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Replacing chloride anions in dyeing enables cheaper effluent concentration and recycling

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Citation: Gupta, Priyanshu Ranjan, Shanmukham, Saravana Perumal, Patel, Chirag G, Lienhard, John H and Swaminathan, Jaichander. 2022. "Replacing chloride anions in dyeing enables cheaper effluent concentration and recycling." Desalination, 533.

As Published: 10.1016/J.DESAL.2022.115761

Publisher: Elsevier BV

Persistent URL: https://hdl.handle.net/1721.1/144376

Version: Author's final manuscript: final author's manuscript post peer review, without publisher's formatting or copy editing

Terms of use: Creative Commons Attribution-Noncommercial-Share Alike

Replacing chloride anions in dyeing enables cheaper effluent concentration1

and recycling2

Priyanshu Ranjan Guptaa, Saravana Perumal Shanmukhamb, Chirag Patela, John H. Lienhardc,3

Jaichander Swaminathana,∗4

aMechanical Engineering Discipline, Dr. Kiran C. Patel Centre for Sustainable Development, IIT Gandhinagar, Gujarat5

382355, India6

bSP Textile Processors Pvt. Ltd., Chithode, Erode, Tamilnadu, India7

cRohsenow Kendall Heat Transfer Lab, Massachusetts Institute of Technology, Cambridge MA 02139, USA8

Abstract9

Reverse osmosis followed by distillation is commonly used to recycle fresh water and concentrated brine in10

the textile dyeing industry to achieve zero liquid discharge. Unlike natural water bodies, the chemistry of11

industrial effluents is governed by the choice of chemicals in various processing steps. This study experi-12

mentally demonstrates that replacing chloride with sulfate anions in the dyeing process baths can lead to13

significant energy and cost savings in effluent treatment by using NF membranes for brine concentration. The14

impact of electrolyte choice and concentration on fabric color is evaluated in a dyeing unit. While achieving15

similar fabric color, thermodynamic minimum energy consumption for sulfate brine concentration is around16

50% lower as a result of its lower osmotic pressure. Practically, we show that while an RO membrane can17

concentrate the sulfate brine upto 60g/kg at 60 bar pressure, nanofiltration membranes can concentrate the18

same to around 99g/kg at the same pressure as a result of its higher permeability. Commercially acceptable19

color reproducibility was achieved for 3-5 cycles for different shades of dyeing when reusing NF brine in the20

dyebath. Ultrafiltration treatment of the concentrated brine reduces color deviation by 50–80% and is a21

promising method for brine polishing.22

Keywords: energy, thermodynamics, nanofiltration, reverse osmosis, reactive dyeing, effluent recycling23

∗Corresponding authorEmail addresses: lienhard@mit.edu (John H. Lienhard), jaichander.s@iitgn.ac.in (Jaichander Swaminathan)P.R. Gupta, S.P. Shanmukham, S. Patel, J.H. Lienhard V, J. Swaminathan, “Replacing chloride anions in dyeing enables

cheaper effluent concentration and recycling,” Desalination, online 20 April 2022, 533:115761, 1 July 2022.

Preprint submitted to Elsevier August 18, 2022

Nomenclature24

Roman symbols25

a∗ CIELAB coordinate on the Red-Green Axis, -26

b∗ CIELAB coordinate on the Blue-Yellow Axis, -27

A Membrane permeability, L/m2-h-bar or m/s-Pa28

Am Area of membrane, m229

ai,x Activity of ith component in stream ‘x’30

C Chroma, -31

∆E Simple distance metric in CIELAB color space, -32

∆Ecmc,2:1 Modified distance metric to compare colors accounting for human visual perception. -33

f(s) Trendline fitted L∗, a∗ and b∗34

H Hue, degree35

j Flux, L/m2-h or m/s36

L Lightness37

L∗ CIELAB coordinate on the Lightness Axis, -38

LR Liquor Ratio, m3 of water/kg of fabric39

mfabric Mass of fabric dyed, kg40

Nbaths Number of baths in the dyeing process41

p1, p2, q2 Coefficients for fitting L∗, a∗ and b∗42

Rm Resistance offered by the membrane to water transport, s-Pa/m43

s Salinity, g/L44

Sgen Entropy generation rate, W/K45

sgen Specific entropy generation, J/m3-K46

S Weight factors for various axes when calculating color difference47

T0 Ambient temperature, K48

V Volume flow rate, m3/s49

w Specific energy consumption, kWh/m350

∆wexcess Excess specific energy, J/m351

W Least Work of Separation, J/s52

Subscripts53

b Concentrated brine reject stream54

2

DB Dyebath55

f Feed56

gen Generated57

i Solute species58

m Membrane59

p Permeate product stream60

w Water61

Greek symbols62

π Osmotic pressure, bar63

Acronyms64

BOD Biological oxygen demand65

COD Chemical oxygen demand66

ED Electrodialysis67

ETP Effluent treatment plant68

FRP Fiber reinforced plastic69

MED Multi-effect distillation70

MF Microfiltration71

MVC Mechanical vapor compression72

NF Nanofiltraiton73

PV Pressure vessel74

R.H.S Right hand side75

RO Reverse osmosis76

TDS Total dissolved solids77

TOC Total organic carbon78

TVC Thermal vapor compression79

UF Ultrafiltration80

ZLD Zero liquid discharge81

3

1. Introduction82

1.1. Zero liquid discharge in textile dyeing83

The presence of textile processing industries in water-scarce and agriculturally active regions has led84

to competition for water withdrawal and restrictions on treated effluent discharge. Effluent treatment and85

recycling plays a critical role in managing the textile industry’s water-footprint. This study focuses on the86

reactive dyeing of cotton fabric which uses around 12–15 tons of water per ton of fiber processed [1, 2].87

Reactive dyes have been gaining popularity ever since their introduction in the 1950s due to the wide range88

of bright colors available and their good wash fastness as a result of forming covalent bonds with the fabric [3].89

Wastewater from dyeing is characterized by high COD, BOD, color, and TDS [4, 5]. Primary and secondary90

treatment steps such as coagulation-flocculation, biological processes (aerobic and anaerobic), and sometimes91

even advanced oxidation processes, such as ozonation, are used to reduce the dye concentrations and organic92

content [6–10]. However, inorganic salts are not removed by these techniques and therefore the discharge of93

such effluents results in soil degradation and increased TDS [11, 12].94

As a result, environmental protection agencies have increasingly mandated zero liquid discharge (ZLD)95

regulations on the textile dyeing industry to manage their impact on the local water resources. In order96

to achieve ZLD, desalination technologies like reverse osmosis, thermal distillation, electrodialysis, and me-97

chanical vapor compression are necessary to recover freshwater and concentrated brine from the secondary98

treated effluent [13–17]. Fig. 1 shows the typical combination of technologies used for water and salt recovery99

in textile units. After desalination, the brine is highly concentrated in salts and also contains dye molecules100

and fragments that were not effectively removed in the initial treatment steps. Therefore, color reappears101

in this desalination brine. Nanofiltration is commonly used as a polishing step to remove color and other102

molecules while allowing NaCl to pass into the polished brine for recycling.103

Being a fragmented and heterogenous sector dominated by small and medium-scale enterprises, the textile104

dyeing industry faces significant hurdles regarding compliance with the zero liquid discharge norms mandated105

by environmental protection agencies. Implementing ZLD usually involves significant energy consumption.106

