«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
18.104.22.168 Mills finishing knitted fabric: mainly synthetic fibres According to data reported in Table 3.21 for thirteen mills in this category, there is considerable variability in the specific waste water flow (35 - 229 l/kg). Lower values are observed in installations using more advanced equipment (automated machines with low liquor ratio) and when a small number of finishing baths is required. Conversely the higher values are typical of companies having older machines where small batches are processed at non-optimal liquor ratios.
Due to the load of preparation agents, the emission of hydrocarbons is significantly higher compared to mills finishing mainly cotton fibres (see Table 3.19).
As with mills finishing mainly cotton, heavy metals emissions are not significant for this category.
COD emission factors vary between 65 and 150 g/kg. The lower COD figures are typical of mills finishing mainly polyamide. In particular, the composition of the emitted COD load for a mill finishing knit fabric consisting mainly of polyamide and polyamide/elastane blends is illustrated as an example in Figure 3.7. 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.
Figure 3.7: Example of composition of the COD load for a mill finishing knitted fabric consisting mainly of polyamide [179, UBA, 2001] As Figure 3.
7 shows, dyeing auxiliaries and in particular levelling and washing agents, account for the highest share of the overall COD load. It is also interesting to notice the significant contribution of the preparation agents. In the given example, preparation agents are responsible for more than 20 % of total emitted COD load. This is in spite of thermofixation being carried out mainly on the greige fabric (preparation agents, based on mineral oil, evaporate to a significant extent during thermofixation and are therefore not found in the waste water). With polyamide 6, caprolactam is also present in waste water; in the presented example it represents 4 % of the total COD load.
The applied chemicals are grouped as dyestuffs, textile auxiliaries and basic chemicals. Typical
ranges (albeit based on limited data) are:
· dyestuffs: 15 - 50 (g/kg textile substrate) · textile auxiliaries: 45 - 150 (g/kg textile substrate) · basic chemicals: 50 - 280 (g/kg textile substrate) The ranges are rather wide, reflecting the variety of processes and process sequences. The total specific energy consumption is in the range of 3.5 - 17 kWh/kg. The consumption of electricity is 1.5 - 6 kWh/kg. The higher values correspond to mills also having spinning and knitting sections (data from three mills).
Table 3.21: Concentration values and textile substrate specific emission factors for waste water from mills finishing knitted fabric consisting mainly of synthetic fibres
22.214.171.124 Mills finishing knitted fabric: mainly wool Data on waste water emissions and energy consumption have been made available for one mill only. The same applied to energy consumption. The precise process sequence is not known.
Table 3.22 contains the values for waste water emissions.
Specific flow and other parameters do not indicate significant differences compared to mills finishing cotton or synthetic fibres.
Information on applied chemicals was not made available.
The specific energy consumption in the analysed company is very high (67 kWh/kg) whereas the specific consumption of electricity is 9.5 kWh/kg. This is because energy consumption for spinning, twisting, coning and knitting is included.
Table 3.22: Concentration values and textile substrate specific emission factors for waste water from one mill finishing knitted fabric mainly consisting of wool 3.
3.2.5 Analysis of some relevant specific processes for mills finishing knitted fabric
Process-specific information has been submitted on:
· heat-setting (Section 126.96.36.199.2) · pretreatment of cotton knit fabric (continuous and discontinuous processes) (Section 188.8.131.52.1) · pretreatment of knitted fabric consisting of synthetic fibres (very limited information available) (Section 184.108.40.206.2) · exhaust dyeing of knitted fabric consisting of cotton and synthetic fibres (Section 220.127.116.11.3)
· printing (Section 18.104.22.168.5) · finishing (Section 22.214.171.124.6) · coating (Section 126.96.36.199.7).
188.8.131.52.1 Pretreatment of cotton knitted fabric Pretreatment for cotton knitted fabric includes bleaching and washing. The intensity of bleaching mainly depends on the kind of cotton quality and the degree of whiteness to be achieved. For subsequent exhaust dyeing with dark shades a less intensive bleach process is sufficient (pre-bleach), whereas for pale shades and non-dyed products the degree of whiteness must be higher and consequently a more intensive bleaching treatment is required.
