# Treatment of Distillery Industrial Wastewater Using Ozone Assisted Fenton’s Process: Color and Chemical Oxygen Demand Removal with Electrical Energy per Order Evaluation

Authors: Perumal Asaithambi; Mamuye Busier Yesuf; Seifu Kebede Debela; Rajendran Govindarajan; N. M. Hariharan; Esayas Alemayehu
Journal: International Journal of Chemical Engineering
 (2022)

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## Abstract

Ozonation is one of the most effective and efficient advanced oxidation processes (AOPs) and has shown great potential in the treatment of industrial effluent and wastewater. In the present work, the ozone-Fenton process for % COD and color removal together with electrical energy per order (EE/O) determination for distillery industrial wastewater (DIW) was established. The process was developed by combining the ozone (O3) with the Fenton (Fe2+/H2O2) process. The ozone-Fenton (O3/Fe2+/H2O2) was compared with other treatment processes such as O3, Fe2+, H2O2, O3/Fe2+, O3/H2O2, and Fe2+/H2O2 for EE/O together with % COD and color removal efficiency for DIW. The removal of color at 100% and chemical oxygen demand (COD) of 96.875% were achieved with a minimum of EE/O of 0.5315 kWh/m3 using the O3/Fe2+/H2O2 process by operating at optimum conditions. The % COD and color values obtained using O3/Fe2+/H2O2 were significantly higher than those obtained using O3, Fe2+, H2O2, O3/Fe2+, O3/H2O2, and Fe2+/H2O2 processes. The % color, % COD removal, and its associated EE/O were evaluated by varying Fe2+, H2O2, O3 inlet and COD concentration, and initial wastewater pH using the O3/Fe2+/H2O2 process. The synergy effect of the O3 and Fe2+/H2O2 processes was evaluated and reported. Our experimental findings suggest that combining O3 with the Fe2+/H2O2 process could effectively treat industrial effluent and wastewater.


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## Body

## 1. Introduction

Nowadays, the ozonation (O3) process is commonly used to treat water, wastewater, and industrial effluents in a variety of applications, including disinfection of swimming pool [1], drinking [2], domestic [3], distilleries [4, 5], fermentation [6], dyeing [7], slaughterhouse [8], laundry [9], municipal [10], pharmaceutical [11], synthetic textile [12], RB 5 dye [13], laboratory [14], and petrochemical [15]. The use of O3 in wastewater treatment to disinfect, deodorize, decolorize, and oxidize is becoming essential, particularly when a high degree of treatment is required [6, 16]. It has high efficiency of pollutant removal, an absence of secondary contamination, and a short residence time for wastewater treatment. In comparison to other methods of wastewater treatment such as physical, chemical, biological process, electrochemical [17, 18], and advanced oxidation processes (AOPs) [18, 19], the O3 process has advantages such as high oxidizing ability, nonselective simple reaction conditions, without the need for high temperature or pressure [20], etc.Ozone has a high oxidation potential and produces the hydroxyl radical (•OH), a highly reactive oxidative species [21, 22]. While O3 has high oxidizing power, its conversion to •OH is inefficient, and thus O3 alone has a lower capacity for pollutant removal than •OH [23]. Consequently, in recent years, new O3 hybrid processes based on AOPs and electrochemical processes have been developed, including catalytic, nanocatalyzed, photocatalytic, and sonolytic ozonation, O3/UV, O3/H2O2, O3/Fenton, O3/electrochemical, and O3/UV/H2O2 [21, 24]. The addition of Fe2+ and H2O2 to O3, referred to as the O3/Fe2+/H2O2 process [25], improves the decolorization/degradation/mineralization of organic/inorganic matter present in effluent and wastewater [26, 27]. The O3/Fe2+/H2O2 process produced additional •OH radicals via the reaction of O3 with Fe2+/H2O2, and the reactions are described elsewhere [28, 29].Owing to the synergic effect, the combination of O3 with Fe2+/H2O2 reagent exhibits a high oxidation rate. The O3/Fe2+/H2O2 process produces excessive •OH via the reaction of O3 and Fe2+/H2O2, resulting in increased pollutant removal at a reduced treatment time. The O3/Fe2+/H2O2 has been used to treat water present in various types of wastewater, such as urban [30] and landfill leachate [31]. A comparison showed that O3 and Fe2+/H2O2 processes were both less effective than the O3/Fe2+/H2O2 combination [32].Feng et al. examined the degradation of spent resin using the O3–Fe2+/H2O2 and Fe2+/H2O2 processes and concluded that both processes run according to first-order kinetic equations, and the O3–Fe2+/H2O2 process had the maximum removal competence and cost savings [32]. Li et al. compared the Fe2+/H2O2 and O3/Fe2+/H2O2 processes for amoxicillin degradation. The COD–65% removal rate was achieved in the O3–Fe2+/H2O2 process as opposed to the Fe2+/H2O2 process. The synergistic effects worked in harmony with each other, thereby giving the O3/Fe2+/H2O2 process the edge [33]. Goi et al. evaluated individual processes such as Fe2+/H2O2, O3, and the combination of these treatment processes using leachate waste. They discovered that the coupled Fe2+/H2O2 and O3 processes eliminated the most COD (77 %) as compared to other approaches [34].Substantial research has concentrated on the elimination of pollutants from simulated wastewater using O3 and AOPs, with only a few studies using real industrial effluent and wastewater. Furthermore, prior research has concentrated on the efficiency of hybrid O3-based AOPs in terms of % COD and color removal (%) but has not put an emphasis on pollutant elimination together with electrical energy per order (EE/O). It was crucial to establish the EE/O of O3-based AOPs to determine the process’s operating costs and feasibility. As a result, the current research concentrated on the determination of EE/O while removing color and COD from DIW utilizing the O3 and Fe2+/H2O2 coupled AOPs process.The primary goal of this study is to assess the efficiency of coupled O3 and Fe2+/H2O2 based AOPs in terms of % COD and color removal, as well as the determination of EE/O in distillery industrial wastewater (DIW). This study investigated the influence of various operating conditions on the O3/Fe2+/H2O2 process such as Fe2+ (5–30 mM) and H2O2 concentration (20–140 mM), initial pH of wastewater (1–11), COD (800–4800 ppm), and O3 inlet concentration (0.80–4 g/hr). The synergy between the combined O3 and Fe2+/H2O2 processes was investigated and recorded.


