«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
COD emission factors for finishers of woven fabric are considerably higher than for yarn or knitted fabric finishers (two to three times higher). This is mainly due to the removal of sizing agents which are present on woven fabrics up to 15 wt-%. The load of sizing agents on the fabric and therefore the COD emission factor vary strongly according to the type of fabric and quality of sizing agents applied. For example, TFI 5 treats light, open fabrics with a low load of sizing agents. In addition, this company only carries out pretreatment without dyeing the material (and the same is valid also for TFI 15), which explains the low emission factor. On the contrary, TFI 2 finishes woven fabric with a high load of sizing agents and therefore has a higher specific COD load (300 g/kg). Also mills finishing cotton frotté or fabrics consisting mainly of viscose have low factors.
It is interesting to see the variability of the COD/BOD5 ratio, which can be considered an indicator of the biodegradability of the sizing agents applied. Synthetic sizing agents, such as carboxymethylcellulose (CMC), polyacrylates (PA) and polyvinyl alcohol (PVA) are all poorly biodegradable and CMC and PA, in particular, are non-biodegradable or only to a very low extent. Having said this, it can be inferred that companies such as TFI 1 and TFI 2 which have a COD/BOD5 ratio of about 3:1 have a relatively high percentage of biodegradable sizing agents.
Meanwhile TFI 3 and TFI 4 have about 5:1, which is indicative of the predominance of hardlyor non-biodegradable sizing agents.
Little further comment is needed on the high ammonia concentration of TFI 4, which is due to the printing section of this mill (this mill has not been categorised with mills having a significant printing section because the percentage of printed textile substrate is lower than 30 %). Furthermore a high AOX concentration and emission factor is observed for TFI 2, but no reasonable explanation is found for this.
Figure 3.8 shows as an example the composition of the COD load released to waste water by a mill finishing woven fabric consisting mainly of cotton.
Data have been calculated using
information reported in the safety data sheets and known/assumed retention factors (amount of chemical that remains on the fibre instead of being released to the waste water). Figures have been cross-checked with the measured COD concentrations and loads in the effluent.
It is obvious that sizing agents and cotton impurities are the biggest contributors to the total COD load. In the example, the company uses sulphur and vat dyes predominantly. Because of the dispersing agents present in the applied formulations, the contribution of the dyes to the total COD is higher than when reactive dyes are mainly used.
Figure 3.8: Composition of the COD load of a mill finishing woven fabric consisting mainly of cotton; semi-continuous and continuous dyeing is carried out with sulphur, vat and reactive dyestuffs [179, UBA, 2001] The total specific energy consumption is in the range of 8 - 20 kWh/kg.
The higher value is for mills also having spinning, twisting and coning sections. The consumption of electricity is about 0.5 - 1.5 kWh/kg (data from eight mills).
Information about the contribution of the different steps of the process to the overall energy consumption is limited. Two examples are available where energy consumption levels have been analysed in detail. These relate to the finishing of viscose fabric (Figure 3.9) and the finishing of woven fabric consisting of viscose/polyester blend (Figure 3.10).
The first example clearly indicates that high temperature processes such as thermal treatment in stenters and drying operations contribute most to the overall energy consumption. Electric energy is needed at all stages and there is no process that consumes significantly more than the others.
Figure 3.9: Analysis of thermal and electric energy consumption for the finishing of viscose fabric [179, UBA, 2001] with reference to “Eutech/ITA/LTT, 2000” The second example shows that when HT dyeing is carried out, which is the case for polyester fibres, this process requires significant amounts of thermal energy and accounts for a considerable share of the total energy consumption.
Figure 3.10: Analysis of thermal and electric energy consumption for the finishing of viscose/PES fabric [179, UBA, 2001] with reference to “Eutech/ITA/LTT, 2000” The basic conclusions drawn from these two examples may be transferable to the textile sector.
These types of assessments unfortunately represent very rare examples in the textile industry. It is obvious, that only such detailed analysis will allow the identification of the processes most significant for minimising energy consumption.
188.8.131.52 Mills finishing woven fabric: mainly CO and CV, with a significant printing section Table 3.34 presents the values of twelve cotton finishing mills each with a significant printing section (more than 30 % of the textile substrates are printed). In the twelve companies surveyed, printing with dyes is dominant in ten (TFI 1 to TFI 10, particularly reactive printing and etch printing). In the other two (TFI 11 and TFI 12) pigment printing is the larger part of the business. This influences water consumption levels, because higher amounts of water are needed in dye printing compared to pigment printing, and explains the relatively high specific waste water flow in comparison to all other categories of mills.
