Electrochemical treatment of industrial wastewater and effluent reuse at laboratory and semi-industrial scale

Journal of Cleaner Production 65 (2014) 458e464

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Electrochemical treatment of industrial wastewater and effluent reuse at laboratory and semi-industrial scale Mireia Sala, M. Carmen Gutiérrez-Bouzán* Institut d’Investigació Tèxtil i Cooperació Industrial de Terrassa (INTEXTER), Universitat Politècnica de Catalunya (UPC), C/Colom 15, 08222 Terrassa, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 April 2013 Received in revised form 2 August 2013 Accepted 3 August 2013 Available online 14 August 2013

This study is based on the electrochemical decolouration of exhausted dyeing effluents which contain dyes and salts. The treated effluents were reconstituted and reused in a new dyeing process. Initially, synthetic effluents containing one of the reactive dyes Novacron Yellow, Ruby and Deepnight were treated in the laboratory pilot. In all cases, the dye decolouration follows a pseudo-first order reaction. Subsequently, seven industrial effluents which contain mixtures of these dyes were collected in a Spanish mill and treated in the laboratory pilot. Two methods for the electrochemical treatment and further effluent reconstitution and reuse were studied. The first method consisted of an electrochemical treatment followed by an acidification and a stripping step to remove the carbonate ions. In the second method, the acidification was carried out before the electrochemical treatment; subsequently, the generated CO2 was removed during the decolouration process. Finally, the optimised process was applied in situ in a Spanish mill by means of a semi-industrial pilot plant (400 L). No significant colour differences were appreciated between reference dyeings and the fabrics dyed with the treated exhausted effluents. The reuse of the dyeing effluent achieves the reduction of 70% process water consumption and 60% salt discharge. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical treatment Decolouration Bath reconstitution Effluents reuse Colour removal Salt reuse

1. Introduction Textile processing industry which comprises different operations such as pre-treatment, dyeing, printing and finishing, is one of major environmental polluters. In order to process a ton of textile, one might have to use as much as 230e270 t of water (Tahri et al., 2012). For this reason, environmental issues are being increasingly taken into account in textile dyeing and finishing industries because of strict legislations and a growing ecological concern (Hoque and Clarke, 2013). Main environmental impacts of textile dyeing and finishing industries involve high water consumption, high energy use and also input of wide range of chemicals such as dyes, surfactants, salts... (Pasquet et al., 2013). This work is focused on reactive dyes effluents, especially on the spent dyeing and first washing baths because they contain higher amount of residual dyes and salts. Reactive dyes were selected because of their high worldwide consumption: they represent about 20e30% of the total market (Carneiro et al., 2005). They are extensively used in the cotton industry due to their washing * Corresponding author. Tel.: þ34 93 739 80 08; fax: þ34 93 739 82 72. E-mail addresses: [email protected] (M. Sala), [email protected] (M.C. Gutiérrez-Bouzán). 0959-6526/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jclepro.2013.08.006

fastness and brilliant colour. In their structure, they contain one or several reactive groups (able to react with the fibre) and a chromophore group (which is the main responsible of the colour). The most extensive chromophore group is the azo (ReN]NeR0 ), followed by the anthraquinone group (Lee and Pavlostathis, 2004). The azo group constitutes more than half of worldwide dyes production (Oliveira et al., 2010). In particular, Guaratini and Zanoni (2000) indicate that approximately 65% of dyes production corresponds to azo dyes and this finding was also supported by and Carneiro et al. (2010). The conventional biological treatments are not effective in reactive effluents colour removal because this type of dyes have aromatic rings in their large molecules that provide chemical stability but also resistance to the microorganisms attack (Dos Santos et al., 2007). Although the effluent colour regulations are very variable depending on the country, the biological treatments are always insufficient to degrade reactive dyes (Reemtsma and Jakobs, 2001) and to satisfy current regulation. Consequently, in all cases, the application of tertiary treatments to remove colour is required because dyes are usually toxic (Baliarsingh et al., 2012) and, in addition, small amount of them (ppm) are sufficient to produce an intense colouration which can impact or even impede the aquatic life in the riverbeds. Current methods for colour removal are based

