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

"Tanning" refers to the process by which collagen fibers in a hide react with a chemical agent (tannin, alum or other chemicals). However, the term leather tanning also commonly refers to the entire leather-making process. Hides and skins have the ability to absorb tannic acid and other chemical substances that prevent them from decaying, make them resistant to wetting, and keep them supple and durable. The flesh side of the hide or skin is much thicker and softer. The three types of hides and skins most often used in leather manufacture are from cattle, sheep, and pigs.

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

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.

Out of 1000 kg of raw hide, nearly 850 kg is generated as solid wastes in leather processing. Only 150 Kg of the raw material is converted in to leather. Tannery generated huge amount of waste as follows:

Over 80 per cent of the organic pollution load in BOD terms emanates from the beamhouse (pre-tanning); much of this comes from degraded hide/skin and hair matter. During the tanning process at least 300 kg of chemicals (lime, salt etc.) are added per ton of hides. Excess of non-used salts will appear in the wastewater. Because of the changing pH, these compounds can precipitate and contribute to the amount of solid waste or suspended solids. Every tanning process step, with the exception of finishing operations, produces wastewater. An average of 35 m3 is produced per ton of raw hide. The wastewater is made up of high concentration of salts, chromium, ammonia, dye and solvent chemicals etc.

Gasification of Tannery Wastes

The implementation of gasification has the potential to provide significant cost benefits in terms of power generation and waste disposal, and increase sustainability within the leather industry. The gasification process converts any carbon-containing material into a combustible gas comprised primarily of carbon monoxide, hydrogen and methane, which can be used as a fuel to generate electricity and heat.

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.

All of the wastes created by the tannery can be gasified following pre-treatment methods such as maceration, drying and subsequent densification or briquetting. A combined drying and gasification process could eliminate solid waste, whilst providing a combustible gas as a tax-exempt renewable energy source, which the tannery can directly reuse. Gasification trials have illustrated that up to 70% of the intrinsic energy value of the wastes currently disposed can be recovered as “synthesis gas” energy.

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

Biomethanation (or anaerobic digestion) systems are mature and proven processes that have the potential to convert tannery wastes into energy efficiently, and achieve the goals of pollution prevention/reduction, elimination of uncontrolled methane emissions and odour, recovery of bio-energy potential as biogas, production of stabilized residue for use as low grade fertilizer.

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.

 

Biogas produced in anaerobic digesters consists of methane (50%–80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide.  Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. A combined heat and power plant system (CHP) not only generates power but also produces heat for in-house requirements to maintain desired temperature level in the digester during cold season. CHP systems cover a range of technologies but indicative energy outputs per m3 of biogas are approximately 1.7 kWh electricity and 2.5kWh heat. The combined production of electricity and heat is highly desirable because it displaces non-renewable energy demand elsewhere and therefore reduces the amount of carbon dioxide released into the atmosphere.

Conclusions

The energy generated by anaerobic digestion or gasification of tannery wastes can be put to beneficial use, in both drying the wastes and as an energy source for the tannery’s own requirements, CHP or electricity export from the site. A large amount of the energy recovered is surplus to the energy conversion process requirements and can be reused by the tannery directly. Infact, implementation of waste-to-energy systems have the potential to make the industry self-sufficient in terms of thermal energy requirements. Tanneries are major energy users, and requires up to 30 kW of energy to produce a single finished hide. Thus, waste-to-energy plant in a tannery promotes the production of electricity from decentralized renewable energy sources, apart from resolving serious environmental issues posed by leather industry wastes.

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