Each year some 25 million m3 of mineral sludge are dredged from rivers, canals and harbours in the Netherlands alone, twenty percent of which is polluted. The unpolluted sludge is usually dumped at sea. The remaining five million m3 may only be dumped in a few selected landfill locations, at considerable cost.A sludge flock, hotographed at the DelftLaboratory of Fluid Mechanics. Sludge consists of a collection of small particles, including clay, fine sand, organic material, and a lot of water, and if the dredgings came from anywhere near the sea, silt. The clay particles give sludge its characteristic properties, which differ from, say, sand. The clay particles are cohesive, so when they meet they tend to stick together. The size of a flock varies from a few dozen to many hundreds of micrometers (see also Delft Outlook 2003.2).In the late nineteen-eighties the processing of mineral sludge from soil remediation projects and dredgings was still too costly. So the Slufter, a 260 hectare basin that can hold 150 million cubic metres of sludge, was created at Europoort near Rotterdam. Each year, some 3 to 5 million tons of dredgings are added to the 50 metre deep reservoir.