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 (Fe 2+ ) and ammonium (NH + ) in groundwater treatment. Most often, biological NH + oxidation is delayed and starts only upon complete Fe 2+ depletion. However, the mechanism(s) responsible for the inhibition of NH + oxidation by Fe 2+ 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 Fe 2+ oxidizing mechanisms and the resulting products on biological NH + oxidation. Modelling of the obtained datasets allowed to quantitatively assess the hydraulic implications of Fe 2+ oxidation. Dissolved Fe 2+ and the reactive oxygen species formed as byproducts during Fe 2+ oxidation had no direct effect on nitrification. The Fe 3+ 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 NH + oxidizing organisms by the Fe 3+ flocs generated during Fe 2+ oxidation as the main cause for the commonly observed nitrification delay. The addition of Fe 3+ flocs inhibited NH + oxidation both in batch and column tests, and the removal of Fe 3+ flocs by backwashing completely re-established the NH + 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. Graphical abstract Highlight Dissolved Fe 2+ and reactive oxygen species do not affect NH + oxidation Fe oxide coating aids sand grain colonization by NH + -oxidizing bacteria Fe 3+ flocs inhibit NH + oxidation by reducing the nitrifying capacity of AOB Changes in transport patterns due to clogging do not play a major role in NH + oxidation The inhibition of NH 4 + oxidation is reversible and reduced by backwashing