A critical review of the current water conservation practices in textile wet processing

Accepted Manuscript A critical review of the current water conservation practices in textile wet processing

Tanveer Hussain, Abdul Wahab PII:

S0959-6526(18)32018-3

DOI:

10.1016/j.jclepro.2018.07.051

Reference:

JCLP 13505

To appear in:

Journal of Cleaner Production

Received Date:

18 April 2018

Accepted Date:

05 July 2018

Please cite this article as: Tanveer Hussain, Abdul Wahab, A critical review of the current water conservation practices in textile wet processing, Journal of Cleaner Production (2018), doi: 10.1016 /j.jclepro.2018.07.051

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Words count: 13877 A critical review of the current water conservation practices in textile wet processing Authors: Tanveer Hussain a, Abdul Wahab b, * a Faculty

of Engineering & Technology, National Textile University, Faisalabad, Pakistan

b Research *

and Innovation center, Interloop Limited, Faisalabad, Pakistan

Corresponding author: [email protected] , Abdul Wahab

Key Words: Water conservation, Textile wet processing, Eco friendly dyeing, Water less dyeing, Foam dyeing, Effluent treatment Abstract: Textile wet processing industry accounts for a huge proportion in the consumption and pollution of fresh water. Increasing consumer awareness on the environmental issues, tightening environmental legislations on the effluents generated by textile industry and water scarcity in different areas of the world have compelled textile industry to review, restructure and reduce its water consumption and the associated effluent hazards. In this paper, a critical review of the latest water conservation practices in the textile wet processing industry is presented. Water conservation efforts in different segments of the textile industry have been classified into five major categories. These include waste water treatment and reuse, machine innovations, process innovations, chemical innovations, advanced water analysis and water saving tools. Waterless dyeing using supercritical carbon dioxide (SC-CO₂) and the use of low liquor ratio machines in textile wet processing are two very promising approaches for water conservation. But waterless dyeing needs further working to dye natural fibers in a reliable way. The huge capital investment required for SC-CO₂ dyeing machines and conversion of conventional dye houses into low liquor ratio dye houses is also a major hindrance in the way of the wider acceptance of these techniques in the industry. 1. Introduction:

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Clean water is indispensable for the sustenance of life. Adequate availability of clean water for drinking, sanitation as well as industrial use is one of the greatest challenges of the 21st century. Rapid increase in global population and industrialization has put a lot of strain on our natural resources, and posed tremendous challenges to our eco-system (Dobilaitė et al., 2017; Morrison et al., 2009). Textile industry is a water intensive industry that puts a high strain on the global water resource. (Saxena et al., 2017). The increasing concern about the textile wet processing industry is its extremely high water consumption, huge wastewater discharge and high pollution potential (Gomes De Moraes et al., 2000). The major culprit in this regard comprises of the bleaching, dyeing, printing and finishing segments of the textile industry, which use water as a primary medium to apply dyes and chemicals on the fabrics (Tong et al., 2012). In literature, different levels of water requirement have been reported to dye one Kg of textile material, depending upon the fiber type, dyestuff chemistry and machinery used for dyeing. Traditional aqueous dyeing requires 100-180 liters of water to dye 1 Kg of fibers (Petek and Glavic, 1996; Zheng et al., 2016). In a conventional dyeing and finishing mill, on average, about 150 m3 of water is consumed for every ton of textile processing. More than 80% of the industrial waste water is discharged by textile wet processing mills (Lu et al., 2010; R.M. Christie, 2007). Textile industry is the worst polluter of clean water after agriculture (Vandevivere et al., 1998). Around 3600 different dyes and 8000 different chemicals are being used by textile industry today in various processes including bleaching, dyeing, printing and finishing. Many of these chemicals pose a direct or indirect threat to human health, aquatic life and cause water and soil pollution. An average-sized textile mill with processing capacity of about 8000 kg of fabric per day consumes around 1.6 million liters of water every day. To produce enough finished fabric to cover a sofa, around 500 gallons of water is used on average (Kant, 2012). According to an estimate, around 280,000 tons of textile dyes are discharged as industrial effluent every year (Pang and Abdullah, 2013). Effluents from textile processing mills pose a direct threat to aquatic life. Suspended solids in the effluent can clog fish gills with potential to reduce their growth rate or even kill them. Highly colored discharge has ability to reduce light

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penetration that in turn reduces the ability of different Algae species to produce food and oxygen (Tüfekci et al., 2007). Environmental Toxicity and allergic reactions caused by synthetic dyes are now being increasingly reported and criticized by textile consumers (Savvidis et al., 2013). The discharge of dyes into the environment may be a small part of water pollution, but even in small quantities, dyes are highly visible and have capacity to stain huge quantities of water. New environmental legislations have put demand on textile industry to reduce and/or treat their effluents and minimize waste water discharge in the ecosystem(Brik et al., 2006; Robinson et al., 2001). An indirect impact of high water consumption in dyeing and finishing is the higher energy cost, as more energy is needed to heat up larger volumes of water to the dyeing temperature (Hasanbeigi, 2010). Around 24.9% of the total thermal energy used in a dyeing plant is lost by waste water (Hasanbeigi and Price, 2012). The color of a fabric plays an important role in purchase decision of any clothing item. This color comes at some cost. And if it costs us the planet, we need to rethink our strategy to impart color to our fabrics. The focus of global policy makers has been on how to increase water supplies to meet projections of future demand. Different approaches have been in practice like building water infrastructure, reservoirs, dams, etc. Failing to meet the water demand can lead to economic crisis, industrial flight and fall of agriculture which may result in massive unemployment. Water consumption can be broadly categorized into industrial, urban and agricultural water consumption (Gleick et al., 2003). Focus on the conservation of water coming from existing resources and water infrastructure has been less as compared to that on building new resources and infrastructure. Since mid of the 20th century, the question of water conservation has come to spot light. In 1950, the US President’s Water Resources Policy Commission published “A Water Policy for the American People,” which stated: “We can no longer be wasteful and careless in our attitude towards our water resources. Not only in the West, where the crucial value of water has long been recognized, but in every part of the country, we must manage and conserve water if we are to make the best use of it for future development” (Gleick et al., 2003).

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Last two decades of the 20th century witnessed a sharp increase in awareness regarding environmental aspects of the industry. The published work regarding water impact of textile industry became highly visible. Now, textile industry is under tremendous pressure as the legislations regarding environment protection are tightening (Avnir et al., 1985; Chao et al., 2017; Laing, 1991; Smith and Rucker, 1987). The purpose of this paper is to critically review different strategies, approaches and methods for water conservation in textile wet processing industry, highlighting their comparative advantages, limitations and future prospects. The aim of this study is to identify the best water conservation practices that are commercially viable and easy to implement on bulk scale. This review is also meant to evaluate the current state of the art and to identify the research gap for future work. 2. Methodology An effective literature review provides a firm foundation for advancement in knowledge. It facilitates theory development, identifies areas where a plethora of research exists and uncovers areas where research is needed (Webster and Watson, 2016). Water conservation in textile industry is an area where rapid scientific progress on different water conservation methods has highlighted the need to adopt a systematic approach to assess and aggregate research outcomes in order to provide a balanced and objective summary of current state of knowledge. In pursuit of this literature review, following key words were used for searching online research databases and high impact research journals: Water conservation, Eco friendly dyeing, Water less dyeing, Foam dyeing, Plasma treatment, SC-CO2 dyeing, Cationization, Effluent treatment, Recycling, Low liquor ratio, Water filtration, Nano-filtration, Waste water treatment, Ultrasonic dyeing, Liquid paraffin dyeing, Enzymes, Water foot print, LCA (Life cycle assessment), BAT (Best available technology), Water audit tools. A brief overview of these water conservation practices along with their advantages and limitations is given in table 2. Around 279 articles were shortlisted and after going through their abstracts, 184 research papers along with some patents, reports and book chapters were collected, out of which 117 most relevant pieces of literature have been cited in this review.

