Supercritical Fluid Technology: A Way to Reduce Textile Water Footprint

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Abstract

Excess utilization of fresh water and discharge of wastewater is a major environmental obstruction for the growth of the textile industry besides the other issues like solid waste and energy management. For many years, the textile industry has tried to identify new ways to reduce the water consumption and the wastewater discharge in the industry, such as machine modification (use of low liquor ratio machines), process modification (combined processes), reuse of water and Zero Liquid Discharge (ZLD) processes.

Textile industry has accelerated efforts to reduce or eliminate water consumption in the areas of wet processing. Supercritical carbon dioxide (SC-CO2) is one of the most environmentally acceptable solvents in use, and its applications in textile processing have many advantages. Positive environmental effects range from drastically reduced water consumption to eliminating hazardous industrial effluent. Furthermore, economic benefits include increased productivity and energy savings. This paper highlights various areas of textile processing where the SC-CO2 technology can be used and what are the developments so far.

Keywords: Textile wet processing, Supercritical Carbon dioxide Technology, Water footprint, Wastewater, Zero Liquid Discharge.

  1. Introduction

Demand for freshwater is increasing worldwide due to population growth, industrialization and globalization of the world economy. The world’s population has tripled since 1900, but its water demand has increased six fold [1, 2]. This is a tremendous challenge considering that India has merely 4% of the world’s renewable water resource. The facts of increasing water scarcity and declining water quality have brought sustainable water resources management in the forefront of the global development agenda [3].

1.1 Water footprint

The idea of Water Footprint (WF) was introduced by Arjen Hoekstra, a Professor of Water Management at University of Twente in the Netherlands. “The water footprint of an individual, community or business is defined as the total volume of freshwater used to produce the goods and services consumed by the individual or community or produced by the business. Water use is measured in terms of water volumes consumed (evaporated or incorporated into a product) and/or polluted per unit of time [4].

The concept of ‘Water Footprint’ (WF) is an important step in the direction of evolving methodologies, approaches, and indicators for measuring freshwater consumption and assessing the impacts of water pollution. It is a comprehensive indicator, on the one hand, it accounts for the total amount of water consumption that reflects the production process; on the other hand, it accommodates for the total amount of pollutants in the water, which reflects the environmental impact [5].

1.2 Water footprint of the Textile Industry

Textiles are important necessity of our daily life. The life cycle of textiles involves many stages, such as agriculture, industry, distribution, use and recycling. The production and consumption of textiles in relation to environmental impacts have received political and social attention in recent years. Studies addressing the impacts of the textiles on freshwater resources concluded that the majority of impacts result from raw materials (e.g. cotton, hemp and wool) production, wastewater emissions from textile processing and final products maintenance [6].

Since textile industry is highly water intensive and India had been identified as a highly water-scarce region, the long term viability of the Indian textile industry hinges heavily on sustainable water management. As per one of the study, the water consumption of Indian Textile industry alone is about 200 – 250 m3/tone of cotton cloth [7, 8],Textile industry in general has an enormous water footprint right from the stage of cultivation of cotton to the stage when it reaches the stores. It can be divided

  • Production of fibres (cultivation of cotton)
  • Spinning it into yarns
  • Weaving it into fabric
  • Processing the fabric (pre-treatment, dyeing, printing, finishing, washings)

The quantity of water required for textile processing is large and varies from mill to mill depending on fabric produce, process, equipment type and dyestuff. The longer the processing sequences, the higher will be the quantity of water required. Water requirements for cotton as well as other synthetic textiles like Nylon, Polyester, Acetate, Acrylic are given in Table 1 [9, 10]. In the Table 2 characteristics of waste streams after each wet processing are given [11].

Table 1: Water requirements for cotton as well as other synthetic textiles

.

