You are here

Leather processing

In an age of plastics, metals and synthetics, leather has kept its place as a product of superior quality. As a result, tanning remains an essential economic activity. Leather processing can be done at the small-scale or large-scale level, all to varying degrees of sophistication.

The tanning industry has been subject to important challenges and changes. Foremost has been the introduction of processing technologies with less impact on the environment. As the production of finished leather is concentrated in developing countries, UNIDO, together with other partners, have provided support to enhance tanning industry practices in developing countries.

Cleaner leather production technologies remain UNIDO’s main focus in the field of leather processing. Cleaner production applications include green hide and skin processing (supply of raw material from slaughterhouses without preservation, e.g. salting), water management (use minimum volume of process water), recycling (e.g. in liming) and chromium recovery (after tanning), hair saving (to reduce dissolved solids in effluent) and application of environmentally friendly chemicals (e.g. enzymes). Special attention is also given to occupational health and safety (OHS) in tanneries.

The essential part of any tannery waste audit is assessing the efficiency of existing operations carried out during the leather manufacturing process. Typically, tannery staff have a good idea of, and comparatively accurate figures on the waste resulting from specific operations such as fleshing, splitting, trimming or chrome tanning. Only rarely, however, they have a proper overview of the entire range of waste generated. Thus, when considering various cleaner technologies or waste treatment systems, having access to a complete computation of the overall mass balance certainly makes it easier for a tanner facing arduous choices. Dialogue with environmental authorities is also simpler if such figures are readily available. This paper attempts to provide a comprehensive computation of a mass balance and the efficiency of the leather manufacturing process for a tannery, seen as a closed entity. The calculations are deliberately based on operations in a hypothetical tannery processing bovine hides and producing upper leather for shoes. With minor exceptions (batch washing instead of continuous rinsing, splitting in lime, roller coating), it follows the conventional process.

Utilization and/or safe disposal of sludges generated in the course of effluent treatment still represents a great challenge; worldwide many methods have been explored and proposed. This report describes the attempt made under UNIDO Regional Programme in South-East Asia to test another path. The idea was to convert the hazardous tannery sludge  into an inert, physically stable mass, with very low leachability and sufficient strength to allow making building materials like nonfired bricks for fencing or for landfilling or land reclamation. Solidification  (cementation) was achieved by mixing the sludge with various materials to form a solid product hoping that immobilisation/chemical stabilisation) will also be achieved.

Conventional treatment of tannery effluents does not affect the TDS content (colloquially: salinity); they remain unsuitable for lifestock watering or irrigation which, especially in arid areas, represents a great loss of natural resource. This paper reports on successful irrigation trials with treated effluent from a CETP servicing a cluster of tanneries processing wet blue and crust leather into finished leather and with TDS not exceeding 5000 mg/l and chlorides not exceeding 900 mg/l. Eventually a plot of barren land was converted into a pleasant park-like area.This paper, based on the project implemented by a women-only team, provides information on saline resistant plants and assess their growth properties; and the impact that the continuous application of treated effluent had on the soil and ground water.

Based on new data and requests from interested users, the revised second edition of the paper Pollutants in tannery effluents was prepared drawing on technical inputs by J. Buljan, I. Král, M. Bosnić, R. Daniels. This training material is primarily intended to meet the needs of tanners and people of different profiles associated with environmental protection in the leather industry in developing countries.

The environment is under increasing pressures from solid and liquid wastes as by-products from leather manufacture and tannery effluent create significant pollution unless there has been a form of treatment before discharge.  The industry has gained a negative image in the society with respect to its pollution potential and therefore the leather processing activity is facing a serious challenge.

The paper presents the main sources of pollution and typical pollution loads generated by tanning processes adopted by the tanneries in developing countries, volume(s) of wastewater discharged, the corresponding concentrations of main pollutants as well as the the table of widely prevailing discharge standards.

