DEPOSITION AND RETENTION OF BACTERIA IN BACKWASHED FILTERS

Experiments tracked the deposition of two model bacteria Pseudomonas a eruginosa and Klebsiella pneumoniae-onto clean filter (i.e., glass bea ds and filter sand) and the subsequent removal of the bacteria by back washing. The ability of bacteria to accumulate and be retained in a fr equently backwashed filter is critical for effective removal of biodeg radable organic matter in a process termed biofiltration. Column filtr ation experiments showed that the bacteria, especially P. aeruginosa, readily attached to the filter media during filtration, despite unfavo rable chemical conditions. Water backwash was not effective at removin g bacteria from the filter media, with-only 20-40 percent removal duri ng 10-min backwash cycles at hydraulic loading rates ranging from 23 t o 53 m/h (9.3 to 22 gpm/sq ft) and bed expansions ranging from 15 to 5 0 percent. Incomplete removal of biomass during backwashing was benefi cial for removal of biodegradable total organic carbon, typically allo wing a backwashed biofilter to maintain treatment at levels similar to the period immediately preceding the backwash.