Suspended Particle Size Distribution Research Articles

Ultrasonic wave propagation in concentrated slurries – The modelling problem

The suspended particle size distribution in slurries can, in principle, be estimated from measured ultrasonic wave attenuation across a frequency band in the 10s of MHz range. The procedure requires a computational model of wave propagation which incorporates scattering phenomena. These models fail at high particle concentrations due to hydrodynamic effects which they do not incorporate. This work seeks an effective viscosity and density for the medium surrounding the particles, which would enable the scattering model predictions to match experimental data for high solids loading. It is found that the required viscosity model has unphysical characteristics leading to the conclusion that a simple effective medium modification to the ECAH/LB is not possible. The paper confirms the successful results which can be obtained using core–shell scattering models, for smaller particles than had previously been studied, and outlines modifications to these which would permit rapid computation of sufficient stability to support fast particle sizing procedures.

A new method for separation, characterization, and quantification of natural nanoparticles from soils

Properties and distribution dynamics of nanoparticles are pertinent to ecological and human health. Wide variation in particle density confounds their accurate separation for study. The objective of this study was to develop a fine-particle separation protocol requiring no constant density assumption. Six soils with contrasting properties were chosen for study. Size distributions of suspended particles following centrifugation were assessed using dynamic light scattering. Mineralogy was determined by X-ray diffraction. Particle characteristics were examined using transmission electron microscopy. A significant pooled logarithmic relationship was found between centrifugation speed and particle-size distribution. Nanoparticles (<100 nm) became dominant by fractional volume at widely differing separation energies among the six soils. However, relations were strong between separation energies and fractional volume for individual soils. Bi- and tri-modal-size distributions within suspensions were related to density differences of soil components. Re-aggregation at high separation energy was apparent for the tropical soils. Light-scattering, UV–Visible absorbance, and chemical analyses distinguished samples of high hematite content from those without hematite. These results indicate that dynamic light scattering in conjunction with a standard centrifuge regimen provides a practical means of collecting colloidal particles within a targeted size range, compensating for natural variations among the soil samples.

Clogging evaluation of open graded friction course pavements tested under rainfall and heavy vehicle simulators

In this study a new procedure is developed to obtain core samples from field sections to assess clogging mechanisms of open graded friction course (OGFC) pavements using X-ray computed tomography (CT) imaging. The approach compared X-ray computed tomography (CT) images taken before and after: (1) rainfall simulations without trafficking to investigate particle-related clogging and (2) full-scale accelerated pavement rutting tests (APT) to investigate deformation related clogging of OGFC layers. Rainfall simulations were performed with runoff water of known total suspended solids (TSS) and particle size distributions (PSDs). Full-scale accelerated rutting tests were performed under controlled temperature and loads. Both investigations were performed for three different OGFC pavements with different layer thicknesses and mix types. The clogging of rutting test sections were also evaluated by comparing the surface permeability measurements performed before and after APT testing.The results of X-ray CT image processing revealed a significant reduction in air-void content of core samples after APT rutting tests. The highest air-void reduction was concentrated at the bottom of the OGFC layers. Permeability measurements also showed a 40%–90% reduction in permeability after APT trafficking. X-ray CT image processing of core samples tested under simulated rainfall showed that air void content reduction is concentrated in the lower part (2–6 mm from the bottom) of the OGFC layers as a result of particle accumulation. Small changes in air void contents were observed in the upper part of the OGFC layers (10–15 mm) while these reductions in air void contents were not significant to cause surface overflow and hence it is expected that the tested OGFC pavements will have sufficient permeability to infiltrate water during most average storm events.