While RO is an energy-efficient process for desalination compared to thermal distillation processes [18, 19],107

it is limited by membrane fouling and by the higher feed pressures required to overcome the concentrated108

brine’s osmotic pressure [20]. The thermal concentration processes are highly energy intensive and inefficient109

technologies [21–24]. Around 50% of the energy consumed by the textile dyeing units today goes into110

the water treatment plants alone [25–27]. Consequently, identifying suitable process interventions that can111

reduce the cost of water treatment and recycling while maintaining, if not improving, the final dyed fabric112

quality is essential for the industry’s sustenance and growth.113

4

Figure 1: Typical layout and operation of various technologies in a textile dyeing effluent treatment plant for implementingZLD.

1.2. Recovery and reuse of salt and water in the dyeing process114

As shown in Fig. 1, in addition to the pure water, salt or saline brine is also recovered for reuse in the115

dyebath. Ranganathan et al. [28] and Vishnu et al. [15] have reported on multiple dyeing units in Tirupur,116

India, that employ a combination of RO and MED in their effluent treatment plant (ETP) for recovering117

water and salts.118

A simplified schematic representation of the various steps involved in reactive dyeing of cotton is shown119

in Fig. 2. The fabric is subjected to a sequence of baths, and, in each bath, the fabric is placed inside120

an aqueous solution containing specific additives at a particular temperature. In Fig. 2, e.g., a total of 8121

steps are considered, with the first three being preparatory steps prior to the actual dyeing. Bath 4 is the122

critical dyebath step, wherein reactive dyes are bound to the fabric. Baths 5 to 8 are used post-dyeing to123

remove unbounded dyes, chemical treatment to ensure colorfastness, and impart other favorable properties124

to the fabric. The amount of water used per bath is proportional to the mass of fabric being dyed, and the125

proportionality constant is referred to as the liquor ratio (mass of water per bath/mass of fabric). The liquor126

ratio (LR) is a function of the geometry of the dyeing apparatus and ranges from 7–12, with newer machines127

being designed at lower LR. Thus, the total amount of water used in dyeing is Nbaths × LR×mfabric, where128

Nbaths is the number of baths and mfabric is the mass of fabric dyed.129

In order for concentrated brine to be reused in the dyebath, the combined effluent has to be desalinated130

until the brine concentration exceeds the required concentration of the dyebath (sDB). Usually, in dyeing131

units, the combined effluent starts at around 7 g/L salinity and is thus concentrated to beyond 80 g/L or132

solid salt is recovered so that it can be used in the dyebath even for the darkest shades of fabric. For lighter133

shades of dyeing, the solution is diluted as required. The need and role of salt in dyeing and some recent134

5

Figure 2: Effluent streams from various baths in textile dyeing are mixed to form the combined effluent. Our focus is on thedissolved solutes which are handled in the tertiary desalination treatment steps.

studies evaluating elimination of salt from the dyeing process are discussed in Appendix A.135

1.3. Separate collection, treatment, and reuse of the high salinity dyebath effluent?136

The effluents from the dyebath and the wash bath that immediately follows it are salty, whereas the137

effluents from the other baths leave at relatively low salinity. Therefore, it would be advantageous to collect138

and treat these solutions separately. There has been significant research interest focused on decolorization of139

the dyebath effluent, and reuse of the saline solution. Li et al. [29] reported acceptable color reproducibility140

for up to 3 cycles by recycling the dyebath effluent after treatment using a novel heterogeneous Fenton system.141

However, the process requires the addition of new salt to increase the concentration of the treated solution142

upto the level of the dyebath requirements. Grimau et al. [1, 30] used electrochemical decolorization of the143

dyebath and post-dyebath wash solutions to reuse the decolorized solutions with dissolved salt in dyeing,144

saving up to 72% of the NaCl consumption. Bilinska et al. [31] used ozonation to remove the color from145

the effluent to produce a brine that may be reused in the dyebath. However, the build-up of transformation146

products from ozonation limited the reusability of the brine to a single reuse cycle.147

The recent academic literature has been very focused on the development and testing of novel membranes148

for efficient separation of dyes and inorganic salts. Various loose NF and tight UF membranes have been149

formulated [32–40] with a range of additives [41, 42] to achieve high rejection of dyes and low rejection of150

inorganic salts simultaneously. While commercial dyeing units using NF for brine polishing and are therefore151

restricted to recovery of chlorides in the brine recovery solution, these membranes are developed to allow152

6

passage of all inorganic salt species. Ye et al. [43] used a loose NF membrane as an anion exchange mem-153

brane in electrodialysis and demonstrated dye-salt fractionation using ED. Buscio et. al. [44] demonstrated154

electrochemical decolorization of the colored retentate obtained after nanofiltration-based separation of dyes155

and salt.156

Although it would be advantageous from an energy standpoint to separate and treat the effluent different157

steps of the dyeing process separately (to avoid the entropy generation associated with mixing streams at158

different salinities), it is a common practice that all the effluent streams are combined. One of the reasons159

for this is that the commonly used conventional biological processes for color removal are ineffective at high160

feed salinity. The other reason is that a set of small dyeing units would not have individual water treatment161

infrastructure, and therefore rely on subscription to a centralized common effluent treatment plant, to which162

they send all their effluents and from where they would get back clean water and saline brine for reuse.163

1.4. Objectives of the present study164

The focus of this study is on the tertiary treatment stage (desalination steps) of an ETP where the165

secondary treated effluent is separated into a concentrated brine and clean water for reuse in the dyebath and166

other processing steps respectively. For the first time, we explore how controlling the electrolyte used in the167

upstream processes (particulary the dyebath) can influence the energy consumption in the effluent treatment168

plant. Using a model textile dyeing plant layout, the impact of replacing chloride with sulphate anions on169

the energy consumption for desalinating the secondary treated brine is analyzed. Since reproducibility of170

the fabric colors is a major factor in making such an intervention acceptable to the industry at scale, we also171

look at the impact of reusing the concentrated brine as the liquor for the dyebath on the resulting color of172

the dyed fabric.173

Unlike natural water bodies such as seawater and brackish groundwater, whose composition is deter-174

mined by nature, the composition of industrial effluents can be tuned by changing the chemical used in each175

processing step. Historically, the choice of chemicals would likely have been governed by local cost opti-176

mization, i.e., among the choices available, the option that provides satisfactory performance at the lowest177

cost would be chosen to maximize profits. The impact of chemical selection on ZLD implementation would178

not have been considered prior to ZLD regulations. After ZLD was mandated, water treatment has often179

been considered an add-on to the core dyeing operations and hence a system level optimization has not been180

undertaken, though effluent treatment imposes a significant economic burden on the dyeing units.181

