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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.

In view of ever increasing legal and social pressures, no tanner can afford to be unfamiliar with the main issues and principles of environmental protection pertaining to tannery operations. Among these, preventing pollution and promoting cleaner leather processing, which ultimately leads to lower treatment costs, clearly remain a priority. Through the application of industrially proven low-waste advanced methods - such as using salt-free preserved raw hides and skins, hair-save liming, low-ammonia or ammonia-free deliming and bating, advanced chrome management system, etcetera - it is possible to decrease the pollution load expressed as COD and BOD5 by more than 30%, sulphides by about 60 to 70 %, ammonia nitrogen by 80%, total (Kjeldahl) nitrogen by 50%, chlorides by 70%, sulphates by 65 % and chromium by 90%. Yet, despite all preventive measures, there is still a considerable amount of pollution load to be dealt with by the end-of-pipe methods. The purpose of this booklet is to help a tanner or a tannery manager (possibly a well-trained leather technologist) to get familiarized with basic principles and methods of treatment of tannery effluents. This knowledge should make him better equipped for communications with the factory’s environmental unit, environmental authorities and NGOs. To keep the manual short and concise, there are many simplifications and omissions of details; for in-depth understanding of the complexities of treatment of effluents and solid wastes (sludge) we recommend you to consult extensive literature on this subject. Finally, and contrary to the widespread misperception that vegetable tanning is environmentally harmless (in reality its effluents have very high, difficult-to-treat COD), the manual basically refers to the combined chrome tanning (i.e. chrome tanning supplemented by vegetable and synthetic tanning agents) because it is by far the most prevailing leather tanning method.

This paper attempts to discuss the main, predominantly technical, issues with regard to benchmarking; it is intended to assist those who are willing to admit to the fact that despite all explanations and definitions available, they are not quite sure what it is all about and whether and how it could be applied in the tanning industry. The paper outlines "cock-pit" check lists for ten areas - components: 1. Tannery location, infrastructure 2. Production parameters 3. Cleaner technologies 4. Energy management and consumption 5. Quality assurance, reprocessing, delivery time, failures 6. Product development, strategies 7. Occupational safety and health at work, maintenance 8. Effluent treatment, solid waste, air emissions 9. Financial indicators 10. Human resources and staff welfare, CSR. Some typical production parameters are also provided.

The Tannery of the Future Foundation developed self-assesment tool for tanneries

 

Towards the end of the 20th century the tanning industry has made a considerable progress in controlling the environmental pollution caused by its activities, yet the situation varies from country to country and even from region to region within some large countries. Some tanners in industrialized countries hold the view that lax environmental regulations and poor enforcement account for lower production costs, higher competitiveness and hence further expansion of the tanning industry in developing countries.

This study compares the costs of treatment of tannery effluents, including indicative investments costs in selected industrialized and developing countries. While the figures concerning the investment and operational costs by now are quite obsolete and technologies change, the comparisons of the cost structures are still quite elightening.

Typically only a small part of fleshings is used for manufacture of glue and animal protein while the major part is dumped as waste at landfill or disposed of along with other solid wastes. The unutilised fleshings, containing high concentration of lime and sulfide, putrefy and produce obnoxious odour. They also cause groundwater pollution, attract flies, rodents and stray dogs and thus represent a public nuisance. Due to high moisture content handling and transportation of fleshings is quite difficult. On the other hand, one tonne of wet fleshing with 85% moisture is estimated to generate 20-30 m3 of biogas. To solve the disposal problem of fleshings, one of the options considered and tested during UNIDO Regional Programme in South-East Asia was biomethanation. The results of testing at the pilot plant, the first of its kind in the region, are given in this report.

With increasing pressure from the pollution control authorities, tanners in many countries of South East Asia region are faced with the urgent task of utilization or safe disposal of solid wastes from tanneries, particularly fleshings. Likewise, sludge generated by tannery effluent treatment plants has to be either put to use or safely disposed. These two issues were highlighted by the industry and government representatives of countries participating in the Regional programme for South-East Asia.

