Enhancing textile wastewater treatment for subsequent biological processes by integrating electrooxidation and electrocoagulation

The textile industries contribute significantly to the contamination of the surface water bodies by the release of effluents that are laden with complex and persistent dyes and chemicals (Behera et al. 2021; Wang et al. 2022; Jorge et al. 2023). Of various operations such as mercerization, sizing, dyeing, finishing, bleaching, printing performed in these industries, the process of dyeing and finishing produces a majority of contaminants (Wang et al. 2022; Jorge et al. 2023; Kallawar & Bhanvase 2024). The textile industries utilize a variety of hazardous dyes such as vat, acidic, reactive, azo, disperse that do not fully adhere to the fabric and are released into the aquatic ecosystem with wastewater (Yaseen & Scholz 2019; Kishor et al. 2021; Jorge et al. 2023). In addition, the textile effluent contains a huge quantity of binders, salts, dispersants, surfactants, dioxins, phthalates, detergents, heavy metals (Bakaraki Turan et al. 2021; Kishor et al. 2021; Islam et al. 2023). The effluent from textile industries is characterized by the presence of high salinity, inorganic solids content, chemical oxygen demand (COD), total organic carbon (TOC), low biodegradability, and variable pH (Gilpavas et al. 2018b; Zazou et al. 2019; Kuleyin et al. 2022). The highly pigmented effluent generated by these industries also contains a variety of toxic persistent organic pollutants that can travel large distances without degradation, which results in the deterioration of the aquatic ecosystem. Hence, the effluent from these units should be treated prior to discharging into aquatic sources (Badmus et al. 2018; Kishor et al. 2021; Selvaraj & Arivazhagan 2024). Recent technologies such as electrochemical processes are emerging as promising techniques for the remediation of wastewater that contains an extensive amount of non-biodegradable organics (Gunawan et al. 2018; Yakamercan et al. 2023; Barcelos et al. 2024). The electrocoagulation (EC), electro-oxidation (EO) and a combination of these processes with other physical, chemical and biological processes are considered as the most efficient techniques for the treatment of difficult wastewater (Asfaha et al. 2022a; Tokay Yilmaz et al. 2023; Babu Ponnusami et al. 2023).

The EO process is gaining prominence as a robust technology for the remediation of intricate and persistent organic compounds (Gilpavas et al. 2018a; Yao et al. 2022; Navarro-franco et al. 2022). This can be accomplished either through complete mineralization or by transforming them into less toxic substances (Zhu et al. 2011; Song Sun et al. 2022; Navarro-franco et al. 2022). In this process, hydroxyl radicals (●OH), an extremely reactive species, are produced when water at the anodic surface oxidizes. These radicals mineralize organic pollutants indiscriminately. However, complete mineralization of the complex pollutants requires a lot of energy, thus making it impossible to apply the EO process practically (Navarro-franco et al. 2021; Yakamercan et al. 2023; Barcelos et al. 2024). To overcome this challenge, researchers have combined EO with different biological processes. The EO process is used to enhance the biodegradability of the toxic waste so as to prepare it for treatment by biological means (Lindholm-lehto & Knuutinen 2015; Yakamercan et al. 2023; Barcelos et al. 2024). The combination synergises the capability of EO to oxidize recalcitrant and non-biodegradable organic pollutants with the capacity of biological processes to decompose any remaining biodegradable organic matter (Paździor et al. 2019; Navarro-franco et al. 2021; Babu Ponnusami et al. 2023).

Wre et al. (2017) investigated the concentrated stream collected at four different time intervals from the dyeing industry, which consists mostly of effluents from bleaching and dyeing units. The process of ozonation in conjunction with a sequential batch reactor with a 48-h hydraulic retention timewas used to remediate the dyeing effluent. For the four different samples, the combination yielded an overall COD and color removal efficiency ranging between 59–62% and 47–88%, respectively. In their research, Santhanam et al. (2017) explored the treatment of textile dyeing wastewater obtained from a Common effluent treatment plant through a combination of EO and bioprocess. A RuO₂–TiO₂/Ti mesh anode and a bacterial consortium were used for EO and biological processing, respectively. No visible was discovered following EO, and the combined treatment resulted in a COD removal efficiency of 73%.

