pH-based control of NH₄⁺ and Mn²⁺ oxidation sequence in low-oxygen groundwater filters

Iron (Fe²⁺), manganese (Mn²⁺), and ammonium (NH₄⁺) are the three most common contaminants in anaerobic groundwater and are typically removed in rapid sand filters in a series of simultaneous, uncontrolled, and interconnected redox reactions. In this study, we demonstrated separation of these oxidation processes, including reversing the order of NH₄⁺and Mn²⁺oxidation, allowing Mn²⁺to oxidize before NH₄⁺. To achieve this uncommon sequence, the filter was operated with low O₂ concentrations (∼0.02 mmol/L, ∼0.5 mg/L) and a high pH (∼8). Under these conditions, Mn²⁺ oxidation is consuming all available O₂, suppressing the occurrence of NH₄⁺oxidation. In the filter with low O₂ (0.08 mmol/L, ∼3 mg/L) and low pH (∼6.8), the opposite was observed, as Mn²⁺ oxidation was delayed under these conditions, resulting in complete O₂ consumption by NH₄⁺-oxidizing bacteria. Reactive transport modelling and parameter estimation revealed that Mn²⁺ oxidation is one order of magnitude faster in absence of NH₄⁺ oxidation (1.4 × 10^-2 vs 2.5 × 10^-3 mmol/L), whereas NH₄⁺ oxidation seemed to be accelerated by simultaneous Mn²⁺ oxidation (6.8 × 10^-3 vs 2.9 × 10^-2 s^-1). This interconnection between Mn²⁺ and NH₄⁺ oxidation was further emphasized by the observation of Mn²⁺ release in the presence of NO₂^-. In conclusion, this study has shown that a shift from conventional aerated groundwater treatment to sequential oxidation in separate filters offers (i) a more controllable system, (ii) the potential to optimize the rates of each oxidation process separately, which would ultimately result in higher flows and less backwashing.