## 2. Materials and Methods

### 2.1. Wastewater Collection and Characterization

Distillery industrial wastewater was collected from distilleries in Erode, Tamil Nadu, India. The wastewater had the following characteristics: dark brown color, burn sugar odor, pH: 4.1–4.3, chemical oxygen demand (COD) of 80,000–90,000 mg/L, biochemical oxygen demand (BOD) of 7000–8000 mg/L, total dissolved solids (TDS) of 5550–5750 mg/L, and total suspended solids (TSS) of 15.44 g/L.The chemicals used in the experiments were H2O2–50% (w/w), FeSO4.7H2O, NaOH, H2SO4, Na2S2O3, KI, K2Cr2O7, etc. The analytical reagent (AR) grade chemicals were purchased from Merck, India. They were used as received without any purification.


### 2.2. Experimental Setup

The experimental setup for the O3/Fe2+/H2O2 process is depicted in Figure 1. The O3 was generated with the aid of an ozone generator (Ozonetek Limited, Chennai). The air was pumped at a rate of 20 liters per minute (LPM) and generated up to 4 g/hr of O3. The generated O3 is directed into the reactor, which has a capacity of 500 mL. The O3 was purged via a diffuser at the reactor’s bottom. The residual O3 in the gas stream leaving the reactor was destroyed by the 2–5% KI solution. The DIW was adjusted to the necessary pH and COD concentrations and was loaded into the reactor along with the measured amount of Fe2+ and H2O2.Figure 1
Experimental setup of the O3/Fe2+/H2O2 process.The concentration of O3 was determined using the iodometric titration process. After the process, the samples were taken from the reactor and immediately added to Na2S2O3 to stop the reaction. After centrifuging for 15 minutes at 15000 rpm, the supernatant was collected and analyzed for color using a UV/Vis–Spectrophotometer (TR300 Spectroquant®) and for COD using the principle of the closed reflux method (TR320, Spectroquant®).


### 2.3. Analysis

#### 2.3.1. COD Removal (%)

The COD removal efficiency was measured using the following equation:(1)COD removal efficiency%=CODIni−CODFinCODIni100,where CODIni and CODFin are the chemical oxygen demand (mg/L) values of DIW before and after the treatment process.


#### 2.3.2. Color Removal (%)

The color removal efficiency was calculated using the following equation:(2)color removal%=1−AbsFinAbsIni100,where AbsIni and AbsFin are the absorbances of the before and after treatment process of DIW.


#### 2.3.3. Electrical Energy per Order (EE/O)

Electrical energy per order has emerged as an additional and effective way to determine the suitability of wastewater treatment, and it must be economical for both individual and combined processes [35].The equation proposed for the determination of EE/O for COD removal is as follows:(3)EEOkWh/m3order1=P∗t∗1000V∗60∗logCODIni/CODFin,(4)logCODIniCODFin=kt,where P is the power (kW) for O3, t is the treatment time (min), V is the volume of the reactor (L), k is the pseudo-first-order rate constant (min−1) for the deterioration of the pollutant concentration.Combining equations (3) and (4), EE/O becomes(5)EPOkWh/m3order1=38.4∗PV∗k.


#### 2.3.4. Synergistic Effect (SE)

The synergy effects (SE) of the O3/Fe2+/H2O2 process can be determined from the COD and/or color removal rate constants of the coupled and standalone processes using the following equation (6) [36]:(6)SE=kO3/Fe2+/H2O2kO3+kFe2+/H2O2−1100,where, kO3/Fe2+/H2O2, kO3, kFe2+/H2O2 are the rate constants of the O3/Fe2+/H2O2, O3, and Fe2+/H2O2 system, respectively.ASE value ≥ 1 indicates that the coupled process surpasses the sum of the individuals, instead of SE ≤ 1 means that the coupled process produces a negative effect in combining the individuals.


## 2.1. Wastewater Collection and Characterization

Distillery industrial wastewater was collected from distilleries in Erode, Tamil Nadu, India. The wastewater had the following characteristics: dark brown color, burn sugar odor, pH: 4.1–4.3, chemical oxygen demand (COD) of 80,000–90,000 mg/L, biochemical oxygen demand (BOD) of 7000–8000 mg/L, total dissolved solids (TDS) of 5550–5750 mg/L, and total suspended solids (TSS) of 15.44 g/L.The chemicals used in the experiments were H2O2–50% (w/w), FeSO4.7H2O, NaOH, H2SO4, Na2S2O3, KI, K2Cr2O7, etc. The analytical reagent (AR) grade chemicals were purchased from Merck, India. They were used as received without any purification.


