Ajay Shankar Joshi 1,Shyam Barhanpurkar2 & Saurabh Suryavanshi3
1MBA,BE,Faculty, Faculty, Department of Textile Technology, Shri Vaishnav Institute of Technology and Science, Indore, M.P., India
2Mtech. GCTI Kanpur, Faculty, Department of Textile Technology, Shri Vaishnav Institute of Technology and Science, Indore, M.P., India. E-mail: [email protected]
3Mtech. IIT Delhi, Faculty, Department of Textile Technology, Shri Vaishnav Institute of Technology and Science, Indore, M.P., India.
This article focuses on various waste minimization options and techniques that are available for the textile industry. These are divided into two sections; General waste minimization suggestions for reducing water and Chemical consumption & Specific waste minimization suggestions for each textile process.
General waste management for reducing water consumption includes attention to minor leakages and faulty valves problems to reducing number of process steps and reusing water for auxiliary processes. For reducing chemical consumption; control over dosing, chemical recovery and reuse and improving scheduling are highlighted.
The second section highlights in the specific areas of textile processing, which includes sizing, desizing, scouring, bleaching, dyeing, printing and many more batch and continuous processes where consumption of water and chemicals may be reduced drastically by following simple steps like using synthetic sizes, recipe optimization, installing counter-current washing, careful selection of surfactants, removing excess printing paste from drums, screens and pipes by dry techniques before washing to reduce the color load discharged to drain, and many such ideas which also bring down the overall cost involved in processing the material.
The application of a systematic approach to reducing the generation of waste at source is called waste minimization. Water is used extensively throughout textile processing operations. Almost all dyes, specialty chemicals, and finishing chemicals are applied to textile substrates from water baths. In addition, most fabric preparation steps, including desizing, scouring, bleaching, and mercerizing, use aqueous systems . The amount of water used varies widely in the industry, depending on the specific processes operated at the mill, the equipment used, and the prevailing management philosophy concerning water use. Reducing water consumption in textile processing is important for furthering pollution prevention efforts, in part because excess water use dilutes pollutants and adds to the effluent load. Mills that currently use excessive quantities of water can achieve large gains from pollution prevention. A reduction in water use of 10 to 30 percent can be accomplished by taking fairly simple measures. This review paper contains detailed descriptions of the various waste minimization options that are available for the textile industry. They are divided into two main sections:
- General waste minimization suggestions for reducing water, chemical and energy consumption; reducing solid waste; reducing air and noise pollution; and minimizing the emission of toxic substances.
- Specific waste minimization suggestions for each textile process.
The purpose of this paper is to enable the textile industry to become more efficient through minimizing wastes and emissions, thereby decreasing costs.
- LITERATURE REVIEW
- Reducing Water Consumption
Water consumption in a textile factory can be reduced by implementing various changes ranging from simple procedures such as fixing leaks, to more complex options such as optimizing water use and reducing the number of process steps. Some suggestions are outlined in this section [2-3].
- Repair Leaks, Faulty Valves, etc.
A simple method of determining if leaks exist is to take incoming water meter readings before and after a shut-down period when no water is being used. A difference in the readings could indicate a leak.
- Turn off Running Taps and Hoses
Encourage workers to turn off taps and hoses when water is not required. The fixing of hand triggers to hoses also reduces water consumption.
- Turn off Water when Machines are not running
Encourage workers to turn off machines and water during breaks and at the end of the day. Avoid circulating cooling water when machines are not in use.
- Reduce the Number of Process Steps
This involves a study of all the processes and determining where changes can be made. For example, fewer rinsing steps may be required if a dye with high exhaustion is used.
- Optimize Process Water Use
Examples include using batch or stepwise rinsing rather than overflow rinsing, introducing counter-current washing in continuous ranges, and installing automatic shut-off valves.
- Recycle Cooling Water
Cooling water is relatively uncontaminated and can be reused as make-up or rinse water. This will also save energy as this water will not require as much heating.
- Re-use Process Water
This requires a study of the various processes and determining where water of lower quality can be used. For example, final rinse water from one process can be used for the first rinse of another process.
- Using Water Efficient Processes and Equipment
Although replacing outdated equipment with modern machines which operate at lower liquor ratios and are more water efficient requires capital investment, the savings that can be made ensure a relatively short pay-back period.
- Sweeping Floors
Instead of washing the floors of the dye-house and kitchens, rather sweep up any spillages and wash down only when essential. Not only will this reduce water use, but also the concentration of contaminants to drain as the waste is disposed of as solids.
