The applicability of an electrochemical Fenton-type process (EF-HOCl-ReFe) to the treatment of three actual wastewaters, namely wastewater from an automobile factory (automobile wastewater), metal scrap-cleansing wastewater, and municipal wastewater, is discussed in this research. The EF-HOCl-ReFe successfully removed the chemical oxygen demand (COD) from automobile wastewater pre-treated by a coagulation process without any inhibition. The apparent current efficiency reached 86%, 46% of which was ascribed to the electrochemical Fenton-type mechanism. The metal scrap-cleansing wastewater had a yellow colour and high concentrations of COD (6550 mg/L) and Cl (1560 mM). The EF-HOCl-ReFe could achieve almost complete COD removal and decolourization after 48 h of treatment, although a temporary intensification of colour was observed before the decolourization. The EF-HOCl-ReFe was also effective in the removal of 1,4-dioxane from municipal wastewater pre-treated by activated sludge and coagulation processes, which were unable to remove 1,4-dioxane. The 1,4-dioxane removal efficiency after 30 min of treatment reached 68.5%. Thus, the EF-HOCl-ReFe was applicable to the treatment of these actual wastewaters.

INTRODUCTION

Tomat & Vecchi (1971) first proposed the electrochemical Fenton process, in which hydrogen peroxide (H2O2) and ferrous ions (Fe2+) are concurrently produced by the cathodic reduction of oxygen and ferric ions (Fe3+), respectively. Thereafter, various types of electrochemical Fenton processes were proposed. These processes are characterized by the combination of H2O2 and Fe2+ supply systems (Qiang et al. 2003). The electrochemical Fenton processes can solve the disadvantages of the classical Fenton process, namely handling the dangerous H2O2 reagent and the high costs of separating and disposing of iron sludge after treatment. However, there is still the disadvantage of competition for the production of H2O2 when Fe2+ is regenerated at the cathode. Therefore, a new electrochemical Fenton-type process has been proposed, using Fe2+ and hypochlorous acid (HOCl) as a Fenton-like reagent (Kishimoto & Sugimura 2010). This new electrochemical Fenton-type process (EF-HOCl-ReFe) is outlined as follows. 
formula
1
 
formula
2
Reaction in bulk solution: 
formula
3
 
formula
4

In Equation (4) HOCl substitutes for H2O2 in the Fenton reaction (Candeias et al. 1994). Since HOCl can be generated by the oxidation of chloride ions (Cl) at the anode, it does not compete with the Fe2+ regeneration at the cathode. This is advantageous, especially as it can stoichiometrically generate 1 mol of hydroxyl radicals (•OH) using 2 mol of electrons, whereas the conventional electrochemical Fenton process requires 3 mol of electrons per 1 mol of •OH.

The electrochemical Fenton process has been extensively studied for the destruction of various organic pollutants (Nidheesh & Gandhimathi 2012). However, reports on its application to actual wastewater treatment are far fewer than the reports on synthetic wastewater treatment. Actual wastewater usually contains various coexisting substances that function as radical scavengers, like carbonates, which deteriorate the performance of advanced oxidation processes (AOPs) (Riga et al. 2007). Although some papers have demonstrated the effective application of electrochemical Fenton processes to actual wastewater treatment (Khoufi et al. 2006; Zhu et al. 2011), there is no report on the application of the EF-HOCl-ReFe to actual wastewater treatment. Accordingly, to gain a better understanding of the EF-HOCl-ReFe, it is beneficial to investigate its performance using various actual wastewaters.

This research focused on the EF-HOCl-ReFe, and explored its applicability to actual wastewater treatment by investigating its performance in the treatment of three actual wastewaters, namely wastewater from an automobile factory (automobile wastewater), a metal scrap-cleansing wastewater, and a 1,4-dioxane-contaminated municipal wastewater. The automobile wastewater mainly comes from painting and manufacturing processes. A conventional activated sludge process and a coagulation process are applied to treat this wastewater at present. However, these processes are not very effective in removing chemical oxygen demand (COD). The metal scrap-cleansing wastewater is discharged from a metal scrap-recycling facility, which accepted various types of metal scrap, such as cans used for beverages and industrial metal wastes. A classical Fenton process is applied to the COD removal from this wastewater in the facility, but it is costly and dangerous because of the rise in temperature, to around 80 °C, caused by the heat of reaction during the operation. It is known that conventional biological processes are not effective in removing 1,4-dioxane (Zenker et al. 2003). Thus, a more effective treatment process is desired for these wastewaters.

