### 1. Introduction

_{2}, H

_{2}O, N

_{2}, and sulfite ions [6]. However, these processes have some difficulty in degradation related to persistent organic matters in wastewater [3].

##### (1)

$${\text{S}}_{2}{{\text{O}}_{8}}^{2-}+{\text{e}}^{-}\to {{\text{SO}}_{4}}^{\u2022-}+{{\text{SO}}_{4}}^{2-}$$##### (4)

$${\text{S}}_{2}{{\text{O}}_{8}}^{2-}+{\text{M}}^{\text{n}+}\to {{\text{SO}}_{4}}^{\u2022-}+{{\text{SO}}_{4}}^{2-}+{\text{M}}^{\text{n}+1}$$##### (5)

$$2{\text{S}}_{2}{{\text{O}}_{8}}^{2-}+2{\text{H}}_{2}\text{O}\stackrel{{\text{HO}}^{-}}{\to}3{{\text{SO}}_{4}}^{-}+{{\text{SO}}_{4}}^{\u2022-}+{{\text{O}}_{2}}^{\u2022-}+4{\text{H}}^{+}$$^{2}[16]. When an electrochemical is combined with persulfate (PS), TOC is degraded by over 60% [17]. The phenol decomposition in the solutions was high at 75.3% when using the persulfate/electrochemical/FeCl

_{2}system (PS/Electro/Fe) at optimal conditions (1000 mg/L Na

_{2}S

_{2}O

_{8}and 500 mg/L FeCl

_{2}at 15 V) [18]. Optimization of atrazine depletion (> 99%) and TOC removal (22.1%) performance was achieved after 35 minutes of treatment with atrazine dose 3.0 g/L, PS concentration 4.0 mM at a current density 4 mA/cm

^{2}and an initial pH of 6.3 [19].

_{2}-Nb

_{2}O

_{5}anode for textile wastewater treatment and make the comparision with traditional superior Ti/BBD anode in the present of persulfate catalysts. In this study, the persulfate assisting electrochemical oxidation was used to treat biologically textile wastewater on Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes. Various factors affecting the electrochemical process, such as NaCl electrolyte concentration, persulfate catalyst concentration, current density, initial pH, stirring speed, oxidant generations, and energy consumption were studied on the pollutant removal efficiencies.

### 2. Materials and Methods

### 2.1. Textile Wastewater Effluent

### 2.2. Research Models

_{2}-Nb

_{2}O

_{5}electrode with dimensions of 3 cm × 2 cm × 0.25 mm (length × width × thickness) were used as anodes. The electrode pair was fixed to the reactor cover, and the distance between the two electrodes was kept at 2 cm. The reactor was then placed in a magnetic stirrer (Velp AREC.X, Italy) with a speed of 100 rpm to get a good diffusion and mass transfer. The Keithley's DC Power Supply 2260B-260-4 (DC, Taiwan) was used to control the current with a potential range of 0–150 V.

_{2}-Nb

_{2}O

_{5}electrode was fabricated using the sol-gel method based on the hydrolysis and condensation reactions of the precursors. 1 mL of mixing SnCl

_{4}0.1 M and NbCl

_{5}0.1 M solution was prepared to get a molar ratio between SnO

_{2}:Nb

_{2}O

_{5}was 4:6, then mixed for 120 min at 60°C. Finally, the precusor solution was coated on the Titanium substrates and calcinated at a temperature of 500°C in 1 hour. The process repeated 10 times until the desired thickness of SnO

_{2}-Nb

_{2}O

_{5}was obtained.

### 2.3. Analytical Methods

_{2}Cr

_{2}O

_{7}with Ag

_{2}SO

_{4}in H

_{2}SO

_{4}concentrated acid using RD - 125 heater, Lovibond (Germany). Absorbance was measured at wavelength 604 nm using UV–1800 spectrophotometer Shimadzu (Japan). TOC was determined using a Shimadzu TOC-L (Japan). In situ Cl

_{2}free chlorine present in the effluent water after electrolysis was determined using DPD method using a DR900 multi-index portable colorimeter (USA) as suggested by Dr. Joonseon Jeong and Prof. Yoon Jeyong’s group at the Seoul National University, South Korea [21]. HO

^{•}and SO

_{4}

^{•−}radicals produced at the anode surfaces during electrochemical processes were consumed in the presence of tert-Butyl alcohol (TBA) and ethanol [22]. The nPS/nEtOH ratio was 1/250 and the nPS/nTBA ratio was 1/250. H

_{2}O

_{2}concentration was determined using DMP reagent and Cu(II) sulfate 0.1M solution. Ozone concentration was determined using the Indigo Colorimetric Method [21]. The data were then statistically analyzed using an ANOVA two-factor replication, with a significance level of 0.05 (p = 0.05). The hypothesis H0 is insignificant different and H1 is significant different; if F

_{fisher}> F

_{crit}then accept H0, in contrast, if F

_{fisher}< F

_{crit}then reject H0.

