratio had a significant impact on TOC removal.