METHODS

Materials

The automobile wastewater was coloured light yellow and had a high concentration of organic matter, but the concentration of the inorganic contaminants was much lower than that of the organic matter. The quality of the automobile wastewater is summarized in Table 1.

Table 1

Water quality of automobile wastewater

pH COD (mg/L] BOD (mg/L] TOC (mg/L] Cl (mM] SO42− (mM] Li+ (mM] Na+ (mM] Ca2+ (mM] Mg2+ (mM] Fe (mM] 
6.2 13,250 3,900 4,500 9.2 0.05 0.02 15.7 0.20 2.2 0.02 
pH COD (mg/L] BOD (mg/L] TOC (mg/L] Cl (mM] SO42− (mM] Li+ (mM] Na+ (mM] Ca2+ (mM] Mg2+ (mM] Fe (mM] 
6.2 13,250 3,900 4,500 9.2 0.05 0.02 15.7 0.20 2.2 0.02 

Note: Chloride concentration increased from 9.2 to 10.7 mM due to the pre-treatment of coagulation using FeCl3.

The metal scrap-cleansing wastewater was coloured yellow and contained high concentrations of organic matter and anions, and trace levels of heavy metals. The quality of the metal scrap-cleansing wastewater is summarized in Table 2.

Table 2

Water quality of metal scrap-cleansing wastewater

pH COD [mg/L] TOC [mg/L] Cl [mM] NO2 [mM] NO3 [mM] SO42− [mM] 
8.9 6,550 3,470 1,580 8.1 11.5 88.9 
pH COD [mg/L] TOC [mg/L] Cl [mM] NO2 [mM] NO3 [mM] SO42− [mM] 
8.9 6,550 3,470 1,580 8.1 11.5 88.9 

Note: Cationic elements of Na, K, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Al, Si, and Pb were detected by inductively coupled plasma spectrometry, but not determined due to the extreme high concentration of Na.

The municipal wastewater was obtained from a sanitary chamber in Ryukoku University. The raw municipal sewage contained 445 mg/L of COD and was pre-treated by a conventional activated sludge process followed by a coagulation process before the EF-HOCl-ReFe. We added 1,4-dioxane as a model pollutant into the municipal wastewater, because 1,4-dioxane can act as an •OH probe. The initial concentrations of 1,4-dioxane and Cl in the mixed liquor of the municipal wastewater and activated sludge were 0.081 and 3.9 mM, respectively.

Electrochemical cell

Two types of electrochemical cells, namely an electrodes-immersed cell (Kishimoto & Sugimura 2010) and electrochemical flow cell (Kishimoto et al. 2015), were used for the EF-HOCl-ReFe. A ruthenium oxide-coated titanium anode (plate) and a stainless steel (ANSI304) cathode (plate) were installed into both cells. Galvanostatic electrolysis was applied. The wastewater pH was regulated at a set value by the addition of H2SO4 and/or NaOH with a pH controller (FD-02, TGK, Tokyo, Japan).

The effective electrode area, the electrode gap, and the effective volume of the electrodes-immersed cell were 72.25 cm2, 2.0 cm, and 1.0 L, respectively. The wastewater was continuously stirred with a stirrer during treatment.

The effective electrode area and the electrode gap of the electrochemical flow cell were 31 cm2 and 1.0 cm for the metal scrap-cleansing wastewater treatment, and 21 cm2 and 3.0 mm for the municipal sewage treatment, respectively. The linear velocity at the electrode surface was regulated by the feed flow rate.

