Effluent Tratement- Kerosene Recovery from Pigment Printing

Effluent Treatment- Kerosene Recovery from Pigment Printing

In India about 46% of the total printing is done using pigment printing method, 27% using reactive dyes, 15% Disperse dyes, 2% napthol, 3% acid and 7% using vat colors.

In a typical pigment printing paste, 75% is keorsene, 10% is binder, 5% each is gum and water, 2% urea and 1% each of DAP and emulsifier. Kerosene water emulsion acts as a thinkner is pigment printing. Its use is prohibitied in most of the developed countries. Alternatives to kerosene are synthetic thickners but they make the printed fabric stiff.

In India alone, about 140 thousand kiloliters of kerosene per annum is consumed during the process of printing and drying. Out of the total kerosene applied, about 22% is lost at various points during this process( 1 % is left out on the fabric, 12% screen printing and wastage, 5% is before dryer, 4% is at curing machine and 78% is released or evaporated in the atmosphere during the process of drying at 120-150 deg C). It signifies a loss of precious kerosene as well as pollution in the environment.

Apart from the effluents generated during wet processing, there is a substantial amount of kerosene vapour that is released in the air in the process of pigment printing.

Any kerosene recovery process should be based on the fact that kerosene is liquid at room temperature and immiscible with water. The kerosene vapour are at 120 deg C and they have to be cooled below 40 deg Celcius. The kerosene and water will separate out in two layers, with top layer can be skimmed for reuse.

An excellent effluent treatment plant is proposed by BTRA. Average recovery is 58% whereas maximum recovery can be 85%. Read more about it here

To view the pigment printing process and alternative to kerosene, please find the link here.

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Process Flow of Effluent Treatment in a Textile Industry-1

Process Flow of Effluent Treatment in a Textile Industry

Typically the following process flow of effluent treatment is followed in a typical textile industry;

Screening

The purpose of screening is to filter out the loose impurities. Thus wood, paper plastic bags etc. can be removed by this method. A method of progressive filtering is followed where first the matter is passed through coarse screens and then through fine screens.

Oil and Grease Removal

Many units discharge water mixed with oil. The purpose of this equipment is to remove the oil. Since surface density of oil and grease is less than water, the oily substance come on the surface of the water and can be skimmed.

Cooling and Equilization

The effluent from the primary treatment is cooled ( cooling towers are employed for this purpose) and then uniformly mixed using equlisation tank. It takes in any sudden gush of effluents as well as slow rate of effluent and feeds to the subsequent processes in a uniform way.

pH Adjustment ( Acid or Alkaline Dosing)

The waste from the Textile Industry is rarely pH neutral. To increase the efficiency of biological treatment and coagulation/flocuculation a pH of 6-8 is needed. Generally Sodium hydroxide is used to neutralise acidic waste and hydrochloric acide is used to neutralize alkaline waste. Generally the effluents from the dyeing industry have high pH and hence an acid addition is required.

Aeration 

It is reuqired to control BOD. Two main methods are used: one in which water is mechanically agitated so that air from atomoshphere may enter into the water, second method is introducing the air in the water through blowers and using diffusers to diffuse the air uniformly.

Prechlorination

Generally organic matter present in the effluent takes oxygen from the water, which increases its oxygen demand, to avoid that chlorine is added to oxidise the matter.

Clarification

The purpose of clarification is to remove any suspended solids by coagulation and flocculation. It is done using flash mixer. In flash mixer, alum solution is dosed as coagulant.

The flocculated water flows upwards towards tube settler. The suspended solids settle down.

This settling can also be done using lamella filter

Lamella Filter

It achieves solid liquid separation by directing the liquid between a seris of inclined plates called lamellae. It settles suspnded solids by gravity.



