Characteristics and Sources of Wastewater in Textile Industry

Introduction:
Textile wastewater contains a large variety of dyes and chemical that make the environmental hazardous for textile industry not only as liquid waste but also in its chemical composition. Dyeing and finishing industry are mainly responsible to produce a large amount of waste water. This processes done by the input of a wide range of chemicals and dyestuffs or pigments, which generally are organic or inorganic compounds in nature. Water is applied as the medium to apply dyes and various chemicals for finishes.

Most of the pollutants in textile waste waters from textile industry are high suspended solids, chemical oxygen demand, heat, color, acidity, and other soluble substances. Materials which need to be eliminated from textile wastewater are mainly COD, BOD, nitrogen, heavy metals and dyestuffs or colorants.

Waste water from cotton textile mills:

  • Cotton textile mill produces yarn from the raw cotton. This process done by the several process likes opening & cleaning, picking, carding, drawing, spinning, winding and warping.
  • All these are operation do not contribute to the liquid waste from the mill.
  • Several type of wastes generated are Color, Heavy Metal, Suspended Solid, Total Dissolved Solid, Chemical Oxygen Demand (COD), Bio Chemical Oxygen Demand (BOD), Oil & Grease, Surfactant.
The wastes water from the textile mills comes from the following operation of:
  1. Sizing (caboxymethyl cellulose (CMC), polyvinyl alcohol (PVA).
  2. Desizing (mineral acid)
  3. Scouring (Caustic soda, soda ash, detergent )
  4. Bleaching(sodium hypochlorite (NaOCl) or hydrogen peroxide (H202))
  5. Mercerizing (caustic soda solution)
  6. Dyeing (vat dyes, developing dyes, naphthol dyes, sulfur dyes, basic dye, direct dyes etc.)
  7. Finishing (starches, dextrines,natural and synthetic waxes, synthetic resins)
In textile pre-treatment process:

Sizing: 

Sizing is the process by which the warp thread or yarn is sized with starch to give the necessary tensile strength and smoothness required for weaving. Cotton and some man-made yarns are need to be sized before weaving, to gain require properties likes stiffness, strength and minimize breaking of the fibers. The yarn is passes between rollers, the size is applied into the fiber and then the yarn are dried. In 75% of the sizing operations starch or starch derivatives are used. Another substances used for sizing are polyvinyl alcohol (PVA), polyacrylates and carboxymethyl cellulose. Sizing liquid is wasted, usually in small quantity but with very high levels of chemical oxygen demand (COD), biochemical oxygen demand (BOD), and suspended solids. Synthetic sizes contain to a lower BOD.

Desizing: 

Desizing is the process used for removing the sizing materials present in the grey cloth with the help of 0.5% H2SO4 or with the help of enzymes. After weaving, the size materials must be removed before next processing of the fabric. It form a coating into the yarn and therefore act as a barrier to penetrate dyes and other chemicals. Depending on the uses size, alkalis, acids, enzymes or surfactants are used. Washing with detergents can be sufficient for some sizes, whereas starch is usually removed by using enzymes. After desizing, the fabric is rinsed and then cleans.

Waste water contains to the overall BOD and Total Solids, load of a textile wastewater can be very high. Up to 50%-55% of the total BOD in woven fabric processing can produce from the desizing step when starch is applied as size.

Scouring

It is the process used to remove natural or organic impurities like greases, waxes, oil, fats, dirt's, dusts etc by boiling with NaOH, soda ash, sodium silicate, Na202 with small amount of detergent.

The waste water contains form the scouring steps have higher ph, BOD, COD that hampers the ecosystem of aquatic organism.

Bleaching:
Bleaching is used to remove natural color from the fabric. It is done using alkaline hypochlorite or Na202 or H2O2.

Bleaching is usually occured by washing with water and then by scouring treatment with sodium bisulfate to remove traces of alkali and Cl2. The waste water after bleaching contains chlorine, hypochlorite and peroxides, alkalis, chromium, phenolics, oils and waxes.

Cotton Mercerizing: 

Mercerization is the process of boiling the cloth wing 20% caustic soda solution followed by washing with water. This process gives luster and strength to the fabric. Mercerizing imparts the require properties to the fabric likes strength, lustre, absorbency and dye affinity of cotton fabrics. A cold sodium hydroxide solution is applied which causes the fibres to swell and adopt a circular cross-section from the oval shape.

The solution is washed out away in an acid wash. Most mercerization units have their own caustic recovery systems to help minimize waste. Wastewater originated by mercerizing is usually low in BOD and solids.