### 3. Results and Discussion

### 3.1. Effects of NaCl Electrolyte Concentration

_{2}-Nb

_{2}O

_{5}anode (D–F). As can be seen in Fig. 2, color, COD, and TOC removal performances in both anodes significantly increase with the increase of NaCl concentration as the supporting electrolyte. Since the textile wastewater was collected after the activated sludge process of the current wastewater treatment plant of the factory, it was still lacking in conductivity for electrochemical reactions. In overview, high color, COD, and TOC removal efficiencies at 3 g/L and 5 g/L NaCl were achieved with both anodes. However, the Ti/BDD electrode at NaCl concentration 3 g/L produced an equivalent removal yield at NaCl 5 g/L. The removal yields on the Ti/BDD anode increased from 39.69 ± 2.16% to 100% for color, 29.78 ± 1.57% to 54.66 ± 1.44% for COD, and 11.95 ± 1.31% to 27.77 ± 1.07% (F

_{fisher}< F

_{crit}) for TOC when measured at 30 minutes and 180 minutes of electrolysis at 3 g/L NaCl. When using the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode, the removal efficiencies increased from 33.91 ± 1.27% to 80.43 ± 2.15% for color, 19.83 ± 1.14% to 49.82 ± 0.85% for COD, and 3.43 ± 1.99% to 36.63 ± 1.23% (F

_{fisher}< F

_{crit}) for TOC at NaCl concentration of 3 g/L after 30 minutes to 180 minutes of electrolysis. The color, COD, and TOC removal efficiencies when using the Ti/BDD anode were higher than when using the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode. At the anode surface, chlorine is made up of chloride ions, which forms hypochlorous acid and is further dissociated into hypochlorite ions, as shown in Eq. (6–8) [23]:

_{2}and H

_{2}O which causes a decrease in color, COD, and TOC values after the electrochemical process, as shown in Eq. 9–10:

### 3.2. Effects of Persulfate Concentration

_{2}-Nb

_{2}O

_{5}anode (D–F). Fig. 3A shows that the decolorization efficiency of Ti/BDD anodes was very high, almost reached 100%. When increasing PS concentration, the decolorization rate occurs quickly. At a [PS] 2 g/L after 60 minutes of electrolysis, the color decreased from 230 Pt-Co to 0 with a removal yield of 100%. The COD removal efficiency when using PS 3 g/L after 30 minutes to 180 minutes of electrolysis increased from 64.47 ± 1.29% to 69.26 ± 1.09% (F

_{fisher}< F

_{crit}), and in the absence of PS, changed to 54.66 ± 1.44% (F

_{fisher}< F

_{crit}). TOC decomposition efficiency was highest at 2 g/L PS, and the removal yield increased from 22.66 ± 1.88% to 49.48 ± 2.42% (F

_{fisher}< F

_{crit}) after 30 minutes to 180 minutes of electrolysis. The oxidants generated in the electrochemical processes as sulfate radicals and hydroxyl radicals could oxidize organic compounds in wastewater, as shown in Eq. 11–13 [25]:

##### (11)

$${\text{S}}_{2}{{\text{O}}_{8}}^{2-}+\text{R}\to {{\text{SO}}_{4}}^{2-}+\to {{\text{SO}}_{4}}^{\u2022-}+\text{P}$$##### (12)

$${{\text{SO}}_{4}}^{\u2022-}+{\text{H}}_{2}\text{O}\to {{\text{SO}}_{4}}^{2-}+{\text{HO}}^{\u2022}+{\text{H}}^{+}$$##### (13)

$$\{\begin{array}{c}{SO}_{4}^{\u2022-}\\ {\text{HO}}^{\u2022}\end{array}+\{\begin{array}{c}R\\ P\end{array}\to {\text{CO}}_{2}+{\text{H}}_{2}\text{O}$$_{4}

