Organic carbon uptake and mineralization in slow sand filters and their relation to process variables

Slow sand filtration (SSF) is a widely used biofiltration method for drinking water treatment, yet quantitative measures of biological activity, specifically in terms of carbon uptake and mineralization, are not well established. This study assessed biological activity in mature SSF systems operated by three Dutch drinking water companies using carbon concentration measurements, isotopic signature analyses, and ¹³C-labeled glucose tracer experiments. Our results revealed measurable differences in carbon concentrations and isotopic signatures between influent and effluent waters. Specifically, DOC concentrations decreased by up to 0.13 mmol L⁻¹, while DIC concentrations increased by up to 0.84 mmol L⁻¹. Additionally, δ¹³C-DOC and δ¹³C-DIC values exhibited shifts of up to +1.9 ‰ and 2.9 ‰, respectively, indicative of carbon uptake and mineralization. Biological activity varied across filters, as reflected in the time required for complete removal of glucose, which ranged from 5 to over 48 h and correlated with assimilable organic carbon (AOC) removal rates. AOC loading appeared to be the primary driver of bioactivity, with the lowest activity found in a filter fed with dune-infiltrated water and the highest in a filter receiving ozonated influent water. These findings highlight the importance of considering source water characteristics and the preceding treatment chain to understand their potential impact on biological activity in SSF.