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Introduction
In textile mills mainly, the waste water discharge is the effluent which contains :
• Organic matter
• Inorganic matter
• Dissolved solids
• Suspended solids
• Dyes
• Chemicals & Auxiliaries
• Metal Toxicants
They directly or indirectly shows effect in the color, alkalinity, pH, hardness, BOD, COD
values of water.
• Primary goal is to reduce the effect on ENVIRONMENT
-- by determining the type of pollutants.
-- and giving treatment accordingly.
Raw Effect Samples Treated Effluent Samples
Need of Effluent Treatment Study
Classification of textile wastes
• Hard to treat
- Colours, Metals, Phenol
• Toxic organic compounds
• Phosphates
• Non-biodegradable surfactants
• Hazardous or toxic
• Dispersible
• Primary
Removes identifiable suspended solids and floating materials.
• Secondary
▫ Also known as biological treatment.
▫ Removes organic matter that is in soluble form or colloidal form.
• Tertiary
▫ Removes sodium and chlorides ions.
▫ Removes additional suspended solids including algae, fungal bacteria, etc.
Stages of Effluent treatment process
Diagram for the treatment of dyeing effluents
Primary Treatment
Screening
Sedimentation
Equalization
Neutralization
Coagulation
Secondary Treatment
Aerated Lagoon
Trickling filtration
Activated sludge process
Oxidation-Ditch & Pond
Anaerobic digestion
Tertiary Treatment
Evaporation
Reverse Osmosis
Dialysis
Ion Exchange
Chemical precipitation
Removal by algae
Treatment stages
Sedimentation
• This process is particularly useful for treatment of wastes containing high percentage
of settable solids or when the waste is subjected to combined treatment with sewage.
• The sedimentation tanks are designed to enable smaller and lighter particles to settle
under gravity.
Screening
Coarse suspended matters such as rags, pieces of fabric, fibres, yarns and lint are
removed. Bar screens and mechanically cleaned fine screens remove most of the
fibres.
Equalization
These effluent streams are segregated and stored in separate tanks, and this liquid
effluent may be dosed at regular and uniform rate which is known as equalization.
This process is carried out to equalize the pH.
Screening Equalization
Neutralization
• Neutralization helps in maintaining the pH range of 6-9.
• Mercerizing discharges are highly alkaline liquors, and carbonizing discharges
are highly acidic nature. Diluted by adding of H2SO4 or CO2 or flue gas.
Chemical Coagulation
• To remove colour, suspended solids, colloidal particles, the effluents are treated
with coagulants like Alum, Ferrous Sulphate, Ferric Chloride, Sodium Aluminate
and activated Silica in a clariflocculator
• Acids such as polyelectrolytes are used along with coagulants to improve
coagulation. Sludge is separated and dried on sand beds. Treated effluent is
subjected to secondary treatment.
BIOLOGICAL TREATMENT
Aerated lagoons (tanks)
• Large cement tanks having 3-5 m depth. Effluent from Primary Treatment are stored in
these tanks for 2-6 days which are aerated mechanically.
• After 2-6 days of aeration, a healthy flocculent sludge forms, which carry out oxidation
of organic matter. It removes 90% of BOD.
Trickling filtration
• Effective aerobic biological oxidation method widely used. The effluent is sprinkled
over a bed of broken stones. Bacterial slimes formed on the stones oxidizes Organic
matter during the passage. Effluent is finally settled and discharged.
Activated Sludge Process
• Here the effluent is continuously exposed and subjected to biological degradation
carried out by ‘Microbial Floc’ suspended in reaction tank into which oxygen is
introduced by mechanical means. The effluent from this tank is allowed to settle
and a portion is recycled.
Comparison of trickling filters with activated sludge systems
Trickling Filters Activated sludge systems
Bacterial growth is fixed on the media. Bacterial growth is suspended as a dispersed floc.
All solids from the settler are wasted. Solids from the settler are partially recycled.
Less sensitive to shock loading - more
stable.
More sensitive to shock loadings, require closer
process control.
Produce insects and odors Produce spray clouds.
Less effective in removing disease causing
organisms.
More effective in removing pathogens than
trickling filters.
Low operating costs. High operating costs.
Oxidation ponds
• An oxidation pond is a large shallow pond. Stabilization of organic matter is brought out
by bacteria. Oxygen is required for this purpose of metabolism and is supplied by algae.
The algae utilizes the carbon dioxide released by bacteria for photosynthesis.
• For effective treatment : 1)Maximum sunlight penetration (for photosynthesis),
2) Wind action for mixing , 3) Neutral Aeration
Sludge treatment & disposal
• The Concentration of solids in the primary sewage sludge is about 5%, Activated sludge
contains less than 1 % solids and the sludge from trickling filters contain 2% solids.
• In addition to reducing water content sludge must be stabilised, conditioned to reduce
biological activity disinfected before disposal.
Sequence of Operation for Sludge Treatment
Concentration
•Gravity Thickening•Floatation
Digestion
•Anaerobic digestion•Aerobic digestion•Sludge lagoons
Conditioning
•Chemical Addition•Heat Treatment
Dewatering
•Centrifuging•Vacuum Filtration•Pressure Filtration
•Drying Beds•Heat Drying
Oxidation
•Incineration•Wet air oxidation
Ultimate Disposal
Evaporation
• It is a simple process of boiling the waste and vaporizing the waste water. This process is useful
only when the recovered water and solids are re-used and recycled.
• Various fuel systems like coal, wood, use of stem are employed for evaporating these effluents.