In textile dyeing process:

Dyeing: 

It is the process that is used for the coloration of different textile materials. In this step lots of dyes or pigment and different types of auxiliaries are used for coloration of textiles. In the environment it supply most of the waste water that is leads to inhibition of the sunlight penetration and therefore of the photosynthesis. In addition, many dyes and their degradation products are very toxic and harmful. It is not only things that dyes cause problems in the environment, as they were developed to resist harsh environmental factors for the lifetime of the colored textile. Pigments are used for textile coloration have a very complex structure, with the intention to make them resistant to breakdown by for instance exposure to sunlight, water or soap.

The quantity of water use in dyeing is higher then other textile process. Water is used not only in the dyeing process, but also for rinsing o f the dyed products. Water using is generally dependent on process characteristics like the used equipment, fabric and dyestuff. So that in waste water contains dyes, different auxiliary chemicals are used that also end up in the wastewater. Typical pollutants generated in the dyeing step are color and different auxiliaries, such as organic acids, fixing agents, defoamers, oxi dising/ reducing agents, and diluents.

While a large amount of dyes leaves the process in an unfixed state, the exact amounts and kind of pollution depends on the used dyes and the used process. Dyeing contributes to most of the metals and almost all of the salts and color present in the overall textile effluent. For some dyeing processes likes cotton with reactive dyes, about 70% to 75% of the salts end up in the wastewater.

Rinsing:
Rinsing generally used after several processes likes dyeing, different types of finishing. Although rinsing is not only a separate textile process, but also it is interesting to regard the generated wastewater separately when looking at reuse possibilities. The wastewater from the rinsing steps is less polluted than the other several process wastewaters from the preceding baths o f the same process. Many of the processes that take place before dyeing or after dyeing of the fabric involve a number of hot or cold rinses. Rinsing is require after all several steps that leave substances on the fabric that can influence the subsequent processes, most importantly after dyeing or printing to remove surplus (unfixed) dyestuffs from the fabric.

Depending on the using processes, rinsing can make up for around 50% of the total water usage in textile wet processing. For several dyeing processes, the rinsing water around 70- 80% of the total water consumption. A comparatively easy way of taking advantage of the quality (and in some cases temperature) of water from several rinsing steps in a process is counter-flow rinsing. Counter-flow rinsing can be applied in many processes, depending on the process and the used machinery.

In textile printing:

Printing: Printing is the process of produce the attractive design into textile material by applying print paste into the fabric. Generally in printing, dyes and auxiliaries are near similar to those used in fabric dyeing, with the difference that the color is only applied to specific parts of the fabric .

An important chemical in textile printing is the print pastes, which consist of water, thickeners, dyes, urea and various other chemicals like surfactants and solvents. Urea is mainly used when printing cotton, viscose and silk, to improve the solubility of the hardly water-soluble dyes and fix them on the fabric.

Large quantity of residual pastes is produced in paste preparation and cleaning of the equipment. Residual print pastes mainly result from washing the printing matrices, tanks for paste preparation and matrix protective tissues. Other sources are leakage of dyestuffs and washing of textile products.

In textile finishing:

  • Generally finishing require to impart some important characteristics likes make the textiles more softer or stiffer, improve the appearance of cloths, improve the strength of fabric, also improve the weight of the fabric.
  • Several types of chemicals and auxiliaries are used in textile finishing likes soil release finisher, flame retardant finisher, water proof finisher, anticrease finisher etc.
  • The chemicals are used generally they are derivatives of organic compound likes organic solvents, phenolic, sulfonate, nitrogen contain compound, laryl sulfate, etc.
  • The waste water from textile finishing 25% to 40% but it contain higher BOD, COD and suspended solids.
  • Several chemicals are containing some carcinogenic compound that are responsible for cancer.
  • A common disposal method is dilution of the residual pastes and disposal with the other waste water streams, where they considerably increase COD, nitrogen and dye loads. 
  • The printing method determines the wastewater characteristics. Printing wastewaters are small in volume, but difficult to treat. Apart from the presence of solids and solvents from the print paste preparation, textile- printing waste waters resemble textile-dyeing waste waters. In addition, the concentration of the pollutants in printing wastewater is higher than for dyeing wastewater.
Woolen textile mill waste water:
Woolen fiber, unlike cotton is a fiber of animal origin. Wool contains considerable quantities of dirt, grass , etc. Contain 30% fiber and 70% foreign matter. In extreme cases, raw wool may contain as little as 30% fiber, 70% foreign matter, of which 45% is grease. Wooling waste originated from scouring, carbonizing, bleaching, dyeing, oiling, fulling and finishing operations.

