«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Optimising water consumption in textile operations starts with controlling water consumption levels. The next step is reducing water consumption, through a number of often-complementary actions. These include improving working practices, reducing liquor ratio in batch processing, increasing washing efficiency, combining processes (e.g. scouring and desizing) and reusing/recycling water. Most of these measures allow significant savings not only in water consumption, but also in energy consumption because energy is used to a great extent to heat up the process baths. Other techniques are specifically focused on optimising the use of energy (e.g. heat-insulation of pipes, valves, tanks and machines, segregation of hot and cold waste water streams and recovery of heat from the hot stream).
Quality management of incoming fibre Information about textile raw materials is the first step to tackle pollution carried over from upstream processes. Information from the supplier should include not only the technical characteristics of the textile substrate, but also the type and amount of preparation agents and sizing agents, residual monomers, metals, biocides (e.g. ectoparasiticides for wool) present on the fibre. Various techniques are available that can significantly reduce the environmental impact originating from upstream processes.
As for pesticides residues on raw wool fibre, a number of organisations maintain information on the pesticides content of greasy and scoured wool. Manufacturers can use this information to minimise at source any legally used pesticides such as OP and SP ectoparasiticides, and to avoid processing wool contaminated with the most hazardous chemicals, such as OC pesticides, unless an analytical certificate is provided. In the absence of information, samples should be assayed to confirm their pesticide content, but this option entails higher costs for the manufacturer. Currently co-operation programmes between trade associations and leading grower countries have resulted in a progressive reduction of the average OP and SP residues on wool, along with the development of low-residue certification schemes.
Improvements are also possible for auxiliaries, such as, preparation agents, spinning lubricants and knitting oils. Substitutes for mineral oils are now available for most applications.
Alternative compounds have a high level of biodegradability or at least bioeliminability; they are also less volatile and more thermally stable than mineral oils. This helps to reduce odour nuisance and air emissions, which can occur when the substrate is submitted to hightemperature treatments such as thermofixation.
iv Textiles Industry Executive Summary The combination of low add-on techniques such as pre-wetting of the warp yarns or compact spinning, with the targeted selection of sizing agents helps to reduce the environmental impact of the desizing process. It is now accepted that readily biodegradable or bioeliminable compounds are available, covering all needs. Moreover, latest generation-polyacrylates are highly efficient with lower add-on and can be completely and easily removed from the fabric.
In general, integrated mills have the means to control the source of their raw material and chemicals applied on the fibre. For non-integrated companies (particularly for commission companies), it is more difficult to influence the up-stream suppliers. Conventional formulations are typically cheaper. Raw material suppliers (e.g. spinning, knitting mills) look mainly at the economic aspects and at the performance of the given substance in their own process, rather than at the environmental problems produced in the downstream processes (at the finishing mill). In these cases it is necessary to work with clients to eliminate these materials from the supply chain.
Selection and substitution of chemicals used A number of schemes for ecotoxicological assessment and classifications of chemicals have been proposed by the TWG for consideration in the determination of BAT. Based on these tools, substitution of the harmful substances is often an available option to reduce the environmental impact of a process.
Surfactants are used for many different purposes in the textile industry (e.g. detergents, lubricants, etc.). Some surfactants are considered problematic because of their poor biodegradability and toxicity to aquatic species. Concerns currently focus on APEO and in particular NPE. The main alternatives for APEO are fatty alcohol ethoxylates, but also for other surfactants substitutes are often available that are readily biodegradable or bioeliminable in the waste water treatment plant and that do not form toxic metabolites.
Complexing agents can often be avoided. Nevertheless, when they need to be used, compounds are available as an alternative to conventional sequestering agents that are readily biodegradable or at least bioeliminable and that do not contain N or P in their molecule (e.g. polycarbonates, polyacrylates, gluconates, citrates and some sugar-acrylic acid copolymers). Costs are comparable, although higher quantities may be necessary in some cases.
Antifoaming agents are often based on mineral oils. Typical active ingredients in mineral oilfree products are silicones, phosphoric esters, high molecular alcohols, fluorine derivatives and mixtures of these components. Silicones are eliminated only by abiotic processes in waste water and above certain concentrations they hinder the transfer/diffusion of oxygen into the activated sludge. Tributylphosphates are odour intensive and strongly irritant and high molecular-weight alcohols are odour intensive and cannot be used in hot liquors.
