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Treatment And Disposal Of
Textile Effluents
Presented By:Mona VermaPh.D. Research ScholarDeptt. of Textile and Apparel Designing, CCSHAU,[email protected]
Effluent:Effluent is liquid discharged from any source. Effluent can
originate from municipalities industries, farms ,ships, parking lots and camp ground. There is a connotation that effluent contains contaminants but in the strictest sense ,it could be pure water.
The term effluent refers to the left over dyes and auxiliaries which get washed during the manufacturing processes and create pollution.
Sludge:Sludges are the product of biological treatment of waste water.
Sludge comprise solids found in waste water plus organism used in the treatment process.
pH
Temperature
Total suspended solids (TSS)
Total dissolved solids(TDS)
Biological oxygen demands(BOD)
Nature of the effluent depends on:
Chemical oxygen demands(COD)
POLLUTION PROBLEMS IN TEXTILE INDUSTRY
ColourPresence of colour in the waste water is one of the main problems in textile industry.
Colours are easily visible to human eyes even at very low concentration. Hence, colour from textile wastes carries significant aesthetic importance.
Most of the dyes are stable and has no effect of light or oxidizing agents.
They are also not easily degradable by the conventional treatment methods.
Removal of dyes from the effluent is major problem in most of textile industries.
Dissolved Solids:
Dissolved solids contained in the industry effluents are also a critical parameter.
Use of common salt and glauber salt etc. in processes directly increase total dissolved solids (TDS) level in the effluent.
TDS are difficult to be treated with conventional treatment systems.
Disposal of high TDS bearing effluents can lead to increase in TDS of ground water and surface water.
Toxic Metals
Waste water of textiles is not free from metal contents. There are mainly two sources of metals.
1. The metals may come as impurity with the chemicals used during processing such as caustic soda, sodium carbonate and salts.
2. The source of metal could be dye stuffs like metalised mordent dyes. The metal complex dyes are mostly based on chromium.
Others
Textile effluents are often contaminated with non-biodegradable organics termed as refractory materials.
Detergents are typical example of such materials. The presence of these chemicals results in high chemical oxygen demand (COD) value of the effluent.
Organic pollutants, which originate from organic compounds of dye stuffs, acids, sizing materials, enzymes, tallow etc are also found in textile effluent, Such impurities are reflected in the analysis of bio-chemical oxygen demand (BOD) and COD.
EFFLUENT PRODUCTION FROM DIFFERENT PROCESS OF TEXTILE INDUSTRIES
Categorization of Waste Generated in Textile Industry:
Textile waste is broadly classified into four categories, each of having characteristics that demand different pollution prevention and treatment approaches. Such categories are discussed in the following sections:
Hard to Treat Wastes
Dispersible Wastes
Hazardous or Toxic Wastes
High Volume Wastes
Categorization of Waste textile
industry
This category of waste includes those that are persistent, resist treatment, or interfere with the operation of waste treatment facilities.
Non-biodegradable organic or inorganic materials are the chief sources of wastes, which contain colour, metals, phenols, certain surfactants, toxic organic compounds, pesticides and phosphates. The chief sources are:
Colour & metal and dyeing operationPhosphates - preparatory processes and dyeingNon-biodegradable organic materials ,surfactants
Since these types of textile wastes are difficult to treat, the identification and elimination of their sources are the best possible ways to tackle the problem.
Some of the methods of prevention are chemical or process substitution, process control and optimization, recycle/ reuse and better work practices.
1. Hard to Treat Wastes
Hazardous or Toxic Wastes
These wastes are a subgroup of hard to treat wastes. But, owing to their substantial impact on the environment, they are treated as a separate class. In textiles, hazardous or toxic wastes include:
metals,
chlorinated solvents,
non-biodegradable or volatile organic materials.
Some of these materials often are used for non-process applications such as machine cleaning.
High Volume Wastes
Large volume of wastes is sometimes a problem for the textile processing units. Most common large volume wastes include:
High volume of waste water
Wash water from preparation and continuous dyeing processes and alkaline wastes from preparatory processes
Batch dye waste containing large amounts of salt, acid or alkali
These wastes sometimes can be reduced by recycle or reuse as well as by process and equipment modification.
