«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Ø use a separate aftertreatment process to minimise emissions from dyeing processes which are carried out under less than optimum conditions for mothproofer uptake Ø use semi-continuous low-volume application machinery or modified centrifuges Ø recycle low-volume process liquor between yarn batches and processes designed specifically to remove active substance from spent process liquor. These techniques may include adsorptive or degradative treatments Ø apply mothproofer directly to the carpet pile (when mothproofing during carpet manufacture) using foam application technology.
· Softening treatments BAT is to apply the softening agents by pad mangles or better, by spraying and foaming application systems, instead of carrying out this treatment by exhaustion directly in the batch dyeing machine (see Section 4.8.3).
BAT is to:
· substitute overflow washing/rinsing with drain/fill methods or “ smart rinsing” techniques as described in Section 4.9.1
· reduce water & energy consumption in continuous processes by:
Ø installing high-efficiency washing machinery according to the principle described in Section 4.9.2. The associated values for high-efficiency continuous washing of cellulosic and synthetic fabric in open-width are reported in Table 4.38 Ø introducing heat recovery equipment · when halogenated organic solvent cannot be avoided (e.g. with fabrics that are heavily loaded with preparations such as silicone oils that are difficult to remove with water), use fully closed-loop equipment. It is essential that the equipment fulfil the requirements described in Section 4.9.3 and provisions be taken for in-loop destruction (e.g. by advanced oxidation processes) of the persistent pollutants in order to avoid any possible contamination of groundwater arising from diffuse pollution and accidents.
5.3 Effluent treatment and waste disposalWASTE WATER TREATMENT
Waste water treatment follows at least three different strategies:
· central treatment in a biological waste water treatment plant on site · central treatment off site in a municipal waste water treatment plant · decentralised treatment on site (or off site) of selected, segregated single waste water streams All three strategies are BAT options when properly applied to the actual waste water situation.
Well-accepted general principles for waste water management and treatment include:
· characterising the different waste water streams arising from the process (see Section 4.1.2) · segregating the effluents at source according to their contaminant type and load, before mixing with other streams. This ensures that a treatment facility receives only those pollutants it can cope with. Moreover, it enables the application of recycling or re-use options for the effluent · allocating contaminated waste water streams to the most appropriate treatment · avoiding the introduction of waste water components into biological treatment systems when they could cause malfunction of such a system · treating waste streams containing a relevant non-biodegradable fraction by appropriate techniques before, or instead of, a final biological treatment.
According to this approach, the following techniques are determined as general BAT for the
treatment of waste water from the textile finishing and carpet industry:
· treatment of waste water in an activated sludge system at low food-to-micro organisms ratio as described in Section 4.10.1, under the prerequisite that concentrated streams containing non-biodegradable compounds are pretreated separately · pretreatment of highly-loaded (COD5000 mg/l) selected and segregated single waste water streams containing non-biodegradable compounds by chemical oxidation (e.g. Fenton reaction as described in Section 4.10.7). Candidate waste water streams are padding liquors from semi-continuous or continuous dyeing and finishing, desizing baths, printing pastes, residues from carpet backing, exhaust dyeing and finishing baths.
Certain specific process residues, such as residual printing pastes and residual padding liquors are very strong and, where practicable, should be kept out of waste water streams.
These residues should be disposed of appropriately; thermal oxidation can be one suitable method because of the high calorific value.
For the specific cases of waste water containing pigment printing paste or latex from carpet backing, precipitation/flocculation and incineration of the resulting sludge is a viable alternative to chemical oxidation (as described in Section 4.10.5).
For azo-dyes, anaerobic treatment of padding liquor and printing pastes as described in Section 4.10.6 before a subsequent aerobic treatment can be effective for colour removal.
If concentrated water streams containing non-biodegradable compounds cannot be treated separately, additional physical-chemical treatments would be required to achieve equivalent
overall performance. These include:
· tertiary treatments following the biological treatment process. An example is adsorption on activated carbon with recycling of the activated carbon to the activated sludge system: this is followed by destruction of the adsorbed non-biodegradable compounds by incineration or treatment with free-radicals (i.e. process generating OH*, O2*-, CO2*-) of the excess sludge (biomass along with the spent activated carbon) (see plant 6 in Section 4.10.1) · combined biological, physical and chemical treatments with the addition of powdered activated carbon and iron salt to the activated sludge system with reactivation of the excess sludge by “wet oxidation” or “wet peroxidation” (if hydrogen peroxide is used), as described in Section 4.10.3 · ozonation of recalcitrant compounds prior to the activated sludge system (see plant 3 in Section 4.10.1).
