Oxidation of ferrous iron in aerobic and anoxic conditions in activated sludge systems

Understanding iron (Fe) speciation dynamics in wastewater is essential for controlling Fe redox reactions, phosphate removal and predicting vivianite formation in wastewater treatment plants. This study investigates the oxidation rate of ferrous iron (Fe²⁺) in activated sludge systems under aerobic and anoxic conditions using controlled batch experiments at different total suspended solids (TSS) concentrations. Ferrous iron disappearance from the liquid phase was attributed to rapid adsorption onto biomass immediately after dosing, followed by oxidation; subsequent Fe²⁺ disappearance from the liquid phase followed pseudo first-order behavior and was governed by oxidation. Experimental results show that increasing TSS increased the adsorbed fraction of Fe²⁺, while the subsequent disappearance rates were primarily governed by oxidation and accelerated the apparent Fe²⁺ disappearance rate. The anoxic oxidation rate was measured for the first time under typical activated sludge conditions and was only 2.7% of the aerobic oxidation rate at comparable TSS. Experimentally derived apparent Fe²⁺ oxidation rate constants were subsequently implemented in both a generic continuous stirred tank reactor (CSTR) model and in a full-scale wastewater treatment process model, substantially improving the prediction of soluble and total Fe concentrations in the effluent and achieving closer agreement with full-scale observations. The results demonstrate that accurate representation of Fe²⁺ oxidation kinetics is critical for modelling Fe behavior and predicting effluent Fe and plant-wide Fe partitioning. However, adsorption primarily controls short term phase partitioning and has a limited influence on long term plant-wide Fe fate under typical operating conditions.