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  Waste minimisation in textile industry

During the last 50-75 years, there has been ever-increasing efforts to somehow arrange manufacturing processes in such a way that they cause minimal damage to the environment. Efforts focused on the technology of waste treatment development at the same time, and an uneasy adversarial relationship developed between regulators and industry. Waste minimisation is the application of a systematic approach to reducing the generation of waste at source. In other words, waste minimisation prevents the waste from occurring in the first place, rather than treating it once it has been produced by end-of-pipe treatment methods. 

Waste minimisation applies to hazardous materials, non-hazardous materials, water, energy, raw materials, all waste emissions, and other resources. It is NOT a one-off activity, but an ON-GOING programme. That is, it is a technique that can be applied to all inputs to and outputs from, a process. Waste minimisation is important because it reduces operating costs, risk of liability and end-of-pipe treatment, improves process efficiency, enhances public image, protects health and environment, and improves employee moral. Waste is not only materials that are excess to requirements, but represents a loss of company profits. 

This article makes an attempt to review the various waste minimisation techniques and possibilities that are available for the textile industry today. 

Source reduction 

Waste minimisation is achieved through source reduction by making process and product changes. Product changes include increasing product life, and designing for less environmental impact. Process changes include improved operating practices, improved housekeeping, change in raw materials, change in technology, in-house reuse or recycling. End-of-pipe treatment is only considered after source reduction. 

Four main types of wastes are particularly amenable to source reduction: 
(1) Hard to treat, 
(2) Dispersible, 
(3) Offensive and 
(4) High volume. 
For each, there are major advantages in source reduction. Hard-to-treat wastes are persistent or resistant to normal treatment and include colour, metals, phosphates, and certain organic materials especially certain types of surfactants which resist biodegradation. These are often extremely expensive and difficult to remove or destroy via conventional wastewater treatment, and therefore source reduction is an economical and attractive alternative. Hard-to-treat wastes can be reduced at the source by chemical substitution, control and conservation, waste segregation, capture, reuse and recycle. In doing this, the goal is to make changes which either reduce the amount of hard-to-treat waste or make the wastes more treatable and less persistent. 

The second category, dispersible wastes, usually exists in concentrated form when they are created and at that point they can be captured or reduced by process modification at the source, or recycled. But, once discharged, they tend to become widely dispersed and hard to recapture and collect for treatment. Machinery design, chemical substitution, procedural remedies or other primary control measures can often accomplish excellent results at low cost for these wastes. 

In addition, reclaimed wastes in concentrated form have its highest potential for resale or recycle. Textile examples of this type of waste include lint (eg, from air and water filters), print paste (eg, foam drum washers), waste foam coating operations (eg, foam back coatings), air emissions (eg, breathing losses from tanks), waste solvents from machine cleaning (eg, shop), still bottoms from solvent recovery (eg, dry cleaning), and batch dumps of unused mixes (eg, finishes). Even if not reused, collection, handling and disposal is usually much easier if the material is captured when least diluted or contaminated. Offensive or hazardous textile wastes include toxic air or water emissions, eg, metals, various organic materials and surfactants. 

In many instances, chemical substitutions can effectively reduce production of undesirable process by-products or reduce their offensive nature. For these wastes, treatment often leads to undesirable waste treatment solids, eg, metal bearing sludges. Hazardous or toxic wastes are particularly offensive and they deserve special attention to reduce either volume or offensiveness. For textiles, hazardous or toxic wastes include metals, chlorinated solvents, non-degradable surfactants, and other non-degradable or volatile organic materials. These often arise from non-process sources, eg, machine cleaning, biocides. Appropriate reduction strategies include conservation, -- substitution, process or equipment modification maintenance and housekeeping improvements. 

