A direct viable counting method was used to rapidly assess the effectiveness of chlorination of biofilms on rapid sand filter gravel. A total of 50% of the cells were nutrient responsive after exposure to 0.5 mg of chlorine per liter, while this value was 25% after exposure to 25 mg of chlorine per liter. A large variation was seen in the numbers of nutrient-responsive cells on different rocks. More cells attached to the sandblasted side of marbles than to the smooth side, but there was no difference in eight of nine cases in the proportion of survival to chlorination between the two different sides. The effectiveness of chlorination appeared to be influenced by the species of bacterium in the biofilm
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