For effluent treatment in the wool scouring sector (water-based process)
BAT is to:
· combine the use of dirt removal / grease recovery loops with evaporative effluent treatment, with integrated incineration of the resulting sludge and full recycling of water and energy for: 1) new installations 2) existing installations with no on-site effluent treatment 3) installations seeking to replace life-expired effluent treatment plant. This technique is described in Section 4.4.2 · use coagulation/flocculation treatment in existing mills already using it in conjunction with discharge to sewerage system employing aerobic biological treatment.
Whether or not biological treatment can be considered as BAT must remain an open question until better information on its costs and performance can be assembled.
SLUDGE DISPOSALFor sludge from waste water treatment of wool scouring effluent
BAT is to:
· use sludge in brick-making (see 4.10.12) or adopt any other appropriate recycling routes · incinerate the sludge with heat recovery, provided that measures are taken to control emissions of SOx, NOx and dust and to avoid emissions of dioxins and furans arising from organically bound chlorine from pesticides potentially contained in the sludge.
6 EMERGING TECHNIQUESExcept where otherwise specified the information in this chapter has been provided by [179, UBA, 2001] and [77, EURATEX, 2000].
Enzyme catalysed finishing processes Enzymes are proteins that act as biocatalysts activating and accelerating chemical reactions which would otherwise normally need more energy. Their excellent substrate selectivity allows more gentle process conditions compared to conventional processes. Enzymes are present in bacteria, yeasts and fungi.
At present enzymes are used and under study only for natural fibres, the use of enzymes for man-made fibres is not mentioned in literature. Some enzymes, such as the amylases in the desizing process, have been widely applied for a long time; other enzymes are still the object of investigations. Table 6.1 lists the main enzymatic processes already in use or currently emerging in the textile sector.
Table 6.1: Enzymatic processes in textile finishing [77, EURATEX, 2000], [179, UBA, 2001] Energy savings (lower processing temperatures) and lower water consumption (reduced number of rinsing steps) are some of the promising advantages of enzymatic processes, along with the omission, in some cases, of the use of hazardous/harmful substances.
Also enzymes can be used in catalytic amounts and as a biocatalyst they can be recycled.
Plasma technology A plasma can be described as a mixture of partially ionised gases. Atoms, radicals and electrons can be found in the plasma. The electrons in low temperature plasmas are able to cleave
covalent chemical bonds, thereby producing physical and chemical modifications of the surface of the treated substrate.
Two types of plasma are generally used: the corona plasma and low-pressure plasma.
Plasma treatment can be performed on natural fibres as well as on synthetic fibres, to achieve
the following effects:
· wool degreasing · desizing · change of fibre wettability (hydrophilic, hydrophobic properties) · increase in dyestuff affinity · improved dye levelling properties · anti-felt finishing in wool · sterilisation (bactericidal treatment), etc.
Anti-felt finishing effects for wool is one of the most studied applications of plasma technology in the textile sector. Plasma treatment, instead of the conventional anti-felt treatment (see Section 126.96.36.199), is very attractive because this technique causes less degradation of the wool fibre and avoids the presence of AOX in the waste water.
In general the main advantages of plasma technology are the extremely short treatment time and the low application temperature, along with the fact that water and solvents can be avoided and no or less chemicals are required.
Electron-ray treatment Electron-rays start free-radical initiated polymerisation reactions that can then be used for coating, lamination and for graft co-polymerisation reactions on textiles pre-coated with monomers or pre-polymers.
The advantage over thermal curing is that solvent-free formulations can be used. This reduces the emissions of VOCs during drying operations. The technique is already established in other sectors and therefore its implementation in the textile sector is foreseeable in the next five years.
Use of supercritical CO2 in dyeing processes Supercritical fluids are capable of dissolving organic molecules of low to medium polarity.
CO2 has the advantage over other gases of being non-flammable, non-explosive and non-toxic.
CO2 dyeing of PES and PP fibre is already developed on an industrial scale, however the application of this technique on wool, PA and cotton is still problematic due to the polar nature of the dyestuffs used to colour these fibres.
