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Danish experience. Best Available Techniques – BAT - in the clothing and textile industry

2. Cleaner technology in reactive dyeing of cotton

This chapter presents Danish know-how with reclamation and re-use of process water from reactive dyeing of cotton knitwear in batch. Reactive dyeing of cotton is the most used textile dyeing process world-wide, both with regard to cotton textiles or all kinds of textiles, and the share of the market is increasing.

The overall strategy of the research was to identify environmental improvements by a stepwise procedure:

1. Process optimisations – e.g. savings in chemicals, energy and water.
2. Reclamation and re-use of chemicals, energy and water.

Not respecting this order of priority could lead to wrong dimensioning of water reclamation equipment, and in worst case, total unsuccessful investment.

In textile dyeing the recipe is the fundamental specification of the processes. Water consumption, chemical consumption, temperature, salinity, pH etc. are all specified step by step in the recipe.

The recipe for reactive dyeing of cotton can be divided into three steps: the pre-treatment, the dye-bath and the rinsing after dyeing. Traditionally, the consumption of energy, chemicals and water in rinsing is crucial; approximately half of the total energy consumption and up to three-quarters of the total COD discharge and of the total water consumption have relation to the rinsing after the dyeing processes. The potential for environmental improvements in the rinsing procedures are in this way considerable and the process of recipe development focused on the rinsing after dyeing.

The advantages and limitations of different water reclamation techniques have been identified in lab-scale and documented in pilot-scale. An overall solution has been chosen, based on membrane technology for the rinsing water and activated carbon adsorption for the dyebath itself. The solution implies hot water reuse in rinsing, reuse of filtration remains in anaerobic digesters, and reuse of dyebath water and salts.

In this way, Danish experience includes development of new rinsing recipes and of water reclamation techniques, leading to large savings in time, water, energy, and chemicals. For reclamation and reuse of the rinsing water from the optimised rinsing recipe, a comparative study of the environmental impacts before and after rinsing water reuse has been carried out. The study concerns a reuse solution based on membrane filtration. All changes in use of water, energy, and chemicals are included in the study, also including the membrane plant, reservoirs etc. from raw material extraction, to production, use, and disposal of the equipment. This study is a so-called life-cycle assessment of the solution, performed according to international practice. The study concerns only environmental impacts from the rinsing process and the results are outlined in figure 2.1.

In figure 2.1, there is a column for the "before" situation and for the "after" situation, and the potential for improvements explain mostly itself. In the environmental benefits there are three columns. This is because the dye-house, where the membrane plant is implemented, has heat production on the basis of natural gas – but the membrane plant is driven by electricity based on coal. The difference in the environmental load due to different energy source blur the real environmental improvement potential – this is the reason why the third column is calculated as if the electrical power for the membrane plant was produced on natural gas.

Figure 2.1
Environmental benefits (on top) and resource savings (below) by introducing the new rinsing recipe and reuse of water and energy by membrane filtration.

A more detailed description of the research is presented in the following Chapter "2.1 Recipe optimisation" and Chapter "2.2 Reclamation and reuse of chemicals, energy and water".

2.1 Recipe optimisation

The reactive dyeing process is outlined in table 2.1 by a representative recipe selected by one of the dye-houses included in the project. During the pre-treatment, the cotton fabric is washed and degreased, and treated with lye in order to open the fibre structure. When the pre-treatment is part of a light shade recipe, the pre-treatment process includes a bleaching. After some rinses, the dyestuff is poured into the dyebath and a diffusion of the dyestuff molecules between the cellulose fibres takes place. After some time, salt is added to obtain adsorption of the dyestuff to the cellulose fibre. After this, adjusting temperature (50-80°C) and pH (10,5-11,5) completes the reaction between the dyestuff and the cellulose. Some of the dyestuff will be hydrolysed during this dyeing process, and the adsorbed hydrolysate must be removed in the succeeding rinsing after dyeing.

Table 2.1
Original recipe, typical example. The dyeing machine is drained after each batch.