M. Sala, M.C. Gutiérrez-Bouzán / Journal of Cleaner Production 65 (2014) 458e464

on the separation between dye and effluents: physico-chemical methods (coagulation-flocculation), absorbent materials (such as active carbon, silica gel or alumina) or filtration with membranes. However, these methods produce a residue which requires an additional treatment to be destroyed and in some cases, regeneration or cleaning step (López-Grimau et al., 2011). With the increased awareness of environmental protection, the need for green wastewater technologies is growing fast. Advanced oxidation methods such as ozonation, Fenton, photoFenton, photocatalysis are an advanced alternative, as residues are not produced (Chon et al., 2012), but they are rather expensive and involve some operational difficulties. Enzymatic decomposition of dyes requires an exhaustive control of temperature and pressure to avoid enzymes denaturalisation. For these reasons, the electrochemical methods are nowadays the subject of a wide range of investigations at laboratory and pilot plant scale, as the electron is a clean reagent and they do not generate residues. As some industrial wastewaters contain large amounts of chloride ions, the indirect electrochemical oxidation method with active chlorine is a very suitable technique to treat this kind of effluents. The indirect electro-oxidation occurs when strong oxidants are generated in situ during the electrolysis and react with the organic pollutants such as dyestuffs, producing its total or partial degradation, according to the reactions 1e3: 2Cl ➔ Cl2(aq) þ 2e

(1)

Cl2(aq) þ H2O ➔ ClO þ Cl þ 2Hþ

(2)

Dye (C, H, O, N) þ ClO ➔ intermediate compounds ➔ CO2 þ H2O þ Cl þ N2

(3)

Sanjay et al. (2005) suggested a mechanism involved in decolouration and chemical oxygen demand (COD) reduction for the azo dyes, according to the reactions 4e6: ReN]NeR’ þ 2ClO ➔ ReN(ClO) ¼ N(ClO)eR’ (intermediate) (4) ReN(ClO) ¼ N(ClO)eR’ ➔ ReN(O) ¼ N(O)eR’ (intermediate)

(5)

ReN(O) ¼ N(O)eR’ ➔ RO þ R’O þ N2

(6)

On the other hand, McCallum et al. (2000) discussed the possible reaction pathways of anthraquinone textile dyes through the oxidative degradation process of Uniblue A. In the case of reactive dyes effluents, the addition of any chemical product in the electrolysis is not required because the chloride ion contained in the effluents acts as electrolyte. The electrochemistry method using chlorine as indirect oxidant has noted to be effective in the degradation of several kinds of dyes, such as azo dyes (LopezGrimau and Gutierrez, 2006), acid dyes (Oliveira et al., 2007) or disperse dyes (Osugi et al., 2009). Its combination with photoelectrochemistry has also provided good results for phtalocyanine dyes degradation (Osugi et al., 2006), but in this case, the metal ions liberated (i.e. copper) have to be removed. Del Rio et al. (2009a) studied the efficiency of electrochemical treatment with DSA, both in a divided cell (for oxidation and reductions processes separately) and in an undivided cell (oxidereduction processes). The greatest decolouration rates were obtained with the second process (Del Rio et al., 2009b). According to those results, in the current study the studies are focused on an oxide-reduction process carried out in an undivided cell. Moreover, the current policies concerning water and energy consumption conduce to the recycling and reuse treatments (Kurta et al., 2013). In this sense, recent studies on textile effluents (Riera-

459

electrodes recipient

b) Semi-industrial pilot

a) Laboratory pilot

Fig. 1. Electrolytic cells.

