Aerobic spherical granules were observed in zeolitic dual-layer intermittent percolating filters treating domestic septic tank effluent. The diameter of the granules ranged from 0.05 to 0.5 mm and when they predominate in the biofilms the mean SVI value was of 54 ml g-1 instead of 232 ml g-1 when they were absent.\r\n Flow behavior in these filters was characterized by the dimensionless Reynolds number. The values obtained (40 less than Re less than 60) were representative of intermediate flow conditions and invalidated the current hypothesis of laminar flow conditions. These values were of the magnitude of those estimated from studies on SBR reactors presenting granular sludge formation.\r\n The possible influence of granular sludge formation on long-term performances and interfacial clogging of intermittent dual-layer percolating filters is also discussed.
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