Sediment Height Research Articles

Correction of wall adhesion effects in the centrifugal compression of strong colloidal gels

Several methods for measuring the compressive strength of strong particulate gels are available, including the centrifuge method, whereby the strength as a function of volume‐fraction is obtained parametrically from the dependence of equilibrium sediment height upon acceleration. The analysis used conventionally due to Buscall & White (1987) ignores the possibility that the particulate network might adhere to the walls of the centrifuge tube, even though many types of cohesive particulate gel can be expected to. The neglect of adhesion is justifiable when the ratio of the shear to compressive strength is small, which it can be for many systems away from the gel‐point, but never very near it. The errors arising from neglect of adhesion are investigated theoretically and quantified by synthesising equilibrium sediment height versus acceleration data for various degrees of adhesion and then analysing them in the conventional manner. Approximate correction factors suggested by dimensionless analysis are then tested. The errors introduced by certain other approximations made routinely in order to render the data‐inversion practicable are analysed too. For example, it shown that the error introduced by treating the acceleration vector as approximately one‐dimensional is minuscule for typical centrifuge dimensions, whereas making this assumption renders the data inversion tractable. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1520–1528, 2017

Experimental study on settling velocity of soil particles in dredged slurry

ABSTRACTStudying sedimentation and consolidation of dredged slurry has significant implications to the design of storage yard and subsequent ground improvement. In this study, settling velocity of soil particles in dredged slurry during sedimentation and consolidation processes was investigated using an improved multilayer extraction sampling (MES) method. A series of sedimentation column tests were performed on dredged slurry with three different initial water contents. Distributions of volume of soil particles and density of dredged slurry were first obtained by the MES method, settling velocity of soil particles was then calculated by volume flux function approach. It was found that the density and velocity inflection points can be used to distinguish the settling zone and the consolidation zone. The experimental results reveal that the velocity of soil particles was quite low and monotonically decreased with sedimentation height at low initial water content throughout the whole test period, whereas it was increased at 0–1 hours and almost remained constant at 1–7 hours in the settling zone at high initial water content. The effects of initial water content on sedimentation and consolidation mode of dredged slurry and the settling velocity of soil particles were discussed. The relationship between settling velocity of soil particles and particle diameter was also studied. It is indicated that the measured velocity of soil particles was much lower than that calculated by the Stokes equation, and it was related to 0.4881–0.5906 order of particle diameter at 0–1 hours and 0.1117–0.1825 order of particle diameter at 1–7 hours for the test slurries.

Effects of electrolyte concentration and pH on the sedimentation rate of coagulated suspension of sodium montmorillonite

The sedimentation behavior of semi-dilute suspensions of sodium montmorillonite in the flocculation regime was studied as a function of ionic strength at pH 4.0 and 9.5 to elucidate the response of variation of macroscopic sedimentation behavior with microscopic particle association. The maximum velocity of the boundary between the supernatant and sediment and ultimate sediment height were chosen as indexes to evaluate the sedimentation behavior. Increment ratio of effective volume fraction of the particles (α) caused by the formation of flocs and hydrodynamic floc diameter were evaluated by Michaels and Bolger method on the basis of the Richardson⿿Zaki formula. The results clearly demonstrate that the maximum velocity of the boundary is strongly affected by both pH and salt concentration. The two observed patterns of variation of maximum sedimentation rate and ultimate sediment height can be clearly defined using 0.25⿿0.5M NaCl as an inflection range. This range is consistent with the transition point at 0.3M NaCl for the swelling behavior of sodium montmorillonite reported by Norrish. Below 0.25M NaCl, the effect of pH on the sedimentation rate is remarkably enhanced compared with that at higher salt concentration. This effect is ascribed to the dominant appearance of edge⿿face (EF) particle interactions, which can be regarded as negligible at higher salt concentration. The reason for this is that the breakage of weak EF bonds is induced by the dominant appearance of face⿿face particle interactions at higher salt concentrations. That is, the decrease of repulsion strengthening the attraction between neighboring particles is responsible for EF bond breakage. The small influence of pH on ultimate sediment height at high salt concentration can be ascribed to the residual strong EF bonds. The Michaels and Bolger method can be used qualitatively at range of NaCl greater than 0.5M.

New technique for continuous monitoring of sediment height

A sewer deposit is a complex mix of minerals and organic materials whose dynamics remain difficult to ascertain. One way to improve the knowledge held on sewer processes consists of obtaining detailed long-term measurements. This paper will present the technique developed for long term monitoring of sediment height within a combined sewer. The aim of this device is to acquire long-term records of the sediment height evolution. The current state of the art is discussed and the technical constraints are exposed. Next feasible technical choices are explained and then results are provided and discussed.