Table 2: Characteristics of wastewater after each textile wet processes

In addition to water use, the impact of textile wastewater on the quality of water resources (the streams, lakes, rivers) must also be considered. It is unadvisable to discharge highly polluted wastewater without any treatment to the water bodies. This severely impacts the flora and fauna dwelling in the water resources. The indirect consequences would be on the animals and humans would consume such infected water.

1.3 Strategies for reducing water footprint

There are best available techniques available for reducing intake of fresh water and primarily aim at reduction in the quantity and quality of waste with a view to combat water pollution. The various water and waste minimization options that are available for the textile industry can be listed out as follows,

  1. Improved working practice (good housekeeping, incorporation of automation)
  2. Process modification (combining processes)
  3. Machine modification (reduced liquor ratio)
  4. Chemical management (reduction and reuse in process chemicals, substitution of hazardous chemicals with the eco-friendly ones)
  5. Efficient washing techniques (counter current and using final rinse of one process as a first rinse for another one)
  6. Segregation of waste streams (coloured and non coloured streams, less polluted from more polluted, biodegradable and non biodegradable) and then effluent treatment

‘Green production’ is a preventive business strategy in textile dyeing and finishing industry and nowadays more research is carried out on topics like, use of ultrasonic energy, use of microwave energy, use of plasma technology, use of supercritical fluids in textile dyeing and finishing to advance the textile processing industry.

This paper highlights the supercritical carbon dioxide technology, the developments done and its varied applications in textile processing.

  1. Supercritical Carbon dioxide Technology or ‘Waterless technology’

Textiles have undergone chemical wet processing since time immemorial. Now, the general concept of environmental protection is shifting from a reactive approach referred to as “pollution control” to a proactive approach referred as “pollution prevention”. It is still imperative to change the mindset of many industries which is still to control the pollution rather than preventing it.

Usage of water as solvent for chemicals is mostly because of its abundant availability and low cost. Problems associated with usage of water are effluent generation and an additional drying step is needed. The amount of energy spent to remove the water is also huge, adding to the woes of processors; making processing the weakest link among the entire textile chain. The unspent dyestuffs remain in liquor, thus polluting the effluent.

New waterless dyeing technologies are being developed and deployed that could help reduce the vast quantities of pollution generated by textile dyeing. Application of ultrasonic waves, microwave dyeing, Plasma technology, supercritical carbon dioxide, and electrochemical dyeing of textiles are some of the revolutionary ways to advance the textile wet processing

A supercritical fluid (SCF) is defined as a substance above its critical temperature (Tc) and critical pressure (Pc). The critical point represents the highest temperature and pressure at which the substance can exist as a vapor and liquid in equilibrium. The phenomenon can be easily explained with reference to the phase diagram of carbon dioxide, CO2 (Figure 1). This shows the conditions where CO2 exists as a gas, liquid, solid, or as a SCF; i.e. above 7.4 Mpa and 31.1 °C.

Figure 1: Phase diagram of CO2

For a long time the supercritical carbon dioxide (SC-CO2) technology has been known for only extraction purpose i.e. extraction of essential oils, fragrances, coffee, tobacco etc. Supercritical fluid dyeing (SFD) was first established by Professor Schollmeyer and his research groups from Deutsches Textil for Schungszentrum Nord-West e.V. (DTNW) in Krefeld (Germany) in the late 1980s and, since then, many research activities and experiments, from laboratory scale to pilot scale, have been developed. In 2002 the above-mentioned research group published a comprehensive review about this technology. In the last decade, further research activities have been carried out with the aim of studying the impregnating properties of supercritical fluids [12].

The advantages of using a SCF can therefore be summarized as follows:

  1. SCFs have similar solvating powers to liquid organic solvents, but their higher diffusivities, lower viscosity and lower surface tension make them more effective in many cases.
  2. Since their density is pressure-tuneable, separation of substances from solvents is easy to achieve.
  3. The ability to add modifiers to a SCF, for example to change the polarity, gives them more selective separation power.
  4. Negligible harm is done to the environment in terms of residues from processes using SCF compared to volatile organic compounds (VOCs) and ozone depleting substances (ODSs).
  5. SCFs are generally cheap, safe to use and have minimal disposal costs associated with their operation in industrial processes.

