Impacts
Tannery waste waters are highly polluting. It is estimated that in India alone 2000 t to 3000 t of chromium escape into the environment annually with chromium concentration ranging from 2000 to 5000 mg/l in the aqueous effluents as against the permissible level of 2 mg/l. Chromium containing sludge disposal is also one of the major environmental issues. Tannery effluents influence the growth and yield patterns in crops and are bio accumulative poisons. At high concentration, Chromium is toxic, mutagenic, carcinogenic and teratogenic. Out of all the valency states of chromium, the trivalent (which is generally used in India) is the most stable, essential to mammals and relatively immobile in the aquatic system owing to its solubility. The hexavalent form is more toxic for the aquatic environment. While most microorganisms are sensitive to chromium VI toxicity, some may be more resistant (Tolerating high levels). The availability of these strains raises the possibility of employing microorganisms for bioremediation of Cr(VI) contaminated sites in a more economically viable way with respect to chemical remediation.
Agricultural fields in Jajmau area (Kanpur) which have been subject to decades of pollution sites haven been found to be highly contaminated with heavy metals. The mean concentrations reported in soils were 1178 mg/g of chromium, 51.5 mg/g of Nickel, 39.86 mg/g of copper, and 1.09 mg/g of cadmium. Chromium levels are high because of the use of tannery waste water for irrigation purpose. Plants in the area have exhibited accumulation of metals. In the soil system, chromium VI is reduced to chromium III by the organic and microbial ecology of the soil. Chromium III uptake is limited because of the highly impermeable nature of chromium III to the cell membrane. The uptake of the other heavy metals is however more pronounced. 50% to 70% of bacterial isolates have been formed to be resistant to chromium III. Pseudomonas and Rhizobium isolates from the root nodules of Trifolium alexandrinum have evolved a resistance mechanism to deal with metal toxicity.
Tannery Effluent Treatment
Tannery effluents can be treated through physico-chemical treatment followed by biological treatment and further tertiary treatment to meet the standards. Technology is being constantly upgraded to achieve cost effective end of pipe solutions. An improved solar evaporation system has facilitated reduction in time and space. The anaerobic treatment of tannery waste water has utilized technologies based on lagoons, contact filter, UASB (Upflow Anaerobic Sludge Blanket) reactor and high rate biomethenation. The use of closed bio reactors offers a possibility to recover sulphate from H2S and energy from methane. Anaerobic treatments result in only 50 to 60% of BOD, COD reduction. Post anaerobic treatment technologies are utilized to reduce the BOD, COD loads to permissible limits. These include microbial degradation in aerobic conditions through the use of aerators. A new post treatment technology by the name of ‘Chemo Autotrophic Activated Carbon Oxidation’ system has been developed and adopted for treatment of anaerobically treated tannery waste.
Activated carbon filters reedbed and root zone technologies and reverse osmosis methods are being investigated for providing tertiary treatment of tannery waste water.
Treatment of tannery effluents by electro flotation technology has also been studied and found to be very effective especially for the removal of pathogenic bacteria and colors.
In pilot studies at Chennai the separation of suspended solids and chromium by electro flotation was almost complete (> 95%). The removal of COD was 15-20% greater than conventional chemical coagulation and 99.97% of pathogenic bacteria were removed by electro flotation.
Chrome which is let of as effluent ideally has to be recovered for reuse in chrome recovery plants in order to economize on production costs and also for an efficient management of toxic substances. It is precipitated by an alkali and precipitate treated with sulphuric acid to get the basic chrome sulphate which can be reused. Commercial polyelectrolytes are also used for facilitating chrome precipitation. The technology is capable of recovering 98 to 99% of chrome. The cost of installation can be recovered in 12 to 24 months depending on the scale of operation.
Paddle based processes for small tanneries, batch type processes for medium and large tanneries and some continuous processes for common chrome recovery plants have been standardized.
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