## 2.2. Experimental Setup

The experimental setup for the O3/Fe2+/H2O2 process is depicted in Figure 1. The O3 was generated with the aid of an ozone generator (Ozonetek Limited, Chennai). The air was pumped at a rate of 20 liters per minute (LPM) and generated up to 4 g/hr of O3. The generated O3 is directed into the reactor, which has a capacity of 500 mL. The O3 was purged via a diffuser at the reactor’s bottom. The residual O3 in the gas stream leaving the reactor was destroyed by the 2–5% KI solution. The DIW was adjusted to the necessary pH and COD concentrations and was loaded into the reactor along with the measured amount of Fe2+ and H2O2.Figure 1
Experimental setup of the O3/Fe2+/H2O2 process.The concentration of O3 was determined using the iodometric titration process. After the process, the samples were taken from the reactor and immediately added to Na2S2O3 to stop the reaction. After centrifuging for 15 minutes at 15000 rpm, the supernatant was collected and analyzed for color using a UV/Vis–Spectrophotometer (TR300 Spectroquant®) and for COD using the principle of the closed reflux method (TR320, Spectroquant®).


## 2.3. Analysis

### 2.3.1. COD Removal (%)

The COD removal efficiency was measured using the following equation:(1)COD removal efficiency%=CODIni−CODFinCODIni100,where CODIni and CODFin are the chemical oxygen demand (mg/L) values of DIW before and after the treatment process.


### 2.3.2. Color Removal (%)

The color removal efficiency was calculated using the following equation:(2)color removal%=1−AbsFinAbsIni100,where AbsIni and AbsFin are the absorbances of the before and after treatment process of DIW.


### 2.3.3. Electrical Energy per Order (EE/O)

Electrical energy per order has emerged as an additional and effective way to determine the suitability of wastewater treatment, and it must be economical for both individual and combined processes [35].The equation proposed for the determination of EE/O for COD removal is as follows:(3)EEOkWh/m3order1=P∗t∗1000V∗60∗logCODIni/CODFin,(4)logCODIniCODFin=kt,where P is the power (kW) for O3, t is the treatment time (min), V is the volume of the reactor (L), k is the pseudo-first-order rate constant (min−1) for the deterioration of the pollutant concentration.Combining equations (3) and (4), EE/O becomes(5)EPOkWh/m3order1=38.4∗PV∗k.


### 2.3.4. Synergistic Effect (SE)

The synergy effects (SE) of the O3/Fe2+/H2O2 process can be determined from the COD and/or color removal rate constants of the coupled and standalone processes using the following equation (6) [36]:(6)SE=kO3/Fe2+/H2O2kO3+kFe2+/H2O2−1100,where, kO3/Fe2+/H2O2, kO3, kFe2+/H2O2 are the rate constants of the O3/Fe2+/H2O2, O3, and Fe2+/H2O2 system, respectively.ASE value ≥ 1 indicates that the coupled process surpasses the sum of the individuals, instead of SE ≤ 1 means that the coupled process produces a negative effect in combining the individuals.


## 2.3.1. COD Removal (%)

The COD removal efficiency was measured using the following equation:(1)COD removal efficiency%=CODIni−CODFinCODIni100,where CODIni and CODFin are the chemical oxygen demand (mg/L) values of DIW before and after the treatment process.


## 2.3.2. Color Removal (%)

The color removal efficiency was calculated using the following equation:(2)color removal%=1−AbsFinAbsIni100,where AbsIni and AbsFin are the absorbances of the before and after treatment process of DIW.


## 2.3.3. Electrical Energy per Order (EE/O)

Electrical energy per order has emerged as an additional and effective way to determine the suitability of wastewater treatment, and it must be economical for both individual and combined processes [35].The equation proposed for the determination of EE/O for COD removal is as follows:(3)EEOkWh/m3order1=P∗t∗1000V∗60∗logCODIni/CODFin,(4)logCODIniCODFin=kt,where P is the power (kW) for O3, t is the treatment time (min), V is the volume of the reactor (L), k is the pseudo-first-order rate constant (min−1) for the deterioration of the pollutant concentration.Combining equations (3) and (4), EE/O becomes(5)EPOkWh/m3order1=38.4∗PV∗k.


## 2.3.4. Synergistic Effect (SE)

The synergy effects (SE) of the O3/Fe2+/H2O2 process can be determined from the COD and/or color removal rate constants of the coupled and standalone processes using the following equation (6) [36]:(6)SE=kO3/Fe2+/H2O2kO3+kFe2+/H2O2−1100,where, kO3/Fe2+/H2O2, kO3, kFe2+/H2O2 are the rate constants of the O3/Fe2+/H2O2, O3, and Fe2+/H2O2 system, respectively.ASE value ≥ 1 indicates that the coupled process surpasses the sum of the individuals, instead of SE ≤ 1 means that the coupled process produces a negative effect in combining the individuals.