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Sustainable and environmentally responsible manufacturing is a rapidly developing field and there is a growing body of knowledge in this area. Review of the collected literature suggested that more focus of sustainability work is on waste avoidance, improved production technology, supply chain and product design (Despeisse et al., 2012). Multidimensional efforts in context of water conservation in textile industry were found in the literature, which can be broadly classified into following five categories: I.

Water conservation through textile waste-water treatment and reuse

II.

Water conservation through innovations in textile processing machines

III.

Water conservation through innovations in textile processing methods

IV.

Water conservation through innovations in textile chemicals & auxiliaries

V.

Tools for processing water use analysis and conservation 3. Water conservation through textile waste-water treatment and re-use 3.1.

Handling textiles effluent complexity issues:

Waste water reclamation and reuse is a promising approach towards a more sustainable and environment friendly textile industry. The huge quantity of the water used in textile industry is just one part of this complex problem. The discharge of effluents contaminates the water environment which has in turn heavily accelerated the scarcity of clean water. In the way of recycling the textile waste water there are challenges like higher values of TDS (Total dissolved solids), strong colors, highly fluctuating pH, high COD and higher temperature of effluent (Banat et al., 1996; Sanmuga Priya and Senthamil Selvan, 2017). Possible recycling of textile waste-water has attracted attention of many researchers. But a wide variety of textile effluent colors, pH, heavy metals temperatures and a huge diversity of the dissolved chemicals therein makes textile effluent recycling a daunting task. Nevertheless, if textile waste water is thoroughly processed, there is a possibility to reuse it up to 90%(Correia et al., 1994; Fibbi et al., 2012; Wasatex, 2016). A combination of chemical coagulation, ion exchange and electrochemical methods have been suggested to treat wastewater effluent from secondary waste water treatment plant of a dyeing and finishing mill to make it suitable for possible reuse in textile dyeing or finishing. Electrochemical and chemical coagulation methods effectively minimize turbidity, COD and color

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in the effluent. Ion exchange method is effective to further reduce the COD, Fe concentration, total hardness and conductivity. Addition of Hydrogen peroxide, even in the smaller quantities like 200 mg/L, can enhance the efficiency of electro chemical treatment up to 100% (Lin and Chen, 1997). Although a combination of the three techniques described above is quite effective to make textile effluent re-usable, it has limited commercial viability at bulk scale due to intricacy and heavy cost of the processes. Xujie Lu et. al., has reported the performance of a pilot waste water treatment plant installed in Ningbo Sihua hosiery dyeing and finishing plant, located in Zhejiang province, China (Lu et al., 2010). This waste water treatment plant had capacity of processing 600m3 of textile waste water per day. This plant has three stages of waste water treatment to make it suitable for reuse in textile dyeing and finishing. (i)

Biological process consisting of two step anaerobic and aerobic treatment

(ii)

Biological Aerated Filter process (BAF)

(iii)

Membrane filtration

Xujie Lu et. al., reported following results. The treated effluent had no suspended solids, and 93% COD removal efficiency, 94.5% color removal efficient, and 92.9% turbidity removal efficiency(Lu et al., 2010). If the initial set up cost is excluded, the cost of the treated water was 0.25 USD/m3 as compared to municipal tap water cost of 0.67 USD/m3 including water discharge cost in Ningbo city area, indicating good commercial viability of the waste-water treatment and use at the pilot scale (Lu et al., 2010). 3.2.

Two step water conservation:

Russel F. Dunn et al. worked on a two-step water conservation approach. The first step was water use minimization during the process and the second step was the reuse of waste water in land application technology (Dunn et al., 2001), known as “Slow rate irrigation” or “Spray field”. They separately identified “Source water streams” and “Sink water streams”. Four different process

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design scenarios were considered for evaluation. For all the four scenarios, a mathematical optimization program was developed using commercially available Lingo Optimizer. 3.3.

Split/Segregated waste water streams:

Textile industry uses a variety of wet processes that generate a very complex mixture of dyes and chemicals in the effluent. If the waste water streams are segregated, their treatment and reuse becomes much easier and cost effective (Baban et al., 2010; Kurt et al., 2012). The so-called ‘split flow treatment’ has multiple advantages over mixed flow treatment, such as higher water recovery, higher recycling rates and higher water quality with a smaller investment (Rott and Minke, 1999). 3.4.

Ozonation for effluent treatment:

Gianluca Ciardelli and Nicola Ranieri used a Ozonation can remove color and decrease COD (chemical oxygen demand) to an extent which is sufficient for water reuse for dyeing in lighter colors. A combination of electro-flocculation and ozonation has been reported to make textile waste water reusable for dyeing. The estimated operating costs are also affordable for the color removal and COD reduction by Ozonation (Ciardelli and Ranieri, 2001). A bubbling reactor with Ozone system has been reported to achieve 90% de-colorization of textile waste water just after 15 minutes of reaction. Direct Ozonation provided same level of decolorization as compared to the combination of Ozone with Photo-catalysis or Photo-electrocatalysis and consumed less energy. Ozonation process is reported to be faster, cheaper, simpler, more efficient and has ability to treat huge volumes of textile effluents (Cardoso et al., 2016). 3.5.

Filtration for effluent treatment:

In textile industry, different filtration techniques are also in use for making waste water reusable. These include reverse osmosis, ultra-filtration, nano-filtration and micro-filtration. Microfiltration is considered as the most cost-effective alternative for waste water pretreatment as the larger pore size in microfiltration helps to remove larger impurities in effluent before more sensitive nano-filtration and reverse osmosis process (Pang and Abdullah, 2013). Membrane separation process having reverse osmosis and ultrafiltration units can result in more than 55%

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reduction of fresh water requirement in a small scale textile factory (Nandy et al., 2005). But the high cost, frequent membrane replacements and different pretreatments depending upon influent water are some critical problems in the way of wide adoption of membrane filtration technology by textile industry (Verma et al., 2012). Nano-filtration membranes show better COD removal efficiency as compared to the reverse osmosis membrane filtration which might be attributed to its sieving removal mechanism (Liu et al., 2011). Crossflow nano-filtration using thin film composite polysulfone membrane has been investigated to recover the electrolyte solution and remove the color from reactive dyeing waste water. The effect of feed pressure, initial dye concentration, electrolyte concentration and crossflow velocity was examined on the nano-filtration mechanism. The results have shown a decolorized permeate that was reusable in textile processing, with higher water flux recovery of 99% (Tang and Chen, 2002). 3.6. Use of Ultrasound for effluent treatment: G. Tezcanli Guyer and N.H. Ince studied the degradation pattern of azo dyes in effluents has been investigated by 520 kHz ultrasonic radiation and its combinations with ozone and ultraviolet light (UV). C.I. Acid Orange 7 was used as a probe dye. Operation parameters such as ozone flow rate, ultrasonic power density, UV intensity, type and injection method of the bubbling gas were optimized to get maximum rate of absorption decay. It was found that ultrasound can be used for de-colorization of textile azo dye solutions but overall dye degradation or complete mineralization of the dyes cannot be achieved unless ultrasound is used in combination with Ozone and/or UV irradiation. (Tezcanli-Güyer and Ince, 2004) Despite some success stories on recycling of textile waste water, some concerns have also been raised such as possible contamination, inadequate treatment levels and some quality issues in the reuse of recycled water (Schäfer and Beder, 2006). Due to complexity of the waste water coming from the textile processing industry, often a single water treatment process is not considered adequate to make water reusable. A combination of different water treatment approaches is used which in turn adds to the cost and complexity of water treatment (Blanco et al., 2012; Chen et al., 2015; Hayat et al., 2015; Oller et al., 2011).

