Published on 05/06/2011
First Update 16/03/2017
Clean Process technologies and waste utilization 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.