|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ/BMZ, 1995, 736 pages)|
|Trade and industry|
|59. Textile processing|
2.1 Fibre conditioning
Before they are processed, all natural fibres must be conditioned. This involves the removal of all manner of extraneous matter.
The by-products of conditioning (cottonseed and linseed, wool fat, silk gum) are in some cases commercially viable substances, and dry waste can in theory be returned in the soil as fertiliser, although another method of treating it is to compost it. A large proportion of the "waste" produced when native fibres are cleaned may therefore be regarded as commercial commodities, although all this requires additional treatment.
With regard to emissions, only cotton ginning and raw wool washing are of special significance.
Only about one third of the weight of a cotton boll is in the form of spinnable fibres. When the cotton is ginned (cleaned) - an operation normally carried out in situ - cotton seeds are obtained as a by-product, from which cotton seed oil and meal are produced. Another by-product is known as linters, which are used amongst other things in the manufacture of synthetic silk and viscose (spun rayon). The actual husks and waste constitute some 15% of the weight of the cotton boll and can be ploughed back into the soil.
Ginning is a dry, mechanical process which generates considerable noise and quantities of dust. While the latter of these two problems can be greatly relieved in modern mills by the use of extractor and filter systems, the wearing of ear protection is absolutely essential to combat the former.
Emissions from raw wool washing are far more problematic. In contrast to cotton ginning, raw wool is washed centrally in large industrial facilities remote from the place where the wool is obtained. The operation yields between 300 and 600 g attendant material per kg of washed wool. In addition to the wool fat which is a saleable commodity per se, being used for technical and cosmetic purposes, biocides and the like from sheeps wool are present in the washing water. Raw wool washing may therefore be regarded as one of the major wastewater pollution problems in the textile industry.
Today, wool fat is generally extracted before the wastewater is discharged.
A further factor to be considered is that complex purification plants are used to condition the still highly polluted wastewater (COD around 15,000) so that it can be discharged into the drains (on the subject of wastewater purification, see also the environmental brief Mechanical Engineering, Workshops, Shipyards and Wastewater Disposal).
2.2 Spinning and yarn production
Because of the high spindle speeds reached on new machines (ring spindles up to 20,000 rpm, rotor up to 110,000 rpm), spinning mills can generally be assumed to generate a great deal of noise.
Noise levels of 70 to 100 dB(A) are commonly recorded in work rooms.
As the spinning process calls for a specific room climate, i.e. temperature and relative humidity, which must be as constant as possible and not affected by time of day or night, or season, practically all spinning mills today are fitted with powerful air-conditioning systems. To limit the cost of air-conditioning, the production plant must be well insulated against external temperature changes. In the sixties and seventies this led to a windowless architecture for textile mills with extremely good values for insulation against temperature variations and against noise.
Since the eighties, windowless designs have been frowned upon by the building authorities in the light of the physiological and psychological problems caused to employees. To improve workplace design fields of vision of around 2 to 5% of the floor area of the workrooms must be provided, and special glazing is needed for this.
The high mechanical stress on fibres during the spinning process results in the production of considerable quantities of dust, which must be carefully extracted for industrial safety reasons and in order to keep the product clean. Emissions can be prevented and dust extracted by means of special machine enclosures and extraction systems and via the air-conditioning system which keeps the air in the rooms circulating. Air is not reintroduced until it has been passed through automatic filter installations. The filter dust is not dangerous and its disposal is therefore not a problem.
2.3 Weaving and knitting
Although considerable progress has been made in the weaving sector over the last twenty years, the whole area of noise nuisance and, closely associated with it, vibrations coming from looms, cause major problems.
Noise levels of 85 to 107 dB(A) must be expected in weaving rooms, according to the design, type, fitting, erection and number of looms used, fabric structure, building type and size etc. The vibrations transmitted from the running looms to the building can, under certain circumstances, cause a nuisance to the local population and damage to nearby buildings, and to avoid this special vibration absorbers are now provided. Generally speaking, the comments made about noise and dust emissions with regard to spinning mills apply here too.
The sizing of warp threads in weaving mills gives rise to emission problems.
Natural substances such as starch and cellulose products and synthetic products such as polyvinyl alcohol, acrylates, PVC, oils and fats are used as sizing material.
The evaporation fumes produced during the drying stage consist mainly of steam. The small amounts of sizing agents contained are not regarded as environmental pollutants.
