A difficult coexistence: resolving the iron-induced nitrification delay in groundwater filters

Rapid sand filters (RSF) are an established and widely applied technology for the removal of dissolved iron (Fe2+) and ammonium (NH4+) in groundwater treatment. Most often, biological NH4+ oxidation is delayed and starts only upon complete Fe2+ depletion. However, the mechanism(s) responsible for the inhibition of NH4+ oxidation by Fe2+ or its oxidation (by)products remains elusive, hindering further process control and optimization. We used batch assays, lab-scale columns, and full-scale filter characterizations to resolve the individual impact of the main Fe2+ oxidizing mechanisms and the resulting products on biological NH4+ oxidation. Modelling of the obtained datasets allowed to quantitatively assess the hydraulic implications of Fe2+ oxidation. Dissolved Fe2+ and the reactive oxygen species formed as byproducts during Fe2+ oxidation had no direct effect on nitrification. The Fe3+ oxides on the sand grain coating, commonly assumed to be the main cause for inhibited nitrification, seemed instead to enhance nitrification by providing additional surface area for biofilm growth. Modelling allowed to exclude mass transfer limitations induced by accumulation of iron flocs and consequent filter clogging as the cause for delayed nitrification. We unequivocally identify the inhibition of NH4+oxidizing organisms by the Fe3+ flocs generated during Fe2+ oxidation as the main cause for the commonly observed nitrification delay. The addition of Fe3+ flocs inhibited NH4+ oxidation both in batch and column tests, and the removal of Fe3+ flocs by backwashing completely re-established the NH4+ removal capacity, suggesting that the inhibition is reversible. In conclusion, our findings not only identify the iron form that causes the inhibition, albeit the biological mechanism remains to be identified, but also highlight the ecological importance of iron cycling in nitrifying environments. ### Competing Interest Statement The authors have declared no competing interest.