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Processes for reducing the environmental impact of fabric finishing
Processes for reducing the environmental impact of fabric finishing
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D. Weydts, D. De Smet, M. Vanneste Centexbel, Textile Competence Centre, Ghent, Belgium
2.1 Introduction Conventional fabric finishing processes require huge amounts of energy and water. New finishing processes are developed using limited amount or no water and reducing the energy consumption substantially. While some of them are still on a laboratory level, other are about to be used at industrial scale. In this chapter, an overview will be given of dry finishing techniques with potential to be implemented in textile finishing. Finishing companies need to deal with water discharge and its chemical load, energy consumption, air and water emissions, waste, odors, and noise. Waste management should focus primarily on prevention, secondly on reuse, recycling, and revaluation and only if those options fail go to external treatment of waste. Regarding emissions to water, the final effluent is often composed of streams coming from different processes that are mixed together resulting in a complex effluent depending on the type of fiber, the process, and the functionalities applied (i.e., the type of chemicals and auxiliaries used). The reduction of emissions to water is driven by the Water Framework Directive that aims at protecting and restoring the water quality in Europe. It established a precise timetable, with 2015 as the target date for getting all EU waters into good condition. To avoid water emission and its chemical load and to reduce energy and water consumption, and waste production more environmental friendly and water-free processes such as hotmelt, plasma coating, digital finishing, and radiation-curing are developed for fabric finishing.
2.2 Waste minimization in fabric finishing Minimization of waste can be realized by the reduction of water, chemical, and energy consumption, as well as by reducing solid waste, air and noise pollution, and by minimizing the emission of toxic substances (Barclay and Buckley, 2000a). Waste minimization requires good management of environmental practices, taking into account the following: ●
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Correct choice of chemicals, that is, substitution of hazardous substances by reusable, biobased, and/or biodegradable materials. This part has been dealt with in Chapter 1. New technologies and processes to diminish the impact of fabric finishing on the environment (in particular on the use of energy and water) have been investigated, of which some have proven their usefulness in other sectors.
Sustainable Apparel. http://dx.doi.org/10.1016/B978-1-78242-339-3.00002-9 Copyright © 2015 Elsevier Ltd. All rights reserved.
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Source reduction and minimization of used chemicals. Effluent segregation and reuse of chemicals.
2.2.1 Energy-efficient and water-saving finishing processes For many years, research at Centexbel on textile functionalization and surface modification focused on energy-efficient and water-free finishing technologies. As some of these techniques are established in other applications, such as the coating of wood, paper, and plastics, they need to be tuned to textiles processes. In addition, adapted formulations need to be developed to maintain textile flexibility and haptics. The overall advantage of these techniques is the absence of water eliminating the long, energy-consuming oven and, as a consequence, reducing the floor space needed for the machines. Another advantage is the possibility to produce short runs since a quick start and stop of the machine is possible. The hotmelt process for textiles (Fung, 2002; Sen, 2008) was primarily used in lamination applications (e.g., glue in collars of men’s shirts). Later on it was seen that hotmelts could also be used as a coating. In a hotmelt process, the material used is a polymer that is molten and put onto the textile as a coating layer. The hotmelt polymer material can vary from granules to blocks. By means of functionalized hotmelt, the textiles can be made flame retardant, antimicrobial, hydrophobic, and more. Various hotmelt application systems exist such as slot-die, spray, gravure roller, multiroller, and rotary screen. Depending on the type of applicator, continuous or discontinuous layers can be applied—the latter being suitable to impart breathable and melting temperature properties. A similar process is extrusion coating (Singha, 2012), where an extruder is coupled with the (hotmelt) application machine. In general, the polymer materials used in extrusion coating have a much higher viscosity compared to the hotmelt process. In radiation curing processes, using ultraviolet (UV), light-emitting diode (LED), or electronbeam (EB) processes, water-based as well as 100% coatings exist. A substantial reduction in energy is obtained when 100% formulations are used. Other advantages of this technique are the space savings, and the ability to integrate the radiation curing machinery into the existing coating line and reduced run lengths. Initially, the textiles coated by means of radiation curables were hard (as they were developed for hard surfaces), and they lost their typical textile feeling and touch. However, thorough investigation has led to an enormous improvement in acceptable properties. By means of this technique, various functionalities even using conventional additives can be implemented in textiles such as abrasion resistance (Goethals et al., 2014), flame retardancy, and hydrophobicity (Castelein et al., 2011). Digital finishing is an emerging finishing technique in which the digital printers normally used for (local) coloration of the textile are now used for finishing. The advantage of the technique is the possibility to do this locally, resulting in a cost-effective process (Nierstrasz, 2012) with a limited use of chemicals, low amounts of solvent (UV-curable 100% inks are also available), and using less energy compared to the conventional finishing of an entire textile. Similar to digital printing, the production flexibility is an advantage, for instance, it is easier to produce small runs (Van Parys, 2013). This technique also shows great potential for mass customization of clothing
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(Kaiser et al., 2014), specifically for persons with special needs. Digital functionalization via a magnetic valve system was compared with other application techniques (e.g., airless spraying with rotating plate, air-driven jet-spraying, and three-dimensional spray finishing). Some advantages reported in comparison to the other techniques are (Dura et al., 2014a,b) the high resolution, the possibility to realize multifunctional textiles using multiple jets, no failures due to overlap of spray cones, no need for air, the flexibility of the system, and the simple exploitation. Three-dimensional printing or additive manufacturing (AM) techniques generally focus on the production of stand-alone, individual objects often for niche markets such as prostheses for medical applications. Recently, Deleersnyder (2013) started to use this technique for textile coating and even the production of accessories for apparel (see Figure 2.1) and other textiles. Silk fabrics have been made hydrophilic using UV irradiation (Periyasamy et al., 2007). Hydrophilic and oil absorption properties have been generated in wool fabrics by use of a UV treatment (Samanta et al., 2014). Plasma technologies have been investigated for quite some time with the aim of improving wettability and thus the adhesion of coatings. However, a much broader range of applications is feasible with plasma (Buyle, 2009): these applications include the improvement of printability and dyeability (Rahman and Nur, 2014), and antishrink treatment of wool, the scouring of wool (Rahman and Nur, 2014), sterilization, and the desizing of cotton. Plasma coating, that is, the deposition of a nanolayer of functionalities on textiles, has been investigated thoroughly by means of low-pressure plasma, atmospheric plasma, or plasma sputtering. Using these techniques various functionalities can be implemented into the textile (Buyle et al., 2010). Durable, PFOA- and PFOS-free, water- and oil-repellent protective clothing have been produced using low-pressure plasma nanocoatings of C6 fluorine chemistry (Rogge, 2013). Atmospheric pressure plasma with fluorocarbon has been used to impart hydrophobic properties in cotton
Figure 2.1 PLA button deposited on polyester knitting by fused deposition modeling.
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(Samanta et al., 2013). Hydrophilic, hydrophobic, and dirt-repellent coatings using different plasma technologies were reported by Bhosale et al. (2013). Antibacterial activity was obtained by plasma sputtering of silver and copper on dyed cotton (Shahidi et al., 2010; Shahidi, 2015). At the same time, an improvement in the fastness properties was observed. As both radiation-curing and plasma affect only the outer surface of the fabric, only a minimum of chemicals are needed. The cost is low because of a shorter processing time and (partial) reduction of effluent treatment (Samanta et al., 2014). Both techniques have been compared using conventional methods regarding the process medium, water and energy requirement, time required, consumption of chemicals, and cost.
