Our biosphere is under constant threat from continuing environmental pollution. Impact on its atmosphere, hydrosphere and lithosphere by anthropogenic activities on water, air and land have negative influence over biotic and abiotic components on different natural eco-systems. 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, Textile Engineering has 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 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 washing. Due to the nature of various chemical processing of textiles, large volumes of wastewater with numerous pollutants are discharged. Since the stream 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 demanded.

Thus, a study on different measures, which can be adopted to treat the wastewater discharged from textile chemical processing industries to protect our surroundings from possible pollution problem, has been the focus point of many recent investigations. This communication highlights one of such preliminary studies carried out on Amaravathi Common Effluent Treatment Plant situated in Karur, a place near north-central part of Tamil Nadu, India which has recently been known for producing considerable amount of home textiles.

BASIC FEATURE OF THE TREATMENT PLANT

The Amaravathi effluent treatment plant is the common treatment plant which is founded by the group of dyeing factories. Through pipelines the effluent water was collected from 43 dyeing units and treated here. About 200m3 of effluents were treated per day. A schematic diagram of the Amaravathi Common Effluent Treatment Plant is indicated in Figure 1. Dimensions of different plant components are detailed in Table 1.


TREATMENT OF TEXTILE EFFLUENTS

Composition of the feed to the plant:

The feed consists of the different kind of solid and liquid wastes from different textile processing plants. Wet processing of textiles involves unit operations such as desizing, scouring, bleaching, dyeing and finishing. Different auxiliaries are used either in solid or in liquid form to the textile product to obtain the desired effect. Cotton textiles can not be dyed evenly without removing its natural and added impurities, which inhibit the proper penetration of dyes and chemicals. Thus, treatment with various chemical agents such as enzyme, alkalis, acids, salts, surfactants, solvents, oxidizing and reducing bleaching agents etc. are necessary prior to dyeing. In the process of dyeing, auxiliary chemicals viz. glauber’s salt, sodium chloride and other bio-salts are added to exhaust the dye from the liquor to the substrate at appropriate temperature and pH. Even after dyeing, unfixed dyestuffs and complex organic products arising out of the reaction with textile substrate and dyes are to be adequately removed from the surface of the yarns/ fabrics to achieve the proper fastness requirements. So, waste stream from the dyeing industry which is to be fed into the effluent treatment plant essentially comprised of the above ingredients which are used in the preparatory processes and in actual dyeing process.

The effluent treatment method is broadly classified into three main categories: physical, chemical, and biological treatments. There are four stages, preliminary, primary, secondary, secondary, and tertiary treatments to treat the effluents. The preliminary treatment processes are equalization and neutralization. The primary stages involve screening, sedimentation, floatation, and flocculation. Secondary stages are used to reduce the organic load, facilitate physical / chemical separation and biological oxidation. Tertiary stages are important because they serve as polishing of effluent treatment.

Plant Operation

The effluent water was collected from 43 processing units and treated by the following stages.

1. Screen chamber:

Objective screens are provided to remove relatively large solids to avoid abrasion of mechanical equipments and clogging of hydraulic system.

2. Collection tank:

The collection tank collected the effluent water from the screening chamber and stored it, and then pumped it to the equalization tank.

3. Equalization tank:

The effluents are not having similar concentrations at all the time; the pH will vary time to time. The effluents were stored from 8 to 12 hours in the equalization tank. This will result in a homogenous mixing of effluents and helps in neutralization. In addition, it eliminates shock loading on the subsequent treatment system. Continuous mixing also eliminates settling of solids within the equalization tank.

4. Flash mixer:

Coagulants were added to the effluents. They are
  Lime    (800-1000 ppm) -   To raise the pH 8-9
  Ferrous sulphate    (200-300 ppm) -   To remove colour
  Poly electrolyte    (0.2 ppm)-   To settle the suspended matters

According to the above proportions the chemicals were added and mixed with the effluents. The addition of the above chemicals by efficient rapid mixing facilitates homogeneous combination of flocculates to produce microflocs.

5. Clarriflocculator:

In the clarriflocculator the water was circulated continuously by the stirrer. Overflowed water was taken out to the aeration tank. The solid particles were settled down, and collected separately and dried. Flocculation provides slow mixing that leads to the formation of macro flocs, which then settles out in the clarifier zone. The settled solids i.e. primary sludge was pumped into sludge drying beds.


6. Aeration tank:

The water was passed like a thin film over the staircase arrangement. Here the water got direct contact with the air to dissolve the oxygen into water. BOD value of water was reduced up to 90%.

7. Clarifier:

The clarifier collects the biological sludge. The overflowed water is called as treated effluent and it was disposed out. The outlet water quality is well within the tolerance limit as delineated in the norms of the Bureau of Indian standards. Through pipelines, the treated water was disposed into the river water.

8. Sludge thickener:

The inlet water consists of 60% water and 40% solids. The effluents were passed through the centrifuge. Due to centrifugal action, the solids and liquids were been separated. The sludge thickener reduces the water content in the effluent to 40% water and 60% solids. The effluent was then reprocessed and the sludge was collected at the bottom.

9. Drying beds:

Primary and secondary sludge was dried on the drying beds. Here the sludge was subjected to solar evaporation.


DIFFICULTIES OF OPERATION

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. Therefore, the treatment of waste stream needs to be done by various methods, which include physical, chemical and biological treatment depending on pollution load.

SOLID WASTE DISPOSAL

Sludge management is the final stage in textile effluent treatment process. Huge amount of sludge was generated after textile effluent treatment. The sludge was collected and packed in polythylene bags and finally covered in the water proof sheets. Sludge should be disposed off in an offsite designated landfill area recognized by the State Pollution Control Board of Tamil Nadu, India.

TREATED WATER DISPOSAL

The treatment plant disposed off the treated effluent into the river water. In Karur, availability of soft water is more. So the treated effluents are not reused. This treated water is in compliance with the standards of agricultural usage. The treatment plant strictly follows the rules and regulation of the Bureau of Indian Standards.

CONCLUSION

The treatment of textile effluent is the necessary one to safe guard the environment from the pollution problems. Physical, chemical and biological treatments were given to the effluents. The sludge was remaining as a solid mass; there is no solution for sludge disposal. Technologies have to be developed for proper disposal of the sludge.

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