Rehydration behaviours of high protein dairy powders: The influence of agglomeration on wettability, dispersibility and solubility

Five common high protein dairy powders and their agglomerates produced by fluidised bed granulation were evaluated and compared for their rehydration characteristics in this study. Wettability of powders was measured by immersion wetting time, capillary rise wetting and contact angles methods, while dispersion and solubilisation processes were quantified by the change of particle size and the sediment height after centrifugation. The results showed that these high protein dairy powders generally had poor wettability, especially for whey protein isolate and the caseinates, which formed an impermeable layer separating the water surface and powders just after they contacted the water. However, the casein-micellar dominant powders exhibited prolonged dispersion due to strong interactions inside the micellar structures. The agglomerates with large particle size and high porosity are expected to exhibit increased wettability. However, agglomeration only caused the external structural modification and thus is difficult to accelerate the dispersion process of micellar casein, which can be explained by the milk protein isolate rehydration mechanism. The micellar structure inhibits the release of materials into surrounding liquid phase, which is mainly responsible for the extended rehydration time.

Electrical impedance spectroscopy for monitoring the gum Arabic–chitosan complexation process in bulk solution

Abstract Electrical impedance spectroscopy (EIS) tests were carried out to study the complexation process between gum Arabic (GA; aqueous dispersion, pH 4.75, 1% w/w) and chitosan (Ch; acetic acid dispersion, pH 3.5, 1% w/w) in bulk solution. EIS data were obtained for different GA/Ch mass ratios (RGA/Ch = 1:1–10:1) at frequencies ranging from 1 Hz to 1000 Hz. A fractional-order Randles circuit was used to estimate resistance and capacitance parameters as function of RGA/Ch. Complex coacervates were obtained by subjecting the bulk solutions to centrifugation, and quantifying the sedimentation velocity and height. The equivalent circuit parameters showed sharp changes at the isoelectric point of the complex coacervates. The capability and accuracy of the EIS was compared with that obtained via traditional ζ-potential measurements. The results indicated that the EIS technique can be used at specific frequency ranges as a practical method for monitoring the complexation process between biopolymers.

Stability and Agglomeration of Alumina Nanoparticles in Ethanol-Water Mixtures

Nanofluids have gained much attention in the last decade due to wide range of engineering applications. Agglomeration among nanoparticles in nanosuspensions accelerates settling of nanoparticles due to gravity and reduces overall thermal conductivity of nanofluid. Settling characteristics of alumina nanoparticles in six different concentrations in different proportions of ethanol and water mixtures are studied. Surfactant free nanofluids are prepared with Alumina nanoparticles of average diameter 40nm, 50nm and 100nm using two-step method. Settling behaviour of nanoparticles in nanosuspensions is observed under natural and sonicated conditions. Photographic technique is used to measure sediment height in a batch sedimentation apparatus. Heights of the sediments are observed for 24hours with and without sonication at room temperature. It is found that sonication has significant influence on the stability of nanofluids. Effect of ethanol concentration on the sediment ratio is studied with and without sonication. Aggregation among nanoparticles and interaction of particles with fluid mixtures are investigated by performing various analyses such as Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and particle size analyser.

Comparative study on the behavior of soil fills on rigid acrylic and flexible geotextile containers

Comparative study has been performed to investigate the behavior of dredged fills on rigid (Model 1) and flexible (Model 2) containers. The study was focused on the sedimentation of soil fills and the development of total stresses. Model 1 is made of an acrylic cylinder and Model two is a scale-size geotextile tube. Results indicate that for rigid containers, significant decrease of the sediment height is apparent during the dewatering process. On the other hand, because the geotextile is permeable, the water is gradually dissipated during the filling process on flexible containers. Hence, significant loss in the tube height is not apparent during the duration of the test. Pressure spikes are apparent on rigid containers during the filling process which can be attributed to the confining effect due to hydrostatic pressure. For the flexible containers, the pressure readings gradually increases with time during the filling process and normalize at the end on the filling stage. No pressure spikes were apparent due to the gradual dissipation of pore water pressure.