However, efficient removal of inorganic particles alongside the organic pollutants is crucial for comprehensive wastewater treatment, particularly in textile wastewater where suspended solids are in abundance. These inorganic particles are inert and are not biodegradable. While the EO process effectively targets recalcitrant organic contaminants, it often falls short in addressing the inorganic suspended impurities (Chakchouk et al. 2017; Abbar & Alkurdi 2021; Asfaha et al. 2021). These impurities are also not effectively removed by biological treatment units. Prior to biological processes, wastewater must undergo treatment to eliminate these suspended inorganic particles. Zhu et al. (2011) utilized the dye effluent that had been processed to electrochemically oxidize the organic components. Zero-valent iron and coagulation were used as on-site pretreatments, followed up by up-flow anaerobic sludge blanket treatment. Ling et al. (2016) employed primary sedimentation and activated sludge processes before subjecting the wastewater to electrolysis for refractory organic removal.

EC has developed as an efficient technique for getting rid of various organic and inorganic suspended and colloidal pollutants to reduce the turbidity of various wastewater types (Chakchouk et al. 2017; Bener et al. 2019; Al-Raad & Hanafiah 2021). During the EC process, the in-situ formation of metal hydroxides takes place when a system of electrodes is subjected to an electric current (Chakchouk et al. 2017; Bener et al. 2019; Al-Raad & Hanafiah 2021). These metal hydroxides cause the destabilization of colloidal and suspended particles, allowing attachment when the contact occurs. The gelatinous nature of these hydroxides enables them to entrap the undesirable constituents, which can be further removed from the system through the mechanism of sweep coagulation.

Bener et al. (2019) addressed the pretreated textile wastewater through the EC process. With aluminum electrodes, a maximum COD, color, TOC, turbidity, and total suspended particle removal efficiency of 18.6, 90.3–94.9, 42.5, 83.5, and 64.7% were obtained, respectively, at an optimal current density of 25 mA/cm², pH of 5, and reaction duration of 120 min. Selvaraj & Arivazhagan (2024) treated a lung-producing textile industry effluent that contains turquoise blue dye with EC, followed by an adsorption process. The studies were carried out with aluminum electrodes for EC and algal activated carbon (AAC) adsorbents for adsorption. After EC, under the optimal conditions of current density as 1.5 A/dm² and treatment time of 36 min, a maximum COD removal of 79.97%, color removal of 54.12%, total dissolved solid removal of 14.60% and turbidity removal of 85.91% were achieved, which increased to 91.28, 68.82, 16.04, and 90.96%, respectively, after 12.8 g/L dosage of AAC and time of 44 min.

Despite considerable advances in textile wastewater treatment technologies, there is a substantial gap in the existing literature that discusses the implementation of the EC process as a treatment method post-advanced oxidation process (AOP) to remove suspended pollutants in both carbon and non-carbon forms. While a number of studies have looked into using AOPs in conjunction with other biological processes to address refractory organic pollutants, there has been little investigation into using EC as a sequential treatment step that particularly targets inorganic suspended particles. Given the abundance of inorganic contaminants in textile wastewater and the difficulties in successfully treating them with AOPs and biological processes, investigating the viability as a post-AOP treatment of the EC process might provide a feasible approach for enhancing overall treatment efficiency. By coagulating inorganic particles, EC may reduce the strain on downstream biological processes, filling a critical gap in complete wastewater treatment techniques for the textile sector.

The present study presents a unique approach to the management of organic and inorganic toxic contaminants from textile wastewater. It involves the use of three-dimensional (3D) electrodes for the treatment of wastewater by a hybrid process that involves a combination of EO and EC. The effectiveness of the combination was assessed in terms of color, COD, and TOC removal. The fate of the persistent organic contaminants was analyzed in terms of the biodegradability index (BI). The removal of non-carbonic impurities post EO-EC was assessed in terms of the improvement in COD removal efficiency obtained after EO. In addition, the removal of inorganic carbon from wastewater after the combination was also determined to make sure the complete removal of the inorganic impurities.

The textile wastewater was synthetically prepared in the laboratory using azo red 3BL dye. The azo dyes have the capability to persist in the aquatic ecosystem (Camargo & Morales 2013; Selvaraj et al. 2021; Shi et al. 2021). A variety of chemicals, as reported by many researchers (Mountassir et al. 2015; Punzi et al. 2015; Yaseen & Scholz 2019), including dye, disodium hydrogen phosphate, sodium chloride, and starch were mixed together in the distilled water to prepare the synthetic textile effluent. The characteristics of synthetic textile wastewater are as follows: color – 5015 PCU (Platinum Cobalt Unit), pH – 8.31, conductivity – 7.36 mS/cm, COD – 1010 mg/L, TOC – 320 mg/L and inorganic carbon – 36.37 mg/L.