2.3 Wastewater treatment and management Even if care is taken to minimize waste, there will still be waste to treat. Currently used wet finishing processes produce waste containing organic as well as inorganic compounds. The effluents are rich in chemicals of which some are persistent or resistant to water treatment methods. Table 2.1 lists typical examples of finishing waste that resists biodegradation. Removal of these substances from wastewater is expensive and difficult to achieve. Because of this, the effluent segregation and source reduction methods are preferred as economically attractive alternatives. Before starting an end-of-pipe treatment, effluent segregation is required; this separates contaminated fluid from the cleaner streams. The final wastewater is less voluminous and can be treated more effectively and by more appropriate treatment methods, whereas the more clean stream can be reused in the factory with limited or no treatment needed. Vajnhandl and Valh (2014) reviewed the wastewater reuse programs used over the last 10 years covering the European textile finishing industry, with special emphasis on the FP7 project Aquafit4use. The water treatment methodologies presented enable more than a 40% reduction in freshwater consumption. Table 2.1 Typical substances present in wastewater from wet finishing processes Finishing process
Substances in waste water
Easy-care finishes
Urea and melamine derivatives (noncross-linked) Organo-phosphorous compounds, polybrominated compounds Alkylphosphates, alkyletherphosphonates Fluorochemicals Organic material such as phenols: alkylphenol (AP), alkylphenolethoxylates (APEOs)
Flame-retardant finishes Antistatic finishes Water-repellent finishes Surfactants
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Shaid et al. (2013) suggested that the finishing companies define the quality level of the rinsing waters needed to be able to reuse the waste streams, in some cases even without any treatment. For waste that is widely dispersed and hard to treat when it is discharged, source reduction is an appropriate way to deal with it. Such waste can be captured or the amount can be reduced by modifying the process or the machinery design, choosing a chemical substitution, or remediating the existing procedures, and so forth (Smith, 1988). Concentrated residues (i.e., not dispersed) are in general easier to remove as waste than via wastewater treatment. In some cases they can be reused in the textile finishing process. For chemicals being persistent, bio-accumulative, toxic, and/or highly aquatic toxic, it is strongly recommended to consider chemical substitution. Depending on the type of chemical, the collection of concentrated residues may not be sufficient; separate collection of the (less concentrated) rinsing waters may be needed (Derden et al., 2011). This is the case for chemicals for which very low environmental quality standards are issued, usually in combination with the classification as priority hazardous substances within the Water Frame Directive (see information to come). Examples are brominated flame retardants, fluorosurfactants, nonylphenols, and nonylphenolethoxylates for which concentrated residues and rinsing waters are preferably reused in the process. If this is not feasible, they need to be treated as waste by a third party having the necessary permits to do so. Once effluent segregation, reuse, and/or the substitution of chemicals have been considered, one of the following water treatment techniques (or a combination thereof) can be applied to the remaining wastewater flows: coagulation/flocculation, nanofiltration, chemical oxidation, mechanical vapor recompression, membrane bioreactor, or powered-activated carbon treatment. Barclay and Buckley (2000b) provide an explanation of these techniques together with their pros and cons. In Belgium, the BAT center of Vito developed a methodology to choose the appropriate combination of techniques for a specific wastewater issue.
2.3.1 Coagulation and/or flocculation Colloidal or suspended particles are charged and stable in water (they do not precipitate). One must take into account their variation in size, shape, source, and density when applying coagulation and flocculation processes (e.g., correct choice of coagulants and flocculants). Coagulation aims to destabilize the charged particles present in the wastewater leading to aggregation and the formation of floccules. To achieve this, the addition of a coagulant (with a charge opposite of the particles) is required, reducing the repellency between and thus stabilizing the colloidal particles. Commonly used coagulants are inorganic (Bratby, 2006; Anon, 2014) and are based on aluminum (i.e., aluminum sulfate, aluminum chloride, sodium aluminates), iron (i.e., ferric sulfate, ferrous sulfate, ferric chloride, ferric chloride sulfate), hydrated lime, or magnesium carbonate. They are often the cheapest, most widely available, and effective coagulants.
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Prepolymerized coagulants based on aluminum and iron are used as they have the advantage of working over a much wider range of pH and water temperature. In addition, less dosage is needed and fewer residuals (chloride, sulfate, or metal). Examples are aluminum chlorohydrate, polyaluminum chloride (PAC), polyaluminum sulfate chloride, and polyferric sulfate. The coagulant is added, meanwhile intensively mixing using a short retention time (typically 1–3 min), and generating small floccules surrounded by clear water. The mixing velocity and time are critical parameters. To enable them to grow, gentle stirring is needed (to not destroy the floccules) with a longer retention time (15–20 min), allowing the particles to further cluster. Flocculants (promoting flake formation) can be added to stimulate this process. They are high-molecular polymers functionalized with various groups. The small floccules are attracted to this polymer because of the charges present. This results in a larger floccule. Polymeric coagulants aids or flocculants consist of a broad range of different compounds and are effective over a wider range of pH than the inorganic coagulants. In general they are more expensive than the inorganic coagulants. Anionic polymeric coagulants are often used in combination with the metal coagulants. Cationic ones can be used alone or with iron of aluminum-based coagulants. Natural as well as synthetic polymers are used; the natural polymers are in general nontoxic and biodegradable. The synthetic ones, however, are more widespread since their effectiveness can be largely tuned during manufacturing. As mentioned above, the correct dosing and sequence of the addition of the chemicals (coagulant and flocculant) are critical for the process to work correctly. This is not straightforward with wastewater that is strongly varying in composition. Depending on the type of chemical used the pH will change. For that reason buffering the wastewater (i.e., pH correction) is used. The floccules can then be easily separated via flotation or sedimentation. The colloidal particles formed do not all have the same charge. For that reason the polymer needs to have various charges (anionic, cationic, or nonionic) combined with good distribution thereof over the whole structure to achieve an effective flocculation. The molecular length and the level of (self) cross-linking are also important parameters. Due to the variation of all these parameters a few hundred different polymers result, each with their specific level of effectiveness. The floccules are collected and form a polluted sludge to be further treated (reduced, disposed of, incinerated, etc.) The coagulation/flocculation technique can be used for the removal of various components. The efficiency is determined by a large number of factors in the wastewater (e.g., type and concentration of flocculant and coagulant, final pH, intensity and duration of mixing). In general, an effective removal of 80–95% is seen for solid matter. The reduction of chemical oxygen demand (COD, that is, the measure of pollution that cannot be oxidized biologically), phosphorus and metals results usually in a yield of more than 60% (application dependent). In practice, the chemicals and dosages required depend to a large extent of the wastewater. Thorough lab research and/or pilot studies are recommended to set up the most appropriate procedure and to optimize the process.
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An advantage of the coagulation/flocculation technique is that certain specific pollutants can only be removed by this technique. It is however necessary to determine the correct combination of pollutant, flocculant, and coagulant. Only a limited investment is required for the tanks and dosage units. However, the operational costs of the technique are huge, especially when large amounts of coagulant and flocculant are needed to achieve the required level of flocculation. Also a considerable quantity of physicochemical sludge is formed, that in general needs to be processed externally by a third party. These costs are especially high when large volumes of wastewater need to be treated. Keeley et al. (2014) showed that by means of coagulant recovery (i.e., regeneration and purification), the amount of sludge can be reduced as well as the associated costs.
