Danish experience. Best Available Techniques – BAT - in the clothing and textile industry
4. Cleaner technology in enzymatic desizing
Desizing is only carried out on woven material. As the size chemicals render the material water repellent, they must be removed before dyeing/printing/finishing.
Sizes made of starch and modified starch are normally removed by using enzymes (amylases), which will decompose the starch and make it water-soluble. Starch sizes can also be removed by means of an oxidation with sodium or potassium persulphate. Synthetic sizes e.g. PVA (polyvinyl alcohol) are water-soluble and can be removed by a simple washing process.
The option described in this paper is related to enzymatic desizing of starch.
4.1.1 Description
Wastewater from the desizing bath and the first rinse after desizing of starch based sizes will have a very high COD load, and it will have an elevated temperature, typically 60° C, thus being subject to heat recovery. To reduce the COD-load in the wastewater from such processes, this wastewater can be collected and treated by means of membrane filtration, either nano-filtration or reverse osmosis.
This option has been tested, evaluated and installed at a Danish dye-house, where nano-filtration and reverse osmosis membranes were used. Wastewater from nine jiggers executing desizing is collected in a holding tank, from which the wastewater is led to the membrane filtration plant. As textile wastewater contains a lot of fibres, it is very important to install mechanical filters before the membrane filters.
The permeate (cleaned water) is reused for desizing and washing purposes, where the energy content can be utilised as well. The concentrate should be handled separately. Due to its very high content of broken down starch, the concentrate can serve as a substrate for biogas production or a carbon source for denitrification. The latter option is being considered in the above-mentioned case.
Further details about the project are enclosed in Annex B.
4.1.2 Main achieved environmental benefits
The main achieved environmental benefit is reduction in water consumption. In a Danish dye-house, the implementation of the above option has reduced the daily water consumption by 17 m3 (12%). In addition, there is a non-quantified energy saving. A reduction of COD discharge is estimated at about 70 – 90 g COD per kg textile.
4.1.3 Cross-media (whole environment) effects | Reduction in water consumption. |
| Ability to re-use wastewater. |
| Reduction in energy consumption. |
| Increased electricity consumption for running the membrane filtration plant. |
| Reduction of COD load at wastewater treatment plant (provided concentrate could be handled separately). |
4.1.4 Applicability
This option can be implemented at all dye-houses, which carry out enzymatic desizing of natural or modified starch. A membrane filtration plant with collecting tanks etc. will take up some space depending on the size of plant.
4.1.5 Economics
In the Danish case mentioned above, the total capital costs (tanks, mechanical filters, pumps, pipes and membrane filtration plant) are about DKK 800,000 (» EUR 105,000). Running costs are not calculated. The membrane filtration plant is designed to treat 22 m3 per day and to produce 19 m3 water for reuse per day. Provided the plant is utilised at its full capacity, treating wastewater at 60oC the following savings can be calculated:
Water savings: 19 m3 per day or 4,180 m3 per year. The costs for fresh water and wastewater discharge are approximately DKK 23.76 per m3. Annual savings: approx. DKK 100,000 (» EUR 13,000).
In addition, the implementation of the system is estimated to remove the special COD-load tax at DKK 9.91 pr m3. The tax is charged on the total water discharge at approximately 30,000 m3 per year. Hence additional savings at about DKK 300,000 (EUR 40,000).
Energy savings: 19 m3 per day or 4,180 m3 per year, which must not be heated to 60° C. The gas price is DKK 3.07 per m3. Annual savings: approx. DKK 84,000 (EUR 11,000).
The total annual savings are approx. DKK 484,000 (EUR 64,000). Simple payback time estimated at about 1.7 years.
4.1.6 Driving force for implementation
High costs for fresh water and wastewater. Extra costs for wastewater with a high COD load.
4.1.7 References to literature and example plants
Literature:
Environmental assessment of textiles, 1997. Life cycle screening of the production of textiles containing cotton, wool, viscose, polyester or acrylic fibres. Environmental project no. 369, 1997. Ministry of Environment and Energy. Danish Environmental Protection Agency.
BAT for Textile Industry, 1996. Pre-treatment, dyeing, printing and/or finishing of textile products. TemaNord 1996:558. Nordic Council of Ministers.
Membrane filtration of desizing wastewater in the textile industry. Working report no. 25, 2001. Ministry of Environment and Energy. Danish Environmental Protection Agency (In Danish).
Example plant:
| Nordisk Blege- og Farveri A/S | |
| Industrivej 10 | |
| DK-3000 Helsingør | |
| Denmark |
|
| Att: | Mr Torben Kinch |
| Phone: | + 45 49 22 36 00 |
| Fax: | + 45 49 26 62 36 |
| E-mail: | NBF@NBF.dk |