Modeling bacterial attenuation in on-site wastewater treatment systems using the active region model and column-scale data

Bacterial attenuation in porous media is often higher in columns than in the field. This study investigates whether this inconsistency could be attributed to finger flow, as assessed by the active region model (ARM). It develops a numerical model of flow and transport of the fecal indicator Escherichia coli  in a wastewater infiltration basin from the literature. Modeling was based on the traditional, uniform flow approach (Richard’s equation) as well as the ARM, representing finger flow. The uniform flow model resulted in flow rates that decreased rapidly with filter depth and an underestimation of the observed average relative effluent concentration by three orders of magnitude. With the ARM, the flow rates remained high throughout the filter, more consistently with observations, and the relative effluent concentration (0.018) was relatively accurate in reproducing the field result (0.025). Considering a range of removal rates derived from laboratory studies, the ARM consistently enabled more accurate and conservative assessments of the filter efficiency; thus, results indicated that the ARM provides a more relevant approach to bacterial transport in wastewater infiltration basins with sandy, unstructured soils.