Traditional rinsing after dyeing processes

The rinsing traditionally consists of several rinsing baths, as in table 2.1. The large water consumption in the rinsing after dyeing is primarily caused by the large number of baths but also by the common use of overflow rinses. Before the temperature is raised in the rinse, the dyestuff producers recommend neutralisation to pH around 8, when dyestuffs with vinyl sulphone reactive groups are used. This neutralisation has, however, in some dye-houses, become usual practice for all sorts of reactive dyestuffs.

After neutralisation, the rinsing consists of a number of soaping sequences: hot soaping, warm rinse and overflow rinse. Table 2.1 shows a typical use of two soaping sequences. In the hot soaping – bath no. 10 and 13 in table 2.1 – »soaping« additives are used, covering surface active agents (detergents), complexing agents and dispersing agents. The reasons for the use of these auxiliary agents are protection against hardness in the water and/or the cotton, concurrently with a need for an additive to hold the dyestuff hydrolysate dispersed in the water.

The process is finalised with neutralisation to pH around 7 and treatment with softening agents, necessary for the following sewing process.

Performed investigations

Table 2.2 gives an overview of performed investigations. The dye-houses participating in the investigations have pointed out the recipes, and thus the 24 reactive dyestuffs entering the tests, as »difficult«. Criteria have either been the very accurate and sensitive balance between the used dyestuffs in light shades or problematic fastness for dark shades.

Analyses were made according to international standards and include ash content, hardness in water and extract from textile, spectrophotometer scanning for dye-stuff content in process water, conductivity for salt measuring, and pH.

To assess the quality of the dyed textiles, the skilled quality assessment people at the dye-houses did the normal quality assessments: washing, water, wet rub, and dry rub fastness, evaluated on a scale from 1 to 5 with 5 as best. As always in the dye-houses, colour and shade were assessed by comparing with the customer samples. The main part of the experiments has been performed on production lots.

Table 2.2
Overview of performed investigations.

More than 50 full-scale recipes have been carried out in jets, overflow and drum batch machines. None of the performed experiments caused quality reductions in the finished lots, neither when neutralisation before hydrolysate rinse were omitted, use of detergents or complexing auxiliaries were omitted, nor when cold overflow rinse were replaced by one cold batch rinse followed by a few 95°C hot batch rinses. A suggestion for a new recipe is outline in table 2.3.

Table 2.3
New water saving, chemical free, high temperature and high speed rinsing recipe.

The full-scale tests with the new recipe documented that a chemical free, high temperature rinse, using a reduced number of batch rinses, and thus saving water and process time, can be implemented in the dye-house with no adverse effect on product quality. When implementing the water saving, chemical free, high temperature and high speed rinse after reactive dyeing of cotton in batch, the following CT-options should be considered:

1. Change from overflow rinsing to stepwise rinsing.
2. Omit the use of detergents in the rinsing after reactive dyeing of cotton.
3. Omit the use of complexing agents in the rinsing after reactive dyeing of cotton.
4. Use only neutralisation after dyeing when using VS reactive dyestuffs.
5. Chemical-free high speed rinsing after reactive dyeing of cotton.

2.1.1 Description

Rinsing by overflow, i.e. pouring clean cold water directly into the process water in the machine while excess water is drained out of the machine, is used both for rinsing and for cooling purposes. Overflow is quick but causes unnecessary water consumption.

Changing from overflow rinsing to a stepwise rinsing procedure as outlined in table 2.4 should be considered.

Table 2.4
Stepwise rinsing as substitute for each overflow rinse.

This option is in general relevant and should be investigated wherever overflow rinsing is used.

Surplus and non-fixed reactive dyestuffs are highly water-soluble. Nevertheless, detergents are often used during rinsing after dyeing.

Both in international literature and in the Danish projects, it has been documented that detergents do not improve removal of hydrolysed reactive dyestuffs from the fabric. In the Danish project, 50 full-scale dyeings have been carried out at various dye-houses without the use of detergents. All have successfully proven that detergents can be omitted without negative impact on product quality.

