Published on 16/05/2011
First Update 16/03/2017
The Tannery Industry:
As of estimates made in 2002, India had more than 3000 tanneries with a total capacity of 700000 tonnes of hides and skins per year. The annual income from leather trade in India was about Rs 20000 crores. More than 90% of the tanneries were small or medium with a processing capacity of less then 2 to 3 tonnes of hides/skins per day. Most of the tanneries are located near river banks. The highest concentration of tanneries in India is on the banks of Ganga river (Kanpur, Unnao) in North India and the Palar river system in Tamilnadu.
Leather Production Technology and Pollution:
An animal skin consists of about 61% water, 34% fibrous proteins, 1% globular proteins, 2% lipids, 1% natural salts and some other ingredients including pigments. Out of three layers, the epidermis, dermis and the hypodermis it is the dermis which is later transformed into leather. The epidermis primarily composed of keratin has hair which is removed and the hypodermis has flesh and blood vessels which is also removed. In leather processing, the basic operations revolve round cleaning the skin of unwanted inter fibrillary material through a set of pre-tanning operations in the Beam House, processing the leather permanently by means of tanning and adding aesthetic value during the post tanning process. The starting material in most cases is raw hide or skin which has been preserved temporarily by the addition of common salt.
- The Beam House process involves the removal of salt, dirt and hair in the following processes:
(a) Desalting and Soaking the hides to remove salt and other foreign material such as dirt and also to remove the moisture content. This process uses a large amount of water about 20 m3 per ton of hide and generates conspicuous pollution. Soaking generates about 6-9 m3 per ton of effluents with a BOD from 1100 to 2500 mg/l, a COD of 3000-6000 mg/L, very high total solids and suspended solids, 15000 to 30000 mg/l of chlorides and 800-1500 mg/l of sulphates.
(b) Unhairing and Liming – The process yields one of the most polluting effluent streams from tanneries. Liming opens up the collagen structure by removing interstitial material, fleshing removes excess tissue from the interior of the hide. Unhairing is done by treating soaked hides in a bath containing sodium sulphide / Hydrogen sulphide and lime. About 3 to 5 m3 of effluent per tonne of hide/skin is expected to be discharged with a high pH of 10.0 to 12.8, a BOD of 5000 to 10000 mg/l and COD of 10000 – 25000 mg/l. The concentration of sulphides ranges from 200 to 500 mg/l, the total solids (24000 to 48000 mg/l) and sulphates (600-1200 mg/L) are also high.
(c) Deliming and bating: A bath of ammonium salts and proteolytic enzymes is used to process the pelt. About 1.5 m3 of effluents are generated in the process at a pH of 7 to 9. The pollutants from the process include Calcium salts, Sulphide residues (30 to 60 mg/l), degraded proteins, residual proteolytic enzymatic agents, Chloride (1000 to 2000 mg/l), Sulphates (2000 to 4000 mg/l), BOD (1000 – 3000 mg/l) and COD (2500 to 7000 mg/l). Nitrogen based deliming agents are considered a long term environmental threat because of their impact on soil NOx values.
Sulphates are an important content of pretanning waste waters. They readily get reduced to sulphide under anaerobic conditions in waste water treatment plants like anaerobic lagoons, contact filters or up flow anaerobic sludge blanket reactors. A build up of sulphides makes the biomethanation of organic materials less effective apart from adding to the COD load. Ammonia is also given off as an air pollutant in the process.