^{•−}radical which is a strong oxidizing agent capable of oxidizing refractory pollutants in wastewater. However, increasing the PS concentration beyond the optimal concentration will cause competition between strong oxidizing radicals SO

_{4}

^{•−}, HO

^{•}and the mutual self-reduction of SO

_{4}

^{•−}radicals, as shown in Eq. 14 [26]. In addition, a significant excess of the S

_{2}O

_{8}

^{2−}anion will consume the HO

^{•}generated at the anode, as shown in Eq. 15, which reduces the COD removal efficiency in the electrochemical process [27]:

##### (14)

$${{\text{SO}}_{4}}^{\u2022-}+{{\text{SO}}_{4}}^{\u2022-}\to {\text{S}}_{2}{{\text{O}}_{8}}^{2-}$$##### (15)

$${\text{S}}_{2}{{\text{O}}_{8}}^{2-}+{\text{HO}}^{\u2022}\stackrel{k=1.2\times {10}^{7}\hspace{0.17em}L\hspace{0.17em}{mol}^{-1}\hspace{0.17em}{s}^{-1}}{\to}{{\text{HSO}}_{4}}^{-}+{{\text{SO}}_{4}}^{\u2022-}+0.5{\text{O}}_{2}$$_{2}-Nb

_{2}O

_{5}anode. Fig. 3D shows that after 180 minutes of electrolysis, the decolorization yields at Ti/SnO

_{2}-Nb

_{2}O

_{5}anode were 80.43 ± 2.15%, 82.6 ± 2.2%, 69.57 2.15%, 73.9 ± 2.2%, and 70.87 ± 1.21% (F

_{fisher}< F

_{crit}), corresponding to PS concentrations 0, 1, 2, 3, and 5 g/L. The decline in COD removal efficiency with increasing PS concentration is evident in Fig. 3E, when the COD decomposition efficiency decreased from 49.82 0.85% at PS 0 g /L to 43.97 ± 1.15% (F

_{fisher}< F

_{crit}) while adding PS 1 g/L, and further decreased to 28.31 ± 1.45% (F

_{fisher}< F

_{crit}) when PS concentration increased to 5 g/L. The same trend was reported by Silveira et al. (2017) [17] who studied the decolorization of disperse Blue azo dye 3 (DB3 - C

_{17}H

_{16}N

_{2}O

_{3}) and reached 96% removal efficiency after 60 minutes of electrolysis when using DSA electrode at a current density of 40 mA/cm

^{2}at 50°C, [DB3] = [PS] of 80 mg, [Na

_{2}SO

_{4}] = 0.05 M.

### 3.3. Effects of Current Density

_{2}-Nb

_{2}O

_{5}anode (D–F). Clearly seen in Fig. 4A and Fig. 4D, the higher the current density, the greater the decolorization efficiency. Decolorization performance after 90 minutes of electrolysis at current densities 2.50, 4.16, 5.83, and 7.50 mA/cm

^{2}were 63.91 ± 1.26%, 84.78 ± 2.18%, 100%, and 100% (F

_{fisher}< F

_{crit}) with the Ti/BDD anode, and 29.57 ± 1.21%, 45.65 ± 2.18%, 58.70 ± 2.2%, and 63.04 ± 3.75% (F

_{fisher}< F

_{crit}) with Ti/SnO

_{2}-Nb

_{2}O

_{5}anode, respectively. The decolorization efficiency when using the Ti/BDD anode is higher than that of the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode. Fig. 4B and Fig. 4E show clearly that at the Ti/BDD anode, the COD removal efficiency at current densities 4.16, 5.83 and 7.5 mA/cm

^{2}were the same. The COD removal efficiency from the first 30 minutes to 120 minutes of electrolysis significantly decreased, but after 120 minutes of electrolysis, the COD removal efficiencies at these three current densities appeared stable. The highest COD removal efficiency at the current densities 4.16, 5.83 and 7.5 mA/cm

^{2}were 66.91 0.93%, 69.66 ± 0.55%, and 69.54 ± 0.96% (F

_{fisher}< F

_{crit}), respectively. This result showed that when the critical current density is reached at the Ti/BDD anode, the pollutant removal efficiency of the electrochemical process does not increase further. In contrast to the Ti/BDD anode, the COD removal efficiency on the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode increased sharply after 120 minutes of electrolysis. The peaks of the COD removal efficiencies were 36.03 ± 1.19%, 43.850.83%, 46.211.04% and 47.29 0.60% (F

