«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Textiles Industry vii Executive Summary In general, with pH-controllable dyes (e.g. acid and basic dyes) it is advantageous to dye at isothermal conditions imposing a pH profile. One of the advantages over temperature-controlled dyeing processes is that maximum exhaustion of dyes and insect resist agents can be achieved with only a minimum use of organic levelling agents. When dyeing wool with metal-complex dyes, higher levels of exhaustion and fixation rate can be achieved by controlling the pH and by using special auxiliaries with high affinity for the fibre and dyestuff. The higher exhaustion rate directly correlates with the reduced residual chromium levels in the spent dye bath (10 - 20 mg/kg of treated wool, corresponding to 1 - 2 mg/l of chromium in the spent dye bath with 1:10 L.R.). The referenced technique has been designed for dyeing loose wool fibre and combed tops, but the same performances can also be achieved with other make-ups by using pH-controlled methods to maximise final bath exhaustion.
Various techniques are described in the BREF aimed at improving the environmental performance of batch and continuous dyeing processes in general. A distinct trend has developed among batch dyeing machinery manufacturers toward reducing bath ratios.
Moreover, an outstanding feature of modern machines is that they can be operated at approximately constant liquor ratio whilst being loaded much below their nominal capacity.
This is especially advantageous for commission companies, who typically need high production flexibility. Furthermore, various functions typical of continuous processing have been transferred to batch machines, which allow maximum cut-off between different batches and thereby open up further options for re-use of the dye bath and improved treatment of the concentrated streams.
As for continuous dyeing processes, reduction of system losses can be achieved by carrying out the impregnation step in a nip or by minimising the capacity of the dip trough (e.g. flex-shaft, U-shaft). Additional improvements are obtained by dispensing the dyestuff and auxiliaries as separate streams and by dosing the padding liquor based on measurement of the pick-up. The amount of dyeing liquor consumed is measured by reference to the quantity of processed fabric.
The resulting values are automatically processed and used for the preparation of the next comparable batch in order to minimise residues of unused dyeing liquor. This system, however, cannot avoid the presence of residual dye liquor in the feeding tank. The rapid batch dyeing technique represents a further improvement because, rather than being prepared in a single step (for the whole batch) before starting the dyeing batch, the dyestuff solution is prepared just in time, in several steps, based on on-line measurement of the pick-up.
Printing Minimising the volume of the printing paste supply system (i.e. diameters of pipes and squeegees) has major effects in reducing printing paste losses in rotary-screen printing. A further reduction can be achieved by improving paste recovery from the supply system itself. A recent technique consists in inserting a ball in the squeegee, before filling the system. At the end of a print run, the ball is pressed back, thus pumping the printing paste in the supply system back into the drum for re-use. Today, computer-assisted systems offer more opportunities for recycling printing pastes. Printing paste recovery and recycling systems are applied in textile finishing mills (for flat fabrics), but not for carpets. The main reason is that guar-gum (the most common thickener used for carpets) has a limited shelf-life (biodegradable compound) and therefore it cannot be stored for a long time before re-use.
Screens, buckets and the print paste feed systems need careful cleaning before being used for new colours. There are several inexpensive ways of reducing water consumption (e.g. start/stop control of cleaning of the printing belt, re-use of the rinsing water from the cleaning of the printing belt, etc.).
An alternative to analogue printing is the use of digital techniques, which are gaining importance in the textile and in the carpet sector. In digital printing the selected dyes are dosed on-demand, based on computed requirements. This avoids printing paste residues at the end of each run.
viii Textiles Industry Executive Summary Digital ink-jet printing is suitable for flat fabrics. However, production speeds are still too low to allow this technique to replace traditional analogue printing. Nevertheless ink-jet printing can already offer significant advantages over analogue printing in the production of short runs.
The latest improvement in jet printing machines for carpet and bulky fabrics is now represented by machines in which the colour is injected with surgical precision deep into the face of the fabric without any machine parts touching the substrate. Here, the control of the quantity of liquor applied to the substrate (which may vary for example from lightweight articles to heavy quality fabrics) is achieved by varying not only the “firing time” but also the pumping pressure.