2.3.2 Membranes (nanofiltration) Nanofiltration (NF), characterized by a membrane with a pore size of approximately 1 nm, is situated between ultrafiltration (UF) and reverse osmosis (RO) with respect to the separation level. The sieve effect is based on the difference between the particle size and the pore size of the filter. A typical NF membrane lies in the range of 150–500 Da (weight in grams of mole of the molecule; i.e., the molecular weight of the smallest molecule that can be 90% restricted by the top layer of the membrane). NF membranes are characterized by the retention of charged and neutral particles tested by experimental filtration tests with preselected molecules (see Table 2.2). NF removes most organic molecules, viruses, and the natural organic matter as well as a range of salts. NF does not remove dissolved compounds. An NF membrane, being ion selective, is able to differentiate between various ions. As it collects charged groups, electrostatic repulsion or attraction forces take place between the liquid and the membrane surface. Not the pore size alone determines the diffusion of ions; the charge of the ion is also important. For that reason the retention of sulfates (being larger in size than chlorides, i.e. 0.23 nm vs. 0.12 nm) is over 90% whereas for chlorides the retention is at maximum 90%. NF can be used for the removal of dissolved matter, harmful microorganisms (e.g., bacteria, protozoa, algae, fungi), persistent organic matter and organic compounds. Table 2.2 Molecules used in filtration tests to characterize nanofiltration membranes Type of particles to retain
Preselected molecules
Charged particles
Simple salt solutions such as NaCl or Na2SO4 Polysaccharides (dextrines) or polyethylene glycols (PEG) with various molecular weights
Neutral particles
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Advantages of NF are as follows: the lower discharge volumes; lower retentate concentrations than RO for low value salts, reduced salt content, and dissolved matter content (TDS for total dissolved solids) in brackish water; reduction in color, tannins, and turbidity; chemical free (e.g., needs no salt or chemicals during operation); a generally nonaggressive pH of water after NF; operating at room temperature; and disinfecting. Disadvantages are the higher energy consumption (0.3–1 kWh/m3) compared to UF and microfiltration (MF); the need for pretreatment for some heavily polluted waters (prefiltration 0.1–20 μm). NF membranes are more expensive than RO membranes, and are sensitive to free chlorine (life span of 1000 ppm h) recommending an active carbon filter or a bi-sulfite treatment for high chlorine concentrations. After NF treatment a volume-reduced concentrated flow is obtained, requiring further treatment; the flow often needs to be processed externally.
2.3.3 Chemical oxidation This treatment involves the oxidation of organic pollutants and as such makes them less harmful. In the best-case scenario, complete oxidation of organic substances results, that is, only CO2 and H2O. The most active oxidant is the hydroxyl radical (OH°) that is able to attack organic compounds and cleave bonds. It can be generated by a combination of ozone (O3), hydrogen peroxide (H2O2), natriumhypochlorite (NaOCl), chlorine dioxide (ClO2), chlorine gas (Cl2), peroxy acetic acid (C2H4O3), pure oxygen (O2), and combinations thereof (Kalra et al., 2011). A detailed overview of the advanced oxidation processes and their mechanisms has been given by Rashed et al. (2005). This radical can also be formed by the activation of hydrogen peroxide with a catalyst (e.g., Fe2+; usually sulfate); known as the Fenton reaction. During this reaction, the ferrous ion is transformed into the ferric ion at a pH of 3 simultaneously producing OH° (Barclay and Buckley, 2000b). A disadvantage of the Fenton reaction is that it is very sensitive with respect to pH (Rashed et al., 2005). It is also seen that more sludge is produced when using the Fenton reaction. UV light in combination with a photocatalyst (e.g., TiO2) or other chemical agents (e.g., hypochlorite) is also used as an oxidizing method. Chemical oxidation is used for the removal of persistent organic substances (e.g., dioxins, pesticides, and biocides), organic compounds (e.g., biochemical oxygen demand [BOD] and chemical oxygen demand [COD], adsorbable organic halides [AOX], extractable organic halide [EOX], total organic carbon [TOC], total organic halides [TOX], benzene, toluene, ethylbenzene, and xylene [BTEX], monoaromatic hydrocarbons [MAHs], phenols, and polycyclic aromatic hydrocarbons [PAHs]), nutrients (nitrogen and organophosphorous compounds), and inorganic salts (e.g., CN−, S2− and SO32- ). Recalcitrant COD can be removed up to 100% independent of the input concentrations. In general, a good to excellent yield is obtained when using chemical oxidation. However, for each case the technology should be tested for suitability. To obtain the intended yield, prior treatment steps can be performed or the oxidant dosage can be increased. Chemical oxidation can be used as a pretreatment in combination with biological purification leading to the breakdown of components that are difficult to degrade. Another possibility is a partial oxidation of the sludge resulting in limited sludge production.
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The setup for chemical oxidation requires a buffer tank, an oxidation reactor, and a dosage unit for the oxidant and optionally an UV installation. The main advantages of the chemical oxidation technique (Le Marechal et al., 2012) are the fast reaction rate and the reduction of toxicity. The technique also allows for the treatment of multiple organics all at once, with no creation of sludge and no waste concentration. In addition, the process requires little space. When the wastewater is discharged, excessive amounts of oxidizing should be avoided. Overdosage must be avoided, as it could have a negative impact on subsequent biological purification. Most oxidants are not selective requiring a purification step (e.g., filtration) or leading only to partial oxidation.
2.3.4 Evapoconcentration: Evaporation via mechanical vapor recompression By means of evaporation, dissolved pollution is concentrated with the aim of obtaining distilled purified water from wastewater. In mechanical vapor recompression (MVR), the influent is inserted in the system, where it is distributed across heat elements and as a consequence is partly evaporated. This vapor is compressed by a compressor and is then transported to the inner surface of the heat element where it condenses and is collected. The concentrated wastewater is deposited onto the bottom of the device and is subsequently transported by the concentrate pump, after which the cycle starts all over. The technique is effective (ca. 99%), which is dependent on the influent and the type of pollution. By means of MVR, separation of the nonvolatile substances from the distillate flow is feasible. The high-quality purified water that results can be reused in the process or even discharged into surface water. Volatile compounds cannot be separated. As a consequence a separate (post)treatment is required. In some cases salts may be encountered in the distillate flow. The remaining residual flow contains large amounts of salts and nonvolatile compounds, which makes it impossible to discharge. The flow must be treated externally by a third party. It is possible to further concentrate and dry this flow. Considering the high investment of costs, this technique is feasible only for very low capacities. Corrosion is a potential problem at high salt concentrations. Concentrate (sludge) is released as a by-product. Zhou et al. (2014) investigated the performance of MVR using high-salinity wastewater containing Na2SO4. An optimum value of temperature difference leads to the optimum system with lower power consumption and a smaller heat transfer area.
2.3.5 Biological treatment (membrane bioreactor) A membrane bioreactor (MBR) combines membrane filtration with a biological active sludge system. The membrane can either be positioned outside (= external) or in the biological basin (= internal). Both MF and UF membranes can be used for MBR. In external systems a continuous cross-flow is circulating along the membranes. In internal systems the effluent is extracted from the active sludge using under-pressure.
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The membrane separates the sludge from the effluent and thus retains all floating matter, which avoids the need for sedimentation to concentrate the sludge. For this reason, MBR enables the processing of much higher sludge concentrations (10–20 g/L) and lower reactor volumes, compared to conventional systems. By using the MBR technology, organic compounds (COD and BOD), suspended matter, nutrients (nitrogen and phosphorus), and bioaccumulative or biodegradable micropollutants can be removed. The same biological conversion processes occur as the active sludge systems remove COD, nitrifying or nitrifying/denitrifying active sludge. The membranes ensure total retention of the biology as well as total removal of the suspended matter. The technique is better suited for the degradation of larger or hard-to-degrade compounds due to the longer presence of the bacteria in the reactor. The COD value is defined by the quality of the influent quality. In general a lower COD is seen in comparison to a conventional system. MBR is a compact process resulting in a high-quality effluent—in terms of suspended matter, bacteria, and COD—that can be discharged in vulnerable areas or used as process water in various applications. MBR is able to treat highly loaded effluents with a high rate of degradation. However, membrane fouling is limiting the widespread use of MBR. A high oxygen demand is needed because of the high sludge concentration and the high degradation rates. Because of the high mass and the high viscosity of the sludge mass, the transfer of oxygen is difficult. For that reason modern aeration systems with efficient oxygen transfer are required.