_{fisher}< F

_{crit}) at current densities of 2.50, 4.16, 5.83 and 7.50 mA/cm

^{2}. The COD removal efficiency has an insignificant difference at current densities 4.16, 5.83 and 7.50 mA/cm

^{2}. In another aspect, at a current density of 4.16 mA/cm

^{2}, TOC removal efficiencies between 30 minutes to 180 minutes increased from 36.88 ± 1.14% to 43.78 ± 1.09% (F

_{fisher}< F

_{crit}) at the Ti/BDD anode and 15.11 ± 1.58% to 34.64 ± 0.73% (F

_{fisher}< F

_{crit}) on the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode. Free HO

^{•}and persulfate radicals were generated more and more at the anode surface as the current density gradually increased, becoming involved in the oxidation of pollutants, as shown in Eq. 16–17 [28]:

##### (16)

$$\text{S}[\hspace{0.17em}]+{\text{H}}_{2}\text{O}\to \text{S}[{\text{HO}}^{\u2022}]+{\text{H}}^{+}+{\text{e}}^{-}$$##### (17)

$$\text{S}[{\text{HO}}^{\u2022}]+\text{R}\to \text{S}[\hspace{0.17em}]+\text{RO}+{\text{H}}^{+}+{\text{e}}^{-}$$^{•}radicals may combine to form hydrogen peroxide, as seen in Eq. 18:

_{2}O

_{2}) according to Eq. 20 [29]:

##### (22)

$${\text{O}}_{3}+\text{Organic}\to {\text{nCO}}_{2}+{\text{mH}}_{2}\text{O}+{\text{O}}_{2}+\dots $$_{2}O

_{2}, according to Eq. 23 [32]:

^{2}to achieve COD removal efficiency of 62%, 70%, 72% and 68% (F

_{fisher}< F

_{crit}) in textile effluent electrolysis at 28°C, pH 9.5 with a support electrolyte NaCl concentration 1 g/L. It is also comparative to the treatment of coking wastewater using electrochemical oxidation. Here, COD removal efficiencies were 66.4%, 78.7%, 82.3% and 84.8% (F

_{fisher}< F

_{crit}) after 60 minutes of electrolysis at current densities of 6, 8, 10 and 12 mA/cm

^{2}[34]. TOC decomposition efficiency upon electrochemical oxidation of methylene blue dye solution in 20 minutes were 40%, 56%, 73%, 78%, 82% and 83% (F

_{fisher}< F

_{crit}) at current densities 2, 4, 6, 8, 10 and 12 mA/cm

^{2}, respectively [35].

### 3.4. Effects of Initial pH

_{2}-Nb

_{2}O

_{5}anodes. In general, the electrochemical pollutant removal efficiencies in a weak base environment better than in the acid environment. Fig. 5 (A) and Fig. 5 (D) show that the decolorization of textile effluent on the Ti/BDD anode occurs very quickly with 100%, 94.35 ± 1.27%, 81.74 ± 1.27%, 79.57 ± 1.21%, and 63.91 ± 1.27% (F

_{fisher}< F

_{crit}) after 30 minutes of electrolysis at pH 2–10. The decolorization yields at Ti/SnO

_{2}-Nb

_{2}O

_{5}anode were 81.71 ± 1.27%, 77.39 ± 1.25%, 72.61 ± 1.25%, 66.09 ± 125%, and 63.04 ± 2.15% (F

_{fisher}< F

_{crit}) after 180 minutes of electrolysis at pH 2–10. The highest COD and TOC decomposition yields at pH 2 were 56.51 ± 1.1% and 57.12 ± 0.82% (F

_{fisher}< F

_{crit}) at the Ti/BDD anode and 51.18 ± 1.18% and 51.76 ± 1.5% (F

_{fisher}< F

_{crit}) at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode after 180 minutes of electrolysis. At a higher pH (6–10), COD, and TOC removal efficiencies decreased to 43.36 ± 1.18% and 50.08 0.57% at the Ti/BDD anode and 37.26 ± 0.8% and 31.06 ± 0.81% (F

_{fisher}< F

_{crit}) at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode after 180 minutes of electrolysis. This is because OCl

^{−}radicals and HClO acid exist and function best in a low pH environment [2]. This hypochlorous acid is a strong oxidizing agent as a major agent in the oxidation of dyes leading to the high decolorization of textile effluents, as shown in Eq. 24 [36].