2.1 Applications of SC-CO2 technology in textile processing

The applications of supercritical fluid originated from replacing traditional solvent extraction of caffeine from coffee bean or nicotine from tobacco. This technique has reached the economic scale of mass production. The application of SC-CO2 has extended continuously in various domains including Textile industry [12]. The following are some of the areas,

  1. Dyeing of fibers/Fabrics
  2. Treatment of contaminated/soiled substrates
  3. Extraction of impurities with supercritical fluids

Dyeing with SC-CO2 has already been proven for synthetic fibers. However, it lacks the specific work flow towards the processing of the textile materials before and after the dyeing i.e. pre-processing and post-processing.

2.1.1 Supercritical Carbon-dioxide dyeing

Conventional water based dyeing of textile fabrics has intrinsic environmental problems due to the inevitable use of water and the discharge of various chemical additives. Moreover, a subsequent drying process with high energy consumption is necessary [13].

Researchers have worked and are still working on various aspects related to dyeing of textiles with supercritical fluids. Some of these include,

  • Solubility of dyes in SC-CO2 medium

 

  • Selection of substrates for processing

 

  • Studying the effect of SC-CO2 on morphology of substratea

Solubility of dyes in supercritical fluids

In general, CO2 displays the properties of a typical solvent; beyond its critical point, CO2 has unique properties. SC-CO2 exhibits densities and solvating powers similar to those of liquid solvents yet has extremely rapid diffusion and viscosity similar to those of a gas [14, 15].

In order to develop compatible dyes and design a proper SC-CO2 dyeing process, the solubility of dyestuffs in SC-CO2 fluid is one of the most important parameters which determine their appropriate selection and dosage.

Draper et al. in their detailed study performed solubility relationships of ten disperse dyes in supercritical carbon dioxide. These were made over a pressure range of 200 – 400 atm at 50 – 100 °C. The Authors concluded that melting point, molar heat of fusion show little correlation with solubility. Dye structure is the only factor that correlated consistently with SC-CO2 solubility. Higher polarity generally led to lower SC-CO2 solubility of the dye [16].

In another study, the solubility of dyes in the SC-CO2 medium, kinetics of dyeing processes or SC-CO2 flow characteristics in textile substrates was studied. It was concluded that the dye solubility is the function of solvent (SC-CO2) density. An increase in pressure increases the fluid density and eventually increases the solubility of solute [15].

In a similar study, Long et al. reported the solubility of the reactive disperse red dye in a batch system at pressures 7.5–25.0 MPa (75 to 250 bar) and temperatures ranging over 60 to 140 °C. The study shows that the solubility of the dye increased with the system pressure and decreases with rise in temperatures, especially above 100°C [17].

Solubility behavior of dyes in SC-CO2 is important for level or uniform dyeing. This would minimize the problem of unleveled dyeing, shading and streaking might be minimized, with less time required for correction of the uniformity problems.

b) Dyeing of different substrates

Dyeing of any textile material in SC-CO2 depends on (but not limited to) dissolution of dyes in SC-CO2 medium and the diffusion of SC-CO2+dye into the polymer matrix. Thus study of polarity of dyes and the textile substrate is important.

Non-polar dyes are used to enable proper dissolution in SC-CO2 medium. Synthetic substrates like polyester, nylon, polypropylene, etc are also non-polar or hydrophobic and during the dyeing process, the dye molecules can diffuse into the polymer matrix, where they are physically bonded. Because of its non-polarity, these substrates can be dyed in SC-CO2 with non-reactive, so-called disperse dyes. On the other hand, natural textile substrates like cotton, silk and wool are polar or hydrophilic and therefore have no affinity for the non-polar dye molecules. It is only possible to dye these textiles in SC-CO2 when the dyes are reactive towards the reactive groups in these materials. These non-polar reactive dyes are generally called disperse reactive. Another way is to impart hydrophobicity onto the natural substrates making them non-polar and can be dyed with disperse dyes in SC-CO2 [18].

