«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
First of all, compared to the other sites, this mill treats almost half of the material in hank form (which requires higher liquor ratios). Secondly (like TFI 3) it also has a high production of shrink-resist treatment, which entails an additional step in the production sequence. The fact that this company is an integrated mill producing its own high-quality yarn and a premium product (at a premium price) may also explain why they appear to take a more relaxed view of the costsavings to be had from reducing water consumption.
154 Textiles Industry
Chapter 3As already stated, both TFI 3 and TFI 4 have high production on shrink-resist treatment. The higher specific COD shows this difference with respect to the other two mills. More representative, however, would be the level of AOX in the discharged waste water. High levels of AOX are typical of effluents from wool shrink-treatment, but data for this parameter were not made available.
TFI 2 has a very high production on chrome dyeing, which is confirmed by the higher emission factor for chromium (124 g/kg).
Source [193, CRAB, 2001]
Blank cells mean that relevant information is not available (1) Data are measured in homogenisation tank before on-site treatment (2) after on-site waste water treatment Table 3.10: Overview of consumption and emission levels from four mills finishing WO tops/yarn 184.108.40.206 Mills finishing yarn: mainly CO Table 3.11 contains the concentration values and emission factors of four mills that finish yarn consisting mainly of cotton. Specific water flows are between 100 and 215 l/kg. These values
are confirmed by “FhG-ISI, 1997” reporting lower and higher values (68, 73, 78, 83, 120, 128, 149, 181, 271 l/kg). In this respect, the high value of 271 l/kg (from a small mill processing less than 0.5 t/d) has to be closely questioned. The high specific water flow reported for TFI 4 corresponds to a company that dyes a high percentage of material in hank form. The site is also reported to not fully recover cooling water (when leakage occurs from the process water, the cooling water has to be sent to the waste water treatment plant). This probably explains the high figure reported in the table.
Table 3.11: Concentration values and textile substrate specific emission factors for waste water from four mills mainly finishing yarns consisting of CO It is interesting to note the differences in COD emission factors associated with the patterns of dyestuffs usage.
Mills like TF1 and TF2, dyeing mainly with reactive dyes, show a significantly lower COD (about 70 g/kg) compared to TF3 which uses predominantly vat dyes (nearly 100 g/kg). The higher COD emission factor from dyeing with vat colourants is partly explained by the use of additional textile auxiliaries, such as dispersing agents in the vat dyeing process.
Also, since some reactive dyestuff-chromophores absorb 2 - 3 times more light per molecule than the corresponding vat dyestuffs, COD for vat dyeing may be higher because a higher amount of dyestuff is required to achieve the same depth of shade compared to reactive dyestuffs.
Note that for companies dyeing mainly with reactive colourants the consumption of basic chemicals can be higher because of the high neutral salt input.
The total specific energy consumption is about 11 kWh/kg, where the consumption of electricity is about 2 kWh/kg.
220.127.116.11 Mills finishing yarn: mainly PES Table 3.12 contains the concentration values and emission factors for waste water for eight mills mainly finishing polyester yarn.
Table 3.12: Concentration values and textile substrate specific emission factors for waste water from eight mills mainly finishing yarns consisting of PES
Apart from TFI 8, the reported specific water flows vary between ca. 65 and 148 l/kg. They are confirmed by “FhG-ISI, 1997” reporting for three similar mills 63, 86 and 122 l/kg. The figures are in the same range as mills finishing cotton yarn.
The upper value in the range (148 l/kg) is for a mill also processing yarn in hank form (this is normally associated with higher consumption of water due to higher liquor ratios). The higher water consumption is also justified by the fact that this company also treats some cotton yarn which is mercerised and dyed with azoic (development) dyestuffs, both operations requiring relatively high amounts of water.
The finishers of PES yarn generally have higher COD emission factors (97 - 125 g/kg) than the
corresponding finishers processing cotton. This is mainly attributable to two different factors:
· the use of disperse dyes · the removal of preparation agents present on the incoming fibre.
Concerning the first factor, the use of disperse dyes involves relevant emissions of dispersing agents (see Sections 18.104.22.168 and 8.6.3). These auxiliaries (usually naphthalene sulphonatesformaldehyde condensates and lignine sulphonates), besides contributing to the increase of the COD load of the final effluent, are water-soluble and hardly biodegradable.
