The functioning of biological slow sand filtration in relation to the presence and the role of Annelids in the schmutzdecke

At Waterbedrijf Groningen, The Netherlands for the purification of surface water to drinking water slow sand filtration is used. There are five slow sand filters, and slow sand filter three (SSF3) has different behavior in head loss (clogging). SSF3 had run for more than 25 years without renewing the schmutzdecke due to clogging. Experiments showed that certain type of annelids give the schmutzdecke a different structure than the schmutzdecke found in the other filters. The presence of the annelids had no effect on the log removal of pathogenic microorganism of the filter. Identification of the worm is doubtful. Comparison of the annelid found in the water of SSF3 and annelids found in the surrounding ground of the filters showed that the origin of the "water"-annelid lies somewhere different. The annelid is not found in the other SSF and therefore slow sand filter four is inoculated with approximately a thousand annelids. In the next years yearly the structure of the schmutzdecke is examined.