Modeling the Ammonium Removal Processes in Household Sand Filters

Elevated ammonium (NH₄⁺) concentrations in groundwater (GW) pose significant challenges to existing GW treatment systems, particularly in simplified systems such as household sand filters (HSFs), which are widely used in developing countries. We previously conducted a series of column experiments (sand filter materials collected in Hanoi, Vietnam) mimicking HSFs. These experiments revealed limited and fluctuating NH₄⁺ removal, highlighting the need for a comprehensive process-based model to elucidate the complex interplay of physical and biochemical processes that influence NH₄⁺ concentration dynamics in these systems. Here, we established a one-dimensional advective-dispersive-reactive model conditioned on data from column experiments under laboratory (artificial GW inflow with sand materials from local HSFs) and field conditions (natural GW inflow with sand materials from local supplier), accounting for temporal variations in reaction kinetics, transport processes, and a previously unconsidered inter-phase transfer process for nitrate (NO₃^-). The modeled breakthrough curves capture the complex dynamics of NH₄⁺, nitrite (NO₂^-), and NO₃^- concentrations under both laboratory and field conditions. The reaction rates of the nitrogen species show strong hysteresis in response to substrate (NH₄⁺ and NO₂^-) concentrations, suggesting that potential lags in the biochemical reactions caused by inhibitions and low retention time lead to the incomplete NH₄⁺ removal. Our scenario analysis indicates that, without inhibition effects, the current bio-reactive environment could reduce NH₄⁺ concentrations to the legal target level (within up to eight hours retention time under field conditions). This study represents one of the few process-based modeling efforts mimicking HSFs. Future modeling research should parameterize various inhibition effects into the existing reactive transport models in order to gain quantitative insight into enhancement methods for HSFs.