To be continued

An Excellent Document on Denim Effluent Treatment Process is here

Sources:

http://www.scribd.com/doc/4678823/Etp-Manual-Std-Denim

http://www.sharpengineering.co.in/Iffluent_Equipments.aspx

http://www.jfc-cetp.com/envinfo.html

http://www.iichemrc.org/Lamella.pdf

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Effluent Generated at various Processes of Textile Manufacturing

Effluent Generated at various Processes of Textile Manufacturing

Textile manufacturing involves conversion of Fibers into garments. At each process some amount of liquid is used. However, it is not that each process leads to generation of efflents. Typically coversion to yarn involves no generation of effluents. It is the sizing and processing where most of the efflent is generated which needs to be treated. Given below is a list of the processes along with the information about effluent generated.

1. Fiber Preparation and Yarn Spinning: 

There is very little effluent generated in these processes.

2. Slashing/Sizing

The Effluent contains BOD, COd, metals, cleaning waste and size.

3. Weaving, Knitting and Tufting

There is very little effluent generated in these processes.

4. Desizing

The effluent contains BOD from water-soluble sizes, synthetic size, lubricants, biocides and anti-static compounds.

5. Scouring

The effluent contains disinfectants and insecticide resudes, NaOH, detergents, fats, oils, pectin, wax, knitting lubricants, spin finishes and spent solvents.

6. Bleaching

The effluent contains hydrogen peroxide, sodium silicate or organic stabilizer. The effluent also contains high pH.

7. Singeing and Heat Setting

There is very little effluent generated in these processes.

8. Mercerising

The effluent contains high pH and Sodium Hydroxide.

9. Dyeing

The effluent contains metals, salts, surfacftants, toxics, organic processing assistants, cationic material, color, BOD, sulfide, acidity or alkalinity and spent solvents.

10. Printing

The effluent contains suspended Solids, urea, solvents, color, metal, heat , BOD and foam.

11. Finishing

The effluent contains BOD, COD, suspended solids, toxics and spent solvents.

Read Also

Textile Effluent Treatment-1
Textile Effluent Treatment-2

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Effluent Treatment in Textile Industry-2

Bleaching and Dyeing

It is estimated that to dye 1 kg of cotton with reactive dyes, 0.6-0.8 kg of NaCl, 30-60 grams of dyestuff and 70-150 liter of water is required. Once the dyeing operation is over, the wastewater must be treated before reuse. Coagulation and Membrane Technique ( nanofilteration  or reverse osmosis) are among the processes suggested for the treatment of water. 

Coagulation and Flocculation

Natural and wastewater contain small particles. They are suspended in water in a form called as a colloid. These particles carry the same charges, and repulsion prevents them from combining into larger particles to settle. Thus, some chemical and physical techniques are applied to help them settle. The phenomenon is known as coagulation. A well known method is the addition of electrolyte. Charged particulates combine with ions neutralizing the charges. The neutral particulates combine to form larger particles, and finally settle down. Historically Alum is used for this purpose but it makes the pH of the solution slightly basic. 

Another method is to use high-molecular-weight material to attract or trap the particulates and settle down together. Such a process is called flocculation. Starch and multiply charged ions are often used.

Here the basic advantage is that the dye molecules themselves are removed which is better than other methods where dye molecules are decomposed and produce harmful and toxic aromatic compounds. 

The disadvantage is that in coagulation process, large amount of sludge is created which may become  a pollutant itself and increase the treatment cost.

This method is useful for removing the insoluble dyes, but the cost of treating the sludge increases.

Ultrafilteration and Nanofilteration


Ultrafilteration filters substances with sizes less than  than 10^-7 to 10^-8 m . It can effectively remove suspended organic solids. It can not remove multivalent ions. It needs low water pressure to operate.

Nano filteration filters substances with size less than 10^-8 to 10^-10 m. It can remove multivalent ions. 

Reverse Osmosis

It can remove substances with size less than 10^-9 to 10^-11m. It can remove multivalent as well as monovalent ions.

When a compartment containing a dilute solution is connected to another compartment containing a concentrated solution by a semipermeable membrane, water molecules move from the dilute solution to concentrated solution. This phenomenon is called osmosis.

By applying pressure in the higher concentration solution, water molecules migrate from a high concentration solution to a low concentration solution. This method is called reverse osmosis water filter system. ( Source )

An excellent FAQ on Reverse Osmosis can be found here .

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