Torres et al., 2011) demonstrated the possibility of reusing these discoloured effluents for new dyeing processes. The reuse of 70% discoloured dyebaths after the electrochemical treatment assisted by UV irradiation provides, in most of cases, acceptable colour differences (limit of acceptance in the textile industry: DECMC (2:1)  1) with respect to the original dyeing with decalcified tap water. The current study was carried out with the support and assistance of a Spanish mill focussed on knitwear dyeing and finishing processes. According to environmental policies, the main interest of this enterprise was to reduce the salts content in their wastewater. In this sense, the electrochemical treatment-reuse system was a promising alternative to avoid the discharge of the reactive dyeing effluents due to their high salinity. The main goal of this study is to optimise the electrochemical method for the treatment of dyeing effluents (dyeing and first washing effluents) and their reuse in a new dyeing process, with the aim to reduce the water consumption and the salts discharge in the industrial effluents. Several authors have published studies about dyeing effluents treatment but reuse experiments have not been included in their manuscripts (Kim et al., 2005). As far as we know, in addition to our research group, only Lu et al. (2009) reused printing and dyeing effluents after biological-filtration treatments and Tahri et al. (2012) achieved the reuse of 50% of water and 30% of salt after microfiltrationenanofiltration treatment of reactive dye effluents. With respect to these studies, the main advantage of the electrochemical treatment is the possibility of reusing higher amounts of water and salt without the generation of wastes. 2. Experimental 2.1. Dyes and reagents Three azo dyes containing three reactive groups in their molecule, kindly provided by the mill Hidrocolor SA were selected: Novacron Deepnight (ND), Ruby (NR) and Yellow (NY). Their maximum absorption wavelengths are 583 nm, 543 nm and 417 nm, respectively (see calibration curves in section 2.4.1). These three dyes, used at different rates, are the basis of many industrial trichromies as they provide a wide range of colour shades. For the effluents and the dyebath preparation, analysis quality reagents were used: Na2CO3 (Merck) and NaCl (Fluka). Dyeing was performed on 100% cotton fabrics, kindly provided by the mills TIPSA and Hidrocolor SA. The industrial effluents studied correspond to 3 trichromies collected at the mill, namely: Purple (PpT), Violet (ViT) and Midnight Blue MbT). Their dyeing formulas were not specified. 2.2. Electrochemical treatment The industrial effluents were treated in the undividable electrolytic cells shown in Fig. 1.

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In the laboratory pilot (Fig. 1a) the treated volume was 2 L and studies were carried out under galvanostatic conditions (power supply: Grelco GVD310 0-30Vcc/0-10A) at 10A. The cathode was made of stainless steel and the anode was made of Ti/Pt, both of them with 60 cm2 of active surface. The current density was 177 mA/cm2. In the semi-industrial pilot (Fig. 1b) the treated volume was 400 L and the experimental studies were also carried out under galvanostatic conditions, by means of a power supply provided by the mill. The intensity applied was 72A, which was the highest available by this industrial source. The electrodes were made of Ti/ PtOx, with 1000 cm2 of active surface. In this case, the current density was 70mA/cm2. 2.3. Effluent reuse Reuse dyeing tests were performed in a laboratory TI-COLOR dyeing machine (ICL, Prato, Italy) under the following conditions: 10 g of cotton fabric, dyestuff concentration 3% o.w.f (over weight of fibre), liquor ratio 1:10 (1 g fibre/10 mL dyeing bath), 80 g/L of salt (NaCl) and 16 g/L of alkali (Na2CO3) for monochromies: ND, NY or NR. When the effluent was reused for a trichromie (DyT) the total dye concentration was 4.5% o.w.f (3% ND, 0.7% NY and 0.8% NR), 100 g/L of salt (NaCl) and 25 g/L of alkali (Na2CO3) for 10 g of cotton fabric and a liquor ratio 1:10. The dyeing method was performed by steps, according to the dyes supplier and the mill procedures. After the dyeing process, the cotton dyed fabrics were washed to remove the unfixed dye. This process is constituted by five successive washings at different temperatures. The first one was performed at 50  C with tap water (10min), then a neutralising step with acetic acid at 50  C was carried out (10min) followed by a soaping step with 1.5 g/L COTEMOLL TLTR (Color Center, Barcelona, Spain) at 95  C (15min). Finally two additional tap water washings were conducted at 50  C and cold water, successively during 10min. All steps were performed at liquor ratio 1:10 and all the experiments were run in triplicate. 2.4. Analyses and instruments 2.4.1. Spectroscopic analyses The decolouration process was followed by spectroscopic analysis, where the initial dyes absorbance (Abs0) was compared with the absorbance of samples collected during the treatment (Abst). Absorbance measurements were carried out with a UVeVis spectrophotometer (Shimadzu UV-2401 PC). The absorbance was measured at the visible maximum dye absorption wavelength (583 nm for ND, 543 nm for NR and 417 nm for NY). The dye absorbance has a linear behaviour versus the dye concentration into the working range, according to Lambert Beer equation (7).