On the other hand the sizing liquor may no longer be discharged into wastewater in Germany as it is a "concentrate" according to Anhang 38 [Appendix 38] to the Rahmen-Abwasserverwaltungsvorschrift [General Administrative Framework Regulation on Wastewater].
However, the sizing coating itself has much more serious repercussions in the subsequent textile finishing stage where the size first has to be completely removed. In finishing works which handle primarily woven goods, up to half the wastewater pollution may derive from dissolved sizes during washing.
There is currently a trend in the USA towards the use of sizing agents which can be recycled; this aids the work of the finisher in terms of disposal, and also saves up to 90% of sizing agent due to recycling. In Europe, where the textile industry, in contrast to the USA, is largely horizontally structured, this trend has yet become established. From the point of view of recycling, which is attracting increasing economic interest, this could provide a certain environmental benefit in the woven products sector.
Emissions in the knitting industry are substantially lower than in the weaving sector, with noise levels of around 77 to 90 dB(A), and dust and vibration emissions are a rarity. More problematic are the slip agents applied to the yarn which, as in the case of the warp sizes, only appear at the textile finishing stage, where they pollute either the wastewater (from washing) or the waste air (in thermofixing).
2.4 Textile finishing
In contrast to textile production, noise plays only a very minor role in textile finishing. On the other hand odour emissions are generated from drying and thermofixing processes, and particularly wastewater pollutants, which transfer into the water during cleaning and the various textile finishing processes. The textile finishing industry both consumes relatively large quantities of water and generates large quantities of wastewater.
Only general comments can be made about water consumption in the finishing stage (see table 1), as this is determined not only by the type of fibres processed, but also by the article, type and extent of finishing, as well as the technology applied (continuous/discontinuous process) and batch size structure. For the textile finishing industry in the Federal Republic of Germany the latest figures of the TVI-Verband for 1988 set the specific water consumption at 120 l/kg goods (3).
Table 1 - Water consumption in the textile industry (amended per (1))
|Fibre type/make-up||Mean water consumption in l/kg material|
|a)||by fibre type|
|cotton||50 - 120|
|wool||75 - 250|
|synthetic fibres||10 - 100|
|flock/yarn||100 - 200|
|knit||80 - 120|
|printing||0 - 400|
2.4.1 Wastewater contamination
If textile finishing mills are sited in the catchment area of efficient municipal sewage works, textile wastewater should preferably undergo mechanical biological treatment before being fed to these works and discharged into the drains (indirect discharge). If this option is not provided, and the wastewater therefore has to be discharged directly into the drains, it must first be treated in the mills own treatment plant to meet legal requirements (direct discharge). (For more detailed information on the requirements applicable in Germany, see 3.3).
Although most of the substances in the wastewater are biodegradable, discharges into open drains can in some circumstances, during the biological decomposition stage, reduce the oxygen content of the drain water to below the level required for a healthy water quality and lead to fouling of the water.
The textile finishing industry also uses a range of compounds which are not biodegradable per se.
Below, we look briefly at the following types of wastewater pollution:
For dyes and many surfactants which do not degrade readily (but which are increasingly being replaced by more easily biodegradable ones), the said mechanical biological treatment is inadequate if there is insufficient biomass to absorb the dyes (the bonding of excess sludge to bacterial protein is the principle method by which water-soluble dyes are eliminated, see also 3.3). Past experience has shown that a combination of physico-chemical and biological wastewater treatment is required in most cases to achieve a satisfactory treatment level, with a facility for treating heavily polluted partial flows (e.g. dye liquor) separately.
· Settleable solids
The values normally found in the wastewater relating to settleable solids are subject to enormous fluctuations; they are dependent on a number of factors such as finishing process, fibre type, fibre make-up and whether batch or continuous treatments are used. Sometimes the undissolved substances remain in suspension and cannot simply be filtered off. Most values should be below 50 ml/l.
· Heavy metals
The pollution of textile wastewater with heavy metals is limited by the current state-of-the-art. Cadmium is hardly ever present and mercury is present only insofar as it is introduced via soda lye and hydrochloric acid (manufactured with mercury electrodes). Chromium, cobalt and copper may penetrate the wastewater from a number of dyeing processes, and zinc from more sophisticated finishing processes (wash and wear cotton articles) (zinc in the lower milligram range, the others mostly under 1 mg/l).
Hydrocarbons are more serious pollutants. They derive mainly from yarns which are coated with oil to give them the right slip properties, and to a lesser extent from the residues of sizing agents (impregnation).
