In a component of an extensive pilot distribution system (PDS) study, the effects of four different water qualities on biological stability in distributed water were investigated through identical (parallel) single-pass pipe arrangements. Through 24 months of monitoring, a number of key observations were made. Incorporation of a biological treatment step reduced the overall dissolved organic carbon (DOC) loss through the PDS by reducing biodegradable DOC (BDOC) within the water prior to distribution. In the absence of chlorine residuals, the proliferation of culturable organisms was favoured with considerably higher heterotrophic plate counts in samples at the outlet of the PDS. Despite different bacterial cell counts (measured by flow cytometry) entering each PDS from the four treatment streams, equivalent outlet cell numbers were achieved in all systems after 8 months' operation; however multi-step treatment streams took longer to reach equilibrium.