THE POPULATION-STRUCTURE OF A PEAT FILTER BED SYSTEM FOR TREATING DOMESTIC WASTE-WATER, AND COMPARISONS WITH CONVENTIONAL ACTIVATED-SLUDGE

The protozoan population growing in a peat filter bed located at an ur ban wastewater treatment plant (''La Cabrera'', Madrid, Spain) was stu died. A high density of microorganisms (2.7-10(6) No./1) was found ind icating a considerable biological activity. Comparing the mean charact eristics of the protozoan population of a peat filter bed with that of a conventional activated-sludge pilot plant, it was found that 80% of the total population corresponded to free swimming ciliates in the fi rst case, whilst sessile or fixed ciliates dominated in the activated- sludge system. These differences in population structure between purif ication systems could indicate, that the ciliate population in the pea t bed did not reach an equilibrium phase, due to the relatively short operation time used in a pond (25-30 days), and was more similar to th e transitional phase in an activated-sludge system. Paramecium sp. was found to be the predominant ciliate in the peat bed although a reason ably diverse population structure was found, which was indicative of a erobic conditions.