«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Needle felt carpet Almost all fibres may be used for the production of needle felts (PP, PA, PES, PAC, wool, cotton jute/ sisal, coconut fibre and viscose). However, mostly man-made fibres are used.
Needle felts finishing involves:
· dyeing (rarely done) · coating · mechanical finishing (rare) · chemical finishing.
Woven carpet Both natural and synthetic fibres are used in woven carpet production.
Carpets are woven using dyed yarns (so piece dyeing is not applied in woven carpet production). The final carpet is then submitted to mechanical and chemical finishing treatments.
3 EMISSION AND CONSUMPTION LEVELS
3.1 Introduction The main environmental issues relevant for the textile industry have been dealt with in detail, process by process, in Chapter 2.
The textiles industry has always been regarded as a water-intensive sector. The main environmental concern is therefore about the amount of water discharged and the chemical load it carries. Other important issues are energy consumption, air emissions and solid wastes and odours, which can be a significant nuisance in certain treatments.
Air emissions are usually collected at their point of origin. Because they have been controlled for quite a long time in different countries, there are good historical data on air emissions from specific processes.
This is not the case with emissions to water. The various streams coming from the different processes are mixed together to produce a final effluent whose characteristics are the result of a
complex combination of factors:
· the types of fibres involved · the types of make-ups processed · the techniques applied · the types of chemicals and auxiliaries used in the process.
Furthermore, since the production may vary widely not only during a year (because of seasonal changes and fashion) but even over a single day (according to the production programme), the resulting emissions are even more difficult to standardise and compare.
The ideal approach would be a systematic analysis of the specific processes, but data availability is very poor for many reasons, including the fact that legal requirements have tended to focus on the final effluent rather than on the specific processes.
Mindful of these limitations on the characterisation of waste water emissions, it has proven appropriate to identify narrow categories of finishing industries and then to compare the overall mass streams between mills belonging to the same category. This approach allows a preliminary rough assessment in which, by comparing the consumption and emission factors of mills within the same category, it is possible to verify given data and identify key issues and macroscopic differences between the similar activities.
Input/output considerations will therefore be addressed step by step, starting from overviews of the overall mass streams and ending in a more detailed analysis of single processes and/or issues that are of some concern.
This is the approach that will be followed in this chapter for all categories of industries identified in Chapter 2 (Section 2.14). One exception is represented by odours and solid wastes issues which will be dealt with in Section 3.5 and Section 3.6 at the end of this chapter. It should be noted that the data sets given within the tables reported in this chapter represent examples only.
3.2 Wool scouring mills3.2.1 Cleaning and washing with water
This section refers to a well defined category of companies whose general features are briefly described in Section 2.14.1, while the scouring process itself is described in Section 22.214.171.124. The information reported in this section reflects an industry survey of raw wool scouring and effluent treatment practices in the European Union, undertaken by ENco in 1997/98 on behalf of INTERLAINE.
In addition, a completed questionnaire was received from an Australian subsidiary of a European company and a set of data was submitted in a second stage by some Italian mills ([193, CRAB, 2001]).
Production volumes varied greatly, from 3000 to 65000 tonnes of greasy fibre per year.
Working patterns also varied, ranging from companies working 24h per day on 7 days per week, to companies working 15 - 16h per day on 5 days per week.
As already highlighted in Section 126.96.36.199, the arrangements for circulating the scour and rinse liquors may vary widely. There are also significant differences in processing conditions due to the nature of the wool processed (fine or coarse) and the contaminants present. All these factors, combined with the type of waste water treatment adopted, influence the quality of the effluent from the scouring mill. Table 3.1, Table 3.2 and Table 3.3 summarise the data collected at different wool scouring sites. Some companies have been grouped together in an attempt to find a relationship between the liquor handling system adopted and the resulting consumption and emission levels. The original identification letters for the different companies have been kept.
Fine, extra-fine and coarse wool processors appear in separate groups to enable easier comparison.
The wool scouring industry has a reputation for high water consumption. [18, VITO, 1998] reports 10 – 15 l/kg greasy wool as the range of water consumption for traditional installations, although lower values were observed in the surveyed companies.
Net specific consumption can be reduced by installing a grease and dirt recovery loop, through which water is recycled to the scouring bowls. It is also possible to apply similar recycling technology to waste rinse water. Mill L had such an arrangement, using ultrafiltration to treat the rinse water.
