«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Potential for pollution of land Two main "wastes", grease and sludge, are produced as a consequence of the scouring activities (and related effluent treatment).
Depending on its oxidation extent, it may be possible to recover from 20 to 40 % of the grease initially present on the raw wool. This is to be regarded as a by-product rather than a waste, since it can be sold to lanolin refiners for the production of high-value products in the cosmetic industry. However, high levels of pesticides residues in the grease can also be a problem for the lanolin refiners, especially for the production of lanolin-based pharmaceuticals and cosmetics, since more expensive and sophisticated techniques have to be used to reduce the pesticides to acceptable levels. Acid-cracked grease has no market value and has to be landfilled.
The sludge produced as a result of physico-chemical treatment of waste water also contains grease, dirt and the portion of pesticides which are strongly associated with either grease or dirt.
Concentrates and sludges from evaporation or membrane filtration may also contain suint, which is mainly potassium chloride and potassium salts of fatty acids. Suint is a by-product which can be used in agriculture.
Sludge and concentrate disposal may follow several routes:
· incineration (with heat recovery) · pyrolysis/gasification · brick manufacturing · composting or co-composting with other organic material · landfill.
The first three sludge disposal routes destroy the organic material in the sludge, including grease and pesticides. The ash from incineration may contain potassium salts, derived from suint, and heavy metals characteristic of the soil on which the sheep producing the wool have grazed. The ash is normally disposed of to landfill. The characteristics of the char from pyrolysis/gasification are unknown and this char is also typically disposed of to landfill. The use of wool scour sludges in brick manufacture results in no residues for immediate disposal. These three sludge disposal methods probably have the least potential to pollute land.
Wool scour sludges cannot be composted alone, but require the addition of carbon-rich organic material. Green waste from agriculture or horticulture has been used. Composting is not yet regarded as a fully developed, fail-safe technique and only partly degrades the pesticides present in the sludge. However, since the pesticides present in the sludge are there because of their lipophilicity or their strong propensity to absorb onto solids, they are likely to be immobile in soil, and spreading of compost derived from wool scour sludges on agricultural land is unlikely to pose an environmental risk of any significance.
Landfill is the simplest and often cheapest method of disposal of sludges. In the longer term, however, landfill is not believed to be either economically or environmentally sustainable. The fate of wool scour sludges in landfill is not known, but there is a small potential for the ectoparasiticides present to arise in leachate. Anaerobic degradation of the organic material in the sludge will give rise to methane emissions [187, INTERLAINE, 1999].
Potential for pollution of air Air pollution is not a main issue for wool scouring processes. Nevertheless two issues can be mentioned.
Hot acid cracking, which involves heating the scour effluent with sulphuric acid, when used near residential areas, has been the subject of odour complaints.
Incineration is used in conjunction with evaporation of the effluent because the surplus heat from the incinerator can be used in the evaporation process. Incineration of wool scour sludges has potential for air pollution. Since sludges contain relatively high levels of chloride (from suint) as well as organically bound chlorine from ectoparasiticides etc., there is potential for the production of polychlorodibenzodioxins and furans, when they are incinerated (catalytic and high temperature incinerators are now available to prevent these emissions). The sludges also contain relatively high levels of sulphur and nitrogen and the combustion process therefore produces SOX and NOX. Dust and odours should also be taken into account.
126.96.36.199 Cleaning and washing with solvent Various solvent processes exist that use a non-aqueous solvent for scouring wool.
The Wooltech wool cleaning system involves the use of trichloroethylene and does not use any water in the washing process. A schematic layout of the process is shown in Figure 2.5.
The following information was submitted by [201, Wooltech, 2001].
Wash bowls Wool is received in bales, unpacked and then fed into the reception area. This wool is lightly broken up and fed through a series of solvent wash bowls (typically 3 or 4) and washed with a countercurrent flow of solvent. Up to 10 kg of solvent is added for the production of 500 kg of clean wool, however this is a function of plant management and maintenance, the exact plant arrangement, and the quality of wool being processed.
Clean, solvent saturated wool is taken from the last wash bowl to a centrifuge where the solvent concentration is reduced to around 4 wt%. A centrifuge has been found to be particularly effective in this duty owing to the low surface tension and significant density of trichloroethylene. The wool with a small quantity of solvent is taken to a dryer where warm air is used to evaporate the last quantity of solvent. The processing area from the wash bowls through to the centrifuge and the dryer is all fully enclosed and is kept under a slight negative pressure by evacuating air to a vapour recovery system.