Dynamic process of turbidity generation triggered by the 2011 Tohoku‐Oki earthquake

We analyzed the distribution of turbidity in seawater near the epicenter of the 2011 Tohoku earthquake (Mw 9.0) as well as the mineral composition and grain size distribution of suspended particles and of shallow sediments in cores from the same area. One month after the earthquake in an area where large changes in bathymetry accompanied the earthquake, we observed a strong turbidity anomaly above the seafloor (maximum 5 mg/L) that greatly exceeded turbidity measured several years before the earthquake (average 0.59 mg/L). The suspended particles were composed of material similar to the surface material of the sediment cores, and we inferred that they were generated locally by a disturbance due to slope failure. We estimated slope stability on the basis of the geophysical characteristics of the sediment cores as well as the acceleration of the earthquake. Our results showed that a submarine landslide could have been induced by the very large ground acceleration of the Tohoku earthquake, as high as 10–15 m/s2, even if the sediment layer on the sliding surface was thin. We interpreted the turbidity anomaly observed one month after the Tohoku earthquake as the result of shallow submarine landsliding and stirring up of unconsolidated sediment around the epicentral area.

Particle sorting and formation and elimination of pseudofaeces in the bivalves Mulinia edulis (siphonate) and Mytilus chilensis (asiphonate)

Mulinia edulis and Mytilus chilensis are suspension-feeding bivalves with homorhabdic gills that live in different sedimentary habitats in the lower and upper intertidal, respectively, in Yaldad Bay, Chile. They are faced with different suspended particle size distributions when feeding, and both eliminate most of the inorganic particles by pseudofaeces production. This study used histology, scanning and transmission electron microscopy, and video endoscopy to compare particle processing on the labial palps and the mechanisms of particle sorting, acceptance, and rejection in the two species. In both species, disaggregation of mucus-bound particles occurs on the plicate surface of the palps. Particles destined for ingestion pass anteriorly from crest to crest and reach the mouth via the palp acceptance tract. Those destined for rejection enter the troughs between the plicae and move ventrally to the palp rejection tract. The palps manipulate the pseudofaeces into a mucous ball, which is transferred to the mantle rejection tract. In Mulinia edulis (an infaunal, siphonate mactrid), the pseudofaeces are stored in a chamber at the base of the inhalant siphon until expelled by intermittent contraction of the siphon wall. In contrast, Mytilus chilensis (an epifaunal, non-siphonate mytilid) releases pseudofaeces continuously when submerged.

Mass-specific scattering cross sections of suspended sediments and aggregates: theoretical limits and applications

The spectral mass-specific scattering cross section σ[PIM](λ) is most important for the remote sensing inversion of the concentration of suspended mineral matter in the coastal ocean. This optical parameter is also important in optical theory and therefore the theoretical limits of this parameter are important. There are differing reports in the literature on the magnitude of σ[PIM](λ) and its spectral slope in different coastal ocean systems. To account for and predict these differences, I have applied a model of the size distribution of primary suspended mineral particles and aggregates of these particles to theoretical calculations of σ[PIM](λ). I utilized a model of mineral particle aggregates by Khelifa and Hill [Khelifa, A. and P.S. Hill, J. Hydraul. Res. 44, 390 (2006)] and Latimer's optical model of aggregates [Latimer, P., Appl. Opt. 24, 3231, (1985)]. I have been able to account for the variations in magnitude and spectral slope of σ[PIM](λ). This analysis will apply to not only inverting the concentration of suspended mineral matter but also provides the basis for inverting the processes of coagulation and aggregation of primary mineral particles in determining sedimentation rates, budgets, etc.

Hydro-meteorological influences and multimodal suspended particle size distributions in the Belgian nearshore area (southern North Sea)

Suspended particulate matter (SPM) concentration and particle size distribution (PSD) were assessed in a coastal turbidity maximum area (southern North Sea) during a composite period of 37 days in January–April 2008. PSDs were measured with a LISST 100X and classified using entropy analysis in terms of subtidal alongshore flow. The PSDs during tide-dominated conditions showed distinct multimodal behaviour due to flocculation, revealing that the building blocks of flocs consist of primary particles (<3 μm) and flocculi (15 μm). Flocculi comprise clusters of clay minerals, whereas primary particles have various compositions (calcite, clays). The PSDs during storms with a NE-directed alongshore subtidal current (NE storms) are typically unimodal and characterised by mainly granular material (silt, sand) resuspended from the seabed. During storms with a SW-directed alongshore subtidal current (SW storms), by contrast, mainly flocculated material can be identified in the PSDs. The findings emphasise the importance of wind-induced advection, alongshore subtidal flow and highly concentrated mud suspensions (HCMSs) as regulating mechanisms of SPM concentration, as well as other SPM characteristics (cohesiveness or composition of mixed sediment particles) and size distribution in a high-turbidity area. The direction of subtidal alongshore flow during SW storm events results in an increase in cohesive SPM concentration, HCMS formation, and the armouring of sand; by contrast, there is a decrease in cohesive SPM concentration, no HCMS formation, and an increase in sand and silt in suspension during NE storms.