_{2}O

_{2}generated during the electrochemical process decreases as the pH of the solution increases, according to Eq. 25 [36]:

^{−}ion dominates at pH over 8 [39].

### 3.5. Effects of Stirring Speed

_{fisher}< F

_{crit}) at the stirring speeds 200, 400, 600 rpm after 180 minutes of electrolysis. The results show that the pollutant removal efficiencies at the Ti/BDD anode had an insignificant difference depending on the stirring speed, meaning that the mass transfer at the electrode surface is high. When using the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode, the decolorization yields were 74.78 ± 1.26%, 68.70 ± 1.26%, and 64.35 ± 1.26% (F

_{fisher}< F

_{crit}); COD removal yields were 44.22 ± 0.61%, 40.94 ± 0.4%, and 37.95 ± 1.17%; and TOC removal yields were 39.05 0.68%, 38.07 0.92%, and 35.56 0.74% (F

_{fisher}< F

_{crit}) at stirring rates 200, 400, 600 rpm after 180 minutes of electrolysis. The results show that the pollutant removal efficiencies decrease as the stirring speed at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode increases too much. The influence of the stirring speed on the methylene blue (MB) dye decolorization was also reported through the color removal increases alongside the increased the stirring speed [40]. After 240 minutes of electrolysis, the color removals reached 91.84%, 99.35%, and 99.49% at stirring speeds 200, 400, and 700 rpm, respectively. However, the color removal efficiency between the stirring speed 400 rpm and 700 rpm showed an insignificant improvement.

### 3.6. Oxidants Radical Generation

_{2}-Nb

_{2}O

_{5}anodes. The results of Fig. 7 (A) show that when only NaCl 1g/L is present in the electrolyte, the Cl

_{2}concentration produced at the Ti/BDD positive electrode is higher than at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode. Generated Cl

_{2}concentration was 0.62 ± 0.025 mg/L and 0.36 0.042 mg/L at the Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes. In the presence of PS concentration 2 g/L, the capability of both electrodes to produce Cl

_{2}is roughly the same. When the PS in the textile wastewater effluent was added, Cl

_{2}concentrations at Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes were 0.43 ± 0.02 mg/L and 0.46 ± 0.035 mg/L. In this case, PS could be a scavenger to consume Cl

_{2}. The free Cl

_{2}produced during the electrochemical process is an important agent for producing hypochlorous acid and hypochlorite ions that are involved in the oxidation of organic compounds in textile wastewater effluents.

_{2}-Nb

_{2}O

_{5}anodes after 20 minutes of electrolysis. This demonstrates that in the electrochemical process, ozone is produced and will also participate in the oxidation of pollutants. The concentration of ozone produced at the Ti/BDD anode was higher than that at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode under the same electrochemical condition. This could contribute to the higher pollutant decomposition at the Ti/BDD anode than seen at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode.

^{•}and SO

_{4}

^{•−}radicals were generated in the oxidation of organic pollutants resulting in a high CN yield: 68.54 0.61% and 65.76 ± 1.58% at the Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes, respectively. With the addition of tert-Butyl alcohol (TBA) inhibitor, CN removal on both anodes were markedly reduced, indicating that the produced HO

^{•}radical in both anodes were consumed in the oxidation of pollutants. The CN removal efficiency at Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes decreased to 31.73 ± 0.39% and 23.22 ± 2.81% (F

_{fisher}< F

_{crit}), respectively. Furthermore, when the inhibitor of both HO

^{•}and SO

_{4}

^{•−}radicals is ethanol (EtOH), the CN removal efficiencies were 29.74 ± 0.2% and 19.56 2.59% (F

_{fisher}< F

_{crit}) at the Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes. This demonstrates the presence of the SO

_{4}

^{•−}radical at both anodes. Since EtOH further inhibits the SO

_{4}

^{•−}radical, the CN removal efficiency is lower than when inhibited with TBA. Upon continuing to add both inhibitors (TBA and EtOH) in the textile wastewater effluent, the CN removal at both anodes continues to decrease. Specifically, CN removal efficiency decreased to 22.420.53% and 16.27 ± 2.6% (F

_{fisher}< F

_{crit}) at the Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes, which demonstrated the HO