The influence of different electrolytes on the uptake of anionic dyes has been studied in the past. Madan et182

al. [45] observed that at an equal equivalent concentration of dyebath solutions, the sorption of hydrolyzed183

dyes (CI reactive red, brown, blue 4, and 9) to the fabric is usually higher for those electrolytes with a184

7

larger cation size. Sakata [46] and Sivaraja Iyer [47] observed similar trends in their respective studies with185

other dyes. However, since the primary and secondary treatment steps do little to deal with the inorganic186

electrolytes, the electrolyte chosen for the dyebath shows up in the brine that is concentrated through the187

tertiary treatment step.188

Mistry et al. [48] considered desalination of various saline streams and showed that the thermodynamic189

limit of the minimum energy required for desalination is a strong function of the ions present in water for190

solutions of the same overall salinity or ionic strength. Hence, the choice of electrolyte in the dyeing process191

can be expected to have an impact on the energy requirement as well as technologies necessary for effluent192

recycling. This study looks at the relation between these two factors for a model textile dyeing plant as193

summarized in Fig. 3.194

Figure 3: Schematic representation of the proposed interventions in comparison to a conventional system.

1.4.1. Manuscript layout195

Initially, we compare the energy required for brine recycling based on literature data for dye uptake196

as a function of salinity of various electrolytes in the dyebath (Section 3.1). In the industry, NaCl and197

Na2SO4 are recommended and commonly used in reactive dyeing. Therefore we focus on these two salts198

in subsequent sections. The potential advantages, both based on separation-process thermodynamics and199

kinetics, associated with replacing all the Cl− ions in solution with SO2−4 ions are discussed in Section 3.2.200

The impact of electrolyte concentration on the fabric color produced with both NaCl and Na2SO4 at various201

concentrations is discussed in Section 3.3. Based on these results, the thermodynamic energy requirements202

are compared in Section 3.4.203

8

Practically, further energy savings are possible if the effluent has SO2−4 as its primary anion since NF204

membranes can be used for salt-water separation rather than RO. This hypothesis is evaluated experimentally205

in Section 3.5, and the reusability of the NF brine for dyeing is also studied. Finally, the potential of206

using ultrafiltration for Na2SO4 brine polishing to improve color quality during brine reuse is demonstrated207

experimentally.208

2. Methods209

2.1. Thermodynamic minimum energy evaluation210

The secondary treated combined effluent needs to be split into a relatively clean product and a saline211

brine for complete recycling. As shown in Fig. 2, this involves concentrating the effluent from the combined212

effluent feed salinity of sf = sDB/Nbaths up to a brine salinity of sb = sDB. For each choice of electrolyte,213

once sDB is known, the thermodynamic minimum energy required per unit volume of the combined effluent214

can be calculated as w = W/Vf , where Vf is the volume of the combined effluent and W is the least work215

required for separation. W can be evaluated by combining the first and second laws of thermodynamics216

applied to the separation system and setting entropy generation (losses) to be zero [49]:217

W = RT[{

Nw,b ln(aw,b) + Nw,p ln(aw,p)− Nw,f ln(aw,f)}

+∑i

{Ni,b ln(ai,b) + Ni,p ln(ai,p)− Ni,f ln(ai,f)

}](1)

where N is the mole flow rate, a is activity, w denotes water, and i refers to various solute species. The218

subscripts b, p, and f denote the brine, product, and feed respectively. The activity of various species in219

Eq. 1 are obtained using the pyEQL package [50] or PHREEQC [51, 52].220

Note that the effluents from multiple dyeing operations are mixed in common practice. In each oper-221

ation, sDB is chosen based on the depth of fabric shade desired, so there are multiple values of sDB, for222

example at shighDB , smediumDB , and slowDB. Therefore the combined effluent salinity will be a weighted average of223 {

shighDB

/Nbaths, s

mediumDB

/Nbaths, s

lowDB

/Nbaths

}, with the weighting being based on the number of loads of dark,224

medium, and light shades dyed. However, the final brine concentration after separation should still be shighDB225

so that even for the darkest shades, the brine can be reused in the dyebath as-is, whereas for the lighter226

shades, it can be appropriately diluted down as necessary. For simplicity, only one dyebath salinity level227

is considered in these calculations. The trends for energy consumption will be similar even if a portfolio of228

dyeing shades and a corresponding weighted average is considered instead.229

9

2.2. Dyeing experiments230

The dyeing tests were carried out at SP Textile Processors, Pvt. Ltd., in Erode, Tamil Nadu, India.231

Dyeing was performed on an infrared lab dyeing machine from RB engineering. Previously bleached and232

washed cotton yarn samples (5 g) were used as the target material. Since the yarn is already prepared for233

dyeing, it is exposed to the dyebath directly. A liquor ratio of 8 is used in this device (40 ml of water per bath234

for 5 g of fabric). After the dyebath, 8 additional wash baths are performed. The temperature and duration235

of various baths were matched with the settings used in the production machines. Yarn samples were pre-236

treated by demineralizing, peroxide boiling, hot washing, and neutralizing. These pre-treated samples were237

then dyed in the lab dyeing machine. The dyebath was followed by washing, neutralizing, soaping, hot238

washing, and finishing baths. The dyebath pH was maintained at around 11 using Na2CO3 and NaOH.239

Following this, fixation of dyes was performed using 15 g/L solution of Na2CO3 for 28, 28, and 21 minutes240

for olive, light blue, and light purple recipes, respectively. In tests using Na2SO4 as the electrolyte in the241

dyebath, the combined effluent pH was adjusted using H2SO4 acid. Also, high-grade auxiliary chemicals242

such as soda ash were used to reduce Cl− addition in the form of impurities.243

To compare the impact of electrolyte choice (NaCl vs. Na2SO4) on the color produced, dyeing was244

performed at constant dyestuff concentration while changing the concentration of each electrolyte from 10245

to 80 g/L, in steps of 10 g/L. This way, we tried to isolate the electrolyte type and concentration impact246

when holding other parameters constant. Note that both the dye and salt concentration would be varied247

simultaneously to produce darker or lighter shades in practice.248

2.3. Color analysis and comparison249

The color of the dyed cloth was measured using a color meter under simulated D65 illumination conditions250

[53]. The color is measured along the L∗, a∗, and b∗ coordinates (in Cartesian space) or equivalently, in251

terms of lightness (L∗), chroma (C) and hue (H) coordinates (in cylindrical polar space). The interconversion252

between the L∗a∗b∗ and the L∗CH coordinates is done as shown in [54]. Fig. 4 gives a visualization of the253

