Sizing agents are applied to warp yarn in order to protect it during the weaving process and have to be removed during textile pretreatment, thus giving rise to 40 - 70 % of the total COD load of woven fabric finishing mills.

Water-soluble synthetic sizing agents such as polyvinyl alcohol, polyacrylates and carboxymethyl cellulose can be recovered from washing liquor by ultrafiltration. Recently, it has been confirmed that modified starches such as carboxymethyl starch can also be recycled.

The principle of recovery by ultrafiltration is shown in Figure 4.9. After sizing and weaving, sizing agents are removed during textile pretreatment by hot washing with water in a continuous washing machine (in order to minimise water consumption, the washing process may need to be optimised). Sizing agents concentration in the washing liquor is about 20 - 30 g/l. In the ultrafiltration plant, they are concentrated to 150 - 350 g/l. The concentrate is recovered and can be re-used for sizing, whereas the permeate can be recycled as water in the washing machine. Note that the concentrate is kept at high temperature (80 - 85°C) and does not need to be reheated [179, UBA, 2001].

Figure 4.10 shows the mass balance of sizing agents and water for the process with and without recovery in a representative case study. It can be noticed that, even with recovery, some losses of sizing agent still occur at various steps of the process, especially during weaving. Furthermore, a certain amount of sizing agent still remains on the desized fabric and a fraction ends up in the permeate. In conclusion, the percentage of sizing agents which can be recovered is 80 - 85 %.

Figure 4.10: Representative example of mass balance for sizing agents and water with and without recovery

COD load of waste water from finishers of woven fabric is reduced by 40 - 70 %. Sizing agents are recovered by 80 - 85 %. In addition, sizing agents in waste water do not need to be treated. Thus energy consumption for treatment is reduced significantly as well as quantity of sludge to be disposed of [179, UBA, 2001].

Ultrafiltration is very efficient in reducing high organic load from textile mills. However, it has to be remembered that the polymers used for recoverable sizing agents are also widely applied in products such as household detergents, which are found in great quantities in other effluents. [61, L. Bettens, 1999].

In order to minimise scaling and fouling, fibres have to be removed before ultrafiltration. The same applied to fine particles, such as singeing dust. A pre-filtration step is carried out for this purpose.

When desizing coloured woven fabric (dyed warp yarn), the desizing liquor becomes slightly coloured. Dyestuff particles are more difficult to remove and the liquor needs to be submitted to microfiltration (which is more complex, but still feasible) [179, UBA, 2001].

The operation/management of ultrafiltration units for recovery of sizing agents requires qualified staff and accurate maintenance.

Ultrafiltration needs energy, but the amount consumed is much less than the energy required to produce new sizing agents (if they are not recovered) and to treat them in a waste water treatment plant [179, UBA, 2001].

As explained earlier, this technique is suitable only for specific types of sizing agents. These are water-soluble synthetic sizing agents such as PVA, polyacrylates and carboxymethyl cellulose. Recently, it has been confirmed that some modified starches such as carboxymethyl starch can also be recycled.

Re-use in the weaving plant is not always without problems. Stock and the recovered size need to be kept under sterile conditions when stored and mixed with virgin size. In the past, failure of protection against bacterial growth (biological degradation of concentrates and contamination of the ultrafiltration equipment) resulted in the shutdown of a recycling plant in Belgium [61, L. Bettens, 1999]. Nowadays, recovered sizing agents are kept at temperatures above 75 °C. It is reported that under these conditions there are no problems of microbial attack and therefore no addition of biocides is needed to maintain sterile conditions [280, Germany, 2002].

Limitations in the applicability of this technique may arise from cases where the auxiliaries applied to the yarn are not only sizing agents, but also waxes, antistatic agents, etc. These compounds remain in the concentrate after UF. The concentrate can be re-used for sizing, but limitations can be found when re-using the same concentrate for different kind of yarns (with different applications and end-uses) which may need specific additives [281, Belgium, 2002]. To date, the weavers' acceptance of recovered sizes is still limited. Weavers are concerned about the quality of the recovered size. Furthermore, certain effects such as minting can only be carried out with non-desized fabric. For these reasons, re-use of the concentrate is typically applied in integrated companies with a uniform production.

