Integrated electrochemical processes for textile industry wastewater treatment: system performances and sludge settling characteristics
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Textile wastewater containing toxic dyes needs efficient treatment before being released into the environment. Certain dyes are known or presumed to have carcinogenic potential for humans. In this work, hybrid electrochemical processes including electrocoagulation (EC) alone and combined with electro Fenton (EF), anodic oxidation (AO) and peroxi-coagulation (PC) were tested to treat real textile wastewater using a batch reactor. A sequential EC and EF (EC-EF) process was found to be more effective. The experimental results indicated that the effectiveness of the treatment decreases in the following order: EC-EF > EC-AO > EC-PC > EC. EC-EF results showed a decrease in chemical oxygen demand (COD, 97%), total organic carbon (98%), total suspended solids (98%), and the concentration of metal species; showing that the treatment of such wastewater type can be achieved by combined EC-EF process in a one-pot bench-scale reactor. The electrical energy consumption, the iron dissolution, and the biological oxygen demand/COD ratios of EC and EC-EF processes were evaluated. Characterization of the sludge generated during EC treatment at current density of 20 mA cm− 2 was carried out. Precipitation, adsorption, and electrochemical oxidation/reduction of organic dyes and metallic ions occurred during the treatment. This investigation shows the efficiency of combined EC-EF to treat textile wastewater.
KeywordsTextile wastewater Electrocoagulation Electro-Fenton COD TOC BOD
The textile industry requires very large quantities of water during manufacturing and processing procedures, being major consumer of water . This industry uses about 10,000 different dyes, and more than 0.7 Mt of these dyes are annually produced worldwide . Ten to fifteen percent of these dyes are released to the environment which constitutes one of the biggest environmental problems of the twenty-first century. These products cause serious damage to the environment due to the high concentration of color and dissolved matter in wastewater [3, 4]. The textile wastewater contains a wide range of pollutants including organic persistent and toxic substances, heavy metals either in the free form or adsorbed onto the suspended solids, and inorganic compounds . Azo dyes, characterized by the presence of at least one azo group (−N=N-), are one of the largest groups of synthetic dyes used in industrial applications . These azo dyes are metabolized to colorless, possibly carcinogenic amines in living beings . Therefore, the primordial tasks were focused to treat these wastewater-containing dyes. The treatment method developed and proposed in this paper aims to eliminate dye contamination in the most efficient manner, both from a technical and economic point of view (cost-effectiveness must be taken into account).
Electrocoagulation (EC) is one of the most applied electrochemical methods in wastewater treatment . EC is based on the electrochemical dissolution of sacrificial metal electrodes (Iron/Aluminum) into soluble or insoluble species according to the pH of the solution, as described in a seminal review article . The coagulant (dissolved metal hydroxides) is generated continuously by applying an electric current that forms the flocs . Therefore, these flocs create a blanket of sludge that entraps and bridges colloidal particles still remaining in the solution to float or to settle.
Recent studies on integrated treatment of industrial wastewater
Cell configuration (Anode/Cathode)
Maximum mineralization efficiency, %
Oil and grease industry
Various chemical and textile industries
Tannery plastics and textile industries
The main objective of this study is to treat industrial textile wastewater to minimize their pollution load and to enhance their biodegradability using integrated electrochemical processes.
Materials and methods
Characteristics of textile wastewater
The textile industry wastewaters were collected from Mohammedia city in Morocco and contained a mixture of azo dyes with methylene blue as the main dye, and inorganic compounds (nitrogen, phosphorus, potassium, etc.). The wastewater was filtered using a pre-filtration grid to remove large suspended solids before being used for the subsequent study.
Experimental apparatus and operating conditions
In each experiment, approximately 400 mL of real textile wastewater were placed in the electrolytic reactor. The pH of the solution was adjusted to the desired level using a dilute solution of sulfuric acid and sodium hydroxide (initial wastewater pH was 8.75 for EC and adjusted to pH 3.00 for EF) prior to the experiment and agitated with a magnetic stirrer at 200 rpm. In the case of EF, prior to electrolysis, compressed air was bubbled for 10 min in order to saturate the aqueous solution with O2. The electrochemical cell was operated with imposed current densities for both EC and EC-EF (20 and 10 mA cm− 2, respectively) during 60 min of each treatment. These operating conditions (current densities and pH values and electrolysis time) were previously optimized and the operating conditions for other techniques are described, in our previous work .