3D color space using either of these systems. L∗ values in the CMC colorspace vary from 0 to 100, where254

0 indicates a dark shade and 100 indicates bright white. The a∗, and b∗ values range from −150 to +150.255

Negative values of a∗ indicate greener, whereas positive values indicate more redness. Similarly, a negative256

value of b∗ coordinate indicates a more yellow color while positive values corresponds to a more blue color.257

Comparisons between a measured color and a standard were performed by calculating the ∆Ecmc,2:1258

value, which is an appropriately weighted distance measure between the two points in LCH space:259

∆Ecmc,2:1 =

√(∆L∗

lSL

)2

+

(∆C

cSC

)2

+

(∆H

SH

)2

(2)

10

Figure 4: 3D Color space with showing the different axes (L, a∗, b∗ for the CIELAB system and L, C (chroma), H (hue) for theLCH system); the point indicates the position of a sample point with respect to each axis. (For representative purposes only)

The weighting factors (S) along each axis are functions of the point coordinates and are used to account260

for variations in human visual perception. Further, since the eye’s sensitivity to differences in lightness and261

chroma are not equal, l and c are numeric parametric factors that allow these individual differences to be262

independently weighted relative to the difference in hue. The choice of l and c values are usually based on263

industrial practices. For textile industries, l is set to 2 and c is set to 1. ∆Ecmc,2:1 is calculated according264

to UNE-EN ISO 105-J03: 2010 [55] following the steps outlined in [54]. A value of ∆Ecmc,2:1 below 1 is265

considered to be visually indistinguishable from the comparison color standard.266

2.4. Membrane brine concentration and effluent recycling267

The water flux, permeability, and salt rejection of various membranes were compared using a Sterlitech268

HP4750X stirred cell setup capable of withstanding up to 172 bar of applied pressure. The applied pressure269

was increased with time using a pressure regulator at the outlet of a pressurized N2 tank to maintain water270

flux at around 15–20 L/m2-hr, even as the feed salinity and osmotic pressure inside the system increased.271

Water flux was calculated by taking the derivative of the total permeate mass collected with respect to time.272

The permeate conductivity was measured after every 15 mL of permeate collected. Based on the cumulative273

water and salt lost in the permeate, the feed salinity inside the system was also inferred. Using the permeate274

and feed salinity, the salt rejection during that time-frame was calculated as 1− (sp/sf).275

In order to assess the reusability of the dyeing effluent, the combined effluent was first treated with ferrous276

sulfate and aeration for color removal. The decolorized solution was then concentrated using a Desal-5 DK277

nanofiltration membrane (GE/Suez) in the Sterlitech stirred cell HP4750 setup (for the multi-cycle trial in278

the dyeing unit) or CF042 cross-flow setup. The pressure was manually adjusted with time to ensure that279

the flux was maintained around 15–20 LMH. A PAN50 ultrafiltration membrane (Nanostone Water) was280

11

used for the polishing experiment. The color of fabric produced using these (suitably diluted) brines in the281

dyebath was compared with the color produced using a pure salt solution at the same conductivity.282

3. Results and Discussion283

3.1. Ranking electrolytes based on energy for effluent recycling284

To understand the energy implications of the choice of dyebath electrolyte, in the dyebath, we use data285

reported by Madan et al. [45] for dye uptake vs. solution concentration for CI Reactive Red at 40 °C (Fig. 5a).286

The dyebath concentration was varied over the range of 0.1–1 equivalents/L for chloride salts of ammonium,287

magnesium, sodium, and lithium chloride, as well as ammonium sulfate. Significant differences in dye uptake288

were observed amongst the different salts at the same normality. At low concentrations, the dye uptake of289

MgCl2, NH4Cl and (NH4)2SO4 are much higher than that of NaCl, which in turn is higher than LiCl.290

Uptake in the case of MgCl2 stabilizes beyond a particular solution concentration, whereas the trend among291

the other salts remains consistent up to higher concentrations.292

0.0

0.1

0.2

0.3

0.4

0 3 6 9 12Dye uptake [g/kg-fabric]

Spec

ific

ener

gy

[kW

h/m

3] LiCl

NaClMgCl2NH4Cl(NH4)2SO4

(a) Dye uptake vs. dyebath concentration (sDB) forvarious electrolytes reported in [45].

0

3

6

9

12

0 0.2 0.4 0.6 0.8 1

Dye

upta

ke [

g/k

g-f

abric]

Dyebath normality [eq/L]

NH4CL

MgCl2NaClLiCl

(NH4)2SO4

(b) Comparison of least work for desalination for dif-ferent salts to achieve the same dye uptake.

Figure 5: Impact of dyebath electrolyte on dye uptake and energy cost of effluent recycling

At each value of dye uptake, therefore, we have the sDB required for each salt choice. Therefore, for293

each dye uptake and salt combination, the minimum work for concentrating the combined effluent starting294

at sDB/Nbaths up to sDB by recovering pure water is calculated using Eq. 1, for Nbaths = 8. The activity295

of various species is evaluated using the phreeqc.dat database using the PHREEQC software. There is a296

large difference in the specific energy consumption among the salts to achieve the same level of dye uptake.297

At low dye uptakes (≲ 5 g-dye/kg-cotton), the order of least work requirement is: LiCl > NaCl > NH4Cl298

> MgCl2 > (NH4)2SO4. Although MgCl2 seems to provide energy savings, divalent cations causing water299

hardness are usually removed and avoided in dyeing plants. Beyond a dye uptake of 5 g-dye/kg-cotton, the300

least work requirement for MgCl2 appears to shoot up due to the dye uptake saturating after about 0.5 eq/L.301

12

The ammonium salts promise lower energy, but they are unstable and would convert into ammonia at low302

pH before being lost from the solution. Additionally, they are more expensive and are not used commonly303

in the industry. In the textile dyeing industry, NaCl and Na2SO4 are recommended as potential electrolytes304

for use in the dyebath by dyestuff manufacturers. Therefore, a comparison between NaCl and Na2SO4 is305

our focus going forward. Note that (NH4)2SO4 requires lower energy for recycling than NH4Cl, although a306

higher concentration of (NH4)2SO4 is required in the dyebath than NH4Cl to achieve the same dye uptake.307

Our hypothesis based on these trends is that replacing Cl− with SO2−4 as the anion of choice in textile308

dyeing can provide cost savings when considering the overall process, including brine concentration and309

recycling. The potential advantages are explored from both a theoretical and practical perspective in the310

following section.311

3.2. Potential advantages of replacing Cl− with SO2−4312

3.2.1. Thermodynamics: osmotic pressure, least energy.313

Recall that the energy required for brine concentration was lower in the case of (NH4)2SO4 compared314

to NH4Cl. The reason for the lower energy consumption in the case of the sulfate salt at nearly equal315

cation concentration can be understood as follows: at equal molality of the cation scat, assuming complete316

dissociation, the total concentration of solutes is 2 × scat in the case of NH4Cl whereas in the case of317

