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TECHNICAL REPORT
WASTE MANAGEMENT

Transport of Escherichia coli in Sand Columns with Constant and Changing Water Contents

Department of Environmental Sciences, Clark Hall, Univ. of Virginia, Charlottesville, VA 22903

Corresponding author (dkp9e{at}virginia.edu)

Received for publication January 13, 2000. Understanding how changes in volumetric water content () affect bacterial adsorption could help reduce transport of pathogenic and indicator bacteria that may be present in infiltrating wastewater. Three flow regimes that simulated infiltration from a household septic system were evaluated: saturated, unsaturated with a constant volumetric water content (constant unsaturated flow), and unsaturated with cyclic changes in (variable unsaturated flow). Escherichia coli was suspended in artificial sewage (AS) and applied as step inputs to sand columns, with regular interruptions in input for variable unsaturated flow. A transport model was fit to the saturated and constant unsaturated flow breakthrough curves to determine retardation (R), the first-order filtration coefficient (µ), and the maximum outflow relative concentration (C_max). The total cells transported as a fraction of input () in all three flow regimes was calculated. Constant unsaturated flow resulted in a significantly lower C_max (0.633) in comparison with saturated flow (0.803, P 0.05), although unsaturated µ (0.0693 h^-1) was not significantly different from saturated µ (0.0259 h^-1). Constant unsaturated flow also resulted in a significantly smaller (0.617) than saturated (0.806) or variable unsaturated flow (0.734). In variable unsaturated flow, cell concentrations were out of phase with —as the column drained, cell concentrations in the outflow increased; and when a pulse of suspension was applied, cell concentrations decreased. Constant unsaturated flow is probably the best for removal of pathogenic bacteria because this regime resulted in lower maximum concentrations of E. coli and greater cell removal, in comparison with saturated and variable unsaturated flow.

Abbreviations: A, cross-sectional area • AS, artificial sewage • AWI, air–water interface • c, concentration • C_max, maximum relative concentration • D, dispersion coefficient • L, column length • pv, pore volume • N, number of cells • Q, volumetric flow rate • R, retardation coefficient • s, suspension volume • t, time • TPC, triple-phase contact • v, average linear velocity • x, distance • µ, first-order filtration coefficient • , volumetric water content • , fraction of cells transported

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