Construction of analytical solutions and numerical methods comparison of the ideal continuous settling model

Continuous sediment has a wide application in chemical engineering process, such as wastewater treatment, water reuse, mineral waste manage and processing. This paper provides analytical solutions of the ideal continuous settling model by using the method of characteristics. The analytical solutions are compared with experiment data to show that the solutions accurately predict the sediment height and concentration as a function of time and loading conditions. Additionally, three alternative methods, using finite differences are compared to the analytical solutions, and their accuracy and efficiency are evaluated. It is shown that all three methods are reliable for solving sedimentation problems but have varying efficiency. Method YRD is the most accurate but also has the greatest computation and implementation cost. Method SG is the least accurate but is the easiest to implement with lowest computation cost. Method G is a compromise between the two methods, providing acceptable accuracy and low computation cost.

Wet-jet milling-assisted exfoliation of h-BN particles with lamination structure

The ability to exfoliate hexagonal boron nitride (h-BN) as laminated particles has been discussed by various mechanical high shear devices, such as vortex fluid (VF), planetary homogenizer(PH), ultrasonication (US) and wet-jet milling (WJM) with effective shear flow. The sedimentation test showed that the height of sediment of h-BN prepared by WJM was much higher than that of h-BN prepared by other mechanical methods, indicating that WJM process led to exfoliation of the laminated h-BN effectively. Also, the particle size distributions showed that the particle size of wet-jet milled h-BN was as similar as the initial particle size, whereas the particle sizes of h-BN prepared by other mechanical devices were less than 70% of the initial particle size. These results suggested that the difference of particle size after mechanical methods were based on the method of applying shearing force on the mechanical methods. In the mechanical methods such as VF, PH and US, the exfoliated h-BN would be destroyed even if the exfoliation of h-BN was progressed. On the other hand, in the case of the WJM, h-BN must be uniformly aligned in the thickness direction by pushing the slurry out at a high pressure, and the large impact energy is applied in the direction of the thickness of the h-BN particles. Therefore, it was found that the exfoliation of h-BN progressed by WJM was effective.

Settlement prediction model of slurry suspension based on sedimentation rate attenuation

This paper introduces a slurry suspension settlement prediction model for cohesive sediment in a still water environment. With no sediment input and a still water environment condition, control forces between settling particles are significantly different in the process of sedimentation rate attenuation, and the settlement process includes the free sedimentation stage, the log-linear attenuation stage, and the stable consolidation stage according to sedimentation rate attenuation. Settlement equations for sedimentation height and time were established based on sedimentation rate attenuation properties of different sedimentation stages. Finally, a slurry suspension settlement prediction model based on slurry parameters was set up with a foundation being that the model parameters were determined by the basic parameters of slurry. The results of the settlement prediction model show good agreement with those of the settlement column experiment and reflect the main characteristics of cohesive sediment. The model can be applied to the prediction of cohesive soil settlement in still water environments.

Numerical modelling of mixed-sediment consolidation

Sediment transport modelling in estuarine environments, characterised by cohesive and non-cohesive sediment mixtures, has to consider a time variation of erodibility due to consolidation. Generally, validated by settling column experiments, mud consolidation is now fairly well simulated; however, numerical models still have difficulty to simulate accurately the sedimentation and consolidation of mixed sediments for a wide range of initial conditions. This is partly due to the difficulty to formulate the contribution of sand in the hindered settling regime when segregation does not clearly occur. Based on extensive settling experiments with mud-sand mixtures, the objective of this study was to improve the numerical modelling of mixed-sediment consolidation by focusing on segregation processes. We used constitutive relationships following the fractal theory associated with a new segregation formulation based on the relative mud concentration. Using specific sets of parameters calibrated for each test—with different initial sediment concentration and sand content—the model achieved excellent prediction skills for simulating sediment height evolutions and concentration vertical profiles. It highlighted the model capacity to simulate properly the segregation occurrence for mud-sand mixtures characterised by a wide range of initial conditions. Nevertheless, calibration parameters varied significantly, as the fractal number ranged from 2.64 to 2.77. This study investigated the relevance of using a common set of parameters, which is generally required for 3D sediment transport modelling. Simulations were less accurate but remained satisfactory in an operational approach. Finally, a specific formulation for natural estuarine environments was proposed, simulating correctly the sedimentation-consolidation processes of mud-sand mixtures through 3D sediment transport modelling.

Dispersion behaviour and agglomeration effects of zinc oxide nanoparticles in ethanol–water mixtures

Nanofluids are the new engineered fluids with particles of nanometre size as an additive to alter the effective thermal and viscous properties for different applications. Settling of nanoparticles because of agglomeration among particles in the liquid is a big challenge towards the stability of nanofluids. The dispersion behaviour of nanofluids using zinc oxide nanoparticles of 30 and 40 nm size in ethanol–water mixtures has been examined. Sediment heights are measured at different time intervals with and without ultrasonication using conventional batch sedimentation apparatus. The effect of different nanoparticle concentrations in variety of ethanol-water proportions has been studied using the photographic method. Agglomeration effect has been studied with and without sonication in terms of sedimentation ratio and with the support of transmission electron microscopy. It is found that sonication has a significant influence on the nanofluid stability. An important evaluation technique for stability of nanofluids has been presented.

