By: Wasif A.I. and Chinta S. K
There is no doubt that the impact of industrialization on environment is far reaching and even disturbing. The three basic requirements of human beings are food, clothing and shelter. Human activities for these requirements more or less disturb the ecological balance.
The toxic effects of chemicals and physical agents on living organisms and their interactions with the environment lead to phenomenon like acid rains, depletion of ozone layer and global worming due to green house effect.
Ecology is the study of interactions between living organisms and their environment, which includes atmospheres, water and pollutants.
The main raw materials used in the textile industry are the textile fibres, which are spun into yarns, which, in turn, are woven into fabrics. They are further processed to get bleached, dyed / printed or finished before introduced into the consumer market. The textile industry consumes substantial volumes of water and chemicals for wet processing of textiles. The chemical reagents used by the dye house are very diverse in chemical composition, ranging from inorganic compounds and elements of polymers and organic products.
About 50% of the textile mills are using approximately 200-300 litres of water per Kg of fabric produced while about 20% of the mills are using below 200 litres of water per kg of cloth depending on the process sequence and water utilization practices adopted in the mill. Water pollution by the textile mills in mainly attributable to various waste liquor coming out of the unit operations in wet processing such as desizing, scouring, bleaching, mercerizing, dyeing, printing and finishing.
Adverse effects of textile waste
- The waste water discharged by textile mills is of highly alkaline nature, which adversely affects the aquatic life.
- The toxic chemicals like sulphides, chlorine, chrome and aniline dyes also affect aquatic life.
- The textile water is discharged in the server line, which gets corroded due to presence of sulphur dyes and sulphur compounds in the effluent.
- The municipal sewage if exposed to acid and hydrogen/sulphides, will result in rustication of pipelines.
With respect to ecology and testing, the following three aspects should be considered.
1. Production Ecology
- Cultivation and harvesting of natural fibres
- Manufacture of man made fibres
- Manufacture of threads and fabrics
- Bleaching, dyeing, printing and finishing of textiles, and
- Garments manufacture, with the application of fertilizers, growth, regulators, pesticides, dyes, pigments, and textile chemicals auxiliaries and finishing agents.
2. User Ecology - referring to the effect of clothing textiles and the substances that give them their beauty and performance characteristic on human beings during use i.e. when the clothing is worm.
3. Disposal Ecology - referring to the disposal of the textiles after use i.e. recycling, composting, dumping or incinerating in a manner that ensures the least possible environmental impact.
Classification of Textile Wastes
Waste generated in textile industry can be classified into four categories:
- Hard to Treat
- Hazardous or toxic
- High volume waste
Hard to Treat Waste
These include primarily colours, metals, phenols, toxic organic compounds, phosphates, etc. Hard to treat waste include non biodegradable organic materials such as certain surfactants and solvents. These can resist biological effluent treatment process, pass through standard activated sludge systems and produce aquatic toxicity when the effluent treated water is discharged into downstream. Since the primary problem associated with these waste toxicity, they can be included in hazardous or toxic category.
Hazardous or toxic Waste
These are generally sub group of hard to treat waste and include materials such as metals, chlorinated solvents, non biodegradable surfactants and volatile organic materials. Some of these wastes can also come from non textile processes such as machine cleaning, boiler chemicals etc.
These include waste stream from continuous operations, print paste, waste from back coating operations, batch dumps of unused process chemicals, etc. Thus, the sources of dispersible waste are wide spread in textile wet processing.
High Volume Waste
The most common high volume waste includes wash water from preparatory, dyeing and printing operations and the exhausted dye baths. These can be reduced by recycle process and equipment modification.
Pollution Control Strategies
There are two major pollution control strategies:
- End of pipe treatment
- Cleaner production techniques and processes.
Having reviewed the improved production processes to eliminate and reduce waste as much as possible, it is necessary to select the best effluent treatment plant.
Cleaner Production Techniques and Processes
Complex environmental issues demand a comprehensive integrated approach to tackle the problem of pollution.
End - of - Pipe Treatment
Effluent Treatment Plant
The textile industry consumes large quantities of water for different processes and the effluent generated is discharged into various receiving bodies like public sewers, lakes, ponds, rivers and nallahs etc. This effluent is contaminated with dyes and chemicals. While some of these contaminants are non-biodegradable others have high oxygen demand. This waste water destroys the quality of water in which they are disposed thereby affecting the flora, fauna and human life also.