The report contains details of the large scale pilot project implemented under Programme, characteristics of fleshings and sludge charged to the digestors, volume of gas generated vis-à-vis projection, coping with the hydrogen sulphide gas (H2S) and the many valuable lessons learnt. Details of the problems encountered – both process-related and mechanical – have been narrated. At the time of the report preparation (2002) it was the only operational plant of its kind in the world

Conventional technologies for treatment of tannery effluent are generally energy & chemical intensive and continuous process monitoring and control are required to achieve optimum results. In search for alternatives, robust, easy to operate and low maintenance technologies, constructed wetland system, also known as root zone treatment system, using reeds for treatment of effluent, has been considered a possible option. This system is widely used in Europe and elsewhere to treat municipal sewerage. However, there was no practical experience, at least at semi-industrial scale, about its applicability in purification of tannery effluents. Accordingly, in cooperation with willing tanneries and management of common effluent treatment plants (CETP)  in Tamilnadu, India, UNIDO, under its Regional Programme, established four pilot and demonstration reed beds, each with different features, to deal with effluent of different characteristics. Practical experience and results are reported in this paper.

Under the Regional Programme for pollution control in the tanning industry in South-East Asia UNIDO has been actively looking for methods to improve conventional treatment processes which simultaneously reduce the nitrogen content and give the possibility of dealing with TDS/chlorides present in the effluent. The following technologies relating to the issues mentioned were implemented in pilot demonstration units:

  •  Mechanical/manual removal of excess salt from wet salted hides and skins
  • Reverse osmosis (RO) of treated tannery effluent
  • Improved solar evaporation
  • Carbon dioxide (CO2) deliming in a small scale tannery to reduce ammonical nitrogen
  • Constructed wet land treatment system (reed beds) possibly resulting in nitrification/denitrification
  • Ultrafiltration

A study with preliminary estimates of costs of multistage evaporation system to recover salt from reject generated by RO has also been prepared.

In another study, the scope of replacement of secondary clarifier in the biological treatment stage by ultrafiltration has been assessed. (Mladen Bosnic, December, 1997).

This report deals specifically with ultrafiltration.

Due to climatic conditions the scope for green processing is limited in many countries, sodium chloride is widely used to preserve raw hides and skins. It contributes to a high volume of total dissolved solids (TDS) in the soak waste liquor. No commercially viable technology for treating effluent has been developed to date. A large amount of the salt sticking to the hide and skin surface can be removed by shaking the hides mechanically or manually.
Within the framework of the UNIDO regional programme for pollution control in the tanning industry in South-East Asia, a pilot demonstration unit was set up to demonstrate different options for

(a) desalting hides and skins prior to soaking and

(b) reusing dusted salt in the pickling operation after purifying the salt recovered.

This report covers the demonstrations carried out during the period January 1997 to February 2001 of desalting of salted raw stock and use of the recovered salt in pickling.

What is future of the chrome tanning? What will be used as a tanning agent in horizont of 50 years? In this paper prepared in 1999 Mr. Frendrup analyzed possible trends and scenarios of leather manufacture. Many issues and predictions are still relevant, especially those concerning the recent EU regulations. For additional information see also IULTCS papers concerning chrome tanning http://www.iultcs.org/pdf/IUR-1_Chromiumandleatherresearch_Abalancedview...

 


 

Chrome tanning is the most common type of tanning in the world. Chrome tanned leathers are characterised by top handling quality, high hydro-thermal stability, user-specific properties and versatile applicability. Waste chrome from leather manufacturing, however, poses a significant disposal problem. Throughout the world, chrome discharge from tanneries is subject to strict regulations. That notwithstanding, chrome is a component that has to be strictly monitored. The environmental impact of chrome discharged from tanneries has been a subject of extensive scientific and technical dispute. Although the legislative limits on the disposal of solid chrome-containing waste have been relaxed in some countries, liquid emissions remain strictly regulated throughout the world. Given the close link between chrome tanning and the environmental impact of leather manufacturing, chrome management is of primary importance in tanning operations. This paper provides information on chrome management and those techniques most frequently used to reduce the amount of chrome in tannery wastewater. By providing information and citing practical experience, the paper aims at contributing to sustainable development of leather manufacture without avoidable harm to environment.

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