- Reusing Water from Auxiliary Processes
The water used in the rinsing of ion-exchange columns and sand filters can be reused elsewhere in the factory.
- Reducing Chemical Consumption
The majority of chemicals applied to the fabric are washed off and sent to drain. Therefore, reducing chemical consumption can lead to a reduction in effluent strength and therefore lower treatment costs, as well as overall savings in chemical costs . Various options for reducing chemical use are listed below:
- Recipe Optimization
Recipes are generally fail-safing designed which results in the over-use of chemicals. Optimizing the quantity of chemicals required will lead to more efficient chemical use and lower costs. Continual updating of recipes should be carried out when new dyestuffs enter the market as, in general, less of these chemicals are required.
- Dosing Control
Overdosing and spillages can be reduced by mixing chemicals centrally and pumping them to the machines. Check that manual measuring and mixing is carried out efficiently and automatic dispensers are properly calibrated.
- Pre-screen Chemicals and Raw Materials
Avoid dyestuffs containing heavy metals, solvent-based products and carriers containing chlorinated aromatics. Safety data sheets should be obtained from the chemical manufactures to obtain information such as toxicity, BOD and COD. Check that raw materials do not contain toxic substances. Check that companies will accept expired raw materials for disposal.
- Chemical Substitution
Review chemicals used in the factory and replace those hazardous to the environment with those that have less of an impact. Use dyes that have high exhaustion rates and require less salt. Specifically:
- replace metal-containing dyes
- use bi-reactive dyes in place of mono-reactive
- avoid the use of APEO detergents and replace with more biodegradable
- replace stilbene optical brightners with alternatives, or eliminate all together
- dye wool with dyes that do not require after-chroming
- Correct Storage and Handling
More effective control of the storage and handling of chemicals will results in less spillage reaching the drains.
- Chemical Recovery and Reuse
Chemical use may be reduced through recovery and reuse. For example, sodium hydroxide from mercerizing can be recovered through evaporation. Dye baths may be reused and size can be recovered for reuse.
- Process Changes
Investigate the feasibility of changing to cold-pad batch dyeing. This results in less chemicals being used (and in particular, salt) and reduces water consumption significantly.
- Improve Scheduling
Review the scheduling of continuous processes such as sizing, desizing, padding etc. to ensure that the same chemical bath is used as many times as possible, thus reducing the number of dumps to drain per day.
- Waste Minimization in Specific Textile Processes
The following sections will describe various waste minimization techniques that can be implemented in specific textile processes .
Replace starch-based sizes with synthetic sizes. The advantage of this is a reduced pollution load as synthetic sizes have lower BOD levels, and they can be recycled for reuse.
Test incoming raw materials for toxic compounds. Purchase size in bulk in drums rather than bags etc. as this produces less solid waste and reduces the chances of spills due to breakages.
Ensure that only the minimum required size is added onto the yarn. This reduces chemical consumption as well as the pollution load to drain during desizing.
- Preparation Department
Preparation includes desizing, scouring, bleaching and mercerizing. Desizing accounts for greater than 50% of the pollution load of preparation while scouring contributes between 10 and 25%. Good preparation is essential for subsequent processing as any impurities remaining on the fabric will interfere with the dyeing and finishing processes [6-7]. Some waste minimization options for the preparation department are listed below:
The effluent from desizing will contain the sizes that were added onto the yarn before weaving/knitting. Using and recycling synthetic sizes in place of starch sizes will reduce the pollution load from desizing.
- Incoming raw material should be screened for toxic chemicals as these will be removed during the scouring process.
- Detergents should be easily biodegradable. Avoid the following detergents: linear alkylbenzenesulphonate; nonylphenoletoxylate; dialkyldimthylammonium chloride; distearyl dimthyl amoniumchloride; di-dimithylammonium chloride; sulphosuccinates; alkylphenolethoxylates; complexing agents with poor biodegradability (e.g. EDTA; phosphonic acid; NTA; phosphonates).
- Reuse scours wash water for desizing. Recycle continuous scour wash water to batch scouring.
- Replace the use of chlorites and hypochlorites with hydrogen peroxide.
- Ensure that bleaching is carried out efficiently.
- Recycle bleach wash water for scouring.
- Use vacuum slots to remove excess solution which can then be reused.
Little or no pollution arises from singeing. Ensure that air scrubbers are installed to trap particles that are burnt off the fabric. Cooling water can be reused elsewhere in the factory. Remove lint from the pad solution to reduce the frequency of dumping.