Dyeing of synthetic hydrophobic textiles using SC-CO2 dates back to 1982 where Chou and Wessinger studied the absorbance and swelling capacity of SC-CO2 for polystyrene, polycarbonate, polyvinyl carbonate, polypropylene, poly methyl methacrylate (PMMA) polyester and polyurethane polymers. The results showed that PMMA and polycarbonate have maximum affinity for CO2 due to carbonyl group [19, 20].

Long et al. investigated the level dyeing of polyester fabrics in SC-CO2 by employing an improved beam. The level dyeing properties of fabrics were studied under different conditions including dyeing pressure, dyeing temperature, dyeing time, the time ratio of fluid circulation to static dyeing, different fabric layers on the beam, and the properties of dyestuffs. The results obtained showed that the level dyeing properties of polyester fabric samples were enhanced with the new beam [21].

Santos et al. compared the dye incorporation process of non-modified and N, N-dimethylacrylamide modified PET fibers with SC-CO2. They investigated the effect of dyeing

conditions and modifying agents on the PET films and fibers dyeing properties. The results concluded that the amount of incorporated dye increases 3.8 times with the pre-treated N, N-dimethylacrylamide PET fibers [22].

There is a limited published research about dyeing of polyamide fabrics using SC-CO2. Nylon 6, 6, was dyed with synthesized hydrophobic reactive dyes using SC-CO2. A covalent bond was formed between the dye and the fibre which was confirmed by FTIR, MS and NMR analysis. Dye uptake was moderate to good and the dyed fabrics had very good fastness properties [23].

In an another study on dyeing Nylon-6 fabrics using SC-CO2 reported by Abou Elmaaty et al; they demonstrated the ability of a new series of hydrazonopropanenitrile azo dyes with potential antibacterial activity in dyeing Nylon-6 fabrics under SC-CO2 conditions. The results were promising, as excellent wash and rubbing fastness, and good light fastness compared with conventional dyeing. Also, excellent antibacterial effectiveness of the dyed samples using SC-CO2 was reported [24].

The properties of polypropylene (PP) like low density, high toughness and resilience, poor moisture adsorption, excellent chemical and abrasion resistance makes the PP fibers/fabric very difficult to dye. In the research by Liao et al. PP fibers were dyed successfully in SC-CO2 and claimed that the results were comparable with the water based dyeing [25]. The disperse dye had better dyeing effect on polypropylene fiber in SC-CO2 conditions as it helps to reduce the Tg of polypropylene thus plasticizes the polymer for making it dyeable [26, 27].

One step dyeing process for dyeing polypropylene with commercial acid dye was studied by Garay et al. in their research. The result showed that the use of co-solvent is necessary because of the dye insolubility in SC-CO2, and water was chosen because of environmental reasons. Good scCO2 dyeing results were obtained for pressures above 175 bar [28].

It is a well established study that synthetic fabrics can be dyed using SC-CO2 technology. Dyeing of natural fabrics such as cotton, wool, silk are difficult to dye with this technology. Various modifications in the dye as well as the substrate are necessary prior to dyeing. The inability of CO2 to swell and promote the diffusion of dyes into the interior of polar natural fibres is one of the main problems. Furthermore, the interactions of polar fibres with disperse dyes is very low,

while the polar dyes that are generally used to dye natural fibres in conventional aqueous dyeing (direct dyes, acid dyes, basic dyes, vat dyes, etc.) are nearly insoluble in SC-CO2.

