Removal of particle‐associated bacteria by prefiltration and ultraviolet irradiation

The combined effect of prefiltration and ultraviolet (UV) irradiation on the removal of bacteria associated with plankton fragments was investigated in an experimental flowthrough system. The system was supplied by sea water containing fragments of Artemia nauplii in the size range 10–240 μm diameter (mean = 66 μm), as determined by particle size analysis. By applying an UV dose of 10 mJ cm–2, the number of colony-forming units (CFU) was reduced from 7.4 × 105 per 100 mL to 2.0 × 103 per 100 mL. Increasing the dose to 13 or 22 mJ- cm–2 did not result in any further reduction, suggesting that bacteria associated with Artemia fragments were protected. This protection hypothesis was supported by improved overall bacterial removal efficiency by prefiltration. At the highest dose (22 mJ cm–2), more than 5 log10 units total reduction was obtained by prefiltration using 50-μm mesh sizes. The reductions relative to the unfiltered sample were 1.4, 2.4 and 3.0 log10 units using mesh sizes of 355, 80 and 50 μm respectively. It is concluded that sea water for aquacultural purposes should be filtered to remove particles before UV disinfection, thereby improving the overall bacterial removal efficiency and reducing the risk of introducing UV-shielded bacteria.

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