2.3.6 Powdered activated carbon treatment Hard-to-degrade and toxic compounds in wastewater can be adsorbed by using activated carbon. Powdered activated carbon (PAC) treatment is mainly used to remove persistent organic compounds (e.g., pesticides and biocides) and organic compounds (e.g., AOX, BTEX, MAH, and PAHs). In this PAC treatment, the biological degradation process occurs undisturbed and with changing influent compositions. PAC is dosed into the aeration tank; the optimum is determined via experiments. Because during the process the sludge quantities increase, an additional system for sludge treatment may be required. Since the retention time of the added activated carbon depends on the age of the sludge in the system, the PAC process is difficult to control. The presence of PAC improves sludge sedimentation. The more activated carbon that is added, the more sludge is produced. Care should be taken regarding dust formation. A disadvantage is the color (black sedimentation). The presence of activated carbon can influence the disposal possibilities for the excess sludge.
2.4 Regulations Various regulations are valid when dealing with fabric finishing processes. Some important directives are listed below. More information about these directives can be found via the links in the paragraph on “sources of further information and advice”, the references or in Chapter 1.
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The Water Framework Directive (European Union, 2010) aims at achieving a good ecological and chemical status of surface water by 2015. The Industrial Emissions Directive (IED) 2010/75/EC, minimizes the environmental impact of industrial sources. Permits provided to companies need to concern the whole environmental impact of the plant; they also need to be based on the best available techniques and are subject to mandatory inspections.
2.5 Future trends The general trend in fabric finishing is to use less energy and less water. The realization is not obvious, however, since it demands the replacement of often cheap and well-known processes. Such a change often imparts, especially at the start of the switch-over, quality differences, process instabilities, and the like. Regarding the processes used in fabric finishing, the water-free techniques will gain importance in the future. Currently, many of these techniques are still under development or are evaluated at the laboratory level. Once the feasibility has been proven (i.e., requirements regarding quality, properties achieved as well as substantially lower energy use) techniques such as hotmelt, UV-curable coating, digital finishing, and plasma coating will be implemented.
2.6 Conclusion Fabric finishing companies will face huge challenges in the future: they need to comply with all regulations and keep their activities economically feasible. To achieve this they need to decrease the energy and water consumption of their machinery. This implies the search for new technologies, efforts to define the correct process parameters of the new machinery, developments to obtain the properties required, training of employees, and more.
Acknowledgments The authors are indebted to IWT (Flemish Agency for Innovation by Science and Technology) and AO (Enterprise Flanders) for the grants in support of their work (IWT070660, IWT080725, IWT 095024, IWT110391, IWT120626, NIB.CALL.2012.044, IWT135101).
Sources of further information and advice Treatment of wastewater AquaFit4Use (2010) FP7 project, Sustainable water use in chemical, paper, textile and food industries. Water Quality Demands in Paper, Chemical, Food and Textile Companies. http:// www.aquafit4use.eu/.
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WASS (Water treatment Selection System), http://emis.vito.be/wass-waterzuiveringsselectiesysteem (accessed 01.12.14).
Waste minimization RESITEX, Life project (Life05 ENV/E/000285) http://www.fomentex.eu/resitex http://ec.europa.eu/environment/life/project/Projects/index.cfm?fuseaction=home. showFile&rep=file&fil=LIFE05_ENV_E_000285_LAYMAN.pdf.
Legislation Typically for Belgium-Flanders: Stroomgebiedbeheerplannen 2016-2021 http://www. volvanwater.be/ http://ec.europa.eu/environment/water/water-dangersub/lib_dang_substances.htm http://ec.europa.eu/environment/water/water-framework/index_en.html http://ec.europa.eu/environment/water/water-framework/priority_substances.htm http://www.eea.europa.eu/policy-documents/directive-2010-75-ec-on
References Anon, 2014. Coagulation and Flocculation—Process Fundamentals. http://www.uacg.bg/filebank/att_1846.pdf. (accessed 30.11.14). Barclay, S., Buckley, C., 2000a. Waste Minimisation Guide for the Textile Industry: A Step Towards Cleaner Production. Waste Minimisation Options for the Textile Industry, vol. 1, Appendix 1. Barclay, S., Buckley, C., 2000b. Waste Minimisation Guide for the Textile Industry: A Step Towards Cleaner Production. End-of-pipe Treatment Methods, vol. 1, Appendix 4. Bhosale, N., Pareek, V., Jadhav, B., Mujawar, S., 2013. Innovative Plasma Treatment in Textiles—Applications, Advantages and Surface Functionalization. http:// textilelearner.blogspot.be/2013/10/plasma-treatment-in-textiles.html?goback =.gde_1784801_member_5799026601677045760. Bratby, J., 2006. Coagulation and Flocculation: in Water and Wastewater Treatment. IWA Publishing, London, Seattle. Buyle, G., 2009. Nanoscale finishing of textiles via plasma treatment. Mater. Today 24, 46–51. Buyle, G., Heyse, P., Ferreira, I., 2010. Tuning the surface properties of textile materials. In: Rausher, H., Perucca, M., Buyle, G. (Eds.), Plasma Technology for Hyperfunctional Surfaces—Food, Biomedical & Textile Application. Wiley-VHC, Weinheim, pp. 135–182. Castelein, P., Paul, J., Van de Vyver, D., Vanneste, M., Godefroidt, F., 2011. Speeding up the development of coated textile products. Proceedings Radtec-Europe 11—18-20 October 2011, 1–7. Deleersnyder, K., 2013. FDM4TP—Potential of Fused Deposition Modelling in Textiles and Plastics. http://www.centexbel.be/projects/fdm4tp-0 (accessed 01.12.14). Derden, A., Vanassche, S., Huybrechts, D., 2011. Best Available Techniques (BAT) for the textile industry—reducing emission of some micro-pollutants via waste water. http://emis.vito.be/ sites/emis.vito.be/files/pages/1142/2014/BAT_for_the_textile_industry_full_version.pdf (accessed 01.12.14).
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Dura, J.V., Caprara, G., Cavallaro, M., Ballarino, A., Kaiser, C., Stellmach, D., 2014a. New technologies for the flexible and eco-efficient production of customized products for people with special necessities. Paper presented at 20th ICE Conference—IEEE TMC Europe Conference, 23–25 June 2014, Bergamo, Italy. Dura, J.V., Cavallaro, A., Chiodi, M., Ballarino, A., Brondi, A., Arnim, V. and Stellmach D. A., 2014b. New technologies for customizing products for people with special necessities. Project FASHIONABLE. IJCIM, accepted. European Union, 2010. Water is for life: ‘How the Water Framework Directive helps safeguard Europe’s resources’. Publications Office of the European Union, Luxembourg. http:// www.eurosfaire.prd.fr/7pc/doc/1301470143_kh3109164enc_002.pdf (accessed 01.12.14). Fung, W., 2002. Coated and Laminated Textiles. Woodhead Publishing in Association with the Textile Institute, Cambridge. Goethals, F., Eufinger, K., De Vrieze, S., Vanneste, M., Schumann, A., Wypkema, A., 2014. Eco-efficient hybrid coatings for durable textile applications by UV curing—Hybritex. Unitex 1, 4–6. Kaiser, C., Fischer, T.V., Schmeltzpfenning, T., Stöhr, M., Artschwager, A., 2014. Case study: mass customization of individualized orthotics—the FASHION-ABLE Virtual Development and Production Framework. Paper presented at the 24th CIRP Design Conference, 14–16 April, 2014, Milano, Italy. Procedia CIRP 21, 105–110. Doi: 10.1016/j. procir.2014.03.189. Available at: http://ac.els-cdn.com/S2212827114007550/1-s2.0S2212827114007550-main.pdf?_tid=37375066-e069-11e4-87a1-00000aab0f6c&acdnat= 1428770503_a183fdaaf7a428dbe58ee33c26e9e2c8. Kalra, S.S., Mohan, S., Sinha, A., Singh, G., 2011. Advance oxidation processes for treatment of textile and dye wastewater: a review. In: 2nd International Conference on Environmental Science and Development, IPCBEE, 4, 271–275. Keeley, J., Jarvis, P., Judd, S.J., 2014. Coagulant recovery from water treatment residuals: a review of applicable technologies. Crit. Rev. Environ. Sci. Technol. 44 (2014), 2675–2719. Le Marechal, A.M., Križanec, B., Vajnhandl, S., Valh, J.V., 2012. Textile finishing industry as an important source of organic pollutants. In: Puzyn, T., Mastrag-Szlichtyng, A. (Eds.), Textile Finishing Industry as an Important Source of Organic Pollutants, Organic Pollutants Ten Years After the Stockholm Convention—Environmental and Analytical Update. InTech, ISBN: 978-953-307-917-2. Available from: http://www.intechopen.com/ books/organic-pollutants-ten-years-after-the-stockholm-convention-environmental-and- analytical-update/textile-finishing-industry-as-an-important-source-of-organic-pollutants (accessed 01.12.14). Nierstrasz, V., 2012. Surface modification and functionalisation of textile materials: Digital finishing for functional and smart textile materials. In: EMRS Fall Meeting 2012, Warsaw, Symposium B: Renewable Polymers as Multifunctional Materials: Properties, Processing, Applications. Periyasamy, S., Gulrajani, M.L., Gupta, D., 2007. Preparation of a multifunctional mulberry silk fabric having hydrophobic and hydrophilic surfaces using VUV excimer lamp. Surf. Coat. Technol. 201, 7286–7291. Rahman, M. and Nur, M.G., 2014. Feasible application of modern eco-friendly treatment of wool fabric before coloration. Int. J. Sci. Res. Publ., 4, 7, available from http://www.ijsrp. org/research-paper-0714/ijsrp-p3141.pdf (accessed 10.01.15). Rashed, I.G., Hanna, M.A., El-Gamal, H.F., Al-Sarawy, A.A., Wali, F.K.M., 2005. Overview on chemical oxidation technology in wastewater treatment. Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt, 115–132.