Automobile wastewater treatment

When the EF-HOCl-ReFe was directly applied to the automobile wastewater, organic matter removal was observed at the sedimentation step for the residual iron in the water sample (data not shown). Therefore, the raw wastewater was pre-treated by coagulation using 0.5 mM FeCl3 as the coagulant. The coagulation was performed as follows: ferric chloride was added into the wastewater at a final concentration of 0.5 mM. Then, the wastewater pH was set to 8.0 by adding 0.1 M NaOH. Next, 2 min of rapid mixing with a G-value of 125 s−1 followed by 15 min of slow mixing with a G-value of 30 s−1 was applied. After the slow mixing was finished, the wastewater was filtered with a glass fibre filter with a particle retention of 1.0 μm (GF/B, Whatman, Tokyo, Japan). Finally, the obtained filtrate was treated by the EF-HOCl-ReFe using the electrodes-immersed cell.

Ferrous sulphate was added to the pre-treated wastewater as an iron source for the EF-HOCl-ReFe at a final concentration of 1.0 mM, and the pH was set at 3.0 by adding 3.6 M H2SO4. Then, 1.0 L of the filtrate was poured into the electrodes-immersed cell and the EF-HOCl-ReFe was started. The electrolytic current was set at 0.20 A (current density: 2.8 mA/cm2).

A control experiment was also performed. The experimental conditions were the same as for the aforementioned EF-HOCl-ReFe except there was no addition of FeSO4.

For comparison purposes, the EF-HOCl-ReFe was applied to a synthetic wastewater containing 20 mM 1,4-dioxane, 1.0 mM FeSO4, and 10 mM NaCl at pH 3.0, using an electrolytic current of 0.20 A.

Metal scrap-cleansing wastewater treatment

When the pH of the metal scrap-cleansing wastewater was adjusted to 2.0, a white precipitate was observed. Therefore, the acidic wastewater was filtered with a glass fibre filter (GF/B, Whatman, Tokyo, Japan) before use in the EF-HOCl-ReFe. The acidification-filtration removed 10% of the initial COD and 13% of the initial total organic carbon (TOC).

The electrochemical flow cell was applied to the EF-HOCl-ReFe of the pre-treated wastewater at an electrolytic current of 0.68 A (current density: 22 mA/cm2). A 200 mL quantity of pre-treated wastewater was stored in a reservoir tank and was circulated between the tank and the electrochemical flow cell with a peristaltic pump at a flow rate of 9.3 mL/s (linear velocity: 4.65 cm/s). Ferric chloride was added to the pre-treated wastewater at a final concentration of 8 mM.

1,4-Dioxane-added municipal wastewater treatment

The 1,4-dioxane-added municipal wastewater was treated with a conventional activated sludge process followed by a coagulation process before the EF-HOCl-ReFe. The activated sludge process was operated at a mixed liquor suspended solids concentration of 1,930 mg/L, a temperature of 20 °C, and an aeration time of 8 h. After 8 h of aeration, the activated sludge in the mixed liquor was settled for 4 h. Then, coagulation using FeCl3 coagulant at the dose of 2.0 mM and pH 7.0 was applied to the supernatant water. The rapid mixing was continued for 5 min at a G-value of 100 s−1. Then, the slow mixing was continued for 12 min at a G-value of 50 s−1. The floc formed by coagulation was separated by centrifugation. Ferric chloride was added to the supernatant water after the centrifugation at a final concentration of 2.0 mM and the pH was adjusted to 2.0 by adding H2SO4 before starting the EF-HOCl-ReFe. Consequently, the final Cl concentration in the supernatant water was 15.9 mM due to the FeCl3 added as the coagulant and the iron source.

An electrochemical flow cell was applied to 200 mL of the supernatant water for 30 min at an electrolytic current of 0.30 A (current density: 14 mA/cm2), and a flow rate of 6.0 mL/s (linear velocity: 20 cm/s).

Chemical analysis

The water sample was immediately adjusted to a pH above 10 by adding NaOH for the removal of dissolved iron. Then, the supernatant of the sample was filtered with a membrane filter with 0.20 μm pores. The filtrate was neutralized by the addition of H2SO4 and the residual chlorine was removed by the addition of Na2SO3. The excess sulphite ions were oxidized to sulphate ions by aeration. The residual chlorine and sulphite-free samples were then applied to analyses of COD, biochemical oxygen demand (BOD), TOC, inorganic ions by ion chromatography, 1,4-dioxane by high performance liquid chromatography (Kishimoto et al. 2013), and pH. The absorption spectrum of the diluted sample was measured with a spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) using a quartz cell with a 10-mm optical path. The analytical data were corrected by diluting the sample during treatment and by adding a reagent after sampling.