Dispersible Wastes:
The following operations in textile industry generate highly dispersible waste:
Waste stream from continuous operation (e.g. preparatory, dyeing, printing and finishing)
Print paste (printing screen, squeeze and drum cleaning)
Lint (preparatory, dyeing and washing operations)
Foam from coating operations
Solvents from machine cleaning
Still bottoms from solvent recovery (dry cleaning operation)
Batch dumps of unused processing (finishing mixes)
Classification of waste water treatment process
Primary Treatment
After the removal of gross solids, gritty materials and excessive quantities of oil and grease, the next step is to remove the remaining suspended solids as much as possible.
This step is aimed at reducing the strength of the waste water and also to facilitate secondary treatment.
Primary Treatment
Sedimentation
Neutralisation Equalization
Screening
Chemical coagulation
and Mechanical flocculation
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.
The suspended fibres have to be removed prior to secondary biological treatment; otherwise they may affect the secondary treatment system.
They are reported to clog trickling filters, seals or carbon beads.
Sedimentation:
The suspended matter in textile effluent can be removed efficiently and economically by 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.
The most common equipment used includes horizontal flow sedimentation tanks and centre-feed circular clarifiers.
The settled sludge is removed from the sedimentation tanks by mechanical scrapping into hoppers and pumping it out subsequently.
Equalization:
Effluent streams are collected into ‘sump pit’. Sometimes mixed effluents are stirred by rotating agitators or by blowing compressed air from below.
The pit has a conical bottom for enhancing the settling of solid particles.
Neutralisation:
Normally, pH values of cotton finishing effluents are on the alkaline side. Hence, pH value of equalized effluent should be adjusted.
Use of dilute sulphuric acid and boiler flue gas rich in carbon dioxide are not uncommon.
Since most of the secondary biological treatments are effective in the pH 5 to 9, neutralization step is an important process to facilitate.
Chemical coagulation and Mechanical flocculation:
Finely divided suspended solids and colloidal particles cannot be efficiently removed by simple sedimentation by gravity.
In such cases, mechanical flocculation or chemical coagulation is employed.
Specialized equipment such as clariflocculator is also available, wherein flocculation chamber is a part of a sedimentation tank.
In order to alter the physical state of colloidal and suspended particles and to facilitate their removal by sedimentation, chemical coagulants are used.
It is a controlled process, which forms a floc (flocculent precipitate) and results in obtaining a clear effluent free from matter in suspension or in the colloidal state.
The degree of clarification obtained also depends on the quantity of chemicals used. In this method:
80-90% of the total suspended matter, 40-70% of BOD, 30-60% of the COD and 80-90% of the bacteria can be removed.
Most commonly used chemicals for chemical coagulation are alum, ferric chloride, ferric sulphate, ferrous sulphate and lime.
Secondary Treatment
The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by simple sedimentation.
It also removes appreciable amounts of oil and phenol.
In secondary treatment, the dissolved and colloidal organic compounds and colour present in waste water is removed or reduced and to stabilize the organic matter.
This is achieved biologically using bacteria and other microorganisms. Textile processing effluents are amenable for biological treatments.
These processes may be:
aerobic or
anaerobic.
In Aerobic Processes, bacteria and other microorganisms consume organic matter as food.
They bring about the following sequential changes:
(i) Coagulation and flocculation of colloidal matter
(ii) Oxidation of dissolved organic matter to carbon dioxide
(iii) Degradation of nitrogenous organic matter to ammonia, which is then converted into nitrite and eventually to nitrate.
Anaerobic Treatment is mainly employed for the digestion of sludge.
The efficiency of this process depends upon pH, temperature, waste loading, absence of oxygen and toxic materials.
Secondary Treatment
Aerated lagoons
Trickling filters
Oxidation ditch and
pond
Anaerobic digestion
Activated sludge process
Aerated lagoons:
These are large holding tanks or ponds having a depth of 3-5 m and are lined with cement, polythene or rubber.