Regarding the preparation agents, it has already been mentioned in Section 2.6.4 that they are applied during fibre and yarn manufacturing, but they need to be removed before dyeing. The formulations applied may still have a significant content of refined mineral. The amount of mineral oils, which is defined with the parameter "hydrocarbons" (HC), was measured in only two companies. On inspection of the HC emission factor reported for TFI 2 (1.2g/kg, corresponding to a COD of 3g/kg), the contribution of mineral oils to the overall COD load does not seem to be significant. However, the reported value is to be closely questioned. If a typical load-on of 20g/kg for preparation agents is considered, with an average COD content of 3000 mg/g of product, assuming 90 % removal from the fibre, a COD of about 50g/kg or more coming from the preparation agents would be expected in the waste water (see also Section 8.2).
The applied chemicals are grouped as dyestuffs, textile auxiliaries and basic chemicals. Typical
ranges (albeit based on limited data) are:
When high amounts of softening agents are applied, the consumption of textile auxiliaries can be up to 175 g/kg.
The total specific energy consumption is in the range of 11 - 18 kWh/kg. The higher value applies to companies which also have spinning, twisting and coning sections. The consumption of electricity is about 1 - 2 kWh/kg.
22.214.171.124 Mills finishing yarn: mainly WO, PAC and/or CV Table 3.13 contains data for emissions to water for seven mills finishing yarn consisting mainly of wool and PAC blends along with some viscose in blend with wool and acrylic fibre.
Table 3.13: Concentration values and textile substrate specific emission factors for waste water from seven mills mainly finishing yarns consisting of WO/PAC/CV Specific water flow may vary widely (ranges between 43 and 212 l/kg have been observed).
Various factors influence the water consumption and therefore the waste water flow of a mill.
The age of the equipment is one factor, but the make-up of the yarn is also to be taken into account (hanks involve considerably higher consumption of water than cones). For example, TFI 6, which process 100 % yarn in package has a lower specific waste water flow compared to TFI 7, where 10 % of the production is dyed in hank form.
COD emission factors are similar to mills finishing cotton yarn. All parameters are quite similar to the other categories except for chromium, which is encountered in the effluent in significantly higher amounts. This is due to the use of metal-complex and chrome dyes, the latter involving the use of potassium or sodium dichromate. Chromium emissions may vary widely depending on the amount of wool processed and the dyeing method applied. In the case of companies with a high percentage of wool among the other fibres, emissions factors up to 100 mg/kg (89 mg/kg in TFI 5) are observed.
The applied chemicals are grouped as dyestuffs, textile auxiliaries and basic chemicals. Typical
ranges (albeit based on limited data) are:
· dyestuffs: 13 – 18 (g/kg textile substrate) · textile auxiliaries: 36 – 90 (g/kg textile substrate) · basic chemicals: 85 - 325 (g/kg textile substrate)
The total specific energy consumption is in the range of 4 - 17 kWh/kg. The higher value is for finishing mills that also have spinning, twisting and coning sections. Of the overall energy consumption the actual consumption of electricity is about 0.9 - 6.5 kWh/kg, the higher value relating to mills with the above-mentioned additional processes.
126.96.36.199 Analysis of some relevant specific processes for mills finishing yarn and/ or floc The main environmental concerns for yarn finishing mills arise from emissions to water, while air emissions are not significant.
It is interesting to show the wide variability of the characteristics (particularly in concentration) of the different baths involved in a typical dyeing process. This is put in evidence by showing the behaviour of five different parameters, namely COD, pH, conductivity, temperature and colour (determination of adsorption coefficients at wavelengths 435, 500 and 620 nm) in three
different typical dyeing processes:
· exhaust dyeing of CV yarn on cones (L.R. 1:8 - 1:12) with reactive dyestuffs (Table 3.14) · exhaust dyeing of PES yarn on cones (L.R. 1:8 - 1:12) with disperse dyestuffs (Table 3.15) · exhaust dyeing of CO yarn on cones (L.R. 1:8 - 1:12) with vat dyestuffs (Table 3.16).
Table 3.15: Sequence of discharged baths from exhaust dyeing of PES yarn with disperse dyestuffs along with values for COD, pH, conductivity, temperature and colour (spectral absorption coefficients, SAC)
Table 3.16: Sequence of discharged baths from exhaust dyeing of CO yarn with vat dyestuffs along with values for COD, pH, conductivity, temperature and colour (spectral absorption coefficients, SAC) The examples reflect the typical sequences applied in exhaust dyeing of yarn with reactive, disperse and vat dyes.
