Settling Velocity Distribution Research Articles

The practical use of wastewater characterisation in design

The design of conventional settlement devices used for primary treatment at wastewater treatment works has traditionally been based on generalised loading criteria. These generalised loadings do not necessarily take into account the nature and settling characteristics of the suspended material in influent wastewater streams. The paper describes the practical use of wastewater characterisation, in the form of settling velocity distributions, for the design of primary sedimentation devices, such as the hydrodynamic separators used in the Swirl-FloTM process. Results of settling velocity distributions for diverse wastewater sources (including municipal and industrial sources) are presented together with comparisons between steady-state model predictions with observed performance for waste streams at different sites. The observed differences in settling characteristics for the wastewater streams investigated, together with the implications these have for design and performance assessment, are used as basis for advocating the need to incorporate wastewater settling characteristics in the design process for wastewater sedimentation devices.

The practical use of wastewater characterisation in design

The design of conventional settlement devices used for primary treatment at wastewater treatment works has traditionally been based on generalised loading criteria. These generalised loadings do not necessarily take into account the nature and settling characteristics of the suspended material in influent wastewater streams. The paper describes the practical use of wastewater characterisation, in the form of settling velocity distributions, for the design of primary sedimentation devices, such as the hydrodynamic separators used in the Swirl-Flo™ process.Results of settling velocity distributions for diverse wastewater sources (including municipal and industrial sources) are presented together with comparisons between steady-state model predictions with observed performance for waste streams at different sites. The observed differences in settling characteristics for the wastewater streams investigated, together with the implications these have for design and performance assessment, are used as basis for advocating the need to incorporate wastewater settling characteristics in the design process for wastewater sedimentation devices.

The decomposition of polymodal settling velocity distributions for a comprehensive sedimentological description of sand‐sized samples

ABSTRACTThe composition of sediment samples and/or the influence of current activity on the sediment can be easily determined using the dissection of settling velocity distributions. A new dissection method for the analysis of settling velocity distributions has been developed. The initial settling velocity distribution is decomposed by means of settling velocity distributions of the main biogenic constituents, three planktic and three benthic foraminifera species. A divergence from the natural size distribution of a foraminifera species can be attributed to current influence. Thus, negative or positive skewness of current‐affected foraminifera populations indicate erosional or depositional sediment influence. The dissection of Holocene samples from the northern North Atlantic demonstrates the advantages of this method in the interpretation of sand‐size material.

Evaluation of the potential to dispose of sewage-sludge. 2. Potential for off-site movements of solids and solutes

This paper reports a study of the potential for off-site movements of pollutants from sewage sludge broadcast onto the soil surface in Pinus plantations established on the coastal lowlands of south-east Queensland. Laboratory studies of size and settling velocity distributions of rainfall-wetted sludge showed that it is relatively coarse and non-erodible. Field rainfall simulation studies at three sites in pine forests near Beerburrum found no significant increase in interrill erosion due to broadcasting of sludge. (Most or all of the sediment was observed to come from mineral soil exposed when a gutter to collect runoff was installed at the downslope ends of the plots.) However, there was considerable movement of solutes in runoff from rainfall simulator plots that had received sludge-either freshly applied or 'consolidated' sludge (that had been broadcast on the plots 6 months prior to the rainfall simulation study and exposed to rain and weathering). Electrical conductivity (EC) of runoff from freshly applied sludge was initially high but decreased steadily during the 30 min rainfall event. Runoff from consolidated sludge had lower ECs, though significantly (P < 0.05) higher than those of runoff from control plots. Concentrations of nitrate-N and ammonium-N were initially high in runoff from freshly applied sludge, but decreased rapidly during the rainfall event. In contrast, concentrations of mineral N in runoff from consolidated sludge were low throughout the rainfall event. These results indicate a significant risk of off-site N pollution if runoff occurs during the first rains following broadcasting of sludge. By contrast, concentrations of total phosphorus, copper, and zinc in runoff from both fresh and consolidated sludge were relatively high, and showed only small decreases during the rainfall event applied. This suggests that sludge will contribute significant quantities of these elements to runoff for extended periods after broadcasting. The implications of these findings become important in terms of the timing and method of sludge application to the soil.

Size and settling velocity distributions of flocs in the Tamar estuary during a tidal cycle

Data extracted from video recordings of individual estuarine flocs near the estuary bed during the advance and retreat of the salt intrusion show changes in size and settling velocity distributions. The recordings were taken using INSSEV —IN Situ SEttling Velocity instrument. Size coupled with effective density variations due to both changes in floc structure and ambient salinity result in changes in the settling velocity during the tidal cycle. In particular, just after high water slack, the appearance of high settling velocity medium size flocs and individual particles suggest that the lower density flocs have been broken up by the intense vertical shear in the currents caused by the salt wedge intrusion. Current shear is shown to have a significant influence on floc effective density.