A further issue to consider is the transport distances. Long-distance shipments cancel out any ecological advantages because the liquor needs to be transported in adequate conditions in insulated tankers [179, UBA, 2001]. Although, there are mills where recovery is carried out in spite of a considerable distance between the weaving and finishing departments (up to 300 km in one company in the USA), sizing agents are usually recovered in integrated mills having a weaving and a finishing section at the same site.

When weaving and finishing (desizing) take place in completely different places, a more practicable option would probably be to remove and recover the sizing agents directly in the weaving mill, which would therefore produce desized fabric. However, while the quantity of processed fabric must be higher than 1000 t/yr to make the process cost-effective in an integrated mill, the minimum amount in a weaving mill producing desized fabric is much higher (about 5000 - 8000 t fabric/yr) because, in addition to the ultrafiltration plant, a washing machine and a dryer have to be installed [179, UBA, 2001]. Additionally the textile finishers' acceptance of already de-sized fabric is still limited and certain effects such as minting can only be carried out with non-desized fabric.

A cost/benefit assessment should take into account not only the costs of ultrafiltration, but also the recipe and overall process and treatment costs, especially when considering that changing over from starch and starch derivatives to synthetic sizing agents also has implications for weaving efficiency. Synthetic sizing agents are more expensive than starch-based sizing agents, but they are applied in lower amounts and the weaving efficiency may be higher.

The following table presents a typical example of the annual savings achievable when introducing recovery of sizing agents [179, UBA, 2001].

Input for sizing	Without recovery (annual basis)			With recovery (annual basis)
			EUR			EUR
Produced woven fabric	8750	t		8750	t
Quantity of warp yarn	5338	t		5338	t
Load of sizing agents ⁽¹⁾	13.8	%		10	%
Recovered sizing agent	-			427	t	76095
Starch derivative	470	t	261435
PVA	264	t	722500	75	t	205100
Polyacrylates (100 %)				32	t	158400
Wax	59	t	133040	26.7	t	30485
Fresh water	5075	m³	5840	755	m³	830
Steam	890	t	10780	350	t	4235
Electricity	155680	kWh	8560	32000	kWh	1760
Manpower	4450	h	58700	1680	h	22180
Total cost			1200855			499085
[179, UBA, 2001]

Table 4.15: Typical example of annual savings achievable when introducing recovery of sizing agents

In the example given in the table, there will be additional savings because of the higher weaving efficiency and the reduced cost of pretreatment (time saving and significantly reduced consumption of chemicals for degradation and removal of sizes compared to starch-based products) and waste water treatment. The payback time of an ultrafiltration plant may then be less than one year [179, UBA, 2001], which suggests that in most cases companies primarily invest in this technique not because of the environment, but because of the economical benefit.

The investment costs for the ultrafiltration plant referenced above illustrated above are the following [179, UBA, 2001]:

Waste water problems and cost reductions have been the most important driving forces to implement recovery of sizing agents [179, UBA, 2001].

The first plant for recovery of polyvinyl alcohol went into operation in 1975 in the USA. Meanwhile there are two plants that have been in operation in Germany for many years and various plants are now in operation in Brazil, Taiwan and USA. There are not many suppliers of ultrafiltration plants [179, UBA, 2001].

4.5.2 Application of the oxidative route for efficient, universal size removal

Many woven fabrics contain a variety of different sizing agents, depending on the origin and quality of the substrate. Most textile finishers deal with many different types of fabrics, and therefore sizing agents, so they are interested in fast, consistent and reliable removal of non-fibrous material (be it the impurities and fibre-adjacent material or any preparation agent) independent of the origin of the fabric.

Enzyme desizing removes starches but has little effect in removing other sizes. Under specific conditions (above pH 13), H₂O₂ generates free radicals which efficiently and uniformly degrade all sizes and remove them from the fabric. This process provides a clean, absorbent and uniform base for subsequent dyeing and printing, no matter which size or or fabric type is involved [189, D. Levy, 1998].

Recent studies ([203, VITO, 2001]) show that above pH 13 the oxide radical anion O^*- is the predominant form. This species is highly reactive, but it will attack non-fibrous material (sizing agents, etc.) rather than cellulose, for various reasons. First because it is negatively charged like the cellulose polymer in strongly alkaline medium (coulombic repulsion effect) and secondly because, unlike the OH* it does not react by opening the aromatic rings.