In this edition the main pollutants' parameters are elaborated in great detail, together with descriptions of their negative environmental impact. Air pollution and toxicity aspects are expanded and a concise chapter on Substances of Very High Concern, SVHC ( carcinogenic, mutagenic, bioaccumulative, persistent etc.) introduced. One can also find photos of equipment used for laboratory analysis.

For the country-wise overview of discharge standards (admittedly somewhat obsolete) please refer to Part II of the first edition.

The globalization of the leather industry means that all tanners face the same problems of minimizing the environmental impact of processing and selling into the global market. Regulatory pressures oblige tanners to make continuous improvements in the processing operations. The authorities concerned and consumers look more closely whether hazardous substances such as certain preservatives, some azo dyes and Cr(VI) are present in leather and leather products. The presence of potentially harmful substances attract attention of public media with the risk of developing unfavorable perception about health safety of leather products. In that context materials imported from developing countries are especially critically judged in some reports. Closer monitoring of this aspect has revealed that leather and leather products sometimes contain some hazardous substances like Cr(VI) although only chromium compounds in the form of Cr(III) were used in the tanning process. It has been concluded that this might be result of some undesired reactions in leather itself but the cause was unclear. In this paper, in a very brief form, some results of the investigations about conditions conducive to or inhibiting generation of Cr(VI) in leather are summarized. Also, the results of a series of tests carried out on leathers received from several countries included in UNIDO Regional Programme of Pollution Control in the Tanning Industry in South-East Asia are presented.

Processing of one tonne of raw hides results in approximately 100 kg of wetblue shavings, the utilization and/or safe disposal of which is globally a serious challenge. Currently a part of the chrome shavings is used in the manufacture of leather board by combining with shavings of vegetable tanned leather. There have been also other methods tested and used to convert shavings into sellable product, e.g. application in paper, wood and other industries.

The method described in this report is enzymatic digestion.

Conversion of chrome shavings into usable products employing the technique of enzymatic digestion as developed in the United States of America and already implemented in a 3 tonnes/day commercial plant in the Czech Republic. Three products, namely, gelatable protein, protein hydrolyzate and filter cake are obtained from enzymatic digestion of chrome shavings. While the products obtained find use in construction and plywood industry and also as nitrogenous fertilizer, the chrome-containing filter cake can be used as a reducing agent in the preparation of basic chromium sulphate.

The method was successfully demonstrated at pilot scale in India using wooden tanning drums.

The Regulation of the European Union (EU) on Registration, Evaluation, Authorisation and Restriction of Chemicals, REACH entered into force on 1st June 2007. Its proclaimed aims were to ensure a higher level of protection of human health and the environment as well as free movement of substances, on their own, in preparations and in articles, while enhancing competitiveness and innovation.

This Regulation should also promote the development of alternative methods for the assessment of hazards of substances. REACH was also expected to streamline and improve the former legislative framework on chemicals in order to improve the protection of human health and the environment from the risks that can be posed by chemicals, especially by those listed as Substances of Very High Concern (SVHC) The purpose of this desk study was to give some background information and practical advice to the leather industry so as to maintain or establish business in the EU market.

 

Generally 35-60% of the total solids in tannery sludge is organic matter. A number of solutions for utilization and/or safe disposal of tannery sludge have been proposed, practiced, tested and applied at pilot and industrial scale. Composting is one of these options and this report describes results of tests and application of sludge composting on low, non-mechanized scale together with its utilization as soil conditioner for nonedible plants.

Utilization or safe disposal of sludge generated by tannery effluent treatment plants poses a challenge worldwide; landfill disposal should be considered only in case when no other viable option is possible.  Unfortunately, in some areas and/or developing countries properly designed and constructed landfills are not available either.
With the technical assistance of UNIDO, CETP-Ranitec in Ranipet, Tamil Nadu, India, established a low cost pilot scale demonstration landfill in October 1997, the first of its kind in the region. The report describes requirements for a safe landfill disposal and practical recommendations for replication.

Case Study prepared for the Conference on Ecologically Sustainable Industrial Development (ESID), Copenhagen, Denmark, 14 - 18 October 1991.

Pages