Genbrug af procesvand fra reaktivfarvning af bomuld

English Summary

Background

The project Reuse of Process Water from Reactive Dying of Cotton is sponsored by the Danish Enviromnental Protectiori Agency, and is part of a major effort during the nineties towards environmental improvements within the Danish textile industry.

Aim of project

The overall goal was to develop a concept and a solution for reuse of process water from reactive dying of cotton, and to establish a demonstration plant documenting the solution.

Dyeing technology

In this project, focus has been on reactive dyeing of cotton for several reasons. Firstly, cotton dyeing is the most commonly used dyeing process both in Denmark and abroad, and nearly all cotton dyeing is reactive dyeing. Secondly, reactive dyeing has a large consumption of water, energy and chemicais and a large production of waste water. Moreover, the project focuses particularly on batch dyeing (in contrast to continuous dyeing), because this is the most common textile wet treatment process both in Denmark and worldwide, and because the use of batch dyeing is expected to increase in the future.

The cleaner technology concept

The project has followed an overall concept of cleaner technology development. This implies an implementation of environmental improvements according to a list of priorities as follows:

1) Optimization
where the possibilities for savings in water, energy and chemicals are investigated within the frames of existing equipment.

2) Modernization
where the possibilities for obtaining improvements by rebuilding or renewing old equipment are assessed.

3) Chemical substitution
where environmental improvements by substituting hazardous chemicals by less hazardous chemicals are sought.

4) Reclamation and reuse
where finally the possibilities for reclaiming and reusing water including its content of energy and chemicals are looked into.

Following this procedure, large environmental gains have been implemented by optimization of the dyeing process itself, before the solutions for reclamation and reuse have been introduced.

The extent of the work

As mentioned above, the work has partly contained the issue of optimization and partly the issue of reuse of water, energy and chemicals. At an early stage, it became obvious, that there were great possibilities for optimizations. This particular work was then isolated from the main project in a self-contained sub-project, where the possibilities for water and chemical savings were demonstrated in full scale. The work is reported in Miljøstyrelsen (1995).

The work with reclamation and reuse comprised literature studies, laboratory tests, pilot scale tests and establishment and operation of a full seale demonstration plant. All together eleven man years went into this work, including five man years of graduation work at universities and technical colleges. The public financing of the project is equivalent to four man years plus expenses for pilot scale and a demonstration scale plants.

Tests and choice of technique

The optimal solution comprises separation of the process water into two types: 1) dye bath + first rinse as one water type and 2) rinse water as the second water type. Water type 1 has an extremely high salinity and dyestuff content, while water type 2 has a low salinity and a moderate dyestuff content.

Activated carbon adsorption

The high salinity of the dye bath and first rinse renders chemical precipitation, membrane filtration and evaporation impossible. However, the high salinity has a positive effect on the adsorption on activated carbon, and the high dyestuff concentration increases the capacity of the activated carbon because the adsorption is driven by the concentration gradient. Furthermore, the adsorption produces colourless water with a high content of salts and thermal energy which enables reuse of water, salts and energy in subsequent dye baths.

Membrane filtration of rinse water

The low salinity rinse water can be treated with all four techniques.
However, membrane filtration is economically and technically advantageous compared to the other techniques. Through testing, membranes specially suitable for separating dyestuff from water under the given conditions were identified. lt has been demonstrated possible and even favourable to operate the filtration at high water temperatures (approx. 90°C) which allows for direct hot water reclamation and reuse. This has a positive effect on the rinsing process, and it has proven to speed up the rinsing by a factor two when hot water is used. Membrane filtration produces colourless hot water and this technique therefore makes it possible to reuse water and energy.

Reuse of dye baths

The reuse of the dye bath has been investigated by laboratory and full scale tests for a number of recipes. All tests have proven it possible to reuse water. However, not all types of recipes were tested.

Reuse of rinse water

Rinsing water has been reused both in pilot scale and full scale for a large number of recipes. The conclusion was that reuse is possible irrespective of the recipe. There is a potential that certain chemicals will damage the membrane, and especially cationic agents used as softeners should be avoided. Optimally, softening should be performed »dry« e.g. the softener to be applied after rinsing by spraying the fabric before it is dried.

Remanence from membrane filtration

Membrane filtration produces a remanence containing a high concentration of dyestuff. The volume of this remanence is around 1% of the original volume of process water, and the remanence must be disposed of. Lab scale tests have shown that the remanence can be treated in anaerobic digesters. The results showed a complete colour removal with no negative impact on the digester when remanence was fed up to 20% of the total digester intake. This treatment is an environmental improve ment compared to the present situation, where the content of dyestuff is typically discharged to conventional aerobic waste water treatment, giving only 50% colour removal. When the remanence is treated in anaerobic digesters at waste water treatment facilities, it is exposed to both the anaerobic treatment and a subsequent aerobic treatment, because the digester overflow is fed to the aerobic reactor. Analyses have shown that the remanence complies with the standards for heavy metals stipulated in sludge regulations given by the Danish Ministry of Environment and Energy (Miljøstyrelsen (1996)), which is a condition for the feeding of anaerobic digesters and the subsequent agricultural use of the sludge.
The sludge regulations comprise also standards for the compound groups PAH, LAS, DEPH and NPE, and these compounds are on the chemical lists of the textile industry. They can potentially be present in the remanence, but they have not yet been analyzed for. This should be done before further implementation of the solution and treatment of the remanence in anaerobic digesters. It should, however, be noted that by the combined anaerobic/aerobic treatrnent, the compounds in question will most probably be subject to improved degradation compared to the present situation with only aerobic treatment.

An alternative is incineration of the remanence, either direct or after spray drying or activated carbon adsorption. This has not been investigated in this project.

Environmental assessment

The developed cleaner technology solution has been subdued to a so called life cycle assessment. The membrane filtration part is assessed very thoroughly according to the EDIP-method (Wenzel et al., 1997), (Hauschild and Wenzel, 1997) while the activated carbon adsorption part is assessed by estimation. The environmental assessment of the solutions show large environinental improvements: the water consumption can be reduced by up to 90%, and energy consumption and energy related impacts by up to 70%. In addition, a substantial reduction in consumption of chemicals is achieved including complete omission of detergents and complexing agents and a substantial reduction in the use of salts.

Economic evaluation

The investment in the membrane filtration plant is by far the greatest.
Operation costs for time plant will amount to approx. 5 DKK/m³ rinsing water, and the investment will be of a maximum of 5 DKK/m³ including pipes, plumming etc. when written off over a period of 5 years. This implies a total of 10 DKK/m³ over the 5 year period. These expenses should be compared to the savings in water and energy, which as an average in the Danish County of Ringkjøbing is 27 DKK/m³. The economical gains obtained by reducing production time in the rinsing processes, when using hot water, are 50% (» 30% of the total production time for the recipe). The reduced costs due to this reduced production time are estimated to be around 5-10 DKK/m³.

The investment of 10 DKK/m³ can thus be compared to the savings of 32-37 DKK/m³ as an average for Ringkjøbing county. A simple calculation of the pay back time for the membrane plant, as investment divided by savings minus operation costs, shows a pay back time of 8 months.

The activated carbon adsorption plant is a smaller investment.
Costs for investments and operation and savings on water, chemicals and energy are about equal, and the pay back tirne is estimated to be around 5 years.

Demonstration plant

A demonstration plant has been built at Martensens Fabrik, and the experience from running the plant is positive.