The specific waste water flow of the ten predominatly dye-printers varies from 155 to 283 l/kg, except for TFI 4 which is not directly comparable (because it does not carry out pretreatment, but performs only printing and finishing on already pretreated fabric). The high figures for specific waste water flow are confirmed by “FhG-ISI, 1997” which reports for seven more mills values of 282, 288, 327, 450, 261, 189 and 302 l/kg.
COD emission factors are also high because, in addition to sizing agents, the high organic load from the printing section (cleaning of printing equipment and after-washing operations) has to be considered.
High ammonia concentrations and emission factors are also typical of dye-printing. This is due to the presence of urea and ammonia, especially in printing pastes (up to 150 g urea/kg printing paste). Urea hydrolyses to ammonia in waste water. Furthermore, concentrations and emission factors for copper are significantly higher compared to other kinds of mills because of the low fixation rates of copper-phthalocyanine-complex reactive dyestuffs. The higher AOX values are mainly attributable to vat and phthalocyanine dyes containing halogens (green shades).
There are two main causes of the high specific consumption of dyestuffs. First, colouration is performed twice: for dyeing of the fabric and for printing. Second, many liquid dyestuff formulations are in use and, as already stated in the introduction, the water content is included for calculating the consumption factors.
The high specific consumption of basic chemicals is due to the high chemical demand of operations such as pretreatment and printing.
Energy consumption data have been made available for one company only. The total consumption for this site is 18.8 kWh/kg (2.3 kWh/kg for electricity, 16.5 kWh/kg as thermal energy).
Table 3.34: Concentration values and textile substrate specific emission factors for waste water from mills mainly finishing woven fabric consisting mainly of CO, with a significant printing section
184.108.40.206 Mills finishing woven fabric: mainly WO The values for waste water emissions from six mills finishing fabric consisting mainly of wool are reported in Table 3.35. The specific waste water flow are usually higher than for mills finishing cellulosic fibres. The values quoted are confirmed by five values reported by “FhGISI, 1997” (133, 156, 253, 142 and 243 l/kg). Compared to the other kinds of mills, the emission factors for chromium can be noticeable (e.g. 54 and 71 mg/kg for TFI 1 and TFI 2) due to the application of after-chrome and metal-complex dyestuffs. Note also the high copper emission factor for TF5 (603 mg/kg). This comes from the fact that at the time when the data were collected, the company used to have copper pipelines in the heat recovery system. The mill has now replaced them with new pipelines in stainless steel.
Table 3.35: Concentration values and textile substrate specific emission factors for waste water from six mills finishing woven fabric consisting mainly of wool The applied chemicals are grouped as dyestuffs, textile auxiliaries and basic chemicals.
ranges, albeit based on limited information (data from two mills) are:
Also energy consumption data have been made available for two companies only. The total consumption ranges from 11 to 21 kWh/kg (0.5 - 0.8 kWh/kg for electricity, 10 - 20 kWh/kg for natural gas).
220.127.116.11 Mills finishing woven fabric: mainly synthetic fibres Data on finishing of woven fabric consisting of synthetic fibres are presented in Table 3.36 for six mills. Figures vary according to the type of blend and the percentage of natural fibres (mainly cotton, flax and silk) often present in the blend.
Specific waste water flows are all above 100 l/kg, except for TFI 1 with only 7 l/kg. This mill, however, represents a particular situation not comparable with the other sites presented in this section. TFI 1 treats 100 % PA fibres and only carries out pretreatment in continuous modern washing machines for the removal of preparation and sizing agents.
As for the other companies, higher values for specific waste water flows can be explained partly by the equipment used and partly by considering that some companies process significant amounts of cellulosic fibres together with synthetic fibres (e.g. TFI 3 and TFI 5).
COD emission factors vary between 110 and 200 g/kg (one higher value, 286, only in TFI 2). In TFI 1 a lower value is reported (14 g/kg), but this is because no dyeing is carried out.
Figure 3.11 shows as an example the composition of the COD load of waste water from a mill finishing woven fabric consisting mainly of polyamide.
