Time variation of floc properties in a settling column (original) (raw)
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Time Variations of Sediment Floc Size and Density by using Settling Column Data
Civil Engineering Research Journal, 2021
The current study was conducted to examine the sediment floc size and density changes over time in quiescent water by experimenting with a plexy glass settling column with 0.19 m in inner diameter and 3 m in height. The experiments were done with 5 initial concentrations of 3, 5, 10, 15 and 20 g/l and suspended sediment concentration was measured at different time and height intervals. Mclauglin differential equation was used to obtain the settling velocity of the sediments and Kranenburg's equation, and Stokes' Law relationship were solved to estimate the geometrical characteristics of the flocs. In all experiments, the maximum settling velocity of particles occurred 15 minutes after the beginning of the settling process, which was in agreement with results obtained by other researchers. The results show that the maximum settling velocity of sediments is about 8 times the average settling velocity of the sediments. The results of floc density calculations show that the floc density gets to the minimum value after 15 minutes of the start of the experiment, and it is concluded to the primary particle density. Also, the floc diameter reaches to the maximum value after 15 minutes of the start of the experiment because the flocs reach their maximum size at this time.
Journal of Sea Research, 1998
The fate of fine particulate material in aquatic environments is closely linked to aggregation and disaggregation processes. Understanding the mechanisms controlling these processes is fundamental to the development of predictive models of fate and effects for particulate discharges in the coastal zone from such sources as offshore hydrocarbon exploration and development. One of the variables required for the development of these models is maximal floc size. Using a non-invasive imaging technique, the significance of turbulence, composition, and concentration on maximal floc size in an inverting column flocculator was determined for materials commonly discharged during offshore hydrocarbon development. The settling velocity of the suspension was determined from volume concentrations of samples obtained by pipette during still water settling in a manner similar to that of Owen tubes. After 20 h, both maximal floc size and settling velocity showed a highly significant dependence on turbulence and type of material in suspension, but showed no effect from concentration.
A MODEL FOR THE SETTLING VELOCITY OF FLOCS; APPLICATION TO AN AQUACULTURE RECIRCULATION TANK
Computational Methods and Experimental Measurements
A general model for fl ocs settling velocity is still an open fi eld of research in the scientifi c literature. In this work, a reduced model of an aquaculture recirculation tank was used to validate a model for fl oc settling velocity. Cohesive sediments from non-used food and fi sh excreta are a main concern in those tanks design. Excess concentrations of sediments can cause fi sh death or additional costs of energy for aeration. This research is aimed to understand the settling behavior of fl ocs when subjected to a liquid shear rate. A reduced scale model of an aquaculture recirculation tank was build in Plexiglas in order to use particle image velocimetry and particle tracking velocimetry techniques to measure fl uid velocities, solid settling velocities, fl ocs shape and size. Different fl ow rates and solid concentrations were used to develop varied confi gurations in the system; models for fl oc settling velocity based on fractal theory were calibrated. Cohesive sediments from fi sh food were observed in long-term experiments at constant fl uid shear rate in the recirculation tank. A group of 50 images were obtained for every 5 min. Image analysis provided us with fl oc settling velocity data and fl oc size. Using fl oc settling velocity data, fl oc density was obtained for different diameters at equilibrium conditions, after 1 h or larger experiments. Statistical analysis of fl oc velocities for different fl oc sizes allowed us to obtain an expression for the drag coeffi cient as a function of fl oc particle Reynolds number (R ep). The results were compared with fl oc settling velocity results from different researchers. The model is able to defi ne the general behavior of fl oc settling velocity, which shows a reduction for larger fl ocs that is not taken into account in classical models. Only two parameters of the drag coeffi cient model for a permeable spherical particle are needed to be calibrated, for different types of sediments, in order to have more general applicability.