The fourth group, high volume wastes, can sometimes be reduced by process modification or chemical substitution, or reused either on- or off-site. The most common examples of these in textiles are hydraulic loading (high volume of wastewater), wash water from preparation and continuous dyeing, sizes and alkaline waste from preparation, air from hot air dryers, and dye waste containing large amounts of salt, acid or alkali. These can often be reduced by segregation and subsequent recycle, as well as by process and equipment modifications. Especially important are wasted chemicals (batch dumps and cleanup activities) of starch size, knitting oils and degradable surfactants. 

Each of the above four types of waste may originate from a variety of textile operations and can be addressed by reduction conservation strategies, process modifications, chemical substitutions and other techniques which reduce the sources of these wastes in textile wet processing, including preparation, dyeing, printing, finishing and other sources. Since every operation is different, it is not possible to specify exact universally applicable method for source reduction activities. The main ingredient in a successful source reduction program is to encourage individuals to look for waste reduction opportunities and take appropriate action. 

Source reduction techniques can generally be viewed in terms of several classifications as shown in Figure 1. These include common sense, good work practices, known technologies already in commercial use, known technologies not in commercial use, new technologies based on known science and, finally, new science. The worst mills do not even use good work practices and common sense. Average mills use good work practices as well as some known technologies. A "state of the art" mill will use all economically proven technologies. "Best available technology" would involve the application of all know technologies. Development of new technology or new science can be very expensive. 

A common mistake of textile mills is to jump into the middle of the waste reduction curve, thus encountering high cost and poor results. The best strategy for a source reduction program is to take things one step at a time, keeping the pollution source reduction program simple and efficient at first and avoiding any attempt to substitute high tech approaches for common sense and good work practices. Only after laying a proper foundation of common sense, good attitudes and work practices can a mill expect to successfully move on to the "high tech source reduction approaches. 

Steps onvolved in source reduction 

The first and most important step is to audit the wastes from an operation, including both process and non-process items. In many cases, non-process sources account for over half the pollution, especially the most offensive types. Most often found is non-process cooling water, solvents from machine cleaners and shop use, metals, herbicides, fungicides, boiler chemicals, insecticides and the like. Once all wastes have been listed, each should be viewed as a potential recycle product, and markets should be sought. 

The next step is to identify specific reduction strategies for each identified waste. Some broadly applicable techniques include raw material control, conservation and optimisation of chemicals, chemical substitution, process modification, equipment modification, maintenance procedures, housekeeping, waste recovery (for reuse and recycle), and segregation. Using a combination of the above techniques has produced documented annual savings in the millions of dollars. These savings will persist and even increase year after year as time passes, in contrast to waste treatment losses and costs which produce an ever increasing burden of cost and liability as time passes. 

In any manufacturing operation, there are two fundamental sources of product variation: Raw material variation and processing variations. In order to achieve process optimisation and product uniformity, consistent raw materials must be fed into the manufacturing process. To try to optimise processes and equipment without first controlling raw materials leads to constant adjusting of the process and equipment with little if any permanent improvements. Thus the concepts of source reduction, process efficiency, cost reduction and quality control go hand-in-hand. 

Raw materials coming into a textile process generally include commodities such as water, substrate (yarn, fibre, and fabric), salt and size, as well as speciality process chemicals and dyes. Raw material control can be exercised both by prescreening materials before use and by testing shipments as they are received. These precautions provide many important benefits, not the least of which are reduction of off-quality goods, lower reworks, and improved product uniformity. Each of these increases manufacturers’ profits while reducing waste associated with inefficient processes, reworks and remakes. These techniques also allow the mill to screen out raw materials which will ultimately produce offensive wastes. 

In this process, be sure to include shop and maintenance chemicals. It is not unusual to find dyeing and other procedures in textile wet processing which use excessive amounts of chemicals or unnecessary chemicals. Often chemicals are unnecessarily added to procedures to counteract the side effects of other chemicals. For example, defoamer is frequently added to reduce foaming caused by other chemical specialties. In many cases, it is more judicious to adjust, substitute, or to remove offending chemicals from a process than to add more chemicals to offset undesired side effects. Even better, it is often possible to accomplish the desired result by machine set up, time temperature and other processing parameter adjustments, without using chemicals at all. Such conservative use of chemicals can significantly reduce waste loads, cut processing costs and increase quality.

