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.