|Enzymatic Bleach Cleanup in Cotton Dyeing||Denmark||1994||Full scale|
MANUFACTURE OF TEXTILES # 14
Raw cotton fabric is usually bleached using hydrogen peroxide. The bleaching chemical has to be removed before the dye is applied in order to prevent reaction between the bleach and dye. The traditional method of removing the bleach is by rinsing the fabric with water a number of times. Alternatively, a mild reducing agent can be used to neutralize the bleach. In either case, large amounts of water (up to 40 liters per kg of fabric) are required for rinsing resulting in discharge of large volumes of wastewater.
The present case study illustrates use of an eco-friendly alternative for bleach neutralization imlemented at the Skjern Tricotage-Farveri, Skjern in Denmark.
Skjern Tricotage-Farveri is a modern dyehouse with an annual production of around 5000 tons of fabric. 225 employees are working at the company. The annual turnover is about 115 million DDK. The company has a stated policy of reducing water and air pollution risks . This dyehouse is a major supplier to Novotex, a Danish company that sells ecologically sound cotton under the brand name "Green Cotton".
At this dyehouse, an enzyme was used to replace a chemical agent (such as thiosulfate) in bleach neutralization leading to reduction in water and energy consumption. The enzyme, Terminox Ultra is manufactured and marketed by Novo Nordisk. The company manufactures a variety of enzymes, pharmaceuticals and biotech products. It employs 11,600 people and is headquartered in Denmark.
The cleaner production technique was fully implemented in summer 1994. The data on economic and environmental analysis given in this case study are based on a full-scale trial at Skjern Tricotage-Farveri. The trial is done with 470 kg knitted 100% cotton fabric with a machine capacity of 500 kg. The results of this test are passed on to the 1500 tons dyed in a year to calculate the yearly savings .
Cleaner Production Principle
Cleaner Production Application
Enzymes provide a more convenient alternative because they are easier and quicker to use. A small dose of Catalase is capable of decomposing hydrogen peroxide to water and oxygen. Compared with the traditional clean-up method, the enzymatic process results in reduced water consumption and reduced energy consumption. The enzyme can be applied directly after the bleaching stage.
The Terminox Bleach Cleanup process recommended for batch processing in e.g. Yarn dyeing equipment, Jet dyers and winches is as follows :
The rinsing step is not required when using the enzyme because the bleaching agent is decomposed and the residues of the enzyme have no effect on textile fibers or dyestuffs. In cases where the fabric is very dirty it is recommended to include a rinsing step between bleaching and dyeing.
Terminox Ultra (commercial name of the Novo enzyme) is a stabilized liquid catalase, found in nature and produced by submerged fermentation of a non-pathogenic microorganism. The enzyme specifically breaks down hydrogen peroxide into non-active oxygen and water under mild temperature conditions.
2 H2O2 ----> O2 + 2 H2O
The activity of the enzyme is 10 KCIU/g. One KCIU activity unit (Kilo Catalase International Unit) is the amount of enzyme that breaks down one millimole of hydrogen peroxide per minute under standard conditions (25oC, pH 7.0, 10 millimole H2O2) . Under industrial conditions, Terminox Ultra takes 10 to 15 minutes to breakdown the hydrogen peroxide completely and then the dye can be added.
The various advantages of using the enzyme Terminox Ultra are that there is no:
|adverse effect on dyestuffs|
|need for heating|
|need for rinsing prior to dyeing|
|risk of harmful overdosing|
|formation of by products in waste water|
Environmental and Economic Benefits
A thorough economic and environmental analysis of the enzyme treatment has been made in full-scale trials at Skjern Tricotage Farveri. Earlier the company used thiosulfate for Bleach Cleanup but switched to Terminox Ultra in 1994.
The dye house tested the color shade and fastness, and found no difference between the processes. The bursting strength of samples was tested internally, and again no difference could be found between the two samples .
Neither Terminox, nor the reducing agent applied as alternative, had any significant impact on the wastewater quality. Impurities from the cotton fabric (pectines and spinning oil) were the main contributors to BOD5 and COD.
The enzyme Terminox Ultra works in mild temperature conditions, whereas thiosulfate requires a raised temperature in order to react efficiently with hydrogen peroxide. The bleaching liquor therefore had to be heated. Using Terminox Ultra for bleach cleanup has the advantages of:
|reduced water consumption|
|reduced energy consumption|
|reduced process time|
|ease of implementation and control|
The comparison of the process based on Terminox Ultra and the old reducing agent is summarized below:
|Consumption per kg fabric||Reducing agent||Terminox Ultra|
|Water||19 liters||10 liters|
|Energy||1.9 MJ||0.04 MJ|
|Reducing agent||8.5 g||-|
|Terminox Ultra 10L||-||3.5 g|
|Processing time||105 min||50 min|
Estimated savings when using the enzyme instead of a reducing agent are presented in the table below:
|Savings per year|
|CO2 emissions||160 t|
|Water||13 500 m3|
|Natural gases||70 800 m3|
The quantity of gas is converted to the quantity of energy in GJ. The basic heating value of natural gas is assumed to be 39.1 MJ/ m3.
The natural gas is used for the steam generation at the dyehouse. As no heating is required when the enzymatic treatment is used, considerable amounts of gas can be saved giving reductions in carbon dioxide emissions of an estimated 160 tons per year. This is the main advantage from the environmental point of view. However, from the economical point of view, the savings in water charges are most significant. Instead of 19 liters per kg of fabric, only 10 liters were required for the Bleach Clean up stage .
Existing equipment could be used, so there was no investment costs [1,3]. The exact savings will vary depending on current water charges and the cost of energy.
There are no additional operational and maintenance costs. The difference in price of terminox and thiosulfate depends on the supplier (of thiosulfate) as well as the geographical market. The cost savings in a particular country will depend on local water charges and the costs for energy.
Cost estimates are based on following assumptions:
|Basic heating value natural gas||39.1 MJ/ m3|
|Specific CO2-emission factor natural gas||57 kg/GJ|
|Energy efficiency||70 %|
|Price natural gas||1 DKK/ m3|
|Price water||15 DKK/ m3|
Using these assumptions the cost savings, as a result of the use of Terminox, are 27500 DKK per year or in other words the estimated saving is 15-30 US $/ton fabric.
While the investment costs are zero the payback time is also zero. The new process starts to give benefits as soon as it is used.
The company did not experience any problem when using the enzyme. There are no technical constraints to change over to the biotechnological process by applying Terminox Ultra.
Type of Source Material: Leaflet of the company, organization report, interviews.
This case study was submitted to UNEP IE by the Institute for Applied Environmental Economics (TME) of the Netherlands on the part of the UNEP IE working group on Biotechnology. The case study was edited for the ICPIC diskette in August 1995. Subsequently the case study has undergone a technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.