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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Effluent Treatment in Textile Industry-1

The idea for this topic is triggered by the news of discovery of treatment of textile effluent by a combination of biological and physico-chemical methods.

Speaking simply, the effluent control is effected by two approachs, physcial and chemical. However these methods are expensive and non effective. Using both biological and chemical methods, one can achieve effective effluent control. 

Read the full story here .

Industrial Textile processing comprises several processes which include pretreatment, dyeing, printing and finishing processes. Besides consuming large amount of energy and water, these processes generate lot of waste products.  

Generally the effluents generated in textile processes have the following shorcomings:

- Heavily Colored

- Contain High concentration of Salts

- Exhibit High BOD ( Biochemical Oxygen Demand) and COD ( Chemical Oxygen Demand)

Understanding COD and BOD

COD is the total measurement of all chemicals in the water that can be oxidised.  BOD is supposed to measure the amount of food ( or organic matter ) that bacteria can oxidise. Permissible limit of COD is 250 to 500 ppm and BOD is 30mg/l.

To explain it further, the microbes present in polluted water consume the dissolved oxygen for respiration and nitrification. These bacteria consume pollutants and then use dissolved oxygen to convert the pollutants into energy. Other bacteria consume ammonia to nitrate a process called nitrification. Please see the pictures here for illustration.

This is important to control COD and BOD as a water high in BOD can deplete oxygen in the receiving waters, causing fish kills and ecosystem damages. Low BOD also helps in further treatment.

A typical textile mills effluent has a pH of 9.8 to 11.8, Total alkalinity as CaCO3 17-22 mg per liter, BOD 760-900 mg/l, COD 1400-1700 mg/l, total solids 6000-7000 mg/liter and total chromium 10-13 mg/l.

Three methods are suggested to reduce pollution

1. Use of new, less polluting technologies

2. Effective Treatment of effluent

3. Recyling waste several times over before discharge. 

The following are the most common causes of generation of effluent wasts in textile industry:

Desizing

In general 50% of the water pollution is due to waste water from desizing. 

Problem with Effluent from Desizing

It has high BOD which renders it unsuitable. 

Solutions to Control

- Using enzymes that degrade starch into ethanol. This can be recovered by distillation

- Using oxidative system like H2O2 that degrade startch to CO2 and Water. 

- Using Electro-oxidation

- Treatment with Mixed activated Sludge system. 

In mixed activated sludge system, microorganisms are introduced which convert carbon in the effluent into suspended solids and carbon dioxide and water. The solids are then separated from the wastewater in the settling tank and then recovered.

As most of the dyes are not biodegradable, this method of using activated sludge is not always successful. 

Mercerisation

Problems:

The effluent has high concentration of NaOH

Solutions:

- Recovering NaOH from waste water using membrane techniques. 

- Use of Zinc Chloride during mercerisation

Source: 1

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