The measurement of spores of aerobic spore-forming bacteria (ASFB) is becoming a widely accepted method for validating the effectiveness of treatments applied in drinking water treatment plants. These bacteria have been proposed as indicators of soil intrusion in distribution systems. There are, nevertheless, some limitations to the measurement of ASFB spores by membrane filtration with respect to the rate of recovery of spores present in distributed water. This is mainly due to the low concentrations of these spores. Two factors may decrease spore recovery: the presence of a cake on the filter and the aggregation of spores. For distributed water, our data suggest limiting the volume of water filtered (V in ml) as a function of its turbidity, according to the relation: V = 500/turbidity. The addition of a surfactant, Tween 80®, can increase the recovery of spores very significantly (1000 X). The experimental results show that a triple manual inversion will ensure a significantly higher rate of recovery.

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