The influence of ambient light intensity on in situ laser diffractometers

In situ laser diffractometers characterize the suspended particle size distribution (PSD) by measuring laser‐generated light scattered off small particles over a range of small forward angles. In environments with low particulate concentrations or high ambient light conditions the ratio of natural downwelling sunlight to scattered laser light sensed by the photodetectors is high and measurements are influenced. Here, we evaluate the effect of the ambient light field intensity on measurements made with a Laser In Situ Scattering and Transmissometry (LISST) 100X type B instrument. Paired light‐dark scattering distributions are recorded over a range of underwater light intensities in high‐turbidity and low‐turbidity water. Light measurements displayed large erroneous concentrations of particles in the smallest size bin (1.25–1.48 μm) and showed effects over the full range of the PSD. Ambient light was found to exhibit the same constant distribution over the instrument photodetectors in both water samples, although the magnitude of the response, in laser counts per unit ambient light intensity, was PSD dependent. A technique for postprocessing data to remove the influence of light is presented for moored deployment and vertical profile data collected at Lake Tahoe, California‐Nevada, United States. While measurements removed of the light effect were successfully reconstructed, the technique may not be applicable to data where the PSD or the LISST orientation relative to the sun direction change rapidly or when light intensities are high enough to quench the instrument photodetectors. Ambient light was found to have negligible effects on PSD measurements in Lake Tahoe was below intensities of ∼30 W m−2.

A heuristic examination of cohesive sediment bed exchange in turbulent flows

Prediction of the concentration of suspended cohesive sediment in the marine environment is constrained by difficulties in interpreting experimental evidence on bed exchange, i.e. erosion and deposition of particles, which remains sparse in mechanistic details. In this paper, conditions under which bed exchange in turbulent flows collectively determines the concentration of suspended matter have been examined in the heuristic sense based on selective experimental data. It is argued that interpretation of such data can be significantly facilitated when multi-class representation of particle size, collisional interaction between suspended particles and probabilistic representations of the bed shear stress along with variables describing particle behavior (critical shear stress for deposition, bed floc shear strength) are taken into account. Aggregation—floc growth and breakup kinetics—brings about shifts in the suspended particle size distribution; bed exchange is accordingly modulated and this in turn determines concentration dynamics. Probabilistic representation of the governing variables broadens the suspended sediment size spectrum by increasing the possibilities of inter-particle interactions relative to the mean-value representation. Simple models of bed exchange, which essentially rely on single-size assumption and mean-value representation of variables, overlook the mechanistic basis underpinning particle dynamics.

Analysis of the disperse composition of dust in a gas flow at the outlet from a deduster

A method is proposed for analysis of the disperse composition of suspended particles in the discharge from an inertia-type deduster; the method is based on compliance of the dust composition at the inlet to the vessel and the fractional efficiency of the deduster to the logarithmic size distribution of suspended particles.

Simultaneous particle size and concentration measurements using a back-lighted particle imaging system

A back-lighted particle imaging system (BLPIS) is developed to simultaneously measure particle size distribution and volume concentration of suspended particles in water. Based upon optical principles, a modified in-focus parameter, incorporating optical conditions, particle shape, overlapping, and depth-of-field variations due to particle size, is developed to identify particles. BLPIS is calibrated with a precise graticule with an accuracy of particle sizes within the measurement uncertainty of image pixel size resolution. For volume concentration, BLPIS employs an adaptive sampling volume methodology that varies with each particle size based upon a critical in-focus parameter value. Experiments show that under lower volume concentration, measured and actual particle size distributions are in good agreement. For higher volume concentration, measured particle concentration is corrected using a Poisson-based distribution to account for overlapping. Overall the volumetric concentration measurement is reliable and accurate with a relative error of approximate 10%. Finally BLPIS is applied to accurately measure both size and concentration of flowing particles near the bottom boundary with resuspension and sedimentation processes.

