In a biological dual-media (anthracite-sand) filter, backwashing with
nonchlorinated water did not affect the biomass attached to the media
at the top of the filter or the control of total organic DBPs. Backwas
hing a similar concentrations of biomass attached to the media at the
top of the filter and lower steady-state removal of the same parameter
s. A loss of some of the attached biomass at the top of the filter occ
urred immediately after backwashing with chlorinated, water, but later
in the filter cycle the biomass recovered to its prebackwashing conce
ntration. Concurrent with the loss of biomass was an increase in the c
oncentrations of ozone DBPs in the filter effluent: these also recover
ed to prebackwashing concentrations later in the filter cycle.