## 3. Results and Discussion

### 3.1. Process Comparisons

The individuals such as Fe2+, H2O2, O3, and hybrid processes such as Fe2+/H2O2, O3/Fe2+, O3/H2O2, and O3/Fe2+/H2O2 was carried out under the following operating conditions: COD—1600 mg/L, pH—6, Fe2+—20 mM, H2O2—100 mM, and O3 flow rate and concentration of 20 LPM and 4 g/hr, respectively. The efficiency of these processes was compared in terms of % COD and color reduction, accompanied by an estimate of EE/O for DIW, and the findings are depicted in Figures 2(a) and 2(b). As represented in Figure 2(a), the single Fe2+, H2O2, and O3 methods were ineffective at removing color and COD. The Fe2+/H2O2 process removed % COD and color at a moderate rate. The % COD and color removed by combining O3 with Fe2+ and H2O2 processes such as O3/Fe2+, O3/H2O2, and O3/Fe2+/H2O2 were approximately 46.87%, 68.75%, and 96.87%, and 57.43%, 79.74%, and 100%, respectively. As predicted, the hybrid O3/Fe2+/H2O2 process is more efficient at removing COD and color than the O3/Fe2+ and O3/H2O2 treatment processes. The abovementioned results suggested that adding Fe2+ and H2O2 to the O3 process significantly increases COD and color removal. Perhaps this is due to the influence of concurrent pathways capable of producing plenteous •OH radicals for COD and color removal from DIW [37, 38].Figure 2
Comparison of various AOPs such as Fe2+, H2O2, Fe2+/H2O2, O3, O3/Fe2+, O3/H2O3, and O3/Fe2+/H2O2 on (a) % color and % COD removal and (b) EE/O (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).
(a)(b)The suitability of the O3/Fe2+/H2O2 process for wastewater treatment is primarily determined by the EE/O, which relies on COD removal using equation (3). A minimum of 0.2 kWh/m3order1 of EE/O was needed to remove COD and color from DIW using the O3/Fe2+/H2O2 process. In comparison to the O3/Fe2+/H2O2 process, the other single and combined processes such as O3 and O3/Fe2+, O3/H2O2 required a high amount of EE/O to remove COD and color.


### 3.2. Various Operating Parameters

Experimental operating parameters including the Fe2+, H2O2, COD, and O3 inlet concentration, and initial wastewater pH [39, 40], and so on, are found to have a major effect on the efficacy of the combined O3/Fe2+/H2O2 process in terms of COD, color removal, and EE/O of DIW.


#### 3.2.1. Effect of Fe2+

The concentration of Fe2+ and H2O2 is an important operating parameter, affecting the efficiency of pollutant removal through the O3/Fe2+/H2O2 process and preventing the excessive use of Fe2+ and H2O2 [29, 41]. Figure 3 illustrates the significance of the amount of Fe2+ on the % COD reduction and EE/O in the O3/Fe2+/H2O2 method. As illustrated in Figure 3, around 96.87% COD removal and 0.5315 kWhr/m3 EE/O were observed in the O3/Fe2+/H2O2 process at a Fe2+ concentration of 20 mM in comparison to other Fe2+ concentrations. The combination of Fe2+ and H2O2 increases the development of •OH and thus the oxidation efficiency. This may be because the increased Fe2+ concentration facilitated the formation of •OH radicals, thereby accelerating DIW degradation [41]. As Fe2+ concentrations exceeded 20 mM, the excess Fe2+ absorbed •OH radicals, resulting in a small decline in % COD reduction and a rise in EE/O of DIW [40]. Thus, the optimal Fe2+ concentration was estimated to be 20 mM.Figure 3
Effect of Fe2+ on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; and pH–6).


#### 3.2.2. Effect of H2O2

The influence of varying the H2O2 concentration from 20 to 140 mM on the effectiveness of the O3/Fe2+/H2O2 process in terms of % COD reduction and EE/O for DIW was studied, with the findings shown in Figure 4. The H2O2 dose added had a major effect on the studied process. According to Figure 4, the % COD removal increases from 40.63 to 96.87%, and the EE/O decreases from 3.53 to 0.53 kWhr/m3 as the initial H2O2 dose increases to a certain point, reaching a maximum at an initial H2O2 dose of about 100 mM, resulting in a substantial increase of process performance. At high concentrations of H2O2, it functions as an efficient •OH scavenger [41, 42], depending on the pollutant in concern. The observation is consistent with the following empirical equation (7):(7)H2O2+•OH⟶HO2•+H2O(8)H2O2⟶H2O+12O2.Figure 4
Effect of H2O2 on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; Fe2+–20 mM; and pH–6).WhileHO2• promotes radical chain reactions and is an effective oxidant in its own right, it has a much lower oxidation potential than •OH. Thus, when H2O2 concentrations are too high, the treatment efficiency is reduced, and its concentration must be tailored for each form of wastewater. The increase in H2O2 dosage increases the number of active sites on the surface, facilitating the decomposition of O3 molecules into additional •OH. Thus, the O3/Fe2+/H2O2 process demonstrates an improvement in COD efficiency as the H2O2 dose is increased.


