«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
The achievement of performances typical of highly efficient washing machines requires investment in new equipment. However, the application of low-technology measures such as flow control devices, automatic valves, etc. can also produce some reduction in water and energy consumption.
Detailed information was not made available. In order to indicate the order of magnitude, a continuous IPPC-capacity preparation line for cotton fabric, allowing for minimal water consumption (9 l/kg in total, including odour removal and lint filtration) by combined application of various technical optimisation measures, costs 2.5 million euros [247, Comm., 2001].
Driving force for implementation The main driving forces for the implementation of the described techniques are the increasing cost of water supplies and waste water treatment, and the desire for increased productivity (in the case of new highly efficient washing machinery).
Reference plants Many plants.
Reference literature [167, Comm., 2000], [179, UBA, 2001], [11, US EPA, 1995], [146, Energy Efficiency Office UK, 1997] 4.9.3 Use of fully closed-loop installations for fabric washing (scouring) with organic solvent Description Continuously operating solvent scouring installations for open-width fabrics have been known since the late 1960s. They have been used by the textile finishing industry in different production sectors for over thirty years mainly because of the advantages of organic solvents over water in solving technical and qualitative problems related to fabric cleaning.
The specific heat of PER (the solvent most commonly used) is about 1/5 of that of water and the latent heat of evaporation is over 10 times lower, resulting in about 90 % reduction of the total heat requirement for evaporation in favour of PER. This means much faster and cheaper evaporation with significant savings in time and energy during drying.
The lower surface tension of PER results in a quicker and deeper fibre impregnation, thereby making any cleaning or finishing treatment more complete and uniform.
Nevertheless, the application of PER requires extreme care and sophisticated techniques for reducing and minimising its harmfulness potential for the environment and humans.
The following gives an insight into the features of the new generation of technologically advanced solvent treatment installations compared to the traditional ones.
In general terms, the components of a typical installation of the 1970s, schematically
represented in the diagram below (see Figure 4.32), are:
S. scouring unit D. drying unit C. cooling section
Solvent recovery equipment:
LR liquid recovery / sludge disposal (1 distillation, 2 condensation, 3 water separation, 4 solvent tank) GR gaseous recovery (5 open-loop active charcoal filters)
Figure 4.32: General layout of a conventional solvent scouring installation [197, Comm.
, 2001] The components of a typical modern installation, schematically represented in the figure below,
S. scouring unit D. drying unit C. cooling section
Solvent recovery equipment:
LR liquid recovery / sludge disposal (1 main distillation, 2 sludge distillation, 3 condensation, 4 water separation, 5 solvent tank) GR gaseous recovery (6 closed-loop active charcoal filters)
W. water treatment:
WD decantation WS air stripping WA charcoal absorption
Figure 4.33: General layout of a modern solvent scouring installation [197, Comm.
, 2001] The following solutions to main emission and pollution sources have been developed in the new generation equipment.
Air emissions (Outside atmosphere) Problem Open-loop active charcoal filters used for the purification of the air stream release to the outside atmosphere between 500 and 1000 g/h PER, depending on the equipment size (at a solvent concentration in the region of 500 - 600 mg/m3).
Solution The new installations are fitted with closed-loop active charcoal filters. The exhaust duct has been eliminated and the purified air is now recycled to the fabric deodorising / cooling section of the machine: this avoids any air-stream exhaustion to the outside environment.
Moreover, with the closed-loop filters it has been possible to re-design more efficient sealing systems at the machine inlet and outlet sides with consequent benefits for the workplace as well.
Water emissions Problem The so-called “separation water” produced by the solvent recovery system with an average flow of about 0.5 m³/h and a content of PER of between about 150 and 250 g/m³, gives rise to an emission of 75 - 125 g/h PER. This effluent used to be drained to the sewer (in the worst case), or to the central waste water treatment plant. Since the solvent is not biodegradable, once it reaches the aquifer it accumulates, and lasts indefinitely.
Solution A built-in, dedicated piece of equipment is now available to pretreat, extract and recover most
of the water-dissolved PER, through a two-stage process involving:
1. stripping by means of an air-stream
2. absorption through active charcoal cartridges, periodically changeable and rechargeable.
Again, the closed-loop active charcoal filters are involved in purifying the polluted air-stream from the 1st stage and recovering the extracted solvent.