In addition to the above process-integrated recycling arrangements, it is also possible for mills with evaporative effluent treatment plants to re-use the evaporator condensate for feeding scour and/or rinse bowls. Five of the mills surveyed treat effluent by evaporation, but only three of these recycle the condensate. One of those that does not recycle the condensate cites problems with build-up of ammonia and odours as the reason for not recycling.
In fine wool scouring, gross water flow in the scour varies greatly, from 5 l/kg in the case of Mill N to more than 10 l/kg for Mill E. The latter apparently operates in similar conditions to Mill G, but it has an old and complex system for collecting, settling and filtering effluent, which probably explains its lower performance. Mill G recycles scour liquors at three times the rate of Mill N.
140 Textiles Industry
Chapter 3Net water consumption varies even more widely than the gross liquor circulation (from 10 l/kg in the case of Mill E to 0.36 l/kg for Mill J). In the latter this very low net specific water consumption is achieved through total recycling of condensate from the effluent treatment plant (anaerobic lagoon/evaporation), plus an unstated amount of process-integrated recycling via a grease/dirt removal loop.
Of the coarse wool scourers, two have dirt removal/grease recovery loops recycling to the first scour bowl; one of these two also has a rinse water recycling loop (Mill L). All three scourers bleach in the last bowl of the scour train, using hydrogen peroxide and isolating the bleaching bowls from the countercurrent.
It is possible to calculate gross circulation in the scour at two of the mills. In both cases, it is significantly higher than encountered in all but one of the fine wool scourers. This may be because the coarse wools contain more dirt than fine wools [187, INTERLAINE, 1999].
Net water consumption varies considerably in the three mills. Mill C has the highest net water consumption of any of the surveyed mills that process fine and coarse wool. This mill recycles no liquor at all. Mill H has a moderately low net water consumption, which is achieved by using the highest capacity dirt removal/grease recovery loop encountered in this survey. Mill L recycles rinse water and also presumably has other recycling arrangements to achieve its low net consumption.
Another factor playing a potential role in net water consumption is the production volume.
Figure 3.1 shows, by plotting net consumption against production volume, a tendency for net specific water consumption to fall as production volume increases.
There are clearly mills whose net water consumption is below the norm, which is represented by the drawn curve.
Figure 3.1: Net specific water consumption plotted against production volume [187, INTERLAINE, 1999] There may be several reasons for this relationship between water consumption and production volume.
Besides economies of scale in larger companies, possibly the most important reason is the mill’s perception of the economics of reducing water consumption. Some of the mediumsized mills may feel unable to make the required investment or may not have the staff resources to devote to the task [187, INTERLAINE, 1999].
No detailed information has been submitted about the characteristics of the grease recovery loops applied in the mills mentioned in Table 3.3. Therefore it is not possible to draw conclusions about the water consumption levels reported in the table.
Grease recovery One scourer (Mill C) has no grease recovery plant, while the remaining mills recover between 8 and 71 kg grease per tonne of greasy wool processed. The scourers at the bottom end of this range (Mill H, 13 kg/tonne and Mill L, 8 kg/tonne) are wholly or predominantly scourers of coarse wools, which contain lower percentages of grease in a more oxidised (less hydrophobic) form, which is more difficult to separate centrifugally. At the top end of the range is Mill E, which recovers 71 kg of grease per tonne of wool processed. This mill is a fine wool scourer with a centrifugal grease recovery plant and an acid cracking plant. The acid cracking plant produces a lower quality grease which must now be regarded as a waste rather than a byproduct, since it cannot usually be sold and has to be landfilled. The remaining mills, wholly or predominantly fine wool scourers, recover between 22 and 42 kg of grease per tonne of raw wool (average, 30 kg/tonne).
The most important chemicals used by scourers are detergents and builders. As for the data reported in Table 3.1 and Table 3.2, seven of the scourers use alcohol ethoxylate detergents and five use alkylphenol ethoxylates (the data are reported for only two mills). Two UK scourers also report the use of “solvent assisted detergent” for the removal of marking fluids from fleeces. Eight scourers use sodium carbonate as builder, two use sodium hydroxide and two use no builder.
No information has been submitted about the types of detergents used by extra fine scouring mills referred to in Table 3.3.
