The interactions between organic and inorganic particles in the context of flocculation is an on-going topic of research. Most current researches do not distinguish between the effects of EPS (produced by microorganisms) and living microorganisms (like algae). In this study, the effect of salinity, EPS and living algae on sediment flocculation are investigated separately. Several types of measurements were performed, which can be divided into the following categories: sediment at different salinities, sediment in the presence of EPS at different salinities, sediment in the presence of living algae at a given salinity. Results show that increasing salinity enhances slightly sediment flocculation. In the presence of EPS there was hardly any flocculation in demi-water, but the flocculation was significant in saline water. The living algae cells were shown to flocculate with themselves and form large flocs. These algae flocs can bind to sediment particles to form larger flocs, both in demi-water and sea water. Size-wise algae-sediment flocs were largest, EPS-sediment flocs came second, and salt-sediment flocs were smallest.

@en

The characterization of flocculation rates is often done through floc size distribution over time. This distribution in size also affects settling rates. Besides sizes, settling velocities are dependent on floc density. This density is estimated using Stokes law, by recording the velocity and equivalent radius of a settling particle. This work compares two experimental setups used to calculate the floc size distribution and their settling velocities. The two floc measurement instruments as shown in figure 1, one is called LabSFLOC-2 [1,2] and the other is referred to as FLOCCAM [3]. Both setups have a telecentric lens which effectively reduces the pixel distortion. Polystyrene particles are used to benchmark the results. Different clay types are used to create flocs with the addition of industrial flocculant zetag 4110. The post-processing of the recorded floc videos for size, settling velocities and shapes of flocs is done with the help of an image analysis tool based on Matlab for LabSFLOC-2, whereas FLOCCAM analysis is done with the help of Safas [4].@en

In situ floc size and turbulent shear stress were measured together with suspended sediment concentration to investigate the floc properties under changing hydrodynamic forcing over the intertidal mudflat. A tripod system was established in the field for a period of approximately one month, including ~ 6 days of stormy conditions in the middle of the investigation period. Mean floc size exhibited strong temporal variations within a tidal cycle, and inverse relationship was found between mean floc size and shear stress. Suspended sediment concentration (SSC) can modulate the flocculation dynamics when shear stress decreases down to enhancing flocculation. Asymmetrical behaviors of floc sizes between flood and ebb phases were identified, with overall larger floc sizes in flood than in ebb tide under the same shear stresses. Floc structure showed different properties under calm and stormy conditions, and the variable fractal dimension and variable primary particle size were more convincing in simulating the variation of floc effective density with mean floc size during the storm period, which was inferred to be related to the resuspension of bed sediment as well as organic matter. A total of 110 mm bed erosion was measured during the storm, and erosion events occurred only around low water, due to the high current-wave combined bed shear stress and off-shore current. After the storm, ~ 40% of the erosion recovered within one week, and the fast settling of large flocs around high water plays significant role in the deposition process, leading to ~ 60% of the recovery.

@en