· Organic halogen compounds
Other serious pollutants are chloro-organic compounds. The AOX total parameter (adsorbable organic halogen compounds) introduced for wastewater covers a spectrum of substances which are hardly comparable in terms of their ecological and toxicological properties (highly volatile chlorinated hydrocarbons, PVC, non-toxic green pigments, toxic chlorophenols etc.).
Sources for the AOX total parameter are primarily chlorinated bleaches, anti-felting finishing of wool, dye accelerators (carriers) used to dye synthetic fibres, chlorinated reactive dyes and solvent soaps with solvents from the chlorinated hydrocarbon range (per), which are incidentally also used as the sole "dry cleaning" agent for degreasing polyester articles.
The number of compounds used which are regarded as particularly environmentally pollutant has already fallen considerably. Whereas up to 5% carrier (colour accelerators for polyester fibres), based on product weight, was previously used, dyeing is now carried out mainly in high-pressure installations where only around 0.5% carrier is needed, basically as a precautionary measure, and even then only aromatic ester-based substances are used.
Pentachlorophenol (PCP), formerly used occasionally as a preservative for heavy fabrics, has been banned since 1986. However, products of a similar composition are used with native fibres, frequently from the range of pesticides manufactured; examples of such products are chlorinated phenoxyacetic acids, hexachlorocyclohexane, DDT and allied substances.
An equally serious pollution problem is posed by surface-active agents, called surfactants or detergents, which are used as washing, emulsification and wetting agents, as adjustment agents for dyeing processes, as auxiliaries to improve smoothness and softness and for a range of other purposes, and to a far larger extent as dyes, and which in some cases are not fully biodegradable either. However, in the Federal Republic of Germany and other central European countries, a minimum 80% degradability under the conditions prevailing in treatment plants is required for products used specifically as washing agents. Since this ban has been imposed there has been a definite improvement in water quality.
Surfactant water pollution is due not only to its organic load, but also its surface-active action, which on the one hand hampers the self-purifying capacity of rivers and on the other causes problems for the micro flora and fauna, and fish.
Water-soluble dyes are a further environmental pollutant specific to the textile finishing industry. If heavily coloured - something which cannot generally be determined in textile effluent after biological treatment - the light reaching plants is reduced. If wastewater from dyeing plants constitutes 20% or less of the municipal wastewater to which it is added, a mechanical biological treatment plant is generally able to bind this dye content to the excess sludge (by a sort of dyeing process) and then degrade it in the digestion tower.
Where this is not the case, substantial quantities of dye may pass through the treatment plant into the outlet and cause a perceptible coloration of the drain water.
Now that certain limit values for the density of colour of textile wastewater have been specified in Anhang 38 (appendix 38) of the Rahmen-Abwasser-VwV [General Administrative Framework Regulation on Wastewater], this wastewater now requires additional decolouring in some cases.
A partially anaerobic treatment stage, comprising the addition of ferrous (II) salts in conjunction with lime, active carbon biology and, more recently, processes using membrane technology (ultra-filtration, reverse osmosis) have proved to be effective processes. In special cases, partial dye recovery and process water recycling are possible in conjunction with membrane technology.
· Water temperature
Wastewater temperature is another major form of pollution. In dyeing processes, so much hot water is discharged that, in the absence of any counter-measures, total wastewater temperatures may exceed 40°C, even though 35°C is the maximum permissible temperature. In many cases this heat can be recovered in heat exchangers, then returned to the process.
A pH of between 6 and 9 is prescribed for wastewater discharged from treatment plants in the Federal Republic of Germany, as in most other European countries.
Because partially acidic and partially alkaline wastewater is produced in the mill, according to the treatment stage and process, a balancing tank to hold around 50% of the daily wastewater quantity is normally required by the approving authorities. Following a partial mutual neutralisation of the various flows, the wastewater is routed from here to the wastewater plant at a fairly balanced pH and at a constant quantity.
While wastewater from wool processing plants generally has excess acidity, which requires to be buffered with alkali, the pH of wastewater from cotton processing plants is usually in the alkaline range. In this case the simplest, and at the same time, most environmentally friendly method of neutralising the wastewater involves the use of flue gas.
· Major incidents
In principle major incidents may only be expected as a result of negligence, and a useful preventive measure is to appoint an "industrial water pollution control officer".
In some of the German Federal states, Abwasser-Eigenüberwachungsverordnungen [AbwEV - wastewater self-monitoring ordinances] have been issued which oblige businesses in the textile finishing industry to fit control and measuring installations to keep an internal record of certain wastewater parameters and to report major incidents (27).