The effluents from primary treatment processes are collected in these tanks and are aerated with mechanical devices, such as floating aerators, for about 2 to 6 days.
During this time, a healthy flocculent sludge is formed which brings about oxidation of the dissolved organic matter.
BOD removal to the extent of 99% could be achieved with efficient operation.
The major disadvantages are the large space requirements and the bacterial contamination of the lagoon effluent, which necessitates further biological purification.
Trickling filters:
The trickling filters usually consists of circular or rectangular beds, 1 m to 3 m deep, made of well-graded media (such as broken stone, PVC, Coal, Synthetic resins, Gravel or Clinkers) of size 40 mm to 150 mm.
On prepared bed, wastewater is sprinkled uniformly on the entire bed with the help of a slowly rotating distributor (such as rotary sprinkler) equipped with orifices or nozzles.
Thus, the waste water trickles through the media. The filter is arranged in such a fashion that air can enter at the bottom; counter current to the effluent flow and a natural draft is produced.
A gelatinous film, comprising of bacteria and aerobic microorganisms known as “Zooglea”, is formed on the surface of the filter medium, which thrive on the nutrients supplied by the waste water.
The organic impurities in the waste water are adsorbed on the gelatinous film during its passage and then are oxidized by the bacteria and the other micro-organisms present therein.
CONS….
Activated sludge process:
This is the most versatile biological oxidation method employed for the treatment of waste water containing dissolved solids, colloids and coarse solid organic matter.
In this process, the waste water is aerated in a reaction tank in which some microbial flock is suspended.
The aerobic bacterial flora bring about biological degradation of the waste into carbon dioxide and water molecule, while consuming some organic matter for synthesizing bacteria.
The bacteria flora grows and remains suspended in the form of a floc, which is called “Activated Sludge”. The effluent from the reaction tank is separated from the sludge by settling and discharged.
An efficient aeration for 5 to 24 hours is required for industrial wastes. BOD removal to the extent of 90-95% can be achieved in this process.
Oxidation ditch:
This can be considered as a modification of the conventional Activated Sludge process.
Waste water, after screening in allowed into the oxidation ditch.
The mixed liquor containing the sludge solids is aerated in the channel with the help of a mechanical rotor.
The usual hydraulic retention time is 12 to 24 hrs and for solids, it is 20-30 days.
Most of the sludge formed is recycled for the subsequent treatment cycle. The surplus sludge can be dried without odour on sand drying beds.
Oxidation pond:
An oxidation pond is a large shallow pond wherein stabilization of organic matter in the waste is brought about mostly by bacteria and to some extent by protozoa.
The oxygen requirement for their metabolism is provided by algae present in the pond. The algae, in turn, utilize the CO2 released by the bacteria for their photosynthesis.
Oxidation ponds are also called waste stabilization ponds.
Anaerobic digestion:
Sludge is the watery residue from the primary sedimentation tank and humus tank (from secondary treatment).
The constituents of the sludge undergo slow fermentation or digestion by anaerobic bacteria in a sludge digester, wherein the sludge is maintained at a temperature of 35oC at pH 7-8 for about 30 days. CH4, CO2 and some NH3 are liberated as the end products.
Tertiary Treatment Processes
It is worthwhile to mention that the textile waste contains significant quantities of non-biodegradable chemical polymers. Since the conventional treatment methods are inadequate, there is the need for efficient tertiary treatment process. Oxidation techniques: A variety of oxidizing agents can be used to decolorize wastes. Sodium hypochlorite decolourizes dye bath efficiently. Though it is a low cost technique, but it forms absorbable toxic organic halides (AOX) .
Ozone on decomposition generates oxygen and free radicals and the later combines with colouring agents of effluent resulting in the destruction of colours .
The main disadvantage of these techniques is it requires an effective sludge producing pretreatment.
