Conventionally organic carbon is assumed to be the limiting nutrient of regrowth. Inorganic phosphorus in comparison to carbon should be a good candidate for regrowth control because it can be measured and controlled. It may be able to limit regrowth in drinking water distribution systems. In order to clarify whether inorganic phosphorus can really control or limit regrowth, three kinds of bioassays were performed: 1. for different combinations of acetate, phosphorus, and other inorganic nutrients added to different test tubes containing pure water, 2. for different tap water dilutions with pure water, and 3. for different phosphorus addition to test tubes containing all nutrients but phosphorus. The inoculum used was taken from a drinking water distribution system carrying chlorinated groundwater. Results indicated the following: 1. tap water bacteria cannot grow on simple organic carbon source (acetate) alone, 2. not organic carbon but inorganic nutrients limited the growth in tap water, and 3. tap water bacterial growth was suppressed when phosphorus was not present and it was not affected by over-presence of phosphorus when carbon or other nutrients became limiting. This gave a clue that phosphorus might play a major role in controlling regrowth in drinking water distribution system.

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