Direct filtration tests were carried out on a pilot plant (20 m3/h) in order to examine the relationships between the removal efficiency of the total particle count, the spores of aerobic spore-forming bacteria and turbidity. Samples were taken from the raw water of the St Lawrence River (Canada), the turbidity of which is between 1 and 5 NTU, and the treatment objective was to maintain the turbidity of the filtered water at a value less than or equal to 0.10 NTU at all times. The treatment comprises a coagulation step using polyaluminium chloride (PACl) or Percol LT35, followed by filtration (10 m/h) on a bilayer filter of sand and activated carbon. The results show that linear correlations exist between the removal of total particle count, aerobic spores and turbidity. There is a progressive increase in the total particle count in filtered water, which begins just after the ripening period of the filter and continues to the end of the filtration cycle. Increase in the aerobic spores is not evident until much later, and there is only a weak increase in the turbidity.

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