Ozone treatment of the raw water results in strong bacterial growth in the rapid sand filters during the winter months at Zurich's lake-water treatment plants. The source of the assimilable organic carbon (AOC) needed for this growth is not known. Evidence is provided that AOC is generated by ozone treatment of cyanobacteria present in the raw water which are disrupted during ozonation. Firstly, bacterial growth correlates with the presence of cyanobacteria at the intake during the winter months and secondly, the quantification of specific phytoplankton pigments, as a parameter for intact biomass, suggests that cells of cyanobacteria are readily disrupted by ozone treatment whereas other phytoplankton cells, such as diatoms, are more resistant to ozone. It is proposed that the released cellular content of phytoplankton provides the AOC source for bacterial growth.

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