Shutdown of BAC filters: effects on water quality

Biological activated carbon (BAC) filtration is used in drinking water trea tment to biodegrade dissolved organic carbon (DOC) and to nitrify ammonia. This study evaluated BAC filters before, during, and after shutdown. Concen trations of five substrates were measured: ammonia, nitrite, nitrate, DOC, and dissolved oxygen. Media samples were taken simultaneously at similar de pths to evaluate the effect of a shutdown on the densities of fixed bacteri a. Dissolved oxygen concentrations quickly dropped below 2 mg/L within the BAC filters during the first 2 h of filter shutdown. At the same time, nitr ite and bromide concentrations increased rapidly. Fixed bacterial biomass d ensities, measured at several depths in the filter after a 24-h shutdown, d eclined. Concentrations of DOC and ammonia, measured in the effluent after the filter was restarted, were higher than those measured before shutdown. Backwashing after shutdown, before the filter was returned to normal operat ion, eliminated these negative effects and restored the biodegradation perf ormance of the BAC filters.