In the recent times the textile finishing sector on one hand has to pay substantial amount towards fresh water charges and effluent treatment charges and on the other hand has to strive hard to meet more stringent effluent regulations. The treatment options are based on the concentrations of dyes, auxiliaries & chemicals, types of chemicals, and also norms of pollution control board based on types of disposal.
It is now widely accepted by the dye house managements, on environmental and economic grounds, that they must treat the effluent before discharging into water bodies on a continuous basis. Therefore, it becomes essential to opt for an efficient working effluent treatment plant. Various methods of effluent treatment are known but in most of the methods, the treatments costs are high.
Designing of effluent treatment plant is a complex work because varying nature of effluent coming from various operations of textile processing and has variable characteristics. Hence it becomes necessary to consider the following factors before the construction/designing of effluent treatment plant.
- Quantity or the profile of the effluent to be treated.
- End use of the water reclaimed.
- Pattern of hourly flow and daily flow of effluent.
- Characteristics of effluent from each drain.
- Characteristics of combined effluent from the drain.
- Possibility of segregation of widely varying effluents and treating them separately.
- Treatment options available by pilot trials.
- The available outlets for removal of the dewatered sludge.
- Possibility of reuse of treated effluent.
Following are some of the more common methods of treatment followed throughout the world:
1. By using various flocculation decolourisation of the effluent can be carried out and it can be partially reused.
2. Various oxidation treatments like spurge pipes, surface aeration ejectors, diffusers, helixors, and oxygen and also chemical oxidation treatments can be employed for decolourisation of the effluent.
3. Fenton reagent used for waste water treatment is based on it's oxidizing efficiency which is highest at pH ranging from 2 to 5, and molar H2O2 to Fe2+ ratio is about 1:1.
4. The combination of UV radiation and hydrogen peroxide (UV/H2O2 involves an oxidative degradation of the noxious substances.
5. Bagasse, the waste material generated in sugar mills has been us for the removal of chromium nickel, copper and lead from aqueous solutions. Bagasse has been pretreated with 0.1 N NaOH followed by 0.1 N CH3COOH for effective removal.
6. Membrane process now-a-day plays an important role in considerations of waste water treatment concepts and from the current trend it may become even more important in the future. Based the pore size of thin membrane film and corresponding rejection extent the membrane processes are vided into four groups:
- Micro filteration (pore size 0.07 microns to 2 microns)
- Ultra filteration (pore size 0.002 microns to 0.1 microns)
- Nano filteration (pore size 0.001 microns to 0.07 microns)
- Hyper filteration (pore size < 0.001 microns)
7. Reverse osmosis is the most widely used desalination process and preferred system for removal of dissolved salts. About 85% of membranes used worldwide are for RO applications. RO implies the finest level of filteration wherein only water and very low molecular weight compounds <40 Dalton) pass through as ions.
8. Discoloration of reactive dyeing waste water can be made possible by using oxidative decolourisation of reactive dyes with persulphate. It is very effective due to formation of free radicals.
9. Cleaner production techniques can impart decolourisation of dyehouse effluents by using various flocculants, speciality chemicals and substitution products.
10. Biological treatments are very much essential wherein biomass generation for reduction of BOD and COD level of the effluent is used. Various biological treatments are opted for efficient treatment of effluent.
11. Various techniques are used for colour removal based on physical methods like by using charcoal, activated carbon, sawdust, tea waste, sugarcane waste (bagasse).
Problems of Effluent Treatment Plant
The problem with processors is about effluent treatment plant and the cost of the plant. However, it can not be assumed that a simple system can be placed at any textile plant and successfully treat the waste water. The problem faced by processors regarding effluent treatment can be summarized as follows:
- High initial investment cost.
- Daily recurring expenses Chemicals, electricity. etc.
- Daily & Annual Maintenance Cost.
- Stringent Norms of Pollution Control Board.
- Complex and variable nature of effluent.
- Extra manpower (Technical & NonTechnical) to supervise the plant & the system.
- Separate Testing Laboratory.
- Huge Space requirement.
- Solid & Liquid waste disposal.
- Public awareness regarding environmental issues.
- Local Bye Laws and media.