Abs ¼ l$ε$conc

Fig. 2. Absorbance spectra of the three trichromies (MbT: Blue, ViT: Violet and PpT: pink). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

were collect each 30min. The decolouration was reported in % according to Equation (8).

Dð%Þ ¼ ðAbs0  Abst Þ$100=Abs0

(8)

2.4.2. TOC and CI analyses The Total Organic Carbon was determined with a Shimadzu TOC5050A instrument. The working range was 20e80 ppm. Before the effluent reuse step, it is very important to establish the chloride concentration in order to know the amount to be added to perform the new dyeing process. An Ionic Chromatograph Dionex ICS-1000 with conductivity detector was used to determine the chloride ion concentration. The column was IonPac AS23 4 mm for anions determination and the eluent was a Na2CO3/NaHCO3 buffer solution. 2.4.3. Colour evaluation The final dyed fabric colour was measured by means of a spectrophotometer Macbeth Color Eye 7000, with illuminate D65 and 10 standard observers. The chromatic coordinates of each dyed fabric were evaluated. These coordinates are defined by three parameters (Lightness DLcmc; Chrome DCcmc, and Hue DHcmc) which establish the colour difference between the standard fabrics and the fabrics dyed with the reused effluent. According to the Standard UNE-EN ISO 105-J03 (AENOR, 1997), the colour difference was calculated with DECMC(2:1) formula (Equation (9)):

DECMCð2:1Þ ¼

2  2  2 i1=2 h DL* =2SL þ DC*ab =Sc þ DH*ab =SH

(9)

(7)

For each dye, the absorbance calibration curve is:

ND : Abs ¼ 34:837$conc þ 0:0157 and R2 ¼ 0:9999 NR : Abs ¼ 23:629$conc þ 0:0051 and R2 ¼ 0:9998 NA : Abs ¼ 18:686$conc þ 0:0004 and R2 ¼ 0:9999 The absorbance in the industrial effluents (trichromies) was measured at the maximum absorption wavelength of each trichromie (PpT: 529 nm; MbT 554 nm; ViT: 606 nm and 555 nm). Their absorbance visible spectrum is represented in Fig. 2. In the laboratory pilot, samples were collected each 5min during the electrochemical treatment whereas in the industrial pilot, they

In general, the acceptance limit for colour differences in the textile industry is one unit (DECMC(2:1)  1). This criterion is widely used in dyeing quality control to evaluate the colour differences between two fabric samples (López-Grimau et al., 2011). 2.4.4. Power consumption Power consumption values C (expressed in W h/L or kW h/m3) were calculated according to the Equations (10)e(11):

P ¼ V$I

(10)

C ¼ P$t=vol

(11)

Where P is the Power (W), V the Voltage (V), I the Intensity (A), t the time (h) and vol the volume (m3).

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461

Table 2 Characterisation of industrial effluents corresponding to three trichromies: exhausted dyeing baths and first washing effluents. (lmax. ¼ maximum absorbance wavelength). Parameter

PpT Dyeing bath

Fig. 3. Electrochemical treatment at 10A: decolouration evolution of the three trireactive dyes (ND, NR, and NY).

