Chemical osmosis in compacted dredging sludge

Sediments of high clay content are known to exhibit semipermeable properties when compaction is sufficiently high, resulting in osmotically induced water and solute transport. The semipermeability of these materials is quantified by the reflection coefficient, cr. In the design of large dredging sludge depots in the Netherlands, osmotic transport is rarely taken into account. In a flexible wall permeameter, a bentonite and two dredging sludge samples were subjected to a chemical potential gradient to monitor water transport and to obtain values for cr. Chemical osmosis was observed in the bentonite and one of the sludge samples (similar to 56% clay), but was absent in the sludge with a relatively low clay content (similar to 26%). The measured cr are low in comparison with values obtained with the Fritz-Marine Membrane Model (FMMM) and a model presented by Bolt. The discrepancy between the theoretical and experimental values is explained by the assumptions made in both models. Several of these assumptions do not hold for the samples. If the observed semipermeability of the sludge is applied to an existing depot, chemical osmosis induces a water flux close to the maximum allowed advective nux laid down in Dutch legislation.