Wastewaters from industrial application possess heavy metals and anonymous organics, causing horrendous environmental pollution worldwide. Chromium is one of the toxic heavy metals that is widely used in electroplating, leather tanning, textile dyeing, and metal processing industries. Chromium discharge in wastewater contaminates water bodies; endangers aquatic lives, posing a serious health hazard. Elevated organics load (as high COD) causes depletion of dissolved oxygen in water, results the aquatic life to suffocate and die. Hence, it is a public health concern to solve these problems by reducing the pollution load from industrial effluents. In Bangladesh, about 250 leather processing industries at Hazaribagh, South Western part of the Dhaka city, are discharging both liquid and solid wastes into canals and rivers, and thus increasing the amount of chromium and chemical oxygen demand (COD) in the surrounding waterbody . Human Right Watch reported that tannery wastewater from 47 tanneries of Hazaribagh contained extremely elevated levels of chromium (4,043 mg/L) and COD (9,300 mg/L) . It was found that the COD of the tannery effluents at Hazaribagh were ranged from 510 to 2555 mg/L . On the other hand, the tannery effluents in Chittagong contained hexavalent chromium (70.33 mg/L) and higher COD (6650 mg/L) . Cr (VI) was also found in the deep tube wells from Hazaribagh that exceeded the Bangladesh standard limit (0.05 mg/L) of chromium in drinking water .
Cr (VI) is toxic as it has higher solubility in water, rapid permeability through biological membranes and subsequent interaction with intracellular proteins and nucleic acids [5, 6]. Thus accumulation of toxic heavy metals in human lead to the growth and developmental of abnormalities, carcinogenesis, neuromuscular control defects, mental retardation, renal malfunction and other illnesses . The prevalence of diseases among the tannery workers were studied and reported as the gastrointestinal problem (71.7%), diarrhea (71.7%), blood pressure (52.2%), asthma (49.9%), eye problem (46.7%). scabies (73.9%), nail discoloration (69.6%), urticaria (59.7%), miliria and foliculities (56.5%) .
Therefore, it is of utmost importance to treat the effluents before discharging into the environment for the reduction of environmental pollution and the disease burden. Numerous technologies are available for the remediation of chromium-contaminated effluents which includes precipitation, coagulation, reverse osmosis , and redox chemical processes . But the conventional methods for the removal of metals from contaminated sites are expensive and require large amounts of energy and chemical reagents with the possible generation of secondary pollution [10, 11]. The use of microorganisms, on the other hand, is an alternative method for the treatment of metal-contaminated effluents. It allows the application on a large volume of effluent, low cost of operation and energy efficiency of metal removal [11, 12]. Due to their high surface area to volume ratio, they can provide a large contact interface, which would interact with metals from the surrounding environment .
Various microorganisms including bacteria, yeast, algae, protozoa, and fungi are found in waters receiving industrial effluents and have developed the capabilities to protect themselves from heavy metal toxicity. They use different mechanisms such as adsorption, uptake, methylation, oxidation, and reduction in order to survive in heavy metal polluted environment . These microorganisms including Acinetobacter & Ochrobactrum , Arthrobacter , Serratia marcescens , Bacillus spp. , Desulvofibrio vulgaris , Cellulomonas spp. , Intrasporangium sp. Q5-1, Bacillus sp. ES29, Escherichia coli, Enterobacter cloacae, Pseudomonas fluorescens LB300 [21-25] have capacity to reduce the highly soluble and toxic Cr (VI) to the less soluble and less toxic Cr (III). Previous study showed that a wide variety of chromium resistant bacteria (Micrococcus luteus APBS5-1, Bacillus pumilus APBS5-2, Bacillus flexus APBE3-1, Virgibacillus sediminis APBS6-1 can be used for the reduction of COD from tannery effluents .
The application of microbes for bioremediation of heavy metals and other contaminants is still in its infancy in the developing countries like Bangladesh. There is no concrete study on the isolation and application of chromium resistant bacteria in the treatment of wastewater. Therefore, the present study was conducted for the isolation and characterization of Cr (VI) resistant bacteria from tannery effluents and their application in the removal of hexavalent chromium and COD from wastewater.