It is recommended to first remove the catalyst that is not evenly distributed over the fabric (e.g. iron particles, copper, etc.). One possible process sequence would therefore be: removal of metals (modern pretreatment lines are equipped with metal detectors), oxidative desizing (peroxide and alkali), scouring (alkali), demineralisation (acid reductive or, better still, alkaline reductive/extractive), bleaching (peroxide and alkali), rinsing and drying.

The proposed technique allows significant environmental benefits: water & energy consumption along with improved treatability of the effluent.

The oxidative route is a very attractive option where peroxide bleaching is carried out. Taking advantage of hydrogen peroxide also being used as an active substance for bleaching, it is advantageous to combine alkaline bleaching with scouring and regulate the countercurrent flow of alkali and peroxide through the different pretreatment steps, so as to save water, energy and chemicals.

Because of the action of free radicals generated by activation of hydrogen peroxide,the size polymers are already highly degraded. The process produces shorter and less branched molecules, glucose, more carboxylated molecules such as oxalate, acetate and formate, which are easier to wash out with a reduced amount of water in efficient washing machines.

The pre-oxidation of size polymer is also advantageous at waste water treatment level (improved treatability). With enzymatic desizing, starches are not completely degraded (the long molecules are not completely broken down after desizing). This means higher organic load to be degraded in the biological plant and it is often the cause of problems such as the production of bulky difficult-to-settle sludge.

It is well known that in an oxidative alkaline medium (with hydrogen peroxide) there is potential risk of fibre damage during bleaching if OH* formation is not controlled. Size and the cellulose have similar molecular structure and therefore the attack of the cellulose polymer from non-selective OH* is possible. To achieve good results and avoid damage to the fibre when removing starch-like size, it is essential to add hydrogen peroxide at pH >13. These operating conditions minimise OH* radicals, which are responsible for cellulose damage.

The technique is particularly suitable for commission finishers (independently of their size), who need to be highly flexible because their goods do not all come from the same source (and consequently they do not have goods treated with the same type of sizing agents). In the interests of high productivity, these companies need to operate with a universally applicable technique to enable a right-first-time approach.

There is no need for sophisticated control devices as these are already be available for control of oxidative bleaching. Equipment is no different from modern preparation lines.

The steps and liquors are combined so that the resource consumption is optimised at overall minimal cost.

With the increased usage of hydrogen peroxide as a replacement for hypochlorite in bleaching, the cost of hydrogen peroxide will continue to drop relative to other oxidants. Selective use of hydrogen peroxide (minimising non-selective reaction pathways) will be important for reducing overall costs, including raw material, energy and environmental clean-up.

«Ref. 1995, Catalytic oxidations with oxygen: An Industrial Perspective, Jerry Ebner and Dennis Riley

«Ref. 1998, Peroxide desizing: a new approach in efficient, universal size removal, David Levy»

«Ref. 1995, Environmentally friendly bleaching of natural fibres by advanced techniques, Ludwich Bettens (SYNBLEACH EV5V - CT 94- 0553) - Presentation given at the European Workshop on Technologies for Environmental Potection, 31 January to 3 February 1995, Bilbao, Spain - Report 7»

4.5.3 One-step desizing, scouring and bleaching of cotton fabric

For cotton woven fabric and its blends with synthetic fibres, a three-stage pretreatment process has been the standard procedure for many years, comprising:

New auxiliaries' formulations and automatic dosing and steamers allow the so-called «Flash Steam» procedure which telescopes desizing, alkaline cracking (scouring) and pad-steam peroxide bleaching into a single step [180, Spain, 2001].

Combining three operations in one allows significant reductions in water and energy consumption.

Within the space of 2 - 4 minutes (with tight strand guidance throughout) loom-state goods are brought to a white suitable for dyeing. This is a big advantage, especially when processing fabrics that are prone to creasing [180, Spain, 2001].

The chemistry is simple and completely automated with full potential for optimum use.

Companies with new machinery suitable for this process can apply this technique [180, Spain, 2001]. No more detailed information was made available.

[180, Spain, 2001] with reference to «International dyer, October (2000), p.10»

4.5.4 Enzymatic scouring

Enzymatic desizing using amylases is an established process that has been in use for many years. More recently, pectinases have shown promise in replacing the traditional alkaline scouring treatment. Some auxiliaries suppliers have introduced an enzymatic process to remove hydrophobic and other non-cellulosic components from cotton. The new process operates at mild pH conditions over a broad temperature range and can be applied using equipment such as jet machines.

