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

Iron (Fe 2+ ), manganese (Mn 2+ ), and ammonium (NH 4 + ) 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 NH 4 + and Mn 2+ oxidation, allowing Mn 2+ to oxidize before NH 4 + . To achieve this uncommon sequence, the filter was operated with low O 2 concentrations (∼0.02 mmol/L, ∼0.5 mg/L) and a high pH (∼8). Under these conditions, Mn 2+ oxidation is consuming all available O 2 , suppressing the occurrence of NH 4 + oxidation. In the filter with low O 2 (0.08 mmol/L, ∼3 mg/L) and low pH (∼6.8), the opposite was observed, as Mn 2+ oxidation was delayed under these conditions, resulting in complete O 2 consumption by NH 4 + -oxidizing bacteria. Reactive transport modelling and parameter estimation revealed that Mn 2+ oxidation is one order of magnitude faster in absence of NH 4 + oxidation (1.4×10 −2 vs 2.5×10 −3 mmol/L), whereas NH 4 + oxidation seemed to be accelerated by simultaneous Mn 2+ oxidation (6.8×10 −3 vs 2.9×10 −2 s −1 ). This interconnection between Mn 2+ and NH 4 + oxidation was further emphasized by the observation of Mn 2+ release in the presence of NO 2 − . 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.