One possibility is to develop reactive disperse dyes that could be soluble in SC-CO2 medium and later react with cotton fabrics to form covalent bond. Gao et al. reported the high efficiency of the reactive disperse dyes in the dyeing of cotton fabrics; and the results indicated that the solubility of the dyes increased with increasing pressure at the same temperature, and decreased with increasing temperature at a given pressure. Meanwhile, the solubility for these dyes was also influenced by the polarity of the dye molecules, in such a manner that higher solubility was displayed by less polar molecules and lower solubility by more polar molecules. These dyes were applied on cotton fabrics in SC-CO2, and the appropriate dyeing condition were 100 °C at a 20.0 MPa pressure, for 60 min. The colour fastness properties were between 4 – 5  [29].

Schmidt could successfully dye wool and silk fibers by synthesizing a reactive disperse dye. The dyeing was carried out without pre-treating the fibers and various fastness properties were above grade 4. However the dyeing parameters were 280 bar pressure at 160 °C for 4 hour, which was not ideal as the fibers were damaged [30].

Promising reports for the supercritical dyeing of poly (lactic) acid (PLA) fibres with disperse dyes can be found in the literature. PLA is a sustainable fibre that has been receiving increasing attention from the textile industry, but its use is limited because of its poor resistance to conventional textile processing conditions, such as dyeing and scouring. SC-CO2 dyeing of PLA gave comparable dyeing and fastness properties with aqueous dyeing. Wen and Dai conducted experiments on PLA fibers and recommended a maximum working temperature of 110 °C and concluded that SC-CO2 dyeing resulted in much better retention of the mechanical properties of PLA fibres with respect to the conventional aqueous treatment [31].

Zheng et al. investigated the effect of water on the dyeing of wool fabrics in SC-CO2. Addition of water significantly improved the dyeing process at low temperature and pressure. The SEM results obtained indicated that the solubility of dyes on the wool fibers surface was remarkably improved by adding water. In addition, increasing the amounts of water in the SC-CO2 and the textiles clearly improved the results of K/S and color difference of wool fibers [32].

c) Effect of SC-CO2 on structural/morphological change of the substrate

It should be emphasized that exposure to supercritical fluids have significant effects on the structure of substrate depending on the conditions applied. Thermal and solvent treatments of hydrophobic synthetic samples lead to reduced glass transition temperature but increased molecular chain mobility followed by crystallization induced in the samples. While exposure of such textiles to SC-CO2 allows desirable structural changes in them without any environmental hazards, improvement in their ultimate properties can only be achieved by knowing exactly the mechanism of microstructure development.

Bai et al. detected the changes in molecular structure of PET under SC-CO2 conditions back in 1998 by using Solid state NMR spectroscopy and wide angle X-ray diffraction (WAXD). The WAXD results indicate an increase in crystallinity from essentially zero in the as-received sample to 62% in the SCF-treated sample [33].

Baseri et al. investigated the influence of SC-CO2 on the structural changes and mesomorphic changes in PET fibers. The fibers were exposed to SC-CO2 at 80 °C and 220 bar for 2.5 hr. to study the structural changes. Thermal characteristics of the structure, especially the rearrangement of polymer chains in the amorphous phase induced by exposure to SC-CO2, were evaluated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR). The orientation factor of the fibers was measured using a polarizing microscope. The paracrystalline portion and the crystallite sizes of exposed samples were estimated by wide-angle X-ray diffraction [34].

2.2.2 Pre-treatment (Pre-processing)

Dyeing with supercritical carbon dioxide has already been proven for being able to be widely utilized in the dyeing of synthetic fibers. However, it lacks the specific work flow towards the processing of the textile materials before and after the dyeing. This technique can be used for pre-treatment of fabrics. The principle for the same is that the fluid is compressed to elevated pressures above its critical pressure, to make it supercritical. The polymer is then exposed to the supercritical fluid and swells. As the free volume in the polymer is increased, the SCF can penetrate deeply into the matrix and the impurities are dissolved by the supercritical fluid. In the de-pressurisation phase the pressure is quickly reduced and the supercritical fluid and impurities

diffuse out of the polymer. Very limited amount of literature was available for this application.