Adsorption
Tertiary Treatment
Electrolytic precipitation
& Foam fractionatio
nElectro
chemical processes
Photo catalytic
degradation
Ion exchange method
Thermal evaporation
Effluent Treatment Practices
Membrane technologies
Electrolytic precipitation & Foam fractionation
The mechanism by which synthetic organic polymer removes dissolved residual dyes from effluents is best described in terms of the electrostatic attraction between the oppositely charged soluble dye and polymer molecules.
Many of the most problematic dye types, such as reactive dyes, carry a residual negative charge in their hydrolysed dissolved form, and sopositively charged groups on the polymers provide the necessary counter for the interaction and subsequent precipitation to occur.
The immediate result of this co-precipitation is the almost instantaneous production of very small coloured particles, having little strength and breaking down at any significant disturbances.
The agglomeration of the coloured precipitates by using appropriate high polyelectrolyte flocculants produces stable flocs.
Electro chemical processes:
It has lower temperature requirement than those of other equivalent non-electrochemical treatment and there is no need for additional chemical.
It also can prevent the production of unwanted side products. But, if suspended or colloidal solids were high concentration in the waste water, they impede the electrochemical reaction.
Therefore, those materials need to be sufficiently removed before electrochemical oxidation
Ion exchange method:
This is used for the removal of undesirable anions and cations from waste water.
It involves the passage of waste water through the beds of ion exchange resins where some undesirable cations or anions of waste water get exchanged for sodium or hydrogen ions of the resin .
Most ion exchange resins now in use are synthetic polymeric materials containing ion groups such as sulphonyl, quarternary ammonium group etc.
Photo catalytic degradation:
An advanced method to decolourize a wide range of dyes depending upon their molecular structure.
In this process, photoactive catalyst illuminates with UV light, generates highly reactive radical, which can decompose organic compounds.
Adsorption:
It is the exchange of material at the interface between two immiscible phases in contact with one another.
Adsorption appears to have considerable potential for the removal of colour from industrial effluents.
Owen (1978) after surveying 13 textile industries has reported that adsorption using granular activated carbon has emerged as a practical and economical process for the removal of colour from textile effluents.
Thermal evaporation:
The use of sodium per sulphate has better oxidizing potential than NaOCl in the thermal evaporator.
The process is ecofriendly since there is no sludge formation and no emission of the toxic chlorine fumes during evaporation.
Oxidative decolourisation of reactive dye by persulphate due to the formation of free radicals has been reported in the literature .
Membrane filtration offers potential applications:
Processes using membranes provide very interesting possibilities for :the separation of hydrolyzed dye-stuffs and dyeing auxiliaries simultaneously reduce coloration and BOD/COD of the wastewaterused to treat reactive dye bath effluent, reduce waste volume and simultaneously recovering salt
The advantages of membrane filtration are because it is a quick method with low spatial requirement and the saturate can be reused.
The disadvantage with the membrane filtration method that it has a limited life time before membrane fouling occurs and the cost is also high.
Membrane filtration
Membrane filtration
Reverse Osmosis
Nanofiltration
Ultrafiltration
Microfiltration
Reverse osmosis membranes have a retention rate of 90% or more for most types of ionic compounds and produce a high quality of permeate . Decoloration and elimination of chemical auxiliaries in dye house wastewater can be carried out in a single step by reverse osmosis. Reverse osmosis permits the removal of all mineral salts, hydrolyzed reactive dyes, and chemical auxiliaries. Greater the energy is required for process of the separation of higher concentration of dissolved salt.
MEMBERANE PROCESS AND THEIR EFFECT USE
Nanofiltration has been applied for the treatment of colored effluents from the textile industry. A combination of adsorption and nanofiltration can be adopted for the treatment of textile dye effluents.
Ultrafiltration enables elimination of macromolecules and particles, but the elimination of polluting substances, such as dyes, is never completeit is only between 31% and 76%.
Microfiltration is suitable for treating dye baths containing pigment dyes as well as for subsequent rinsing baths. The chemicals used in dye bath, which are not filtered by microfiltration, will remain in the bath.Microfiltration can also be used as a pretreatment for nanofiltration or reverse osmosis
Advantages and disadvantages of different effluent treatment processes
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