The system is able to ensure a residual PER content into the draining water not higher than 1 mg/l (emission in the water ≤ 0.5 g/h PER).
Nevertheless, since the water flow is fairly low (≤ 0.5 m3/h) advanced oxidation processes (e.g.
the Fenton process) are suitable for treating such low water flows on site [281, Belgium, 2002].
Apart from the above-mentioned contact-water drain, a solvent installation, either old or new, does not generate any other water effluent.
Waste Problem The high water content and the over 5 % by weight residual PER concentration in the sludge makes this waste difficult to manage at mill level and undesirable to most collectors. Landfilling creates soil or aquifer contamination, and PER may still be released to atmosphere in landfill gas. It is possible that implementation of the Directive 99/31/EC on the landfill of waste may prevent this material being landfilled in the future.
Solution The complete redesign of the main distilling group (of the “forced circulation” type) and, particularly, the redesign of the sludge distiller (of the “thin layer evaporator” type), has drastically reduced the solvent residue in the sludge well below 10000 mg/kg (1 %), producing a dry, thick waste. This reduces collection and disposal problems and cost.
Problem Solvent remains absorbed on fibres and this has been evaluated to be in the range of 0.1 - 1.0 %.
Emission of this solvent to the surrounding environment is difficult to control. This also influences the air-stream quality from the drying / heat-setting machines exhaust ducts.
Solution The sealing systems at the machine inlet and outlet sides have been redesigned to further improve the solvent vapours pick-up efficiency. This results in a much safer environment and better preservation of human health. A typical TLV-TWA value all around the installation is now no higher than 50 mg/m³.
As a whole, the total solvent consumption of the solvent treatment installation has been reduced from the 3 – 5 % (by weight of fabric produced) typical of traditional equipment to 0.8 - 1.5 %.
Further improvement is likely in the near future.
Main environmental benefits The environmental benefits of scouring with organic solvent lie essentially in the following
· reduction of both water and energy consumption, due to the dry-to-dry processing and to the heat requirement for solvent evaporation compared to water · reduction in auxiliary usage (e.g. surfactants used as detergents, emulsifiers, etc). A high amount of auxiliaries is needed for difficult-to-remove preparation agents such as silicone oils present on elastane fibres and their complete removal with water washing is not possible. As a result the remaining preparation agents are released to the exhaust air from stenters in the subsequent thermal treatments · reduction of the organic load sent to the waste water treatment plant (the impurities are disposed of in a concentrated form as sludge).
Organic halogenated solvents are non biodegradable and persistent substances. Unaccounted losses from spills, scouring-unit filters, fabric, etc. may give rise to diffuse emissions, resulting in groundwater and soil pollution. In the UK, municipal water abstraction has been stopped
from aquifers contaminated in this way and compensation claims can threaten companies’economic viability.
Moreover, textiles treated with perchloroethylene have a potential to release it in the later thermal treatments. In directly heated stenter frames, dioxins and furans may be formed. In Germany legislation forbids treating textile substrates which have been pre-cleaned with PER in directly heated stenters or comparable equipment. High emissions of PER, in the exhaust gas (0.1 – 0.8 g PER/kg of textile substrate and emission mass flows of up to 0.3 kg PER/h) have been observed in some installations, thereby creating difficulties in meeting the emissions limit values [280, Germany, 2002].
As far as the problem of the solvent retention by the fibres is concerned, studies and experimentation are in progress, which aim at reducing the solvent content basically by means of a final fabric treatment under heat and moisture conditions. It seems that the solvent abatement level in the fabric can reach over 90 %.
Solvent treatments of textiles includes all those applications where the solvent (PER) is able to perform better than water, particularly in terms of solvency power of hydrophobic substances.
The cleaning treatment of loom-state fabrics as a preparation for dyeing or printing is the most typical application of this technique, provided that some hydrophilic (water-soluble) substances, such as warp sizing agent, are not present as well.
The main application of solvent is for knitted fabrics, particularly in man-made fibres.
In the particular case of elastic knits (elastane blended fibres), the solvent pretreatment is particularly indicated because it is able to remove efficiently the silicone oils contained in the elastomeric fibres while conferring optimal shrinkage properties. Thanks to those features, solvent scouring is often extended to the cotton/elastane blends as well.
In woven fabrics processing, scouring of loom-state wool cloth, both grey and tops/yarn dyed, either worsted or woollen, is widely used provided that sizing agents are not present.
