Recently we asked Dr. C.N. Sivaramakrishnan, a distinguished senior scientist from the Society of Dyes & Colorists in India to respond to some questions to give us a sense of the sustainability challenges and opportunities in the dyeing industry in India. The ACS GCI published Part One in January 2015 and below it Part Two.
Q: Is there a particular case study that you would highlight as a success story [of more sustainable dyeing in India]?
A: Arvind Mills is one of the largest integrated textile mills in India and popular for producing world renowned 'Denim' brand jeans. It supplies to almost all the famous jeans, trousers, and garments making units in the world. The unit is located at a distance of approximately 40 kms away from the city of Ahmadabad, Gujarat, India and has been set up on 385 acres of land.
The unit may be an example for end-of-pipe pollution abatement technology for achieving a zero discharge of liquid effluent. It is a fact that integrated textile mills are a water intensive industry. From that point, achieving zero discharge deserves special mention and appreciation. The only source of water here is ground water. Ahmadabad is a developing city, and like all other developing cities, rapid ground water depletion is an important environmental issue in the area. The State Pollution Control Board has restricted the extraction of the ground water. It is because of this compulsion as well as from their quest for making the product cost effective, the unit has adopted suitable measures to bring down the cost of utilities with the support from their research and development wing.
Zero discharge means the entire effluent that is being generated from the different unit operations in the process plants are recycled in the process. For this the unit has adopted a multi-prong approach ranging from the separation of raw effluent on the basis of pollution load to judicious mixing and blending, after primary treatment, with the other highly concentrated pollution load-bearing effluent at the appropriate level of the treatment procedure to the adoption of proper tertiary and polishing treatment technology for making the effluent suitable for reuse in the process without hampering the quality of the product.
Water consumption pattern: Major contributions of raw effluent are from the sizing & desizing, scouring, dyeing and bleaching, and mercerizing sections of the industry. Apart from these main unit operations, a substantial quantity of effluent is generated from the humidification section. Concentration of pollutants and quantity of effluent from these sections may vary depending upon the scale of production, chemicals used and technologies adopted. The following table may give an idea for raw water requirements in similar types of industries. Water loss is approximately 20 percent, mainly from the humidification section.
Considering that the unit produces 180 gsm of fabrics with the width of 140 – 150 cm (approximately 59 inches), water requirements come out to be approximately 24.3 litres kg of fabrics produced. The unit produces 5 million meters of fabric with different product mixes as per customer requirements. It generates 10 MLD of process effluent. The same can also be verified considering the average consumption of 24.3 litres per meter of different type of above mentioned product mix.
Flow rate of the effluent generation from different unit operations are given in the table
Effluent characteristics and its treatment facility: Effluent characteristics of a textile unit depend on the type of fabrics being processed, the colors used, whether printing is done, on the chemicals used during mercerization, and the dye fixing process. Apart from the main processes, a substantial quantity, in fact a major quantity of effluent is generated from the washing carried out in between successive processes. For example, in the dye house, after dyeing activity, fixation of dye is one of the most important stages. Usually, 70-80% of fixation is practicable and the rest, i.e. 20% of the dye used, comes out in the effluent generated due to washing. Effluent generated from the dye house has a high concentration of pollutant as compared to other processes in the textile processing unit. It also contains a high amount of inorganic salts like sodium sulphate or sodium chloride, which is used for dye fixing and acts as an electrolyte.
Another important unit operation is mercerization. Mercerization imparts the shining characteristic to the fabric. Washing after mercerization generates typical effluent containing caustic solution and other impurities. In the case of fabrics, caustic is used, but in the case of polyester, sulphuric acid is used. Bleaching is done by peroxide method.
The unit has an elaborate effluent treatment plant consisting of judicious segregation of effluent stream on the basis of pollution load and mixing of the same at an appropriate stage. The treatment processes may be divided into three parts namely i) main treatment facility, ii) pre-treatment (prior to reverse osmosis) and iii) reverse osmosis.
Main treatment facility: Streams of effluent generated from the sizing & de-sizing, bleaching & mercerizing and humidification sections are subjected to physico-chemical processes, i.e. the effluent is collected in an equalization tank. After pH adjustment and addition of poly electrolytes (as coagulant), the effluent is sent to the clarifier for sedimentation.
Effluent from the dye house is collected separately in an equalization tank where pH is adjusted and a chemical is added in acidic medium (pH 5.5) to decolorize the effluent. The unit uses a chemical (brand name Micro plus) which is claimed to act as a color removal agent. This effluent is then mixed with the entire effluent from the mercerized, sizing & humidification sections. These effluents are then fed into the primary clarifier followed by a conventional biological system comprising of degradation of organic components by microorganisms followed by sedimentation in a clarifier and return of bio-mass to the aeration unit.
In the aeration unit, retention time is approximately 16 hrs, MLSS is kept at 2500 and DO level is maintained at 2.5 to 3.0.
Sludge generated from the biological treatment facilities are sent to a sludge drying bed and leachate is sent back to the aeration unit. Effluent is then sent to the primary or pre-treatment facility. The unit has a large storage tank capable of holding 10ML of treated effluent.
Pre-treatment facility: Pretreatment or primary treatment facilities are adopted before the effluent is subjected to reverse osmosis. This stage comprises of two unit operations in succession: turbocirculator followed by pressure sand filter. The turbo circulator is basically a flash mixer.
After main treatment facility, poly aluminum chloride, poly electrolyte are added in the effluent and are passed to turbo-circulator and then to the sand filter before being subjected to reverse osmosis. The unit has intermediate storage tank (capacity 2400 cu.m) for storage of the treated effluent.
Reverse Osmosis: After pre-treatment the effluent is sent to the reverse osmosis plant. Osmosis, as we know, is a natural process and is the tendency of two liquids of different concentrations separated by a semi-permeable membrane, to move from low to high concentrations for chemical potential equilibrium. But in reverse osmosis (RO), when high pressure is applied, liquid moves from high concentration to lower concentration. RO is a method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a membrane. The result is that the impurities are retained on the pressurized side of the membrane and the pure water is allowed to pass to the other side. The reject of the reverse osmosis plant is fed into the desalination plant (thermal). Backwash of the sand filter is fed into the main treatment facility.
Reject of the reverse osmosis plant is fed into the desalination plant (thermal). Backwash of the sand filter is fed into the main treatment facility.
Other salient features: In the textile industries there are certain processes which generate typical effluent that require special treatment since its recovery gives the cost benefit to the entrepreneur. One such treatment process is the caustic recovery plant.
The caustic recovery plant is a multiple effect evaporator followed by condenser. Recovered caustic is reused and water (effluent) is recycled. It saves approximately 30% of the raw material (caustic soda) cost even after considering the operating cost of the caustic recovery unit. Caustic requirement for this unit is approximately 10 MT/day on 25 % concentration basis. Out of the 10 MT of caustic, 0.5 % is retained by the fabric and the rest goes to the effluent.
Total cost of treating the effluent for the said unit is approximately Rs. 45/ cu.m. of effluent including RO plant cost.
“The Nexus Blog” is a sister publication of “The Nexus” newsletter. To sign up for the newsletter, please email email@example.com, or if you have an ACS ID, login to your email preferences and select “The Nexus” to subscribe.
To read other posts, go to Green Chemistry: The Nexus Blog home.