Zero discharge in textile dyeing

The words of Thomas fuller (Gnomologia,5451)

“we never know the worth of water till the well is dry”

are we  going to wait till that long?

Water is an essential ingredient in the textile wet processing , the availability , quality and cost are issues of concern worldwide at one hand and the issues on other hand like environmental aspects, public health, treatment and recycling cost  and legislative issues regarding disposal of effluent and sludge threatening the textile industry in many countries.

The textile processing industry is mainly now shifted to the developing countries from the developing world creating a huge potential for textile exports and earning foreign currency  , but unless the water is used judiciously , it would not last too long and we will run out of this precious resource very soon.

Therefore the need of the hour is to understand the extent of the problem and implement a proper water management program for the usage , treatment and disposal of effluents. Water is being a major chemical in the textile dyeing , without which dyeing is impossible, therefore we  should take appropriate steps , not only to meet the statutory requirements but also to save this resource for the coming generations. The implementation of water management program also important for the textile industry ,to remain competitive and meet the customer requirement specifically in exports . It is essential to reduce the  overall water requirement , effluent treatment , recycling and disposal of wastes.

CHARACTERISTICS OF TEXTILE EFFLUENT

The effluent from dye houses  consists of a mixture of all the chemicals used in the dyeing process plus materials derived from the fabric/fiber or yarn being dyed. For cotton dyeing small fibers of cotton detached from the fabric during scouring/bleaching, dyeing and washing stages. During the scouring and bleaching process some components of the cotton fibers such as pectin, waxes and oils are also dissolved and dirt will be released. So the effluent from a dyeing unit contain among other things, sizing agents cellulose and synthetic fibers, synthetic dyes, common salt, sodium sulphate, caustic soda and soda ash, surfactants, acetic acid from neutralization stages, finishing chemicals , oxidizing and reducing agents  and any auxiliaries used.

ZERO DISCHARGE

Textile processing industry is one of those identified as a potential threat to the environment because of discharge of huge quantities of waste water loaded with toxic chemicals  in the form of effluents , which cause air , water and soil pollution. Although it is mandatory to install an effluent treatment plant and treat the waste water to conform with the standards laid down by the legislation, but importance of recycling is increasing day by day ,with the decreasing quality and quantity of fresh water and increasing cost.

First of all we should remember the principle of 3 R,

Reduce:- reducing the water and chemical consumption is the first step towards zero discharge. Follow few simple rules such as

1.Repair the water leaks and faulty valves.turn off running taps and hoses.

2.Turn off  water supply when the machines are not running.

3.Reduce and optimise the number of steps in a processing cycle.

4.Reduce the process water , by working at a lower liquor ratios ,where less water levels can be used.use intermiitent washing instead of overflow washing.

5.Implementing maintenance plan for steam traps

6.Installing a digital monitoring system for boilers

7.Installing a peak load generator to minimize power consumption.

8.Installing screens in drain lines to reduce solids in effluents

9.Installing a digital hardness monitor in the softener system to conserve water and chemicals

10. Use counter current washing techniques in continuous washing machines.

11.Delivering training to operators to reduce power and water consumption

12. Reuse standing bath in operations such as sulphur black dyeing.

Recover

Segregate and reuse the water , which can be recycled without any treatment.

Collect hot condensate and reuse it.

Recover heat from the hot waste water and use it for heating fresh water.

Recover chemicals from the waste water , such as sizing chemicals, caustic recovery in the mercerising etc.

Effluent can be treated in a number of different ways depending on the composition of the effluent and the level of treatment required. These levels could be classified into the following types: Preliminary, Primary, Secondary, and T

Recycle

Recycling of cone-dye cooling water

Recycle air compressor cooling water

Recycling of air conditioning system water

Recycling rinse water from the bleaching, washing and softening stage

Stages in textile waste water treatment for zero discharge,

ertiary. The mechanisms for treatment can be divided into three broad categories: Physical, Chemical, and Biological. Many of these processes are used together in a single treatment plant.

Primary treatment:-

Primary treatment is the first step in the treatment process rather than simply preparing the effluent for treatment it actual reduces the pollution potential. In the treatment of dye house effluents settling is not normally used as a primary treatment process. pH control is an important aspect of primary treatment of textile wastes as these often have extreme pHs which need to be adjusted prior to discharge in order to meet effluent standards. pH control is also important to prepare effluent for biological treatment as extreme pH can kill or inhibit microbes in treatment plants. Following are the different steps in primary treatment,

Screening, straining
This first step of treatment is to remove small particles from the process water. In this way the water will be cleaned from fibers, fluff and cotton flock. For these filters, drum screens- and fine filters are used.

