Wednesday, 28 September 2011

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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