Settling Velocity Distribution Research Articles

Submicron size distributions of inorganic suspended solids in turbid waters by photon correlation spectroscopy

Photon correlation Spectroscopy (PCS) is a technique for determining size distributions of submicron‐sized particles. A particle diffusion coefficient, which is dependent on particle diameter, is determined from measurements of the autocorrelation function of the intensity fluctuations of light scattered from a laser beam passing through a suspension of particles. We demonstrate the applicability of the technique to particles likely to be found in runoff or turbid impoundments using soil fractions prepared to have a known size distribution. Mean particle diameters of all soil fractions determined by PCS fell within the expected range. Particle‐size distributions of natural water samples were reproducible when analyzed within minutes of collection. The effects of sample storage and pretreatment varied between water bodies and thus can be a source of uncertainty in the results. Particles from a turbid creek carying storm runoff began to coagulate within 2.5–6 hours, whereas particles from a perennially turbid impoundment remained stable for 10 days. Addition of sodium hexametaphosphate improved the reproducibility of the particle‐size distribution with prolonged storage. Applications of the technique include determination of coagulation rates and critical coagulation concentrations of electrolytes, and calculation of settling velocity distributions of submicron particles in turbid natural waters.

Sample site variation in settling velocity distribution subpopulations using curve dissection analysis

ABSTRACT It is generally believed that grain‐size distributions can be broken down into subpopulations which may be characteristic of a specific mode of transport, some source signature, or the nature of sediment packing in the depositional process. This paper evaluates the variation at a sample site of subpopulation parameters of settling velocity distributions derived using a non‐linear least squares algorithm for curve dissection. Results incorporate artificial samples constructed using spherical glass beads in the 125‐840 μm range and natural samples collected as close as possible from a single sedimentation unit in marine, fluvial and aeolian environments. If subpopulation distributions from the same sample site are compared then 90% of artificial bead samples and 65% of natural sands have cumulative curves diverging from each other by less than 20%. Similarity in means and standard deviations of subpopulations collected at the same site was not easily obtained. However, if it is important to be able to identify a specific process or source signature correctly, then 86% of all subpopulations from the same natural sample site grouped together. If subpopulations of grain‐size distributions are to be used for process identification, it is advisable to analyse more than one sample from a site.

Use of the UASB reactor for the anaerobic treatment of stillage from sugar cane molasses

The feasibility of applying the UASB concept for the anaerobic treatment of stillage of distilleries in the sugar producing area of Argentina was subject to study. Results obtained in a 100-L UASB reactor treating stillages with COD values between 35 and 100 g COD/L are presented. Loading rates of up to 24 g COD/L/day were applied with an average COD removal of 75% and a biogas production of more than 9 L/L/day, with an average methane content of 58%. The settling velocity distribution of sludge particles would indicate a good formation of biomass pellets. System interruptions of months without feed and at ambient temperature (20-24 degrees C) were well tolerated.

Erosion, transport and deposition of fine-grained marine sediments

Summary Fine-grained marine sediments are cohesive but their degree of cohesion is not simply determined by grain size. Cohesion controls erodibility, and water content, mineralogy, cation exchange capacity, salinity of interstitial and eroding fluid, organic mucus content and Bingham yield strength are all parameters relating to cohesion that have been proposed. No unique relation to erodibility has emerged and for erosion of slowly deposited sediment, modified by biota, it seems that measures of surface properties such as aggregate strength should be more relevant than bulk properties such as yield strength. The latter may be more appropriate for rapidly deposited estuarine muds. Once eroded, suspended sediment is subject to flocculation and biological aggregation, which alters size and settling velocity distributions, thereby controlling distribution in the flow and rate of deposition. Disaggregation may also occur through turbulent straining of particles, but in most marine situations this will permit stable particles over 1 mm in diameter, though under higher shear close to the bed the upper limit may be 100–200 μm. For deposition where τ 0 < 0.1 Pa the maximum stable aggregate size is probably ≥ 100 μm. Deposition of fine sediment on smooth beds probably occurs by entrapment in, and settling through, the viscous sublayer. On rough beds particles are trapped in the interstices between roughness elements. We have only a rough idea of critical deposition conditions but can give a fairly good estimate of deposition rate in given conditions. The longer term net deposition rate as measured by radiometric methods over weeks to months probably owes as much to frequency of erosion as to frequency and rate of deposition and is not yet well predicted from fluid and sediment parameters.

Settling velocity distribution of natural aggregates

An analysis of previous experimental determinations of aggregate settling rates for particles <100 μm in diameter shows that these particles fall significantly more quickly than Stokes' law predicts, with the deviations being most pronounced for smaller particles. This implies that previous sediment flux calculations which have used a form of Stokes' law to obtain aggregate velocities may have significantly underestimated the contribution from the smaller aggregates. The measured fall velocity distributions are described as the sum of normally distributed populations (four for the lacustrine data, two for the marine) whose means, standard deviations, and weights have been determined. Sample calculations of collision frequency show that because of the increased settling velocities and the use of a distribution of velocities for a given particle size, collisions due to differential settling rates are the governing nonbiological process in the formation of natural aggregates.

An optical settling tube for the determination of particle-size distributions

The particle-size distribution, the settling-velocity distribution and the concentration-weighted settling velocity are important parameters in the study of sediment dynamics. These parameters can be determined by means of an optical settling tube in which the concentration of particles is determined by means of a beam-attenuation meter and the particle size is determined by Stokes' settling. Results obtained by this method compare very well with Coulter Counter and gravimetric settling tube data. Various correction factors are discussed. The instrument is designed for eventual use in situ.