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4. Water conservation through innovations in textile processing machines 4.1.

Low liquor ratio machines:

Textile material to liquor ratio has the highest impact on water consumption in exhaust dyeing. If liquor ratio is reduced from 1:10 to 1:8, it results in 20% water saving, and 15% reduction in the processing costs. Low liquor ratio dyeing machines not only reduce water consumption in dyeing of textile materials but also reduce the usage of salt and alkali as these are added in grams per liter of the total liquor (Khatri et al., 2015). Lowering the liquor ratio also improves the dye uptake of the cellulosic fibers specially in case of dyeing with direct dyes (Burkinshaw and Salihu, 2017). Overflow rinsing after dyeing is a widely used process for removal of unfixed dyes and dyeing auxiliaries. This consumes huge quantity of water. Rinsing under pressure is an eco-friendly viable alternative to the overflow rinsing (Petek and Glavic, 1996). 4.2.

Use of Ultrasonic energy in dyeing:

From literature review, it is clear that many researchers believe that the use of energy in the form of micro waves, infrared radiations, radio waves and ultrasound is believed to reduce environmental damage caused by the textile processing industry. (Gulzar et al., 2015; McNeil and McCall, 2011). Application of ultrasound appears to be a promising alternative to reduce water, energy, chemicals and time involved in different textile wet processing operations. There are two ways of application of ultrasonic technology in textile wet processing: (i) Direct application to the reaction mixture by using ultrasonic probes (Horn); (ii) Indirect application through the ultrasonic bath using walls of the sample container. Horn type applicators are normally used for sonodyeing and sono-finishing of fabrics (Harifi and Montazer, 2015). Ultrasonic waves are reported to have following three major functions inside dye bath. i.

Dispersion: They break down micelles and dye agglomerates to provide uniform dispersion

ii.

Degassing: They remove the air or gasses entrapped in fiber and fabric capillaries

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iii.

Dye diffusion: They increase the diffusion rate of the dye inside the fiber (Tissera et al., 2016)

Use of ultrasonic waves in the dye bath has potential to reduce water consumption by increasing dye penetration, dye uptake and reducing the number of rinses required after dyeing to remove unfixed dyestuff (Guesmi et al., 2013; Kamel et al., 2010). The use of Ultrasonic waves of 40 KHz frequency is reported to increase dye uptake by 5-6% in batch wise dyeing of bamboo fabric as compared to conventional dyeing process. It also provided 29% reduction of the TDS contents and 13% reduction of the COD in the effluent water (Larik et al., 2015). Cold pad batch (CPB) dyeing is considered as more environment friendly dyeing process as it requires less water and energy. But the drawback of CPB dyeing is its lower production as compared to continuous process. Zeeshan Khatri et. al., exposed the padded fabric batcher to ultrasound waves of 25-40 KHz frequency at 25-30°C temperature. They reported a decrease of four hours in batching time, with around one third reduction in alkali and auxiliaries by using ultrasound waves. Color strength and dye fixation was also reported to be improved without any adverse effect on color fastness values (Nitayaphat and Morakotjinda, 2017). The use of ultrasound (26 kHz, 180 W) has been reported to be more effective in increasing the degree of whiteness in Laccase-Hydrogen peroxide bleaching of Linen fabrics as compared to the bleaching process without ultrasound. It has also been reported that the fabric bleached with Laccase-Hydrogen peroxide in the presence of ultrasound has enhanced dye uptake and color fastness as compared to the fabric bleached without the assistance of ultrasound (Abou-Okeil et al., 2010). Use of ultrasonic energy in the dye bath has also been found effective in achieving ultra-low liquor ratio dyeing by preventing dye agglomeration thus minimizing water use (Mohammad Mahbubul Hassan and Bhagvandas, 2017). Water saving, energy saving and increased speed of processing are promising benefits of the use of ultrasonic waves in wet processing. But the huge capital investment required to upgrade the dyeing and finishing machinery set ups, erosion damage caused by acoustic cavitation and

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unavailability of reliable bulk scale ultrasonic dyeing machines are some of the key hurdles in the way of wide spread use of sono-chemistry in textile wet processing. A significant amount of water is also used in the textile laundering process at the consumer end. An eco-friendly front-loaded washing machine has been developed with a hydraulic system to spray water inside the washing machine drum with high efficiency nozzles. This tremendously improves water distribution inside the washing bath, leading not only to huge saving of water at the washing stage but also savings in detergent and energy used to heat the washing liquor (Gleick et al., 2003; Innovation, 2015). 5. Water conservation through innovations in textile processing methods 5.1.

Ozone Bleaching:

Pretreatment of the textile substrates is a must for proper dyeing. For dyeing textiles in darker shades, a scouring pretreatment is usually considered enough to proceed for dyeing. But for lighter and brighter shades, fabric bleaching is also usually required. Ozone bleaching of textiles, instead of using free radical hydroxyl species, have been reported with very small amount of water requirement as compared to the traditional process. A fabric sample with 80% wet pick up was hanged in a closed chamber comprising a mixture of Ozone and Oxygen to get acceptable whiteness and absorbency levels in the treated fabric (Prabaharan et al., 2000). However, ozone generation set-up is costly and requires leak proof sealed chambers to process. Ozone is composed of three oxygen atoms and has very high oxidation potential. Ozone has oxidation potential of 2.07 electron volt as compared to 1.77 electron volt for Hydrogen peroxide and 1.49 electron volt for Hypochlorous acid. Ozone was first proposed as bleaching agent in 1871. Ozone is highly unstable in water and depending upon the pH of the medium, it liberates radicals that have ability to oxidize various organic and inorganic impurities (Perincek et al., 2007). A novel approach of ozone bleaching of pre-wetted cotton fabric followed by ultrasonically assisted rinsing has also been proposed (Benli and Bahtiyari, 2015). Fabric pick up was adjusted at 50% with water of pH 7 before ozone application for 15 minutes in a closed bath, followed by rinsing for 5 min. at 40 °C without using any chemicals during the washing. Ozone-Ultrasound

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combination method provided comparable desizing efficiency, hydrophilicity and degree of whiteness to conventional water, chemical and energy intensive pretreatment process. (Benli and Bahtiyari, 2015). Perincek et. al, worked on bleaching of greige cotton fabric with a mixture of Ozone in water. They investigated different parameters like water temperature, pH of the fabric, ozone concentration, treatment time and effect of rinsing after ozonation, on the bleaching efficiency of the Ozone. They reported that whiteness of the fabric increases with ozone exposure time. Optimum pH was found to be 6.5-7.5 and pick up of 60% gave best whiteness levels. The optimum temperature for ozone bleaching was reported to be 23-25°C. Overall this process saves water, energy, use of harmful chemicals and process time (Perincek et al., 2007). They have not quantified the water, energy and chemical savings. 5.2.

Enzyme pretreatment:

Polonca Preša and Petra Forte Tavčer worked on possibility of water and energy saving in cotton pretreatment before dyeing.