^{•}residue was produced more than the SO

_{4}

^{•−}residue in both anodes and that persistent HO

^{•}radicals were partly consumed by the inhibitors still involved in the oxidation of the pollutant. When using both TBA inhibitor and EtOH, the SO

_{4}

^{•−}radical inhibitor does not change, the HO

^{•}inhibitor increases, and the CN decreases, meaning that the HO

^{•}radical is still inhibited by TBA but not totally, so that upon addition of EtOH, the HO

^{•}radical continues to be inhibited. Can-Güven et al. (2021) [41] reported the changes in CN removal when TBA and EtOH were added under optimal operating conditions. When TBA was added at a ratio of 1/250 to total oxidizer, CN efficiency decreased from 92.1% to 56.8% after 120 minutes of electrolysis. When EtOH was added at a ratio of 1/250 to total oxidizer, the CN removals reduced to 73.8%.

_{2}O

_{2}produced was 17.5 ± 0.5 mL, more than twice as in the presence of NaCl 1g/L, which was 7.5 ± 0.5 mL. At the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode, when there was no persulfate in the electrochemical process, the volume of H

_{2}O

_{2}was generated at 7.5 ± 1 mL. However, similar to the Ti/BDD anode, when PS was added to the electrochemical process, the volume of produced H

_{2}O

_{2}increased very little, from 7.5 ± 1 mL to 8.5 ± 1 mL. In that case, PS could be a scavenger to consume ozone in the degradation process.

### 3.7. Energy Consumptions

_{2}-Nb

_{2}O

_{5}(B) anodes at different current densities. The energy consumption increases as the current density increases in both anodes. When using the Ti/BDD anode at a current density of 2.5 mA/cm

^{2}, energy consumption increased from 4.8–10.9 kWh/kgCOD with a COD removal efficiency of 22.1% to 59.6%. Energy consumption at the current density of 7.5 mA/cm

^{2}increased from 13.8–36.4 kWh/kgCOD with a COD decomposition efficiency of 29.46% to 69.54% (F

_{fisher}< F

_{crit}). At the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode, the highest energy consumption was achieved when a current density of 7.5 mA/cm

^{2}was applied, at which the power consumption increased from 27.5 kWh/kgCO to 128.9 kWh/kgCOD with a COD decomposition efficiency of 23.42% to 47.29% (F

_{fisher}< F

_{crit}). The applied current density of 2.5 mA/cm

^{2}resulted in the lowest energy consumption. Here, power consumption increased from 28.4 to 43.3 kWh/kgCOD with a COD decomposition efficiency of 6.73% to 36.03% (F

_{fisher}< F

_{crit}). The results reveal that the energy consumption at the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode is approximately four times higher than at the Ti/BDD anode. The energy consumption appears to be different when various experimental conditions are applied. The energy consumption was found to be 11.9 kWh/kg COD for the textile effluent upon removal of 52.86% COD under a circulating reactor with a recirculation flow rate of 100 L/h, a discharge flow rate of 3 L/h, an input COD concentration of 560 mg/L, and an applied current density of 5 A/cm

^{2}[42]. COD removal efficiency was achieved at 90% for synthetic dyed textile wastewater (initial COD 780 mg/L) at a flow rate of 500 mL/h (retention time 6 h) and a flow density of 1.15 mA/cm

^{2}to get an energy consumption of 9.2 kWh/kg COD [33].

### 4. Conclusions

_{2}-Nb

_{2}O

_{5}anodes. The results show that the Ti/BDD anode exhibits a higher pollutant removal efficiency and has less energy consumption than the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode at optimal operating conditions. The Ti/BDD anode achieved the best color, COD, and TOC removal efficiencies at 94,781.25%, 64,570.95%, and 41,570.56% (F

_{fisher}< F

_{crit}), respectively, and energy consumption at 11.1 kWh/kgCOD. At the Ti/SnO

_{2}-Nb

_{2}O

_{5}anode, the best color, COD, and TOC removal efficiencies were 72,041.25%, 41,321.21%, and 38,220.55% (F

_{fisher}< F

_{crit}), respectively, and the best energy consumption was 51.3 kWh/kgCOD,. Persulfate presents the different trends in the electro-oxidation behavior of Ti/BDD and Ti/SnO

_{2}-Nb

_{2}O

_{5}anodes in biologically textile wastewater degradation.