Wool scouring The implementation of dirt removal/ grease recovery loops allows water and energy savings (net specific water consumption figures of 2 - 4 l/kg greasy wool have proven to be achievable for coarse and fine wool). Additionally, a valuable by-product is obtained (25 to 30 % of the grease estimated to be present in the wool scoured), along with a significant reduction of the organic load sent to the effluent treatment plant. If the dirt removal/ grease recovery loop is combined with evaporation of the effluent and incineration of the sludge, with full recycling of the water and energy, additional environmental benefits are achieved in terms of water savings and amount of solid waste to be disposed of. Nevertheless, the technology is complex and is reported to involve very high capital costs and high running costs.
Wool scouring with organic solvents avoids the use of water in the actual cleaning process. The only source of water emission is moisture introduced with the wool, steam used in vacuum ejectors and moisture recovered from air drawn into the equipment. This water is contaminated with perchloroethylene (PER). To avoid any risk of diffuse emissions, the water stream is treated in two steps, comprising a solvent air stripping unit and a residual solvent destruction Textiles Industry v Executive Summary unit. Since pesticides partition strongly to the solvent and are removed with the grease, the clean wool is reported to be pesticide free. This has beneficial implications for the downstream processes where the wool is finished. Another positive effect of this technique is the reduced energy consumption, due to the low latent heat of an organic solvent compared to water.
Pretreatment Water-soluble synthetic sizing agents such as PVA, polyacrylates and CMC can be recovered from washing liquor by UF and re-used in the process. Recently, it has been confirmed that modified starches such as carboxymethyl starch can also be recycled. However, re-use in the weaving plant is not always without problems. To date, the weavers’ acceptance of recovered sizes is still limited. Furthermore, long-distance shipments cancel out any ecological advantages because the liquor needs to be transported in adequate conditions in insulated tankers. For these reasons, sizing agents are usually only recovered in integrated mills which have a weaving and a finishing section at the same site.
For non integrated mills that deal with many different types of fabrics and find it more difficult to have a direct control on the source of the raw fabric, a viable option is the oxidative route.
Under specific conditions (i.e. above pH 13), H2O2 will generate free radicals which efficiently and uniformly degrade all sizes and remove them from the fabric. The process produces shorter and less branched pre-oxidised molecules, which are easier to wash out (with a reduced amount of water) and easier to degrade in the waste water treatment plant. It is desirable to combine alkaline peroxide bleaching with scouring and regulate the counter-current flow of alkali and peroxide through the different pretreatment steps, so as to save water, energy and chemicals.
Hydrogen peroxide is now the preferred bleaching agent for cotton and cotton blends as a substitute for sodium hypochlorite, although it is claimed that sodium hypochlorite is still necessary for high whiteness and for fabrics that are fragile and would suffer from depolymerisation. In these cases, a two-stage process first with hydrogen peroxide and then with sodium hypochlorite can be applied, in order to reduce AOX emissions (the impurities on the fibre – which act as precursors in the haloform reaction – are removed in the first step). A twostage bleaching process using only hydrogen peroxide is also possible today, which completely avoids the use of hypochlorite. This option is however reported to be from two to six times more expensive.
There is also increasing support for peroxide bleaching under strong alkaline conditions, which can achieve a high degree of whiteness after careful removal of catalysts by a reduction/extraction technique. The additional advantage claimed is the possible combination of scouring and bleaching. The reduction/extraction followed by a strong oxidative combined bleaching/scouring step is applicable for bleaching highly contaminated textiles in all make-ups and on all types of machines (discontinuous and continuous).
Chlorine dioxide (from sodium chlorite or chlorate) is an excellent bleaching agent for synthetic fibres and for flax, linen and other bast fibres that cannot be bleached using peroxide alone.
Recent technologies (using hydrogen peroxide as the reducing agent of sodium chlorate) are now available to produce ClO2 without generation of AOX (elemental chlorine-free bleach).
The rinsing water after the mercerising treatment (so-called “weak lye”) can be recycled in the process after being concentrated by evaporation.