Recycle/reuse is not a source reduction activity, it does deserve comment here. It is good economic sense to avoid dumping unused portions of chemical mixes, but this still happens in many types of continuous textile operations such as slashing, preparation, continuous dyeing, printing, coating and finishing. Batch dumps are expensive and account for a major portion of a processor's waste load. To salvage unused mixes, segregation of wastes is the first step in recycle/recovery, since mixed wastes are generally not reusable. Even if wastes are not recycled, there are several reasons that segregation of wastes may be desirable. Specific treatments, such as neutralisation of acid/alkali and oxidation/reduction wastes are more effective prior to mixing. Recovery/reuse systems generally are more effective when the waste stream is most concentrated. Mixing hazardous and non-hazardous (or hard to treat and easily treatable) wastes can create unnecessarily large volumes of hazardous (or hard to treat) wastes. 

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 optimising water use and reducing the number of process steps. They include: 

# Repairing leaks, faulty valves, etc: A simple method of determining if leaks exist is to take incoming water metre 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. 
# Optimise 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 optimisation: Recipes are generally fail-safe designed which results in the over-use of chemicals. Optimising 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 alternatives, replace stilbene optical brighteners with alternatives, or eliminate altogether, 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 mercerising 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. 

Energy conservation 

As with water conservation, reductions in energy use can result in substantial savings and lower emissions from boilers or generating plants. They include optimising compressed air generation, installing compressor control systems, and general housekeeping 

Reduce cooling loads, decrease condensing temperature (as a guideline, reducing condensing temperature by 1oC will yield savings of between 2% and 4% of annual refrigeration cost); Increase evaporating temperature (as a guideline, increasing evaporator temperature by 1oC will yield savings of between 2% and 4% of annual refrigeration cost); Compressor control, incorrect control of compressors can increase costs by 20%, or more; Boiler blowdown, economisers, insulation, flash steam recovery, good housekeeping, installing heat exchangers, optimising plant environmental conditions, shutting off of lighting, air-conditioning, etc. 

Reducing solid waste 

In terms of volume, solid waste is the second largest waste stream in the textile industry next to liquid effluent. There are a number of waste minimisation options available to reduce solid waste, and these include: 
i) Reducing the amount of packaging material by improved purchasing practices such as ordering raw materials in bulk or returnable intermediate bulk containers (IBCs). This reduces spillages, handling costs, exposure of workers to chemicals and the amount of storage space required. ii)Purchasing chemicals in returnable drums. Enquire if vendors will accept unwashed drums as this will reduce the waste water generated in the factory. If possible, ordering chemicals in IBCs rather than bags as these are easily broken, causing spillages. 
iii) Purchasing yarn on reusable plastic cones rather than cardboard cones. 
iv) Reducing seam waste through effective training programmes. 
v) Selling waste fibres, sweeps, rags, yarn and cloth scraps. 

Waste minimisation in specific textile processes 

The following sections will describe various waste minimisation techniques that can be implemented in specific textile processes: 

# Size selection in sizing: Replace starch-based sizes with synthetic sizes. The advantages of this are, a reduced pollution load as synthetic sizes have lower BOD levels and they can be recycled for reuse. 
# Sizing raw materials: 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. 
# Sizing recipe optimisation: 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 mercerising. Desizing accounts for > 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.
 
Some waste minimisation options for the preparation department are listed below. 

* Desizing: 

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. 

* Scouring: 

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; dialkyldimthyl ammonium chloride; distearyl dimethyl ammonium chloride; di dimethyl ammonium chloride; sulphosuccinates; alkylphenolethoxylates; complexing agents with poor biodegradability (eg, EDTA; phosphonic acid; NTA; phosphonates). Reuse scours washwater for desizing. Recycle continuous scour washwater to batch scouring. 

