Biofiltration performance: part 2, effect of backwashing

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 carbon, disinfection by-product (DBP) precursors, or ozone DBPs. Backwashing a similar filter with chlorinated water, however, resulted in lower concentrations of biomass attached to the media at the top of the filter and lower steady-state removal of the same parameters. A loss of some of the attached biomass at the top of the filter occurred immediately after backwashing with chlorinated water, but later in the filter cycle the biomass recovered to its prebackwashing concentration. Concurrent with this loss of biomass was an increase in the concentrations of ozone DBPs in the filter effluent; these also recovered to prebackwashing concentrations later in the filter cycle.