#### 3.2.3. Effect of Wastewater pH

The pH of wastewater at its initial state is critical because it affects the reaction of organic/inorganic compounds with the O3/Fe2+/H2O2 system [25]. Figure 5 shows the results of an investigation into the impact of the initial values of pH on the % COD reduction and EE/O in the O3/Fe2+/H2O2 process for the DIW. As shown in Figure 5, with the O3/Fe2+/H2O2 process, the highest % COD removal efficiency and the lowest EE/O occurred at an initial pH of 6. As the pH increased up to 6, COD removal increased and EE/O decreased. Lower values of pH reveal the H+ ion’s scavenging effect on •OH, the creation of H3O2+ by reaction between H2O2 and H+, resulting in increased H2O2 stability and decreased % COD removal efficiency. The % COD reduction improved as the pH increased from 1 to 6, owing to the increased formation of •OH (a more powerful oxidant than O3) via the hydroxylation reaction with O3.Figure 5
Effect of pH on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).Furthermore, raising the wastewater pH to 6 in the combined O3/Fe2+/H2O2 process resulted in coagulation, which increases contaminants through the complexation reaction induced by the transformation of Fe2+ and Fe3+ to Fe(OH)n-type structures [25]. When the pH was greater than 6, COD removal decreased marginally with increasing pH. It is deduced that COD removal exists as a eupterotid that is energetic and easily reacts with the hydroxyl ion in acidic conditions, but it acts as a stable molecule in basic conditions, resulting in a decrease in % COD reduction and an increase in EE/O as the pH increases.


#### 3.2.4. Effect of COD Concentration

The initial pollutant content is critical in the O3/Fe2+/H2O2 process wastewater treatment process [43, 44]. The increased initial COD concentration from 800 to 4800 mg/L, then decreased the % COD elimination from 100 to 45.83%, with a rise in the EE/O from 0.07 to 3 kWh/m3order1 for DIW using the O3/Fe2+/H2O2 process, as presented in Figure 6. Since, raising the initial COD content promoted the creation of intermediate products that compete with the O3-consuming pollutant, thus decreasing % COD removal performance, and increase of EE/O for DIW [45].Figure 6
Effect of COD on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).


#### 3.2.5. Effect of O3 Inlet Concentration

It is critical to select the optimal O3 inlet concentration for pollutant removal from wastewater when using the O3/Fe2+/H2O2 process [40, 46]. The effect of O3 inlet concentration on the % COD removal efficiency and EE/O with the O3/Fe2+/H2O2 process was explored by bubbling ozone into the DIW solution at different gas concentrations ranging from 0.8 to 4 g/h. As shown in Figure 7, the % COD reduction improved from 52.18 to 96.87% and the EE/O reduced from 2.49 to 0.535 kWh/m3 order1 as the O3/Fe2+/H2O2 system’s O3 concentration increased from 0.8 to 4 g/hr. This can be explained by the fact that the two key parameters, O3 concentration and O3–liquid mass transfer resistance, have a massive effect on the mass transfer rate of O3 [40, 46]. With a higher O3 concentration, the driving factor for O3 mass transfer is increased, allowing the DIW solution to absorb more O3. As a consequence, the excess O3 in the solution interacted with radical initiators (Fe2+, H2O2, •OH, etc.) to produce more •OH, which eventually improves COD and color removal efficiency while lowering EE/O. Zhao et al. [46] reported similar results for the removal of Ni-EDTA using O3-based oxidation processes.Figure 7
Effect of O3 inlet concentration on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate–20 LPM; H2O2–100 mM; Fe2+–20 mM; and pH–6).


### 3.3. Synergy Effect

The O3 and Fe2+/H2O2 processes were carried out under the best experimental conditions to determine the synergy among each process for the removal of % color and % COD from DIW. The experimental findings confirmed a synergy index among O3 and Fe2+/H2O2 processes for COD removal of DIW. Thus, as opposed to other processes (O3 and Fe2+/H2O2), the O3/Fe2+/H2O2 process greatly improves COD and color removal. Equation (6) was used to determine the synergy index of the O3 and Fe2+/H2O2 processes [36]. The synergy index was 21.50%, suggesting that the effectiveness in terms % color and % COD removal was sufficiently greater for the combined process than for the individual O3 and Fe2+/H2O2 processes. The reported synergy index is due to the fact that combining O3 and Fe2+/H2O2 provides a larger volume of •OH, which enhances the rate of % color and COD removal from DIW. Thus, the coupled O3 and Fe2+/H2O2 processes offer an alternative and novel approach for industrial wastewater treatment.


