The specific objectives of this research included: ranking the relative importance of various media characteristics and operational conditions on biofiltration removals of E. coli bacteria; assessing the effect of a sudden removal of the schmutzdecke on pathogen removal in the event of scouring or cleaning and a filter's ability to recover from such an event; comparing the role of protistan predation to that of adsorption and straining as a pathogen removal mechanism; and, determining the extent to which biological activity contributes to pathogen removal in a biological filter. Specifically, do the extracellular polymeric excretions of the biofilm present in drinking water treatment enhance the "stickiness" of filter media? Methods included six studies conducted on laboratory scale sand columns under varying operational and design conditions with spiked concentrations of E. coli to assess removal efficiency. Results confirmed that E. coli removals in slow-rate biological filters occur primarily at the interface. These interfacial removals appear to be related to schmutzdecke ripening state, empty bed contact time, biological activity, temperature, and protistan abundance. Includes 11 references, tables, figures.