Some of them are explained below.

One of these usage applications of supercritical carbon dioxide (SC-CO2) is in surface modification of polyester fabrics as a pretreatment. In their study, glycerol polyglycidyl ether was impregnated as a cross-linking agent into polyester fabric through supercritical carbon dioxide. It is reported that glycerol polyglycidyl ether can penetrate to the surface of polyester fabric in supercritical carbon dioxide pretreatment process. The modified polyester fabric exhibited progression in surface hydrophilicity and wettability, moisturisation efficiency, and antibacterial activities [35].

C.Wang et al. studied the scouring possibility of polyester fibers by utilizing SC-CO2 as a medium, the oil removal efficiency from polyester fibers reached to +99% [36].

The bleaching possibility of knitted cotton fabric by using SC-CO2 as a medium was studied by Eren et al. at 80 °C and 120 °C for 20 min using SC-CO2 fluid technology. Bleaching was carried with either solely hydrogen peroxide or combination usage of hydrogen peroxide with necessary related auxiliaries such as peroxide stabilizer and caustic soda. When compared with conventional aqueous bleaching, the waterless bleaching resulted in higher whiteness index [37].

In a patented technology by DeSimone et al., SC-CO2 system was used to clean synthetic fibre i.e. nylon employed solely or in combination with other types of fibers in various non woven and woven fabrics. Contaminants such as dirt, dust, grease, and sizing aids used in textile processing are removed from the synthetic fiber. Methanol was used as co-solvent in order to increase the efficiency of removal [38].

2.2.3. Finishing (Post Dyeing)

Supercritical carbon dioxide has recently found different application types also in textile finishing. For instance, A.L.Mohamed et. al. studied supercritical carbon dioxide assisted silicon based finishing on cotton fabric. In here, researchers used supercritical carbon dioxide as a medium for finishing cotton fabrics with modified dimethylsiloxane polymers terminated with silanol groups. 3- isocyanatepropyltriethoxysilane and tetraethylorthosilicate were utilized as cross-linkers for covalent bonding formation between silicon and cellulose polymers of cotton

fiber. It is reported that all cotton fibers applied with silicon (PDMS) and 3-isocyanatepropyltriethoxysilane possess larger silicon amounts than those applied with tetraethylorthosilicate. Supercritical carbon dioxide medium procures nice cotton surface coating via a 3D network of DMS compound and cross linker leading to the highest DMS concentration formation in a layer between 1 and 2 micron under the cotton fiber surface [39].

In the case of antimicrobial finishing attainment, T. Baba et al. impregnated chitin and chitosan to polyester (PET) fabric using supercritical carbon dioxide in order to achieve high anti-bacteria property durable to washing. The chitosan was fixed in the PET fabric as evidnced by the durability rate of 70 % after 50 washes [40].

SC-CO2 medium was used to impart water/oil repellent property on polyester fabrics with a solution of organic fluorine. The finishing experiments were conducted at temperatures and pressures ranging from 50 °C to 110 °C and 10 MPa to 26 MPa, respectively. FTIR confirmed the presence of –CF2– group on the surface of polyester fabrics. SEM images confirmed that fluorine was evenly distributed on the fiber surface [41].

Functional active agents such as functional dyes, antimicrobial agents, flame retardant, antioxidants, fragrances, pharmaceutical drugs, and others can be impregnated into a polymer by exposing the polymer to SC-CO2 medium containing these agents based on the mechanisms explained in Figure 2 [42].

Pharmaceutical drugs can be impregnated into a swollen polymer matrix (like polyester, PMMA, polyurethane) at operating temperature low enough to avoid thermal degradation of temperature-sensitive drugs (35 – 55 °C) and pressure 90 to 200 bar. After impregnation and depressurization, the impregnated drug materials slowly diffuse out from the polymer matrix at a slower rate than the rate it was diffused into the polymer which can be used to form a novel controlled release of drugs [43].