Continuous pretreatment of cotton knitted fabric
Continuous pretreatment is typical of larger installations. A process is described below for
continuous bleaching/washing with hydrogen peroxide. It consists of the following steps:
· padding of the bleaching liquor with a pick-up of 130 % · bleaching reaction in a steamer (30 min) with saturated steam at a temperature of 95 - 98 °C · countercurrent rinsing · padding of liquor containing complexing and washing agents followed by steaming (3 - 5 min with saturated steam) · rinsing and drying (in the case of non-dyed finished products, softening agents are applied before drying)
A typical recipe is given below:
Table 3.23: Typical recipe for the continuous bleaching/washing of cotton knitted fabric The specific COD-input can vary between 20 and 30 g/kg textile.
This example is confirmed by the standard recipe reported in Chapter 11.
The specific water consumption and waste water flow from the whole process is about 30 l/kg (±7 l/kg).
Typical values in the rinsing water from the first step are:
· COD: 4000 - 8500 mg/l · Conductivity: 2.5 - 4.5 mS/cm · pH: around 10.
Typical values in the rinsing water from the second step are:
· COD: 1000 - 3000 mg/l · Conductivity: 0.5 - 1.2 mS/cm · pH: around 8 - 10 As reported in Table 3.23, the COD-output-factor attributable to applied chemicals is estimated in about 20 g/kg textile (the wetting agents are the auxiliaries that contribute the most to the final COD in the waste water).
Measurements in total waste water showed COD output factors between 80 and 100 g/kg textile.
The reason for this can be found in the additives (e.g. knitting oils) and the adjacent material present on the raw cotton knitted fabric, here 60 to 80 g/kg textile is attributable to these compounds. However, it should be noted that hydrogen peroxide has not been accounted for in the calculation (the CODcr method measures 0.45 g COD/g H2O2).
Data are available for a second process which consists of the following steps:
· padding of the demineralisation liquor with a pick-up of 130 % (inorganic and organic acids) with subsequent reaction at 40 °C and rinsing · padding with the bleaching liquor (H2O2 as bleaching agent) · bleaching reaction in a steamer with saturated steam at a temperature of about 97°C · countercurrent rinsing · addition of softening agents (if required).
The specific consumption of chemicals and the associated COD values are reported in Table
3.24. It is obvious that this process needs significant lower amount of water.
Discontinuous pretreatment of cotton knitted fabric For discontinuous bleaching/washing of cotton knitted fabric, Section 11 provides the standard recipes for bleaching with hydrogen peroxide (H2O2). Data for COD, pH and conductivity for exhausted bleaching bath and rinsing water for pre- and full-bleaching with H2O2 are reported in Table 3.25 (batch process). Thereby emission factors cannot be calculated because figures for waste water flow corresponding to the single batches were not made available. Only the overall specific waste water flow is known (30 - 50 l/kg, including rinsing).
Table 3.25: Data for COD, pH and L for exhausted bleaching bath and rinsing water from pre- and full-bleaching with H2O2 of cotton knitted fabric; the specific waste water flow for the whole process including rinsing is 30 - 50 l/kg For cotton knitted fabric a combination of sodium hypochlorite and hydrogen peroxide used to be widely applied.
Since hypochlorite has been largely replaced by hydrogen peroxide this process is now less common. Information from 1992 shows values for the combined application of hypochlorite and peroxide and indicates the big difference of AOX in the waste water (Table 3.26). Thereby the AOX value in the exhausted H2O2-bleaching bath (up to 6 mg Cl/l) derives from the fact that no rinsing is performed after hypochlorite bleaching and therefore the knitted fabric carries over the by-products from the previous bath to the peroxide bleaching bath.
Table 3.26: Data for COD, AOX, pH and conductivity for exhausted bleaching bath and rinsing water from combined bleaching of cotton knit fabric with NaOCl/H2O2 (L.
R. = 1:15) – “ITV, 1992”; the overall specific waste water flow (for the whole process including rinsing is 30 - 50 l/kg)
184.108.40.206.2 Pretreatment of knitted fabric consisting of synthetic fibres The availability of specific data on the input/output of processes for the pretreatment of knit fabric consisting of synthetic fibres is limited. The components which are removed from the fibres along with quantities can be seen in Section 8.2. Using the information for specific water consumption and waste water flow, the concentration of COD and hydrocarbons can be reliably estimated. Bigger companies have continuous pretreatment processes (usually washing processes) with low specific water consumption resulting in high COD and hydrocarbon concentrations. In particular, for hydrocarbons, concentrations in the range of g/l-order are typical.
220.127.116.11.3 Exhaust dyeing of knitted fabric Usually knitted fabric is dyed in batch (exhaust dyeing). In a few cases fabric is dyed in semicontinuous mode (generally by cold pad batch).