Bio-scouring has emerged as an ecofriendly alternative to

conventional alkaline scouring. The quantity of water used in alkaline scouring and Hydrogen peroxide bleaching has been compared with the quantity of water used in Bio-scouring and subsequent bleaching with Peracetic Acid. It has been reported that the water used in the bioscouring route was 33.3% less as compared to the alkaline process. When alkaline scouring and Hydrogen Peroxide bleaching is done, the fabric must be neutralized afterwards. While neutralization is not required if we do Bio scouring and Peracetic acid bleaching because the pH value in this process is slightly acidic and close to the pH level required in finished fabric (Presa and Forte Tavcer, 2009). 5.3.

Spun dyeing of yarns:

Polymerization of PLA (Ploylactic Acid) along with simultaneous coloration in one step is an ecofriendly alternative to conventional PLA dyeing which is water and energy intensive. In this method, a catalyst-containing chromophore is used which accelerates the polymerization and also incorporates color to the PLA polymer (Hussain et al., 2015).

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N. Terinte et. al., compared the environmental foot print of dyeing modal fabric in conventional jet dyeing with Black reactive dyes with spun dyed modal fibers with carbon black. They reported that the production of spun dyed modal fabric requires only 50% of water , , 50% lower energy and 60% lower carbon footprint as compared to conventionally dyed fabric (Terinte et al., 2014). 5.4.

Micelle Dyeing:

Kongliang Xie et al., Micelle dyeing has been reported as an eco-friendly alternative to conventional dyeing process (Xie et al., 2011). The reactive dyeing using dye micelle solution was carried out at 1:5 liquor ratio. The control sample was dyed at conventional 1:15 liquor ratio. The properties of the micelle dyed fabric at low liquor ratio were compared to conventionally dyed sample. Dye exhaustion, fixation, reactivity and Fastness properties like washing, wet crocking, perspiration fastness were found to be comparable to the conventionally dyed fabric. Dyeing water saving of 60%, 50% saving in steam, 60 % electrolyte saving during dyeing were reported. 5.5.

Super critical fluid dyeing:

The first dyeing of polyethylene terephthalate (PET) in supercritical fluid is a remarkable innovation in textile processing aimed to replace water as a dyeing medium altogether (Long et al., 2014). The super critical Carbon dioxide (SC-CO2) is the fluid of choice, thanks to its special behavior beyond its critical point. SC-CO2 has solvation power and density that is comparable to liquid solvents but it has an added advantage of having extremely rapid diffusion and viscosity which is closer to a gas. SC-CO2 dyeing not only conserves water, but also increases productivity due to shorter dyeing cycles, reduces energy consumption due to dyeing at lower temperature, minimizes the use of chemicals and auxiliaries in dyeing and drastically reduces air emissions. The key factors in successful SC-CO2 dyeing are the dye solubility in SC-CO2 solution and the flow behavior of SC-CO2 inside the complex porous structure of textile fabrics. The behavior of the disperse dye in the SC-CO2 solution is also of utmost importance. In SC-CO2 dyeing, disperse dyes are seen as either temperature controllable dyes or as density controllable dyes. In temperature controllable dyes, the dye solubility decreases with a predetermined decrease in temperature. While in density controllable disperse dyes, the dye solubility on SC-CO2 decreases with a

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controlled decrease in density. This decrease in the dye solubility is desired to optimize dye exhaustion on the textile substrate out of the solution (Montero et al., 2000). SC–CO2 is a viable alternative dyeing medium to both water and organic solvent media. SC-CO2 is cost effective, nontoxic, nonflammable and requires relatively gentle critical conditions (TC=31.1 C, PC=7.38 MPa). SC-CO2 can easily be recaptured and recycled after use (Jun et al., 2004). Unfortunately, for hydrophilic dyes, SC–CO2 is a poor solvent because of its low permittivity. Due to this reason, SC-CO2 dyeing is limited to dyeing of synthetic fibers which use disperse dyes soluble in SC-CO2 . In order to realize the dyeing of natural fibers with SC-CO2 technology, solubilization of water soluble dyes like reactive and acid dyes in SC-CO2 is required. Jun et. al., tried to solubilize the water soluble dye in a perfluoropolyethe (PFPE) reverse micellar/SC-CO2 system by dispersing a small amount of water in SC-CO2. These reverse micelles solubilized small amount of water inside the micelles thus producing micro water pools for hydrophilic dye solubilization in non-aqueous medium. This PFPE reverse micellar system was used in dyeing wool fabric. Using this method, acid dye was effectively exhausted on wool fibers without the addition of any auxiliary, at lower temperature of 55⁰C and in a short time. Density of CO2 did not drastically influence dye-ability of acid dyes, however the effect of temperature of the system on dye-ability was significantly high (Jun et al., 2004). Zheng et.al., designed and built an industrial scale multiple apparatus to dye yarn bobbins in SCCO2 solution with capacity 500 L × 2. This SC-CO2 dyeing plant consisted of following seven main subsystems: (i) CO2 storage system, (ii) Refrigerating system (iii) Conduction oil system (iv) Dyeing circulation system (v) Separation recycling system (vi) Solvent system (vii) Automatic control system and safety interlock system With this system, they were able to achieve uniform dyeing with bright colors, with color fastness to washing 5, color fastness to crocking 4-5 and light fastness 5-6. At dyeing temp 120°C, dyeing pressure of 24 MPa and dyeing time of 60 minutes, lower processing cost was obtained with this new SC-CO2 dyeing plant as compared to conventional aqueous dyeing(Zheng et al., 2016). Solubility of different dyestuffs in SC-CO2 is the critical point for successful dyeing. Alwi and coworkers investigated the solubility of 1,4-diaminoanthraquinone (C.I. Disperse Violet 1) and

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1,4-bis(ethylamino) anthraquinone (C.I. Solvent Blue 59) in SC-CO2 over the temperature range of 323.15 to 383.15 K and the pressures from 12.5 to 25.0 MPa. It was reported that the solubility increases with the rise of temperature and pressure of SC-CO2 and the solubility of 1,4bis(ethylamino)anthraquinone was higher than 1,4-diaminoanthraquinone (Alwi et al., 2014). One major hindrance in the way of bulk implementation of SC-CO2 dyeing in industry is the lack of availability of special dyes for waterless dyeing of natural fibers. Reactive dyes were introduced for the first time in 1956 and have become the preferred choice for cotton dyeing (Haque et al., 2015). Now-a-days, reactive dyes constitute the largest dye class for cotton. Zhang et. al., developed three special Azo Disperse Reactive dyes using Dichlorotriazine reactive group. These disperse reactive dyes were used for waterless dyeing of wool in SC-CO2 medium using pressure of 20.0 MPa and a temperature of 120 ⁰C for 90 min. They developed a safer and ecofriendly method for synthesizing these special disperse reactive dyes by diazotization of the alkalescent arylamine of 2-chloro-4-nitroaniline in the presence of a safer and milder acid of hydrochloric acid instead of concentrated sulfuric acid, in an eco-friendly media of polyethylene glycol 200 mixed with water. Good rubbing fastness (4-5) and medium level washing fastness with good levelling properties were reported (Zhang et al., 2016). However, washing fastness and cross staining specially on Polyamide 6,6 and acetate needs to be improved to meet minimum acceptance fastness level in industry. The only way forward to dye cotton by SC-CO2 seems to be the development of specifically synthesized disperse reactive dyes and the use of proper swelling agents for cotton. Other suggested alternatives can significantly increase the complexity of the dyeing system and minimize the environmental advantage due to complex chemistries involved (Banchero, 2013). 5.6.