Rehydration characteristics of milk protein concentrate powders

Low-(MPC35, MPC50), medium-(MPC60, MPC70) and high-(MPC80, MPC85, MPC90) protein content milk protein concentrate (MPC) powders, manufactured at pilot-scale, were evaluated for their rehydration characteristics. Optical tensiometry confirmed that water droplets were imbibed more slowly as protein content of the MPCs increased, indicating impaired wetting. Casein micelles comprised only<2% of the particle population by volume in MPC70, MPC80, MPC85 or MPC90 after 90min of rehydration at 25°C, as primary particles which had not dispersed fully remained in suspension. The quantity of sediment, measured using analytical centrifugation, increased in the order MPC70<MPC80<MPC85<MPC90 after 90min of rehydration at 25°C, with lower protein MPCs forming no sediment. No sediment formation was observed in any of the MPCs after 24h of rehydration at 25°C, despite the predominance of primary particles in suspensions of high-protein MPCs. Increasing the temperature of reconstitution from 25 to 50°C during 90min of rehydration caused a 41.4% decrease in sediment height for MPC90 in water; however, reductions in sediment height of 89.9% and 99.5% were achieved when MPC90 was rehydrated in milk permeate or 80mM KCl, respectively. It is evident that low ionic strength (confirmed using conductimetry) has a strong negative effect on the rehydration properties of high-protein MPCs, and that the synergistic effect of increasing ionic strength and temperature can substantially accelerate rehydration.

Seagrass response to burial and breakage of expanding horizontal rhizomes: implications for clone spread

In seagrasses, elongation of horizontal rhizomes, and hence clone expansion, is supported by physiological integration. However, horizontal rhizomes are subjected to breakage by physical disturbances and to burial, often simultaneously. Little is known about the combined impact of such factors on the growth of apical rhizome regions. We factorially manipulated connections on runners of Cymodocea nodosa and sediment height for 1 mo at 2 sites within a meadow to test the hypothesis that breakage of interconnections close (10 cm) to the rhizome apex causing premature loss of physiological integration would negatively affect the performance of runners and reduce their ability to tolerate burial (60% of shoot height). Shoot survival, length of vertical internodes and horizontal rhizome network, biomass production, partitioning, and costs were unaffected by the investigated factors. Rhizome breakage interacted with burial for some characteristics. Breakage reduced branch length and rhizome diameter only in unburied runners, while burial reduced branch length only in intact runners. Moreover, burial stimulated leaf sheath elongation and greater branching, irrespective of the status of connections. These findings indicate that breakage only delays lateral spread and increases the risk of fragmentation by subsequent disturbances, and, contrary to our hypothesis, has no substantial influence on the capacity of runners to withstand transitory and moderate burial. The findings also reveal that runners may escape from burial not only vertically, but also horizontally, changing growth pattern and direction from predominantly unidirectional to multidirectional. This additional strategy could allow buried runners to accelerate space occupation even if disconnected from clones.

Potential for landscape-scale positive interactions among tropical marine ecosystems

Fluxes of energy, materials and organisms among ecosystems are consequences of their openness to exchange and lead to the consideration of reciprocal connections among adjacent ecosystems. Reciprocal connectivity may have implications for ecosystem functioning and manage- ment but it is generally studied only for a single factor, rather than for multiple factors. We examined the extent to which these fluxes may apply at the landscape scale for 3 ecosystems: mangrove forests, seagrass beds and coral reefs. From a literature review and analysis, we semi- quantitatively assessed fluxes based on attenuation of wave height and exchanges of sediments, nutrients and al- givores. We found that coral reefs and seagrass beds significantly attenuated wave height and that this effect depended on specific physical conditions. In the case of coral reefs, the attenuation ca- pacity depended on the section of the reef the wave breached, whilst for seagrass beds, we hypoth- esised that the density of the plants was the controlling factor. Mangrove forests' ability to reduce sediment fluxes was related to the mangrove forest area. Seagrass plants have a capacity to decrease sediments in the water column. Both mangrove forests and seagrass beds retained nutri- ents within the ecosystems, which was a positive interaction. Isolated reefs showed a decrease (30 to 95%) in algivore biomass compared to situations where the 3 habitats were in proximity to each other. The findings show that there is potential for reciprocal connections among coastal eco- systems. Our results indicate that these exchanges at the ecosystem scale can be placed into the context of facilitation in the field of community ecology. Future research should focus on which natural and anthropogenic factors determine reciprocal facilitation between these ecosystems and determine how ecosystem-based management can be improved with this knowledge. The consid- erable potential for reciprocal facilitation implies that ecosystem managers may need to place greater emphasis on the landscape scale.