(NH4)2SO4 it is only32×scat (since there is only

12 the number of SO2−

4 ions for every NH+4 ion in (NH4)2SO4).318

At low solute concentrations, if the solution behavior is close to ideal, the osmotic pressure is directly319

proportional to the total concentration of solutes, and so correspondingly, the least work of separation is320

also lower.321

From discussions with the industry, we learn that sodium sulfate and chloride are used in approximately322

similar concentrations in terms of g/L to achieve light and dark shades of fabric, i.e., both in the case of323

NaCl and Na2SO4, a dark shade requires around 70–80 g/L concentration in the dyebath, and a light shade324

needs only about 15–20 g/L, with medium shades requiring concentration levels in between these limits. Due325

to the differences in molecular weight and charge between the two anions (SO2−4 and Cl−), a similar g/L326

concentration results in ionic strength values that are around 20% different in the two cases. For the same327

concentration, s, in g/L, the corresponding concentration of dissociated ions in mol/L is 2×(s/58.44) = 0.034s328

for NaCl and 3× (s/142) = 0.021s for Na2SO4 which is about 38% lower.329

Figure 6 compares the osmotic pressure of Na2SO4 and NaCl solutions at similar concentration in g/L.330

The osmotic pressure of the Na2SO4 brine is significantly lower than that of NaCl brines. Permeate flux is331

a function of the difference between the applied hydraulic pressure and the solution osmotic pressure. Using332

sodium sulfate brines would yield significant energy savings since the pump needs to pressurize the feed to333

13

Figure 6: Comparison of osmotic pressure of NaCl and Na2SO4 solutions as a function of (a) solution ionic strength, and (b)concentration in g/L

a much lower hydraulic pressure to drive water across the membrane at the same flux.334

3.2.2. Kinetics: higher permeability membranes.335

The above thermodynamic calculation helps quantify the energy for desalination when using an ideal

separation device devoid of any losses wleast. The specific energy consumption of a real system w would

exceed this ideal limiting value, by a magnitude proportional to the specific entropy generation, sgen, in the

system:

w = wleast +∆wexcess = wleast + T0sgen (3)

For water transport across the membrane, sgen is proportional to the product of the water flux and the336

resistance offered to transport, as shown below (see also [56]):337

sgen =Sgen

Vp

=j × (∆P −∆π)Am/T0

jAm

=1

T0(∆P −∆π) =

1

T0

j

A(4)

∆wexcess = T0sgen = j × 1

A= j ×Rm (5)

Therefore, a lower membrane resistivity (equivalently, higher permeability A) results in less excess work338

∆wexcess, and therefore lower specific energy consumption, for the same wleast.339

Replacing Cl− with SO2−4 in the effluent stream enables the use of membranes with larger pores to separate340

pure water and saline brine. While ROmembranes offer high rejection of monovalent ions, nanofiltration (NF)341

membranes are characterized by larger pores than RO and, therefore, higher water permeability at the cost342

of lower rejection of monovalent ions. However, if monovalent anions are replaced with multivalent anions,343

NF membranes can be used in place of RO for the solution concentration [57–59], since these multivalent344

14

anions are rejected effectively due to charge-based exclusion by the membrane active layer. Hilal et al. [60–345

62] have investigated this idea in the form of Cl−-SO2−4 ion-exchange resins coupled with NF membranes for346

lower-energy desalination. If SO2−4 is rejected, so is Na+ to satisfy electroneutrality in the product stream.347

These NF membranes are usually 2-5× more permeable than RO due to their larger pores, which reduces348

the excess work required to operate the desalination process at the same overall flux.349

Pres

sure

[bar

]

Feed salinity [g/kg]

Osmotic pressure

SW30HR (RO)

NF90

NF270

(a) Applied pressure (dead-end setup). (b) Na2SO4 rejection (dead-end setup).

(c) Steady-state applied pressure in cross-flow mem-brane system with a Desal DK membrane.

(d) Na2SO4 rejection in cross-flow membrane systemwith a Desal DK membrane.

Figure 7: (a,b) Dead-end filtration experimental comparison of membranes for partial desalination of Na2SO4 solution up to 120g/kg. (c,d) Steady-state pressure and rejection data for Na2SO4 concentration using a Desal DK NF membrane in a cross-flowoperation mode. In all cases, the applied pressure is adjusted with changing feed concentration to maintain an average fluxbetween 15–20 L/m2-hr.

Commercially-available NF membranes, NF90 and NF270 (Dow), and an RO membrane, SW30HR, were350

tested for their ability to desalinate pure sodium sulfate solutions up to a salinity of 110 g/kg or 1.75 mol351

Na+/kg-w, which is higher than sDB required for the darkest shades. These tests were performed in a352

Sterlitech HP4750X stirred cell setup. The stirrer speed was set at 300 rpm and kept constant across the353

experiments. In order to test the membrane flux and rejection under realistic conditions over a wide range of354

salinities, the permeate flux was maintained within a range of 15–20 LMH by increasing the feed pressure as355

the feed salinity inside the dead-end apparatus rose with time. The measured flux with all three membranes356

is shown in the inset of Fig. 7b.357

While the SW30HR membrane achieves a better salt rejection, the NF90 also consistently achieves358

15

more than 98% rejection of Na2SO4 while requiring a much lower applied pressure as a result of its higher359

permeability (Fig. 7a). The NF270 membrane has a lower rejection than NF90, which gradually increases360

from 94% to 97% at higher salinity and correspondingly has slightly better permeability. Note that the lower361

rejection seen at 30 g/kg and 60 g/kg for NF270 (and similar deviations in the other cases) are aberrations362

due to refilling and restarting the experiment with fresh feed (this is necessary to ensure sufficient solution363

volume in the system so that the stirring action is effective). Another problem with the dead-end stirred364

cell setup is that since salinity is changing continuously with time, the results may not indicate steady-state365

values, which would be observed during long-term operation in the industry.366

Brine concentration was studied using a cross-flow membrane test to correct for these effects. At each feed367

salinity level considered, the steady-state membrane performance was evaluated by emptying the permeate368

back into the feed tank to maintain constant feed salinity until the flux and salt rejection values stabilized.369

The applied pressure and rejection data for these tests using a Desal DK NF membrane are shown in Figs. 7c370

and 7d respectively. The average rejection achieved by the DK membrane is around 96%, and it decreases371

slowly with an increase in salinity. This trend may result from a reduction in membrane water permeability372

due to compaction with time at high applied pressures, whereas the salt permeability may remain relatively373

constant. Up to a feed salinity of 80 g/kg, a feed pressure of 40 bar is sufficient, and we need to apply 50374

bar pressure to exceed 90 g/kg solution salinity.375

An important takeaway from these tests is that concentrating the brine beyond 90 g/kg is possible using376

the NF membranes with applied pressure lower than 60 bar while still maintaining water flux greater than377