If soft water with a quality of below 5° dH is used, complexing agents can be omitted. In the Danish project, the 50 full-scale dyeings included dyeing without the use of complexing agents. No negative effects on the dyeing results were observed.

However, if hardness builders e.g. calcium and magnesium are present in the dyeing processes and in the rinsing after dyeing, they might have a negative effect on the dyeing result, e.g. change in shade or problems with reproducibility. For that reason, soft water is recommended as standard procedure in the dyeing processes. However, water softening in the dyeing machine by using complexing agents, forming bonds with the hardness-builders, are both economically and environmentally a bad solution.

Water softening can profitably be done in a separate plant by the ion-exchange technique or the membrane filtration technique.

Referring to the chemical suppliers, neutralisation in the first rinse after dyeing can be restricted to the vinyl sulphone (VS) reactive groups. Some VS dyestuffs have poor alkaline washing fastness and thus sensitive to high pH and high temperature simultaneously. Nevertheless, it is not uncommon that all recipes for reactive dyeing in a dye-house include neutralisation in the first rinse after dyeing, whether VS reactive dyestuffs are used or not.

In the Danish project, the dyeing was successfully carried out without the use of neutralisation in the first rinse after dyeing. This in spite of the fact that more than half of the dyeings were carried out with dyestuffs based on VS-groups. As it is not possible to put forward general guidelines on when to neutralise dyestuffs based on VS-groups, it is recommended always to neutralise these. There is no reason to neutralise in this step when all other sorts of reactive dyestuffs are used, e.g. based on monochlorotrazine (MCT), monofluorotriazine (MFT), dichlorotriazine (DCT), trichloropyrimidine (TCP) or difluorochloropyrimidine (DFCP).

In general, it is recommended to select dyestuffs with a superior alkaline washing fastness when selecting VS-dyestuffs for the dye-house.

Danish tests have shown that rinsing is more effective and faster at elevated temperatures – e.g. around 30% more unfixed hydrolysed reactive dyestuff is rinsed out after 10 minutes at 95°C than at 75°C.

Danish full-scale tests using hot 90-95°C rinsing after reactive dyeing of cotton have proved that the technique has no negative effects on the dyeing results. Most often the fastness of the goods were better after the hot rinsing than after the traditional rinsing with overflow, detergents, complexing agents and neutralisation in the first rinse (referring to option 2.1.A-D). Furthermore, when using 90-95°C rinsing water, a few stepwise rinses (table 2.3) can reduce the rinsing time with around 50% compared to a standard recipe (table 2.1). The tests covered 9 different recipes and 13 different reactive dyestuffs including very bright and dark shades.

2.1.2 Main achieved environmental benefits

The benefit is reduction in water consumption and wastewater generation. By replacing each overflow rinse by 2-4 stepwise rinses, a reduction rate at 50-75% per overflow rinse can be achieved.

The benefit is reduction in consumption of resources for the production of chemicals and reduction in pollution load of the wastewaster. Obviously, the potential for reduction will vary according to the existing dyeing procedure at the company. The Danish project was performed at two dye-houses mainly engaged in dyeing knitted piece goods and one mainly engaged in garment dyeing. The average potential load reduction at these dye-houses was documented to be at approximately 1 kg detergent, 1 kg complexing agent and 1 kg acetic acid per 100 kg of textile.

Best available technology on textile dyeing should include energy reclamation – especially when using large volumes of hot process water. If the company do not operate with energy reclamation, there is a risk of enlarged environmental load due to energy production, consumption and discharge when substituting cold rinsing with hot rinsing.

Energy reclamation can be done either by heat exchange between hot outgoing process water and cold incoming clean water or by reclamation of hot water and reuse of both the energy and the water.

In addition, the environmental benefits from option 2.1. E are the combined benefits for option 2.1. A – D.

2.1.3 Cross-media (whole environment) effects

Reduction in water intake, consumption and discharge.

Reduction in production, consumption and discharge of chemicals.