It is claimed that, due to a better bleachability of enzyme-scoured textiles, bleaching can be carried out with reduced amounts of bleaching chemicals and auxiliaries. Enzymes actually make the substrate more hydrophilic (which could explain better bleachability), but they are not able to destroy wax and seeds, which are therefore removed in the subsequent bleaching process.

Sodium hydroxide used in conventional scouring treatment is no longer necessary. Furthermore, the following advantages are reported over the traditional procedure (see next table).

Table 4.16: Environmental benefits achieved with an enzymatic scouring process

A typical process for a pad-batch process combining scouring and desizing in one single step is as follows [179, UBA, 2001]:

The environmental benefits remain unclear as enzymes contribute to the organic load and their action is based on hydrolysis rather than oxidation. The organic load not removed with enzymatic scouring may appear in the later wet processing steps. A more global balance would probably reveal no significant improvement.

The enzymatic scouring process can be applied to cellulosic fibres and their blends (for both woven and knitted goods) in continuous and discontinuous processes.

The process can be applied using jet, overflow, winch, pad-batch, pad-steam and pad-roll equipment.

Price performance is claimed to be economical when considering the total process costs.

Quality aspects (good reproducibility, reduced fibre damage, good dimensional stability, soft handle, increased colour yield, etc.), technical aspects (e.g. no corrosion of metal parts) as well as ecological and economical aspects are reported as reasons for the implementation of the enzymatic scouring technique [179, UBA, 2001].

4.5.5 Substitution for sodium hypochlorite and chlorine-containing compounds in bleaching operations

The application of hypochlorite gives rise to subsidiary reactions leading to the formation of a number of chlorinated hydrocarbons such as the carcinogenic trichloromethane (which is also the most frequently formed as it is the end of the reaction chain). Most of these by-products can be detected as adsorbable organic halogens by means of the sum parameter AOX. Similar contributions to the formation of hazardous AOX come from chlorine or chlorine-releasing compounds and strong chlorinated acids (e.g. trichloroacetic acid). Halogenated solvents are a different category of problematic AOX (see also Section 2.6.1.2).

Sodium hypochlorite was for a long time one of the most widely used bleaching agents in the textile finishing industry. Although it has been largely replaced in Germany and many other European countries, it is still in use not only as a bleaching agent, but also for cleaning dyeing machines or as a stripping agent for recovery of faulty dyed goods.

In certain conditions, sodium chlorite may also give rise to the formation of AOX, although to a lesser extent than hypochlorite. However, recent investigations have shown that the cause is not sodium chlorite itself, but the chlorine or hypochlorite present as impurities (from non-stoichiometric production) or used as activating agent. Recent technologies (using hydrogen peroxide as the reducing agent of sodium chlorate) are now available to produce ClO₂ without generation of AOX [18, VITO, 1998], [59, L. Bettens, 2000].

Hydrogen peroxide is now the preferred bleaching agent for cotton and cotton blends as a substitute for sodium hypochlorite.

When a single-stage process using only hydrogen peroxide cannot achieve the high degree of whiteness required, a two-stage process with hydrogen peroxide (first step) and sodium hypochlorite (second step) can be applied, in order to reduce AOX emissions. In this way the impurities on the fibre - which act as precursors in the haloform reaction - are removed, thus producing a reduction in AOX in the effluent. Nevertheless, a two-stage bleaching process using only hydrogen peroxide is today possible, thus completely eliminating the use of hypochlorite (cold bleaching at room temperature followed by a hot bleaching step).

There is also increasing support for peroxide bleach under strong alkaline conditions, which achieves a high degree of whiteness after careful removal of catalysts by a reduction/extraction technique. The additional advantage claimed is the possible combination of scouring and bleaching. The reduction/extraction followed by a strong oxidative combined bleaching/scouring step (high alkali and high active oxygen concentration) is applicable for bleaching highly contaminated textiles in all make-ups and on all types of machines (discontinuous and continuous). This method takes the oxidative route and uses the active oxygen.

The presence of hazardous AOX such as trichloromethane and chloroacetic acid in the effluent is avoided.

