Electrochemical treatment of real cotton fabric industry wastewater using copper and stainless steel electrodes Open Access

Fabric manufacturing is one of the largest and oldest industries in the world. India is the third largest exporter of textiles worldwide, preceded by China and the European Union. The Indian fabric industry is the world's largest base raw material producer (Yadav et al. 2013). In India, there are 3,400 textile mills; the fabric industry contributes to 14% of the annual industrial production with 4% of the GDP, 18% of employment and 27% of foreign exchange (Prajapati et al. 2016). The fabric manufacturing process steps include sizing, desizing, scouring, bleaching, dyeing and finishing. The dyeing and finishing processes during fabric manufacture generates large quantities of wastewater loaded with chemicals contributing to non-biodegradable matter (Sarayu & Sandhya 2012). Cotton fabric industry wastewater (CFIWW) is loaded with organic and inorganic materials, heavy metals and many colourants. The different types of dyes used in textile wet processing are acid dyes, basic dyes, direct dyes, mordant dyes, sulphur dyes, azoic dyes, vat dyes, reactive dyes, disperse dyes, synthetic dyes, pigment dyes, macromolecular dyes, metalized dyes, premetallized dyes, naphthol dyes, developed dyes, aniline dyes and anthraquinone dyes (Benkhaya et al. 2020). More than 10,000 dyes are currently available commercially, of which azo dyes are the largest group commonly used in textile industries because of their chemical stability.

Currently, chemical and biological processes for the treatment of CFIWW are applied in real scale treatment: chemical coagulation and flocculation (Verma 2017), adsorption (Rashid et al. 2021), oxidation (Hassaan et al. 2017) and biological methods such as activated sludge process (Haddad et al. 2018), aerobic and anaerobic treatment (Shoukat et al. 2019), and biological degradation using photocatalysis (Ceretta et al. 2020). These conventional treatment methods are neither effective nor efficient in the complete removal of contaminants/pollutants; they require a large number of unit operations/processes, high hydraulic retention time (HRT), high energy foot print, high capital cost (Singh et al. 2018), operational and maintenance cost, and most importantly, they are non-robust and less reliable than conventional treatment systems (Mahesh et al. 2022).

In the past two decades, ECT is found to be effective for the treatment of a wide variety of wastewater, namely coffee processing wastewater (Sahana et al. 2018), bakery industry wastewater (de Santana et al. 2018), tannery wastewater (Basha et al. 2009), sugarcane process industry wastewater (Sahu et al. 2017), food processing industry wastewater (Barrera-Díaz et al. 2006), distillery wastewater (Thakur et al. 2009), dairy wastewater (Venkata Mohan et al. 2010), petroleum industry wastewater (Treviño-Reséndez et al. 2021), hospital wastewater (Singh et al. 2018; Mahesh et al. 2022), pulp and paper mill wastewater (Mahesh et al. 2006a, 2006b). Most research work pertaining to fabric industry wastewater focuses on single waste dye streams or the use of synthetic wastewater. The challenge is to deal with real wastewater by removing pollutants and applying the best treatment methods. Therefore, in this work, we propose the use of real wastewaters (CFIWW) for in-depth characterization followed by ECT using two different electrode materials, Cu and SS, for the removal of COD, salts, alkalinity and TDS from raw CFIWW. The electrochemical coagulation (ECC) sludge characteristics obtained after treatment are also reported.

The CFIWW was collected from Nanjangud, an industrial hub 15 km south of Mysuru city. The initial characterization of raw CFIWW before ECC was carried out as per standard procedures for the analysis described in Standard Methods (APHA 2017).

Pre-characterization of raw CFIWW was required before ECC, as it provided input for choosing the appropriate electrode materials. The grab sample technique was used to collect CFIWW at the end of the pipe (EOP). The characteristics of raw CFIWW are shown in Table 1. All quality parameters were analysed in duplicate and the range of values is reported. Table 2 also provides EOP discharge standards in general, not specific to fabric manufacturing wastewaters.