Due to the above mentioned reasons it becomes difficult for processors to the treat the effluent and meets the norms laid down by the pollution control board. Due to eco friendly processing zero discharge is gaining importance day by day which is too costlier to install and operate.
The statistics of cost of the effluent per litre is as follow.
The cost of effluent treatment per 1000 litres is around RS.10/- which is exclusive of demineralization and reverse osmosis process. This includes electricity charges, chemical charges, labour charges, maintenance charges, etc.
The solid sludge disposal expenses are around Rs. 5/- Kg.
Practical Approach Towards Water Pollution Control
The objective of waste minimization is to produce minimum air, water and noise pollution. Waste minimization is both economically sensible and environmentally responsible. Accordingly, a chemical used should be effective at low concentration level, at low level of pollution load and must ensure performance optimization. It automatically conserves all inputs by eliminating wasteful practices and thus ensures cleaner environment for workers, consumers and for the future generation, too.
The motto is to conserve water and substances that are exhausting (depleting) fast. 23 Considering the problems out of two types of pollution control strategies instead of end of pipe treatment, if processors concentrate more on cleaner production techniques and processes which can tackle the problems of pollution.
Recent technological advances are mainly aimed at cost reduction, quality upgradation, and energy and water conservation and to decrease the quantity and upgrade the quality of effluent considering the break up of water consumption in wet preparing the fabric for dyeing & printing. Hence more attention can be given to conserve the water from pretreatments.
For efficient control of water pollution in textile industry, a practical approach is to take the following necessary steps:
- Reduction on the waste volume.
- Reduction in concentration of chemicals used and thereby reduction in their harmful effects.
- Reduction in waste concentration by recovery and reuse.
- Reduction of waste concentration by chemical substitution.
- Reduction of waste concentration by process modificat ion.
- Segregation of drains.
- Devising suitable treatment based on point of ultimate disposal to meet the requirement.
The water consumption varies in the range of 7010 150 lit res per kg. of fabric processed and the waste water discharged correspondence to 58 to 81 % of water consumed. Thus, any economy affected in water consumption will result in reduction of water volume.
By taking the appropriate measures, textile process house may be able to achieve a reduction of around 20%-40 % in the volume waste water generated.
This is applicable wherever possible in some of the unit operations e.g. reduction in no. of washings or standing bath methods.
Reduction in Concentration of Chemicals:
Normally, in industry, higher concentration of chemicals like acids, alkalis, hydrosulphite, bleaching agents and various auxiliaries are used. The manufacturer's recommendations in order to provide a large margin of safety also affect the effluent. After judicious study, minimum amount necessary for process should be used. This not only helps in reducing the pollution load but also useful in effecting considerable economy.
This reduction is not possible foe standard recipes set for dyeing and other processes.
The change in/or reduction in chemicals may not be permitted due to obvious reasons. This may change the tone and/or depth of the shade.
Reduction of Waste concentration by Recovery & Reuse of Chemicals:
To effect this system processor must have conservation attitude. Here many more storage tanks, pumping motors and sufficient space are involved e.g. caustic recovery, standing bath, finishing left over, spent acid etc.
In this case conservation philosophy is very useful and practical problem here is storage for chemicals to be reused, space for storage, additional pumping system, piping and reutilization.
Reduction of Waste Concentration by Chemical Substitution and Process Modification:
The substituents are available for some of the chemicals and not for all chemicals and hence it poses limitations but process modification is the area where processors can contribute and can achive very good results not only in effluent reduction but savings in time, energy, water and reduction in labour.
This may increase in production also. Textile industry commonly uses various chemicals / agents in their different processes like desizing, cotton softening, denim washing, silk degumming. These chemicals cause pollution in the effluents; some of them are corrosive, causing damage to the equipments and fabrics.
Different stain-removing agents are used to remove the various stains from the fabric. Sodium bromide has been used as an efficient oxidative desizing agent in the industry but has got limitations as far as strength of the fabric is concerned. However, with the introduction of enzymatic process in textiles, the above problems could eliminated, as enzymes are biodegradable and safe for the fabric and the environment.
Today, enzymes have become an integral part of the textile processing. Though the use of enzymes in desizing application was established decades ago, only in recent years the applications have widened with the introduction of new products. With the creased awareness and regulation about the environment concerns, the enzymes are the obvious choice. This is because the enzymes are biodegradable, work under mild condition and save the precious energy.
