The amount of assimilable organic carbon (AOC) and bacterial regrowth was studied in drinking waters from seven Finnish surface and groundwater works. Bacterial regrowth in drinking waters was studied by in vitro laboratory incubation tests and by enumerating the microbes in distribution networks. Bacterial numbers were counted as heterotrophic viable counts and as total direct counts (AODC). The bacterial production activity was studied as 3H-thymidine uptake. AOC, measured with a bioassay was 390±130mg/l and 170±60mg/l in drinking waters produced from surface waters and from groundwaters respectively. The content of AOC was slightly reduced in distribution pipelines of groundwater works whereas in the distribution pipelines of surface water works it was reduced by 40%. Bacterial regrowth occurred in all drinking waters. Surprisingly, the heterotrophic regrowth was greater in drinking waters produced from groundwaters than in those produced from surface waters. AOC levels correlated poorly with the in vitro bacterial growth tests and with occurrence of bacteria in distribution networks.

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