BIOPROCESSING OF POLLUTED SUSPENDED MATTER FROM THE WATER COLUMN BY THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA PALLAS)

Measurements of pseudofaeces production of Dreissena polymorpha were carried out with the aim of developing a biological filter at the freshwater inlet of Lake Volkerak-Zoommeer, the Netherlands. Bioprocessing of polluted suspended matter by suspended cultures of D. polymorpha occurs by filtration and sedimentation of the suspended matter as pseudofaeces. The measurements were conducted under semi-natural conditions. Pseudofaeces production was mainly determined by the dry matter content of the water; the relation is linear. Temperature was of much less importance. This agrees with earlier investigations of the filtration rate of D. polymorpha. Even at the lowest temperature measured during the experiments (6.4-degrees-C) no large decrease in activity was observed. The relation of pseudofaeces production with shell length was sigmoid in shape, in accordance with measurements of the filtration rate. The pseudofaeces produced was slightly more polluted than suspended matter, partly due to a finer grainsize. D. polymorpha from Lake IJ sselmeer exposed for 217 days at the intended location of the filter showed bioaccumulation of toxicants, especially organic pollutants and Polycyclic Aromatic Hydrocarbons (up to 10-fold accumulation). The required number of D. polymorpha in the biological filter to treat the waterflow of 14 m3 s-1 entering Lake Volkerak-Zoommeer is 1.24* 10(9). The purification efficiency of the filter, the reduction of the amount of toxicants, partly depends on the binding properties of the toxicants and is highest for those strongly bound to suspended matter.