Urea content in reactive printing paste can be up to 150g/kg paste. Urea can be substituted in the one-step process by controlled addition of moisture either by the foaming technique or by spraying a defined quantity of water mist. However, for silk and viscose articles, it is not possible to avoid the use of urea with the spraying system. The technique is not reliable enough to ensure a uniform dosage of the low moisture add-on required for these fibres.
The foaming technique, on the contrary, has proven successful for viscose in complete elimination of urea. This technique should in principle be technically viable also for silk, although it has not yet been proven. Silk is known to be less problematic as a fibre than viscose, but it is typically processed in smaller runs. Without using the foam technique, the amount of urea consumed can be reduced to about 50 g/kg of printing paste for silk and 80 g/kg for viscose.
Another option for avoiding the use of urea, although more complex and slower, is the two-step printing method.
Although water-in-oil thickeners seem no longer to be applied in Europe and half-emulsion printing pastes (oil in water) are only occasionally used, hydrocarbons (predominantly aliphatic) are still found in exhaust air, mainly arising from mineral oils contained in synthetic thickeners.
Their emission potential can be up to 10 g Org.-C/kg textile. New generation thickeners contain minimal amounts of volatile organic solvents, if any. Furthermore, optimised printing pastes are APEO-free, have a reduced ammonia content and contain formaldehyde-poor binders.
Finishing In order to reduce pick-up, so-called minimum application techniques (e.g. kiss-roll, spray and foaming application systems) are gaining importance as substitutes for padding systems.
In addition, various techniques are available for reducing energy consumption in stenter frames (e.g. mechanical dewatering equipment to reduce water content of the incoming fabric, optimising control of exhaust airflow through the oven, installation of heat recovery systems).
For each finishing process there are techniques for the reduction of the environmental impact associated with the specific substances used. The BREF focuses only on a few finishing processes. In easy-care treatments, emissions of formaldehyde (suspect carcinogenic) can be significantly reduced with low-formaldehyde or formaldehyde-free products (75 mg/kg textile, or even lower than 30 ppm for consumer requirement).
General techniques to minimise emissions of mothproofing agent include handling procedures to minimise spillage during dispensing and transport of mothproofing agent concentrates within the dyehouse, as well as special operating techniques to achieve lowest residues of active substance in the spent dyeing liquor and rinse water. Two effective measures are 1) to ensure that a pH4.5 is reached at the end of the dyeing process (when and if this is not possible, apply the insect resist agent in a separate step with re-use of the bath) and 2) to avoid the use of dyeing auxiliaries that exert a retarding action on the uptake of insect resist agents, (e.g. levelling agent, PA blocking agent).
Textiles Industry ix Executive Summary Other techniques include proportional overtreatment, application of the mothproofer from the low-volume bowl at the end of the yarn scouring line, application of the IR agent directly to the pile of the carpet during back-coating or latexing operation, etc. The application of these techniques is specific for each of the three identifiable routes for yarn manufacture, i.e. by the “dry spinning route”, “loose fibre dyed/ yarn scoured production” and “yarn dyed production”.
The application of softeners by pad mangles or by spraying or foaming application systems give better environmental performance than batch softening directly in the dyeing machine after dyeing. The use of cationic softening agents can be avoided and any chemical loss can be reduced to a few percent. Another advantage is that it is then possible to re-use the dyeing or rinse baths as there is no longer a problem with the presence of residual cationic softeners, which would otherwise limit the adsorption of the dye in the subsequent dyeing process.
Washing “Drain and fill” and “smart rinsing” are both more efficient batch washing techniques than conventional overflow rinsing. Moreover, modern machines are equipped with time-saving devices and other special systems in order to avoid typical limitations of the traditional “drain and fill” method (e.g. longer production cycle time, etc.). With both “smart rinsing” and “drain & fill” it is possible to keep the exhausted concentrated dye liquor and the rinsing waters as separate streams (waste streams segregation and water and energy recovery).
In continuous washing, water and energy conservation should start from the application of simple good housekeeping measures. These can range from the definition of the optimum flow by means of flow control devices on washers, to the installation of stop valves that shut off the water flow as soon as a stoppage occurs. Further improvements can be achieved by increasing the washing efficiency, mainly by counter-current washing and reduction of carry-over (e.g.
vacuum extractors). Installing heat recovery equipment on a continuous washer is usually a simple and effective measure.