3. Results and discussion

pH Conductivity (mS/cm) TOC (ppm) l max. (nm) Absorbance at l max (UA) Additional l max. abs. (nm) Absorbance at add. l max. (UA)

10.46 79.3

MbT 1rst Dyeing washing bath 10.62 26.8

10.54 92.7

ViT 1rst Dyeing washing bath 10.76 24.8

10.57 71.9

1rst washing 10.78 17.3

120.44 117.42 529 529 0.675 0.436

166.76 74.32 554 554 2.608 0.939

290.72 74.52 555 555 2.014 0.838

e

e

e

e

606

e

e

e

e

1.710

606 0.900

3.1. Study of dyes behaviour with the laboratory cell Before the trichromies study, it is important to know thoroughly the behaviour of each dye versus the electrochemical treatment. For that reason, three synthetic effluents were prepared with the three trireactive dyes most frequently used in industrial mills. Trireactive dyes can reach exhaustion ratios (fibre fixation ratios) around 90% (ND: 84.36%; NR: 91.79%; NY: 92.55%) whereas it is well-known that the mono and bireactive dyes could give exhaustion of the order of 60e80%. The decolouration evolution of the three selected dyes (ND, NR and NY) is shown in Fig. 3. In all cases, decolouration greater than 90% was achieved at 10A. Moreover, the exhaustion percentage and the exhausted dyebath decolouration kinetic rate towards the electrochemical treatment are plotted in Table 1. The colour differences of fabrics dyed with the reused effluent versus reference dyeing are also included. These results are in accordance to the previous studies performed with mono and bireactive dyes (Sala et al., 2012). As it is known, in general, the organic pollutant abatement can be treated following pseudo-first order kinetics (Torrades et al., 2008). In particular, in our previous studies it was verified that the electrochemical dye degradation follows a pseudo-first order reaction (Lopez-Grimau and Gutierrez, 2006; Riera-Torres et al., 2010). The decolouration rate constants (Kd) were calculated from the slope of semilogarithmic absorbance versus exposition time (t), according to the kinetic Equation (12). The decolouration kinetic rates values of trireactive dyes are in accordance with these references and the other studies carried out with mono and bireactive dyes (Sala et al., 2012).

lnðAbst =Abs0 Þ ¼ kd $t

(12)

Finally, the decoloured effluents were reused to perform a new dyeing. The colour differences of the new dyeing with respect to a reference (Table 2) were determined in DE(CMC 2:1). In all cases, Table 1 Monochromies: dyeing parameters, decolouration kinetic rates and colour differences. Dye

Process Electrochemical treatment

ND NR NY

Effluent reuse

Kinetic rate

R2

Colour difference

0.1301 min1 0.1593 min1 0.1295 min1

0.9885 0.9975 0.9973

DE(CMC DE(CMC DE(CMC

2:1) 2:1) 2:1)

¼ 0.38 ¼ 0.25 ¼ 0.22

values lower than 1 were achieved, which is into the industrial tolerance limit. Consequently, it can be concluded that the selected conditions are appropriate for the treatment and reuse of the mill effluents in new dyeing processes. A reuse of 70% water and 60% electrolyte in the new dyeing processes is reached. This result provides important reduction of costs due to the minor water and electrolyte consumption and the lower effluents discharge cost. Therefore, the mill improves in two areas: from economic and environmental point of view. In addition, the electrochemical treatment can be considered as a clean technology with respect to other new decolouration technologies such as membrane filtration which produce dye concentrates (Tahri et al., 2012) or wastes (Lu et al., 2009). 3.2. Industrial effluents characterisation Six industrial effluents were collected in the mill. They correspond to three exhausted dyebaths and their subsequent first washing effluents. The pH, conductivity, TOC and maximum wavelength absorbance values were determined. These results are shown in Table 2. All of them contained mixtures of three dyes (trichromies) but their composition was not specified. For that reason, the decolouration was calculated at their maximum absorbance wavelength. Therefore, the absorbance can correspond to the contribution of one, two or three dyes. The remaining 4 washing baths were not studied as they are not relevant from the industrial point of view due to their low dyes and salts content (Sala, 2012). As it is obvious, dyeing bath effluents always contain higher amount of dyes, salts and alkalis than the first washing effluents. Therefore, their electrochemical treatment and further reuse were carried out separately. Initially two treatments were applied to the exhausted dyebaths (sections 3.3 and 3.4). The first washing effluents study was carried out subsequently (section 3.6). 3.3. Exhausted dyebath reuse: electrochemical treatment followed by carbonate removal (METHOD 1) The method 1 was based on an initial electrochemical decolouration treatment followed by an acidification step to remove carbonate. Then, the effluent was reconstituted and finally reused in a new dyeing process. The exhausted dyebaths were treated at 10A with the laboratory pilot. The decolouration evolution of three industrial dyeing effluents provide in all cases decolouration values higher than 90%, as can be seen in Fig. 4.