Recycling of sodium hydroxide through evaporation for reuse in mercerizing or scouring will decrease the pollution load and chemical consumption.
- Use modern equipment.
- Replace batch processes with continuous processes.
- Install counter-current washing.
- Combine processes such as desizing, bleaching and scouring.
- Replace harmful chemicals with those of lower environmental impact.
- Reuse wash water for cleaning equipment and screens.
- Batch Processing
There are a number of waste minimization options for batch processing. These include:
- Cascading multiple rinsing operations.
- Reusing softening baths with reconstitution.
- Reusing preparation baths (scouring and bleaching) with reconstitution after filtration to remove impurities.
- Segregating colored effluent streams from clean streams (preparation and rinsing) to ensure that only concentrated effluent is treated. This clean effluent may be used elsewhere in the factory.
- Installing automatic shut-down of water in overflow cooling when the required temperature has been reached.
- Replacing outdated machines with high liquor ratios with more modern equipment.
- Carrying out softening on a pad mangle.
- Replacing batch-wise rinsing with continuous rinsing with counter current flow.
- General water, chemical and energy conservation measures as listed at the beginning of this paper.
Careful selection of dyes is important. Dyes should have high fixation/exhaustion, low toxicity, absence of metals, and be appropriate for the end use . Correct and efficient application procedures must be used and right-first-time production should be achieved.
The main areas for waste minimization include:
- Using low liquor ratios.
- Using automated dye and chemical dosing systems.
- Reusing dye baths, rinse water and softening baths.
- Ensuring a good cloth preparation.
- Optimizing pH and salt for each recipe.
- Avoiding the use of auxiliaries that reduce or retard exhaustion.
- Using bireactive dyes.
- Using the newer low-salt reactive dyes.
- Optimizing dyeing temperatures.
- Avoiding the addition of more chemicals to offset the effects of other chemicals -use other non-chemical methods such as procedural or mechanical alterations or change the dye selection.
- Replacing the use of acetic acid in neutralizing after dyeing with formic acid or dilute hydrochloric acid (acetic acid adds to the COD of effluent).
The main waste minimization strategies in continuous dyeing are to:
- Maximize dye fixation.
- Minimize wash – off.
- Avoid discards and machine cleaning wastes during start-up, shut-down and changes of color and style.
- Minimize the number of times a dye bath has to be dropped and cleaned due to a color change by careful scheduling.
- Use automated color kitchens to minimize the working losses and discards.
- Improve washing efficiency through the installation of flow restrictors to control water volumes. Use counter current washing procedures.
- Optimize dosing of chemicals through monitoring of relevant parameters such as pH, absorbance, turbidity etc.
General waste minimization options for dyeing
- Operate at lowest possible bath ratio. This leads to a reduction in operating costs, water consumption, chemical use, energy use and less effluent discharge.
- Minimize stripping and / or redyeing procedures.
- Avoid shading additions.
- Avoid the use of detergents to wash fabric after reactive dyeing; high temperatures are just as effective.
- Minimize auxiliary use. Some auxiliaries interfere with dye fixation and should be replaced with alternatives or removed as this will reduce the color load of the effluent. Some auxiliaries are added to compensate for inefficiencies in the process, equipment, or substrate design. Therefore, optimizing these factors will reduce auxiliary use.
- Right-first-time dyeing. Corrective measures are chemically intensive and have much less chance of achieving the required quality. The greatest costs in reprocessing are associated with the cost of dyes and chemicals – typically, the costs can increase by as much as 30%. Right-first-time dyeing leads to an increase in productivity and more efficient use of resources (e.g. Labour, capital).
- In dyeing polyester, avoid the use of carriers by upgrading dye machinery or replace with less harmful alternatives.
- Good fabric preparation. This increases the chance of right-first-time dyeing as fixation is improved.
- Improved dye fixation. Dye fixation onto cotton can be improved by mercerizing the yarn or fabric prior to dyeing.
- Pad-batch dyeing. This is a cold dyeing method used mostly on cellulosic that results in a reduction in pollution, energy use, and costs. The advantages include :
- no salt or chemical specialties are required,
- more efficient use of dye,
- improved quality of dyeing,
- can be used on wovens or knits, and
- Low capital investment results in savings in dyes, chemicals, labour and water.
- Reuse dye baths, especially those using dyes with high exhaustion such as acid or basic dyes. There are 4 main steps to follow:
- Save the exhausted dye bath – this can be done by pumping it to a holding tank (or identical machine doing the same processes) and returning it to the machine for use in the next dyeing procedure.
