Detachment of particles during biofilter backwashing

A model explains why bacteria and organic polymers are more difficult to remove than nonbiological particles during biofilter backwashing. Granular activated carbon (GAC) biological filters accumulate both biological and nonbiological particles on the filter bed. The difference in the way these groups of particles detach during backwashing is an important consideration in biofilter operation. This study compared the detachment of biological and nonbiological particles during filter backwashing and analyzed experimental results in terms of a microscopic force model. Filtration and backwashing experiments were completed in a glass column packed with GAC–sand media and an indigenous bacterial population. Compared with hydrophilic colloids, hydrophobic colloids produced a deeper primary minimum in interactions between particles. Consequently, hydrophobic bacteria were more strongly attached to the GAC surface than were hydrophilic clay particles. During backwashing of the biological filter, maximum turbidity occurred close to zero time, whereas maximum bacterial detachment occurred later.