Scourers of coarse (carpet) wools are often asked by customers to bleach the fibre by adding hydrogen peroxide and acid to the last rinse bowl. Five of the scourers do this routinely or occasionally.
The seven users of alcohol ethoxylates consume an average of 9.1g detergent per kg greasy wool (range 3.5 – 16g/kg), whilst the five users of alkylphenol ethoxylates use an average of
8.0g detergent per kg greasy wool (range 5 – 16g/kg). There is therefore no evidence of economies of scale, nor of the often-claimed greater efficiency of alkylphenol ethoxylates over alcohol ethoxylates.
It is also frequently claimed that fine wools require more detergent for scouring than coarse wools. The survey shows that the fine wool scourers use an average of 7.5g detergent per kg greasy wool (range 5 – 10g/kg) while coarse wool scourers use an average of 8.5g detergent per kg greasy wool (range 3.5 – 16g/kg), so this claim also seems to be without foundation.
Figure 3.2 shows that there is a relationship between detergent feed rate and the rate at which effluent is discharged to the mills’ effluent treatment plants.
Detergent which is discharged in the effluent from the scour is lost, whilst recycling detergent via the grease recovery/dirt removal loop conserves much of it within the scour. Note that the values used in compiling this figure are calculated from annual usage divided by total wool processed and may differ from detergent feed rates to scour bowls used in the tables reported earlier (Table 3.1 and Table 3.2).
Figure 3.2: Relationship between the detergent feed rate and the rate of discharge of effluent to treatment [187, INTERLAINE, 1999] For builders, on the other hand, there is no obvious pattern related to wool type, detergent consumption or type or size of operation.
Several of the scourers also reported using varying quantities of acids and alkalis for cleaning purposes. These included hydrochloric, nitric, phosphoric and sulphuric acids, a mixture of organic and inorganic acids, and caustic soda. The use of sodium chloride for regeneration of the water treatment plant was also mentioned. Significant quantities of chemicals are used by some scourers in effluent treatment processes, but few data are available [187, INTERLAINE, 1999].
Energy Consumption The mills in this survey were not asked to give energy consumption figures. Data presented here come from a survey carried out in the UK in 19983.
Figure 3.3 shows the specific energy consumption (MJ/kg greasy wool) and the specific net water consumption (l/kg greasy wool) of the 11 mills which supplied data (the reported data refer only to the scouring process and do not include energy consumption for the waste water treatment plant).
The relationship between energy and water consumption is immediately obvious and is emphasised in Figure 3.4, where energy consumption is plotted against water consumption. As far as possible, the consumption figures used relate only to the scouring and related processes, such as effluent treatment.
Energy and water consumption both vary widely. Energy consumption ranges from 4.28 to
19.98 MJ/kg (average 9.29 MJ/kg) and water consumption varies from 1.69 to 18.0 l/kg (average 8.16 l/kg). R2 for the correlation is 0.906.
M Madden, ENco, personal communication, 1999.
Figure 3.4: Energy consumption plotted against water consumption for 11 UK scouring mills [187, INTERLAINE, 1999] The variation in water consumption in the UK study bore no relationship with throughput (as it did in the Europe-wide study).
See Figure 3.5.
There are probably two reasons for the relationship between energy and water consumption. The more obvious is that much of the thermal energy consumed in wool scouring is used for heating water. Rather less obviously, it is likely that the mills which have expended time and effort (and perhaps capital) on reducing water consumption will also have given attention to reducing energy consumption. This assumption is supported by the fact that heating 1 litre of water to scouring bowl temperature consumes 0.21 MJ, whilst the slope of the regression line in Figure
3.4 above is 1.09 MJ/l [187, INTERLAINE, 1999].
Chemical Oxygen Demand Specific COD loads before waste water treatment are indicated in the INTERLAINE document for only a few mills (see Table 3.1, Table 3.2 and Table 3.3). A global COD range of 150 - 500 g/kg of raw wool has, however, been estimated in the final summary (see Figure 3.6).
COD in the effluent immediately after the process is also influenced, apart from the quantity of contaminants present on the raw material, by the efficiency of the grease and dirt recovery system. Wool grease, dirt and suint are in fact the main contributors to the COD load, while detergent can be regarded as the smallest contributor. In this respect the specific COD loads
could be roughly estimated, using the data available, by considering: