«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Figure 3.12: Typical continuous process for pretreatment of cellulosic fibres, including desizing (first two compartments), scouring (padding of scouring liquor, steam treatment, washing, drying), bleaching (padding of the bleaching liquor, steaming, washing and drying) [179, UBA, 2001]
Source: [179, UBA, 2001] Notes: (1) for washing after cold bleaching the reference values are: water 4 - 6; steam (without heat exchanger) 0.8 - 1.2; steam (with heat exchanger) 0.4 - 0.6 Table 3.40: Specific input for water, steam and chemicals in a modern continuous pretreatment line, including desizing, scouring (padding of scouring liquor, steam treatment, washing, drying), bleaching (padding of the bleaching liquor, steaming, washing and drying The figures reported in Table 3.40 refer to the amounts of water, steam and chemicals used in each step of the pretreatment process. However, these figures do not take into account possible re-use and recycling options and do not necessarily correspond to the actual consumption levels in the process. It is reported ([281, Belgium, 2002]) that in modern pretreatment lines levels of 6 litres for total water consumption (water and steam/kg of fabric) or 9 l/kg (including heat exchangers and filters cleaning) were observed. As for the organic load discharged, a large fraction comes from the desizing bath. The COD concentration and load can be calculated knowing the amount of sizing agents applied on the fabric and the specific COD value of the size reported in Section 8.3. Considering the specific input for water reported for desizing in Table 3.40 (4 l/kg), and assuming a 6 wt-% load of sizing agent on the fabric, with a specific COD of 1600 g/kg (e.g. polyvinyl alcohol), the resulting COD concentration will be about 24000 mg/l and the corresponding COD emission factor will be 96 g O2/kg fabric.
With starch and modified starch sizing agents, enzymatic or oxidative desizing is normally applied, followed by washing. In Annex IV, typical recipes are given for enzymatic desizing, for cold oxidative desizing and for the removal of water-soluble sizing agents.
Pretreatment of synthetic woven fabric Synthetic woven fabric is pretreated both discontinuously and continuously. The main purpose is to remove preparation agents. Typical recipes are submitted in Annex IV.
In continuous pretreatment, very high concentrations of hydrocarbons may result. For example, with a load of hydrocarbons of 1.5 wt-% and a specific water consumption of 5 l/kg, a highly concentrated stream will result (3000 mg/l). Additional organic load comes from pretreatment auxiliaries.
Pretreatment of woollen woven fabric The availability of detailed information on the pretreatment of woollen fabric is limited. Thus, reference is made to the standard recipes for pretreatment in Annex IV.
126.96.36.199.4 Continuous and semi-continuous dyeing Emission and consumption levels for exhaust dyeing of woven fabric are not considered in this section because the sequence of baths and the operative conditions are very similar to exhaust dyeing of knitted fabric.
In semi-continuous and continuous dyeing, the application of dyestuffs by padding is the most common technique. Therefore the following points should be taken into account.
Very often, the whole quantity of padding liquor is prepared in advance. In order to avoid shortfalls during the process, a surplus of liquor is normally prepared. Discharging the residual liquor contained in the padder and in the preparation tank into the waste water is still practised in many companies. Compared to the overall waste water flow, the quantity of these concentrated dyestuff liquors is very low. However, they contribute to a high extent to the overall dyestuff load in the waste water (see also Section 4.6.7).
The quantity of liquor in the padder mainly depends on width and weight of the fabric and design of the padder. The range is about 10 - 15 l for modern designs and 100 l for old designs and heavy woven fabric (200 g/m2).
The residual amount in the preparation tank depends on applied dosage and control technology and can range from a few litres under optimised conditions up to 150 - 200 l. The latter is not too exceptional.
The quantity of residual padding liquors can be easily estimated on the basis of the number of batches per day. For example, a mill processing 40000 m/d and an average of 800 m per batch will have 50 batches per day. This number multiplied by the average volume of residual padding liquor per batch gives the daily quantity of residual padding liquor to dispose of.
Given a realistic pick-up of 100 % and a typical range of dyeings from 0.2 – 10 %, the dyestuff concentration in the padding liquor varies between 2 and 100 g/l. The specific COD of dyestuffs is in the order of 1 - 2 g/g. Considering only the dyestuff itself, without taking into account the auxiliaries already contained in dye formulations, the COD attributable to the dyestuffs in the padding liquor may vary between 2 and 200 g/l.
Typical recipes for continuous and semi-continuous dyeing are reported in Annex IV.
