Effects of tri-valent (Cr(III)) and hexa-valent (Cr(VI)) chromium on the growth of activated sludge

The effects of Cr(VI) and Cr(III) species on the activated sludge growth rate have been assessed for a batch growth system, for a range of chromium concentration between 0 and 320 mg l−1. Cr(VI) was found to stimulate microbial growth for concentrations up to about 25 mg l−1, exhibiting maximum growth stimulation at 10 mg l−1, whilst the lethal dose was found to be between 80 and 160 mg l−1. On the other hand, Cr(III) was also found to stimulate microbial growth for concentrations up to about 15 mg l−1, (with a maximum stimulation concentration at 10 mg l−1), whilst the lethal dose was found to lie between 160 and 320 mg l−1. The results indicate that Cr(VI) is more toxic to biomass at relatively high concentrations (higher than 70 mg l−1) whilst it has a more pronounced growth stimulation effect at relatively smaller concentrations (less than 25 mg l−1), compared with Cr(III).