Microfauna and filamentous microflora in biological filters for tap water production

A quantitative survey of ciliated protozoa and filamentous microorganisms from biofilters at 12 different treatment plants along the River Po, has been carried out during a period of 1 year from February 1996 to April 1997. Altogether 21 species of ciliates and 13 species of filamentous microorganisms were identified. Other protozoa than ciliates, and metazoa were observed also. The dominating organism group was crawling ciliates. The occurrence and abundance of the most important microorganisms were compared with the physico–chemical and operational parameters of the plant. The backwashing interval seems to influence the protozoan colonization of biofilters whilst the filtering material does not seem to play an important role in the colonization of these microorganisms. Crawling ciliates were associated to ammonium removal efficiency since their grazing activity on heterotrophic bacteria reduced the competition pressure on nitrifying microorganisms, supporting their growth. The filamentous bacterium Crenothrix resulted particularly linked to the Mn removal efficiency. Factor analysis performed on organism groups and physico–chemical parameters, revealed that the removal efficiency of Mn and ammonium were related to the biotic components corroborating the close interdependence of these two processes and their association to the biomass growth. In particular some ciliates, such as Aspidisca lynceus and Litonotus sp., resulted positively correlated to the ammonium removal efficiency; conversely, some free-swimming ciliates showed a negative relationship with the aforenamed operational conditions and were directly associated with the raw water characteristics.