Coupling of the coagulation/flocculation and the anodic oxidation processes for the treatment of textile wastewater


 The increased demand for textile products leads to an increase in the quantity of wastewater discharged. It becomes indeed one of the most critical health and environmental problems in the world. The main challenge, therefore, is to develop innovative techniques for treating this wastewater with low production costs and better efficiency. The major objective of this work was to investigate the efficiency of the coupling of the coagulation–flocculation and the anodic oxidation processes on the platinum electrode in the removal of organic, mineral, and microbial pollution contained in textile effluents. A series of experiments is carried out on samples prepared in the laboratory, in which the textile effluent was mixed with a secondary effluent from an urban wastewater treatment plant. The treatment consists of two steps: a coagulation–flocculation process using aluminum salts as a coagulant and an anodic oxidation on the platinum electrode using photovoltaic panels for the production of electric current. The treatment at optimized conditions reveals that the coupling of the two processes made it possible to achieve satisfactory results. The abatement rates were 95.97% for the turbidity, 90% for COD, 100% for BOD, 100% for , 53.6% for , and 100% for . The coupling of the two processes ensured the complete elimination of fecal germs. Thanks to the satisfactory results, the obtained permeate can be reused in the dyeing process in the textile industry.


INTRODUCTION
Worldwide environmental issues related to the textile industry are typically those associated with water pollution caused by the discharge of wastewater effluent since it contains toxic substances. In addition, the composition of wastewater from dyeing and textile processes varies considerably from day to day and even from hour to hour, depending on dye, fabric, and concentration of fixing compounds that are added (Kim et al. ). This wastewater is very stable in the environment and resistant to oxidation and biodegradation (Croce et al. ). It is considered a source of esthetic pollution and it permits the disturbance of the aquatic ecosystem. Photosynthesis and reduced oxygen solubility result from the discharge of wastewater made up of dye compounds, even at low levels (Rahmani et al. ). However, textile mills are not designed to eliminate all kinds of pollutants such as inorganic and organic substances, which should be included in the treatment priorities. Furthermore, leachate has high values of biological oxygen demand (BOD), chemical oxygen demand (COD) and, because of its toxic potential (Anvari et  The treatment of textile effluents is a major concern since the rejection of such wastewater drastically decreases oxygen concentration in the aquatic ecosystems. Apart from the unsightly appearance, the coloring agents are able to interfere with the transmission of light in the water, thus blocking the photosynthesis of aquatic plants (Mansour et al. ). The works on these azo dyes have shown that these chemical compounds have carcinogenic effects on humans and animals ( In this study, C-F has been chosen, for its simplicity to remove turbidity and reduce the COD value of dye wastewater. El-Gohary & Tawfik () confirmed this idea. The cycle of treatments involved coagulation/flocculation/decantation by using aluminum sulfate as a coagulant. This last neutralizes the charge of the particles, allowing them to agglomerate, and settle at the bottom of the tank (Verma et al. ). This mechanism occurs because the coagulants form monomeric and polymeric species in contact with water, with metallic hydroxides (Alkarkhi et al. ). Depending on the volume of water treatment, chemical coagulation can be very expensive.
One of the possible problems is the difficulty of being able to reduce the solubility enough so that the components can form flocculants to be removed from wastewater (Verma et al. ). Consequently, according to Nabi et al. () and Georgiou et al. (), many attempts were made to combine treatment methods for better and improved to treat wastewater. C-F combines almost all types of treatment methods currently available to treat wastewater (Butler et al. ).
On the other hand, with the ever-increasing level of drinking water supply and strict environmental regulations regarding the discharge of wastewater, electrochemical technologies have regained their importance worldwide over the past two decades (Chen & Hung ). Subsequently, a great deal of research has been devoted to this goal, highlighting the prominent role of a special class of oxidation techniques defined as advanced oxidation processes (AOPs), which generally operate at or near ambient temperature and pressure (Aieta et al. ). These AOPs have been applied in several sectors, such as the treatment of industrial wastewater (Martínez-Huitle & Ferro ). Unlike conventional methods, anodic oxidation does not need any additional processes and chemicals, and the system does not lead to the formation of byproducts (Rahmani et al. ). The AOPs rely on the production of highly reactive hydroxyl radicals (OH • ).
These strongest oxidants are highly oxidizing and nonselective in nature (Asghar et al. ), and they are able to decompose mainly organic matter (Farhataziz & Ross ; Hoigné & Bader ) up to their mineralization (Sirés et al. ; Verma & Samanta ). These compounds react with the double bonds -C-Cand attack the aromatic nuclei, the major components of the refractory compounds (Gogate & Pandit ).
The hydroxyl radicals are produced from a hemolytic rupture of a covalent bond, that is to say, that the two electrons involved during this bond share one electron for each atom (Millet a, b). This characteristic gives it a strongly polar character, and consequently, it is highly reactive with respect to numerous organic (aromatic and aliphatic), inorganic, and bacterial compounds (Zaviska  (1) and (2)).

