This study investigated the effects of free and combined C12 in biofilter influents and the backwash water on the removal of selected biodegradable organic matter (BOM) components and the accumulation of viable biomass in such filters. Data were collected at bench-scale using four parallel 50 mm (2") ID glass filter columns. The influent of the columns was dosed with a cocktail of easily biodegradable substances. The results showed that in the case of the continuous presence of C12, the threshold free C12 concentration above which the removal of easily biodegradable BOM components is inhibited was less than 0.2 mg/L C12. For continuously present combined C12, this threshold concentration was found to be between 0.1 and 0.3 mg/L C12. Chlorinated backwash water (~1 mg/L C12) considerably impaired the removals of acetate and formate, particularly if the chlorine in the backwash water was present as free C12.