The initial pH (pH_o) of raw CFIWW was 7.63 in the near neutral range, slightly alkaline. pH is an important parameter because metal ions form several metal hydroxides and the coagulated species remove the contaminants from the wastewater. The second most important parameter is total alkalinity (TA) which assists in the formation of electro-flocs, thereby aiding the removal of pollutants from the waste stream (Mahesh et al. 2022). The TA present in raw CFIWW was 1,500–1,540 mg/L and chloride 1,107–1,137 mg/L, COD and BOD values were relatively high, i.e. 6,300–6,500 mg/L and 1,500–1,600 mg/L.

The results of the ECC process treating raw CFIWW are reported and discussed in the following sub-sections.

A higher cell current of 4.4 A for SS was able to remove TDS marginally from the solution, whereas 1.5 A current could not remove TDS from the bulk solution. Salt utilization in forming floc and simultaneous release of weakly charged M⁺ ions at low cell current (1.5 A) in the case of SS, resulted in no change of TDS in the ECC treated supernatant. In the absence of sufficient charge, there is a tendency for TDS values to increase during ECC with ET (Singh et al. 2018).

Chloride reduction during ECC is the result of the oxidation of Fe²⁺ ions into Fe³⁺ ions (Sahana et al. 2018). A cell voltage of 4.4 A for SS electrodes showed 94% chloride removal compared to 88% TDS removal for the same cell current using Cu.

The chloride ions in CFIWW were able to dissolve the passive oxide film that had formed on the anode surface causing a reduction in metal dissolution and electron transfer, thereby increasing the COD removal efficiency. Interestingly, for 1.5 A cell current using SS, the COD removal was zero; this condition can occur when the relative applied cell current is low, wherein the metal dissolution is high in a salty environment; and a poor charge on M⁺ ions results in COD addition to the waste stream, making it much more turbid.

Sludge production is an inevitable part of a biological/chemical/ECT process. The ECC process is very effective in controlling sludge volume as desired (Shivaprasad & Mahesh 2022).

In the functional range of 4,000–1,500 cm⁻¹ stretching and vibrations are observed. For SS, at 1,500 and 1,800 cm⁻¹ N–H stretch indicate the presence of amines. At 2,977 cm⁻¹, there is a C–H showing the presence of carboxylic acid. At 3,752 cm⁻¹, the alkyne O–H stretch is found to show alcohol. For Cu at 1,519 cm⁻¹, an aromatic ring stretch is observed. At 3,679, 3,806, 3,972 cm⁻¹, O–H bonding is present. An alkene C–H stretch at 2,973 cm⁻¹ is observed (Aragaw & Aragaw 2020).

Real CFIWW was treated in an ECR using Cu and SS electrodes in a bipolar arrangement for cell voltages 12.7 and 30.2 V, respectively, and their corresponding current at a low HRT of 30 min. Wastewater quality parameters monitored were pH, chloride, alkalinity, COD and TDS. The ED, sludge volume and SVI were determined. This information is useful in estimating the life cycle of an electrode, designing an ECR, evaluating operation and maintenance costs, sludge handling and arriving at logical end disposal/reuse of ECC sludge. SEM, EDS and FT-IR for the ECC sludge were reviewed for morphology, elemental composition and functional groups. Pre-ECC and post-ECC characterization for COD, TDS, chlorides and alkalinity were evaluated for ECC treated effluent. The experimental results showed that ECC is a promising novel technology giving the least HRT for treating cotton fabric industry wastewaters. The specific energy consumption (SEC) worked out to 20–42 kW/kg of COD removed for Cu, and 31–56 kW/kg of COD removed for SS, respectively.

The authors thank JSS Science and Technology University, Mysuru Karnataka State, India for providing research infrastructure in the specialized electrochemical treatment laboratory in the Department of Environmental Engineering to carry out this research work.

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

The authors declare there is no conflict.