Tuesday, April 4, 2017

Manufacturing processes & Wastewater characteristics for Textile industry

Manufacturing processes & Wastewater characteristics for Textile industry
Two types of wastes are generated in textile industries.
1. Process chemicals and
2. Process wastes
The nature of the waste generated depends on:
1. Type of textile facility
2. Processes and technologies and
3. Types of fibers and chemicals

Little or no wastewater is generated during the process of fiber preparation and yarn spinning
During slashing and sizing operations, the wastewater generated contains BOD, COD, metals, cleaning waste and size.
During weaving, knitting and tufting processes little or no wastewater is generated
Wastewater from desizing operations contains BOD from water soluble sizes, synthetic size, lubricants, biocides and anti-static compounds.
Wastewater from scouring operations contains disinfectants and insecticide residues, NaOH, detergents, fats, oils, pectin, wax, knitting lubricants, spin finishes and spent solvents
Bleaching process wastewater mainly contains hydrogen peroxide, sodium silicate and organic stabilizer and has a high pH
Singeing process yields little or no wastewater.
Mercerizing process in textile mills yields wastewater containing high pH and NaOH.
Little or no wastewater is generated during heatsetting processes
The dyeing process generates wastewater containing metals, salts, surfactants, toxics, organic processing assistants, cationic materials, colour, BOD, COD, sulphide, acidity, alkalinity and spent solvents
Printing process in textile industry produces wastewater containing suspended solids, urea, solvents, colour, metals, BOD, heat and foam
Finishing process in textile industry contains BOD, COD, suspended solids, toxics and spent solvents.
Finally, product fabrication produces little or no wastewater

Wastewater from the textile industry is classified into:
1. Dispersible wastes
Such wastes are highly dispersed and mixed with other wastes
2. Hard to treat wastes
Such wastes resist treatment and contain non biodegradable or inorganic substances that cannot be removed by conventional biological treatment methods
3. High volume wastes
Such wastes mainly contain wash water, alkaline waste and warp sizes
4. Hazardous or toxic wastes
Such wastes are a sub-group of hard-to-treat wastes. They contain metals, chlorinated solvents and non-degradable surfactants.

Acute toxicity of textile dyes leads to skin irritation, vomiting and diarrhea. Reactive dyes cause respiratory and skin problems.
The BOD/COD ratio of composite wastewater is around 0.25 implying that the wastewater contains a large amount of non-biodegradable organic matter.


In the primary treatment, the following sequence of operations is carried-out:
1. Screening
2. Sedimentation
3. Equalization
4. Neutralization
5. Chemical coagulation and
6. Mechanical flocculation
Screening and sedimentation helps remove organic color, suspended solids along with primary reduction of BOD and COD.
The conventional treatment is a physico-chemical treatment followed by a biological treatment system.
Sedimentation is sometimes combined with the equalization process.
Textile effluents are highly variable in terms of pH and extent of impurities
Individual process effluents affect the secondary treatment process due to the extreme variation in chemical characteristics from effluents from other processes. Hence, they are collected in a big tank and stirred continuously by rotating agitators or blowing compressed air from below. The tank has a conical bottom for enhancing the settling procces. Dilute sulphuric acid and boiler flue gas which is rich in carbondioxide is commonly used for neutralisation of alkaline effluents from neutralization. Chemical coagulation and mechanical flocculation are used to remove finely divided suspended solids and colloidal particles that cannot be removed by simple sedimentation by gravity.
Secondary treatment consists of aerated lagoon, trickling filter, activated sludge process and an oxidation pond. In this process, the dissolved and residual organic matter is removed in the aerobic or anaerobic mode. This is achieved by the digestive action of bacteria under suitable conditions. This helps to reduce the COD, BOD and toxicity.
Sometimes, an activated adsorption system along with an ozonation unit is used instead of biological treatment process.
The tertiary treatment process involves the following techniques:
Oxidation
Electrolytic precipitation and foam fractionation
Membrane technologies
Electrochemical processes
Ion exchange
Photocatalytic degradation
Adsorption and
Thermal evaporation

Adsorption is the most commonly used method for dye removal. It can also be used to remove toxic chemicals such as pesticides, phenols, cyanides and organic dyes.
It is an expensive process and the adsorbent should be re-activated resulting in 10 – 15 % loss of sorbent.
Other techniques used are:
1. Ion exchange
2. Membrane filtration
1. Activated carbon filter
2. Micron filter
3. Reverse osmosis
4. Post carbon filter
5. Ultraviolet light
Electrodialysis
Distillation
Advanced oxidation processes
Ozonation / UV radiation
Hydrogen peroxide / UV radiation

Hydrogen peroxide and ozone

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