Tanneries

Hides are a by-product of slaughter activities and can be processed into a wide range of end products. For each end product the tanning process can be different and the kind and amount of waste produced may vary enormously. The chemicals traditionally used for tanning have been derived from plants, whereas the most common process nowadays is a combination of chrome salts (chrome tanning) and readily useable vegetable extracts (vegetable tanning). While chrome tanned shoe leather is the most widely produced leather, this kind of leather will receive most attention in the following. In most cases raw hides produced at slaughterhouses are preserved by pickling and drying before transport to tanneries and further treatment. During the tanning process at least 300 kg of chemicals are added per ton of hides.

Mineral tanning, mainly based on chromium, is the most popular process for large scale tanning because it acts quickly and produces leather with desirable physical and chemical properties. In many countries tannery effluents are discharged into sewers or inland surface water or brought onto the land with irrigation water. The high concentrations of salt and hydrogen sulphide affect the quality of water and may cause bad taste and odour. Suspended matter (lime, hair, fleshings, etc.) makes the surface water turbid and settles eventually on the bottom. Both processes create unfavourable conditions for aquatic life. Mineral tannery waste water that is discharged on land, will affect the soil productivity adversely and may cause land to become infertile. As a result of infiltration, the quality of groundwater is affected adversely also. Discharge of untreated tannery effluents into the sewer system causes deposition of calcium carbonate and choking of the sewer. It is possible to treat the waste water effectively before it is discharged into surface water. As a result of this purification the chromium and BOD levels of the purified water is relatively low. The sludge in the waste water systems has to be brought to special dumping grounds because of its chromium content.

The following matrix indicates the most common Industrial risks associated with tanneries

Proceed to the Matrix of Underlying Problems and Potential Impacts

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