The microbial ecology of distribution systems is complex and is influenced by a number of factors. This study assessed whether or not during drinking water treatments it is important to add a chemical disinfectant after ultraviolet (UV) disinfection in order to control the regrowth of microbes in distribution systems. Results showed that low UV doses did not impact assimilable organic carbon (AOC) significantly. No definite relationship between UV dose and steady-state biofilm heterotrophic (HPC) bacteria was evident. AOC and chlorine residual are two important factors affecting biofilm and suspended HPC growth in distribution systems. When the AOC concentration was 0, 5 and 10 μg AC/L (μg acetate-C/L), a chlorine residual was not necessary to keep the fluid HPC under 500 CFU/mL. When the AOC concentration reached between 100 and 200 μg AC/L, the regrowth of microbes in the model distribution system necessitated a chlorine residual of over 0.5 mg/L. At lower AOC levels (0, 10–20 μg AC/L), the AOC appeared to have a strong effect on controlling biofilm formation and suspended HPC concentration. In contrast, at higher AOC levels (50, 100 and 200 μg AC/L), a chlorine residual was indispensable when controlling the regrowth of HPC in the distribution systems. AOC and the required chlorine residual in these systems were interrelated.