More recent production of wool elastic fabrics (elastane/wool and wool-polyester blends) requires an after-dyeing solvent treatment to increase the fabric colour fastness.
Particularly in case of woollen fabrics, solvent scouring can be combined with aqueous carbonising in a single processing line (see Section 126.96.36.199).
Economics The following tables attempt to evaluate the economic aspects of the solvent system by comparing it with the aqueous system in a parallel operation with the same fabric quality and at the same production rate.
Two fabrics with the same fibre composition and different construction (one knitted and one woven) have been selected, both in the range of medium-light weight (240 g/m) thus allowing the two systems to produce about 0.8 tonnes/hour each at the same speed of 55 metres/minute.
Since all the machines in the two systems have a nominal production capacity up to 1 tonne/hour, the comparison has been made at 80 % efficiency.
Both systems comprise a washing installation and a heat-setting installation, but:
· the aqueous process uses heat-setting for drying as well · the solvent process, including a built-in drying unit, uses heat-setting for this purpose only.
The consumption figures have been taken from the technical characteristics supplied by the relevant machinery manufacturers (Sperotto Rimar Spa for the solvent installation and Santex AG for the washing range and the stenter-frame).
Table 4.40: Hourly cost figures: aqueous system and solvent system The thermal energy consumption figures have been calculated from the heating capacity performance and expressed as steam consumption for an easier cost determination.
The detergent usage for the aqueous system and the PER consumption for the solvent system are classified as “chemicals” consumption.
The water purification for the aqueous system corresponds broadly with the sludge disposal for the solvent system.
As stated earlier, the water supply in the solvent system is used for cooling purpose only and can be fully recovered, unpolluted, to be re-used in the dyehouse or even recycled in the solvent installation after suitable cooling.
The unit costs are based on current prices in Italy, although they fluctuate over time and vary with textile type. Local prices can easily be substituted.
The disposal cost for the sludge is compensated by the savings achievable in:
· heating energy (steam) · total water (supply + purification) · chemicals (detergent v/s PER)
On the whole, the solvent system can ensure a total saving of about 17 %, that is 35 euros/working hour.
The high investment cost is sometimes a disincentive, but has a short payback period (usually no longer than 2 - 3 years), particularly for medium-large installations and big companies with an annual production of at least 3000 tonnes of fabric [197, Comm., 2001].
Reference plants It is estimated that at least 200 plants, either old or new, are presently operating world-wide [197, Comm., 2001].
Driving force for implementation Reducing air emissions of pollutants seems to be, at the moment, the priority for an investment decision. Second are market requirements, as new textile products are developed, creating the need for solvent systems development. Running cost reductions are also a factor.
[197, Comm., 2001] with reference to:
“Paolo Zanaroli, Sperotto Rimar S.p.A., Malo, Italy; Optimisation of wool cloth processing with solvent: scouring and carbonising in perchloroethylene - Proceedings of the Aachen Textile Conference, November 1997, pages 417 – 424”
4.10 Final effluent/emission abatement techniques 4.10.1 Treatment of textile waste water in activated sludge system with low food-to-micro organisms ratio (F/M) Description Aerobic biological treatment techniques are widely used to treat mixed textile waste water. In most cases, complete-mix activated sludge systems are used. The description and performance of this type of technique is treated in detail in another BREF ([196, EIPPCB, 2001]).
Textile waste water is a mixture of many different chemical compounds which can roughly be classified into easily biodegradable, hardly biodegradable (recalcitrant) and non-biodegradable compounds. In activated sludge systems, easily biodegradable compounds are mineralised, whereas hardly biodegradable compounds need special conditions, such as low food-to-massratios (F/M) ( 0.15 kg BOD5/kg MLSS · d, or even 0.05 for mineralisation below optimum temperatures), adaptation (when the compounds are discharged very regularly) and temperature higher than 15 °C (which is normally the case for textile waste water).
F/M is the most relevant design parameter. If they remain under the mentioned F/M value, hardly biodegradable chemicals, such as nitrilotriacetate (NTA) (“GDCh, 1984”), mnitrobenzene sulphonate and its corresponding amine (“Kölbener, 1995”), polyvinyl alcohol (PVA) (“Schönberger, 1997”) and phosphonates (“Nowack, 1998”) are degradaded and mineralised.