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industrial point of view, a colour difference higher than 1 (DECMC(2:1)>1) is unacceptable. The decolouration tests results shown that the lower pH (pH3 and stripping step) provided better dyeing results (DECMC(2:1) ¼ 0.56). The colour differences obtained with the treated dyeing effluent at initial pH 3 were into the industrial acceptance limits whereas the dyeing carried out at initial pH 8 exhibited significant colour differences with respect to the reference (DECMC(2:1)>2). This indicates that at higher pH the carbonate was not properly removed and the remaining carbonate resulted in a buffered solution (according to reaction 4) which produced unacceptable dyeing results. 2 þ þ CO2 þ H2 O4H2 CO3 4HCO 3 þ H 4CO3 þ H

Fig. 4. Decolouration evolution of three industrial effluents (dyeing baths containing different trichromies) by applying an electrochemical treatment at 10A with the laboratory pilot.

Consequently, the electrochemical treatment has shown to be an effective method for decolouration of industrial effluents containing trichromies of reactive dyes. Electrochemical decolouration kinetic rates and power consumption were determined for the three industrial effluents. These results are shown in Table 3 and correspond to 86e91% decolouration. In all cases, the decolouration kinetic rates were adjusted to a second order reaction (Equation (13)).

ð1=Abst Þ  ð1=Abs0 Þ ¼ kd $t

(13)

As indicated in section 3.1, each dye should follow a pseudo-first order reaction. For this reason, it can be stated that a second order reaction for the industrial effluents implies the contribution of two dyes in their maximum absorbance wavelength. The third dye of the trichromie has probably a negligible absorbance at this wavelength. The reaction order with respect to each dye can only be verified when the effluent composition is known. To achieve full decolouration, power consumption values from 3.8 to 6.2 kW h/m3 are required. It is a low energy consumption treatment with respect to other technologies. In addition, it must be taken into account that, in this case, only the most coloured effluents are treated whereas in general, the colour removal technologies are applied to the treatment of the whole mill wastewater. After the electrochemical treatment, the treated effluents were prepared for a new dyeing process. This step was based on the acid addition to remove the carbonate ions used as dyeing alkali. Then, the effluent chloride content was analysed and finally, the oxidant species were removed with a reducing agent. After that, 30% decalcified water was added to the effluent, corresponding to the amount lost onto the fibre after the dyeing process. Finally, the effluent was reconstituted with the addition of the required amount of dye (ND in all experiments), electrolyte and alkali according to the dyeing method. Two reuse tests were performed with the ViT trichromie. Different amounts of acid were added in order to remove the carbonate. The first one was adjusted to pH8 and the other one to pH3, followed by a stripping. After the carbonate removal, the effluents were reconstituted and new dyeings were performed. From the Table 3 Trichromies decolouration kinetic rates and power consumption. Trichromie

Kinetic rate (min1)

R2

Power consumption (kW h/m3)

PpT ViT MbT

2.793 0.376 0.099

0.9985 0.9957 0.9917

3.8 4.2 6.2

(14)

