Deposition and retention of bacteria in backwashed filters

Biomass accumulation in laboratory‐scale filters is not impaired by backwash with water alone. Experiments tracked the deposition of two model bacteria—Pseudomonas aeruginosaand Klebsiella pneumoniae—onto clean filter media (i.e., glass beads and filter sand) and the subsequent removal of the bacteria by backwashing. The ability of bacteria to accumulate and be retained in a frequently backwashed filter is critical for effective removal of biodegradable organic matter in a process termed biofiltration. Column filtration experiments showed that the bacteria, especially P. aeruginosa, readily attached to the filter media during filtration, despite unfavorable chemical conditions. Water backwash was not effective at removing bacteria from the filter media, with only 20–40 percent removal during 10‐min backwash cycles at hydraulic loading rates ranging from 23 to 53 m/h (9.3 to 22 gpm/sq ft) and bed expansions ranging from 15 to 50 percent. Incomplete removal of biomass during backwashing was beneficial for removal of biodegradable total organic carbon, typically allowing a backwashed biofilter to maintain treatment at levels similar to the period immediately preceding the backwash.