188.8.131.52.5 Printing Printing paste residues and water emissions from rotary screen printing It is well known that losses of printing pastes are particularly significant for rotary screen printing and analogue printing in general (perhaps less for flat screen printing) compared to digital printing - see also Section 2.8.3. In addition, especially for copper or nickel phthalocyanine dyestuffs, fixation rates can be very low for cellulosic fibres, PES and their blends (less than 50 %).
Typical figures for printing paste losses are:
2.5 – 4 kg for conventional squeegees (depending on diameter and length of the squeegee) 2.5 kg from pipes and the pumps 1-2 kg from the screens 6.5 – 8.5 kg Total Thus, for conventional printing paste supply systems, volumes (= losses) of 6.5 – 8.5 kg per colour applied can be expected Depending on the quantity and pattern of textile substrate to be printed, the losses of printing paste can be even higher than the amount of printing paste applied on the textile substrate. For example, for printing 250 m of fabric (with 200 g/m specific weight) and at a coverage of 80 % (ratio between the total textile area and the printed area), 40 kg printing paste are required. In the case of 7 colours and 6.5 kg of printing paste residues per colour, the loss is 45.5 kg, which is higher than the quantity of paste printed on the textile substrate (without taking into account the residues in the printing paste buckets, etc.).
Printing pastes are concentrated mixtures of different chemicals. Pigment printing pastes are the most concentrated ones, whereas reactive printing pastes have the lowest content of organic compounds. The composition of typical formulations of reactive, vat, disperse and pigment printing pastes are submitted in Section 11.
At the end of each batch the printing equipment (squeegee, pipes, pumps, screens, etc.) is
cleaned. Typical values for water consumption are as follows:
· 350 l per pump and pipes for one printing paste supply system · 35 l per squeegee (modern washing equipment) · 90 l per screen (modern washing equipment).
In addition, water is consumed for cleaning the printing blanket, with typical consumption levels of around 1200 l/h. Normally, the washing facility is coupled with the movement of the blanket, which is only about 25 % of the time (on/off system).
The dryer blanket also needs to be cleaned after the printing process. A typical consumption level is about 400 l/h; here, too, the washing facility is coupled with the movement of the blanket.
Urea consumption levels in reactive printing pastes Urea from reactive printing pastes is the main source of NH3 and NH4+ in the waste water from printing houses.
Data reflecting current industrial practice in three typical mills in Italy are reported for silk and viscose [312, ANT, 2002]. For silk the consumption levels range from 40 to 100 - 110 g/kg printing paste. For viscose the reported figures show even higher dosages (up to 150 g urea/kg of printing). Information about techniques for avoiding or at least reducing the use of urea is reported in Section 4.7.1.
Air emissions from drying and fixation treatments carried out after printing It is well known that printing pastes contain substances with high air emission potential. The relevant pollutants and the possible emission sources are listed in Section 2.8.3.
Table 3.41 shows emission data from measurements carried out in three finishing mills.
The reported figures refer to screen printing of flat fabrics (they do not apply to the printing of bulky fabrics such as carpets).
Table 3.41: Air emission data from drying and fixation after printing (measurements carried out at two finishing mills) 3.
184.108.40.206 Finishing Water emissions from finishing treatments Water pollution from finishing operations may arise from afterwashing operations (which are not always required) and from inadequate disposal of concentrated residues from padding devices, preparation tanks and pipes. The amount of residual liquors is in the range of about
0.5 to 35 % of the total amount of finishing liquor prepared. The lower value is for integrated mills finishing only one type of substrate, whereas the higher value is typical of textile mills processing small lots and different types of substrates.
Many different chemicals and recipes are available in order to finish textile substrates. Notable examples with cellulosic fibres, are the finishing treatments applied with reactive flame retardants (organophophorus compounds) and those with reactive non-iron auxiliaries. For the latter, a typical recipe is presented in Table 3.42.
Table 3.42: Standard recipe for the finishing of cotton woven fabric with reactive non-iron compounds The chemicals applied for both flame retardant and non-iron finishing are non-biodegradable and also adsorption to activated sludge is very low.
This indicates that biological treatment is not appropriate for treating these waste streams.
Another example is given for finishing agents that are widely applied to cotton woven fabric to improve crease and shrink resistance. One typical recipe is presented in Table 3.43.