Exhaust dyeing of cotton knitted fabric Table 3.27 presents typical input factors, distinguishing between light, medium and dark shades, (which influences primarily the specific input of dyestuffs and salt). The wide range reported in the table for liquor ratio is not because different types of machines have been considered (all data refer to machines with a liquor ratio of 1:8). It rather reflects a common situation that occurs when the machines are not fully loaded due to the need to treat small batches and they are operated at a non-optimal liquor ratio. A common feature of modern machines is that small batches can be dyed with approximately the same liquor ratio as for maximum load (see Section 4.6.19).
Table 3.27: Typical input factors for exhaust dyeing of cotton knitted fabric with reactive dyestuffs As concerns the characteristics of the discharged water, the following tables illustrate examples of some typical dyeing processes.
Table 3.28 reports the data for each of the discharged baths from reactive dyeing of a light shade.
For light shades, less rinsing is normally required and soaping is not needed. COD values are very low, especially for rinsing water. Conversely, data presented in Table 3.29 for a dark shade dyeing sequence show significantly higher figures for COD, conductivity and colour. The values for exhaust dyeing with reactive dyestuffs at medium shade will be between these extreme cases.
Table 3.28: Sequence of discharged baths from exhaust dyeing (light shade) of cotton knitted fabric with reactive dyestuffs along with values for COD, pH, conductivity and colour (spectral
absorption coefficients, SAC) L.R. = 1:25; specific water consumption for the whole process:
142 l/kg (including water consumed when loading the material and direct cooling after dyeing)
Table 3.29: Sequence of emitted baths from exhaust dyeing (dark shade) of cotton knit fabric with reactive dyestuffs along with values for COD, pH, conductivity and colour (spectral absorption coefficients, SAC) L.
R. = 1:8.2; specific water consumption for the whole process: 71 l/kg Two additional examples are presented below, one for dyeing with direct dyes (light shade) and one for dyeing with sulphur dyes (dark shade).
Table 3.30: Sequence of discharged baths from exhaust dyeing of cotton knitted fabric with direct dyestuffs (light shade) along with values for COD, AOX, pH, conductivity and colour (spectral absorption coefficients, SAC)
Table 3.31: Sequence of discharged baths from exhaust dyeing of cotton knitted fabric with sulphur dyestuffs (dark shade) along with values for COD, AOX, pH, conductivity and colour (spectral absorption coefficients, SAC) The tables above clearly indicate that high-, medium- and low-loaded baths are discharged from exhaust dyeing.
This shows the importance of separating the different streams in order to allow recycling of the low-loaded baths and more effective treatment of the concentrates (see Sections 4.6.22 and 4.10.7 for more detailed information).
Exhaust dyeing of knitted fabric consisting of synthetic fibres Table 3.32 shows a typical recipe for exhaust dyeing of PES knitted fabric including the application of a UV stabiliser for high light fastness. There are no analytical data available for this or for exhaust dyeing of other synthetic fibres.
The conclusion from the reported data is that disperse dyeing causes COD emission factors significantly higher than, for example, reactive dyeing because of the dispersing agents (present in the dye formulation itself in percentages as high as 40 – 60 % of the weight of the product) and carriers.
3.3.3 Mills finishing woven fabric 18.104.22.168 Mills finishing woven fabric: mainly CO and CV Table 3.33 presents the data for waste water emissions from seventeen mills finishing woven fabric consisting mainly of cotton. Most of these industries carry out pretreatment in continuous or semi-continuous mode. The same is true for dyeing, although some companies perform exhaust dyeing in combination with continuous dyeing or alone (which is the case for example in TFI 16).
Specific waste water flows vary widely, from 45 - 50 l/kg up to more than 200 or even, in two cases, more than 600 l/kg. These values are confirmed by “FhG-ISI, 1997”, reporting similar figures for 25 other mills, also reporting values as high as 240 and 265 l/kg and another very high value (415 l/kg).
Mills with specific waste water flows of about 50 l/kg (e.g. TFI 6 and TFI 9) are known to have more modern and more efficient washing machines than companies showing levels as high as 200 l/kg or more. Two sites in this survey (TFI 12 and TFI 16) show extremely high specific waste water flows (618 l/kg and 646 l/kg). In the case of TFI 12 the reason is to be found in the use of very old machines with poor washing efficiency. In the case of TFI 16 the high water consumption can probably be explained by the fact that on this site dyeing is carried out only in batch, a mode in which water consumption is normally higher than with continuous and semicontinuous dyeing techniques.