Plasma Treatment

Plasma treatment of textiles is an environment friendly process which has attracted attention of researchers due to huge potential of water and energy savings. Plasma is the fourth form of matter comprising partially ionized gas, first discovered by Irving Langmuir in 1929. Plasma has tendency to modify the fabric surface, depending upon the current, temperature, pressure, gaseous matter and the type of textile fiber used (Kumar and Gunasundari, 2018).

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Yan and Zheng investigated the possibility of dyeing Plasma-modified Silk fabric in SC-CO2 medium using C.I. Disperse Blue-77 and C.I. Disperse Yellow E-3G dyes. They identified temperature of 90℃, pressure 40 MPa and dyeing time 60 minutes as the optimum parameters for SC-CO2 dyeing of plasma-modified silk fabric. They reported that surface modification of silk fabric prior to dyeing, improved rubbing and washing fastness (Yan and Zheng, 2011). The first fabric rope dyeing plant equipped with SC-CO2 was designed and constructed by Long and coworkers. The system used an improved circulation of super critical fluid and a special guide tape for dyeing the fabric rope. They used dyeing temperature of 120°C, pressure 20 MPa and time 60 min. with a flow rate of dyeing fluid at 30.0 L/min and a running speed of the fabric rope at 56.0 m/min in clockwise and anticlockwise directions in turn. They noticed that mass transfer was improved between the super critical fluid and polymer system achieving washing and rubbing fastness levels of 4-5 (Long et al., 2014). The pretreatment of textiles with Plasma technology has become a highly active research area as a potential to become eco-friendly alternative to wet pretreatment procedures (Zille et al., 2015). This will save the large quantities of water used in conventional wet pretreatment. When a gas under appropriate pressure is exposed to strong electromagnetic field it can get partially ionized containing electrons, ions and neutral particles in it. In this state it is called Plasma. Low temperature plasma treatments provide opportunity to remarkably change the surface properties of textile materials and increase dye uptake. Plasma treatment before dyeing has been found highly effective for increasing dye ability of cotton, wool and polyester (Ahmed and ElShishtawy, 2010), and the penetration of the natural dye molecules into the fibers (Haji and Qavamnia, 2015). Teli et. al., treated silk fabric with plasma in the presence of Helium and Nitrogen (He/N2) gaseous mixture at a discharge voltage of 5 kV and frequency of 21-23 kHz. They exposed silk fabric to plasma treatment for 1 to 10 min. with a constant nitrogen flow of 50 ml/min. Plasma treated silk fabric demonstrated faster exhaustion and deeper shades of acid dye even at low temperature of 40°C instead of 90°C (Teli et al., 2015).

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Air plasma treatment of wool fabric changes wool surface character from hydrophobic to hydrophilic. This hydrophilicity makes it easy for protease or transglutaminase to catalyze reaction. The anti-felting performance is remarkably improved if protease is applied after air plasma pretreatment and loss of strength is also decreased as compared to conventional chemical pretreatment of woolen fabric (Zhang and Wang, 2015). 5.7.

Dyeing Using Liquid Paraffin:

Suxin Xu et. al., selected liquid paraffin as water-less dyeing medium out of 110 organic solvents by using solvent assessment protocol considering both dye uptake and EHS characteristics of every solvent. Polyethylene terephthalate (PET) fabric was dyed using liquid paraffin medium. They did not use any auxiliaries and did not adjust pH of the dye bath as the results of Liquid Paraffin dyeing are not affected by these parameters. Dyeing temperature of 130°C was used. As compared to chlorinated solvents, liquid paraffin provides higher dye uptake and better EHS properties. It also does not require expensive high pressure equipment which is needed for SCCO2 dyeing. They reported excellent color consistency even after 7 times reuse of the solvent and surface oligomer content was also reduced to 0.02%. Fastness properties, color yield and mechanical properties of the paraffin dyed fabric was found comparable to aqueous dyed fabric (Xu et al., 2016). Successful dyeing of PLA in liquid paraffin medium has also been reported with comparable fastness properties to that of aqueously dyed PLA fabrics. (Xu et al., 2015) 5.8.

Foam Dyeing:

The use of foams as the medium of application of dyes and chemicals to the fabric has widely spread in textile industry in recent years. Foam dyeing, finishing and sizing is an ecofriendly technology that not only provides water conservation but also energy savings thus reducing the carbon foot print of textile manufacturing (Zhu et al., 2016). Mao et. al., investigated the states of water present in cotton fibers, pore size distribution, specific surface area, pore volume and average pore radius under different moisture level conditions by using DSC (Differential Scanning Calorimetry) based on Gibbs Thomson effect. They categorized the water present in wet cotton fabrics in two segments. The water present at the fabric surface and in the interspaces of fibers and yarns was called “Free Water”. This water can

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freely move in the fabric assembly carrying dyestuff and chemicals with it. On the other hand, “Bound Water” is the water absorbed in the pores of cotton fibers and its movement is restricted by pore walls. However, it can carry dyestuff and chemicals of appropriate molecular size into the pores. Keeping porosity parameters and water states in consideration, they compared dyeing results of conventional one bath pad dyeing vs low water add-on foam dyeing and reported higher color strength and dye fixation with foam dyeing as compared to pad dyeing (Mao et al., 2014). Sarwar et. al., compared the application of easy-care finish on denim fabric by padding method and by foam coating method. Dihydroxy ethylene urea (DHEU) was used as zero formaldehyde cross-linker and thickeners were used for enhanced foam stability during application process. They reported that 5.8 times more water was used in case of pad application process vs foam application process. They concluded that foam application is an environment friendly process which saves water and energy (Sarwar et al., 2017). In a recent paper, foam dyeing of cotton and wool using Hydrolyzed Keratin foam was reported (Bhavsar et al., 2017). Hydrolyzed Keratin foam has good foam formation and foam stability for cotton reactive dyeing and acid dyeing of woolen fabric. The bubble diameters for both acid and reactive dyeing were found to be in the range of 0.02- 0.1 mm and 40% wet pick up of the fabric gave uniform dyeing of cotton and wool fabrics. The foam dyeing of cotton and wool fabrics was reported to have superior results as compared to the conventional pad batch process with respect to color strength. Rubbing and washing fastness properties were also equivalent to conventional pad dyed fabrics (Bhavsar et al., 2017). Pigment foam dyeing has also been reported as a tool to save water and energy. A dyeing dispersion containing 8% (w/w) polyoxyethylene ether surfactant as foam controller, 15% (w/w) binder, and 1–6% (w/w) pigment dispersion was stirred for 7 min. The foam was applied on cotton fabrics using a control coater. The dyed samples were dried at 60 °C and cured at 150 °C, both for 5 min. The dyed fabric was reported to have excellent shade levelness and evenness (S. Chen et al., 2017a). The viscosity of polyoxyethylene ether pigment foaming dispersion is higher

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and it results in superior foam stabilization effect with the foam half- life of 172.9 min (S. Chen et al., 2017b). 5.9.