  • Scouring: Wool grease and foreign matters are removed by Scouring with wool in hot alkali, soap and soda ash.
  • Wool Carbonizing: Material in wool that persisted through raw stock scouring and mechanical cleaning is removed by carbonizing. As this material does not dye together with the wool, undyed spots will be present throughout the dyed fabric. In the carbonizing process strong acid is used to degrade cellulose at high temperatures into brittle hydrocellulose, without damaging the wool. Excess acid is squeezed from the wool and the wool is then heated to oxidise the contaminants to gases and a solid carbon residue. Before further processing, neutralisation and rinsing are necessary due to remaining acids in the material. The generated wastewater has low organic matter content, but contains a high dissolved solids amount due to the acid treatment.
  • Dyeing: Hot dye solutions are circulated through the wool packed in a metal container. Waste water generated by dyeing is highly colored and contains many toxic substances.
  • Oiling: Usually olive oil or barg-oil-mineral-oil mixture is mixed with water and sprayed on the wool to increases cohesion of the fibers and aids in spinning operation.
  • Fulling: The loosely woven wool from the loom is shrunk into tight, closely woven cloth. To aid this process, chemicals like soda ash, soap etc are uesd. Excess fulling chemicals, all of the oil etc are washed out of the fabric in a finishing process.
  • Finishing: Waste from a dyeing and finishing process are contributed by the spent liquors and by subsequent washing of wool after bleaching, dyeing, and finishing. 
  • Wool Felting: Woolen fabric is treated to cause the material to mat, shrink, and thus become denser. This process is usually carried out after carbonizing, but it can also take place on raw fabric. Felting involves the use of hot solutions of soda ash or sulphuric acid. The fabric is thoroughly washed afterwards. Wastewater gene rated in this step has a very high BOD content. Traditionally either acid or alkali conditions are applied, but with modern auxiliaries felting can be performed very well at neutral pH.
Synthetic textile industry:
  • Synthetic industry firstly produce synthetic Fiber by the polymerization process in which different types of polymer is used likes PET, polyethylene, HDPE, PETE etc.
  • The wastewater is generally colored and contains alkalis, organic solvents, resins, PVA, etc. The effluent has high values of COD and BOD. The origin of wastes is more or less same as above.
Prospects of Bangladesh:
  • The dyeing and finishing plants are the major pollutants of our water. Turag that flows by Tongi is almost dead with pollution. Its water looks ink black and gives out such a foul smell that one can literally feel their lungs tingling with each breath.
  • Bangladesh has some 789 dyeing and finishing factories to serve some 4,000 garment factories, according to IFC. They are the major consumers of underground water and also contributor to pollution. 
  • IFC looked closely at Konabari area in Gazipur, which houses 33 washing, dyeing and finishing units. They consume about 13 billion litres of fresh water a year.
  • For the average jeans you wear that weigh 1kg, it takes 250 litres of water to wash. And all this has to be sweet water, a precious commodity, pumped out from underground. This implies that the textile factories are also the largest contributors of the cluster’s water scarcity as well as pollution challenges.
  • It does not need to use all that water to wash every kg of apparel. As we said earlier, Bangladesh uses 250 litres of water whereas the global standard is 60 to 70 litres . That is four times less than what we use. Experts say this use of water can be further reduced to 13.5 liters.
  • Every year, around 21.6 million cubic metres of water were saved by the intervention of Partnership for Cleaner Textile (PaCT) programme of International Finance Corporation (IFC), which worked with 200 factories.
  • Two things happen when such a huge quantity of water is pumped out. First, we create a hollow underground. As the hollow grows, it creates chances for the earth to cave in.
  • And secondly, as more and more water is pumped out, the water table goes down. We can find this actually happening in Dhaka where every year the water level is dropping by 2.5 percent. This means we have to dig deeper and deeper to pump the same water out. The water table is not infinite. One day, it will go dry if not recharged. We will not have any more water to pump out. A real disaster will take place.
Effects of mill wastes:
  • The crude waste if discharged into streams causes rapid depletion of the dissolved oxygen of the streams.
  • The alkalinity and the toxic substance like sulphides and chromium affect the aquatic life and also interfere with the biological treatments.
  • The colour often renders the water unfit for use for some industrial purposes.
  • The presence of sulphides makes the waste corrosive particularly to concrete structures.
Characterization values for different textile wastewaters (all values are in mg l-1 unless stated otherwise):


Author of this Article:

Md. Nure Alam
Teaching Assistant at Dept. of Textile Engineering
Daffodil International University
Dhaka, Bangladesh
Email: nure23-4238@diu.edu.bd


 
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Editor-in-Chief:

Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.


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