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

BACKGROUND: Both ferric iron Fe(III) and ferrous iron Fe(II) salts are commonly used for chemical phosphorus removal (CPR) in the activated sludge (AS) process but only a few studies have compared Fe(III) and Fe(II) salts regarding their effect on the physical and biological properties of AS. In this research, the units of a continuous flow laboratory‐scale AS plant were dosed with Fe(III) and Fe(II) salts at a concentration of 25 mg Fe dm^?3 feed and changes in the AS properties were measured as Fe accumulated or washed out during startup, normal operation and withdrawal of dosing. RESULTS: The morphological characteristics of the flocs showed marked differences depending on the type of ion used. Fe(II) dosed flocs were more compact, less filamentous and smaller in size than Fe(III) dosed flocs. The settleability index of the Fe(II) dosed sludge was lower than that of the Fe(III) dosed sludge. The activity of ammonium (NH₄⁺‐N) and nitrite (NO₂^?‐N) oxidizing bacteria was found to be affected by the accumulation of Fe relating products into the sludge. CONCLUSIONS: Fe(II) was a more effective flocculent than Fe(III) and this was attributed to its ability to form stronger ionic bonds with the flocs prior to its oxidation to Fe(III). A hypothesis explaining the mode of action of Fe(II) is proposed. Floc surface properties were enhanced, this being beneficial to the morphological characteristics and settleability with further implications for the operation of AS. However, the effect may be reversed at high Fe contents. Copyright © 2010 Society of Chemical Industry