Electrochemical dyeing:

Conventional Sulphur and Vat dyeing require a redox reaction. The reducing agents used in Vat and Sulphur dyeing are highly polluting, difficult to remove and contribute to a higher level of total dissolved solids in the effluent. The presence of conventional reducing agents makes the recycling of the effluent water very difficult. Nieminen et. al., suggested electro chemical dyeing as an eco-friendly alternative to conventional reducing agents used in Sulphur and Vat dyeing. In electrochemical dyeing, the reducing agent is continuously regenerated at the electrode and give good quality results, allowing full recycling of the dye bath water. (Nieminen et al., 2007). 5.10. Ozone fading of denim: Faded color in jeans is a very popular fashion trend in young consumers. The demand of random fashion and faded look has crossed the boundary of denim to include knitted and other woven apparel. Conventional fading treatments by washing require use of a lot of water and chemicals. Ozone fading provides a waterless and chemical free alternative for having faded fashion effects on cotton apparel. Use of plasma induced ozone treatment is reported to be more effective as compared to simple ozone treatment (Kan et al., 2016). 5.11. Hydro jet technology for denim fading: Faded jeans look is considered as an evergreen fashion trend. But this fading is normally achieved by using water intensive processes like stone wash and/or KMnO4 spray, etc. Recently hydro-jet treatment has been developed to achieve a comparable fading, surface finishing and texture with minimal water use. In this treatment one or both surfaces of the garment are exposed to a high pressure water jet through special nozzles. The degree of fading depends upon the type of the dye used and the amount of the fluid impact energy applied to the fabric. The water used in this process is recycled and fading effects are achieved without the use of any chemical. This makes hydro jet fading a highly environment friendly alternative of conventional denim washing processes (Khalil, 2015).

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6. Water conservation through innovations in textile processing chemicals and auxiliaries 6.1.

Use of Enzymes:

Energy savings and water conservation are some of the promising advantages of enzymatic processes, along with the omission of the use of hazardous/harmful substances (The European Commission, 2003). During cotton pretreatment, scouring is done to remove different natural impurities to make cotton ready for dyeing. The scouring process consumes a lot of water and energy as it is usually done at elevated temperatures under strongly alkaline conditions. Pectatelyase enzyme is an eco-friendly alternative to alkaline scouring process and requires less water and far less energy. (Kirk et al., 2002) (Hasanbeigi and Price, 2015). The bio-scouring of cotton fabrics with acid and neutral pectinases is reported to save water and energy and eliminate effluent neutralization step as compared to conventional alkali scouring process(Pušić et al., 2015). The main classes of enzymes used in cotton finishing and pretreatment are hydrolase and oxioreductase. Hydrolase enzymes group includes amylase, cellulose, protease, cutinase, pectinase, lipase and these are used in bio-polishing and bio-scouring of the fabrics, denim finishing, softening, felting and finishing of wool (Singh et al., 2016). An overview of commonly used enzymes in textile wet processing is provided in Table 1 (Varadarajan and Venkatachalam, 2016). Table 1 Application of enzymes in Eco friendly textile processing (Varadarajan and Venkatachalam, 2016) Textile processing Desizing Scouring Bleaching Polishing

6.2.

Purpose Removal of sizing agents (starch, polyvinyl alcohol) Removal of pectin and lignin Increasing whiteness Removal of fuzz and improving smoothness

Enzymes Amylase, pectinase, polygalacturonase Pectinase, protease, lipase, cellulase Glucose oxidase, laccase Cellulae

Development of special auxiliaries to minimize formation of dye agglomerates to achieve level dyeing at low liquor ratios:

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There are no two opinions on the subject that low liquor ratio dyeing saves water (The European Commission, 2003). But a potential threat associated with Low liquor ratio dyeing is the possibility of aggregation of dye molecules in the dye-bath causing uneven dyeing. Hassan and Bhagvandas studied the effect of different auxiliaries on the dyeing behavior of wool at liquor ratio of 1:10 with three different dyes. They found that “Teric G12A6” from Huntsman was very suitable for the prevention of dye agglomeration during low liquor ratio dyeing of wool in all three dyes (Mohammad M. Hassan and Bhagvandas, 2017). 6.3.

REST Dyeing:

Öner and Sahinbaskan proposed a combined pretreatment and dyeing process named it as “REST” (Rapid Enzymatic Single-bath Treatment). They used starch sized, 100% cotton woven fabric for dyeing trials. During REST process, no fresh water was added to the bath and all the processes like enzymatic desizing, enzymatic scouring, Hydrogen peroxide bleaching, enzymatic anti-peroxide treatment and dyeing were carried out in the same bath. They reported to achieve repeatable shades with 66% water saving and high energy saving as compared to conventional dyeing process. Due to the use of enzymes, control of pH was identified to be a critical parameter and some tone wise variations in the final shade were also observed when compared to conventionally dyed fabric (Öner and Sahinbaskan, 2011). 6.4.

Cationization:

Use of cationization before dyeing of cotton is reported to increase dye utilization, result in higher color values and reduced number of rinsing cycles after dyeing. Pre-cationization of cotton is also reported to be helpful in low-salt or salt-free cotton reactive dyeing (Hasanbeigi and Price, 2015) (Ali et al., 2015). Cationization is also reported to be helpful in achieving ultra-deep dyeing in black color (Fu et al., 2016). When cotton is dyed without the addition of inorganic electrolyte, both the number of wash-off baths and the liquor ratio that is utilized for each wash-off bath, can be reduced (Burkinshaw and Salihu, 2017b). Use of special reactive dyes with cationic soluble groups and nicotinic acid quaternary triazine reactive group has been found to provide a high level of dye exhaustion and fixation on cotton fiber without the addition of salt and alkali (Zhang and Zhang, 2015). Use of cationic starches has

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also been found effective for improvement of dye exhaustion and fixation (Varadarajan and Venkatachalam, 2016). Pre-cationization of the fabric to be bleached has also been reported to give comparable whiteness index to the conventional bleaching process without the addition of Alkali in bleaching bath. Water intensive process of neutralization and removal of residual alkali is also eliminated. The cationic group on the cationized cotton fabric serves as a catalyst for the bleaching process and the second function is that it provides site necessary for activation of hydrogen peroxide bleaching bath instead of NaOH (Hashem et al., 2010). 7. Advanced water analysis and water saving tools 7.1.

Water Saving by water foot print analysis:

Chico et. al., analyzed water footprint (WF) of different fabrics made in Spain. The grey, blue, and green water footprints of the processes in the textile value chain were estimated from the wood and cotton production stage to the industrial processes. They reported the results of a water footprint assessment of five types of textiles which are commonly used for the Jeans manufacturing. This study included two different fibers (cotton and Lyocell fibre) and five corresponding production methods for spinning, dyeing and weaving. They found that cotton fiber production stage is responsible for the highest water consumption. They found that Cellulose-based Lyocell fiber has a notably lower water consumption than cotton fibers. Lyocell fiber required on average 1384, 34.5, and 35.3m3/t water as compared to 263, 2767, and 203 m3/t water for cotton (Chico et al., 2013). The water saving in the case of Lyocell production looks very charming as compared cotton fiber production. But when it comes to the practical implementation of the concept, the manifolds higher price of Lyocell as compared to cotton emerges as the highest hindrance. That idea will have higher chances of success on commercial basis which ensures commercial sustainability along with environmental sustainability. The cotton fiber production has three types of water use impact. These include evaporation of infiltrated rainwater for cotton growth (green water use), ground or surface water use for irrigation or processing (blue water use) and water pollution during growth or processing (grey

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water). Grey water impact is estimated on the basis of dilution volume required to assimilate pollution. For the period of 1997-2001, worldwide consumption of cotton products required 256Gm3 of water per year, out of which about 42% was blue water, 39% green water and 19% dilution water (Chapagain, A.K., Hoekstra, 2005). Life cycle assessments (LCAs) has potential to provide and optimize the real water use in fiber biomass production and during textile processing (Angelis-Dimakis et al., 2016). Sandin et. al., studied water and land use impact of textile fibers and compared the consequential and attributional approach for Life Cycle Assessment (LCA) of textile fibers. They found that industrial use of water dwarfs impacts of water use in biomass production due to extraction from relatively water stressed environments. They suggested that water use impact is highly dependent on geographical location of biomass production and fiber processing units (Sandin et al., 2013). 7.2.

