Azo dye decolorization by halophilic and halotolerant microorganisms

Different types of microorganisms are capable of degrading azo dyes due to their high metabolic potentials. However, many of them cannot be used as degrading agents due to the harsh conditions of dye-polluted environments. Here, halophilic and halotolerant microorganisms can be the best candidates for a practical biodecolorization process as they are able to grow easily at high concentrations of salts. In addition, some of them can tolerate the presence of other stress factors such as toxic oxyanions and heavy metals which are so common in industrial wastewaters. In recent years, several studies have been focused on halophilic and halotolerant microorganisms and their abilities for decolorization of azo dyes. For example, Shewanella putrefaciens was determined to be capable of the complete removal of Reactive Black-5, Direct Red-81, Acid Red-88 and Disperse Orange-3 (all 100 mg l(-1)) within 8 h in the presence of 40 g l(-1) NaCl. Another halophilic example is Halomonas sp. GTW which has shown a remarkable performance in the removal of different azo dyes within 24 h in the presence of 150 g l(-1) NaCl. Although these approaches need to be studied in more detail, some studies have designed different types of fermentation processes and even specific fermentors to provide a practical methodology for industrial wastewater remediation. Sequential anaerobic EGSB (expanded granular sludge blanket) and aerobic reactor was the result of an important attempt to design an effective approach to large-scale biodecolorization.