15 L/m2-hr throughout. On the other hand, for the RO membrane, a pressure of about 80 bar is required.378

Standard FRP pressure vessels for spiral wound membrane elements are available up to a 69 bar (1000379

psi) pressure rating, which would suffice for the brine concentration beyond 90 g/kg using NF membranes.380

This pressure vessel limitation is one of the reasons why dyeing units are forced to use less efficient thermal381

processes to manage their RO brine. The alternative for dyeing units at present is to invest in specialized382

high-pressure RO modules to go up to 150 bar applied pressure to outperform distillation energetically.383

However, the capital cost of such specialized high-pressure vessels and membrane elements is higher than384

conventional PVs and spiral-wound elements.385

3.3. NaCl vs. Na2SO4: color differences386

In the above discussion, we have compared the energy for desalination of NaCl and Na2SO4 at similar387

salinity (in g/L) without explicitly accounting for the color differences when dyeing with these two salts.388

Producing a fabric with an acceptable color is a critical requirement for a dyeing unit, so differences in color389

production between the two salts must be evaluated.390

16

Table 1: Dyes tested

Color Dye, strengthRed Red SPD, 2%Blue Navy blue SPD, 2%Black Black WNN, 8%

We tested the use of Na2SO4 in the dyebath with different dye recipes (Table 1) and compared the391

resulting dyed fabric color with those produced using NaCl at different salinity levels for both salts. The392

salinity was changed from 10–80 g/L in steps of 10 g/L. The dyestuff concentration was kept constant in all393

the dyeing trials to isolate the salt’s impact alone.394

0 0.5 1 1.513

14

15

16

17

18

Ligh

tnes

s (L

*)

NaClNa2SO4

Electrolyte Normality [eq/L]

(a) Black L∗

0 0.5 1 1.5Electrolyte Normality [eq/L]

0

0.2

0.4

0.6

Gre

en-R

ed (a*

)

NaClNa2SO4

(b) Black a∗

0 0.5 1 1.5-4

-3.5

-3

-2.5

-2

Blu

e-Ye

llow

(b*

)

NaClNa2SO4

Electrolyte Normality [eq/L]

(c) Black b∗

0 0.5 1 1.529

30

31

32

33

34

35NaClNa2SO4

Ligh

tnes

s (L

*)

Electrolyte Normality [eq/L]

(d) Blue L∗

0 0.5 1 1.5 Electrolyte Normality [eq/L]

-6.5

-6

-5.5

-5

-4.5

-4

Gre

en-R

ed (a*

)

NaClNa2SO4

(e) Blue a∗

0 0.5 1 1.5-18.3

-18.2

-18.1

-18

-17.9

-17.8

NaClNa2SO4

Blu

e-Yel

low

(b*

)

Electrolyte Normality [eq/L]

(f) Blue b∗

0 0.5 1 1.543

44

45

46

47

48NaClNa2SO4

Ligh

tnes

s (L

*)

Electrolyte Normality [eq/L]

(g) Red L∗

0 0.5 1 1.558.5

59

59.5

60

60.5

61

61.5

NaClNa2SO4

Gre

en-R

ed (a*

)

Electrolyte Normality [eq/L]

(h) Red a∗

0 0.5 1 1.5-2

-1

0

1

2

NaClNa2SO4

Blu

e-Yel

low

(b*

)

Electrolyte Normality [eq/L]

(i) Red b∗

Figure 8: L∗, a∗, and b∗ values obtained with varying concentration of NaCl and Na2SO4.

Figure 8 shows the color produced (in terms of L∗, a∗, and b∗ values) at each sDB level for both the395

salts. For all the three dye recipes, the a∗ and b∗ coordinates follow similar increasing trends with higher396

17

concentrations of both electrolytes, while the decreasing L∗ values indicate that the shade becomes darker397

with increasing salt concentration. The change in the L∗ is the major contributor (> 50%) to the color398

difference observed with increasing dyebath salinity, as shown in Fig. 9 which shows the color difference399

(∆Ecmc,2:1) at various dyebath salinities, compared to the color produced with a 10 g/L solution. The400

relative normalized contribution of each axis is calculated as the ratio of each squared term on the R.H.S of401

Eq. 2 to (∆Ecmc,2:1).402

4

3.5

3

2.5

2

1.5

1

Ecm

c

20 30 40 50 60 70 80NaC l salinity [g/L]

Norm

alis

ed

Contr

ibutio

nof axe

s

1

0.8

0.6

0.4

0.2

0

Lightness (L*)Red-Green (a*)Blue-Yellow (b*)

Ecmc

(a) Black

20 30 40 50 60 70 80NaC l salinity [g/L]

Norm

alis

ed

Contr

ibutio

nof axe

s

Lightness (L*)Red-Green (a*)Blue-Yellow (b*)

Ecmc

4.5

4

3.5

3

2.5

2

1.5

1

Ecm

c

1

0.8

0.6

0.4

0.2

0

(b) Blue

3.2

3

2.8

2.6

2.4

2.2

2

1.8

1.6

Ecm

c

20 30 40 50 60 70 80NaC l salinity [g/L]

1

0.8

0.6

0.4

0.2

0Norm

alis

ed

Contr

ibutio

nof axe

s

Lightness (L*)Red-Green (a*)Blue-Yellow (b*)

Ecmc

(c) Red

Figure 9: Color difference (∆Ecmc,2:1) when using various concentrations of NaCl solution compared to 10 g/L dyebath, andthe relative contributions from the three axes.

The use of Na2SO4 gave shades close to that produced by a similar number of equivalents of NaCl. While403

NaCl is a 1:1 electrolyte, Na2SO4 is a 2:1 electrolyte. However, similar equivalents of NaCl and Na2SO4 will404

result in similar concentrations of the cation, Na+. Sakata [46], and Sivaraja Iyer [47] noted that the nature405

and concentration of the cation has a strong influence on the absorption of direct dyes on cellulosic fiber in406

comparison to the anions, which do not have a strong differential effect on the same.407

3.4. NaCl vs. Na2SO4: energy at similar color408

The color differences between using NaCl and Na2SO4 dyebaths at the same salinity are shown in Fig. 10.409

It may be noted that the ∆Ecmc,2:1 is less than 1 across all the dyes and salinity levels, indicating that410

relatively similar shades are achieved at the same salt concentration in g/L. The plot also shows the411

minimum energy required for brine concentration for both salts. Lower energy is needed in the case of the412

Na2SO4 solution due to its lower osmotic pressure, as shown in Fig. 6. When matching the color achieved413

with NaCl with Na2SO4 the least work is lower by around 30–50% (Appendix B).414

3.5. Na2SO4: use of NF for brine concentration415

3.5.1. Water reusability experiments.416

Textile dyeing is a complex process involving the use of not only the dye mixtures and salts, but also417

many other ‘auxiliary’ compounds. Even though some of these compounds may break down during the418