Mass retailer VS Brand’s Supply chain sustainability:

MacCarthy* and Jayarathne compared sustainability and water saving strategies implemented in the supply chains of a major brand retailer and a supermarket retailer. They noticed that the manufacturers of the brand retailer were more motivated and were carrying out many small and large scale water-based projects. These projects ranged from water conservation to desalination plants. The brand retailer had a comprehensive two-way strategy which on one side helped manufacturers with publishing water conservation guides and on the other hand encouraged end-consumer to use eco-friendly washing processes. In the supply network of a major super market retailer, manufacturer had developed Good Waste Management System (GWM). This GWM system was found to be helpful in minimizing water wastage during the production cycle. The manufacturers for super market retailers were found complying with the minimum levels of national legislations on environment, safety and workforce welfare. However, the evidence of a proactive strategies towards water saving, environment protection and getting accreditations was not found remarkable (MacCarthy and Jayarathne, 2012). This comprehensive study on the comparison of the supply chains of a major brand retailer and a major super market retailer provided much needed insight into the complex calculation of textile industry sustainability drive. It clearly indicates that the level of commitment from the

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brand or super market retailer with its sustainability strategy has a direct impact on the sustainability priorities and practices of the manufacturers. This study suggests a direct relationship of the water conservation, eco friendliness and labor friendliness of a manufacturer with the extent that how religiously its buyers follow the sustainability slogan. However, in this study, only one supply chain of supermarket retailer and a brand retailer has been studied. So, the possible distortion or limitations in the findings could not be neglected. An analysis with a broader base of supply chain comparison could be more interesting. 7.3.

Implementation of best management practices:

Chougule and Sonaje suggested implementation of best management practices (BMP’s) for water conservation and water recycling in textile industry. They identified major flaws in water use practices of textile wet processing industry like use of water in quantities which is not even required by the nature of the process, tendency to use excess water in rinsing after dyeing, use of drinking grade water for all processes despite the fact that all processes do not require the use of drinking grade water. They suggested implementation of BMP’s like carrying out regular water audits, setting plant wide water conservation targets, appointment of dedicated water conservation team, acquiring water conservation information from national and international organizations, vendors, research bodies and educating the human resource on significance of water conservation (Chougule and Sonaje, 2012). 7.4.

Implementation of Cleaner production grading system:

Tong et. al., has proposed to establish a cleaner production indicator system and assessed it by using AHP (Analytical Hierarchy Process) (Tong et al., 2012). The resultant judgement matrix was determined with Delphi Method and Yaahp software was used to assign weight factor to each indicator. This resulted in a comprehensive waste reduction and water conservation cleaner production evaluation indicator system for textile dyeing and printing industry. They identified following four key indicators for water conservation and waste reduction: (i) Water resource indicator, (ii) Main raw materials and additive consumption indicator, (iii) Pollutants creation indicator, (iv) Comprehensive utilization indicator (Agana et al., 2013; L. Chen et al., 2017).

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The judgement matrix was developed based on questionnaire of some enterprises and Delphi method. Weight factor was assigned by using Yaahp software. The weight of water resource indicator was found to be maximum (0.4226). In many industries water audit system doesn’t even exist. The implementation of water audit system and a clear water management strategy has been found effective for water conservation in industries around the world (Agana et al., 2013; L. Chen et al., 2017). 7.5.

Implementation of lean manufacturing tools:

US-EPA developed a toolkit to help companies to improve machine efficiency by water and energy reduction. Lean manufacturing tools like root cause analysis, 5S, Kaizen events, value stream mapping (VSM), 5Why, fishbone diagrams and Visual management are practical strategies and techniques to identify problem sources and improving results related to reducing water use and environmental risk (Maia et al., 2013). 7.6.

European Union’s IPPC Directive implementation:

Kocabas et al., implemented recommendations of BREF (Best Available Techniques; BAT) which included installation of water flow meters at water pipelines, semi counter current rinsing in dyeing and finishing, reuse of treated dyeing and finishing wash waters and reuse of compressor cooling water in production. They achieved 29.5% reduction in total water consumption and 9% reduction in energy consumption was reported (Kocabas et al., 2009). In another study, the application of BAT options like steam boiler optimization, reuse of steam condensate, good management practices, machinery modifications and waste water reuse was reported to reduce the water consumption in the textile mill by 35-65% (Ozturk et al., 2015). In addition to wastewater reuse and machinery modifications, by the application of other BAT options including steam boiler optimization, reuse of steam condensate and good management practices, total water consumption in the mill may be reduced 35-65%. Alkaya et. al., conducted Environmental Performance Evaluation (EPE) of a textile woven fabric manufacturing mill in Bursa, Turkey. After EPE, following five sustainable production alternatives were identified and implemented: Replacement of overflow washing with drop-fill washing;

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Reuse of stenter cooling water; Reuse of singeing cooling water; Improvement of water softening system; Renovation of various valves and fittings in water transmission system It was reported that by the implementation of above alternatives, water consumption decreased by 40.2% while wastewater quantity was reduced by 43.4% (Alkaya and Demirer, 2014). In table 2, all these water conservation practices have been enlisted along with their respective advantages and limitations. Table 2: Water conservation techniques used in textile wet processing with their advantages and limitations Water conservation method A combination of chemical coagulation, ion exchange and electrochemical methods have been suggested to treat wastewater effluent

Advantages

Limitations

Water becomes reusable

Very complex process, (Lin and Chen, 1997) capital intensive

Split/Segregated water streams

Easier effluent treatment and water recycling possibility

Ozonation of waste water

Effluent water may become reusable

Micro and nano filtration

Efficient water cleaning to make it reusable

Use of Ultrasound in effluent treatment

can be used for decolorization of textile azo dye solutions

Separate infrastructure is required, Some machines get limited to specific processes, Dye house can lose process flexibility Capital intensive process, Don’t have ability to handle complex effluent Filtration membranes have to be changed repeatedly, Heavy investment required Overall dye degradation or complete mineralization of the

References

(Rott and Minke, 1999)

(Ciardelli and Ranieri, 2001; Cardoso et al., 2016) (Liu et al., 2011) (Blanco et al., 2012; Chen et al., 2015; Hayat et al., 2015; Oller et al., 2011)

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dyes cannot be achieved

Low liqour ratio machines

Huge water and chemicals saving

Capital intensive

(Khatri et al., 2015; Burkinshaw and Salihu, 2017a)

Use of ultrasonic energy in wet processing

Better washing efficiency, Improved dye penetration

Standardized bulk production machines are not available

(Gulzar et al., 2015; McNeil and McCall, 2011)

Ozone Bleaching

Huge water saving

Enzyme pretreatment

Cost effective and environment friendly

Spun dyeing of yarns

Water saving, lesser carbon foot print

Super critical fluid dyeing

Water less dyeing, no effluent, environment friendly

Micelle dyeing

Plasma Treatment

Foam Dyeing

Water saving of 60%, 50% saving in steam, 60 % electrolyte saving Water saving, environment friendly More than 5 times water saving as compared to conventional

Degree of whiteness needs to be improved to get comparable results to conventional bleaching Degree of absorbency is less in the case of enzyme pretreatment as compared to conventional pretreatment Limited number of shades, Higher MOQs (Minimum order quantity), Longer lead times Very capital intensive, machines have engineering problems that yet need to be resolved Reliability and stability of Micelle formation needs to be improved