The solvent from the first washing bowl is processed through high-speed centrifuge equipment to remove solid particles and recycled back to bowl 1. A proportion of the fluid is drawn off for grease removal and upgrading for recycling.
Dirt separation The dirt slurry from the Dirt Separation stage is sent to an indirect heated Dirt Dryer, where the solvent is evaporated off (and recovered), leaving a warm, dry, and solvent free dirt stream.
Expected pesticide analysis of dirt will result in no organochlorines (OC), less than 1 ppm organophosphates (OP) and less that 0.1 ppm synthetic pyrethroids (SP). Further reducing these levels requires the relatively simple modification of fitting a small Solid Bowl Centrifuge, such that the dirt slurry, on its way to the dirt dryer, is rinsed with fresh solvent. This will remove the grease-associated pesticides from the dirt and send these back to the evaporator where they will leave with the grease stream.
Solvent evaporation system Solvent is recycled by various stages of evaporation in the Solvent Evaporation System. The first stage of evaporation is a multiple effect evaporator, which does the bulk of the solvent recovery work. It boils the spent solvent from a concentration of typically 2 wt% grease up to 20 wt% grease (i.e. 90 % recovery of solvent). To recover all possible solvent from the grease, it has been found necessary to use three stages of evaporation – each at progressively lower pressure and higher temperature. It has been found practical to evaporate the grease down to containing 1 wt% solvent, corresponding to a 99.98 % recovery of solvent through the evaporation stage.
Vapour recovery unit
Other areas where solvent is recovered include when the vapour leaves the dirt dryer, when the solvent laden air leaves the wool dryer, and when the air saturated with solvent is extracted from the wash bowl area/ wool centrifuge/ wool dryer area. The combined stream from these areas is sent to a Vapour Recovery Unit. This consists of a refrigerated primary collection system followed by activated carbon adsorption recovery system.
As discussed, the Wooltech wool cleaning system does not use any water in the washing process. There is, however, a small flow of water into the solvent system. This is due to moisture in the wool, moisture in the air and some input from vacuum raising equipment (steam ejectors). This water, referred to as fleece and steam moisture, is separated from the clean solvent in the solvent recovery section by gravity. Whilst the solubility of the solvent in this water is low, it is nevertheless saturated in solvent, which is thus removed in a two step process.
In the first step, most of the small proportion of solvent in fleece and steam moisture is removed by heating the water and stripping it with air in the Solvent Air Stripping Unit. The small flow of solvent is recovered by condensing it and then by passing it through the Vapour Recovery Unit.
In the second step, a free radical process based on Fenton’s reaction (a redox reaction between iron and hydrogen peroxide) is used to remove the traces of solvent left after stripping in fleece and steam moisture. Using an improved Fenton effect, the Residual Solvent Destruction Unit is effective in eliminating all traces of solvent from water by oxidizing/de-halogenating, thereby destroying the solvent molecules. The solvent is broken down into chloride ions, carbon dioxide and water upon treatment with hydrogen peroxide. Provision is allowed to ensure the water is neutralized prior to discharge. The fine detail of the destruction process is confidential, however processes that use the improved Fenton Effect are well established.
Another source of waste in the Wooltech plant is contaminated liquids from maintenance activities or as a result of recovered spills. These fluids are treated in a very similar manner to process water. The first step of Maintenance/Spill Recovery and Recycle is the recovery of the bulk of the solvent, which is performed by boiling most of the solvent from the water. Finally, the mildly contaminated water is treated in the Residual Solvent Destruction Unit.
It is expected that the Enhanced Fenton Process Residual Solvent Destruction Unit will reduce hazardous substances in water (including solvents, breakdown products and water-solubilized pesticides) to near zero. This is consistent with the long-term objectives (2020) set by OSPAR (protection of the marine environment) and the European Water Framework Directive (for surface waters).
Air emissions Stack emissions of trichloroethylene (TCE) in air will meet new VOC directives of 2ppm using a scrubbing fluid comprising of activated carbon slurry in water. This fluid will then be treated as outlined above in the Residual Solvent Destruction Unit. It should be noted, however, that TCE is very unstable and is readily broken down by free radicals that are photo-induced in the air by ultraviolet radiation (the Photo-Fenton effect). The hydroxyl radicals involved in the destruction are the same as used in the liquid treatment described above.