## 3.1. Process Comparisons

The individuals such as Fe2+, H2O2, O3, and hybrid processes such as Fe2+/H2O2, O3/Fe2+, O3/H2O2, and O3/Fe2+/H2O2 was carried out under the following operating conditions: COD—1600 mg/L, pH—6, Fe2+—20 mM, H2O2—100 mM, and O3 flow rate and concentration of 20 LPM and 4 g/hr, respectively. The efficiency of these processes was compared in terms of % COD and color reduction, accompanied by an estimate of EE/O for DIW, and the findings are depicted in Figures 2(a) and 2(b). As represented in Figure 2(a), the single Fe2+, H2O2, and O3 methods were ineffective at removing color and COD. The Fe2+/H2O2 process removed % COD and color at a moderate rate. The % COD and color removed by combining O3 with Fe2+ and H2O2 processes such as O3/Fe2+, O3/H2O2, and O3/Fe2+/H2O2 were approximately 46.87%, 68.75%, and 96.87%, and 57.43%, 79.74%, and 100%, respectively. As predicted, the hybrid O3/Fe2+/H2O2 process is more efficient at removing COD and color than the O3/Fe2+ and O3/H2O2 treatment processes. The abovementioned results suggested that adding Fe2+ and H2O2 to the O3 process significantly increases COD and color removal. Perhaps this is due to the influence of concurrent pathways capable of producing plenteous •OH radicals for COD and color removal from DIW [37, 38].Figure 2
Comparison of various AOPs such as Fe2+, H2O2, Fe2+/H2O2, O3, O3/Fe2+, O3/H2O3, and O3/Fe2+/H2O2 on (a) % color and % COD removal and (b) EE/O (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).
(a)(b)The suitability of the O3/Fe2+/H2O2 process for wastewater treatment is primarily determined by the EE/O, which relies on COD removal using equation (3). A minimum of 0.2 kWh/m3order1 of EE/O was needed to remove COD and color from DIW using the O3/Fe2+/H2O2 process. In comparison to the O3/Fe2+/H2O2 process, the other single and combined processes such as O3 and O3/Fe2+, O3/H2O2 required a high amount of EE/O to remove COD and color.


## 3.2. Various Operating Parameters

Experimental operating parameters including the Fe2+, H2O2, COD, and O3 inlet concentration, and initial wastewater pH [39, 40], and so on, are found to have a major effect on the efficacy of the combined O3/Fe2+/H2O2 process in terms of COD, color removal, and EE/O of DIW.


### 3.2.1. Effect of Fe2+

The concentration of Fe2+ and H2O2 is an important operating parameter, affecting the efficiency of pollutant removal through the O3/Fe2+/H2O2 process and preventing the excessive use of Fe2+ and H2O2 [29, 41]. Figure 3 illustrates the significance of the amount of Fe2+ on the % COD reduction and EE/O in the O3/Fe2+/H2O2 method. As illustrated in Figure 3, around 96.87% COD removal and 0.5315 kWhr/m3 EE/O were observed in the O3/Fe2+/H2O2 process at a Fe2+ concentration of 20 mM in comparison to other Fe2+ concentrations. The combination of Fe2+ and H2O2 increases the development of •OH and thus the oxidation efficiency. This may be because the increased Fe2+ concentration facilitated the formation of •OH radicals, thereby accelerating DIW degradation [41]. As Fe2+ concentrations exceeded 20 mM, the excess Fe2+ absorbed •OH radicals, resulting in a small decline in % COD reduction and a rise in EE/O of DIW [40]. Thus, the optimal Fe2+ concentration was estimated to be 20 mM.Figure 3
Effect of Fe2+ on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; and pH–6).


### 3.2.2. Effect of H2O2

The influence of varying the H2O2 concentration from 20 to 140 mM on the effectiveness of the O3/Fe2+/H2O2 process in terms of % COD reduction and EE/O for DIW was studied, with the findings shown in Figure 4. The H2O2 dose added had a major effect on the studied process. According to Figure 4, the % COD removal increases from 40.63 to 96.87%, and the EE/O decreases from 3.53 to 0.53 kWhr/m3 as the initial H2O2 dose increases to a certain point, reaching a maximum at an initial H2O2 dose of about 100 mM, resulting in a substantial increase of process performance. At high concentrations of H2O2, it functions as an efficient •OH scavenger [41, 42], depending on the pollutant in concern. The observation is consistent with the following empirical equation (7):(7)H2O2+•OH⟶HO2•+H2O(8)H2O2⟶H2O+12O2.Figure 4
Effect of H2O2 on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; Fe2+–20 mM; and pH–6).WhileHO2• promotes radical chain reactions and is an effective oxidant in its own right, it has a much lower oxidation potential than •OH. Thus, when H2O2 concentrations are too high, the treatment efficiency is reduced, and its concentration must be tailored for each form of wastewater. The increase in H2O2 dosage increases the number of active sites on the surface, facilitating the decomposition of O3 molecules into additional •OH. Thus, the O3/Fe2+/H2O2 process demonstrates an improvement in COD efficiency as the H2O2 dose is increased.


### 3.2.3. Effect of Wastewater pH

The pH of wastewater at its initial state is critical because it affects the reaction of organic/inorganic compounds with the O3/Fe2+/H2O2 system [25]. Figure 5 shows the results of an investigation into the impact of the initial values of pH on the % COD reduction and EE/O in the O3/Fe2+/H2O2 process for the DIW. As shown in Figure 5, with the O3/Fe2+/H2O2 process, the highest % COD removal efficiency and the lowest EE/O occurred at an initial pH of 6. As the pH increased up to 6, COD removal increased and EE/O decreased. Lower values of pH reveal the H+ ion’s scavenging effect on •OH, the creation of H3O2+ by reaction between H2O2 and H+, resulting in increased H2O2 stability and decreased % COD removal efficiency. The % COD reduction improved as the pH increased from 1 to 6, owing to the increased formation of •OH (a more powerful oxidant than O3) via the hydroxylation reaction with O3.Figure 5
Effect of pH on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).Furthermore, raising the wastewater pH to 6 in the combined O3/Fe2+/H2O2 process resulted in coagulation, which increases contaminants through the complexation reaction induced by the transformation of Fe2+ and Fe3+ to Fe(OH)n-type structures [25]. When the pH was greater than 6, COD removal decreased marginally with increasing pH. It is deduced that COD removal exists as a eupterotid that is energetic and easily reacts with the hydroxyl ion in acidic conditions, but it acts as a stable molecule in basic conditions, resulting in a decrease in % COD reduction and an increase in EE/O as the pH increases.