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Rogge, E., 2013. PFOA- and PFOS-free water- and oil repellent low pressure plasma nanocoatings for personal protective equipment. Paper presented at the 5th ETSA Forum on Workwear and Protective Clothing, 26 November 2013 from 9:30 to 16:00, Cologne, Germany. Samanta, K.K., Saxena, S., Aeputharaj, A., 2013. Value-addition to cotton textiles using water-free plasma technology. ICAR News: A Sci. Technol. Newsl. 19, 23–25. Samanta, K.K., Basak, S., Chattopadhyay, S.K., 2014. Environment-Friendly Textile Processing Using Plasma and UV Treatment. In: Muthu S.S., Roadmap to Sustainable Textiles and Clothing—Ecofriendly Raw Materials, Technologies and Processing Methods. Hong Kong, Springer, 161–202. Sen, A.K., 2008. Coated Textiles: Principles and Applications. CRC press, Florida. Shahidi, S., 2015. Plasma sputtering as a novel method for improving fastness and antibacterial properties of dyed cotton fabrics. J. Text. Inst. 106, 162–172. Shahidi, S., Rashidi, A., Ghoranneviss, M., Anvari, A., Rahimi, M.K., Bameni Moghaddam, M., Wiener, J., 2010. Investigation of metal absorption and antibacterial activity on cotton fabric modified by low temperature plasma. Cellulose 17, 627–634. Shaid, A., Osman, S., Hannan, A., Bhuiyan, M.A.R., 2013. Direct reusing of textile wastewater in scouring-bleaching of cotton goods devoid of any treatment. Int. J. Eng. Res. Dev. 5, 45–54. Singha, K., 2012. A review on coating & lamination in textiles: processes and applications. Am. J. Polym. Sci. 2, 39–49. Smith, B., 1988. A Workbook for Pollution Prevention by Source Reduction in Textile Wet Processing. http://www.p2pays.org/ref/02/01091.pdf (accessed 01.12.14). Vajnhandl, S., Valh, J.V., 2014. The status of water reuse in European textile sector-review. J. Environ. Manag. 141, 29–35. Van Parys, M., 2013. Change the course: from wet to dry technologies using digital coating/ finishing technologies. Unitex 2, 6–9. Zhou, Y., Shi, C., Dong, G., 2014. Analysis of a mechanical vapor recompression wastewater distillation system. Desalination 353, 91–97.
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D. Weydts, D. De Smet, M. Vanneste Centexbel, Textile Competence Centre, Ghent, Belgium
2.1 Introduction Conventional fabric finishing processes require huge amounts of energy and water. New finishing processes are developed using limited amount or no water and reducing the energy consumption substantially. While some of them are still on a laboratory level, other are about to be used at industrial scale. In this chapter, an overview will be given of dry finishing techniques with potential to be implemented in textile finishing. Finishing companies need to deal with water discharge and its chemical load, energy consumption, air and water emissions, waste, odors, and noise. Waste management should focus primarily on prevention, secondly on reuse, recycling, and revaluation and only if those options fail go to external treatment of waste. Regarding emissions to water, the final effluent is often composed of streams coming from different processes that are mixed together resulting in a complex effluent depending on the type of fiber, the process, and the functionalities applied (i.e., the type of chemicals and auxiliaries used). The reduction of emissions to water is driven by the Water Framework Directive that aims at protecting and restoring the water quality in Europe. It established a precise timetable, with 2015 as the target date for getting all EU waters into good condition. To avoid water emission and its chemical load and to reduce energy and water consumption, and waste production more environmental friendly and water-free processes such as hotmelt, plasma coating, digital finishing, and radiation-curing are developed for fabric finishing.
2.2 Waste minimization in fabric finishing Minimization of waste can be realized by the reduction of water, chemical, and energy consumption, as well as by reducing solid waste, air and noise pollution, and by minimizing the emission of toxic substances (Barclay and Buckley, 2000a). Waste minimization requires good management of environmental practices, taking into account the following: ●
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Correct choice of chemicals, that is, substitution of hazardous substances by reusable, biobased, and/or biodegradable materials. This part has been dealt with in Chapter 1. New technologies and processes to diminish the impact of fabric finishing on the environment (in particular on the use of energy and water) have been investigated, of which some have proven their usefulness in other sectors.
Sustainable Apparel. http://dx.doi.org/10.1016/B978-1-78242-339-3.00002-9 Copyright © 2015 Elsevier Ltd. All rights reserved.
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Source reduction and minimization of used chemicals. Effluent segregation and reuse of chemicals.