2.2.4 Treatment of soiled substrates / Cleaning of textiles

The application of SC-CO2 in dyeing has been studied by many researchers few of them are discussed already. Apart from dyeing, other area which is gaining ground is cleaning or disinfection of textiles using SC-CO2 technology.

Solvents such as perfluorocarbons, hydrofluorocarbons and other solvents have been investigated and implemented for applications of cleaning of substrates instead of water-based cleaning. However, these solvents still pose environmental and/or health concerns. SC-CO2 cleaning offers several advantages over conventional processes.

There are few patented technologies, wherein the researchers claim of inventing a method with less or no damage to solid components like buttons, zips, etc. with increased performance. They also claim minimum polymer damage. In another claim, it was stated that, SC-CO2 cleaning reduces redeposition of contaminants back onto the substrates. The preferred conditions are 60 to 240 bar at temperatures between 20 to 100 °C. In another trial, the soiled textiles were treated with SC-CO2 along with added formulation (consisting of water, methanol, citric acid and enzyme) for better cleaning efficiency [44, 45].

The disinfection of textiles is of extreme importance when dealing with materials targeted for special uses such as hospital textiles and the proposed treatment must be safe and must not compromise patient’s safety and health. On the other hand conventional textile treatments such as UV-radiation or chemical disinfection seem to be insufficient, time consuming, while they can be held responsible for causing additional health problems to patients such as skin irritation [46].

Successful trials of cleaning of textiles in SC-CO2 plus water and SC-CO2 plus detergent were done which helped in reducing the redeposition of stains onto the textiles. Disinfection properties were also observed wherein inactivation of bacteria took place at 50 – 80 bar [47].

A green textile cleaning process was optimized by Aslanidou et al., wherein the authors managed to remove various stains like oil, beetroot paste, glue and fungi (Aspergillus Niger) from silk fabric using supercritical carbon dioxide and an aqueous suspension of Ca (OH)2 as a co solvent. The proposed operational parameters were 150 bar pressure at 40 °C with the cleaning efficiencies for all three stains were found to be between 95 to 99% [46].

The disinfection efficiency of textiles which were inoculated with microbes and later subjected to treatment using SC-CO2 was studied by Fijan et al. (2011). It was found that, addition of heat to the compressed CO2 treatment of textiles inoculated with microorganisms proved more effective than the addition of detergent or disinfectant with compressed CO2 treatment. Ttreatment at 5 MPa, temperature of 50°C and addition of 4 ml/l of CO2 of specific detergent assures at least a 6 log step reduction of all three chosen microorganisms: Enterococcus faecium, Enterobacter aerogenes and C. albicans [48].

  1. Summary

The review highlights the idea of water footprint, and explains the water footprint of textile industry. It is an important step in the direction of evolving methodologies, approaches, and indicators for measuring freshwater consumption and assessing the impacts of water pollution caused by the industry. Even though there are various developments being done to reduce the water footprints like implementing the Best Available Techniques (BATs); still it is considered as a reactive approach or pollution control approach. Supercritical carbon-dioxide technology is seen as a proactive approach or pollution prevention approach in textile processing industry.

Supercritical carbon dioxide (SC-CO2) waterless dyeing is widely known and applied green method for sustainable and eco-friendly textile industry. However, recently, not only dyeing but also pretreatment processes and different finishing applications take the advantage of SC-CO2 leading to enormous fresh water saving, cleaner and greener way of production and massive amount of contribution for world sustainability. This review provides a brief introduction on the existing circumstances of the developments in the applications of SC-CO2 which can provide a better understanding for further studies.

Acknowledgement

Authors would like to acknowledge Ministry of Textiles, Government of India for providing essential financial support to conduct the research work.

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Dr. U. K. Gangopadhyay, Dr. Manisha Mathur, Dr. Rachana Shukla* and Dr. Rekha R.

The Synthetic & Art Silk Mills’ Research Association (SASMIRA), Mumbai 400030, India