* Bleaching: 

Replace the use of chlorites and hypochlorites with hydrogen peroxide. Ensure that bleaching is carried out efficiently. Recycle bleach washwater for scouring. Use vacuum slots to remove excess solution which can then be reused. 

* Singeing: 

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. 

* Mercerising: 

Recycling of sodium hydroxide through evaporation for reuse in mercerising or scouring will decrease the pollution load and chemical consumption. 

* Batch processing:

There are a number of waste minimisation 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 coloured 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. 

Dyeing 

* Batch dyeing: 

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 minimisation include : Using low liquor ratios; Using automated dye and chemical dosing systems; Reusing dyebaths, rinse water and softening baths; Ensuring a good cloth preparation; Optimising 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; Optimising 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 neutralising after dyeing with formic acid or dilute hydrochloric acid (acetic acid adds to the COD of effluent). 

* Continuous dyeing: 

The main waste minimisation 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 colour and style; Minimize the number of times a dyebath has to be dropped and cleaned due to a colour change by careful scheduling; Use automated colour kitchens to minimise the working losses and discards; Improve washing efficiency through the installation of flow restrictors to control water volumes; Use counter current washing procedures; Optimise dosing of chemicals through monitoring of relevant parameters such as pH, absorbance, turbidity, etc. 

* General waste minimisation 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; Minimise 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; Minimise auxiliary use. 

The greatest costs in reprocessing are associated with the cost of dyes and chemicals -- typically, the costs can increase by as much as 30%. In dyeing polyester, avoid the use of carriers by upgrading dye machinery or replace with less harmful alternatives. Good fabric preparation increases the chance of right-first-time dyeing as fixation is improved. Dye fixation onto cotton can be improved by mercerising the yarn or fabric prior to dyeing. 

Printing 

Pollutants associated with printing include suspended solids, solvents, foam, colour and metals, and in general, large volumes of water are consumed during the washing-off stages. The main areas of waste minimisation in printing include raw material conservation, product substitution, process and equipment modifications, material handling, scheduling and waste recovery. 

Other options include: 

  • Waste minimisation 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 colour 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 colour 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.

Finishing 

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 minimisation 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. 

  • Minimise volatile chemical use. 

  • Avoid mix discards through careful preparation. 

  • Install automated chemical dispensing systems. 

  • Train employees in good housekeeping 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. 

End-of-pipe treatment methods 

Once waste minimisation has been carried out in the factory, effluent will still be produced that will require some form of treatment prior to disposal to sewage, river or sea. 

Effluent segregation: 

Prior to the installation of any end-of-pipe treatment method, it is essential to carry out segregation of the effluent streams to separate the contaminated streams from the relatively clean streams for treatment. This result in a more effective treatment system as a smaller volume of waste water is treated (resulting in lower capital and operating costs) and it allows for the use of specific treatment methods rather than trying to find one method to treat a mixture of waste with different characteristics. The segregated clean streams can then be reused with little, or no, treatment elsewhere in the factory. 

Treatment technologies: 

There are two possible locations for treating the effluents, namely, at the textile factory or at the sewage works. The advantage of treatment at the factory is that it could allow for partial or full re-use of water. The following technologies have all been used: Coagulation and/or flocculation, membranes (microfiltration, nanofiltration, and reverse osmosis), adsorbents (granular activated carbon, silica, clays, fly ash, synthetic ion-exchange media, natural bioadsorbants, and synthetic bioadsorbants), oxidation (Fenton's reagent, photocatalyis, advanced oxidation processes, ozone) and biological treatment (aerobic and anaerobic). 

Since the effluent from the textile industry is complex and variable, it is unlikely that a single treatment technology will be suitable for total effluent treatment and water recycling. 

Coagulation and/or Flocculation: 

Chemicals are added that form a precipitate which, either during its formation or as it settles, collects other contaminants. This precipitate is then removed either through settling or by floating it to the surface and removing the sludge. This is a well-known method of purifying water. Both inorganic (alum, lime, magnesium and iron salts) and organic (polymers) coagulants have been used to treat dye effluent to remove colour, both individually and in combination with one another. With the changes in dyes and stricter discharge limits on colour, inorganic coagulants no longer give satisfactory results. They have the added disadvantage of producing large quantities of sludge. 