In the case of EC, the collected samples were centrifuged at 6000 rpm for 10 min to settle the flocculated material. After each run, the iron electrodes were washed with HCl (10%), deionized water, and dried at room temperature. In addition, the sludge generated by the EC process was dried in the oven at 105 ± 3 °C for 4 h.
Total nitrogen (TN) content was measured by Dumas method Thermo Scientific (Flash 2000) . NH4+ values were obtained according to the standard method for water and wastewater using colorimetric method . An unfiltered sample was taken for the determination of TSS following the standard method; the sample is filtered through a pre-weighed filter. The metal concentration in wastewater samples and the obtained dried sludge were carried out by Optical Emission Spectroscopy (ICP-AES, iCAP 6300 model SERIES THERMO). The floated (foam) and the decanted sludge were recovered separately at the end of the treatment and dried in an oven at 105 ± 3 °C  for 4 h in order to remove water from the samples before weighing to estimate the amount of dried sludge formed by EC. Then, the dehydrated sludge was calcined in the ambient atmosphere in the oven at 300 °C for 1 h. The sludge morphology and its metallic composition were characterized using high resolution scanning electron microscopy (SEM) (FEG Zeiss Gemini 500, Germany) provided with energy-dispersive X-ray spectroscopy (EDS) (DDS Oxford model). EDS allows a simultaneous determination of metallic elements of the sludge. Additionally, transmission electron microscopy (TEM) investigations were carried out using a JEM-ARM 200F Cold FEG TEM/STEM operating at 200 kV and equipped with a spherical aberration probe and image correctors (point resolution 0.12 nm in TEM mode and 0.078 nm in STEM mode) for structure and morphology. Moreover, X-ray diffraction (XRD) analysis was performed to the samples using (Bruker D8 Advance) with Cu Kα1 radiation (wavelength WL = 1.5406 Å).
Results and discussion
Physicochemical analysis of wastewater
Main characteristics of textile wastewater
COD (mg L− 1)
TOC (mg L− 1)
BOD (mg L− 1)
TSS (mg L− 1)
Conductivity (mS cm−1)
TN (mg L− 1)
P (mg L− 1)
Cl− (mg L− 1)
The anodic dissolution of the iron electrode inside the electrolytic cell in the case of EC promotes the generation of ferrous ions (Fe2+/Fe3+) which react with hydroxides ion (OH−) in solution to produce Fe(OH)2(s), and Fe(OH)3(s). These iron hydroxides act as coagulant/flocculent for the suspended solids to form flocs. These flocs have large surface area which is beneficial for a rapid adsorption of organic dye compounds present in textile wastewater and trapping of colloidal particles which sediment or float afterward.
Figure 2 shows the absorption spectra of the textile wastewater between 400 nm and 800 nm versus time of EC treatment. The obtained absorbance decreased continuously to diminish and disappear almost completely after 60 min of electrolysis.
The dissolved iron (sacrificial electrode), mFe, was calculated after considering the experimental conditions which were current intensity I = 1.6 A and time t = 60 min (3600 s) of electrolysis using the Faraday’s law (Eq. 1) :
where MFe is the molecular weight of Fe (55.9 g mol− 1), n (n = 2) is the number of electrons transferred in the reaction at the electrode, and F the Faraday constant (F = 96,500 C mol− 1). The maximum amount of ferrous ions (Fe2+) electrolyzed in these experimental conditions was 0.48 g.
where U, I, t, and V are cell voltage (V), electrical current intensity (A), electrolysis time (h), and volume of wastewater (L) respectively. R is TOC removal efficiency and TOC0 is initial TOC concentration (g L− 1).
In which, C is the TOC, COD or TSS value of treated aqueous solution (mg L− 1) and C0 is the initial relating concentrations (mg L− 1).