RESULTS AND DISCUSSION

Automobile wastewater treatment

Figure 1 shows changes in COD, TOC, and BOD during the EF-HOCl-ReFe of the automobile wastewater. The coagulation pre-treatment removed 310 mg/L of COD and 70 mg/L of TOC, which were equivalent to removal efficiencies of only 2.3% for COD and 1.6% for TOC. Thus, the coagulation process was not very effective in removing organic matter in the automobile wastewater. The COD and TOC were steadily decreased over time by the EF-HOCl-ReFe, whereas the BOD increased until an elapsed time of 6 h and then began to decrease. The temporary increase in BOD at the beginning of AOPs is often observed (Kishimoto et al. 2007) because of the formation of biodegradable organic matter such as acetic acid and oxalic acid through the partial oxidation of biologically persistent organic matter (Tokumura et al. 2013). Therefore, it was considered that the partial oxidation of organic matter in the wastewater proceeded in this experiment.

Figure 1

The changes in COD, TOC, and BOD during the EF-HOCl-ReFe for the automobile wastewater. The error bar indicates the standard error.

Figure 1

The changes in COD, TOC, and BOD during the EF-HOCl-ReFe for the automobile wastewater. The error bar indicates the standard error.

The changes in COD and TOC can be described using zero-order reaction kinetics. Since 1 mmol of •OH can theoretically be generated from 2 mmol of electrons in the electrochemical Fenton-type reaction, the current efficiency (CE) in the EF-HOCl-ReFe was obtained using the following equation: 
formula
5
where F is the Faraday's constant [=96.485 C/mmol], I is the electrolytic current [A], α is a conversion factor from COD to electrons [=1/8 mmol/mg], k is the zero-order removal rate of COD [mg L−1 s−1], and V is the water volume [L]. As the value of k was evaluated to be 25.5 mg L−1 h−1 from the regression line in Figure 2, the CE was calculated to be 86%. However, this value was much higher than the CE of 46% that was observed in the EF-HOCl-ReFe of synthetic wastewater (data not shown), which contained 20 mM 1,4-dioxane, 1.0 mM FeSO4, and 10 mM NaCl. To clarify the cause of this discrepancy, a control experiment without FeSO4 addition was performed, and the observed COD vs. the elapsed time is illustrated in Figure 2. Figure 2 reveals that the COD was removed by the electrolytic oxidation alone. Since the iron concentration in the automobile wastewater was only 0.02 mM, the electrochemical Fenton-type reaction in the control experiment could be ignored. Accordingly, the COD removal was considered to be due to direct oxidation of organic matter at the anode and/or indirect oxidation by free chlorine produced at the anode. Assuming that the same amount of electrolytic COD removal proceeded during the EF-HOCl-ReFe, the COD removal rate by the electrochemical Fenton-type reaction can be estimated by the subtraction of the k for the control experiment from the k for the EF-HOCl-ReFe. As a result, the modified k for the EF-HOCl-ReFe was estimated to be 13.8 mg L−1 h−1. Based on this value, the CE was calculated to be 46%, which was equal to that observed in the treatment of the synthetic wastewater.
Figure 2

The change in COD during the electrochemical treatment of the automobile wastewater with Fe addition (E-Fenton) and without Fe addition (Control).

Figure 2

The change in COD during the electrochemical treatment of the automobile wastewater with Fe addition (E-Fenton) and without Fe addition (Control).

The automobile wastewater generally contains chelating agents and surfactants (Chang et al. 2001; Kim et al. 2002), which are problematic as they mask Fe2+ and Fe3+ (Li et al. 2007) and cause excessive foaming during wastewater treatment, respectively. In fact, the foaming phenomenon was observed during the EF-HOCl-ReFe in this research, although it was not serious. Some papers reported that the chelating agents did not affect the Fenton and Fenton-type reactions (Rastogi et al. 2009; Kishimoto et al. 2013), because most chelating agents do not function well at acidic pH values. Thus, the EF-HOCl-ReFe was effective in removing COD from the automobile wastewater without any inhibition. However, a long operating time will be required for the complete removal of COD from wastewater, because it contains a very high concentration of COD.

