ground water pollution due to tanneries

Answer 1

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Tanning is a process in which by means of the addition of a tanning agent (chromium salts,aluminium, zirconium, etc. or vegetable extracts of mimosa or quebracho tree, etc.) it is possible to stabilise the skin structure by forming transversal bonds among its fibres. The tanning agent blocks carboxylic groups, in the case of mineral tanning agents, or the amine groups, in the case of vegetable tanning agents, and joins the proteinic colloid thus increasing the crosslinking of collagen fibres. This way, the leather is turned into a durable material in respect of physical and biological degradation. The complete tanning process, including finishing treatments, lends the leather the characteristics of resistance, smoothness, softness, colour, etc. required for its industrial application.Currently, chrome tanning is the most widely used technique for leather tanning, as it offers a product with the best performance at a reasonable price. By this technique, around 8% of the leather weight is added as chromium salt. However, a significant share of the pollution resulting from tannery wastewater comes from this stage, due to the chrome that is not fixed to leather amounting 15% of the total chrome added to the tanning bath. Tannery wastewater is usually homogenised with the rest of industrial effluents and chrome precipitates as chromium hydroxide, thus being retained in the sludge of the water treatment plants. Another environmental impact, which cannot be easily corrected in tannery wastewater, is salinity, reaching conductivity values on the order of 10.000-12.000 µs/cm. The main contribution to the effluent salinity is derived from the salt used for the preservation of the skins after flaying, followed by the high salinity of the pickling baths (preparation of skins with salt -sand acids prior to the addition of the tanning agent) and the tanning stage. Therefore, 30% of the chlorides contained in the effluent come from the pickling bath, and 60% of the sulphates come from the tanning bath. For these reasons, both from an environmental and financial point of view, it is necessary to put forward new alternatives for the reduction of chrome content in effluents and their salinity, as well as for the reduction of water and raw material consumption.

 

Answer 2

what hazardous materials were used to cause the water pollution?