2.2.1 Energy-efficient and water-saving finishing processes For many years, research at Centexbel on textile functionalization and surface modification focused on energy-efficient and water-free finishing technologies. As some of these techniques are established in other applications, such as the coating of wood, paper, and plastics, they need to be tuned to textiles processes. In addition, adapted formulations need to be developed to maintain textile flexibility and haptics. The overall advantage of these techniques is the absence of water eliminating the long, energy-consuming oven and, as a consequence, reducing the floor space needed for the machines. Another advantage is the possibility to produce short runs since a quick start and stop of the machine is possible. The hotmelt process for textiles (Fung, 2002; Sen, 2008) was primarily used in lamination applications (e.g., glue in collars of men’s shirts). Later on it was seen that hotmelts could also be used as a coating. In a hotmelt process, the material used is a polymer that is molten and put onto the textile as a coating layer. The hotmelt polymer material can vary from granules to blocks. By means of functionalized hotmelt, the textiles can be made flame retardant, antimicrobial, hydrophobic, and more. Various hotmelt application systems exist such as slot-die, spray, gravure roller, multiroller, and rotary screen. Depending on the type of applicator, continuous or discontinuous layers can be applied—the latter being suitable to impart breathable and melting temperature properties. A similar process is extrusion coating (Singha, 2012), where an extruder is coupled with the (hotmelt) application machine. In general, the polymer materials used in extrusion coating have a much higher viscosity compared to the hotmelt process. In radiation curing processes, using ultraviolet (UV), light-emitting diode (LED), or electronbeam (EB) processes, water-based as well as 100% coatings exist. A substantial reduction in energy is obtained when 100% formulations are used. Other advantages of this technique are the space savings, and the ability to integrate the radiation curing machinery into the existing coating line and reduced run lengths. Initially, the textiles coated by means of radiation curables were hard (as they were developed for hard surfaces), and they lost their typical textile feeling and touch. However, thorough investigation has led to an enormous improvement in acceptable properties. By means of this technique, various functionalities even using conventional additives can be implemented in textiles such as abrasion resistance (Goethals et al., 2014), flame retardancy, and hydrophobicity (Castelein et al., 2011). Digital finishing is an emerging finishing technique in which the digital printers normally used for (local) coloration of the textile are now used for finishing. The advantage of the technique is the possibility to do this locally, resulting in a cost-effective process (Nierstrasz, 2012) with a limited use of chemicals, low amounts of solvent (UV-curable 100% inks are also available), and using less energy compared to the conventional finishing of an entire textile. Similar to digital printing, the production flexibility is an advantage, for instance, it is easier to produce small runs (Van Parys, 2013). This technique also shows great potential for mass customization of clothing
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(Kaiser et al., 2014), specifically for persons with special needs. Digital functionalization via a magnetic valve system was compared with other application techniques (e.g., airless spraying with rotating plate, air-driven jet-spraying, and three-dimensional spray finishing). Some advantages reported in comparison to the other techniques are (Dura et al., 2014a,b) the high resolution, the possibility to realize multifunctional textiles using multiple jets, no failures due to overlap of spray cones, no need for air, the flexibility of the system, and the simple exploitation. Three-dimensional printing or additive manufacturing (AM) techniques generally focus on the production of stand-alone, individual objects often for niche markets such as prostheses for medical applications. Recently, Deleersnyder (2013) started to use this technique for textile coating and even the production of accessories for apparel (see Figure 2.1) and other textiles. Silk fabrics have been made hydrophilic using UV irradiation (Periyasamy et al., 2007). Hydrophilic and oil absorption properties have been generated in wool fabrics by use of a UV treatment (Samanta et al., 2014). Plasma technologies have been investigated for quite some time with the aim of improving wettability and thus the adhesion of coatings. However, a much broader range of applications is feasible with plasma (Buyle, 2009): these applications include the improvement of printability and dyeability (Rahman and Nur, 2014), and antishrink treatment of wool, the scouring of wool (Rahman and Nur, 2014), sterilization, and the desizing of cotton. Plasma coating, that is, the deposition of a nanolayer of functionalities on textiles, has been investigated thoroughly by means of low-pressure plasma, atmospheric plasma, or plasma sputtering. Using these techniques various functionalities can be implemented into the textile (Buyle et al., 2010). Durable, PFOA- and PFOS-free, water- and oil-repellent protective clothing have been produced using low-pressure plasma nanocoatings of C6 fluorine chemistry (Rogge, 2013). Atmospheric pressure plasma with fluorocarbon has been used to impart hydrophobic properties in cotton
Figure 2.1 PLA button deposited on polyester knitting by fused deposition modeling.
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(Samanta et al., 2013). Hydrophilic, hydrophobic, and dirt-repellent coatings using different plasma technologies were reported by Bhosale et al. (2013). Antibacterial activity was obtained by plasma sputtering of silver and copper on dyed cotton (Shahidi et al., 2010; Shahidi, 2015). At the same time, an improvement in the fastness properties was observed. As both radiation-curing and plasma affect only the outer surface of the fabric, only a minimum of chemicals are needed. The cost is low because of a shorter processing time and (partial) reduction of effluent treatment (Samanta et al., 2014). Both techniques have been compared using conventional methods regarding the process medium, water and energy requirement, time required, consumption of chemicals, and cost.
2.3 Wastewater treatment and management Even if care is taken to minimize waste, there will still be waste to treat. Currently used wet finishing processes produce waste containing organic as well as inorganic compounds. The effluents are rich in chemicals of which some are persistent or resistant to water treatment methods. Table 2.1 lists typical examples of finishing waste that resists biodegradation. Removal of these substances from wastewater is expensive and difficult to achieve. Because of this, the effluent segregation and source reduction methods are preferred as economically attractive alternatives. Before starting an end-of-pipe treatment, effluent segregation is required; this separates contaminated fluid from the cleaner streams. The final wastewater is less voluminous and can be treated more effectively and by more appropriate treatment methods, whereas the more clean stream can be reused in the factory with limited or no treatment needed. Vajnhandl and Valh (2014) reviewed the wastewater reuse programs used over the last 10 years covering the European textile finishing industry, with special emphasis on the FP7 project Aquafit4use. The water treatment methodologies presented enable more than a 40% reduction in freshwater consumption. Table 2.1 Typical substances present in wastewater from wet finishing processes Finishing process
Substances in waste water
Easy-care finishes
Urea and melamine derivatives (noncross-linked) Organo-phosphorous compounds, polybrominated compounds Alkylphosphates, alkyletherphosphonates Fluorochemicals Organic material such as phenols: alkylphenol (AP), alkylphenolethoxylates (APEOs)
Flame-retardant finishes Antistatic finishes Water-repellent finishes Surfactants
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Shaid et al. (2013) suggested that the finishing companies define the quality level of the rinsing waters needed to be able to reuse the waste streams, in some cases even without any treatment. For waste that is widely dispersed and hard to treat when it is discharged, source reduction is an appropriate way to deal with it. Such waste can be captured or the amount can be reduced by modifying the process or the machinery design, choosing a chemical substitution, or remediating the existing procedures, and so forth (Smith, 1988). Concentrated residues (i.e., not dispersed) are in general easier to remove as waste than via wastewater treatment. In some cases they can be reused in the textile finishing process. For chemicals being persistent, bio-accumulative, toxic, and/or highly aquatic toxic, it is strongly recommended to consider chemical substitution. Depending on the type of chemical, the collection of concentrated residues may not be sufficient; separate collection of the (less concentrated) rinsing waters may be needed (Derden et al., 2011). This is the case for chemicals for which very low environmental quality standards are issued, usually in combination with the classification as priority hazardous substances within the Water Frame Directive (see information to come). Examples are brominated flame retardants, fluorosurfactants, nonylphenols, and nonylphenolethoxylates for which concentrated residues and rinsing waters are preferably reused in the process. If this is not feasible, they need to be treated as waste by a third party having the necessary permits to do so. Once effluent segregation, reuse, and/or the substitution of chemicals have been considered, one of the following water treatment techniques (or a combination thereof) can be applied to the remaining wastewater flows: coagulation/flocculation, nanofiltration, chemical oxidation, mechanical vapor recompression, membrane bioreactor, or powered-activated carbon treatment. Barclay and Buckley (2000b) provide an explanation of these techniques together with their pros and cons. In Belgium, the BAT center of Vito developed a methodology to choose the appropriate combination of techniques for a specific wastewater issue.
2.3.1 Coagulation and/or flocculation Colloidal or suspended particles are charged and stable in water (they do not precipitate). One must take into account their variation in size, shape, source, and density when applying coagulation and flocculation processes (e.g., correct choice of coagulants and flocculants). Coagulation aims to destabilize the charged particles present in the wastewater leading to aggregation and the formation of floccules. To achieve this, the addition of a coagulant (with a charge opposite of the particles) is required, reducing the repellency between and thus stabilizing the colloidal particles. Commonly used coagulants are inorganic (Bratby, 2006; Anon, 2014) and are based on aluminum (i.e., aluminum sulfate, aluminum chloride, sodium aluminates), iron (i.e., ferric sulfate, ferrous sulfate, ferric chloride, ferric chloride sulfate), hydrated lime, or magnesium carbonate. They are often the cheapest, most widely available, and effective coagulants.