New installations for washing with halogenated organic solvents are fitted with closed-loop active charcoal filters, thereby avoiding any air-stream exhaustion to the outside environment.
In order to minimise emissions of water contaminated with PER, most of the water-dissolved PER is extracted and recovered through a two-stage process involving air-stripping and absorption on active charcoal (PER 1 mg/l in the final effluent). Since the water flow is fairly low (≤ 0.5 m3/h) advanced oxidation processes (e.g. the Fenton process) are suitable for treating this effluent on site. Furthermore the complete redesign of the main distilling section has drastically reduced the solvent residue in the sludge (1 % by weight compared to over 5 % in conventional installations).
Waste water treatment Hardly-biodegradable compounds can still be degraded in biological plants under low food-tomass-ratio (F/M) conditions, but non-biodegradable substances are not degraded in biological plants. Concentrated waste water streams containing such compounds should be treated at source. For the textile finishing industry, advanced oxidation with a Fenton-like reaction is proposed as a viable pretreatment technique (depending on the type of effluent, COD removal can reach 70 – 85 % and the residual COD, which is largely biodegradable because of the modification of the compounds, is suitable for biological treatment). However, very strong residues such as residual printing paste and padding liquors can more conveniently be kept out of the waste water stream altogether and other disposal routes used.
For 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. Moreover, for azo-dyes, anaerobic treatment of padding liquor and printing pastes before a subsequent aerobic treatment is effective for colour removal.
The following techniques are proposed in order to achieve equivalent performance when
treating a mixed effluent:
· tertiary treatments following the biological treatment process, such as adsorption on activated carbon with recycling of the activated carbon to the activated sludge system and destruction of the adsorbed non-biodegradable compounds by incineration or radical treatment of the excess sludge (biomass and spent activated carbon) · 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) · ozonation of recalcitrant compounds prior to the activated sludge system.
For wool scouring waste water a number of different scenarios are discussed. The environmental performance of an evaporation plant is far superior to that of a flocculation plant.
However, the initial cost of the evaporation plant seems to be much higher and payback (versus discharge to sewer) takes 4 – 5 years for small mills (3500 t/yr of wool). For medium-sized mills (15000 t/yr of wool), evaporation is slightly cheaper than flocculation over 10 years. The use of a dirt removal/grease recovery loop in combination with evaporation makes evaporation even more attractive because a smaller evaporator can be installed, which thereby reduces initial capital outlay. The use of a recovery loop also allows a reduction in running costs thanks to the proceeds from the sales of the grease (this effect is more significant for fine wool scouring mills).
The combination of a dirt removal/ grease recovery loop with evaporation of the effluent and incineration of the sludge with full recycling of water and energy is the best option from an environmental point of view. However, the complexity of the technique and the initial capital cost make it more suitable for 1) new installations, 2) existing installations with no on-site effluent treatment and 3) installations seeking to replace life-expired effluent treatment plant.
In the case of effluent treatment by biological processes it is known that there are scourers in Europe (particularly in Italy) using biological processes as their main methods of effluent treatment. However, no precise information has been submitted.
Wool scour sludge has been proven to have excellent technical properties when mixed with clay for brick-making. The economics are greatly dependent on the deal between the scourer and the brick-maker. According to reported information, the technique should be cheaper than landfilling, composting and incineration. No information is submitted in the BREF about other recycling options available.
GENERIC BAT (WHOLE TEXTILE INDUSTRY)
Management It is recognised that technology improvements need to go together with environmental management and good housekeeping. Management of an installation that uses potentially polluting processes requires the implementation of many of the elements of an Environmental Management System (EMS). The implementation of a monitoring system for process input and output is a prerequisite for identifying priority areas and options for improving environmental performance.
Dosing and dispensing of chemicals (excluding dyes) BAT is to install automated dosing and dispensing systems which meter the exact amounts of chemicals and auxiliaries required and deliver them directly to the various machines through pipework without human contact.
Textiles Industry xi Executive Summary Selection & use of chemicals
BAT is to follow certain general principles in selecting chemicals and managing their use:
· where it is possible to achieve the desired process result without the use of chemicals, then avoid their use altogether · where this is not possible, adopt a risk-based approach to selecting chemicals and their utilisation mode in order to ensure the lowest overall risk.