Pollution Control In Dye Industry

Pollution Control In Dye Industry


A  dye  can  generally  be  described  as  a  colored  substance  that  has  an  affinity  to  the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber

Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially.  In contrast with a dye, a pigment generally  is  insoluble, and has no affinity  for  the  substrate.

σ → σ*

π → π*

n → σ*

n → π*

aromatic π → aromatic π*

Chromophore: it is a functional  groups  which  are  unsaturated and  they  cause  a  compound  to become  coloured.

Examples of  chromophores  are  –N=N-,  -C=C-,  -C=N- and -C=O.

  1. Transitions involving σ, π, and n electrons
  2. Transitions involving charge-transfer electrons
  3. Transitions involving d and f electrons


Auxochromes  are  groups  that  does  not  impart  color  to  the  compound  but  increase  the  color of the compound. Functional groups such as hydroxyl (–OH), amino ( -NH2), nitro  (-NO2), alkyl  (-R), —OH, —OR, —NH2, —NHR, —NR2, —SH are common examples  for  auxochrome.  The  effect  of  the  auxochrome  is  due  to  its  ability  to  extend  the  conjugation  of  a  chromophore  by  the  sharing  of  non-bonding  electrons.

Bathochromic Effect

Hypsochromic shift

Hyperchromic Effect

Hypochromic Effect

Classification of Dyes

  • Acid Dye
  • Basic Dye
  • Direct Dye
  • Sulphur Dye
  • Azo Dye
  • Vat Dye

Dyestuff Industry Treatment

The presence of residual color, high  levels of electrolytes, toxic substances (e.g., metals  and  unreacted  raw  materials) in  dye  application processes produce wastewaters that poses unacceptable environmental risks.

In the case of  textile  dyeing  operations,  the  concerns  raised  can  arise  from  incomplete  dye  bath  exhaustion  and  the  presence  of  dyeing  auxiliaries  and  metal  ions  that  are  toxic  to  aquatic  life.

Sources of effluent

  • Dyeing and printing
  • industries
  • Textile industries
  • Paper and ink manufacturing industries
  • Cosmetics
  • Pharmaceuticals
  • Food

Properties of effluent

  • Impart  colour  to  water  bodies  even  if present in small quantity
  • Reduces  light  penetration  and photosynthesis
  • Carcinogenic or mutagenic
  • Azo  dyes  are more  toxic  as  they  affect microbes  thereby  affecting  biological degradation treatment.
  • Dyes  increases  BOD  of  effluent  thereby affecting aquatic life
  • Toxic to fish & microbial organisms
  • The discharge of heavy metals into aquatic ecosystemsIncrease in alkalinity of water
  • The turbidity and colour along with oil  and  scum  create  an  unsighty appearance.
  • The  mineral  materials,  mostlysodium  salts  increase  salinity  of the water.

Volume of effluent

  • The  volume  of  effluent generated  in  dyeing  is comparatively more.
  • It contains dyes, mordants, acids (acetic  acid),  alkalis,  nitrites, chromium  salts,  sodium  chloride and soaps.
  • These  effluents  are  usually  hot, highly  coloured  with  a  high  pH and sulfide content
  • Care  must  be  taken  while neutralising these liquors as acid may  liberate  hydrogen  sulfide gas.
  • Removal  of  Sulfides  by treatment  with  chlorine  or hypochlorites
  • Spent  vat  dyes  are  strongly alkaline  and  have  fairly  high permanganate value

Effluent Treatment Methods

Physical Methods


Activated carbon adsorption has been widely studied as a waste treatment method for the removal of different  classes of dyes  from wastewater.

Factors  such  as  the  choice  of  activated  carbon, temperature, pH, contact time, and dosage must be taken into consideration for optimum removal  of  dyes  from


The most commonly used method of dye removal by adsorption. It can be  Effective for adsorbing cationic, mordant  and  acid  dyes and reactive dyes

Performance  depends  on  the  type  of  carbon  used  and  the  characteristics  of the wastewater.

Adsorptive Bubble Separation

Surface-active material,  which  may  be  ionic,  molecular,  colloidal,  or  macroparticulate  in  nature,  is selectively  adsorbed  at  the  surface of  bubbles  rising  through  the  liquid.