Dyeing Well-known PES dyeing carriers can be avoided (except for PES/WO and elastane/WO blends) by dyeing under high-temperature conditions. Another attractive option is the use of non-carrier dyeable PES fibres, such as polytrimethylene terephthalate (PTT) polyester fibres. However, due to differences in physical and mechanical properties, these fibres do not cover exactly the same product market and cannot be regarded as “substitutes” for PET-based polyester fibres.
When carriers cannot be avoided, conventional active substances - based on chlorinated aromatic compounds, o-phenylphenol, biphenyl and other aromatic hydrocarbons - can be replaced with less harmful compounds such as, benzylbenzoate and N-alkylphthalimide.
vi Textiles Industry Executive Summary In order to avoid the use of sodium hydrosulphite in PES after-treatment, two different approaches are proposed: the use of reducing agents based on a special short-chain sulphinic acid derivatives or the use of disperse dyes that can be cleared in alkaline medium by hydrolytic solubilisation instead of reduction. Short-chain sulphinic acid derivatives are biodegradable, non-corrosive, have very low toxicity and, unlike hydrogen hydrosulphite, they can be applied in acidic conditions without the need for repeated bath changes and shifts in pH (water and energy savings). With alkali-clearable dyes the use of hydrosulphite or other reducing agents can be avoided altogether.
Dispersing agents typically present in disperse, vat and sulphur dye formulations have been improved by: 1) their partial substitution with optimised products based on fatty acid esters, or
2) the use of mixtures of modified aromatic sulphonic acids. The first option is only applicable for liquid formulations of disperse dyes (the dyestuff pallette is currently limited). These dispersing agents are bioeliminable and their amount in the formulation can be significantly reduced compared to conventional formulations. The dispersing agents indicated in the second option show a higher degree of bioelimination compared to the conventional condensation products of naphthalene sulphonic acid with formaldehyde. They can be used both for disperse and vat dyes (solid and liquid formulations).
Pre-reduced sulphur dyestuffs (liquid formulations with sulphide content 1 %) or non-prereduced sulphide-free dyestuffs are available in various different forms (water-soluble in the oxidised, powder, liquid form, or in stable suspension). All these dyestuffs can be reduced without any sodium sulphide, using glucose alone (only in one case) or in combination with dithionite, hydroxyacetone or formamidine sulphinic acid. Stabilised non-pre-reduced sulphidefree dyestuffs are reported to be more expensive than the other types of sulphur dyes.
Poor dye fixation has been a long-standing problem with reactive dyeing in particular in batch dyeing of cellulose fibres, where a significant amount of salt is normally added to improve dye exhaustion. With the use of sophisticated molecular engineering techniques it has been possible to design bifunctional and low-salt reactive dyes that can attain 95 % fixation rate even for cellulosic fibres, with considerably higher performance (reproducibility and level dyeing) than traditional reactive dyes. Hot rinsing avoids the use of detergents and complexing agents in the rinsing and neutralisation steps after dyeing. Substituting cold rinsing with hot rinsing leads to higher energy consumption, unless thermal energy from the rinsing effluent is recovered.
The use of sodium silicate in pad-batch dyeing of cellulosic fabrics can be avoided thanks to silicate-free highly concentrated aqueous solutions, which are ready-made products easily applicable with modern dosing systems. An alternative process is also described, which doesn’t require the addition of substances such as urea, sodium silicate and salt, or long dwell-time to fix the dyes. The process itself is simple and highly versatile and is applicable to a wide variety of fabrics, regardless of the size of the lot. Significant savings can be achieved thanks to higher productivity, reduced consumption of chemicals and energy and the reduced waste water pollution to treat. Nevertheless due to the initial high capital investment, this technique fits better in new installations and in those seeking to replace equipment.
Quite recently, new reactive dyestuffs have come on the market that can provide very good levels of fastness, even equivalent with those achievable with chrome dyes, even for dark shades. However, the importance of reactive dyes is only slowly increasing due to a number of reasons, including difficulties of the operators accepting radical changes to a well-established procedure. Moreover, some finishers still consider that chrome dyes are the only ones that can guarantee the level of fastness required for overdyeing. When chrome dyes are used, lowchrome and ultra-low stoichiometric chrome dyeing techniques can be adopted to minimise the amount of residual chromium in the final effluent. With ultra-low chroming an emission factor of 50 mg chromium per kg of wool treated is achieved, which corresponds to a chromium concentration of 5 mg/l in the spent chroming bath when a 1:10 liquor ratio is used.