Organic polymers show improved colour removal and produce less sludge, but then may have detrimental effects on the operation of the sewage works. Cationic polymers have also been shown to be toxic to fresh water fish. Alum is effective in removing colour from textile effluent containing disperse, vat and sulphur dyes, but is ineffective against reactive, azoic, acid and basic dyes. However, it does have the advantage of reducing phosphorous levels, thereby improving the operation of sewage works. 

Membranes: 

The membrane methods that are available for effluent treatment are microfiltration, ultrafiltration, nanofiltration and reverse osmosis. In general, nanofiltration or reverse osmosis are the most effective processes for removing colour and recovering water. The drawbacks of these processes are the high capital costs, the fact that the concentrated effluent still has to be treated, and membrane fouling. 

The most frequently tested method is reverse osmosis. The effluent is forced under moderate pressure (1.5 to 4 MPa) across a semi-permeable membrane to produce a purified permeate and a concentrate. This process can remove up to 99% of salts and the complete removal of most organic compounds. The concentrate will require further treatment prior to disposal as the level of impurities are up to six times that of the original effluent stream. In nanofiltration, the membrane acts as a molecular filter, retaining polyvalent ions and compounds with a molecular mass greater than 200. The concentrate contains almost all of the organic impurities and a large proportion of the polyvalent inorganic salts and requires further treatment prior to disposal. The permeate contains the monovalent ions (eg, sodium and chloride ions). 

This method of effluent treatment has been found to be effective in the treatment of dyebaths from reactive dyeing where sodium chloride is used as the electrolyte, as the permeate produced contains the salt and is virtually colourless, and therefore, suitable for reuse in the reactive dyeing process, saving both water and the cost of the salt. Ultrafiltration and microfiltration as stand-alone treatment methods are only suitable for reducing COD and suspended solids from solution. They are effective in combination with other treatment methods such as coagulation/flocculation. They are also useful for the partial removal of colour and organics prior to discharge to sewer. Microfiltration removes colloidal material such as disperse and vat dyes.

Adsorbents: 

In order for an adsorbent to work effectively, the concentration of the impurities in the effluent stream must remain fairly constant to prevent the release of the adsorbed material back into the effluent if the concentration falls. Activated carbon is the most commonly used adsorbent and it is effective in removing organic components from the effluent (but not inorganic compounds). Once saturated, it must be regenerated or disposed of.

Regeneration is costly, and in most cases it is trucked off site and disposed of in landfill. Care must be taken with the disposal method as the organics may leach out over time and cause pollution problems at a later date. Other adsorbents include inorganic compounds such as silica, cinder ash and various clays. Trade name adsorbents such as Macro-sorbs and COLFLOC have been shown to be effective at removing colour from reactive dyebath effluent, although disposal of the sludge may be problematic. Bioadsorbants are naturally occurring polymers that are biodegradable and have structures that allow the adsorption of species within them, or which act as ion-exchangers. Synthetic cellulose bioadsorbants have also been developed and preliminary investigations into their use for removing colour due to reactive dyes show promising results 

Oxidation: 

Oxidants decolourise dyes by breaking down the dye molecule. Commonly used processes are ozone and Fenton's Reagent. Ozone has been investigated in a number of studies. It has been found that dye wastewaters react differently depending on the composition. Effluent containing sulphur and disperse dyes are difficult to decolourise, whereas colour due to reactive, basic, acid and direct dyes is removed fairly easily. The main drawback with installing an ozonation plant is the high capital and operating costs. 