However, they should not be taken as fixed sequences, in the sense that they do not apply as such to any finishers of yarn. A finisher may need to apply, for example, additional rinsing baths, the yarn could be mercerised, the application of softening agents in the last bath could be avoided etc.
However, the examples show some key issues:
· first, it is significant (although maybe also obvious) to show how much the concentration values of the mixed final effluent (Table 3.9 - Table 3.13) can differ from the corresponding values reported in the tables above for the single baths. Thanks to a dilution effect, the global COD concentration levels are much lower for mixed effluents than the values for spent dyeing baths · the spent dyeing baths are those that show the highest concentration levels in all three examples. However, the COD of the exhausted dye bath is significantly lower for reactive dyeing than for dyeing processes using disperse and vat dyes. As mentioned earlier in this chapter, the main reason is the application of dispersing agents which are needed to apply the water-insoluble disperse and vat dyestuffs · operations like soaping and reductive aftertreatment, which are carried out in order to remove the unfixed dyestuff, are also associated with high values of COD as well as colour.
In particular, for softening treatment, the high COD concentration is indicative of the poor exhaustion level typical of softening agents · rinsing baths can have a very low concentration, 10 - 100 times lower than the exhausted dyeing bath. This shows the importance of optimising the rinsing process, for example by recycling low-concentration rinsing water.
Table 3.17 and Table 3.
18 illustrate water and energy consumption figures observed in loose fibre and yarn (package) dyeing processes.
Source: [77, EURATEX, 2000] except for (1), [280, Germany, 2002]
(2) Water consumption for washing + rinsing (3) Total water consumption Table 3.18: Water & energy consumption levels in yarn dyeing processes 3.3.2 Mills finishing knitted fabric 188.8.131.52 Mills finishing knitted fabric: mainly CO Table 3.19 shows the data on waste water emissions from seventeen mills finishing knit fabric consisting mainly of cotton. These mills do not have a printing section.
The range of specific waste water flow is relatively small (60 – 136 l/kg), but there are two extreme exceptions at the lower and upper end of the range (see TFI 9 and TFI 17). TFI 9, with only 21 l/kg, performs scouring and bleaching only; in addition these operations are carried in continuous mode, which explains the very low specific waste water flow as well as the highest COD concentration in the final effluent. The reasons for the very high specific waste water flow of TFI 17 (216 l/kg) are reported to be the high quality requirements (more rinsing steps) and the high repairing rate.
The COD emission factors are within a surprisingly small range (70 – 85 g/kg), only TFI 9 carrying scouring and bleaching only has a lower value (48 g/kg). For TFI 10, 11 and 16, higher values are reported (107, 108 and 97 g/kg). The fact that these mills also process some
woven fabric may reasonably explain these higher figures. In addition TFI 10 finishes some polyester knit fabric, which also leads to higher COD emission factors. The processing of PES fibres is also the reason for the higher emission of hydrocarbons in the waste water (concentration 20 mg/l). They come from the removal of preparation agents mainly containing mineral oils. Concentrations between 10 and 20 mg/l may also be observed in the case of mills finishing cotton only (e.g. from antifoaming agents based on mineral oils), but common values are below 10 mg/l. Heavy metals emissions are insignificant for all the seventeen mills analysed.
The very high specific consumption of basic chemicals is due to the application of neutral salts (NaCl or Na2SO4) for reactive exhaust dyeing, which is about 400 g/kg.
The total specific energy consumption is in the range of 6 - 17 kWh/kg. The higher value relates to a site that also has spinning and coning sections. The consumption of electricity is 1 - 3 kWh/kg (data from 9 TFI).
Table 3.19: Concentration values and textile substrate specific emission factors for waste water from seventeen mills mainly finishing knitted fabric consisting of cotton
184.108.40.206 Mills finishing knitted fabric: mainly CO with a significant printing section Data for mills finishing cotton knitted fabric with a significant printing section are presented in Table 3.20. Most of the companies belonging to this category perform pigment printing.
In pigment printing, waste water is only emitted during cleaning-up operations (for drums, screens, etc.). This explains the very low specific waste water flow (usually far below 10 l/kg) typical of these mills. One exception is represented by TFI 6 which also uses reactive printing (for which afterwashing is required).
None of the companies analysed in the table, except for TFI 3, carries out pretreatment, but instead print on already pretreated knitted fabric. This is confirmed by the higher figures for waste water flow and organic load in the final effluent.
Information on chemical input and energy consumption was not made available.