Consequently, to achieve adequate dyeings with the method 1, the effluent should be first decoloured, then acidified until pH3 and finally, the generated CO2 must be removed by means of a short stripping step. 3.4. Exhausted dyebath reuse: electrochemical treatment with simultaneous carbonate removal METHOD 2 In the former method (1), the addition of a high amount of acid is required to reach pH 3. In order to reduce the acid addition, an alternative is to treat the effluent only until pH5 and to carry out a very extensive stripping taking into account the acid carbonic/bicarbonate equilibrium. On the basis of that, a new method at pH 5 to avoid the additional stripping step was proposed (method 2). The treatment was also carried out in the laboratory pilot and it is based on the pH adjustment before the electrochemical treatment. The generated CO2 is removed during the electrochemical treatment by aeration or mechanical stirring, and also by means of the hydrogen bubbles generated during the electrolyses. Two trials at pH 6 and 5.5 were carried out (trials A and B). A new trichromie (NT) with 565 nm maximum absorbance wavelength was collected in the mill to carry out these experiments. The electrochemical treatment was applied at 10A until 85% decolouration. In this case, the consumption of the electrochemical treatment was 5.7 W h/L. After the treatment, the effluent was characterised. According to the CI results, 60% initial chloride was recovered in the treated effluent, which implies that only 40% total chloride must be added to perform the new dyeing. The next step was the preparation of the treated effluent for the new dyeing process. The pH was already adjusted at the beginning of the electrochemical treatment. Then the effluent reconstitution consisted only of removing the oxidant species with a chemical reducing agent followed by the addition of 30% water, the required amount of electrolyte, the total amount of dye (ND in all cases) and the alkali, according to the dyeing method and the effluent analysis. The dyed fabrics were compared with a reference dyeing (with decalcified tap water) and the colour differences obtained are: DECMC(2:1) ¼ 0.41 for the trial A and DECMC(2:1) ¼ 0.26 in trial B. These results indicate that this method (2) is very effective for the treatment and reuse of the dyeing effluents in a new dyeing process (with one dye: ND) as all the dyed fabrics are into the industrial acceptance limit: in both cases the colour differences are lower than 1. In conclusion, when the alkali selected is the carbonate, it must be removed before the new dyeing process in order to obtain appropriate dyed fabrics. For this purpose, two methods have been proposed. In the first method, the pH adjustment and the bath reconstitution was performed at the end of the electrochemical treatment. In the second one, the pH adjustment was done at the begging of the electrochemical treatment, and then, the bath reconstitution was carried out. Both studied methods are efficient for the effluent reuse.

M. Sala, M.C. Gutiérrez-Bouzán / Journal of Cleaner Production 65 (2014) 458e464 Table 4 Colour differences (DECMC(2:1)) of dyeings performed with the reused effluents (method 2) in the semi-industrial and laboratory pilots. New dyeing

DECMC(2

Trichromie (DyT) Monochromie (ND) Monochromie (NR) Monochromie (NA)

1.23 0.79 0.49 0.57

:1)

Lab

DECMC(2

:1)

