Measuring and modelling straining of Escherichia coli in saturated porous media.
Though coliform bacteria are used worldwide to indicate fecal pollution of groundwater, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown. We evaluated the occurrence of both straining and attachment of E. coli ATCC25922 in columns of ultra-pure, angular, saturated quartz sand. The column experiments were conducted over a wide range of porous medium sizes, column heights, input concentrations, and pore water flow velocities. Straining and attachment were examined by modelling the breakthrough curves (with HYDRUS 1D). In addition, model output was compared with measured strained and attached bacteria via column extrusion experiments (in which sand was extruded from the column and placed in excess water) and flow reversal experiments (in which the pore water flow direction was reversed, thereby dislodging strained bacteria). Our model consisted of an attachment rate coefficient and a straining rate coefficient; both of these decreased with transport distance. The straining rate coefficient also decreased in a Langmuirian way, in response to the filling of available pore space, which in turn depended on influent bacteria concentration, quartz grain diameter, and transport distance. The maximum strained fraction was 25-30% of total bacteria mass applied to the column; the maximum attached fraction was 30-35%. The fit between modelled and measured (strained and attached) bacteria masses was acceptable, as was the sensitivity of the model output to fitted parameter values. Our results lead to a new description for the time-dependent mass balance of strained bacteria, which entails using three fitting parameters. The results also imply that column experiments in combination with retention profiles (or various column lengths) are not enough to explain the retention processes in a column. Column extrusion and flow reversal experiments provide vital additional information on the occurrence and magnitude of straining. Our straining model could be of assistance in evaluating the importance of straining and in incorporating the straining process in bacteria transport modelling.
Foppen
Jan Willem
JW
UNESCO-IHE Institute for Water Education, P.O. Box 3015, 2601 DA Delft, The Netherlands. j.foppen@unesco-ihe.org
van Herwerden
Manon
M
Schijven
Jack
J
D003198:Computer Simulation; D004563:Electrochemistry / Q000379:methods; D004926:Escherichia coli / Q000378:metabolism*; D007700:Kinetics; D015233:Models, Statistical; D008962:Models, Theoretical; D016062:Porosity; D012680:Sensitivity and Specificity; D013997:Time Factors; D014867:Water / Q000737:chemistry; D014871:Water Microbiology; D014872:Water Movements; D018508:Water Purification; D014881:Water Supply
D014867:Water