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 )
1. Bleaching(sodium hypochlorite (NaOCl) or hydrogen peroxide (H202))
2. Mercerizing (caustic soda solution)
3. Dyeing (vat dyes, developing dyes, naphthol dyes, sulfur dyes, basic dye, direct dyes etc.)
4. 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.