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Prepolymerized coagulants based on aluminum and iron are used as they have the advantage of working over a much wider range of pH and water temperature. In addition, less dosage is needed and fewer residuals (chloride, sulfate, or metal). Examples are aluminum chlorohydrate, polyaluminum chloride (PAC), polyaluminum sulfate chloride, and polyferric sulfate. The coagulant is added, meanwhile intensively mixing using a short retention time (typically 1–3 min), and generating small floccules surrounded by clear water. The mixing velocity and time are critical parameters. To enable them to grow, gentle stirring is needed (to not destroy the floccules) with a longer retention time (15–20 min), allowing the particles to further cluster. Flocculants (promoting flake formation) can be added to stimulate this process. They are high-molecular polymers functionalized with various groups. The small floccules are attracted to this polymer because of the charges present. This results in a larger floccule. Polymeric coagulants aids or flocculants consist of a broad range of different compounds and are effective over a wider range of pH than the inorganic coagulants. In general they are more expensive than the inorganic coagulants. Anionic polymeric coagulants are often used in combination with the metal coagulants. Cationic ones can be used alone or with iron of aluminum-based coagulants. Natural as well as synthetic polymers are used; the natural polymers are in general nontoxic and biodegradable. The synthetic ones, however, are more widespread since their effectiveness can be largely tuned during manufacturing. As mentioned above, the correct dosing and sequence of the addition of the chemicals (coagulant and flocculant) are critical for the process to work correctly. This is not straightforward with wastewater that is strongly varying in composition. Depending on the type of chemical used the pH will change. For that reason buffering the wastewater (i.e., pH correction) is used. The floccules can then be easily separated via flotation or sedimentation. The colloidal particles formed do not all have the same charge. For that reason the polymer needs to have various charges (anionic, cationic, or nonionic) combined with good distribution thereof over the whole structure to achieve an effective flocculation. The molecular length and the level of (self) cross-linking are also important parameters. Due to the variation of all these parameters a few hundred different polymers result, each with their specific level of effectiveness. The floccules are collected and form a polluted sludge to be further treated (reduced, disposed of, incinerated, etc.) The coagulation/flocculation technique can be used for the removal of various components. The efficiency is determined by a large number of factors in the wastewater (e.g., type and concentration of flocculant and coagulant, final pH, intensity and duration of mixing). In general, an effective removal of 80–95% is seen for solid matter. The reduction of chemical oxygen demand (COD, that is, the measure of pollution that cannot be oxidized biologically), phosphorus and metals results usually in a yield of more than 60% (application dependent). In practice, the chemicals and dosages required depend to a large extent of the wastewater. Thorough lab research and/or pilot studies are recommended to set up the most appropriate procedure and to optimize the process.
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An advantage of the coagulation/flocculation technique is that certain specific pollutants can only be removed by this technique. It is however necessary to determine the correct combination of pollutant, flocculant, and coagulant. Only a limited investment is required for the tanks and dosage units. However, the operational costs of the technique are huge, especially when large amounts of coagulant and flocculant are needed to achieve the required level of flocculation. Also a considerable quantity of physicochemical sludge is formed, that in general needs to be processed externally by a third party. These costs are especially high when large volumes of wastewater need to be treated. Keeley et al. (2014) showed that by means of coagulant recovery (i.e., regeneration and purification), the amount of sludge can be reduced as well as the associated costs.
2.3.2 Membranes (nanofiltration) Nanofiltration (NF), characterized by a membrane with a pore size of approximately 1 nm, is situated between ultrafiltration (UF) and reverse osmosis (RO) with respect to the separation level. The sieve effect is based on the difference between the particle size and the pore size of the filter. A typical NF membrane lies in the range of 150–500 Da (weight in grams of mole of the molecule; i.e., the molecular weight of the smallest molecule that can be 90% restricted by the top layer of the membrane). NF membranes are characterized by the retention of charged and neutral particles tested by experimental filtration tests with preselected molecules (see Table 2.2). NF removes most organic molecules, viruses, and the natural organic matter as well as a range of salts. NF does not remove dissolved compounds. An NF membrane, being ion selective, is able to differentiate between various ions. As it collects charged groups, electrostatic repulsion or attraction forces take place between the liquid and the membrane surface. Not the pore size alone determines the diffusion of ions; the charge of the ion is also important. For that reason the retention of sulfates (being larger in size than chlorides, i.e. 0.23 nm vs. 0.12 nm) is over 90% whereas for chlorides the retention is at maximum 90%. NF can be used for the removal of dissolved matter, harmful microorganisms (e.g., bacteria, protozoa, algae, fungi), persistent organic matter and organic compounds. Table 2.2 Molecules used in filtration tests to characterize nanofiltration membranes Type of particles to retain
Preselected molecules
Charged particles
Simple salt solutions such as NaCl or Na2SO4 Polysaccharides (dextrines) or polyethylene glycols (PEG) with various molecular weights
Neutral particles
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Advantages of NF are as follows: the lower discharge volumes; lower retentate concentrations than RO for low value salts, reduced salt content, and dissolved matter content (TDS for total dissolved solids) in brackish water; reduction in color, tannins, and turbidity; chemical free (e.g., needs no salt or chemicals during operation); a generally nonaggressive pH of water after NF; operating at room temperature; and disinfecting. Disadvantages are the higher energy consumption (0.3–1 kWh/m3) compared to UF and microfiltration (MF); the need for pretreatment for some heavily polluted waters (prefiltration 0.1–20 μm). NF membranes are more expensive than RO membranes, and are sensitive to free chlorine (life span of 1000 ppm h) recommending an active carbon filter or a bi-sulfite treatment for high chlorine concentrations. After NF treatment a volume-reduced concentrated flow is obtained, requiring further treatment; the flow often needs to be processed externally.
2.3.3 Chemical oxidation This treatment involves the oxidation of organic pollutants and as such makes them less harmful. In the best-case scenario, complete oxidation of organic substances results, that is, only CO2 and H2O. The most active oxidant is the hydroxyl radical (OH°) that is able to attack organic compounds and cleave bonds. It can be generated by a combination of ozone (O3), hydrogen peroxide (H2O2), natriumhypochlorite (NaOCl), chlorine dioxide (ClO2), chlorine gas (Cl2), peroxy acetic acid (C2H4O3), pure oxygen (O2), and combinations thereof (Kalra et al., 2011). A detailed overview of the advanced oxidation processes and their mechanisms has been given by Rashed et al. (2005). This radical can also be formed by the activation of hydrogen peroxide with a catalyst (e.g., Fe2+; usually sulfate); known as the Fenton reaction. During this reaction, the ferrous ion is transformed into the ferric ion at a pH of 3 simultaneously producing OH° (Barclay and Buckley, 2000b). A disadvantage of the Fenton reaction is that it is very sensitive with respect to pH (Rashed et al., 2005). It is also seen that more sludge is produced when using the Fenton reaction. UV light in combination with a photocatalyst (e.g., TiO2) or other chemical agents (e.g., hypochlorite) is also used as an oxidizing method. Chemical oxidation is used for the removal of persistent organic substances (e.g., dioxins, pesticides, and biocides), organic compounds (e.g., biochemical oxygen demand [BOD] and chemical oxygen demand [COD], adsorbable organic halides [AOX], extractable organic halide [EOX], total organic carbon [TOC], total organic halides [TOX], benzene, toluene, ethylbenzene, and xylene [BTEX], monoaromatic hydrocarbons [MAHs], phenols, and polycyclic aromatic hydrocarbons [PAHs]), nutrients (nitrogen and organophosphorous compounds), and inorganic salts (e.g., CN−, S2− and SO32- ). Recalcitrant COD can be removed up to 100% independent of the input concentrations. In general, a good to excellent yield is obtained when using chemical oxidation. However, for each case the technology should be tested for suitability. To obtain the intended yield, prior treatment steps can be performed or the oxidant dosage can be increased. Chemical oxidation can be used as a pretreatment in combination with biological purification leading to the breakdown of components that are difficult to degrade. Another possibility is a partial oxidation of the sludge resulting in limited sludge production.