Dialysis
• This is a method of separating solutes from the solution based on the difference in the rates of
diffusion.
• This process is mainly used for recovering NaOH from mercerizing and H2SO4 from carbonizing.
Chemical precipitation
• If the dissolved solids in the effluent can be precipated, they can be easily removed.
• Lime is the effective and cheapest precipating agent.
• Proteins are precipated using sulphuric acid.
Ion exchange
• Ion exchange method is extensively used to remove hardness, iron and
magnesium salts.
Removal by algae
• This process is much similar to oxidation ponds. Algae requires Potassium,
Calcium and magnesium. Traces of Manganese cobalt and Copper are required.
Reverse osmosis
Carding Combing
Drawing
Spinning Sizing Weaving
Effluent
Raw Cotton
Grey cloth
Desizing
Scouring
Bleaching
Dyeing
Finishing
Finished goods
Mercerizing
Printing
Effluent
Effluent
Effluent
Effluent
Effluent
Effluent
Effluent
The Processes carried out in a textile mill
and effluent streams
Dyes in wastewater
• Reduces the depth of penetration of sunlight - decreases photosynthetic activity and
dissolved oxygen.
• dyeing cotton with reactive dyes are highly polluted and have high BOD/COD,
coloration, and salt load.
• Marrot and Roche Review :
1)Physical methods include precipitation (coagulation, flocculation, sedimentation)
2) Adsorption (on activated carbon, biological sludges)
3) Filtration (Micro filtration, Nano filtration)
4) Membrane processes (osmosis and reverse osmosis)
Electrochemical treatment and recovery of chemicals
from the textile effluent
A flow diagram for treatment of cotton
textile mill waste
Analysis of Process Waste water of Textile Mills
Characteristic Desizing Kiering Bleaching Mercerizing Dyeing Printing
pH value 8.6 – 10.0 10.9 – 11.8 8.4 – 10.9 8.1 – 9.8 9.2 – 11.0 6.7 – 8.2
Alkalinity, mg/l 490 - 2480 4740 - 19000 2780 - 6280 930 - 1005 1250 - 3160 2120 – 2750
Total solids, mg/l 7870 - 8920 14220 - 40580 2980 - 8240 2220 - 3030 3600 - 6540 2120 – 2750
Total dissolved
solids, mg/l
5580 - 6250 12260 - 38500 2780 - 7900 2060 - 2600 3230 - 6180 1870 – 2360
Suspended solids,
mg/l
2290 – 2670 1960 - 2080 200 – 340 160 - 430 360 - 370 250 – 390
BOD (5 days at
20°C), mg/l
1000 - 1080 2500 - 3480 87.5 - 535 100 - 1222 130 - 820 135 – 1380
COD, mg/l 1650 - 1750 12800 - 19600 1350 - 1675 246 - 381 465 - 1400 410 - 4270
Standards for effluents from textile industry
• Biological treatments: Self-purification that exists in nature. Most of dyes resist
aerobic biological treatment. BAF-based technologies have been developed to treat
wastewater.
• Coagulation–flocculation treatments: Used to eliminate organic substances.
• Adsorption on powdered activated carbon: Reduction of suspended solids and
organic substances, as well as a slight reduction in the color.
• Electrochemical processes: The removal of dyes from aqueous solutions
results from adsorption and degradation of the dye-stuff. sludge formation
is absent in this method.
1) Electrolytic reactions at the electrodes
2) Formation of coagulants in the aqueous phase
3) Removal by sedimentation and floatation
Under optimal conditions, decolorization yields between 90 and 95%, and
COD removal between 30 and 36% can be achieved
• Ozone treatment: Ozone especially attacks the double bonds. It does not
lead to a significant reduction in COD
• Azo dyes precursors and degradation products (such as aromatic amines) are highly
carcinogenic.
• Removal of dyes from wastewater can be effected by chemical coagulation, air
flotation, and adsorption methods.
• advance oxidation is a potential alternative to degrade azo dyes into harmless
species.
Membrane processes
Recommendations for effluent management in textile industries
• The multiple effect evaporation with crystallizer has been installed for recovery
glauber salt.
• Reverse osmosis membrane filtration can produce colourless treated effluent with
dissolved solids as low as 196 mg/l and zero hardness.
• Recycling and reuse of the treated effluent directly conserve natural resources and a
step towards sustainable development.
• Low polluting stream contains nominal dissolved solids so that it meets quality
requirement for its reuse or disposal after treatment.
• For effective effluent management as follows:
1) The highly polluting effluent stream can be segregated and treated separately. This
stream has low volume and as such, it can be disposed of through solar evaporation
pond where adequate land is available.
2) The other low polluting streams can be given primary/secondary/tertiary treatment to
meet the disposal standards or for use in industry for appropriate operations.
3) Dye bath effluent is to be treated using a nano filtration system and wash water can
be treated separately in primary/secondary/tertiary treatment to meet the disposal
standards or for reuse. It can also be treated with reverse osmosis system to recover and
recycle water.
4) The effluent stream arising out of textile processing can be collectively treated using
primary/secondary/tertiary treatment to meet the disposal standards. In case water is
intended to be reused, the treated water can be further purified with use of reverse
osmosis or other methods.
5) The reject stream of reverse osmosis can be disposed of through solar evaporation
ponds or other evaporation system.
References
• Cotton Textile Processing: Waste Generation and Effluent Treatment
The Journal of Cotton Science 11:141–153 (2007)
• Advanced Methods for Treatment of Textile Industry Effluents
Resource recycling series RERES /7/2007