(Perincek et al., 2007; Benli and Bahtiyari, 2015)

(Presa and Forte Tavcer, 2009)

(Hussain et al., 2015)

(Long et al., 2014; Banchero, 2013)

(Xie et al., 2011)

Special machines are required

(Kumar and Gunasundari, 2018)

Special machines are required, foam stability issues need to be addressed

(Zhu et al., 2016; Sarwar et al., 2017)

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dyeing process, energy saving

Electro chemical dyeing

Ozone fading of denim

Hydro jet technology for denim fading Development of special auxiliaries to minimize formation of dye agglomerates to achieve level dyeing at low liquor ratios Rest Dyeing: During REST process, no fresh water was added to the bath and all the processes like enzymatic desizing, enzymatic scouring, Hydrogen peroxide bleaching, enzymatic antiperoxide treatment and dyeing were carried out in the same bath Cationizaton of cotton Water foot print analysis and Life cycle assessment Implementation of Lean practices

The reducing agent is continuously regenerated at the Process could not be electrode and performed without allows full recycling special equipment of the dye bath water. Ozone is hazardous to workers if they come in direct contact of Water and energy Ozone. Special sealed saving equipment and well ventilated area is required It is difficult to control Water saving level of fading

(Nieminen et al., 2007)

(Kan et al., 2016)

(Khalil, 2015)

Level dyeing

Cost of special chemical adds to the overall processing cost

(Mohammad M. Hassan and Bhagvandas, 2017)

66% water saving and high energy saving as compared to conventional dyeing process

control of pH is a critical parameter and some tone wise variations occur in final shade

(Öner and Sahinbaskan, 2011)

Reduced salt and Excessive alkali use, water use shade variations Better visibility into the supply chain Minimizes wastes, improves water efficiency

(Burkinshaw and Salihu, 2017b) (Chico et al., 2013; Sandin et al., 2013) (Maia et al., 2013)

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Implementation of BAT(Best available technology) Implementation of cleaner production grading system

Water conservation

Capital intensive

Better visibility into the supply chain

(Alkaya and Demirer, 2014) (Agana et al., 2013; L. Chen et al., 2017)

8. Critical analysis of the State of the Art: A plethora of research work is published regarding treatment and recycling of the waste water from textile wet processing industry. Different waste water treatment methods have been studied like chemical coagulation, ion exchange, electrochemical treatment, ozonation, reverse osmosis, ultra-filtration, nano-filtration and micro-filtration. But the major problem in the way of massive adoption of textile waste water recycling is the very complex nature of textile effluent. Textile wet processing mills have effluent which has varying temperatures, pH, TDS (Total Dissolved Solids), electrical conductivity, electrolyte concentration and colorants. This makes it nearly impossible to use a single technique to make textile waste water reusable. Therefore, for this purpose, a combination of the above stated techniques is needed to be used. This adds to the complexity and the cost of the system which in turn reduces commercial and bulk scale viability of these processes. A simple, cost effective and comprehensive solution is needed that can make the waste water recycling a reality at industrial scale. Use of low liquor ratio machines in exhaust dyeing and lower trough volumes in continuous dyeing are considered as a promising source of water conservation in textile industry. Use of lower liquor ratio machines seems to be the most promising solution to the problem of excessive water consumption in textile wet processing. However, the probability of formation of dye agglomerates at lower liquor ratio poses a threat to the uniform dyeing. Further working on optimization of dyes and auxiliaries is needed to ensure uniform dyeing and wet processing at ultra-low liquor ratio. Application of ultrasound waves of different frequencies during pretreatment, dyeing and washing process has also been reported to improve the overall water efficiency of textile wet processing. But different studies have reported varying frequencies of the ultrasound used in textile wet processing. There is a scarcity of research work on the possible

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impact of different ultra sound frequencies on the physical properties of the fabrics. A lot of standardization of the ultrasound process and extensive machine development is required before this process can get acceptance at bulk scale in the industry. A lot of work has been published on water conservation through innovations in textile wet processing methods. Various identified methods are ozone bleaching, enzyme pretreatment, spun dyeing of yarns, micelle dyeing, foam dyeing, supercritical CO₂ dyeing, electrochemical dyeing, plasma treatment, ozone fading and the use of hydro jet technology in denim. Many of these methods have promising water savings, yet they have not got wider acceptance in industry due to higher capital investment required for new machinery set ups and due to the specific limitations of some of these technologies. Like, there is no reliable, bulk scale solution available to dye natural fibers using super critical CO₂ dyeing. Issues of foam stability have been reported in case of foam dyeing. Plasma treatment set ups are available for open width fabric, but more working is needed for satisfactory plasma treatment of apparel. Water conservation through innovations in textile processing chemicals and auxiliaries is also an area of active research. Use of enzymes, development of special auxiliaries to minimize dye agglomerates formation at lower liquor ratio and pre-cationization of cotton is being done in this regard. Although cationization process eliminates use of NaOH during bleaching but significant amount of the alkali is still required in the cationization process itself. Similarly, some saving in water consumption is achieved by elimination of post process neutralization but additional water is still consumed in pre-cationization treatment. This dilutes the claims of the pre-cationization as being the water conserving and environmentally friendly technology. The implementation of proper water audit systems, lean manufacturing tools, best available technology (BAT) and life cycle assessment tools has been reported to have a significant effect on the water conservation in textile wet processing industry. Clear focus of the brands and retailers on water conservation in their supply chains is also reported to compel the mass manufacturers to adopt proactive water conservation practices. However, more visibility and water audit in the supply chains is needed to implement water conservation practices at the grass

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root level. Extensive training and education of textile industry workers on water conservation practices and BAT is the need of the hour. 9. Conclusion: Although in the recent years, there has been a remarkable progress in various treatments for textile effluent yet the waste water minimization approach is much more effective than producing the waste and then treating it. The process changes that minimize the use of water in the process are much more practical and cost effective as compared to recycling, reuse and interception designs for the waste water. The use of advanced machines, processes, chemicals can help textile mills to reduce their environmental impact. But the importance of commitment towards water saving, implementation of water audit system, use of best water management practices along with lean manufacturing concepts cannot be over emphasized. Different large industrial manufacturers have tried some water conservation approaches, but there is still a huge research gap found for the development of such approaches that are cost effective, bulk feasible and commercially viable for textile and apparel industry. Otherwise a lot of published work, unfortunately, doesn’t look practical and viable to industry due to cost, process complexity and lack of infrastructure. The future challenge is to bridge the gap between efforts of the researchers and the direction of the industry. It is necessary to upgrade the machinery infrastructure for water minimization in textile wet processing industry. Authors highly recommend the use of low liquor ratio machines in all wet processing functions. By necessary modifications, existing machines could also be converted to the low liquor ratio machines, and this approach can get wider acceptance in the industry as it is easy to follow and requires far less capital than installing the brand new lower liquor ratio machines. 10. Acknowledgements: Authors acknowledge the proactive and valuable input of following persons. I.

Dr. Munir Ashraf (Head of Textile Chemistry Department, National Textile University Faisalabad)

II.

Mr. Yaser Riaz (Deputy General Manager Research & Innovation, Interloop Limited, Pakistan)

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III.

Mr. Zahid Shafique (Vice President Crescent Bahuman Limited, Pakistan)

IV.

Mr. Ijaz Hussain (General Manager Processing, Interloop Limited, Pakistan)

V.

Mr. Rehman Ali Khan (Assistant Vice President, US Apparel Limited, Pakistan)

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