Particular attention needs to be paid to the combination or sequence of pretreatment operations and to the mixing of streams containing hypochlorite or chlorine. For example, the application of the two-step bleaching method where hypochlorite as well as peroxide is used, is potentially hazardous if the hypochlorite bleaching is performed when large quantities of organohalogen precursors are still present on the substrate. The risk would thus be reduced if hypochlorite bleach came as a last step after an alkaline peroxide bleach that removes the precursors from the fibre. However, no data were made available that show the importance of reversing the sequence of the two steps from hypochlorite peroxide into peroxide hypochlorite. It is actually more important to avoid mixing hypochlorite bleach waste water with certain other streams and mixed effluents, in particular from desizing and washing, even when the right sequence of pretreatment and bleaching is adopted. The formation of organohalogens is highly possible in combined process streams.

For chlorite bleach, handling and storage of sodium chlorite needs particular attention because of toxicity and corrosion risks. Machinery and equipment need to be inspected frequently because of the high stress to which they are subjected (see also Section 2.6.1.2).

Complexing agents (e.g. EDTA, DTPA, phosphonates) are normally applied as hydrogen peroxide stabilisers. The main concerns associated with the use of these substances arise from their ability to form stable complexes with metals (remobilisation of heavy metals), their N- and P- content and their often low biodegradability/bioeliminability. The addition of strong sequestering agents, however, can be avoided by fine control of the pH conditions during the bleaching process (see Section 4.5.6) and with the assistance of silicates, magnesium, acrylates or biologically degradable carboxylates, slowing down the uncontrolled decomposition of hydrogen peroxide (see Section 4.3.4).

Optical brighteners are often applied when peroxide bleaching is not sufficient to achieve the required level of whiteness. The resulting COD load and smoke during fixation in the stenter have to be taken into account. Moreover, optical whiteners are potentially irritating and thus not always acceptable for white goods coming into close contact with the skin (e.g. underwear, bedsheets).

Substitution for hypochlorite as bleaching agent is applicable to both new and existing installations.

Hydrogen peroxide is a valid substitute for bleaching yarn and woven fabric made of most cellulosic and wool fibres and most of their blends. Today a full hydrogen peroxide bleaching process is also applicable to cotton & cotton-blend knitted fabric and a high degree of whiteness (>75 BERBER Whiteness Index) can be obtained (with a strong alkaline scour/bleach after removal of the catalyst).

Exceptions are flax and other bast fibres that cannot be bleached using peroxide alone. Unlike chlorine dioxide, the anionic bleaching agent is not strong enough to remove all coloured material and does not preferentially access the hydrophobic region of the fibre. A two-step hydrogen peroxide-chlorine dioxide bleaching is an option for flax.

It is claimed that a sequence where precursors of halogenation are removed with a peroxide bleach followed by a hypochlorite bleach (or a peroxide pre-bleach followed by a combined hydrogen peroxide/hypochlorite bleach) is still necessary for high whiteness and for fabrics that are fragile and would suffer from depolymerisation.

Sodium chlorite is an excellent bleaching agent for flax, linen and some synthetic fibres.

In general, bleaching with hydrogen peroxide is no more expensive than with hypochlorite because of market saturation.

The two-stage bleaching process with hydrogen peroxide proposed for knitted fabric is reported to be from two to six times more expensive than the conventional process using hydrogen peroxide and hypochlorite [179, UBA, 2001].

If using chlorine dioxide as bleaching agent, investment may be needed (in existing installations) for equipment resistant to the highly corrosive conditions in which this bleaching agent is used.

As far as the production of elemental chlorine-free chlorine dioxide is concerned, this process is fully investigated and described in another BREF (pulp & paper industry).

Market demands for chlorine-free bleached textiles and the requirements set by legislation (regarding waste water discharge) are the main driving forces for the implementation of this technique.

Many plants in Europe and worldwide largely use substitutes for sodium hypochlorite as bleaching agent.

4.5.6 Minimising consumption of complexing agents in hydrogen peroxide bleaching

When bleaching with hydrogen peroxide, oxygen species of differing reactivity may be present in water (O₂**, H₂O₂/HOO-, H₂O/OH-, HOO*/O₂*-, OH*/O*-, O₃/O₃*-). The kinetics of formation and disappearance depend on concentration of oxygen, energy for activation, reduction potential, pH, catalyst and other reagents. These processes are very complex and can only be explained with dynamic simulation models. It is widely accepted that the OH* radical is responsible for attacking the cellulose fibre and leading to its damage (depolymerisation) and that the formation of the OH* radical is mainly due to the reaction of H₂O₂/HOO^- with transition metals such as iron, manganese and copper. The prevention of «catalytic» damage of the fibre as a consequence of the uncontrolled formation of OH* radical is usually achieved by using complex formers that inactivate the catalyst (stabilisers). See also Section 8.5.