18

Figure 10: Energy Requirements and Color differences at the tested salinities

primary and secondary treatment, their byproducts can build up in the system over several cycles of reuse.419

Li et al. [29] noted that further treatment for total organic carbon (TOC) removal would be required for420

reuse of brine treated by their Fenton system beyond three cycles, whereas Bilinska et al. [31] were only421

able to reuse the brine for one cycle. Hence, it is crucial to study the reusability of the treated brine after422

NF-based concentration.423

Figure 11: Color reproduction over brine reusability cycles

Three different dye recipes (Table 2, Fig. 12) producing different shades (bright blue, brown, and olive)

were dyed with Na2SO4 in the dyebath, taking care to reduce Cl− ion addition through the other chemical

additives used. The color obtained initially when pure Na2SO4 is used in the dyebath is taken as the

baseline against which the data from further reusability trials are compared. After decolorization, the

combined effluent was concentrated using the Desal DK NF membrane in a dead-end filtration cell at the

dyeing unit, until the pressure requirement reached 60 bar. An average water recovery of 88% was achieved,

19

Table 2: Recipes and dye concentrations.

Color Dye recipe(% of fabric mass)

Na2SO4

conc.[ g/L ]

Olive Meactive Blue FBS (0.4%),Meactive Red F3B (0.1%),Meactive Yellow F3R (0.6%)

40

Bright blue Meactive Blue FBS (0.1%),Meactive Red F3B (0.1%),Moh. Bril Blue RN (0.6%)

40

Brown Chemifix ultra blue 3D (1.5%),Meactive Red F3B (1.3%),Meactive Yellow F3R (1%)

60

Figure 12: Structures of the reactive dyes used.

20

with an average instantaneous conductivity rejection of 85%. Since the flux is held constant during the

concentration process, the instantaneous rejection (Rinst) was calculated in terms of the overall observed

rejection (Roverall = 1− sp/sf) and recovery ratio (RR) as:

Rinst = 1− ln (1− RR(1−Roverall))

ln(1− RR)(6)

The brine thus produced was then reused in the dyebath after necessary dilution. The color produced424

after each round is compared with the baseline color in terms of ∆Ecmc,2:1, and the results are shown in425

Fig. 11. The number of cycles of acceptable reusability appears to be strongly influenced by the dye recipe.426

The olive dye seems best suited for brine reusability with acceptable dyeing (∆Ecmc,2:1 ≤ 1) for all 5 cycles.427

The blue dye could reuse the brine for up to 3 cycles, but in the 4th and 5th cycles, the color difference is428

quite close to 1. In the case of the brown dye, acceptable color reproduction is achieved only for 2 cycles429

before the color starts to deviate significantly. These color deviations could result from the buildup of the430

TOC and other molecules, which could interfere with the dyeing process.431

3.5.2. Polishing with UF and impact on reusability432

(a) Membrane flux in dead-end UF polishing of NFbrine. (b) Dyed yarn using lab-grade solution and brines.

Figure 13: UF polishing of NF brine.

In order to further eliminate the buildup of other contaminants that could affect the dyeing process, a433

polishing step would be necessary to produce a clean Na2SO4 solution for use in the dyebath. In this regard,434

the concentrated brine was filtered using a UF membrane. The pore size of such a membrane is such that435

it can eliminate other impurities such as color-causing compounds without rejecting the sodium and sulfate436

ions. The use of this polishing step was tested for two dye mixtures, blue and olive.437

The brine stream that was polished using UF showed significant improvement in terms of matching the438

color obtained using a pure salt solution for both the colors (Table 3). The much lower color deviations in439

21

Table 3: Impact of ultrafiltration (UF) on color reproduction.

∆L∗ ∆C ∆H ∆Ecmc,2:1

Bright Blue

UF retentate 0.31 −1.35 0.06 0.73UF permeate 0.41 −0.46 0.03 0.31

Olive

UF retentate 0.17 0.39 0.46 0.61UF permeate −0.09 0.04 0.05 0.08

the case of polished brine (0.31 vs. 0.73 and 0.08 vs. 0.61) show that UF polishing can be a promising method440

to increase the number of cycles for up to which the brine may be reused for dyeing effectively. Much lower441

suspended solids (3 mg/L) in the UF permeate compared to the retentate (50 mg/L) could be the reason442

for the improved color reproduction with the UF permeate.443

Figure 13a shows the flux decline observed in the UF process when operated in a dead-end setup with444

stirring at 300 RPM. A backwash step would be necessary to clean the UF membrane and maintain flux445

periodically. Just as existing dyeing units that use NaCl in their dyebath employ NF membranes for polish-446

ing, replacing Cl− ions with SO2−4 as suggested in this study may be coupled with changing the polishing447

technology from NF to UF to allow sulfate ions to pass through while preventing other larger impurities448

from passing.449

4. Conclusions450

Unlike natural water bodies, the composition of industrial effluents can be adjusted through the appro-451

priate choice of chemicals in the upstream processing steps. In the context of textile dyeing, this provides452

an opportunity to reduce the energy cost for effluent recycling towards implementing zero liquid discharge453

by choosing the appropriate electrolytes for the dyebaths.454

The effect of dyebath electrolyte on dye uptake was reviewed from the literature. The thermodynamic455

least energy for brine concentration was then calculated for a dyeing unit with a total of 8 baths (3 prepara-456

tory, 1 dye, and 4 wash baths) based on the electrolyte concentrations required for similar dye uptake. At457

low dye uptakes (≲ 5 g-dye/kg-cotton), the order of least work requirement is found to be: LiCl > NaCl >458

NH4Cl > MgCl2 > (NH4)2SO4.459

Further, the impact of replacing Cl− ions with SO2−4 ions in the dyebath on the energy consumption for460

brine concentration was calculated. We observe that solutions with SO2−4 ions require around 50% lower461

energy than solutions with Cl− ions. The practicality of this intervention was tested by measuring the462

difference in the color of the final fabrics dyed using NaCl and Na2SO4. This difference came out to be463

