OFF-FLAVORS IN SURFACE WATERS - HOW EFFICIENT IS BANK FILTRATION FOR THEIR ABATEMENT IN DRINKING-WATER

Surface water and bank filtrate were studied from three Berlin lakes of different trophic states, two of which are being restored by phosphate elimination at their main inflow. Surface water showed time patterns of nor-carotinoids, aldehydes, alkanes and octatriene-isomers, which in most cases can clearly be attributed to specific phytoplankton populations. The variety of substances produced by algae was highest in the mesotrophic lake, because as compared to its previous hypertrophic state, restoration has caused an increase in phytoplankton species diversity as well as a more rapid seasonal succession of species. Contrary to expectations, serious odor problems occurred only at this lake, due to Chrysophyceae not found in hypertrophic waters. Therefore it is concluded that further reduction of phosphate concentrations is necessary, if not only the blue-green algae, but also the the Chrysophyte populations are to be reduced below nuisance levels. However, for the purpose of gaining drinking water from surface water, slow bank filtrations showed to be an effective measure for the elimination of terpenoids, ocatrienes and alkane molecules larger than dodecane; therefore it can be recommended as a treatment against odor problems in drinking water gained from surface waters.