Contribution of In‐Channel Processes to Sediment Yield of an Urbanizing Watershed1

Abstract: A study was conducted between September 2003 and September 2006 to obtain baseline sediment inventories and monitor sediment transport and storage along a 3.7 km length of the channel of Valley Creek within Valley Forge National Historical Park, Pennsylvania. Valley Creek is a tributary of the Schuylkill River and drains an urbanizing 60.6 km2 watershed that currently has 18% impervious land cover. Numerous field methods were employed to measure the suspended sediment yield, longitudinal profile, cross‐sections, banklines, and particle size distribution of the streambed. Suspended sediment yield for the watershed was measured at a USGS gage located just upstream of the park boundary between July 2004 and July 2005, the period corresponding to field surveys of bank erosion and channel change. The estimated suspended sediment yield of 95.7 t/km2/year is representative of a year with unusually high discharge, including a storm event that produced a peak of 78 m3/s, the second highest discharge on record for the USGS gage. Based on the median annual streamflow for the 24 years of record at the USGS gage from 1983 to 2006, the median annual sediment yield is estimated to be closer to 34 t/km2/year, considerably lower than median and mean values for other sites within the region. The mass of silt, clay, and fine sand derived from bank erosion along the 3.7 km study reach during the field survey period accounts for an estimated 2,340 t, equivalent to about 43% of the suspended sediment load. The mass of fine sediment stored in the bed along the study reach was estimated at 1,500 t, with about 330 t of net erosion during the study period. Although bank erosion appears to be a potentially dominant source of sediment by comparison with annual suspended sediment load, bed sediment storage and potential for remobilization is of the same order of magnitude as the mass of sediment derived from bank erosion.

Effect of Mixing on Suspended Particle-Size Distribution

Experiments were performed to evaluate changes in suspended particle-size distribution as a function of variations in mixing conditions. Suspensions were taken from Lake Erie near Buffalo, N.Y., and mixed in a standard jar apparatus using six different mixing intensities. These suspensions were monitored for changes in particle size and shape using a noninvasive image analysis technique. A relationship was derived between suspended particle size and mixing intensity, based on estimates of energy dissipation rates. The regression was also compared with mixing of suspensions taken from two other systems in the Buffalo, N.Y. area, a small lake on the University at Buffalo campus, and the Buffalo River (Buffalo, N.Y.). Samples from all three sites behaved similarly in terms of particle size as a function of mixing conditions. Estimates for dissipation rates in natural systems were then used to relate the laboratory data to field conditions. Dissipation rates produced at slow mixing speeds in the jar were similar to those calculated for more energetic riverine and estuarine environments. Results of this study should be useful for understanding suspended particle-size distributions under a variety of mixing conditions, and is of direct interest in a wide range of sediment and contaminant transport problems.

On the effect of flocculation as pretreatment process and particle size distribution for membrane fouling reduction

This work deals with the feasibility of an olive vegetation wastewater purification process consisting of two consecutive steps: coagulation followed by membrane separation. The target of this work was to reach a purification grade of the wastewater compatible with the limits for the discharge into a municipal sewer that is a COD value less than 500 mg L −1. A feedstock with 55 g L −1 of COD was used for the experimental work. In a first step, the feedstock was pretreated by a flocculation process using as coagulant aluminium sulphate or aluminium hydroxide. Then, the pretreated wastewater stream was purified by four consecutive batch membrane processes consisting of microfiltration, ultrafiltration, nanofiltration and reverse osmosis, each with a recovery rate of 90%. For each membrane, at different recovery values, the critical flux values were measured. Finally, all the streams were analyzed by means of a nanosizer, capable to measure the suspended particle size distribution in the samples from 0.8 nm up to 6.5 μm. Flocculation changes the fouling outcome on the membranes by changing sensibly the particle size distributions of the solution. The membrane's performances are strictly dependant on the coagulant type used during the pretreatment operation. In order to develop full advantages on fouling issues, the particle size shift effect of the flocculation process should be carefully studied and evaluated, and needs fine-tuned optimization.

