Overview

The global leather industry produces about 18 billion square feet of leather a year (2003 data) with an estimated value of about $40 billion. Developing countries now produce over 60% of the world's leather needs. About 65% of the world production of leather goes into leather footwear, and the global production of footwear is estimated at around 11 billion pairs (worth an estimated $150 billion at wholesale prices). Conversion of rawhide into leather (an unalterable and imputrescible product) requires several mechanical and chemical operations involving many chemicals in an aqueous medium, including acids, alkalis, chromium salts, tannins, solvents, auxiliaries, surfactants, acids, and metallorganic dyes; natural or synthetic tanning agents; sulfonated oils, and salts. The quantity of effluent generated is about 30 L for every kilogram of hide or skin processed. The total quantity of effluent discharged by Indian tanneries is about 50,000 m ³/day and contains high concentrations of organic pollutants. Tannery effluents can be divided into three types based on the operations carried out in the three different sections (Fig. 12-1):

• Unhairing and liming wastewater with high sulfide and lime content and high pH [accounts for 45% of the effluent volume and contributes to 30% of the overall biological oxygen demand (BOD) and chemical oxygen demand (COD)].

• Tanning wastewater with high salinity and high chrome levels.

• Retanning, dyeing, and fat liquoring wastewater (accounts for ~20% of the total COD).

Figure 12-1: Various steps in conversion of hide to leather.

The first area of operations is the beam...