22

within industrially acceptable limits (∆Ecmc,2:1 < 1).464

In addition to the thermodynamic advantage, we also demonstrated that such an intervention could allow465

us to replace RO systems with more permeable NF membranes in the desalination step. Experimentally,466

we observed that while SW30(HR) can concentrate the sulfate brine up to 60g/kg at 60 bar pressure,467

nanofiltration membranes NF90 and NF270 can concentrate the brine to 87 g/kg and 99g/kg respectively468

at the same pressure. These high salinity brines are especially useful for achieving the darkest shades of469

fabric in the dyebath. Further, the higher water recovery reduces the dependence on less efficient thermal470

processes for brine concentration. Concentrating the brine up to a salinity of 90g/kg using the SW30(HR)471

membrane will require an applied pressure of ∼77bar. NF90 and NF270 membranes can achieve the same472

at pressures as low as ∼58 bar and ∼54 bar, respectively.473

The reusability of the brine concentrated using NF membranes is also tested over multiple cycles. Ac-474

ceptable color reproducibility (∆Ecmc,2:1 < 1) (compared to pure salt solution) are obtained for 3–5 cycles475

for various colors and dye recipes. Finally, ultrafiltration treatment is demonstrated to be a promising476

method for polishing the sulfate brine, with color differences improving by around 50–80%. This can further477

reduce the energy load compared to existing brine polishing methods that use NF membranes. However, the478

observation of different levels of reproducibility highlights that the possible energy savings and acceptability479

are sensitive to the dye recipe used.480

Overall, the ideas presented and developed in this study open up a novel method for textile dyeing units481

to reduce the cost associated with their effluent treatment and recycling through process interventions in482

the dyeing process. Going forward, the idea that the chemistry of the upstream processes and the effluents483

eventually released strongly impacts ZLD costs should be explored in other industries beyond textile dyeing.484

Acknowledgments485

This work was funded by the Department of Science & Technology, Govt. of India. Reference no. DST/486

TMD(EWO)/ OWUIS-2018/ RS-03 (G). JS thanks the Tata Center for Technology & Design at MIT for487

initial financial support. The authors would like to thank Mr. Stalin and Ms. Devika at SP Textile Processors488

Pvt. Ltd. for their help with running the dyeing and reusability experiments, Prof. Lalit Anand for allowing489

the use of the high-pressure dead-end test cell at MIT, and Tiffany Tao for assistance with the dead-end490

filtration tests.491

23

Appendix A. Is salt absolutely necessary for dyeing?492

The dyeing process involves three crucial steps that determine the final product color. The first is the493

exhaustion of the dye into the fiber. When the fiber is dipped into the dyebath, dye molecules from the bulk494

need to move towards the fabric to be available for reaction. The dye molecules then need to migrate within495

the interstitial water between the fibers in order to ensure a more level or uniform distribution of the color.496

The third step is the fixation of the dyes onto the fabric. Once the dye has been sufficiently exhausted and497

migrated, additives are added to the dyebath to start the reaction - the dye molecules in the bulk solution498

undergo hydrolysis and are rendered useless. In contrast, those in the vicinity of the fiber undergo fixation499

onto the fiber. Reactive dyes, being anionic molecules themselves, are repelled by the negative surface charge500

on the hydrolyzed cellulosic fiber and suffer from a low degree of exhaustion and fixation of around 60%501

[63]. Electrolytes in the form of salt are added to the dyebath to increase the dye-fiber substantivity. The502

amount of salt used in dyeing cellulosic fibers depends on not just the dye but also the desired shade of the503

final product. Dark shades use high electrolyte concentrations (60-80 g/L), whereas the lighter shades can504

do with lower salinity (10-30 g/L).505

Two theories have been propounded in the literature to describe the mechanism by which salts aid in506

the exhaustion and fixation of anionic dyes to cellulosic fibers. The first, more commonly cited reason is507

decreased repulsion between the negative surface charge on the fabric and dye molecules with increasing508

solution ionic strength. This enables the dyes to approach closer to the fiber so that hydrogen bonding509

and other short-range forces can start operating. A second less commonly discussed mechanism is that the510

electrolytes reduce the solubility of the dyes in the bulk dyebath solution and promote dye aggregation. This511

pushes the dye into the interstitial water in the fiber and makes more dye available for the fixation step512

[64–67].513

Xia et al. [68] have suggested and demonstrated the use of ethanol-water mixture instead of saline water514

in the dyebath. Burkinshaw and Salihu [65, 66] conducted extensive studies on the possibility of eliminating515

the need for salts in dyeing. The authors proposed that reducing the liquor ratio (LR, mass of water516

per unit mass of fabric) would reduce the amount of water in the dyebath available for dye dissolution,517

consequently pushing the dye towards the fiber. The study achieved a reasonably similar dye uptake at518

LR = 1.5 irrespective of electrolyte addition. Further, the depth of shade at an LR of 1.5 without NaCl519

addition was the same as for an LR of 10 with 50 g/L NaCl addition.520

However, operating at such low liquor ratios in large-scale dyeing is not yet feasible and could result in521

practical problems associated with obtaining a consistent color throughout the fabric. High liquor ratios favor522

dye migration and give a more level (or uniform) finish. Operating at such high liquor ratios necessitates the523

24

use of salt in the dyebath. However, since the salt itself does not have any affinity to the fiber, it is almost524

completely discharged with the exhausted dyebath effluent [69–71].525

Appendix B. Comparing energy based on best fit of color526

Although color differences at the same salinity levels (in g/L) are within acceptable limits, there is an527

opportunity to reproduce even closer colors by further tuning the salinity of Na2SO4. The trends obtained528

for the L∗, a∗, and b∗ coordinates for were fitted using a rational polynomial of the form f(s) = (p1s +529

p2)/(1+ q2s). Since the a∗ value saturated beyond solution normality of 0.5 eq/L in the case of the red dye,530

a good fit was not obtained in this case. Using the fits, the concentration of Na2SO4 required to get the531

closest match in the color based on ∆Ecmc,2:1 was calculated.532

(a) Black (b) Blue

(c) Black (d) Blue

Figure B.14: Comparing the salinity of Na2SO4 required to achieve the closest color to what is produced at a particular salinityof NaCl. The corresponding ∆Ecmc,2:1 value is also shown. Least energy comparison between the two salts when achievingsimilar color.

The concentration of Na2SO4 required to match the color obtained using NaCl has been plotted in533

25

Fig. B.14. In the range of salinity used in this study, the sodium sulfate requirement is lower than that for534

NaCl for the black dye. The color difference between the color coordinates achieved by using NaCl and by535

using Na2SO4is quantified in terms of ∆Ecmc,2:1. The ∆Ecmc,2:1 value decreases from 0.5 at low salinity536

before going through a minima of 0.07, obtained at a salinity of 24.9 g/L Na2SO4. This point corresponds537

to a NaCl-based dyebath of salinity 30 g/L. For blue dye, this trend is opposite in nature, increasing first538

from ∆Ecmc,2:1 of 0.1 at low salinity and reaching a maxima at a salinity of 30 g/L NaCl before decreasing539

again.540

Based on the above estimations, the minimum energy requirements for concentrating the combined541

effluent from sDB/8 up to sDB are calculated using Eq. 1. This desalination process corresponds to a542

recovery ratio of 87.5%. The energy savings on switching from NaCl to Na2SO4 are immediately evident543

for black and blue dyes. Using the above estimates for salt requirements for replacing the sodium chloride544

with appropriate concentration of sodium sulfate, the energy requirement for brine reconcentration can be545

reduced to 45–50%. Although the blue dye requires relatively higher concentrations of sodium sulfate than546

sodium chloride in g/L, an energy savings of about 35–50% is still predicted in this case as well.547

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