2.4.2 Gas and steam emissions
Gas and steam emissions are generated by textile finishing processes when fumes penetrate the exhaust air during the dyeing and drying operations, although these more general operations do not present any real environmental pollution hazard. Table 2 provides an overview of the main sources of exhaust air emissions in textile mills.
Exhaust air from the thermofixing of synthetic fibre articles is not only more unpleasant but also more noxious, for it entrains oligomers of fibres and fragments of smoothing agents (including ethylene oxide) which may constitute up to 0.2% of the weight of the goods. Heat recovery installations - which are a must in all cases for energy reasons - arrest a considerable proportion in the form of a fatty condensate, but these substances nonetheless penetrate the wastewater during the cleaning operation (high-pressure cleaners).
Formaldehyde, which makes the eyes water and irritates the skin, may be produced in connection with the high-grade finishing of cotton articles, but formaldehyde pollution has been dramatically reduced with the introduction of modern, low-formaldehyde, etherified products, which were also required due to the effects on pregnant women.
In Europe, an increasing number of plants are resorting to thermal and/or catalytic afterburning to treat exhaust air from tenters, and all organic substances are therefore combusted to form CO2, CO and NOx.
A further source of gas and fume emissions are coating installations, which yield solvents. There is a simple remedy - provided that chlorohydrocarbons are not used - that of elimination via the combustion air in the boiler plant.
In Germany, the requirements of the 2. BImSchV [Second Ordinance on the Implementation of the Federal Immission Control Act] apply to all the above-mentioned emissions. The plants concerned must be monitored on the basis of emission measurements.
2.4.3 Noise emissions
No significant noise is emitted in the textile finishing industry other than from the ventilation units commonly found elsewhere.
2.5 General impacts on the environment
In addition to textile-specific emissions which are the real object of this brief, forms of environmental pollution which are also found in many other branches of industry can occur.
· Furnace installations
Thermal energy consumption is relatively high, particularly in the finishing stage (~ 13 kWh/kg goods). The firing power from the boiler plant required for process heat (steam, hot water) and for space heating usually lies within the range of 6 - 10 MW (9 - 15 t steam/h). Moreover, because of the high energy efficiency in plants which need power and heat energy simultaneously, power/heat couplings are used. The environmental brief Thermal Power Stations contains further environmental information about these installations.
· Water treatment plants
A certain quality of process water (i.e. it must not contain iron or manganese, it must be not very hard but must be clear) is required for textile finishing (washing and dyeing processes) - a quality which is not often attained in surface, spring or tap water. The rinsing wastewater deriving from regeneration in treatment plants generally has a high salt content and must be fed to partial or complete treatment plants with the process wastewater.
Goods traffic: The large material turnover results in a constant stream of goods traffic for supplying raw materials, taking away finished goods and for transport within the plants from one processing stage to another.
Passenger traffic: Textile factories often operate a two to three shift system, and times of shift changes can result in traffic jams and other problems.
Reference is made to the environmental briefs Planning of Locations for Trade and Industry, and Transport and Traffic Planning, for information on environmental impacts and environmental protection measures.
· Socio-economic and socio-cultural factors
These days, plants for cloth manufacture, i.e. the textile production stages of spinning, weaving, knitting and finishing, are capital intensive operations. (In contrast to subsequent processing in the garment industry, where the wage bill accounts for a high proportion of costs.)
The high capital input means that the machines must run, especially in the spinning and weaving sector, in 3-shift and sometimes even 4-shift operation around the clock, and in many areas at the weekend too. In an average vertical operation, i.e. with a spinning mill, weaving mill and finishing section, and with a production of about 6 million running m/year, about 300 people are now employed in three shifts in industrialised countries.
The proportion of women employed has declined sharply, but traditional family structures are nonetheless greatly affected by multiple shift operation. Furthermore, the personnel structure has been moving in the direction of trained industrial workers, and constant further training is necessary even for supervisory personnel.
The legislative framework and options for the enforcement of provisions in individual countries have a substantial influence on the impacts a textile mill has on the environment.
On the one hand there are statutory regulations and their enforcement for clean water, air and soil and for the rational use of energy, and on the other there are regulations to meet the requirements of the employees in terms of working conditions. Due mention should also be made of environmental and industrial safety provisions which are inadequate or simply do not exist, excessive working hours, low pay levels and child labour. These factors all have an influence on the quality of life of those directly affected, and the economic situation of the textile industry generally.