Flocculation and its effect on the vertical transport of fine-grained sediments
Hydrobiologia, 1992
Recent experimental and theoretical work on flocculation and settling speeds of flocs is reviewed. On the basis of this work, an accurate and computationally efficient model of the aggregation and disaggregation of fine-grained sediments is proposed. This model is then used to predict flocculation times and steady-state floc sizes for a wide range of environmental conditions. The predicted flocculation times are smaller, sometimes by as much as two orders of magnitude, than those predicted by mono-disperse theory. The model is also used to show that the disaggregation of flocs due to increased shear near the sediment-water interface may be a possible mechanism for the increased concentrations often observed near this interface .
Models for effective density and settling velocity of flocs
Journal of Hydraulic Research, 2006
New models to predict settling velocity and effective density of flocs are proposed. The models are based on the concept of fractal geometry, but with the assumption of variable fractal dimension with the floc size. The best results are obtained when the fractal dimension is estimated by a power law function of the floc diameter. The models are compared with observations from 26 published data sets relating floc size to settling velocity measured under various conditions and at different locations. The floc size covered by the data varies between 1.4 and about 25,500 µm. Five commonly used models are also compared to these data and found to reproduce inadequately the full range of the observations. Sensitivity analysis shows that, with the proposed models, the spread in the data may be reproduced by varying the size of primary particles from about 0.05 to 20 µm. The new models are proposed for integration into numerical models to simulate sediment transport of cohesive sediments, contaminants, and biological microorganisms such as phytoplankton. RÉSUMÉ Des modèles nouveaux sont proposés pour prédire la vitesse de sédimentation et la densité effective des flocs d'argile. Les modèles sont basés sur le concept de géométrie fractale, mais avec l'hypothèse de dimension fractale variable avec la taille des flocs. Les meilleurs résultats sont obtenus lorsque la dimension fractale est estimée par une fonction puissance du diamètre des flocs. Les modèles proposés sont comparés avec les observations provenant de vingt-six séries de données publiées. Ces dernières relient la taille des flocs à la vitesse de sédimentation mesurée dans des conditions variées et à des endroits différents. La taille des flocs concernés par les données varie entre 1.4 et 25500 µm. Cinq modèles utilisés fréquemment sont aussi comparés à ces données et sont apparus inadéquats de reproduire les observations. L'analyse de sensitivité montre qu'avec les modèles proposés, les données peuvent être reproduites en variant la taille des particules primaires composant les flocs d'environ 0.05 à 20 µm. Les nouveaux modèles sont proposés pour intégration dans les modèles numériques pour simuler le transport de sédiments cohésifs, de contaminants et de microorganismes biologiques comme le phytoplancton.
A comparison of in situ techniques for estuarine floc settling velocity measurements
Journal of Sea Research, 1996
An Intercomparison Experiment carried out In the turbidity maximum of the Elbe estuary aimed to determine the relative performance of a number of methods of measuring the settling velocity of estuarine floce. These comprised several Owen Tubes, with different sampling protocols, side withdrawal tubes, a settling box, and two in situ video systems. There were significant differences between the results which may partly relate to small-scale spatial and temporal patchiness in the turbidity field. Owen Tubes generally give settling velocities an order of magnitude smaller than the direct video measurements, which may Indicate that the tubes disrupt floce on sampling. Between Owen Tubes different methods of calculating the results may lead to different median settling velocities, especially at low concentrations. However, a well-controlled sampling protocol with the settling tubes gave consistent results. The direct video methods appear to give comparable results, and need to be evaluated further.
2013
The present study was conducted to evaluate the applicability of existing settling velocity models to ballasted flocs. An extensive literature review of the more common equations that represent settling velocity of flocs indicated that little work has been done about modeling the settling velocity of ballasted flocs. However, a general equation was found to be acceptable for this purpose, but the authors suggest the development of a new model that represent more accurately the settling velocity of ballasted flocs. RESUMEN El presente estudio fue realizado para evaluar la aplicabilidad de los modelos existentes de la velocidad de sedimentación para flóculos lastrados. Una extensa revisión en la literatura de las ecuaciones más comunes que representan la velocidad de sedimentación en flóculos indicó que se ha realizado poco trabajo en desarrollar modelos para la velocidad de sedimentación en flóculos lastrados. Sin embargo, se encontró una ecuación general que pudiera ser aceptable pa...
