Ciliates in rapid gravity filters of waterworks exploiting deep groundwaters

Potable water is increasingly produced from deep (>100 m) tertiary groundwaters which often are completely reduced and contain high amounts of ammonium, methane, and hydrogen sulphide. They thus require special treatment which includes oxygenation and removal of the reduced contaminants by the biofilm developing in rapid gravity filters. The biofilm is heavily colonized with ciliates and microinvertebrates. A total of 38 species of ciliates was found in 42 samples taken from 4 waterworks in Germany during a period of 2 years. Only six species occurred in high numbers and in more than half of the samples: Acineria uncinata, Aspidisca lynceus, Cinetochilum margaritaceum, Colpidium colpoda, Glaucoma scintillans, and Holosticha pullaster. Five to thirteen species occurred per sample, and up to 6,000 individuals ml-1 biofilm were counted. There was a considerable fluctuation in the number of species and individuals, which could not be related to specific process parameters. The number of species and individuals decreased markedly from the filter surface to its centre. Colonization of the filters very likely occurs randomly via impure surface waters. The ciliate communities found consist mainly of alphamesosaprobic to polysaprobic species and thus closely resemble those known from activated-sludge processes. This is explained by the specific conditions near and in the biofilm, which is probably microaerobic and highly productive, providing microaerobic bacterial feeders with copious food. Obviously, it is the microenvironment which determines the occurrence of certain species. Thus, future research on the autecology of the indicator species used in the saprobic system should concentrate on their microenvironments.