Metal scrap-cleansing wastewater treatment

The EF-HOCl-ReFe for the pre-treated metal scrap-cleansing wastewater was performed using the electrochemical flow cell to increase the current density from 2.8 mA/cm2 used for the automobile wastewater to 22 mA/cm2, which was expected to enhance the COD removal efficiency during a certain operating time. Figure 3 shows the change in COD during the operation. The COD can be described using zero-order reaction kinetics in the same way as that for the automobile wastewater. The COD removal efficiency reached 91% after operating for 48 h. Thus, almost complete removal of COD was achieved by the EF-HOCl-ReFe.

Figure 3

The change in COD during the EF-HOCl-ReFe for the metal scrap-cleansing wastewater. The error bar indicates the standard error.

Figure 3

The change in COD during the EF-HOCl-ReFe for the metal scrap-cleansing wastewater. The error bar indicates the standard error.

Figures 4 and 5 show the absorption spectrum and pictures of the treated wastewater, respectively. The original wastewater had two absorption peaks at wavelengths of 267 and 285 nm, which were nearly the same wavelengths as are found for the adsorption peaks of some aromatic compounds, namely, 267 nm for nitrobenzene, 269 nm for phenol, 281 nm for o-aminophenol and o-nitroaniline, and 287 nm for hydroquinone (Urano et al. 1981). These peaks rapidly disappeared during the 12 h of operation, but a broad peak at a wavelength of 420 nm appeared instead. The broad peak at 420 nm then disappeared and the absorption spectrum was gradually lowered at all of the observed wavelengths by the continuing operation. As a result of the absorption spectrum changes, the colour of the wastewater changed from light yellow to brown during the first 24 h of operation. Thereafter the colour was weakened and almost disappeared after 48 h of operation.

Figure 4

Changes in the adsorption spectrum of the metal scrap-cleansing wastewater over time.

Figure 4

Changes in the adsorption spectrum of the metal scrap-cleansing wastewater over time.

Figure 5

Pictures of the metal scrap-cleansing wastewater at each sampling time. The number on the bottle shows the sampling time (in h).

Figure 5

Pictures of the metal scrap-cleansing wastewater at each sampling time. The number on the bottle shows the sampling time (in h).

Thus, the EF-HOCl-ReFe was demonstrated to achieve almost complete removal of COD and chromaticity from the metal scrap-cleansing wastewater.

A classical Fenton process has been actually applied to the treatment of metal scrap-cleansing wastewater, in which 83 L of 50% H2O2 solution and 10 kg of FeSO4 · 7H2O are used for COD removal from 1,000 L of wastewater and the final COD is below 1,000 mg/L. The chemical cost of H2O2 and FeSO4 · 7H2O amounts to 10.1 JPY/L-wastewater. When the EF-HOCl-ReFe was applied to the treatment of metal scrap-cleansing wastewater, the COD in the wastewater was decreased to 517 mg/L by the 48 h of operation, which consumed the electric power of 539 Wh/L-wastewater (Figure 3). Since the electric power cost for industrial use in Tokyo is 15.85 JPY/kWh at present, it costs 8.5 JPY/L-wastewater. Thus, the EF-HOCl-ReFe is more economical in feeding Fenton reagents than the classical Fenton process.

1,4-Dioxane-added municipal wastewater treatment

Figure 6 shows the change in 1,4-dioxane concentration at each unit process. The mixed liquor of the municipal wastewater and activated sludge contained 0.081 mM 1,4-dioxane before aeration, which was not removed by the activated sludge process. The coagulation process with FeCl3 as a coagulant was also unable to separate 1,4-dioxane from the water; the observed 1,4-dioxane concentration was 0.080 mM after the coagulation process. The effluent from the coagulation process was then slightly diluted with H2SO4 for the pH adjustment to 2.0. As a result, the 1,4-dioxane concentration before the EF-HOCl-ReFe was 0.073 mM. After operating the EF-HOCl-ReFe for 30 min the 1,4-dioxane concentration dropped to 0.023 mM, corresponding to a removal efficiency of 68.5%. Thus, the EF-HOCl-ReFe was effective in removing 1,4-dioxane, which could not be removed by the activated sludge process and the coagulation process.