MATERIALS AND METHODS
The experimental study lasted 4 months, from November In order to determine the best method of treating textile wastewater to remove organic, mineral, and microbial pollution, the effluent has been treated successively by the C-F and the anodic oxidation (AO) processes on the platinum electrode.

Wastewater characteristics
In order to reduce the high load of pollutants in this industrial sewage effluent, the treatment was carried out using a  Table 1. . The optimal pH of the C-F process is between 6 and 7.4 for aluminum (Bratby ; Gregory ). In this study, the pH was in this optimal range. Practically, the optimal dose of coagulant can be determined by a laboratory test, known as the 'Jar test' (Lounnas ).  for about 120 min. After decantation, turbidity, COD, BOD, NH þ 4 , NO À 3 , PO 3À 4 , and fecal germs were measured in the supernatant to determine the most adequate quantity of coagulant for the abatement of this pollution. In order to study the mode of operation of the AOP and to determine its purification performance, an experimental study was carried out using two electrodes: a platinum anode and a steel cathode ( Figure 1). The platinum electrode used in this treatment has the physical properties, as shown in Table 2. Its oxidation potential is equal to 1.18 (V/SHE) (Daniel ). The steel cathode used in this work is characterized by thickness: 2 mm, width: 2.5 cm, and length: 5 cm.

AOP on the platinum electrode
The characteristics of the panels used, which are of the 'polycrystalline silicon' type, are illustrated in Table 3.
The part of a centimeter of the cathode was placed vertically in the solution while the anode was completely immersed, in order to guarantee that the submerged surface is equal, of which the production of electrons at the level of two electrodes is proportional. The current applied between these two electrodes was imposed by a series of photovoltaic panels. The influence of electric intensity (250, 500, 750, and 1,000 mA) and electrolysis time were investigated. The solutions were homogenized using a magnetic stirrer at a moderate speed. A volume of 1 liter of mixed solution was treated.

RESULTS AND DISCUSSION
Following pH and electrical conductivity The results obtained showed a slight decrease in pH and electrical conductivity (EC) ( Table 4). The study published Removal of turbidity As shown in Figure 3, the abatement rate was 93.43 and 94.36% for water treated by the C-F and the AOPs on the platinum electrode, respectively. After the treatment of the sample

Removal of COD
The evolution of the COD of the raw and analyzed water is illustrated in Figure 4.
In our case, platinum is a nonactive electrode (with a high oxygen overvoltage) whose hydroxyl radicals are weakly bonded to the surface of the anode. This situation allows the hydroxyl radicals to degrade the pollutants into intermediate compounds and to mineralize them in the final stage (reactions (4) and (5)