### 3.2.4. Effect of COD Concentration

The initial pollutant content is critical in the O3/Fe2+/H2O2 process wastewater treatment process [43, 44]. The increased initial COD concentration from 800 to 4800 mg/L, then decreased the % COD elimination from 100 to 45.83%, with a rise in the EE/O from 0.07 to 3 kWh/m3order1 for DIW using the O3/Fe2+/H2O2 process, as presented in Figure 6. Since, raising the initial COD content promoted the creation of intermediate products that compete with the O3-consuming pollutant, thus decreasing % COD removal performance, and increase of EE/O for DIW [45].Figure 6
Effect of COD on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).


### 3.2.5. Effect of O3 Inlet Concentration

It is critical to select the optimal O3 inlet concentration for pollutant removal from wastewater when using the O3/Fe2+/H2O2 process [40, 46]. The effect of O3 inlet concentration on the % COD removal efficiency and EE/O with the O3/Fe2+/H2O2 process was explored by bubbling ozone into the DIW solution at different gas concentrations ranging from 0.8 to 4 g/h. As shown in Figure 7, the % COD reduction improved from 52.18 to 96.87% and the EE/O reduced from 2.49 to 0.535 kWh/m3 order1 as the O3/Fe2+/H2O2 system’s O3 concentration increased from 0.8 to 4 g/hr. This can be explained by the fact that the two key parameters, O3 concentration and O3–liquid mass transfer resistance, have a massive effect on the mass transfer rate of O3 [40, 46]. With a higher O3 concentration, the driving factor for O3 mass transfer is increased, allowing the DIW solution to absorb more O3. As a consequence, the excess O3 in the solution interacted with radical initiators (Fe2+, H2O2, •OH, etc.) to produce more •OH, which eventually improves COD and color removal efficiency while lowering EE/O. Zhao et al. [46] reported similar results for the removal of Ni-EDTA using O3-based oxidation processes.Figure 7
Effect of O3 inlet concentration on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate–20 LPM; H2O2–100 mM; Fe2+–20 mM; and pH–6).


## 3.2.1. Effect of Fe2+

The concentration of Fe2+ and H2O2 is an important operating parameter, affecting the efficiency of pollutant removal through the O3/Fe2+/H2O2 process and preventing the excessive use of Fe2+ and H2O2 [29, 41]. Figure 3 illustrates the significance of the amount of Fe2+ on the % COD reduction and EE/O in the O3/Fe2+/H2O2 method. As illustrated in Figure 3, around 96.87% COD removal and 0.5315 kWhr/m3 EE/O were observed in the O3/Fe2+/H2O2 process at a Fe2+ concentration of 20 mM in comparison to other Fe2+ concentrations. The combination of Fe2+ and H2O2 increases the development of •OH and thus the oxidation efficiency. This may be because the increased Fe2+ concentration facilitated the formation of •OH radicals, thereby accelerating DIW degradation [41]. As Fe2+ concentrations exceeded 20 mM, the excess Fe2+ absorbed •OH radicals, resulting in a small decline in % COD reduction and a rise in EE/O of DIW [40]. Thus, the optimal Fe2+ concentration was estimated to be 20 mM.Figure 3
Effect of Fe2+ on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; and pH–6).


## 3.2.2. Effect of H2O2

The influence of varying the H2O2 concentration from 20 to 140 mM on the effectiveness of the O3/Fe2+/H2O2 process in terms of % COD reduction and EE/O for DIW was studied, with the findings shown in Figure 4. The H2O2 dose added had a major effect on the studied process. According to Figure 4, the % COD removal increases from 40.63 to 96.87%, and the EE/O decreases from 3.53 to 0.53 kWhr/m3 as the initial H2O2 dose increases to a certain point, reaching a maximum at an initial H2O2 dose of about 100 mM, resulting in a substantial increase of process performance. At high concentrations of H2O2, it functions as an efficient •OH scavenger [41, 42], depending on the pollutant in concern. The observation is consistent with the following empirical equation (7):(7)H2O2+•OH⟶HO2•+H2O(8)H2O2⟶H2O+12O2.Figure 4
Effect of H2O2 on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; Fe2+–20 mM; and pH–6).WhileHO2• promotes radical chain reactions and is an effective oxidant in its own right, it has a much lower oxidation potential than •OH. Thus, when H2O2 concentrations are too high, the treatment efficiency is reduced, and its concentration must be tailored for each form of wastewater. The increase in H2O2 dosage increases the number of active sites on the surface, facilitating the decomposition of O3 molecules into additional •OH. Thus, the O3/Fe2+/H2O2 process demonstrates an improvement in COD efficiency as the H2O2 dose is increased.


