Relationships between total particle count, aerobic spore-forming bacteriaand turbidity in direct filtration

Direct filtration tests were carried out on a pilot plant (20 m(3)/h) in or der to examine the relationships between the removal efficiency of the tota l particle count, the spores of aerobic spore-forming bacteria and turbidit y. 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 o r equal to 0.10 NTU at all times. The treatment comprises a coagulation ste p using polyaluminium chloride (PACI) or Percol LT35, followed by filtratio n (10 m/h) on a bilayer filter of sand and activated carbon. The results sh ow that linear correlations exist between the removal of total particle cou nt, aerobic spores and turbidity. There is a progressive increase in the to tal particle count in filtered water, which begins just after the ripening period of the filter and continues to the end of the filtration cycle. Incr ease in the aerobic spores is not evident until much later, and there is on ly a weak increase in the turbidity.