BACTERIAL-POPULATIONS IN THE BIOFILM AND NON-BIOFILM COMPONENTS OF A SAND FILTER USED IN WATER-TREATMENT

The bacterial composition of the biofilm and non-biofilm components of a rapid sand filter used in water purification in Alexandria, Egypt, was determined. The total bacterial population was 64.30 x 10(4) cfu g (-1) at the surface of the filter but this decreased to 21.52 x 10(4) cfu g(-1) at a depth of 50 cm. Representatives of 22 genera were ident ified, with five genera (Micrococcus luteus, Pseudomonas spp., Alcalig enes spp., Citrobacter spp. and Enterobacter spp.) considered to be me mbers of the biofilm consortium as they were present at all tested lev els of the sand filter and were resistant to removal by wash treatment s. The biofilm consortium made up 50.3% of the total bacterial populat ion at the surface of the filter and 71.1% of the population at the 50 -cm level. Bacteria able to reduce nitrate and/or sulfate were present in both the biofilm and non-film populations.