The conventional leather tanning technology is highly polluting as it produces large amounts of organic and chemical pollutants. Wastes generated by the leather processing industries pose a major challenge to the environment. According to conservative estimates, about 600,000 tons per year of solid waste are generated worldwide by leather industry and approximately 40-50% of the hides are lost to shavings and trimmings. Everyday a huge quantity of solid waste, including trimmings of finished leather, shaving dusts, hair, fleshing, trimming of raw hides and skins, are being produced from the industries. Chromium, sulphur, oils and noxious gas (methane, ammonia, and hydrogen sulphide) are the elements of liquid, gas and solid waste of tannery industries.
Renewable Energy Production from Tannery Wastes
Sandeep Goel, Ritika Tewari and Salman Zafar | BioEnergy Consult
Tannery effluents are characterized by high contents of dissolved, suspended organic and inorganic solids giving rise to high oxygen demand and potentially toxic metal salts and chromium metal ion. The tannery effluent, if not treated properly, can cause serious damage to soil and water bodies. The high amount of salt contained in the effluent, for example, can increase soil salinity, reduce fertility and damage farming in large areas. Tanneries also produce harmful gases, dust and a large amount of solid waste. The graveness of the problem can be understood by the fact that these toxic effluents are being discharged into natural water courses without/with minimal treatment in several parts of the world.
Waste Generation in the Tanning Industry
A large amount of waste produced by these tanneries is discharged in natural water bodies directly or indirectly through two open drains without any treatment. The water in the low lying areas in developing countries, like India and Bangladesh, is polluted in such a degree that it has become unsuitable for public uses. In summer when the rate of decomposition of the waste is higher, serious air pollution is caused in residential areas by producing intolerable obnoxious odours.
Solids originate from all stages of leather making; they comprise fine leather particles, residues from various chemical discharges and reagents from different waste liquors. These comprise of large pieces of leather cuttings, trimmings and gross shavings, fleshing residues, solid hair debris and remnants of paper bags.
- Fleshing: 56-60%
- Chrome shaving, chrome splits and buffing dust: 35-40%
- Skin trimming: 5-7%
- Hair: 2-5%
Gasification of Tannery Wastes
A wide range of tannery wastes can be macerated, flash dried, densified and gasified to generate a clean syngas for reuse in boilers or other Combined Heat and Power systems. As a result up to 70% of the intrinsic energy value of the waste can be recovered as syngas, with up to 60% of this being surplus to process drying requirements so can be recovered for on-site boiler or thermal energy recovery uses.
Tanneries generate considerable quantities of sludge, shavings, trimmings, hair, buffing dusts and other general wastes and can consist of up to 70% of hide weight processed. The gasification technology, by virtue of chemically reducing conditions, provides a viable alternative thermal treatment for Chrome containing materials, and generates a chrome (III) containing ash. This ash has significant commercial value as it can be reconstituted.
A proprietary technology designated PyroArc, has been in commercial operation at a tanyard on the West Coast of Norway since mid 2001. The process employs gasification-and-plasma-cracking. The PyroArc process offer the capability of turning the tannery waste problem to a valorising source that may add values to the plant owner in terms of excessive energy and ferrochrome, a harmless alloy that is widely used by the metallurgical industry. The process leaves no ashes but a non-leaching slag that is useful for civil engineering works, and, hence, no residues for landfill disposal
Biomethnation of Tannery Wastes
Anaerobic digestion is a favorable technologic solution which degrades a substantial part of the organic matter contained in the sludge and tannery solid wastes, generating valuable biogas, contributing to alleviate the environmental problem, giving time to set-up more sustainable treatment and disposal routes. Digested solid waste is biologically stabilized and can be reused in agriculture.
The application of an anaerobic treatment for the break down of COD from tannery waste water is an innovative and attractive method to recover energy from tannery wastewater. Until now it was considered that the complexity of the waste water stream originating from tanneries in combination with the presence of chroming would result in the poisoning of the process in a high loaded anaerobic reactor.
When the locally available industrial wastewater treatment plant is not provided by anaerobic digester, a large scale digestion can be planned in regions accommodating a big cluster of tanneries, if there is enough waste to make the facility economically attractive. In this circumstance, an anaerobic co-digestion plant based on sludge and tanneries may be a recommendable option, which reduces the quantity of landfilled waste and recovers its energy potential. It can also incorporate any other domestic, industrial or agricultural wastes. Chrome-free digested tannery sludge also has a definite value as a fertilizer based on its nutrient content.
Ritika Tewari is a Masters student in Natural Resources Management at TERI University, New Delhi, with a passion for improving waste management and renewable energy scenario in developing countries. She can be reached at firstname.lastname@example.org.
Salman Zafar is a serial entrepreneur with expertise in biomass energy, waste-to-energy, waste management, sustainable development, alternative energy systems and cleantech investment. Apart from managing his advisory firm BioEnergy Consult, he is closely associated with a wide array of national and international cleantech organizations. Salman is a freelance author and has been a significant contributor towards popularizing biomass energy and waste-to-energy systems worldwide through his articles. He can be reached at email@example.com.
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