Mitigating NOM competition against micropollutant adsorption through staged dosing of activated carbon

Abstract

Powdered activated carbon (PAC) adsorption is widely applied for the removal of organic micropollutants in drinking water treatment. However, conventional single-dose PAC application requires high dosages to overcome competitive adsorption from natural organic matter (NOM). This study evaluates a multi-stage PAC dosing strategy to mitigate NOM competition and enhance the removal efficiency of representative odorous micropollutants. Experiments with NOM-rich surface water showed that multi-stage dosing increased adsorption capacity and achieved up to 48% PAC savings at an 80% micropollutant removal benchmark, compared to single-stage application. In contrast, no improvement was observed in NOM-free water. Two-stage PAC dosing considerably improved the removal of weakly adsorbing compounds such as 2-methylisoborneol (MIB) and 2-ethyl-5,5-dimethyl-1,3-dioxane (EDD), while incremental gains were observed for strongly adsorbing micropollutants (e.g., 2-butyl-5,5-dimethyl-1,3-dioxane) with three- or four-stage configurations. Mechanistic analysis indicated that early-stage PAC doses preferentially adsorbed competitive NOM components, preserving high-affinity sites for micropollutants in later stages. PACs enriched in narrow mesopores outperformed microporous PACs in the staged dosing configuration. The proposed strategy, as compared to existing designs summarized from the literature, requires minimal infrastructural modifications and offers a cost-effective, scalable approach for improving micropollutant removal under NOM-rich conditions.