Auxiliary water A small boiler unit is also used as a separate utility with a boiler blowdown of approximately 11 kg/h. The water quality in the boiler is directly related to the characteristics of the local water sources.
Energy consumption The consumption of electricity in the cleaning process (not including treatment of emissions) is
0.243 kWh/kg of greasy wool. Natural gas consumption is 0.79 MJ/kg of greasy wool.
188.8.131.52 Environmental issues associated with wool scouring (with organic solvent) The Wooltech system described above, uses trichloroethylene as solvent. Trichlorethylene is a non-biodegradable and persistent substance (trichloroethylene is on the EPER list).
Unaccounted losses of this solvent arising from spills, residues on the fibre, etc., if not adequately treated to destroy the solvent, may lead to diffuse emissions resulting in serious problems of soil and groundwater pollution.
As far as water and energy consumptions are concerned, the Wooltech system shows lower consumption levels compared to a typical scouring process using water. A more accurate balance of the inputs and outputs in this process is reported in Section 3.2.2.
2.3.2 Cotton and flax Raw cotton is a much cleaner raw fibre than wool and initial operations are mainly dry. The fibres are supplied to the spinning mill in compressed bales. The fibres are sorted by grade, cleaned to remove particles of dirt and blended with fibres from different bales to improve the consistency of the fibre mix. Sorting and cleaning is performed in machines known as openers.
With flax, the isolation of the fibre from the flax stem is done in different steps. After crop (plucking) the flax is retted (dew retting, water retting, enzymatic, microbiotic, steam or chemical retting). Retting is a wet process that can result in waste water with high contents of COD and BOD: pectinic and hemicellulosis substances are degraded in this step. Rovings are produced by further mechanical treatment before spinning.
For silk production the silk worm is killed with steam and the filament is unwound directly from the cocoon. The filament is submitted to pretreatment processes to remove the silk gum and other organic impurities (see Section 2.6.3).
2.4 Yarn manufacturing Almost all textile apparel products are made from spun yarns of 100 % natural fibres, 100 % man-made staple fibres or blends. Only a few apparel products, for instance smooth sportswear, are made exclusively of filament yarns (although increasing use is being made of fabrics that contain multifilament yarns, generally textured, and one or more staple fibre yarns).
Secondary spinning is the process by which staple fibres are transformed into yarn suitable for
the textile industry. There are two main spinning processes:
· the wool spinning process · the cotton spinning process.
2.4.1 The wool spinning process The wool spinning process is mainly used to produce wool and wool-blend yarns. A distinction is made between the worsted and woollen process. In worsted spinning, higher-quality and longer fibres are processed and the result is a fine yarn which is used to produce worsted fabric.
In the woollen spinning system, shorter fibres are processed.
In the worsted process the fibres are paralleled in a combing machine and are then drawn and spun. In the woollen system the fibres are only carded and then spun. The resulting yarn is then twisted (if required) and finally prepared for the subsequent treatments (dyeing, weaving, tufting, etc) through an operation called winding.
In both the woollen and worsted systems the various fibres (e.g. wool fibres from different sources, different types of synthetic fibres) are combined during the blending operation. In order to allow efficient mechanical processing in the subsequent operations, spinning lubricants are applied to the fibres at this stage (or later, before or after carding, depending on the system applied).
2.4.2 The cotton spinning process
The cotton spinning process is generally used for cotton and man-made fibres. As has already been described in Section 2.3.2, cotton fibres are first submitted to opening and cleaning
operations. The following steps, which are the same for cotton and man-made fibres, are:
· carding · combing · drawing · roving · spinning · twisting (if required) · winding.
2.4.3 Environmental issues The preparation agents (conditioning agents and spinning lubricants) applied to the fibre during the spinning process have significant environmental implications for the subsequent finishing steps of the textile chain. Since these auxiliaries, together with spin finishes added in primary spinning (in the case of man-made staple fibres), need in general to be completely removed before dyeing, they are found either in the exhaust air from the high-temperature processes or in the water from wet treatments. In the first case they give rise to air pollution, whereas in the second they contribute to the organic load of the final effluent.