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The setup for chemical oxidation requires a buffer tank, an oxidation reactor, and a dosage unit for the oxidant and optionally an UV installation. The main advantages of the chemical oxidation technique (Le Marechal et al., 2012) are the fast reaction rate and the reduction of toxicity. The technique also allows for the treatment of multiple organics all at once, with no creation of sludge and no waste concentration. In addition, the process requires little space. When the wastewater is discharged, excessive amounts of oxidizing should be avoided. Overdosage must be avoided, as it could have a negative impact on subsequent biological purification. Most oxidants are not selective requiring a purification step (e.g., filtration) or leading only to partial oxidation.
2.3.4 Evapoconcentration: Evaporation via mechanical vapor recompression By means of evaporation, dissolved pollution is concentrated with the aim of obtaining distilled purified water from wastewater. In mechanical vapor recompression (MVR), the influent is inserted in the system, where it is distributed across heat elements and as a consequence is partly evaporated. This vapor is compressed by a compressor and is then transported to the inner surface of the heat element where it condenses and is collected. The concentrated wastewater is deposited onto the bottom of the device and is subsequently transported by the concentrate pump, after which the cycle starts all over. The technique is effective (ca. 99%), which is dependent on the influent and the type of pollution. By means of MVR, separation of the nonvolatile substances from the distillate flow is feasible. The high-quality purified water that results can be reused in the process or even discharged into surface water. Volatile compounds cannot be separated. As a consequence a separate (post)treatment is required. In some cases salts may be encountered in the distillate flow. The remaining residual flow contains large amounts of salts and nonvolatile compounds, which makes it impossible to discharge. The flow must be treated externally by a third party. It is possible to further concentrate and dry this flow. Considering the high investment of costs, this technique is feasible only for very low capacities. Corrosion is a potential problem at high salt concentrations. Concentrate (sludge) is released as a by-product. Zhou et al. (2014) investigated the performance of MVR using high-salinity wastewater containing Na2SO4. An optimum value of temperature difference leads to the optimum system with lower power consumption and a smaller heat transfer area.
2.3.5 Biological treatment (membrane bioreactor) A membrane bioreactor (MBR) combines membrane filtration with a biological active sludge system. The membrane can either be positioned outside (= external) or in the biological basin (= internal). Both MF and UF membranes can be used for MBR. In external systems a continuous cross-flow is circulating along the membranes. In internal systems the effluent is extracted from the active sludge using under-pressure.
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The membrane separates the sludge from the effluent and thus retains all floating matter, which avoids the need for sedimentation to concentrate the sludge. For this reason, MBR enables the processing of much higher sludge concentrations (10–20 g/L) and lower reactor volumes, compared to conventional systems. By using the MBR technology, organic compounds (COD and BOD), suspended matter, nutrients (nitrogen and phosphorus), and bioaccumulative or biodegradable micropollutants can be removed. The same biological conversion processes occur as the active sludge systems remove COD, nitrifying or nitrifying/denitrifying active sludge. The membranes ensure total retention of the biology as well as total removal of the suspended matter. The technique is better suited for the degradation of larger or hard-to-degrade compounds due to the longer presence of the bacteria in the reactor. The COD value is defined by the quality of the influent quality. In general a lower COD is seen in comparison to a conventional system. MBR is a compact process resulting in a high-quality effluent—in terms of suspended matter, bacteria, and COD—that can be discharged in vulnerable areas or used as process water in various applications. MBR is able to treat highly loaded effluents with a high rate of degradation. However, membrane fouling is limiting the widespread use of MBR. A high oxygen demand is needed because of the high sludge concentration and the high degradation rates. Because of the high mass and the high viscosity of the sludge mass, the transfer of oxygen is difficult. For that reason modern aeration systems with efficient oxygen transfer are required.
2.3.6 Powdered activated carbon treatment Hard-to-degrade and toxic compounds in wastewater can be adsorbed by using activated carbon. Powdered activated carbon (PAC) treatment is mainly used to remove persistent organic compounds (e.g., pesticides and biocides) and organic compounds (e.g., AOX, BTEX, MAH, and PAHs). In this PAC treatment, the biological degradation process occurs undisturbed and with changing influent compositions. PAC is dosed into the aeration tank; the optimum is determined via experiments. Because during the process the sludge quantities increase, an additional system for sludge treatment may be required. Since the retention time of the added activated carbon depends on the age of the sludge in the system, the PAC process is difficult to control. The presence of PAC improves sludge sedimentation. The more activated carbon that is added, the more sludge is produced. Care should be taken regarding dust formation. A disadvantage is the color (black sedimentation). The presence of activated carbon can influence the disposal possibilities for the excess sludge.
2.4 Regulations Various regulations are valid when dealing with fabric finishing processes. Some important directives are listed below. More information about these directives can be found via the links in the paragraph on “sources of further information and advice”, the references or in Chapter 1.
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The Water Framework Directive (European Union, 2010) aims at achieving a good ecological and chemical status of surface water by 2015. The Industrial Emissions Directive (IED) 2010/75/EC, minimizes the environmental impact of industrial sources. Permits provided to companies need to concern the whole environmental impact of the plant; they also need to be based on the best available techniques and are subject to mandatory inspections.
2.5 Future trends The general trend in fabric finishing is to use less energy and less water. The realization is not obvious, however, since it demands the replacement of often cheap and well-known processes. Such a change often imparts, especially at the start of the switch-over, quality differences, process instabilities, and the like. Regarding the processes used in fabric finishing, the water-free techniques will gain importance in the future. Currently, many of these techniques are still under development or are evaluated at the laboratory level. Once the feasibility has been proven (i.e., requirements regarding quality, properties achieved as well as substantially lower energy use) techniques such as hotmelt, UV-curable coating, digital finishing, and plasma coating will be implemented.
2.6 Conclusion Fabric finishing companies will face huge challenges in the future: they need to comply with all regulations and keep their activities economically feasible. To achieve this they need to decrease the energy and water consumption of their machinery. This implies the search for new technologies, efforts to define the correct process parameters of the new machinery, developments to obtain the properties required, training of employees, and more.
Acknowledgments The authors are indebted to IWT (Flemish Agency for Innovation by Science and Technology) and AO (Enterprise Flanders) for the grants in support of their work (IWT070660, IWT080725, IWT 095024, IWT110391, IWT120626, NIB.CALL.2012.044, IWT135101).
Sources of further information and advice Treatment of wastewater AquaFit4Use (2010) FP7 project, Sustainable water use in chemical, paper, textile and food industries. Water Quality Demands in Paper, Chemical, Food and Textile Companies. http:// www.aquafit4use.eu/.
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WASS (Water treatment Selection System), http://emis.vito.be/wass-waterzuiveringsselectiesysteem (accessed 01.12.14).
Waste minimization RESITEX, Life project (Life05 ENV/E/000285) http://www.fomentex.eu/resitex http://ec.europa.eu/environment/life/project/Projects/index.cfm?fuseaction=home. showFile&rep=file&fil=LIFE05_ENV_E_000285_LAYMAN.pdf.
Legislation Typically for Belgium-Flanders: Stroomgebiedbeheerplannen 2016-2021 http://www. volvanwater.be/ http://ec.europa.eu/environment/water/water-dangersub/lib_dang_substances.htm http://ec.europa.eu/environment/water/water-framework/index_en.html http://ec.europa.eu/environment/water/water-framework/priority_substances.htm http://www.eea.europa.eu/policy-documents/directive-2010-75-ec-on
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