The major operating parameters for the EO and EC processes were identified from the literature. A preliminary analysis was conducted on simulated textile wastewater to determine an appropriate range of values for these parameters. This range of values was used to design the experiments for the combined process.

After the EO process, the wastewater was treated with the EC process, for which the preliminary investigations were done to find ranges of the variables. The experiments for EC were performed on wastewater treated with EO, keeping the current density and electrolysis time during EO constant at 30 mA/cm² and 50 min. During EC, the precipitation of insoluble aluminum hydroxide flocs occurs predominantly near pH 6 (Adeogun & Balakrishnan 2016; Naje et al. 2016; Fajardo et al. 2017). The pH of the wastewater rises to 4.75 after the EO process, which was further increased to 6 before carrying out the EC process to allow the maximum precipitation of the flocs to take place.

With a constant current density and rotational speed of 18 mA/cm² and 60 rpm, respectively, experiments were carried out to evaluate the effect of electrolysis duration on COD removal efficiency for the EC method. The electrolysis duration was changed from 10 to 60 min. From Figure 3(b), it is apparent that the efficiency of COD removal increases up to 40 min and drops thereafter. This is a result of the production of more ions in the system, which subsequently leads to the release of more Al(OH)₃ flocs for pollutant removal (Ehsani et al. 2020; Mariah & Pak 2020; Boinpally et al. 2023). The development of monomeric species and the passivation of cathode were the causes of the efficiency decline that was observed (Bhagawan et al. 2016; Mousazadeh et al. 2021; Asfaha et al. 2022a, b).

A proper mixing inside the reactor is necessary for the mass transfer of the electrocoagulated species formed at the anodic surface. The speed at which the rotation is given is also a crucial factor to consider as too high a speed can cause the disruption of flocs (Tahreen et al. 2020; Al-Raad & Hanafiah 2021; Villalobos-Lara et al. 2021). The impact of different rotating speeds was analyzed by performing experiments keeping the current density and electrolysis time constant as 18 mA/cm² and 40 min, respectively. As evident from Figure 3(c), the efficiency of COD removal rises up to a particular value of agitation speed. This is because when the motion of the produced cations and anions increases, the flocs form significantly earlier, resulting in a higher pollutant removal (Khandegar & Saroha 2013; Tahreen et al. 2020; Manikandan & Saraswathi 2022). Furthermore, rotational speeds beyond a particular value inhibit the increase in pollutant removal due to the disintegration of the flocs at high speeds (Tahreen et al. 2020; Al-Raad & Hanafiah 2021; Villalobos-Lara et al. 2021).

The pH of the treated wastewater after EO + EC ranges between 8 and 8.5.

The preliminary experimental investigation helps in determining the ranges of the operating parameters, based on which the experimental design was constructed using the Taguchi method in Minitab. The design was constructed considering five parameters of five levels each. The experiments were performed in duplicates, and an average of the two values was considered. COD, color, and TOC were considered as the three major responses to study the performance of the hybrid process. Table 2 depicts the experimental design matrix and the values of corresponding responses.

Furthermore, the contribution of each operating variable in COD, color, and TOC abatement is determined using Equations (4)–(7) and shown in Table 3. From Table 3, it is evident that the current density and electrolysis time during the EC process have the greatest impact on COD and TOC elimination. This can be due to the reason that the wastewater has a high percentage of suspended particles, which are majorly removed during the EC process as EO is ineffective against suspended impurities (Chakchouk et al. 2017; Özyurt & Camcıoğlu 2018; Asfaha et al. 2021). However, from Table 3, it can also be seen that the color removal depends largely on the operating variables at the time of EO. After EO, almost 97.5–99.5% of the color is removed from the wastewater in all the experimental combinations.