Complexing agents (see Figure 4.6) that are typically applied in finishing mills are based on polyphosphates (e.g. tripolyphosphate), phosphonates (e.g. 1-hydroxyethane 1,1-diphosphonic acid) and amino carboxylic acids (e.g. EDTA, DTPA and NTA). The main concerns associated with the use of these substances arise from their N- and P- content, their often-low biodegradability/bioeliminability and their ability to form stable complexes with metals, which may lead to the remobilisation of heavy metals (see also Section 8.5).

The use of high quantities of sequestering agents can be avoided by removing the responsible catalysts from the water used in the process and from the textile substrate and by scavanging away the OH*.

Softening of fresh water is largely applied by textile mills to remove the iron and the hardening alkaline-earth cations from the process water (magnesium hydrate has a stabilising effect and techniques that remove transition metals and calcium are therefore preferred).

Iron carried with the raw fibre can be present as fibre impurity, rust or coarse iron particles on the surface of the fabric. The latter can be detected and removed by a dry process using magnetic detectors/ magnets (modern continuous lines are equipped with magnetic detectors). This treatment is convenient when the process starts with an oxidative scouring/desizing step, because otherwise a huge amount of chemicals would be required to dissolve these coarse iron particles in a wet process. On the other hand, the previous removal of coarse iron particles is not necessary when an alkaline scouring treatment is carried out as a first step before bleaching.

Magnetic sensors cannot detect non ferromagnetic particles and magnets cannot remove the iron that is inside the fibre (fibre impurities and rust in heavily contaminated goods). This iron fraction has to be solubilised and removed from the substrate by acid demineralisation or reductive/extractive treatment before bleaching. In the case of acid demineralisation, Fe(III) oxide, iron metal and many other forms of iron (some organic complexes) are solubilised in strongly acid conditions (by hydrochloric acid at pH 3). This means that the metal parts of the equipment must withstand these conditions. The advantage of the reductive treatment is that there is no need to use strong corrosive acids. Moreover, with the new non-hazardous reductive agents (see Section 4.6.5), it is possible to avoid a drastic change of pH.

As mentioned above, OH* radicals can be scavanged away in order to minimise fibre damage without the need for complexing agents.

In-depth research into the reactivity of hydrogen peroxide (SYNBLEACH EV5V-CT94-0553 EC funded research project) has shown that the control of the process is fundamental to prevent uncontrolled decomposition of hydrogen peroxide and to allow optimum use of hydrogen peroxide.

Figure 4.11 shows that under optimal conditions (pH approximately 11.2, homogeneously distributed catalyst and controlled peroxide concentration) the hydroxyl radical OH* is scavenged away by hydrogen peroxide, forming the true bleaching agent, the dioxide radical ion (maximum formation of dioxide radical anion O₂^*- in accordance with the peak). Under these conditions hydrogen peroxide itself acts as a scavenger and the reaction product is the active bleaching agent itself (which allows optimal use of hydrogen peroxide). The addition of formic acid (formate ion) as scavenging agent is also useful to further control the formation of the OH* radical, generating more O₂^*- and even repairing damage to the fibre.

Figure 4.11: Production of the peroxide radical ion by scavenging hydroxyl radicals (OH^*) using hydrogen peroxide

With the proposed technique it is possible to bleach cellulose in full and even to high whiteness, without damage to the fibre with:

As mentioned above, as an alternative to acid demineralisation, pre-cleaning of heavily soiled fabric (rust) is possible in more alkaline conditions using non-hazardous reducing agents, without any need for a drastic change in pH. The reduction/extraction is effective for all types of substrates and qualities of fabrics (highly contaminated, uneven distribution of iron-rust). This step is easy to integrate with discontinuous and continuous pocesses following the oxidative route under mildly or strongly alkaline bleach conditions [203, VITO, 2001].

The measures described in this section may be generally applicable to existing and new plants. However, fully automated equipment is necessary for the application of hydrogen peroxide under controlled process conditions. Dosing of the bleaching agent, controlled by a dynamic simulation model, is still limited [203, VITO, 2001].

Reduction of peroxide consumption by more than 50 % is possible. There is no increase, but rather a decrease in organic load, along with better treatability of the effluent. The chemistry needed is not expensive and is reliable, provided that there is a good knowledge of the complex control parameters [203, VITO, 2001].