- Analyze the dye bath for residual chemicals – most auxiliaries do not exhaust in the dyeing process. T here is approximately a 10% loss due to adsorption onto the fabric. Others however, are used or lost during the process and must be replaced. Unexhausted dyestuffs need to be analyzed to determine the concentration remaining in the dye bath to ensure correct shading in further dyeing. Dye bath analysis can be performed using a spectrophotometer and specific guidelines for such a procedure.
- Reconstitute the dye bath – water is added to replace that which is lost to evaporation or to the product. Auxiliaries are added in proportion to the water volume (these can be estimated) and finally the dyestuff is added for the required shade.
- Reuse the dye bath – check the temperatures to ensure that the reused dye bath is the right temperature to minimize spotting and unlevel dyeing.
If properly controlled, dye baths can be used for up to 15 or more cycles. Use dyes that undergo minimal changes during dyeing (acid, basic, direct and disperse) and reuse dye baths to repeat the same shades .
Dye bath reuse is limited by impurity build-up from, for example, the fabric, salt build-up, steam contamination and surfactants. In addition, specialty chemicals may be lost during the dyeing process and these should be screened for their reuse potential. Close scheduling is also required which may limit the flexibility required for bath dyeing.
- Water reuse. This can be achieved by dyeing multiple lots in the same dye bath.
- This is generally possible for those products where high quality dyeing is not essential (e.g. work gloves, hosiery).
- Install improved machinery that has better controls.
- Screen azo dyes to ensure that they do not degrade to produce amines that are listed as being toxic.
- Metal containing dyes should be replaced where possible.
Pollutants associated with printing include suspended solids, solvents, foam, color and metals, and in general, large volumes of water are consumed during the washing-off stages. The main areas of waste minimization in printing include raw material conservation, product substitution, process and equipment modifications, material handling, scheduling and waste recovery . Other options include:
- Waste minimization in the design stages can eliminate the need for dyes containing metals.
- Careful selection of surfactants.
- Reducing air emissions by replacing solvents with water-based alternatives.
- Routine and careful maintenance of printing equipment.
- Training employees in the practices of good housekeeping.
- Reusing water from washing the print blanket.
- Turning off wash water when machine is not running.
- Installing automated color kitchens.
- Reusing left over print paste.
- Removing excess paste from drums, screens and pipes by dry techniques (wiping with a squeegee etc.) before washing with water. This reduces the color load discharged to drain.
- Careful scheduling to prevent expiration of print pastes before use.
- Investigating alternatives to urea as this increases the nitrogen in the effluent.
There are a number of finishing processes that are carried out on the fabric after dyeing and/or printing. These can be achieved by chemical or mechanical methods. Some waste minimization options are listed below:
- Design fabrics such that the need for chemical finishes are minimised.
- Use mechanical alternatives to chemical finishes.
- Use low add-on methods.
- Minimize volatile chemical use.
- Avoid mix discards through careful preparation.
- Install automated chemical dispensing systems.
- Train employees in good house keeping practices.
- Use formaldehyde-free cross-linking agents.
- Reduce solid waste by reducing the need for selvedge trimming through better width control, training workers and collecting selvedge trim for resale.
- Investigate the use of spray application of finishes as these have a low add-on and require no residual dumping at the end of a run.
- RESULTS AND DISCUSSION
The measures discussed in this paper for water, chemical and waste minimization for textile industries have potential to save:
- Raw Materials 01 to 05 %
- Packaging 10 to 90 %
- Consumables 10 to 30 %
- Ancillary materials 05 to 20 %
- Electricity 05 to 20 %
- Water 20 to 80 %
- Effluent 20 to 80 %
- Solid Waste 10 to 50 %
This paper guides the Textile industry in:
- Identifying areas of waste
- Constructing mass and energy balances
- Identifying suitable waste minimization options
- Determining the feasibility of implementation
The textile industry emits a wide variety of pollutants from all stages in the processing of fibres and fabrics. These include liquid effluent, solid waste, hazardous waste, emissions to air and noise pollution. Waste minimization is important because it: reduces operating costs; reduces risk of liability; reduces end-of-pipe treatment; improves process efficiency; enhances public image; protects health and environment; and improves employee moral . It is important to investigate all aspects of reducing wastes and emissions from the textile industry, as not only will it result in improved environmental performance, but also substantial savings for the individual companies.
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