The combined treatments were found to be more reliable and economical for treating textile industry wastewater than single processes . In the case of EC-EF, the mineralization of organic matter reached 97%. The effectiveness of EC-EF is due to the capacity of the electrolytic cell (in EF step) to produce H2O2 and to regenerate Fe2+ on the CF (electro-generation of Fenton reagent) which leads to the production of hydroxyl radicals in solution. These hydroxyl radicals generated on the anode surface (BDD anode) which has a high oxygen evolution overpotential attacked the organic pollutants to produce CO2, inorganic ions, and water .
Characteristics of treated effluent and metal-ions removal
The main characteristics of textile wastewater before and after treatment
The removal efficiencies of all metals by EC from wastewater were between 54 and 89% and between 60 and 97% after EC-EF treatment (Table 3). The removal amount of heavy metals by EC from the wastewater may be found in the sludge. In these operating conditions, the removal efficiencies of Cu, Mn, Zn, Fe and Cr are 74, 89, 75, 84, and 74% after EC treatment alone and 82, 93, 94, 97, and 81% after EC-EF treatments, respectively.
Characterization of the sludge
Chemical element composition
Sludge characteristics for textile wastewater during treatment by EC
Sludge (mg/100 g)
SEM and EDS analysis
Furthermore, treatment of textile effluents can lead to the formation of other complexes like jarosite . It is a family of iron-hydroxysulphate minerals which are formed along with other compounds during wastewater treatment and acted as scavenger of iron ions and other toxic elements. These compounds are formed in acidic media, iron sulfate-rich environment, high temperature and pressure, and long reaction time . Thus, X ray diffractogram does not show specific peaks of the jarosite, our operating conditions are far from those allowing its formation.
Hybrid electrochemical processes EC, EC-EF, EC-AO and EC-PC were investigated to treat real textile wastewater. The findings indicated that the effectiveness of the treatment decreases in the sequence EC-EF > EC-AO > EC-PC > EC. The combination EC-EF treatment leads to high color and TOC removal of real textile wastewater containing mainly methylene blue (up to over 97%). EC treatment as a single process is not efficient for the elimination of dye organic stuff. The energy consumption using the integrated process was less than that of EC alone. In accordance with this approach, the energy consumption is reduced from 3 kWh kg− 1 of removed TOC in the case of EC to 0.45 kWh kg− 1 of removed TOC when EC is coupled with EF. EC-EF process can reduce the COD and increase the BOD/COD ratio after 60 min of electrolysis, which leads to increasing the biodegradability of the effluent by 75%. Efficient removal for metal ions is obtained by enhanced precipitation and flotation of the sludge during EC treatment for reuse purpose. The floated and decanted sludge are analyzed in terms of metallic element and their morphology characterized. These materials contained heavy metals which initially exist in the wastewater and are mainly composed by NaCl salt. Free salt sludge shows nanostructured shapes with size of less than 1 nm. Thus, sludge issued of wastewater treatment can be a precursor of nanostructure materials. More, the ratio oxygen/metallic elements is in favor of a form of oxide that approaching Fe2O3. Further work is ongoing in our laboratory to better characterize the obtained sludge in terms of composition, specific surface, catalytic and magnetic properties.
The authors would like to thank the University of Ibn Zohr, Agadir, for making all the necessary resources available for this work.
All authors proposed the study and participated in writing the manuscript. HA carried out the lab experimental studies and edit the manuscript, HZ carried out the physicochemical parameters. FET participated in experimental studies. RAA participated in the characterization of materials and drafted the manuscript. YR carried out the wastewater analysis (ICP). AE participated in the design of the study and performed the sludge characterization and participated in its coordination. JG carried out SEM, EDS TEM, XRD. MH conceived of the study and participated in its coordination. All authors read and approved the final manuscript.
This work is done in the framework of the European ERANET MED Water-13_043 project SETPROpER: (Sustainable treatment processes of effluents for reuse of water in agriculture) with the financial support of MOROCCAN MESRSFC.
The authors declare that they have no competing interests.
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