Waste Utilisation in Tanneries

It is being increasingly recognized that end of pipe solutions are not the ultimate strategy for waste management. Economic instruments have a major role to play. These also include cost cuttings through  recovery, reuse and recycle of waste materials as also a more prudent use of resources and a reduction in the quantity of effluents discharged. The ideal being achieving zero or near zero discharge. The use of clean process technologies to achieve these ends is of great advantage to tanners. It has been demonstrated that a Tannery with a production capacity of 2000 kg. of hides /skin per day might potentially save Rs. 1.4 million per month by adopting clean process technologies. A number of clean technology options for tanners have emerged.

Reduction in raw material consumption:
Salt consumption could be reduced by lowering the time between slaughtering and further treatment and by cooling the hides preferably below 4 degrees Celsius for good preservation up to 3 weeks. Fleshing and trimming could be practiced in the slaughter house. Dry salting can also minimize the use of salt for preservation of hides. Low environmental impact antiseptics have also been tried as a substitute for salt as a preservative. Preservatives like TCMTB, Isothiazolone products, potassium dimethyl dithiocarbamate, Sodium Chlorite, benzalkonium chloride, sodium fluoride and boric acid have also been used. Some of these have also been found to be useful for soaking, pickling and wet blue preservation.

In the beam house, a significant reduction in water consumption can be achieved by the use of new drums and processors to facilitate efficient draining and washing and the recycle of low floats.
As a part of clean process initiatives in the soaking process, the use of low polluting antiseptics has been tried. Fleshing of green hides after soaking is a cleaner alternative over fleshing after liming.
Upto 40% of sodium sulphide and 50% of lime can be saved by the direct recycling of the liming float. In order to maintain the quality of leather, unhairing and opening up processes should be done in separate stages. When tanning and pickling floats are separated they result in a saving of about 80% of salt and 20-25% of either formic or sulphuric acid. Salt concentrations in pickling floats can also be reduced by using non swelling agents.
Splitting on the lime is a cleaner technology than chromium tanned splitting as it reduces the amount of chromium used and gives off waste that can be easily used for the production of Gelatine.

Reduction of Pollutants at source:
Mechanical desalting by hand shaking, mechanical brushes or a drum type shaker can remove up to 10 % of salt added to the hides for processing. This can be reused for pickling after dissolution and removal of solids. Desalting of raw hides has resulted in a reduction of up to 15% salt loads at the salt pans in some tanneries at Tamil Nadu. A reduction of up to 15% of T.D.S. has also been observed due to use of enzyme based unhairing processes and better quality lime in tanneries. Segregating and reusing pickle and chrome tanning liquors also has the capacity to reduce the T.D.S by 10% in composite tannery waste waters. Clean processes have resulted in reduction of emission loads in composite waste waters from about 600 to 400 Kg. /ton of raw material.
Solvent recovery, extraction of brines and commercial reuse of recovered grease has been advocated as a clean process technology for degreasing.

Reduction of B.O.D. and C.O.D at source:
Mechanical desalting, use of enzyme assisted sulfide-reduced dehairing and cleaner chrome tanning have resulted in at least 30-40% reduction in the B.O.D. and C.O.D. loads per tonne of leather produced. Recovery of hair either when it is separated during the liming or at the end of the hair saving process and reutilization as a nitrogen source may in itself  bring down the C.O.D. loads by about 15-20% in the mixed effluents and a reduction of 25-30% in total nitrogen.

Reduction of Sulphide Loads:
A 50-60% reduction in the Sodium Sulphide loads required for dehairing has been observed by using enzyme based technologies. This has also demonstrated a net gain of 2% increase in the area of leather and could compensate for the increased cost of using enzymes. The reduction in sulphides has also demonstrated a potential ability to save atleast 8-10% of the cost of end of pipe treatment.

Reduction of Nitrogen Salts:
The use of ammonium salts in deliming is responsible for the generation of about 40% ammoniacal nitrogen.Various Nitrogen free deliming technologies are now available. The use of Carbon Dioxide is one such. The insertion of Hydrogen Peroxide before Carbon Dioxide reduces the creation of Hydrogen Sulphide.

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.

1. 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.