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Introduction 

Great progress has been made in the development and implementation of clean technology in tanneries but we have to do more. The economic value of resources must be maximised and waste must be further reduced. Continuous improvement is now demanded and this is a challenge. A number of issues will become of increasing concern to tanners: 

Water use 
There will be increasing pressure to reduce water consumption in all industries. Some countries are already suffering from aquifer depletion and irrigation water shortages have already reduced world grain production. Reuse of wastewater for irrigation is being promoted but this may not be sustainable when Total Dissolved Salts (TDS) levels are high. 

Salinity 
Agriculture in arid and semi-arid parts of the world results in by of soils and water. Industry must not add to these problems and TDS discharge is therefore of major concern for some tanneries. 

Energy 
All industries should minimise energy use and reduce their contribution to the release of greenhouse gases. Some tanneries make use of solar energy and this should increase as the technologies improve. Photovoltaic cells are already being incorporated into buildings and a photovoltaic roofing material has been developed in Japan. A new development is a cell based on titanium dioxide which is twice as efficient as currently available cells. Solar energy could be used for chilling hides and skins for short-term preservation. 

Occupational Health and Safety (OHS)
In substituting one chemical for another, the real environmental benefits and health and safety effects must be considered. Replacing a regulated chemical with a new chemical is not necessarily clean production. For example, the use of glutaraldehyde in tanning and aziridine or some isocyanates in finishing can be hazardous for the tannery workers. 

Air pollution 
Volatile Organic Carbon (VOC) emissions are now controlled because, in the presence of nitrogen oxides and light, VOC compounds produce ozone in the lower atmosphere and this causes summer smog. 

Tannery emissions controlled for OHS reasons include ammonia, hydrogen sulfide, formaldehyde, glutaraldehyde and particulates, which are generated during combustion of some fuels. 

Solid waste
There is a misconception that cleaner production eliminates industrial solid waste and as a consequence, communities are not providing suitable waste disposal facilities. Wastes must be minimised and utilised where possible but more research and development is needed to enable waste elimination. 

Salinity Reduction 
Salinity or TDS in effluent is the major environmental concern for the disposal of tannery effluent by irrigation. High salinity in irrigation water causes high osmotic pressure which results in reduced water availability and retarded plant growth of crops which are not salt tolerant. In addition, high concentrations of sodium ions in irrigation water can adversely affect soil structure by causing dispersion of clay. The presence of calcium and magnesium ions in tannery effluent ameliorates this effect by stabilising the soils. In addition, hair and lime from unhairing is beneficial for acidic soils. 

Salts and sodium ions are commonly added to all wet-blue processes but effluent loads are being minimised in Australia, as shown in Table 1, using a range of CSIRO developments: 

• processing green hides 
• reducing floats and chemical consumption 
• reusing lime liquors in conjunction with hair-saving processes 
• carbon dioxide deliming · direct recycling of chrome liquors 
• the use of magnesium oxide for basification and 
• eliminating washing after tanning. 

Direct chrome liquor recycling reduces the use of both sodium chloride and chrome powder, which contains up to 30% sodium sulphate. Chrome precipitation from the total volume of spent chrome liquors results in far higher TDS levels and should not be used when salinity is a concern. 

Appropriate technologies
In choosing the best technologies for a particular tannery, the overall environmental and cost benefits need to be assessed, including energy, water and chemical consumption, and solid waste disposal. After such an assessment by Australian tanneries, direct chrome liquor recycling with precipitation of excess chromium containing liquors is usually preferred to high-exhaust systems. Recirculating drums are being purchased although they are more expensive than conventional drums. Appropriate technology should be adopted rather than "best available technology". A particular technology may be best for one situation but not for another. In considering clean technologies, present beliefs should be questioned and new ideas should be generated. 

Towards zero waste 
KKSK Leathers in Tamil Nadu, India, provides an excellent example of new ideas and appropriate choice of technology for particular conditions. Good quality leathers are produced with a process which incorporates the following: 

• Water use for wet-blue production is less than 5 L/Kg hide and half of this is solar evaporated and half irrigated. 
• Floats are minimised and there is only one lime wash, one delime wash and a 20% final rinse. 
• Salted hides are soaked in 175 % float of partially treated effluent which contains some sulphide. The effluent that is treated is 100% lime liquor, small volumes of delime liquor, 30% pickle liquor and 45% supernatant from the chrome recovery plant. 
• The soak liquor is settled, clarified, sand filtered and dried in solar evaporation pans. The recovered salt is used as a pesticide 
• The lime wash liquor is used for 48 hour liming in 175% float 
• Deliming is in no float and there is no bate 
• The 100% delime wash is used for irrigation 
• Pickling is in 30% fresh water 
• Tanning is in 35% fresh water 
• Chromium is recovered from the tanning, wash and rechroming liquors and is reused 
• 50% fresh water is used for fleshine and cleaning and is irrigated 
• The 150% float for irrigation is mixed with liquors from processing of the wet-blue to finished leathers. The mixed liquors have a TDS of 2,100 mg/L, which meets the regulations 
• Sludge from the effluent treatment is dried and used as a fertiliser 
• There is a 30% saving in chemical costs 

Summary of Cleaner Processes