It is evident that preparation agents account for a considerable share of the total COD load. Their removal requires relatively high amounts of washing and sequestering agents, which is confirmed by the high contribution of the pretreatment auxiliaries to the COD load. As for the relatively high COD load attributed to dyes, this is not due to the dyestuffs themselves, but to the levelling agents and dispersants present in dye formulations (especially in the case of disperse dyes, most often used for synthetic fibres).
Table 3.36: Concentration values and textile substrate specific emission factors for waste water from six mills finishing woven fabric consisting mainly of synthetic fibres 3.
3.3.5 Analysis of some relevant specific processes for mills finishing woven fabric
Process-specific information has been submitted on:
· singeing · heat-setting · continuous pretreatment of woven fabric · continuous and semi-continuous dyeing · printing · finishing · coating.
The quality and quantity of air emissions in singeing depend strongly on:
· kind of substrate to be treated · position of burners (angle and distance to the textile; one-sided or double-sided singeing) · kind of emission abatement installed.
Main air emissions are:
· dust from the fibres burned-off · organic-C from volatile substances on the substrate and/or crack-products and methane from incomplete combustion of burner gases · formaldehyde from burner gases.
Emission levels from measurements carried out in five finishing mills are summarised in Table 3.37 [179, UBA, 2001].
The following general considerations apply to the reported data [179, UBA, 2001]:
· if an aqueous scrubber is used for emission abatement in the after-brushing compartment, dust emissions 0.1 mg/m³ can be achieved, but concentrations up to 6 mg/Nm³ can also be detected · Organic-C concentration caused by the process itself (methane emissions not included) varies within a wide range (from 1 to 26 mg C/Nm³). Formaldehyde emission from the burner is in a range of 1 to 3 mg substance/Nm³.
· off-gas temperature depends on sampling point (burner or after-brushing) and whether an aqueous scrubber is installed or not.
Singeing can be a very odour intensive process. An odour value of 60000 OU/kg textile could be measured for an installation singeing cotton without emission abatement systems [179, UBA, 2001] Advanced air treatment for destruction of odorous substances and abatement of dust may be necessary (see also Section 4.10.9). Odour emissions are dealt with in more detail in Section 3.5.
Table 3.38 shows the possible sources of air emissions arising from:
· thermal treatment of raw fabrics · thermal treatment of fabrics which are prewashed in an efficient way.
Table 3.38: Possible sources of air emissions during heat-setting of grey fabrics or inefficiently washed fabrics Typical air emission levels from heat-setting (concentrations, emission factors and mass flows) are reported in Table 3.
39 for a sample of finishing mills. Note that when emission abatement systems are installed, the values shown in the table will correspond to the resulting clean gas. In the case of directly heated stenters, the portion of Organic-C emission attributable to the fuel (methane, propane, butane) is mentioned separately and is not included in the data concerning the Organic-C emission values (concentrations, emission factors and mass flows).
188 Textiles Industry
The following general considerations apply to the reported data:
· heat setting of raw textiles causes significant off-gas load. If heat-setting of PA 6 is carried out, considerable amounts of caprolactam are emitted (see process 1.1 and 13.3).
· in the case of textiles which contain low-emission preparation agents, much lower emission levels are observed (see process 15.1 and 15.2) · Organic-C emissions caused by unburned fuel are in a range of 0.1 g/kg textile up to 5 g/kg textile in the case of poorly maintained burners · formaldehyde emissions may originate not only from the auxiliaries applied, but also from the direct-heated stenters themselves, due to partial burn-out of the gas (methane, propane, butane). Concentration levels from stenters range from 0.1 to 60 mg/Nm3.
The substance-based emission factor for some of the most commonly applied preparation agents are reported in Table 2 (see Section 11.4). As better explained in Section 18.104.22.168.6, the substance-based emission factor is defined as the amount of organic and inorganic substances in grams that can be released at defined process conditions (curing time, curing temperature and type of substrate) from one kilogram of auxiliary.
22.214.171.124.3 Continuous pretreatment of woven fabric Pretreatment of cellulosic woven fabric For cotton, the most common processes are desizing, scouring and bleaching. Today, these processes are often combined.
The next figure illustrates a typical pretreatment process (desizing, scouring and bleaching), in the case of water-soluble sizing agents which can be easily removed with water only. The specific input for water, steam and chemicals in a modern continuous pretreatment line are presented in Table 3.40.