An example of intermittent disinfection occurs in dentalunit water systems (DUWS), which are disinfected only for a specified time per each day. The efficacy of intermittent ozonation was evaluated using a laboratory-scale, membrane- based ozone disinfection system (MBODS), which delivers bubbleless dissolved ozone to the DUWS. A new tool - the weighted Ct value, or Cw, - was applied to interpret heterotrophic plate counts (HPC) data. To achieve the American Dental Association's (ADA's) criterion (<200 CFU{pipe}mL), the required ozone dosage was Cw > 0.07 mg-O3{pipe}L. However, even the highest ozone dosage (Cw > 0.130 mg{pipe}L) allowed biofilm HPC to persist at over 104 CFU{pipe}cm2. Although a higher Cw killed planktonic and biofilm bacteria more thoroughly, it also generated more biodegradable dissolved organic carbon (BDOC). Thus, this research illustrates the inherent trade-off of intermittent ozonation: a higher Cw kills more bacteria during the ozonation period, but creates more BDOC that fosters biofilm regrowth when ozonation is off.

Original languageEnglish (US)
Pages (from-to)436-444
Number of pages9
JournalOzone: Science and Engineering
Issue number6
StatePublished - Nov 2009


  • Biofilm
  • Dental unit water systems
  • Intermittent disinfection
  • Ozone
  • Regrowth
  • Weighted Ct

ASJC Scopus subject areas

  • Environmental Engineering
  • Environmental Chemistry


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