The use of flocculants to enclose silt particles in pore volume of dredged soil
Type:
Presented during:
CEDA Dredging Days 2009 - Dredging Tools for the Future, Rotterdam
Authors:
Mol JN and van Hemmen AJM - Royal Boskalis Westminster; Talmon AM - Delft University of Technology The Netherlands
Abstract: Discharging dredged soil containing a large silt fraction (particle diameter d < 0.063 mm) into a reclamation area causes various problems at the fill area or surrounding environment. Typical problems are 1) exceeding of turbidity requirements with possibly environmental damage. 2) non-homogeneous soil properties of the reclamation area.
The settling behavior of a particle is partly a function of its size and density, the smaller the particle, the lower the settling velocity. Different settling behavior of the particles leads to a separation of particles out of the discharged soil-water mixture over the path of the process water flow between discharge pipeline and weir box of the reclamation area. Very small particles will escape with the process water through the weir box.
To create a homogeneous soil layer over the entire reclamation area the use of flocculants has been tested in a laboratory sand fill model. A solution of fresh water and an anionic or cationic flocculant (1-10-3 kg/L) has been added to the outgoing flow of the discharge pipeline. Doses flocculant per kilogram dry silt ranged between 5-10-6 kg /kg and 1-10-3 kg /kg. The silt fraction flocculated and settled down between the larger particles of the mixture, resulting in a homogeneous soil layer. Due to the pore space of the flocks the permeability of the total soil layer increased compared with a soil layer containing silt pockets. The outgoing process water left the water box without any particles. The possibly environmental load according to the use of a flocculant depends of the type of flocculant used and the dose.
Keywords: dredging, reclamation, silt, environment, turbidity, flocculant