Spatio-temporal variability in suspended particulate matter concentration and the role of aggregation on size distribution in a coral reef lagoon

This paper presents the concentration and size distribution of suspended particles in the South-West Coral reef lagoon of the island of New Caledonia. Data are provided by filtration techniques, Optical BackScattering (OBS) measurements and in situ laser diffraction particle sizing. The concentration of suspended particles increased from reef to land. A bottom nepheloïd layer occurred over the entire lagoon, and was more distinct on the nearshore area. Small particles were more abundant in the bottom nepheloïd layer than in the overlying water column. The concentration of suspended particles showed more variability over space than over time. Conversely, the particle size distribution of suspended particles showed more variability at a given location over a month (time variability) than at a given moment over the lagoon (space variability). Analysis showed that aggregates represented a large fraction of suspended particles. Microscope visualization and chemical analysis of a sample suggest the inclusion of a transparent exopolymeric matrix. The relative abundance of aggregates over suspended particle volume concentration was found to increase as the quantity of suspended particle decreased. The spatial distribution of the relative abundance of aggregates suggests more aggregates proximal to coral reefs. The high concentration of aggregates at low turbidity and the spatial distribution of the relative abundance, infer that aggregation is induced by the presence of organic ligands. Unlike optical backscattering and light attenuation measurements that are size sensitive measurements of suspended particle concentration, in situ laser diffraction particle sizing provides a relevant optical measurement of suspended particulate matter in such an aggregate-dominated system.

Observations of sediment resuspension and settling off the mouth of Jiaozhou Bay, Yellow Sea

We deployed bottom-mounted quadrapod equipped with acoustic Doppler current profiler (ADCP), acoustic Doppler velocimeter (ADV), and optical backscatter sensor (OBS) over two semidiurnal tidal cycles along the western coast of the Yellow Sea, China. In combination with shipboard profiling of CTD and LISST-100, we resolved the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions. During the observations, tidal-induced bottom shear stress was the main stirring factor. However, weak tidal flow during the ebb phase was accompanied by two large SSC and median size events. The interactions of seiche-induced oscillations with weak ebb flow induced multiple flow reversals and provided a source of turbulence production, which stripped up the benthic fluff layers (only several millimeters) around the Jiaozhou Bay mouth. Several different methods for inferring mean suspended sediment settling velocity agreed well under peak currents, including estimates using LISST-based Stokes’ settling law, and ADCP-based Rouse profiles, ADV-based inertial-dissipation balance and Reynolds flux. Suspended particles in the study site can be roughly classified into two types according to settling behavior: a smaller, denser class consistent with silt and clay and a larger, less dense class consistent with loosely aggregated flocs. In the present work, we prove that acoustic approaches are robust in simultaneously and non-intrusively estimating hydrodynamics, SSC and settling velocities, which is especially applicable for studying sediment dynamics in tidal environments with moderate concentration levels.

Measurements of the scattering characteristics of sediment suspensions having broad particle size distributions

Profiles of suspended sediment concentration and mean size can be measured in underwater environments using Acoustic Backscatter Systems (ABS). Inversion of ABS measurements into sediment size and concentration requires knowledge of the backscattering form function, f, and the total normalised scattering cross section, χ. Previous studies have measured these parameters for sediments sieved over narrow size ranges only. Narrow size ranges are unrealistic compared to the broad Particle Size Distributions (PSD) that typically occur in nature however, and it is known that f and χ are significantly altered by changes from narrow to broad PSDs. Theoretically, these changes can be accounted for in ABS inversions by taking values of f and χ obtained from narrow size ranges, and integrating them over the suspended PSD, though this theory has not been validated for real suspended sediments. Here, we compare the results of this integration with measured values of f and χ for suspensions of (irregularly shaped) sandy sediments having broad PSDs. The results show that as the standard deviation of the size distribution increases, acoustic scattering is increased in the Rayleigh regime whilst being reduced in the geometric regime. These changes significantly affect acoustic estimates of suspended sediment concentration and size.