Multistage centrifugation method for determination of filtration and consolidation properties of mineral and biological suspensions using the analytical photocentrifuge

The article describes a new method of multistage centrifugation (MC) for determination of filterability (local filtration and consolidation properties) of mineral and biological suspensions. The method comprises centrifugal compression–permeability and sedimentation–consolidation experiments, performed with the help of analytical photocentrifuge at different centrifugal rotation speeds. The experimental procedure implies determination of the analytical centrifugation curves: dependencies of (i) sediment height on centrifugal acceleration in sedimentation–consolidation experiments and (ii) permeate volume and filter cake dryness on centrifugation time and centrifugal acceleration in filtration–consolidation experiments. The combined analysis of these curves yields the pressure dependencies of local cake dryness, specific cake resistance, and consolidation coefficient in the wide range of solid pressure (5×103–5×105Pa). Application of MC requires a relatively small quantity of suspension (about 2ml).The method was tested for suspensions with different filterabilities and compressibilities: calcium carbonate, kaolin and yeast suspensions. The pressure dependencies of cake dryness, specific cake resistance, and consolidation coefficient, determined by MC, were in correspondence with those determined in the series of conventional constant pressure dead-end filtration–consolidation experiments.

Settling Characteristics of Alumina Nanoparticles in Ethanol-Water Mixtures

Nanofluids are considered as promising heat transfer fluids due to enhanced heat transfer ability as compared to the base fluid alone. Knowledge of settling characteristics of nanofluids has great importance towards stability of nanosuspensions. Sedimentation behavior of Alumina nanoparticles due to gravity has been investigated using different proportions of ethanol-water binary mixtures. Nanoparticles of 40 nm and 50 nm are used in this investigation at 23°C. Sediment height with respect to time is measured by visualization method in batch sedimentation. The effect of sonication on the sedimentation behavior is also studied using ultrasonic agitator. The effect of particle diameter, nanoparticle concentration and ethanol-water proportion on sedimentation behavior of nanofluids has been investigated and discussed.

Stabilization of Ceramics Particles with Anionic Polymeric Dispersants

The dispersion of colloidal ceramic particles in an aqueous media using a charged polymer has been investigated extensively. Most often, such a dispersion is obtained by an electrosteric mechanism. The stabilizing forces that result from electrical double layer repulsion and/or steric interaction must be sufficiently large in magnitude to provide an energy barrier against aggregation. Several factors, such as pH, temperature and ionic strength, nature, structure and molecular weight of the polyelectrolyte and its degree of dissociation can affect the respective contribution of electrostatic and steric interactions. In this study we applied silver nanoparticles prepared by reduction method with modifications. The hydroxyapatite was obtained from pork bone sludge. Polymer system contained poly(acrylic acid) and poly(ethylene glycol). The effects of dispersant structure on particle stabilization were investigated through properties of the suspensions. Viscosity and sedimentation height measurements indicated that addition of the dispersants improve particle stabilization. A dispersants were characterized using Scanning electron microscopy (SEM) with BSE detector (Back Scaterred Electrons), X-ray diffraction (XRD) and pH determination.

Modeling of the Height of Sediment in a Continuous Thickener

Mathematical models for thickeners can be used for equipment design, to simulate the process operation, and to develop control strategies for a continuous operation. In this work, numerical simulation to obtain the final height of the sediment when operating at steady state conditions were performed using two mathematical models: the model introduced by Tiller and Chen (1988) and the one proposed by Arouca (2007). The simulated results were compared to experimental data obtained when the thickener was operated with and without the use of flocculent agent. The equipment used to obtain experimental data was a conventional thickener, built in acrylic. Aluminum sulfate was used in the experiments for the production of flocculated suspensions. The results showed that the Tiller and Chen (1988) model produced a deviation of 39.0% for kaolin suspensions and 14.3% when aluminum sulfate was added to the system. The Arouca (2007) model resulted in values much closer to those obtained experimentally, with deviations of less than 3% for kaolin suspensions regardless of the use of flocculent agent.