Considerations for the reduction of the amount of polluting value of the produced wastewater are:
- a reduction of the total water use by re-use of produced wastewater and by the development of technologies that minimize the quantity of water needed during the tanning process;
- a reduction of the used chemicals such as lime, salt, sulphide etc and a reduction of chromium.
The following gives a more detailed discussion (from Higham, 1991).Water conservation
A reduction of water use can lead to a reduction of the total waste load. Re-use of wastewater with a minimal harmful or even a moderately beneficial effect on earlier processes may be considered as an option.
Curing hides and skins
A reduction of the use of salt for preservation can be considered as an option. Fifteen percent of salt on weight basis may preserve the hides for even 6 weeks, and 5 per cent of salt plus biocide lead to a preservation for two months. Chilling without salt can preserve hides for a few days. Another alternative preservation method is radiation by electron beam or gamma rays. Where possible, biodegradable preservatives (insecticides etc.) should be used instead of derivatives of chlorinated aromatic hydrocarbons. The latter persist in the waste and are highly toxic to the environment.
Hair saving methods are recommended to prevent degraded keratin from entering the waste streams. Unhairing/liming fluids can be recycled after recharging. It is also recommended that the unhairing and liming stages should be seperated. Both liquids can be recharged and hair can be screened out. The intermediate wash can be re-used as a soak liquid.
Low chrome systems, possibly requiring an aluminium salt for pretannage will produce a wet-white leather. Splitting and shaving wastes will contain less chromium. Alternative mineral salts such as aluminium, zirconium, titanium and iron are might be used as substitutes for chromium salts. However, under certain conditions aluminium is known to be more poisonous to aquatic life than trivalent and even hexavelant chromium. Re-use of chromium is a more realistic alternative (see par. 5.2.2). The unused tanning fluids which contain chromium can be collected separately. From these fluids and from the solids that contain chromium, chromium can be recovered. The remainder may be used as source material for glue and animal feedstuff. In countries where discharge of chromium is strictly prohibited, great efforts are made to recover and re-use chrome.
Alternative vegetable tanning methods can replace chrome tanning to a high degree. An example is the ‘Liritan’ process, developed in South Africa. A high chemical uptake, low pollution load, uniform penetration of the tan and a shortened process time with consequent financial efficiency are claimed to be the main advantages of this process (Higham, 1991), but little is known on the practical implications.
A reduction of volatile organic compounds (VOC) can be accomplished by using aqueous finishes for base and middle finishing coatings.
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