Biocidal efficacy of monochloramine against biofilm bacteria

Abstract

Ammonia is one of the major metabolic products of the macrofouling communities which normally colonize the seawater intake systems of coastal power plants. Ammonia reacts with chlorine (used for biofouling control), resulting in the formation of chloramines. At the pH of seawater, the formation of monochloramine (MCA) will be significant. Biofilms (48 h old, 28.5 μm (± 5.4 μm) thick) developed on glass and aluminum brass coupons were exposed to the biocides MCA and free chlorine (FC). Biofilm thickness was measured and biofilm bacterial counts (by epifluorescence microscopy, and also for culturable bacteria by plating on to marine Zobel agar) were made after 15, 30, 45 and 60 min of contact time. The biocidal efficiency of MCA was compared with that of FC at similar doses (1, 2 and 3 ppm) after the same contact times. On glass, the biofilm thickness decreased by 90% (compared to the control) in the case of MCA at 3 ppm after 60 min exposure and the corresponding cell detachment was found to be 94%. With FC the thickness also decreased by 90% but cell detachment was 100%. The study demonstrates that MCA and FC show similar biocidal activity in disinfecting biofilms, and that MCA formed in situ in the cooling circuit could be as significant as FC.

Key words:

• seawater
• biofilm
• monochloramine
• free chlorine
• detachment
• thickness
• microfouling control

Notes

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