Source: [179, UBA, 2001]
Specific fabric weight: 250 g/m2 Fabric width: 1.6 m (1) because of dilution Table 3.43: Typical finishing recipe for crease and shrink resistance In this case, there is no afterwashing, but environmental concerns arise from the potential discharge of the residual finishing liquor in the padder and preparation tank. The reactive component (methyloldihydroxiethylene urea), the optical brightener and the softening agents are non biodegradable and contribute to residual COD in the treated effluent of biological waste water treatment plants.
Potential air emissions from finishing auxiliaries (calculated data)
In most cases the emission potential of a finishing recipe can be calculated on the basis of emission factors given for the individual substances present in the formulation. According to this concept, which is explained in more detail in Section 4.3.2, it is possible to define the
following parameters (the original nomenclature is kept in the following description):
· a substance-based emission factor, and · a textile substrate-based emission factor.
There are two types of substance-based emission factors: 1) fc, which gives the total emission produced by the organic substances present in the formulation, expressed as total organic carbon; 2) fs, which gives the emission attributable to specific toxic or carcinogenic organic substances or inorganic compounds, such as ammonia and hydrogen chloride, present in the formulation.
In Germany, where this concept was developed, auxiliary suppliers provide information on the substance-based emission factors. This is a prerequisite for the calculation of the textile-based emission factors.
The textile substrate-based emission factor (WFc or WFs) is defined as the amount of organic and inorganic substances in grams that can be released under defined process conditions (curing time, curing temperature and type of substrate) from one kilogram of textile treated with a given auxiliary formulation.
The textile substrate-based emission factor can be calculated on the basis of the substance-based emission factors of the individual components of the formulation/recipe (fc or fs), their concentration in the bath (FK) and the liquor pick-up (FA, which normally ranges between
Table 3.44: Two examples for the calculation of air emission factors Based on these examples, the potential of a number of commercial auxiliaries to release Organic-C or specific substances such as formaldehyde during thermal processes carried out after finishing has been calculated and reported in the tables in Section 11.
4. The analysed auxiliaries are taken from the “ Textile Auxiliaries Buyers’ Guide” [65, TEGEWA, 2000]. The various formulations have been divided into different classes according to their functionality.
Considerable differences are also observed among auxiliaries belonging to the same class because of: chemical composition, active ingredients, by-products and impurities. However, the
following general comments can be made:
· formaldehyde is released mainly from auxiliaries based on cross-linking compounds (esp.
cross-linking agents and reactive flame retardants). Formaldehyde emission potential of melamine derivatives is in most cases higher than auxiliaries based on dimethyloldihydroxyethen urea derivatives (see Table 3 and Table 4) · antifoaming agents that contain highly volatile hydrocarbons as the main active compound have a very high emission potential compared to silicon-based types (see Table 5) · for wetting agents based on tributylphoshate, which is characterised by a high vapour pressure, substance-specific emission factors up to about 340 g Organic-C/kg are observed.
Because various additives/by-products not specified in the Material Safety Data Sheets are used and the amount of active ingredients can vary greatly, the emission factors of the other wetting agents (see Table 6) also vary greatly Textiles Industry 199 Chapter 3 · softening agents based on fatty acid derivatives are characterized by emission factors between 1 and 5 g Organic-C/kg. Polysiloxane-based types show higher values. The highest mentioned value in Table 7 is caused by a fatty acid type with an additive of a highlyvolatile wax · carriers are usually highly volatile substances; emission factors above 300 g Organic-C/kg are observed (see Table 8) · levelling agents used in dyeing can – like carriers - be carried over by the textile substrate and cause considerable air emissions · differences in the emission potential of flame retardants are mainly caused by the different types of active substances and amounts of by-products/additives (alcohols, especially methanol for reactive types, glycols, glycol ethers) (see Table 9) · with repellents, too, a wide range of emission factors is observed. This is mainly caused by a different kind and quantity of solvents used for fluorocarbon resins (e.g.buthyl/ethyl acetate, methylethyl/isobutylketone, ethandiol, propanediol) and different amounts of active ingredients if paraffin-based types are considered · for conditioning agents it is clear that products based on paraffins (which have a relatively high volatility) have a higher emission potential than fatty acid derivatives (see Table 11) · optical brighteners and antielectrostatic agents have variable emission potentials due to different active ingredients and differences in the formulations of the auxiliaries (see Table 12) · filling and stiffening agents based on natural or synthetic polymers have low emission potentials · emission levels for aftertreatment agents are low · biocides can contain aromatic hydrocarbons; this leads to increased emission factors (see Table 16) · emission potentials of silicic acid-based non-slip agents are very low.