On the basis of these results and preliminary test, the main experiment was done with 0.05 g l−1 of Evans blue. Biodegradation and biotransformation may be anaerobic, aerobic or a combination of these two. The main experiment was concentrated on Evans blue decolourization in different process conditions. Under anaerobic conditions, many bacteria reduce azo dyes by unspecific cytoplasmic azoreductases. Generally, increase of oxygen concentration decrease colour removal by live bacterial cells, because of direct inhibition of the azoreductase or preferential reduction of oxygen rather than the azo derivatives (Srinivasan and Viraraghavan 2010).