Removal of four groups of indicator bacteria and two groups of bacteriophages in a detention pond receiving Combined Sewer Overflows (CSO) were measured. Also the survival of the bacteria in the sediments was studied. The overall removal rate of the bacteria tended to be somewhat higher than the removal rate for phages. Most values were in the range of 0.7 - 1.0 per day (e-basis). Sedimentation was of limited importance except for a short period following the overflow event. Dilution contributed in a reduction of 0.3 - 0.4 per day and die-off rates were about 0.3 - 0.6 per day, for phages maybe as low as 0.2 per day. Survival of the bacteria in the sediment was much better than in the water, net loss rates were about 0.1 per day, resulting in a more or less continuous contamination of the sediment with pathogenic bacteria as represented by indicator organisms.

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