To accomplish the environmental benefits when using hot process water, the company must as a minimum include reclamation of the energy by heat exchanging hot outgoing process water with incoming cold water. In this situation, the benefits are the combined effects of option 2.1.A, B, C and D. However, if this is not the situation, a negative aspect could be increased environmental load due to energy production, consumption and discharge.

The optimal situation at the dye-house would be to reclaim both energy and water by membrane filtration as described in section 2.2.B.

2.1.4 Applicability

Can be implemented in all types of textile companies involved in reactive dyeing of cotton in batch; new or existing, large or small.

Stepwise rinsing is somewhat slower than overflow rinsing. For a company producing at the maximum dyeing-capacity, the extra production time when changing from overflow to stepwise rinsing can be a problem.

Can only be implemented if the company do have availability to very soft groundwater or is operating with a soft-water system (which is normally the case).

It is recommended always to neutralise in the first rinse after the dyebath when dyestuffs based on VS-groups are used. There is no reason to neutralise in this step when all other sorts of reactive dyestuffs are used, e.g. based on monochlorotrazine (MCT), monofluorotriazine (MFT), dichlorotriazine (DCT), trichloropyrimidine (TCP) or difluorochloropyrimidine (DFCP).

In order to be environmentally and economically feasible, the company must as a minimum perform energy reclamation, as an optimum perform energy and water reclamation.

2.1.5 Economics

The economic feasibility is obvious - 50-70% reduction in the consumption of water for rinsing. Total savings will depend on the number of reactive dyeings at the company.

The only change in operating procedures is to omit the addition of detergents, complexing agents and acetic acid. Savings will depend on the number of reactive dyeings at the company.

If the company is operating with energy reclamation an additional economic benefit (on top of 2.1.A–D combined) would be the economic value of the extra dyeing capacity.

If the company is operating without energy reclamation option 2.1.E is not economically feasible. Best available technology on textile dyeing should include energy reclamation.

2.1.6 Driving force for implementation

High costs for water and wastewater discharge and/or low availability for water of appropriate quality.

High costs for chemicals and wastewater load.

A desire for reduced operation time per lot and increased capacity per machine. High cost for fresh water and wastewater discharge and/or low availability for water of appropriate quality.

Reduction in chemical expenses.

2.1.7 References to literature and example plants

"Cleaner Technology Transfer to the Polish Textile Industry. Idea catalogue and selected options".

DANCEE, Danish Co-operation for Environment in Eastern Europe. ISBN 87-7909-265-9.

"Membrane filtration of textile dye-house wastewater for technological water reuse".

Desalination 119 (1998) 1-10.

"Environmentally friendly method in reactive dyeing of cotton".
Water Science and Technology Vol. 33, No.6, pp.17-27, 1996.

"Reclamation and reuse of process water from reactive dyeing of cotton".

Desalination 106 (1996) 195-20

Example plants:

2.2 Reclamation and reuse of chemicals, energy and water

The strategy for the Danish water reclamation research was to introduce reclamation and re-use closely integrated in the dyeing process. This implies working upstream, where water characteristics are still process specific, and not downstream, where sub-streams have been mixed and water characteristics represent an overall average. This strategy is believed to be optimal, as long as large scale advantages and flexibility are not lost by the tight process integration. For the water types in reactive dyeing of cotton, the strategy was found very suitable, and the Danish experience shows that it will result in the environmentally and economically optimal solution. Furthermore, and not least important, the strategy was to look for reuse not only of water but also of the energy and chemical content in the water.

Water reclamation techniques

Investigated water reclamation techniques were chemical precipitation, membrane filtration, activated carbon absorption, and counter current evaporation/ condensation. The advantages and limitations of each technique, related to the different characteristics of the process water from reactive dyeing of cotton, have been identified in lab-scale and documented in pilot-scale.

Besides the technical tests, economical estimates were given on the basis of not binding offers from suppliers. Economical estimates are expressed as EUR/m³ of process water including both operation costs and investment costs amortised over 5 years. Only the water reclamation equipment is included, not buffer reservoirs, pipes etc. being equal for all solutions. Table 2.4 gives the comparison in a total overview.