Removal of BOD
In Figure 6, it is observed that there is a total degradation of the BOD. After mixing the TE with the UE, the BOD content was decreased to 6.4 mg O 2 /L. These results expressed the good purification performance of this mixture with an abatement rate that exceeds 97.71% compared with the initial value of the TE. The treatment of the M-5% solution by the C-F and AO separately with the optimal conditions favored a total reduction in BOD (100%) which confirms the effectiveness of each of these two processes with respect to the elimination of easily biodegradable organic matter ( Figure 7). After the coupling of the two processes with the optimal conditions, the BOD has been completely degraded. The almost total disappearance of BOD in the water treated by the coupling could be explained by the good role of the coagulant during the C-F process.
This excellent yield also comes back to the oxidation power of hydroxyl radicals (OH • ) which will react instantly with organic components by hydroxylation with a loss of hydrogen atom following a radical mechanism until their total mineralization according to the following reactions: The attack of these radicals on organic pollutants initiates a radical mechanism, leading to mineralization by  Amor () proved that the ammonia could be completely oxidized and the majority of it has been transformed into nitrogen N 2 whose yields were higher in the presence of ammonium chloride, which demonstrated the advantageous contribution of chloride ion to the electroconversion reaction of ammoniacal nitrogen by increasing the time of electrolysis. This is likely due to excessive electrolysis of the chloride ion to chlorine gas at the anode (reaction (9)), to the detriment of its reaction with water to generate hypochloric acid (reaction (10)).
2 Cl À ! Cl 2 þ 2e À (9) In other words, the removal efficiency of ammoniacal nitrogen increases with the current density and the Cl À concentration due to their strong effect of cooperation with the current density.

Removal of NO 3
À After mixing the abatement rate of NO À 3 was almost 15.22% relative to the industrial effluent. This decrease could be explained by the denitrification, which consists in eliminating the nitrates present in the water under the action of the microorganisms of the urban wastewater. The coupling of the two processes allowed a rather large decrease in the nitrate content. It was equal to 53.6% relative to the mixing solution ( Figure 10). The results obtained showed that the C-F allowed a reduction rate equal to 38.35%, while the reduction rate by AO reached 44.93% (Figure 11).

Removal of pathogenic germs
Experimental results showed that the UE is highly loaded with total coliforms (TC), fecal coliforms (FC), and fecal streptococci    (FS). After mixing the two effluents, a reduction in this pathogenic germs content compared with the UE was observed (Figures 14-16). This decrease can be explained by the substances found in the industrial effluent and the products (acid, NaOH, H 2 O 2 , etc.) used during the textile production chain, which are capable of destroying the pathogenic germs.
As shown in Figure 17, the treatment of the M-5% solution by the C-F and AO process separately with the optimal conditions favored a total reduction in TC, FC, and FS (100%).
The coupling of these two processes permits the elimination     bactericidal action. A similar study by Kahoul & Belhachani () has shown that the elimination of pathogenic microorganisms is due to the hydroxyl radicals (OH • ) which are very reactive with respect to many bacterial compounds.

CONCLUSION
The main objective of this work was to study the elimination of organic, nitrogen, phosphorus and microbial pollution contained in the TE by the coupling of the C-F and the AOPs. The experiments showed that combining these two processes improved the purification of the TE mixed with the secondary effluent. The technique of C-F applied in the laboratory uses a Jar test, which seems to be a durable system of the elimination of pollutants such as turbidity, organic matter, nitrogen, phosphorus, and pathogenic microorganisms. In addition, AO is an effective process that can contribute to the satisfactory removal of organic and mineral matter, and micropollutants contained in the mixed solution. These high yields were obtained thanks to the high oxidation power of the water molecule on the surface of the anode to form hydroxyl radicals, and the current applied between these two electrodes, which was produced by a series of photovoltaic panels.
The coupling of the C-F and the AOPs, by using the optimal treatment conditions, has made it possible to increase the purification performances. The results obtained, after the coupled treatment of the urban wastewater mixture with SARTEX textile industrial wastewater in the proportion of 5 and 95%, respectively, are very satisfactory in terms of removal of turbidity, COD, BOD, NH þ 4 , NO À 3 , PO 3À 4 , TC, FC, and FS. All these parameters are in accordance with the Tunisian standard of rejection. In order to recover these effluents after their purification, they can be reused for domestic, agricultural, or industrial purposes. It may become one of the solutions that can respond to the problems of water scarcity in the world, and the growing needs for this subject.

ACKNOWLEDGEMENT
The authors thank the Research Unit of Applied Hydro-Sciences of Gabès for the technical support.

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