The technique described in this section is provided directly by some auxiliaries suppliers. With the help of dynamic simulation models they are able to prepare a recipe that is suitable for the specific substrate, equipment used, etc. under defined process conditions.

4.5.7 Recovery of alkali from mercerising

During the mercerisation process, cotton yarn or fabric (mainly woven fabric but also knit fabric) is treated in a solution of concentrated caustic soda (270 - 300 g NaOH/l, or also 170 - 350 g NaOH/kg textile substrate) for about 40 - 50 seconds. The textile substrate is then rinsed in order to remove caustic soda. This rinsing water is called weak lye (40 - 50 g NaOH/l) and can be concentrated by evaporation for recycling. The principle is shown in the figure below.

Figure 4.12: Representation of the caustic soda recovery process by evaporation followed by lye purification

After removal of lint, fluff and other particles (using self-cleaning rotary filters or pressure micro-filtration), the weak lye is first concentrated, for instance in a three-stage evaporation process. In many cases, purification of the lye is required after evaporation. The purification technique depends on the degree of lye contamination and can be simple sedimentation or oxidation/flotation with injection of hydrogen peroxide.

The alkaline load of waste water is reduced drastically and acid required for waste water neutralisation is minimised.

The concentration of weak lye is usually 5 - 8 °Bè (30 - 55 g NaOH/l) and is increased to 25 - 40 °Bè (225 - 485 g NaOH/l), depending on the mercerising process applied. When mercerisation is carried out on the greige dry textile substrate (raw mercerisation) it is possible to achieve a concentration of caustic soda not higher than 25 - 28 °Bè, whereas a concentration of 40 °Bè can be obtained in non-raw mercerisation. In raw mercerisation, the concentration of impurities is significantly higher, as is viscosity, which makes it difficult to reach higher concentrations (circulation in evaporators is disturbed) [179, UBA, 2001].

The higher the number of stages for evaporation, the more often the heat is re-used, the lower the steam consumption and, therefore, the running cost. Investment, however, obviously increases with the number of stages [179, UBA, 2001].

Evaporation requires approximately 0.3 kg steam /kg water evaporated in a 4-stage evaporation plant. This corresponds to 1.0 kg steam/kg of recovered NaOH at 28 °Bé or 1.85 kg steam / kg of recovered NaOH at 40 °Bé.

Due to the action of active oxygen generated by the decomposition of hydrogen peroxide it is possible to recover and decontaminate coloured alkali for re-use (hydrogen peroxide is already used in the water stream when applying the oxidative route - see Section 4.5.2).

Investment costs mainly depend on plant size and purification technique and typically vary from 200000 to 800000 euros. Payback time depends on plant size and operating time per day. Usually, if mercerisation is practised full-time, the payback period is less than one year. In companies where non-recovered caustic soda lye has to be neutralised with acid, payback time is less than six months. Thus, from the economic point of view, caustic soda recovery may be very attractive [179, UBA, 2001].

High alkali content of waste water and economic aspects of caustic soda losses are the main driving forces [179, UBA, 2001].

The first caustic soda recovery plant went into operation more than one hundred years ago. Today, there are more than 300 plants in operation worldwide, especially for recovery of caustic soda from woven fabric mercerisation and yarn mercerisation and some from knit fabric mercerisation (the latter process is not applied very often).

4.5.8 Optimisation of cotton warp yarn pretreatment

In the production of white, non-dyed cotton sheets (e.g. sheets to be used under bed sheets and table-cloths) cotton warp yarn is bleached before weaving (for the production of this type of article the fabric does not need to be desized after the weaving process).

The conventional process consists of five steps, including wetting/scouring, alkaline peroxide bleaching and three subsequent rinsing steps. The last rinsing water is re-used for making the first bath.

This process can be further improved by combining wetting, scouring and bleaching in one step and performing rinsing in two steps, re-using the second rinsing bath for making the bleaching/scouring bath (as above).

In addition, the energy consumption of the process has been reduced by heat recovery. The heat from the scouring/bleaching bath (110°C) is recovered (by means of a heat exchanger) and used for heating the fresh water for the first rinsing. The bath is therefore cooled to about 80°C, while the fresh water reaches a temperature of 60 - 70 °C.

