Our biosphere is under constant threat from continuing environmental pollution. Impact on its atmosphere, hydrosphere and lithosphere by anthropogenic activities can not be ignored. Man made activities on water by domestic, industrial, agriculture, shipping, radio-active, aquaculture wastes; on air by industrial pollutants, mobile combustion, burning of fuels, agricultural activities, ionization radiation, cosmic radiation, suspended particulate matter; and on land by domestic wastes, industrial waste, agricultural chemicals and fertilizers, acid rain, animal waste have negative influence over biotic and abiotic components on different natural eco-systems. Some of the recent environmental issues include green house effect, loss in bio-diversity, rising of sea level, abnormal climatic change and ozone layer depletion etc.

In recent years, different approaches have been discussed to tackle man made environmental hazards. Clean technology, eco-mark and green chemistry are some of the most highlighted practices in preventing and or reducing the adverse effect on our surroundings.

Among many engineering disciplines Civil Engineering, Mechanical Engineering, Electrical Engineering etc., Textile Engineering has a direct connection with environmental aspects to be explicitly and abundantly considered. The main reason is that the textile industry plays an important role in the economy of the country like India and it accounts for around one third of total export. Out of various activities in textile industry, chemical processing contributes about 70% of pollution. It is well known that cotton mills consume large volume of water for various processes such as sizing, desizing, scouring, bleaching, mercerization, dyeing, printing, finishing and ultimately washing. Due to the nature of various chemical processing of textiles, large volumes of waste water with numerous pollutants are discharged. Since these streams of water affect the aquatic eco-system in number of ways such as depleting the dissolved oxygen content or settlement of suspended substances in anaerobic condition, a special attention needs to be paid. Thus a study on different measures which can be adopted to treat the waste water discharged from textile chemical processing industries to protect and safeguard our surroundings from possible pollution problem has been the focus point of many recent investigations. This communication highlights such studies carried out in the area of textile effluent treatment.

Sources and Causes of Generation of Textile Effluent

Textile industry involves wide range of raw materials, machineries and processes to engineer the required shape and properties of the final product. Waste stream generated in this industry is essentially based on water-based effluent generated in the various activities of wet processing of textiles. The main cause of generation of this effluent is the use of huge volume of water either in the actual chemical processing or during re-processing in preparatory, dyeing, printing and finishing. In fact, in a practical estimate, it has been found that 45% material in preparatory processing, 33% in dyeing and 22% are re-processed in finishing [1]. But where is the real problem? The fact is that the effluent generated in different steps is well beyond the standard and thus it is highly polluted and dangerous.

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:

1.Hard to Treat Wastes

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  dyeing operation
Phosphates  preparatory processes and dyeing
Non-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.

2. 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.

3. 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.

4.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)

SEARCH FOR SOLUTION

Each type of waste / waste stream represents an individual problem which can be solved only by taking into consideration the following factors:

(i)Local conditions
(ii)Dyestuff and chemical used
(iii)Amount and composition of the waste water
(iv)Local drainage conditions
(v)Region
(vi)Main sewage channel
(vii)Sewage characteristics etc.

Our aim is to adopt technologies giving minimum or zero environmental pollution. Effluents treatment plants are the most widely accepted approaches towards achieving environmental safety. But, unfortunately, no single treatment methodology is suitable or universally adoptable for any kind of effluent treatment. For instance, in the past, biological treatment systems had been used extensively but they are not efficient for the colour removal of the more resistant dyes [2]. Therefore, the treatment of waste stream is done by various methods, which include physical, chemical and biological treatment depending on pollution load. The treatment processes may be categorized into preliminary, primary, secondary and tertiary 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.

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, neutralisation 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.

In mechanical flocculation, the textile waste water is passed through a tank under gentle stirring; the finely divided suspended solids coalesce into larger particles and settle out. 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, 5days, 30-60% of the COD and 80-90% of the bacteria can be removed. However, in plain sedimentation, only 50-70% of the total suspended matter and 30-40% of the organic matter settles out. 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 [3]. 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. Some of the commonly used biological treatment processes are described below:

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, over which 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 micro-organisms 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.

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 floc 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. A part of the sludge is recycled to the same tank to provide an effective microbial population for a fresh treatment cycle. The surplus sludge is digested in a sludge digester, along with the primary sludge obtained from primary sedimentation. 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) [4]. Ozone on decomposition generates oxygen and free radicals and the later combines with colouring agents of effluent resulting in the destruction of colours [5]. Arslan et al. investigated the treatment of synthetic dye house effluent by ozonisation, and hydrogen peroxide in combination with Ultraviolet light [6]. The main disadvantage of these techniques is it requires an effective sludge producing pretreatment.

Electrolytic precipitation & Foam fractionation: Electrolytic precipitation of concentrated dye wastes by reduction in the cathode space of an electrolytic bath been reported although extremely long contact times were required. Foam fractionation is experimental method based on the phenomena that surface-active solutes collect at gas-liquid interfaces. However, the chemical costs make this treatment method too expensive [7].

Membrane technologies: Reverse osmosis and electrodialysis are the important examples of membrane process.

The TDS from waste water can be removed by reverse osmosis [8]. Reverse osmosis is suitable for removing ions and larger species from dye bath effluents with high efficiency (upto > 90%), clogging of the membrane by dyes after long usage and high capital cost are the main drawbacks of this process.

Dyeing process requires use of electrolytes along with the dyes. Neutral electrolyte like NaCl is required to have high exhaustion of the dye. For instance, in cotton dyeing, NaCl concentration in the dyeing bath is in the range of 25-30 g/l for deep tone and about 15 g/l for light tone, but can be as high as 50 g/l in exceptional cases. The exhaustion stage in reactive dyeing on cotton also requires sufficient quantity of salt.

Reverse osmosis membrane process is suitable for removing high salt concentrations so that the treated effluent can be re-used again in the processing. The presence of electrolytes in the washing water causes an increase in the hydrolyzed dye affinity (for reactive dyeing on cotton) making it difficult to extract.

In electrodialysis, the dissolved salts (ionic in nature) can also be removed by impressing an electrical potential across the water, resulting in the migration of cations and anions to respective electrodes via anionic and cationic permeable membranes. To avoid membrane fouling it is essential that turbidity, suspended solids, colloids and trace organics be removed prior to electrodialysis.

Electro chemical processes: They have 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 [9].

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 [10]. 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 [11]. In this process, photoactive catalyst illuminates with UV light, generates highly reactive radical, which can decompose organic compounds [12].

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 [13].
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 [14].

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 [15].

Effluent Treatment Practices: Textile industry encompasses a range of unit operations, which use a wide variety of natural and synthetic fibres to produce fabrics. Textile units generally follow various treatment schemes. Two of such are shown in the scheme 1 and 2 as typical cases.

Concluding Remarks

The quality of life depends on the ability to manage available water in the greater interest of the people. Water depletion of good quality water and environmental pollution has given tremendous importance to the water management. Joints efforts are needed by water technologists and textile industry experts to reduce water consumption in the industry. While the user industries should try to optimize water consumption, water technologists should adopt an integrated approach to treat and recycle water in the industry.

Our motto is to save living species and its surrounding environment. Thus we must stop using chemicals and dyes, which produce harmful effect to the biotic and abiotic factors in our eco-systems. Reduction of waste at the source is the preferred strategy instead of the traditional method of end of pipe waste treatment. Apart from problematic chemicals and dyes, the main pollutant is, of course, water. So, the new technologies, which aim to reduce or eliminate water, are to be conceived.