Upgrading Lagoon Effluents with Rock Filters

Rock filtration is an effective, low-cost unit process for removing algae from effluents and correspondingly upgrading lagoon treatment. Sedimentation is the primary mechanism of algal removal within rock filter. A mathematical model of the sedimentation mechanism indicated that total suspendedsolids (TSS) removal efficiency is a function of the distribution of algal settling velocities, the hydraulic retention time, and the effective settling distance in the rock filter. Weekly average effluent biochemical oxygendemand (BOD\d5) and TSS from the Veneta, Ore, lagoon-rock filter treatment facility did not exceed 20 mg/L over a 1-yr study period. Settling rates of algae from the Veneta lagoon averaged about 0.05 m\dy. A linear relationship between TSS removal efficiency and hydraulic loading rate was demonstrated.

Discrimination of depositional environments using settling tube data

The settling-velocity distributions of more than 500 modern and near-modem river, dune, beach, tidal-flat, and turbidite sediment samples were analyzed using a photo-extinction settling tube. Discriminant function analysis was applied in order to investigate the possibility of extracting environmental information from settling velocity distributions using 65 variables that describe distribution curves. The results showed that 1) good discriminations were obtained between reference of river, beach, and dune sediments; 2) reliable answers were obtained through identification tests of unknown sets using reference sets: 3) the Triassic (?) and Jurassic Navajo Sandstone samples were mostly grouped as eolian (dune) when compared with the analyzed of samples; and 4) the variables useful for environmental discrimination were chiefly related to the tails of distributions. Some variables commonly used for environmental analysis (e.g., mean value and sorting) were found to be more influenced by provenance factors than by environmental factors.

Deposition of resedimented sandstone beds in the Pico Formation, Ventura Basin, California, as interpreted from magnetic fabric measurements

Settling velocity distributions and magnetic fabrics in sediments from the Pico Formation were studied in order to determine the relationships between these properties and the observed sedimentary structures, and to evaluate the processes of deposition of turbidites. Three basic types of settling velocity frequency distributions were recognized: P1 profile, a low, flat distribution pattern indicating very poor sorting; P2 profile, a distribution which shows a distinctive mode; and P3 profile, a slope-shaped pattern composed predominantly of fine materials. P1 was found in the graded and massive divisions of turbidites; P2 was found in the lower division of horizontal stratification and the ripple stratification division; P3 was found in the uppermost divisions. Comparison of these patterns with previous results from modern sediments reveals that P2 and P3 are similar to patterns found in fluvial environments, whereas P1 was quite rare in “normal” current- or wave-formed deposits. The uniqueness of the graded and massive division is also evident in the results of the magnetic analysis. Although the magnetic fabrics in the upper parts of turbidites show similarity to other current-formed fabrics, the magnetic fabrics of the graded and massive divisions are quite different. The magnetic fabrics in the graded and massive divisions are characterized by (1) the presence of the current-normal orientation, and (2) less foliated and in-homogeneous fabrics which are indicated by high imbrication and q -value as well as large standard deviations of q -value and K max directions. Comparison with results from modern sediments indicates that fabric characteristics in the sediments of the graded and massive divisions are best explained by a combination of (1) an orientation mechanism related to layer by layer grain collision in a highly concentrated flow and (2) an orientation mechanism related to the suspension of grains in a viscous flow. This evidence indicates that a highly concentrated and partly viscous basal flow in turbidity currents may be responsible for the deposition of the lower part of the graded division and the massive division, whereas a more diluted flow may be responsible for the deposition of the upper divisions.

Origin of Bimodal Sands in Some Modern Environments

Grain settling velocity distributions of sediments from modern fluvial channel, eolian barchan dune, beach, and tidal flat environments of the United States and Mexico were analyzed using a photo-extinction settling tube. The results revealed the occurrence of bimodal distributions in several specific places: (1) trough cross stratification in fluvial channels, (2) the lower slip face, lower stoss side, and crest of barchan dunes, (3) eolian mega-ripples, (4) lower beach foreshores, (5) beach storm layers, and (6) inner parts of tidal fiats. Possible sorting mechanisms for generating bimodal characteristics of these deposits are suggested and are classified into three categories: a) Mixing of two sorting processes which have different sorting tendencies. For example, mixing of avalanche sorting and projection sorting on the dune slip face is responsible for bimodal sediments of types (1) and (2). b) Special hydraulic circumstances yielding high mobility of the coarse fraction; type (3) and (4) bimodal sands are examples. c) Unusual transportation events. Storm transportation of a coarse fraction into ordinarily coarse-sediment-free environments may be responsible for type (5) and (6) distributions. This study has demonstrated that bimodal distributions of grain settling velocity of sediments could be interpreted by simple and basic sorting processes.

Environmental Determination Using Hydraulic Equivalence Studies

Settling velocity distributions for beach, dune, nearshore, fluvial, and deltaic environments, Lake Erie

Settling velocity tube apparatus for successive determination of fall velocities of sand size particles

ABSTRACT Conventional sedimentation methods are impractical for determining settling velocity distribution of a sand sample limited to a few hundred grains. The settling velocity distribution of a very small amount of sand grains may be determined accurately in a one step operation using a modified Hand settling tube and an electrical event recorder.