Changes in the microbiology of activated sludge upon the addition of iron salts with or without nitrite or nitrate.

Iron salts are frequently used in activated sludge treatment as coagulants or to improve reactor performance. However, the effect of the iron salts on the microbiology has received little attention. Furthermore, the interaction of iron ions with nitrite or nitrate present in the sludge has not been considered. In this research, the influence of ferrous and ferric iron additions, and their possible interactions with nitrite or nitrate at levels of a few mM, on the activated sludge microbiology of semi-continuous bench-scale reactors were examined. Conventional as well as molecular techniques have been used to investigate the microbial communities. Microscopic investigations showed that repeated addition of ferric iron at 2 mM gave rise to breaking of the sludge flocs, resulting in massive growth of sessile-protozoa and lowering of the numbers of nitrifiers. Ferrous iron at the same dosage resulted in large coherent flocs. Nitrite hereby seemed to enhance the detrimental effect of the ferric iron on the floc structure, whereas nitrate had a neutralizing character. 16S rRNA gene analysis showed a clear impact of the different iron respectively nitrogen treatments on the composition of bacterial communities. Cloning the sludge samples revealed the increased presence of denitrifying Feoxidizing bacteria in both the Fe(II) and Fe(III) reactors, suggesting a redox-state cycling between Fe(II) and Fe(III) in the mixed liquor.