Leather working is an old technology for processing natural materials. It requires large quantities of water.

Wastewater from tanneries is generally grayish green in color, I is turbid and often putrid. The pH changes from strongly basic (pH 12) to acid (pH 2 to 3) as it goes through the stages of treatment. The wastewater carries a high organic load with insoluble contaminants, sulfides and chromium.

This wastewater goes through several treatment stages, as follows:

-          screens

-          homogenizing in storage tanks equipped with mixing paddles usually arranged in a circle

-          pre-treatment for removing chromium

-          sulfur precipitation

-          final treatment

-          biological cleaning where necessary

iron sulfate and sulfuric acid precipitate any sulfides present. Iron sulfide is then removes in final treatment using further flocculating agents, such as polyelectrlytes.