This cooled scouring/bleaching bath is collected in a tank together with the warm rinsing water from the first rinsing step. This waste water still has a valuable energy content. Therefore, before being drained, this stream is used to heat the water from the second rinsing step (which is then used for making the bleaching/scouring bath as explained above).

Water consumption and waste water discharge before and after optimisation can be seen from the following table: 50 % reduction of water consumption is achieved.

	Process	Water consumption in the conventional process (litres) ⁽¹⁾	Water consumption in the optimised process (litres) ⁽¹⁾
Step 1	Wetting/scouring	6400	6400
Step 2	Bleaching	5000	6400
Step 3	Cold rinsing	5000	5000
Step 4	Warm rinsing	5000	5000
Step 5	Rinsing and pH adjustment with acetic acid	5000	5000
Total		26400	16400
Recycling of last rinsing bath		-5000	-5000
Total water consumption		21400	11400
Specific water consumption (800 kg yarn/batch)		26.8 l/kg	14.3 l/kg
Residual water content in the yarn		1400	1400
Waste water flow		20000	10000
Specific waste water flow		25 l/kg	12.5 l/kg
Source: [179, UBA, 2001] Note: ⁽¹⁾ Data are referred to a 800 kg batch

Table 4.17: Optimisation of warp yarn scouring/bleaching: absolute and specific water consumption and waste water discharge before and after process optimisation

The consumption of chemicals and energy has also been reduced drastically. The following savings are achieved [179, UBA, 2001]:

The operating conditions of the optimised process are illustrated in Table 4.18, which also contains the calculation of COD-input and -output.

Process input and operating conditions	Quantity		Spec. COD		COD-load per kg of yarn
Wetting/scouring/bleaching
· Conditions: pH ca. 12, 110°C, 10 min
· Recipe
- NaOH 38°Bé (33 %)	3.5	g/l	-
- H₂O₂ 35 %	3.0	g/l	-
- Sequestrant and stabiliser	1.0	g/l	85	mgO₂/g		0.6 gO₂/kg
- Surfactant	1.9	g/l	1610	mgO₂/g		24.2 gO₂/kg
- Optical brighteners	0.15	wt-%	2600	mgO₂/g		3.9 gO₂/kg
					Tot. from auxil.	28.7 gO₂/kg
					Extracted from cotton	70.0 gO₂/kg
First rinsing
· Conditions: 70°C, 15 min			3000	mgO₂/l		18.7 gO₂/kg
Second rinsing
· Conditions: 70°C, 15 min			1000	mgO₂/l		6.2 gO₂/kg

					Total	124 gO₂/kg
Source: [179, UBA, 2001]

Table 4.18: Optimisation of warp yarn scouring/bleaching: recipe and operating conditions for the optimised process

The optimisation of the process is possible for both existing and new installations. For the recovery of heat, space for additional tanks is required, which may be a limiting factor in some cases. The quality of the cotton yarn has to be considered (as regards content of iron, seeds etc.) in order to make sure that the process can be applied.

The considerable savings of time, water, chemicals and energy make the process highly economic. The optimised process does not require new equipment for pretreatment, but tanks, heat exchangers, pipes and control devices for energy recovery from waste water are required.

Environmental motivation has been the main driving force for the development of the process, but the economic benefit also justifies the investment of effort.

Two textile finishing plants in Germany are using the described optimised process successfully.

Prozessoptimiwerung durch Wasserkreislaufführung und Abwasservermeidung am Beispiel einer Kettbaumbleiche

Proceedings of BEW-Seminar "Vermeidung, Verminderung und Behandlung von Abwässern der Textilindustrie" on 06.03.2001 (2001)

	EUR
- ultrafiltration plant:	1000000
- equalisation tank:	105000
- installation:	77000
- start up:	27500
- miscellaneous:	27500
- total investment cost:	1237000

	Enzymatic scouring	Enzymatic scouring + bleaching with reduced concentration of hydrogen peroxide and alkali
Reduction in rinsing water consumption	20 %	50 %
Reduction in BOD-load	20 %	40 %
Reduction in COD-load	20 %	40 %
Source: [179, UBA, 2001]

· process time:	about 50 %
· water consumption/waste water discharge:	about 50 %
· NaOH:	about 80 %
· H₂O₂:	no reduction
· complexing agents/stabilisers:	about 65 %
· surfactants:	about 70 %
· optical brightener:	no reduction
· COD load of waste water	about 20 %
· energy:	1.2 kg steam/kg warp yarn

4.5 Pretreatment