Figure 6

The concentration of 1,4-dioxane at each unit process for 1,4-dioxane-added municipal wastewater.

Figure 6

The concentration of 1,4-dioxane at each unit process for 1,4-dioxane-added municipal wastewater.

Figure 7 shows the change in 1,4-dioxane concentration during the EF-HOCl-ReFe. The 1,4-dioxane degradation followed pseudo-first-order reaction kinetics, whereas zero-order reaction kinetics was observed for the COD removal from the automobile and metal scrap-cleansing wastewaters. In the EF-HOCl-ReFe, HOCl and Fe2+ are continuously regenerated from Cl and Fe3+ by the electrode reactions shown in Equations (1)–(3). Accordingly, these chemicals can be regarded as kinds of homogeneous catalysts. Therefore, the electrochemical Fenton-type reaction is thought to approximate to the following kinetics. 
formula
6
where r is the reaction rate, R is the maximum reaction rate, C is the pollutant concentration in water, and K is the half-saturation constant. When C > >K, Equation (6) reduces to zero-order reaction kinetics with a reaction rate of R. This is true of the COD removal from the automobile and metal scrap-cleansing wastewaters, which contained a high concentration of COD (12,940 mg/L for the pre-treated automobile wastewater and 5,900 mg/L for the pre-treated metal scrap-cleansing wastewater). However, when C < <K, Equation (6) reduces to first-order reaction kinetics with a reaction rate constant of R/K. This is true of 1,4-dioxane removal from the municipal wastewater, because the 1,4-dioxane concentration in the pre-treated municipal wastewater was only 0.073 mM, which was equivalent to a COD of 12 mg/L. Thus, pseudo-first-order reaction kinetics was observed in the 1,4-dioxane-added municipal wastewater treatment.
Figure 7

The change in 1,4-dioxane concentration during the electrochemical treatment of the pre-treated municipal wastewater.

Figure 7

The change in 1,4-dioxane concentration during the electrochemical treatment of the pre-treated municipal wastewater.

CONCLUSIONS

The applicability of the EF-HOCl-ReFe to organic matter removal from three actual wastewaters was discussed in this research. The conclusions are summarized as follows:

  1. The wastewater from an automobile factory was characterized by a high concentration of COD (13,250 mg/L). The EF-HOCl-ReFe successfully removes COD from the wastewater pre-treated by a coagulation process. Although the apparent CE reached 86%, the contribution of the electrochemical Fenton-type system was estimated to be 46%, which was equal to the CE observed for synthetic wastewater. Thus, the EF-HOCl-ReFe was applicable to the wastewater treatment without any inhibition.

  2. The metal scrap-cleansing wastewater was characterized by a yellow colour, an intermediate concentration of COD (6,550 mg/L), and a high concentration of Cl (1,560 mM). The COD removal by the EF-HOCl-ReFe followed zero-order reaction kinetics and the COD removal efficiency after 48 h of operation reached 91%. Furthermore, the yellow colour was eliminated, although a temporary intensification of colour was observed before the decolourization. Thus, the EF-HOCl-ReFe was effective in COD and colour removal from metal scrap-cleansing wastewater.

  3. To demonstrate the applicability to the removal of persistent organic matter in municipal wastewater, the EF-HOCl-ReFe was applied to the 1,4-dioxane-added municipal wastewater treatment. Although the activated sludge and coagulation processes used as pre-treatments were unable to remove 1,4-dioxane, the EF-HOCl-ReFe successfully decomposed 1,4-dioxane in the pre-treated municipal wastewater. The removal efficiency after 30 min of operation reached 68.5%.

ACKNOWLEDGEMENT

This work was performed under JSPS KAKENHI no. 23560655, funded by the Japan Society for the Promotion of Science.

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