Transition effects in an unchlorinated drinking water system following the introduction of partial reverse osmosis

It is an increasingly common practice that drinking water distribution systems (DWDSs) may have to deliver new-quality water after decades of service. However, the so-called transition effects when old DWDSs receive new water remain unclear. In this 2 year longitudinal study, transition effects induced by the introduction of partial reverse osmosis in production were observed as marked increases in particle load, elemental concentrations and biomass. This occurred as soon as new-quality water entered the DWDS, lasted for 1 month and started to disappear from the second month. The peak transition window was around 1 month while the restabilization of microbial ecology and improvements in water quality take much longer-between 1 and 2 years. The number of immigrants from loose deposits and biofilm increased by 17.9% during the transition period but had decreased by 79.0% 2 years later. This study provides valuable insights into the occurrence and potential management strategies of transition effects.