## 3.2.3. Effect of Wastewater pH

The pH of wastewater at its initial state is critical because it affects the reaction of organic/inorganic compounds with the O3/Fe2+/H2O2 system [25]. Figure 5 shows the results of an investigation into the impact of the initial values of pH on the % COD reduction and EE/O in the O3/Fe2+/H2O2 process for the DIW. As shown in Figure 5, with the O3/Fe2+/H2O2 process, the highest % COD removal efficiency and the lowest EE/O occurred at an initial pH of 6. As the pH increased up to 6, COD removal increased and EE/O decreased. Lower values of pH reveal the H+ ion’s scavenging effect on •OH, the creation of H3O2+ by reaction between H2O2 and H+, resulting in increased H2O2 stability and decreased % COD removal efficiency. The % COD reduction improved as the pH increased from 1 to 6, owing to the increased formation of •OH (a more powerful oxidant than O3) via the hydroxylation reaction with O3.Figure 5
Effect of pH on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).Furthermore, raising the wastewater pH to 6 in the combined O3/Fe2+/H2O2 process resulted in coagulation, which increases contaminants through the complexation reaction induced by the transformation of Fe2+ and Fe3+ to Fe(OH)n-type structures [25]. When the pH was greater than 6, COD removal decreased marginally with increasing pH. It is deduced that COD removal exists as a eupterotid that is energetic and easily reacts with the hydroxyl ion in acidic conditions, but it acts as a stable molecule in basic conditions, resulting in a decrease in % COD reduction and an increase in EE/O as the pH increases.


## 3.2.4. Effect of COD Concentration

The initial pollutant content is critical in the O3/Fe2+/H2O2 process wastewater treatment process [43, 44]. The increased initial COD concentration from 800 to 4800 mg/L, then decreased the % COD elimination from 100 to 45.83%, with a rise in the EE/O from 0.07 to 3 kWh/m3order1 for DIW using the O3/Fe2+/H2O2 process, as presented in Figure 6. Since, raising the initial COD content promoted the creation of intermediate products that compete with the O3-consuming pollutant, thus decreasing % COD removal performance, and increase of EE/O for DIW [45].Figure 6
Effect of COD on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: reaction time–4 h; O3 flow rate and production–20 LPM and 4 g/h; H2O2–100 mM; Fe2+–20 mM; and pH–6).


## 3.2.5. Effect of O3 Inlet Concentration

It is critical to select the optimal O3 inlet concentration for pollutant removal from wastewater when using the O3/Fe2+/H2O2 process [40, 46]. The effect of O3 inlet concentration on the % COD removal efficiency and EE/O with the O3/Fe2+/H2O2 process was explored by bubbling ozone into the DIW solution at different gas concentrations ranging from 0.8 to 4 g/h. As shown in Figure 7, the % COD reduction improved from 52.18 to 96.87% and the EE/O reduced from 2.49 to 0.535 kWh/m3 order1 as the O3/Fe2+/H2O2 system’s O3 concentration increased from 0.8 to 4 g/hr. This can be explained by the fact that the two key parameters, O3 concentration and O3–liquid mass transfer resistance, have a massive effect on the mass transfer rate of O3 [40, 46]. With a higher O3 concentration, the driving factor for O3 mass transfer is increased, allowing the DIW solution to absorb more O3. As a consequence, the excess O3 in the solution interacted with radical initiators (Fe2+, H2O2, •OH, etc.) to produce more •OH, which eventually improves COD and color removal efficiency while lowering EE/O. Zhao et al. [46] reported similar results for the removal of Ni-EDTA using O3-based oxidation processes.Figure 7
Effect of O3 inlet concentration on % COD removal and EE/O in the O3/Fe2+/H2O2 process (experimental conditions: COD–1600 mg/L; reaction time–4 h; O3 flow rate–20 LPM; H2O2–100 mM; Fe2+–20 mM; and pH–6).


## 3.3. Synergy Effect

The O3 and Fe2+/H2O2 processes were carried out under the best experimental conditions to determine the synergy among each process for the removal of % color and % COD from DIW. The experimental findings confirmed a synergy index among O3 and Fe2+/H2O2 processes for COD removal of DIW. Thus, as opposed to other processes (O3 and Fe2+/H2O2), the O3/Fe2+/H2O2 process greatly improves COD and color removal. Equation (6) was used to determine the synergy index of the O3 and Fe2+/H2O2 processes [36]. The synergy index was 21.50%, suggesting that the effectiveness in terms % color and % COD removal was sufficiently greater for the combined process than for the individual O3 and Fe2+/H2O2 processes. The reported synergy index is due to the fact that combining O3 and Fe2+/H2O2 provides a larger volume of •OH, which enhances the rate of % color and COD removal from DIW. Thus, the coupled O3 and Fe2+/H2O2 processes offer an alternative and novel approach for industrial wastewater treatment.


## 4. Conclusion

The developed O3/Fe2+/H2O2 process is compared to O3, Fe2+, H2O2, O3/Fe2+, and O3/H2O2, Fe2+/H2O2 processes, and the outcomes indicate that the O3/Fe2+/H2O2 process is a promising method for achieving high % color and % COD removal efficiencies while consuming a minimal amount of EE/O from DIW. The findings suggest that to ensure an effective treatment process, operating parameters such as Fe2+, H2O2, COD, and O3 inlet concentration, and initial wastewater pH should be considered. A synergy effect was calculated to exist between the O3 and Fe2+/H2O2 processes at 21.50 percent. The O3/Fe2+/H2O2 method was found to be capable of eliminating pollutants from a range of industrial effluents and wastewater.



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*Source: 5006911-2022-04-20.xml*