semi-ind

1.36 0.65 0.53 0.50

3.5. Treatment and reuse of industrial effluents with the semiindustrial pilot The semi-industrial pilot, built as described in section 2.2, was applied to treat 400 L of an industrial effluent (NT effluent studied in section 3.4) following the reuse method 2: simultaneous electrochemical treatment and carbonate removal. The pH was adjusted to 5.5 with hydrochloric acid and the electrochemical treatment was performed at 72A. The total decolouration was achieved. The power consumption to achieve 85% effluent decolouration was 5.4 W h/L. This value is in accordance with the laboratory pilot (5.7 W h/L). In order to evaluate the differences between the laboratory and the industrial pilot treatments, 2 L of the same industrial effluent were treated in the laboratory pilot. In this case, effluent decolouration higher than 90% was also achieved. Finally both semi-industrial and laboratory treated effluents, once decoloured, were reconstituted to be used in a new dyeing (with a trichromie: DyT, and individual dyes: NY, NR, ND). Their colour differences with respect to a reference dyeing are shown in Table 4. When comparing the DECMC(2:1) of both pilots, it can be seen that the industrial results are in accordance with the laboratory results. In all cases, the colour differences corresponding to the monochromies are into the acceptance limits (DECMC(2 :1) <1). The trichromie is slightly higher than 1 and it can only be accepted in some special cases, i.e. zippers or shoelaces, where the acceptance limit can be 1.5e2. Consequently, the treatment has demonstrated to be effective for dyeing effluents decolouration. The reuse of the treated effluents has shown to be a very promising way to reduce the wastewater salinity and to save water and salts. It is especially interesting in the case of monochromies and it can also be applied for trichromies in some special cases in which the DE acceptance limit is higher than 1. In our knowledge, this electrochemical treatment is the unique colour removal method which provides a simultaneous effluents salinity reduction. 3.6. First washing effluents: treatment and reuse The first washing effluents contain lower concentration of dyes, electrolyte and alkali than the corresponding exhausted dyebath. Consequently, their behaviour with respect to the treatment and reuse is expected to be different due to the lower effect of carbonate-bicarbonate buffer. This behaviour was evaluated for ViT first washing effluent. Taking into account the low carbonate concentration, the method 1 was selected for the treatment (pH adjustment at the end of the decolouration treatment). According to this method, when the electrochemical treatment was applied at 10A, 90% decolouration was achieved in 10min. The decolouration kinetic rate also followed a second order reaction (kinetic rate value: 2.42 min1) which implies, as in the case of exhausted baths treatment, the contribution of two dyes at the maximum absorbance wavelength. The power consumption to reach 90% decolouration was 10.9 W h/L. This value was higher value than the corresponding to

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the exhausted dyebath because the effluent conductivity was much lower and consequently, higher voltage was required. After the decolouration, the pH was adjusted to 8 and then the effluent was reconstituted to perform two new dyeings: the monochromie ND and the trichromie DyT. Colour differences of fabrics dyed with the treated washing effluent versus the reference dyeings were 0.58 for ND and 0.70 for DyT. Both values were into the industrial acceptance limits (DECMC(2:1)  1). Therefore, the electrochemical treatment demonstrated to be useful for washing effluents decolouration and their further reuse in new dyeing processes, both for monochromies and trichromies. In this case 100% water and 15% electrolyte can be reused in the new dyeing processes. Unlike dyebaths effluents, in the case of washing effluents the acidification at pH3 and the stripping step are not necessary due to the lower concentration of residual carbonate ion. Consequently, for this type of effluents, the method 2 was not considered. 4. Conclusions An electrochemical treatment was applied in two scale cells (laboratory and semi-industrial pilots). Different effluents were studied (dyeing and washing baths) and the dyes decolouration was followed by UVeVis spectrophotometer determination. Two methods for the carbonate removal were also optimised. From these results, the main conclusions are:  Two electrochemical methods were developed for the treatment and reuse of industrial reactive dyeing effluents. Both methods have shown to be effective: the method 1 (treatment and further pH adjustment) was the best option for the treatment of washing effluents, whereas the method 2 (pH adjustment preceding the treatment) was more appropriate for the treatment of dyeing effluents.  All dyeings performed with the treated and reused effluents were into the acceptance limit (colour differences  1 versus a reference dyeing).  The laboratory and the semi-industrial pilot provided high decolouration rates and similar dyeing results. Therefore the two pilots can be considered to operate similarly.  The dyeing effluent treatment and reuse was effective for new monochromie dyeing processes, saving 70% water and 60% electrolyte.  The first washing effluent treatment and reuse was effective for both monochromies and trichromies dyeings. That procedure enables savings 100% water and 15% electrolyte.  In any case, the reuse procedure implies the saving of a high amount of the dyeing water and leads a considerable reduction of industrial effluents salinity. In summary, the electrochemical treatment was effective to remove colour of both dyeing and washing effluents containing reactive dyes trichromies. In the case of washing effluents which contains lower amounts of carbonate, the first method was the most appropriate due to low amount of acid to be added. But with the dyeing effluents (which contains much higher carbonate amount), the second method was considered as the best option because it is not necessary to reach pH 3 and the additional stripping step can be avoided. Acknowledgements This work was supported by CIDEM-ACC10 (Valtec09-2-0040) and the Spanish Ministry of Economy and Competiveness (CTM2010-18842 and CTM2012-31461).

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