Invertebrate colonization of granular activated carbon filters

Results of biological sampling of granular activated carbon (GAC) filtrate taken between May 1994 and August 1995 at three different treatment plants along the river Rhine indicate that GAC filters are colonized by invertebrates. Dominating organism groups were rotifers and nematodes. Depending on operational characteristics, mainly regeneration intervals and backwashing procedures, the filter colonization can lead to an output of organisms in high numbers with the filtrate. Examples are given for the development of filter colonization and for the influence of different backwashing procedures to reduce numbers of invertebrates in GAC filters. These findings underline the establishment of a complex food web in a GAC filter, the consequences of which still remain to be investigated in detail.