Invertebrate colonisation of granular activated carbon (GAC) filters is one of the most frequently occurring and less studied biological problems in the process of water potabilisation. In this study we monitored invertebrate presence in the GAC filters of a potabilisation plant located in the province of Ferrara (Northern Italy), treating groundwater only. In particular, we studied the temporal evolution from November 1999 to January 2001 in relation to the duration of backwashing procedures used to cleanse the filters. The study focused on two separate GAC filter lines at the same time which work in identical conditions. The results showed that the community was dominated by nematodes and, with lower densities, gastrotrichs, rotifers and anellids. Invertebrate density did not follow a clear time pattern in either line, in relation to abiotic factors. It seems to depend on plant management practices, such as backwashing effluent recycling within the plant. Nematode generation time, in combination with the time span between two backwashings, is apparently the principal factor in controlling filter colonisation, while backwashing duration did not seem to have a significant effect.

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