Table 2.4 Comparison of 4 water reclamation techniques in reactive dyeing of cotton.

Signatures: " = not influenced significantly", " = positive influence", " = negative influence", " = specific compounds, e.g. cations, can influence negatively".

Surplus costs can be expected for chemical precipitation, as heat exchange and polishing of suspended solids and excess precipitation chemicals may be necessary. The cost estimates in table 2.4 concerns the rinsing water, except for activated carbon for which it concern the dye-bath.

An overall solution has been chosen, based on the relatively cheep membrane technology for the high volume, high temperature and low salt rinsing water, and the relative expensive activated carbon adsorption technique for the exceptional high in salinity and high in dyestuff process water from the exhausted dyebath. Both solutions have been demonstrated at a Danish commission dye-house. The recycling system is connected to five Jet-dyeing (batch) machines with a capacity of 100 kg each. Results are more closely described in the two following chapters:

1. Reclamation and reuse of dyebath and first rinse by activated carbon.
2. Reclamation and reuse of rinsing water after dyeing by membrane filtration.

2.2.1 Description

By treating the highly coloured and salty process water types with activated carbon, the carbon will retain the dyestuff and other organic components by adsorption. The higher the content of dyestuffs and organics, the higher the capacity of the activated carbon, and the ions from the salt significantly improves the adsorption capacity of the activated carbon.

Figure 2.2
Principle in the activated carbon demonstration plant.

The dimensioning parameters from the test plant were a retention time of 2 hours and a consumption of 4 kg activated carbon/kg dyestuff. The used carbon type was F400 from Chemviron Carbon. A full-scale plant can consist of two columns (see figure 2.2) connected in series and with reversible flow. The flow is from the start from column 1 to column 2. When column 2 has the first break through of dyestuffs, column 1 is totally saturated and can be replaced with a new. The flow is reversed so that the flow is now from 2 to 1.

The activated carbon technique provides clear, warm water with sodium chloride and lye for reuse. Test dyeings showed that reusing warm, saline and de-coloured dye-baths as the basis for new dye-baths was possible with no adverse effects on fabric shade or fastness. Both the water, the energy content and the very high content of salts (up to 80 g sodium chloride per litre) and sodium hydroxide are utilised again by this option.

By treating the large volume of coloured rinsing water by membrane filtration, the dyestuff and other components will be retained in a low volume concentrate and a large volume of clear, hot and soft permeate water for reuse is produced.

Figure 2.3
Principle in the cross flow membrane filtration demonstration plant.

The membrane technology in question will have to be nano-filtration or reverse osmosis to be able to produce to a sufficient water quality. The dimensioning parameters from the test plant based on spiral wound elements were an average production of 25 l/m²h at 25°C and 7-10 bar. The selected elements in use were 50 mil Duratherm elements from OSMONICS DESAL. The operational parameters depend heavily and directly proportionally on the temperature of the water. Operation at 90°C will increase flux from 100% to 300% at the same pressure, but it is recommended to reduce pressure to 1/3 and save a very substantial amount of electricity.

The rinsing water reclaimed by membrane filtration was successfully tested in both standard recipes (table 2.1) and the chemical-free high-speed recipe (table 2.3 and option 2.1.E) for rinsing purposes.

2.2.2 Main achieved environmental benefits

Reduction in consumption and discharge of chemicals – too much salt (sodium chloride) and too high pH (sodium hydroxide) are most often the essential problems for cotton dye-houses.

Energy recovery by using warm process water for the new dye-baths gives reductions in consumption of energy.

Large reduction in consumption of water.

Hot water reuse gives large reduction in consumption of energy.

2.2.3 Cross-media (whole environment) effects

Option 2.2.A:

Reduction of the total emission of salt with the wastewater.

Reduction of the emission of dyestuff with the wastewater.

In Europe, an efficient line of suppliers and regeneration plants are prepared to receive the saturated carbon.

Alternatively, the saturated carbon can be incinerated and thereby the heat energy in the carbon can be utilised.