98—99.8% of Direct Blue1 was removed from wastewater

Chemical methods

Fenton’s  reagent  (Fe2/H2O2)  has  been  used  commercially  to  oxidize  and  decolorize effluents containing a number of azo, anthraquinone, and reactive dyes.

Dyes removed in these studies include reactive azo and anthraquinone dyes.


 Titanium Dioxide (TiO2)

The  cationic  dye  Rhodamine  B,  Basic  Violet  10  and  the  anthraquinone  mordant  dye Alizarine Red  (Mordant Red 3)  can  be decomposed  by Ti02  in  the presence of  visible light. Degradation occurs via the interaction of dye with hydroxyl radicals (OH.). Two of the final degradation products are phthalic acid and carbon dioxide.


Borohydride Reduction

  • Sodium borohydride  is one of  the  strongest water  soluble  reducing agent commercially available
  • SO2  is  produced  when borohydride  is  used  in  combination with  bisulfite  catalysis  in  the  pH  rang e  of  5  –  8 which readily reduces azo dyes
  • The  utility  of  this  process  involving  a  variety  of  azo  dyes,  giving  percent colour  reduction  of  83-99%  for  acid,  direct,  and  reactive  dyes  and  74-99.9%  for metalized direct and phthalocyanine dyes
  •  This method was extended to industrial wastewater containing either a mixture of azo disperse dyes or azo reactive black dyes.
  • These solutions were treated with a mixture of 400—500 mg/L Na2S2O5, 12—25 mg/L NaBH4, and 0—200 mg cationic flocculant, reducing color by 93 – 99%

Bioaugmentation and Bioremediation

These  methods  are  currently  used  by  the  textile  and  dyestuff  industries  to  decolorize waste water using enzymes.

In  this  regard, enzymes  such as  laccases,  lignin peroxidases, and manganese peroxidases have proved  effective  in  cleaving  aromatic  rings

Metal Complexed Dyes

One of the most serious environmental problems in the dye, textile, and leather industries is  associated with  the manufacture  and  use  of metallized  azo  dyes  that  are  complexed with chromium or cobalt to obtain desirable fastness properties

various  chemical, physical,  and  biological methods,  such  treatments  can  be  expensive  and may  result  in sludges that must be disposed by  incineration or  land  filling

In view of an emphasis on pollution prevention  instead of waste  treatment,  the merits of  substituting  iron  (Fe)  for chromium (Cr) and cobalt (Co) in a group of commercially important acid dyes.

Fe-complexed  dyes  as  environmentally friendly alternatives  to widely used Cr and Co complexed acid dyes


The  ionic  components  (heavy metals) are  separated  through  the  use  of semi-permeable  ion­selective membranes.

Application  of  an  electrical  potential between  the  two  electrodes  causes  a migration  of  cations  and  anions towards respective electrodes

Because  of  the  alternate  spacing  of cation  and  anion  permeable membranes, cells of concentrated and dilute salts are formed

Membrane Bioreactor in Textile Waste Water Treatment

Utilization of membrane filtration results in the retention of active micro-organisms, extra cellular  enzymes  generated  by  these  micro-organisms  for  degradation  of  the  organics present  in  the effluent

Some  micro-organisms,  especially nitrifiers, are slowly growing one, their loss shall reduce the efficiency of the treatment  system  and  nutrient  removal

In  the MBR, these  organisms  are  retained  and  a  better  treatment  is  achieved

In  addition retention of active enzymes secreted by mico organisms taking part in the metabolization  of organics present in the textile waste water is an important aspect of MBR technology

Maintenance  of  higher  concentration  of  these  enzymes  shall  result  in  rapid  and  better  degradation of complex organic molecules present  in  the  textile waste water.

The overall  efficiency  of BOD  (Biological Oxygen Demand)  and COD  (Chemical Oxygen Demand)  removal  is  improved

Advantages of MBR


  1. www.wikipedia.org
  2. Y. R. Sharma, Elements of organic  spectroscopy, S. Chand & Company LTD, New Delhi, India.
  3. Kirk – Othmer, Encyclopedia of chemical technology, 5th

Edition, 9th Volume, Wiley Interscience, John Wiley & Sons, Inc., Publication

  1. S.Eshwaramoorthi, K. Dhanapal and D.S.Chauhan, Environment With People’s Involvement & Co-ordination in India, Coimbatore, India, www.ecpconsulting.in
  2. http://www.scribd.com/doc/12949482/Dyestuff-Industry-Treatment, 2003