Effects of flushing of dental waterlines in portable dental units on water quality management

Background/purpose: Daily flushing of dental unit waterlines is important for infection control. However, the effect of flushing on water quality management in portable dental units (PDUs) for mobile dental treatments remains unclear. In this study, we aimed to investigate the factors affecting the effectiveness of PDU flushing.

Materials and methods: Free residual chlorine concentration (FRCC) and heterotrophic plate counts (HPCs) in handpiece discharge water were evaluated as water quality indicators to determine the effectiveness of flushing PDUs, considering flushing duration, periods of PDU use, FRCC in the water used, pre-water drainage in water tanks and waterlines, and season. The effect of flushing on heterotrophic bacterial flora was analyzed by 16S rRNA gene sequencing.

Results: PDUs with longer usage periods required a longer flushing duration for FRCC recovery. FRCC reached the standard level for water quality in Japan after at least 20 s of flushing. Flushing changed heterotrophic bacterial flora close to that of fresh tap water by reducing Sphingomonas. Flushing with purified water or stagnant tap water with insufficient FRCC did not allow the HPCs to reach the target level. Without pre-water drainage of the water tanks and waterlines in the summer, the HPCs did not always reach the target level, even with flushing.

Conclusion: Periods of PDU use, pre-water drainage, water temperature, and FRCC substantially affect the effectiveness of PDU flushing. Flushing for at least 20 s with water containing appropriate free residual chlorine effectively restores the water quality of PDUs.