Option 2.2.B:

Reduction in consumption of water.

Reduction in consumption of energy.

An environmentally profitable solution to handle the concentrate is anaerobic degradation. The method has been successfully tested in laboratory scale.

Alternatively, the concentrate can be dried and incinerated and thereby the heat energy in the waste components can be utilised.

A detailed LCA has been worked out according to international standards by the EDIP method. The LCA compare option 2.1.E (Chemical-free high speed rinsing after dyeing) in combination with option 2.2.B (Reclamation and reuse of rinsing water by membrane filtration) with the old traditional recipe, including the use of detergents, complexing agents and overflow rinsing (table 2.1).

The results are outlined in figure 2.1 and points out the following improvements:

	Energy consumption reduced by 70%.
	Water consumption reduced by 90%.
	Chemical consumption reduced by 100%.
	Time consumption per lot reduced by 60%.
	Global warming reduced by 70%.
	Acidification reduced by 70%.
	Nutrient enrichment reduced by 70%.
	Photochemical ozone reduced by 70%.

2.2.4 Applicability

Activated carbon is relatively "low tech", relatively easy to operate and relatively low in investments and can be implemented in all types of textile companies involved in reactive dyeing of cotton in batch; new or existing, large or small. Activated carbon adsorption is relatively high in operation costs and the limiting parameter is the accepted pay-back time at the dye-house – this includes of course the costs for wastewater discharge and limitations in discharge of salt and dyestuff, if any. Piping, pumps, tanks and separation plant will demand some space but rarely constitute a critical problem.

Before implementation of a activated carbon plant, it is very important to test the retention time and the capacity of the carbon type with the actual process water.

When the salt is in the process water from the very start, the process is a so-called "all-in dyeing". The dyestuff is subsequently added on a time or flow basis. This is contrary to the "normal" way, where the dyestuff is evenly distributed on the fabric before salt is added. Not all types of recipes have been tested, and problematic recipes may exist. Installation of chemical dosing equipment on the dyeing machines facilitates the "all-in dyeing" considerably.

Principally membrane filtration can be implemented in all types of textile companies involved in reactive dyeing of cotton in batch; new or existing. However, membrane filtration is relatively "high tech", relatively high in investment and the option do involve some monitoring of the applicability of the used chemicals at the dye-house to the membrane type. The membrane filtration technique addresses in this way to dye-houses of a reasonable capacity and a reasonable critical minimum volume of water to be treated to give an acceptable pay-back time. On the other hand, membrane filtration is relatively low in operation costs.

Piping, pumps, tanks and separation plant will demand some space but rarely constitute a critical problem. Piping can advantageously be done above the dyeing machines, and the collection pumps can be the existing pumps on the machines. A major problem can be the production stop during the piping.

2.2.5 Economics

This economy assessment is based on typical prices in Ringkjøbing County in Denmark, where the majority of dye-houses in Denmark are situated.

Investments: 1,3 EUR/m³.
Operation and maintenance: 10 EUR/m³.
Saved expenses: 7-11 EUR/m³.
Assessed pay back time: Maximum of 5 years.

Investment: 0,6 EUR/m³.
Operation and maintenance: 1,3 EUR/m³.
Saved expenses: 4 EUR/m³.
Assessed pay back time: 8 months.

2.2.6 Driving force for implementation

High costs for fresh water and wastewater discharge.

2.2.7 References to literature and example plants:

Literature:

"Cleaner Technology Transfer to the Polish Textile Industry. Idea catalogue and selected options".

DANCEE, Danish Co-operation for Environment in Eastern Europe. ISBN 87-7909-265-9.

"Membrane filtration of textile dye-house wastewater for technological water reuse".

Desalination 119 (1998) 1-10.

"Environmentally friendly method in reactive dyeing of cotton".
Water Science and Technology Vol. 33, No.6, pp.17-27, 1996.

"Reclamation and reuse of process water from reactive dyeing of cotton".Desalination 106 (1996) 195-20

Example plants:

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