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Mode of action of ferric and ferrous iron salts in activated sludge

This paper is significant because it presents the results of detailed, long-term laboratory experiments to demonstrate that the choice to use either Fe(II) or Fe(III) salts for phosphorus precipitation (considered interchangeable by the water industry except in price) alters the floc structure and biological responsiveness of an activated sludge plant. Although Fe(II) is the cheaper chemical, our results showed that it produced tightly iron-bound flocs with reduced biological activity. We hypothesise this is caused by slower oxygen transfer into the flocs, increasing the need for aeration at full scale, so increasing electricity costs and the carbon footprint of the process.