It has been reported that "Cellulase' the enzyme having cellulose degrading capabilities. Cellulase treatment of textiles made from cotton and cotton blends is carried out for a particular length of time, pH and temperature conditions so as to ensure only softening and no eventual degradation. This treatment is called as Enzymatic Biopolishing.
Chinta S.K. et al have carried scouring process by incorporating Anthraquinone as a reducing age whereby total time of scouring has been reduced to 4 hrs as compared to conventional method thereby saving considerable energy.
Thorough scouring carried out in batch or continuous pad-steam process with no addition of scouring assistants based on emulsified chlorinated solvents can completely remove any traces of harmful pesticides, used during production of cotton. Bleaching with chlorine containing agents should be avoided due to problems of AOX (Absorbable Organic Halogen).
ICI has made an attempt to remove Fats and waxes by using pure solvent. This technique is knows as ICI solvent scouring process. The ICI Mond division first introduced solvent desizing, wherein after impregnation with solvent and enzyme, the cloth is steamed.
Kalinowsky has carried out work using paraffin hydrocarbons in an aqueous emulsion form as a scouring aid. A further development in solvent scouring is the totally enclosed dry-to-dry treatment of fabric using perchloroethylene, in which the solvent is recovered by dry distillation rather than by steam distillation.
Bhide S.N. et al have modified the kier by injecting live steam at the bottom. The fabric is padded with alkali and other constituents of scouring recipe is piled in kier and steamed for 90 minutes. Scouring efficiency is reported to be very good.
Chinta S.K. et al have reported various developments in the bleaching process for shortening of the processing sequences thereby saving energy and water and reduction in pollution load.
Chinta S.K., Wasif A.I., et al have reported scouring and bleaching cotton fabric at room temperature using emulsified solvents like perchloroethylene and trichloroethylene and Sodium hypochlorite. The authors have further reported that hypochlorite decomposition in presence of solvents hamper the whiteness to certain extent and hence recommended that concentration of hypochlorite be increased to 3.0 g/I available chlorine.
Guha S.B. and Shah S.R. have developed a scouring process by using cellulase and protease enzymes. It is claimed that the results are quite comparable with those of kier boiled fabric samples.
In peroxide bleaching, organic stabilizers can be used in place of silicate and phosphate types since the latter are non bio-degradable. It has been reported by Sharma M. that by using amide based speciality chemicals; it is possible to carryout bleaching with hydrogen peroxide in cold condition, by employing cold pad-batch method. The pre-bleach system uses enzyme as a hydrogen peroxide killer thereby allowing quicker processing times, coupled with a clean substrate for drying.
Ramisch Kleinwefers has developed latest preparatory machine "Reco-Yet" for one step desizing, scouring and bleaching. A heated steam in the form of aerosol and chemicals including a new multi-functional auxiliary agent is applied directly to the fabric with the chemical and thermal energy acting simultaneously.
The advantages claimed of this one-step process system are reduction in time (one to three minutes), reduced consumption of water, steam and energy and less volume of waste water.
Continuous pretreatment machines provide increased washing efficiency thereby reducing water consumption.
Broglin has reported a one step desizing, scouring and bleaching process. This process not only optimizes the use of water, but also provides ample scope for water recycling, thereby reducing the waste water load, facilitating a better and easier handing along with ecological and economical advantages.
On account of high BOD and non-reproducible results, use of acetic acid for pH correction of bleached fabrics is losing favor and is being substituted by products such as Neutra acid NV.
Environmental friendly permanganate bleaching process for natural and synthetic fabrics has been discussed by Lokhande et al. The permanganate process has been found to be comparable with the conventional bleaching processes in terms of the physical and chemical properties of the bleached materials.
It is mainly aimed at cost reduction, quality upgradation and consequently better environmental protection.
Deo H.T., Wasif A.I. and Chinta S.K. have developed system of ecofriendly desizing, scouring and bleaching in one bath based on redox system for cotton fabric, cotton yarn, polyester-cotton blended fabric and carbonized fabric. This method has reduced the effluent load almost by 40%-50% and treatment cost drastically reduced.
About the Authors:
Wasif A. I. and Chinta S. K. are associated with O.K. T.E'S Textile & Engg. Institute, Ichalkaranji.
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