Tanning industry involves conversion of animal hide or skin into an end product useful for making leather goods such as shoes, bags, purses, belts and clothing.

Tanning of the raw hide is done either by vegetable tanning process, which yields thick, heavy duty leather, or by chrome tanning process, which give light, thin leather.

TREATMENT OF WASTE WATER

Preliminary and primary treatment consists of:

• screening to remove hair
• settling, to remove lime and other settle-able solids from waste water

Plain sedimentation for 24 hours gives a BOD removal of 40% - 50% and suspended solids removal of 80% - 90%.

Chemical coagulants used are alum, ferric chloride and lime, while pH correction, when necessary, is done with sulphuric acid or carbon dioxide from flue gas. The waste water can be effectively treated by coagulation with alum, pH correction with sulphuric acid and mixing with compressed air to give 93% suspended solid reduction and 57% BOD reduction.

Biological treatment by trickling filters requires pretreatment of spent tan liquor with lime, settling for 24 hours, treating the clarified waste water with 140 mg/l of ferrous sulphate and then spreading the waste water on tricking filter using coke as a medium. It achieves 88-90 % BOD removal with recirculation ratio of 2:1 or 3:1.

In view of the high oxygen demand of vegetable tan liquor, it is possible to subject it to anaerobic treatment for knocking out a sizeable portion of the oxygen demand.

It is known that nearly 90% of the tannin is absorbed in the hide during vegetable tanning process, while only 60 - 65% chromium compounds are absorbed in the hide in chrome tanning, the rest finding their way into the effluent. In view of the toxic property of chromium and in the interest of economy, it is essential to recover and recycle chromium from chrome tanning wastes.

Chromium can be recovered in one of the following ways:

1. Direct resuse of spent liquor for the next batch. The drawback of this method is accumulation of salts and other impurities due to repeated use, which has an adverse effect on the tanning process.
2. Indirect resue in which chromium is precipitated from the spent liquor with an alkali and the precipitate is dissolved in sulphuric acid to acid to get back chromium sulphate. This is an efficient method and the recycled chromium does not adversely affect the tanning process.
3. Separating chromium compounds from the spent liquor by electrodialysis and membrane separation. Although the chrome liquor is cleaner than the produced by direct method, it is very costly.