Three analogous laboratory-scale water pipe systems were constructed to study the effects of three chlorine levels of finished water on the growth of biofilm. The experimental results indicated that the heterotrophic plate counts (HPCs) of biofilm for chlorine-free water were approximately 2 and 3 orders of magnitude higher than those for low-chlorine water (0.3-0.5 mg/l Cl2) and high-chlorine water (1.2-1.5 mg/l Cl2), respectively. The difference in HPCs between low-chlorine water and high-chlorine water was below 1 order. Average biofilm formation rates are 325 ATP pg/cm2 per day for chlorine-free water, 159 ATP pg/cm2 per day for low-chlorine water and 118 ATP pg/cm2 per day for high-chlorine water. Scanning electron microscope (SEM) observation showed that bacterial species separated from the biofilm were more complex than from the finished water. The Gram-negative bacteria were dominant.

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