CO2 dyeing of PES and PP can be carried out under optimal isothermal and isobaric conditions at 120 and 300 °C. Dye uptake and fastness properties are very similar to water dyeing.
Nevertheless some precautions need to be taken.
Excess dye dissolved in the dyeing medium must be extracted with fresh supercritical CO2 at the end of the dyeing cycle.
In conclusion, only special dye formulations can be used because dispersing agents and other auxiliaries typically present in conventional dye formulations strongly influence dye uptake in supercritical CO2.
Hydrophobic preparation agents should be extracted before dyeing because of their solubility in supercritical CO2. They are first extracted from the fibre during the dyeing process, and then precipitate as oily droplets at the end of the process.
CO2 dyeing has a number of advantages:
· almost zero water consumption · zero off-gas emission (CO2 can be recycled) · no drying step necessary after dyeing · levelling and dispersing agents are not needed at all or, in some cases, they are added in very small amounts · the dyestuff residues can be recycled.
Nevertheless, the investment cost for the equipment is high and this is a significant drawback, especially when considering that PES textiles are normally low-price products.
Ultrasonic treatments Ultrasonic treatments improve the dispersion of dyestuffs and auxiliaries and enhance their ability to emulsify and solubilise. This allows improved liquor homogenisation, which then results in higher bath exhaustion and level dyeing properties. In addition, ultrasounds produce a de-aration effect in the liquor and on the fabric, which is normally obtained by adding special auxiliaries (de-aerating agents).
The main environmental benefits achievable with ultrasonic treatments in textile finishing are:
· energy savings (lower process temperatures and shorter cycle times) · reduction in auxiliaries consumption.
Electrochemical dyeing Vat and sulphur dyeing involves both a reducing and an oxidising step, which are carried out with chemical oxidants and reducing agents. The environmental concerns associated with the use of these chemicals are described in Section 188.8.131.52. An attractive alternative technique is to reduce and oxidise the dye by means of electrochemical methods.
With direct electrolysis the dye itself is reduced at the surface of the cathode. In indirect electrolysis the reducing power of the cathode is transferred to the solution by a soluble reversible redox system (e.g. based on antraquinone chemistry or iron complexes). With this reversible redox system the reducing agent is continuously regenerated at the cathode, which thus allows full recycling of the dye bath and the reducing agent.
Direct cathodic reduction in an electrochemical cell is applicable to sulphur dyes. Vat dyes are reduced by indirect electrolysis.
Alternative textile auxiliaries Complexing agents The use of polyasparginic acid as a substitute for conventional dispersing and complexing agents is under study.
Cross-linking agents Polycarbonic acids can be used as an alternative to N-methylol-based cross-linking agents, which are responsible for formaldehyde emissions.
Biopolymers Besides cellulose, chitin, the main structural component of crustacean shells (crabs, lobster, etc.) and insects, is the second main biopolymer. Its deacetylated derivative, chitosan, which is easier to handle due to its higher solubility, is increasing in importance.
Some examples of potential applications of chitosan and its derivatives in the textile sector
· antimicrobial treatment for textiles: a permanent effect can be obtained by blending 10 % of chitosan fibres with cotton fibres to produce a mixed fibre yarn or by spraying chitosan solutions on non-woven fabrics. Compared to other commonly used antimicrobiotics, chitosan is not toxic to aquatic life nor to humans (it is therefore of special interest for those fabrics that stay in close contact with the skin) · aftertreatment to improve fastness properties when dyeing with direct dyes: cationic modified derivatives of chitosan are reported to be suitable for this application Furthermore, chitosan increases the dyestuff uptake and can act as a softening agent or binding agent for non-woven fabrics. It can also be used as an additive in printing pastes and in sizing agents. Its application can also be interesting in waste water treatment.
Fuzzy logic Significant improvements in process reliability are achievable with the use of fuzzy logic (i.e.
expert systems based on self-learning software systems, which auto-enlarge their knowledge by algorithms). The application of fuzzy logic in the textile industry is the object of a number of research projects. Two examples are reported concenring the control of the sizing process and the control of the condensation reaction of cross-linking agents.
The main advantages to be expected are the improved process control, which subsequently can result in increased productivity and enhanced quality of the final product.