Continental Shelf Research, 2010
The flocculation properties of a natural silt-clay mud taken from the San Jacinto estuary near Houston, TX were investigated over a range of suspended sediment concentrations, salinities, and turbulent shear rates. The study was conducted in a laboratory using a paddle mixer to create a turbulent shear field for driving the flocculation process; floc settling velocity and size attributes were measured in a settling column with a camera system and image analysis. In general, maximum floc sizes were observed at turbulent shear rates less than 30 s À 1. For turbulent shear rates less than 50 s À 1 , flocs originating in saline water at 10 and 15 ppt were larger and more loosely packed than those created in freshwater at identical turbulent shear rates and suspended sediment concentrations. At turbulent shear rates of 50 s À 1 , floc properties of size, submerged specific gravity and fractal dimension were approximately equal for all conditions. Two ranges of behavior were observed in regards to the fractal dimension of flocs at a constant turbulent shear rate. In the first region, for floc sizes less than 200 mm, a variable fractal dimension was needed to describe the submerged specific gravity as a function of floc size. It is suggested that this variability in fractal dimension may physically be the result of ploysized primary particles. In the second region, for floc sizes greater than 200 mm, a constant fractal dimension was found to suffice in describing the submerged specific gravity. The constant fractal dimension for this second region was n f ¼2.3 for freshwater flocs and n f ¼1.95 for saltwater flocs. Based on the observation of the two-region behavior, a new variable fractal dimension model is presented using a simple exponential decay which asymptotically approaches a constant fractal dimension at a specified floc size. The model itself is based on the uniform-size primary particle formulation of the relationship between floc size and fractal dimension, but the equation was able to model the average shift from variable to constant fractal dimension, which was thought to primarily be due to polysized primary particles.
Chapter 15 Settling velocity of sediments at high concentrations
Proceedings in Marine Science, 2008
This paper describes two semi-empirical formulas for the hindered settling velocity at high concentration. The first formula is based on the Richardson and Zaki formula (1954) with the inclusion of the effect of the maximum volume concentration of matter cmax≈0.65. The second formula is based on the solid-fluid mixture theory. Both expressions produce the best results among the studied formulas in the case of non-cohesive particles. The formula based on the fluid-mixture theory tends, however, to be sensitive to the estimation of the viscosity of the mixture. In the case of cohesive particles, as larger uncertainties exist due to unknown parameters as the density and the size of the flocs, no conclusion could be found. The Winterwerp (1999) formula gave similar results to the two proposed formulas. The estimation of the gelling concentration is also an important challenge, and appeared to correspond to for a volume concentration of the flocs =max≈0.8.
Effect of hydrodynamics factors on sediment flocculation processes in estuaries
Journal of Soils and Sediments
Purpose Cohesive sediment is able to flocculate and create flocs, which are larger than individual particles and less dense. The phenomenon of flocculation has an important role in sediment transport processes such as settling, deposition and erosion. In this study, laboratory experiments were performed to investigate the effect of key hydrodynamic parameters such as suspended sediment concentration and salinity on floc size and settling velocity. Results were compared with previous laboratory and field studies at different estuaries. Materials and methods Experimental tests were conducted in a 1-L glass beaker of 11-cm diameter using suspended sediment samples from the Severn Estuary. A particle image velocimetry system and image processing routine were used to measure the floc size distribution and settling velocity. Results and discussion The settling velocity was found to range from 0.2 to 1.2 mm s −1. Settling velocity changed in the case of increasing suspended sediment concentration and was controlled by the salinity. The faster settling velocity occurred when sediment concentration is higher or the salinity is lower than 2.5. On the other hand, at salinities higher than 20, in addition to increasing SSC, it was found that the situation was reversed, i.e. the lower the sediment concentration, the faster the settling velocity. Conclusions Sediment flocculation is enhanced with increasing sediment concentration but not with increasing salinity.