The synergistic effect of excimer and low-pressure mercury lamps on the disinfection of flowing water.

Microorganisms in flowing water were disinfected by UV radiation from two excimer (excited dimer) lamps (emitting at 172 and 222 nm) in combination with two low-pressure mercury lamps (emitting at 254 nm). Synergies were investigated among the three types of radiation in the treatment of water spiked in turn with Escherichia coli, Listeria innocua, Shewanella putrefaciens, and spores of Bacillus subtilis and Bacillus cereus. Synergy was demonstrated between radiations at 222 and 254 nm in the treatment of E. coli, L. innocua, and S. putrefaciens, but little or no synergy was observed in the treatment of B. subtilis and B. cereus. At maximum flow rates (60 liters/min), 5-log reductions in E. coli were achieved at 254 nm, although at 222 nm, less than 1-log reductions were observed. No bacterial kill was observed with 172-nm radiation alone, despite increasing exposure time by reducing flow rates to less than 3 liters/min.