For a comprehensive wastewater treatment, inorganic impurities must also be removed alongside the organic contaminants. It was observed after every experimental set that the inorganic carbon content of wastewater after treatment with EO is approximately identical to the inorganic carbon content of the raw wastewater, i.e., 36.37 mg/L which is an average of 25 sets. This indicates that little or no removal of inorganic carbon is taking place during EO. This is owing to the fact that EO is only efficient against dissolved contaminants and cannot remove substantial volumes of suspended particles from wastewater (Chakchouk et al. 2017; Özyurt & Camcıoğlu 2018; Asfaha et al. 2021). To determine the amount of inorganic carbon removed in the EC process post EO process, the experiments for EC were performed after EO at a constant speed of rotation of 50 rpm and varying current density and electrolysis time between 14–22 mA/cm² and 30–50 min. For all the experiments, the operating parameters during EO were kept fixed as: current density = 25 mA/cm², electrolysis time = 50 min.

The improvement in the BI and total energy consumption during EO + EC treatment of the textile wastewater is presented in Figure 7(b). For 50 min electrolysis during EO and 60 min electrolysis during EC, the energy consumption was 32.28 and 6.25 KWh/m³, respectively. This indicates that the EO process alone to achieve a greater removal is not a cost-effective method of remediation. Also, it falls short of addressing the suspended impurities (Chakchouk et al. 2017; Özyurt & Camcıoğlu 2018; Asfaha et al. 2021). However, EC as a sequential step after EO resulted in a greater pollutant removal (both dissolved and suspended) without a significant rise in energy consumption (Özyurt & Camcıoğlu 2018; Asfaha et al. 2021; Tanti & Patel 2023). The total energy consumption for the combined EC + EO at the optimum condition was 36.52 KWh/m³. It is also indicated by Figure 7(b) that following hybrid treatment, the BI of wastewater considerably increases. The BI determines the toxicity of the wastewater. The effluent with BI less than 0.3 is toxic and cannot be remediated biologically (Nagar & Devra 2019; Dhanke & Wagh 2020; Bader et al. 2022). During EO, the BI improved significantly from an initial value of 0.098–0.350. However, for complete biodegradation to take place, the wastewater must have a BI of more than 0.4 (Selvakumar et al. 2010; Rudaru et al. 2022; Yakamercan et al. 2023). From Figure 7(b), it can be seen that the BI increased progressively during the EC process to 0.737 after 40 min of electrolysis. This increase can be accounted for by the elimination of inorganic suspended particles during EC (Chakchouk et al. 2017; Al-Raad & Hanafiah 2021; Asfaha et al. 2021). This suggests that EC after EO eliminated the inorganic suspended contaminants to enhance the BI of the wastewater and make it amenable for biological processes that EO alone cannot.

The efficacy of the EO process in eliminating inorganic pollutants from the wastewater is limited. These pollutants are also inert to biological degradation. Consequently, to prepare the wastewater for biological treatment, a secondary treatment post EO is necessary to eliminate inorganic pollutants (carbon and non-carbon). The efficiency of the combined EO + EC process is accessed in terms of COD, color, and TOC removal. L₂₅ experimental design was obtained by performing a parametric analysis to identify the proper ranges for the operational variables. This design was analyzed in Minitab to obtain the optimal combination of the operating variables. According to the S/N ratio plot, for maximum pollutant removal, the optimal combination for EO is current density = 25 mA/cm², electrolysis time = 50 min followed by EC with current density = 18 mA/cm², speed of rotation = 50 rpm ,and electrolysis time = 40 min. A maximum enhancement in the removal efficiencies of COD and TOC were obtained as 65.11 and 63.57%, respectively, post EC over EO, indicating the efficiency of the EC process in eliminating non-carbonic suspended impurities. The inorganic carbon content also decreased from its value of 36.37 mg/L after EO to 0.1 mg/L post EC, and the BI has improved from an initial value of 0.098–0.737 post EO + EC. The quality of treated wastewater has improved significantly in terms of refractory organic and inorganic pollutant removal. The improved biodegradability made the wastewater fit for further biological treatment. Therefore, it becomes necessary to incorporate EC as an additional treatment process post EO to lessen the load on the biological treatment units. The maximum percentage removal efficiencies of COD, color, and TOC after EO + EC at the optimal conditions were 74.69, 99.64, and 75.49%, respectively.

The authors gratefully acknowledge the Public Health Engineering (PHE) lab of MNIT Jaipur for fulfilling our requirements timely.

P.A. developed methodology, performed experimental analysis and investigation, wrote the original draft, reviewed and edited. B.G. performed experiments, experimental analysis and investigation, reviewed and edited the original draft. S.M. conceptualized, supervised reviewed and edited the original draft.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.