Oil and grease removal

If during the step of textile treatment, solvents like white spirit or others are used they have to be removed from the wastewater. Oil Separators and oil skimmers are used to remove the oil and the other organic solvents from the wastewater so to avoid the microorganisms from being killed.

Equalization or Homogenization

This step is useful in order to mix and homogenize the wastewater. With this step, the pollution is better distributed. That makes it easier for microorganisms to treat the water. The result is a more effective biological treatment

Neutralization
After homogenization, the solution has a pH of around 9 to 10. Neutralization of the water can be done by acid or air flow injector depending on the pH value.

Physical- chemical- treatment

If the concentration of dissolved solids is very high (sulfides, chromates, etc.) and/ or color is also in the water, the kind of treatment is various.Possible procedures:

• Oxidation of sulfides
• Flocculation
• Color-removal with flotation

Secondary treatment

Secondary treatment biologically purifies settled effluent. Mainly bacterial micro organisms digest the effluent and remove the majority of organic compounds. Once wastewater has gone through the primary treatment stage the effluent will undergo a secondary treatment in order to remove both small suspended solids and BOD₅ (five day biochemical oxygen demand) that pass through the primary treatment stage. All secondary treatment systems use a biological process to break down organic matter. Microorganisms are introduced to the wastewater and consume the organic matter, oxygen is delivered to the system ensuring microorganism survival. Oxygen delivery differs among the various systems. This biological process occurs naturally in nature, but is accelerated in secondary treatment systems. Typically 85% of BOD and suspended solids are removed during this process. Water exiting secondary treatment will still carry nitrogen, phosphorus, heavy metals, pathogens, and bacteria. For further removal of pollutants the water is transported to a tertiary treatment system and disinfection. There are a variety of secondary treatment processes; the following are conventional processes used by treatment plants:

Activated sludge

Surface-aerated basins

Trickling filter

Oxidation ponds

There are pros and cons to each of these three processes. Operational and initial costs along with space are three factors that will often determine which technique is appropriate. Space is influenced by population size and cost of land. For example, oxidation ponds require large areas of land if land is costly or needed for housing oxidation ponds are not a likely option. Additionally wastewater treatment plants need to consider maintenance, reliability, and effectiveness of the system.

Tertiary treatment

Tertiary treatment describes any additional treatment process designed to achieve even higher standards of water quality. Tertiary treatment processes can further remove suspended solids, microorganisms, inorganic minerals (like ammonia, nitrate or phosphate) and  colour. Such tertiary treatments are often used to create very high quality effluents from ETPS which discharge it environmentally sensitive areas or recycled.

Ultarfiltration and Reverse osmosis

Ultra filtration and RO are based on the selective permeability of some membranes that are selective toward water and dissolved matters: at a pressure higher than the osmotic one water passes through membrane and dissolved matter is retained RO: membrane permeable to water, not to dissolved solids.

Ultra filtration membrane selectivity is due to a mechanism similar to the filtration: the dissolved matter molecules, of bigger size, don’t pas through the membrane .

In the RO, dissolved matter and water have very different transport velocity through the membrane.

Conclusion

The primary and secondary treatments are given to meet the statutary requirements . However the water after secondary treatment in not recyclable ,in order to get water for recycling the process of ultrafiltration and reverse osmosis are bieng used. This treatment is also known as tertiary treatment .The water after tertiary treatment is almost free from color,odour, COD,BOD,TDS,TSS and heavy matter impuriteis. The recovery of the water for recycling depends upon the number of stages in ultrafiltration , filtration media, and number of stages at RO. By employing a proper set up of different techniques , approximately 85%-97% of water can be reused.

Read Articles on Zero Discharge
Reuse of treated water in Textiles
Reuse of textile ETP Sludge in building material Approaching zero discharge with optimised resource management
Toward Zero Discharge: Innovative Methodology and Technologies for Process Pollution PreventionThe challenge of zero discharge: from water balance to regeneration [An article from: Desalination]
A chemo-enzymatic pathway leads towards zero discharge tanning [An article from: Journal of Cleaner Production]