In the Netherlands large quantities of contaminated sediment have to be dredged. Because it cannot be relocated directly in the marine environment, the dredged material must be placed in special contained depots which are expensive to construct. In 1989 a programme to reduce the volume of storage space required was started. This paper includes the results of research into the physical aspects of sediment cleaning.
Heavy metals are often found in the silt fraction, while organic micro pollutants occur in the silt fraction and sometimes also as tar particles in the sand fraction. The sediment can be cleaned by physical means or by the selective separation of some of the contaminants by froth flotation.
Research into the efficiency of using separation on the basis of particle size to clean contaminated sediments from five different sites and also the experience gained during three sediment clean-up operations, indicated that positive effects could only be achieved in sandy sediments. In some cases, owing to the presence of tar particles in the sediment, the quality of the sand fraction was not improved after separation in hydrocyclones. They can be removed from the sand fraction by flotation. Recent developments in selective separation by means of froth flotation, using equipment specially designed for the fine fraction, are promising.
The use of duo-flocculants has a favourable influence on the consolidation behaviour of the fine fraction after separation.