Effects of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on particle size distribution and phytoplankton abundance during a mesocosm bloom experiment (PeECE II)

Abstract. The influence of seawater carbon dioxide (CO2) concentration on the size distribution of suspended particles (2–60 μm) and on phytoplankton abundance was investigated during a mesocosm experiment at the large scale facility (LFS) in Bergen, Norway, in the frame of the Pelagic Ecosystem CO2 Enrichment study (PeECE II). In nine outdoor enclosures the partial pressure of CO2 in seawater was modified by an aeration system to simulate past (~190 μatm CO2), present (~370 μatm CO2) and future (~700 μatm CO2) CO2 conditions in triplicates. Due to the initial addition of inorganic nutrients, phytoplankton blooms developed in all mesocosms and were monitored over a period of 19 days. Seawater samples were collected daily for analysing the abundance of suspended particles and phytoplankton with the Coulter Counter and with Flow Cytometry, respectively. During the bloom period, the abundance of small particles (&lt;4 μm) significantly increased at past, and decreased at future CO2 levels. At that time, a direct relationship between the total-surface-to-total-volume ratio of suspended particles and DIC concentration was determined for all mesocosms. Significant changes with respect to the CO2 treatment were also observed in the phytoplankton community structure. While some populations such as diatoms seemed to be insensitive to the CO2 treatment, others like Micromonas spp. increased with CO2, or showed maximum abundance at present day CO2 (i.e. Emiliania huxleyi). The strongest response to CO2 was observed in the abundance of small autotrophic nano-plankton that strongly increased during the bloom in the past CO2 mesocosms. Together, changes in particle size distribution and phytoplankton community indicate a complex interplay between the ability of the cells to physiologically respond to changes in CO2 and size selection. Size of cells is of general importance for a variety of processes in marine systems such as diffusion-limited uptake of substrates, resource allocation, predator-prey interaction, and gravitational settling. The observed changes in particle size distribution are therefore discussed with respect to biogeochemical cycling and ecosystem functioning.

Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction

We present new observational data on small‐angle light scattering properties of natural, random shaped particles, as contrasted with spherical particles. The interest in this “shape effect” on scattering arises from the need for a suitable kernel matrix for use in the laser diffraction method (LD) of particle sizing. LD is now used broadly for measuring size distribution of suspended marine particles. LD involves the measurement of small‐angle forward scattering at multiple angles. This data is inverted using the kernel matrix to produce size distribution. In the absence of a suitable matrix for random shaped particles, past practice has been to use a model based on Mie theory, applicable strictly only to homogeneous spheres. The present work replaces Mie theory with empirical data. The work was motivated in part by anomalous field observations of size distribution and settling velocity distributions reported in literature. We show that a kernel matrix for random shaped particles results in improved interpretation of field multiangle scattering observations. In particular, a rising edge at the fine particle end of the size spectrum is shown to be associated with shape effects.

LISST‐100 measurements of phytoplankton size distribution: evaluation of the effects of cell shape

Information on cell size is central to studies of phytoplankton ecology, yet in situ measurements of particle size distribution remain a challenge. The LISST‐100 (Laser In Situ Scattering and Transmissometry; Sequoia Scientific, Inc.) is one of few commercially available instruments that provide autonomous measurements of size distributions of suspended particles in situ. Its capability to size phytoplankton needs to be evaluated, however, for two reasons. First, size is not measured directly; rather, information on size is obtained from near‐forward scattering measurements that are inverted to obtain size distributions. The inversion assumes that particles are homogeneous spheres. Phytoplankton, however, display a wide range of cell shapes and their scattering behavior is likely to be different from that of spheres. Second, the LISST‐100 was originally designed for sediments and its inversion assumes an index of refraction typical of inorganic particles. We compared size and volume concentration distributions obtained by the LISST‐100 to those derived via microscopy for phytoplankton cultures of different cell sizes and shapes. Results demonstrate the success of the LISST‐100 in measuring size and volume concentrations of cells with a cell aspect ratio that is close to 1 and illustrate its limitations when nonspherical cells are present in the sample. Uses of the LISST‐100 in laboratory and field studies are discussed in light of the results from this study.