Assessing Escherichia coli removal in the schmutzdecke of slow‐rate biofilters

A series of studies conducted on laboratory‐scale sand columns confirmed that Escherichia coli (E. coli) removals in slow‐rate biological filters occur primarily at the interface (schmutzdecke) and are statistically related to operational (schmutzdecke ripening state, biological activity, temperature, and protistan abundance) and design (empty bed contact time and media grain size) parameters. Biological activity, as measured by carbon dioxide respiration in the top 2.5 cm, as well as protistan abundance in the top 0.5 cm of the schmutzdecke, correlated positively to E. coli removal. The role of predation deserves further investigation. The ability of a slow‐rate biofilter to reestablish an effective schmutzdeckeremoval layer after a scouring or scraping event was not strongly related to biomass concentrations or biological ripening conditions of the supporting media prior to scraping or scouring. Slow‐rate biological filters should be operated in such a way as to enhance biological activity (in addition to biomass accumulation alone) and increase protistan abundance in the schmutzdecke.