However, improvements in generator and contacting equipment design, together with increasingly strict environmental legislation will probably lead to a more widespread application. Fenton's Reagent consists of ferrous salt (usually sulphate) and hydrogen peroxide. The reaction is carried out at a pH of 3 and involves the oxidation of ferrous ion to ferric ion with the simultaneous production of the hydroxyl radical. This radical is a powerful oxidizing agent and will attack organic compounds and cleave the bonds. In the case of dye molecules, this would lead to decolourisation. 

A disadvantage (in terms of costs for the discharger) is the production of ferric hydroxide sludge, but it is thought that this sludge is advantageous to the biological treatment system. Other oxidation methods include the use of ultraviolet light in conjunction with a photocatalyst (titanium dioxide), or other chemical agents such as hypochlorite (the use of which is not encouraged as chlorinated organic species may be formed which are themselves toxic to the environment). 

The main drawback of these above methods is that it is not known what degradation products are formed from the oxidation process and it may be the case that these end products, although colourless, may be more toxic than the original dye molecules. 

Biological treatment 

Aerobic treatment: 

The majority of sewage works are based on the principle of aerobic treatment, where the incoming effluent is exposed to bacteria which convert the components into carbon dioxide and sludge, which is then sent to an anaerobic digester for further treatment. It has been found by a number of researchers that aerobic treatment methods are not sufficiently able to treat the colour from the textile industry, and any colour removal that does take place is due to adsorption onto the sludge, rather than degradation of the dye molecule. 

Anaerobic digestion: 

Anaerobic digestion is the biodegradation of complex organic substances in the absence of oxygen to yield carbon dioxide, methane and water. It is an effective process for treating high COD wastes (eg, size, desize washing and scouring) and the methane that is produced can be utilised as energy for heating, etc. The reducing conditions in an anaerobic digester have been found to cause decolourisation of azo dyes through cleavage of the azo bond and subsequent destruction of the dye chromospheres. Complete mineralisation of these degradation products does not take place and aromatic amines may be present in the effluent from the digester.

Conclusion

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. The consumption of energy must also be taken into account as the fuel used to provide this energy contributes to the pollution load. In general, effluents that are high in COD are most effectively treated by biological methods, either aerobic or anaerobic.

There are a number of methods for removing colour from effluents, depending on the class of dye used, but the most effective over the range of dyes is oxidation methods (such as Fenton's Reagent) or membrane treatment using reverse osmosis. Effluents that are high in BOD and SS are best removed through coagulation and flocculation methods followed either by settling or dissolved air flotation. Those effluent streams containing alkaline (mercerising and bleaching) can be treated by membranes (ultrafiltration) or evaporation and reused in the same process.

The same is true for synthetic sizes where they can be recycled after filtration. As mentioned previously, there is no one single treatment technology that can effectively treat the final effluent from the textile industry and a combination of the available methods is necessary in order to achieve the required discharge standards. It is important to investigate all aspects of reducing wastes and emissions from the textile industry, since this will result in not only improved environmental performance, but also substantial savings for the individual companies

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Note: For detailed version of this article please refer the print version of The Indian Textile Journal September 2008 issue.

N Karthikeyan
Department of Textile Technology,
Bannari Amman Institute of Technology,
Sathyamangalam, Tamil Nadu.
Email: karthikn15@gmail.com.

J Joshuva Alexander
Department of Textile Technology,
Bannari Amman Institute of Technology,
Sathyamangalam, Tamil Nadu.

published September , 2008
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A Raw Deal
China, the world's largest cotton importer, accounting for more than 60 per cent of total raw cotton exports from India, has upset the calculations of Indian cotton sector following a marked fall in demand from that country. Cotton exports from India have been
Testing the world's favourite fibre
Cotton is the world's favourite fibre, and a superb raw material for many textile end-uses. But it is by no means easy to work with - as a natural product, its many variabilities present some extremely complex challenges.
Arvind's fire protection solutions
Since 1931, Arvind Ltd stands tall as the flagship enterprise of the $14 billion Lalbhai group. Arvind has marked its presence throughout the apparel value chain - from fibre to retail. The company has recently made a strong foray
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