The isolation of settling-velocity fractions of sand-size material; new possibilities for the settling tube technique

A new method for fractionating sand-size granular material by settling velocity makes it possible to obtain up to 24 desired settling-velocity fractions as accurately and easily as obtaining settling-velocity distributions with a sedimentation balance. The complete sample is retained in the fractions. An example using typical polymodal marine sediments demonstrates the advantage of compositional analysis of settling-velocity fractions for the interpretation of sedimentation processes.

Settling Characteristics of Feedlot Cattle Feces and Manure

Increased community awareness has created pressure on feedlots to improve their environmental performance. Australian feedlot sedimentation systems which are failing currently use empirical designs. These designs could be improved by basing them on settling characteristics of Australian feedlot manure. Settling velocity distributions of fresh feces and pen manure were measured using a settling column. We found that settling velocities of fresh feces are similar to those of pen manure and that rations that used barley or greater proportions of roughage had smaller components of slow-settling solids. All samples showed distinct fast and slow settling components. Some 35 to 75% of the material settled quickly and the remainder settled so slowly that it is not practical to design sedimentation systems to retain it. The data suggest that sedimentation system design should use a maximum settling velocity of 0.003 m/s.

Settling velocity characteristics of sediment detached from a soil surface by raindrop impact

Summary The aim of the study was to ascertain whether the detachment of sediment from a soil surface by raindrop impact is a selective or aselective process. Relationships between particle size and settling velocity were established. Two contrasting soil types with a mean surface water depth of 5 mm were subjected to simulated rainfall at a rate of 56 mm h−1 in modified splash-cups. Mean drop diameter was 2.24 mm. The two soil types were a cracking clay (black earth or vertisol) and a slightly dispersive sandy clay loam (solonchak or aridisol). For both soil types, sediment that was ejected into the air from drop-impact sites had a selective preference for removing sediment with settling velocities corresponding to fine sand-sized primary particles, and fine to coarse sand-sized aggregates. Investigations of the settling velocity distributions of detached sediments sampled from the surface water before ejection or deposition could occur suggested that, within experimental uncertainties, the impact-detachment process is aselective with respect to sediment size or settling velocity for the drop diameter and water depth investigated.

Hydrodynamic properties and grain-size characteristics of volcaniclastics deposits on the Mid-Atlantic Ridge north of Iceland (Kolbeinsey Ridge)

Surface sediments from a transect across the mid-ocean ridge north of Iceland (Kolbeinsey Ridge) have been analyzed according to their compositional, textural and hydromechanical characteristics. The results were used to reconstruct sediment formation and depositional processes. The ridge sediments are dominated by volcaniclastic particles of hyaloclastic and pyroclastic origin. These particles show a wide variety in size, shape and density. Single-grain settling velocities of the different glass types reveal the suitability of this parameter as a reflector of the particle properties of size, shape and density, which are also known to be relevant to grain transport. Observations concerning different current expositions of central ridge sediments, combined with the parameters of settling velocity distribution, grain-size distribution and sediment particle composition, were applied to distinguish between transport association with rare, easily movable glass shards and poorly sorted sediments in sheltered ponds. A bimodal settling velocity distribution of steep ridge-flank sediments probably indicates the effect of sediment admixture from poorly sorted mass flows. Alternating coarse- and fine-grained layers characterize the transition between ridge-glass sands and the ridge-adjacent plain, which is dominated by slow-settling pelagic material.

Further results for convection driven by the differential sedimentation of particles

When a well-mixed suspension of small particles is emplaced below a clear fluid whose density is greater than that of the interstitial fluid, but less than that of the bulk suspension, the subsequent settling of the dense particles releases buoyant interstitial fluid and drives convection in the overlying layer. Mixing of interstitial fluid and some entrained particles into the overlying fluid causes the density of the overlying fluid to evolve with time and changes the rate of descent of the interface between the sedimenting and convecting regions. These effects are investigated experimentally in a simple rectangular geometry using suspensions of spherical glass particles. It is found that the convecting region is well mixed in both composition and particle concentration and that the interfacial velocity may be predicted from the instantaneous (uniform) bulk density of the upper layer and the distribution of the particle settling velocities. In the case of an overlying density gradient, the convection does not extend through the depth of the overlying fluid but erodes the base of the gradient to form a well-mixed layer between the gradient and the sedimenting fluid. On completion of the first cycle of sedimentation-driven convection, sedimentation from this well-mixed layer produces further cycles of sedimentation-driven convection, which are of successively decreasing intensity and increasing duration. Whether the overlying fluid is uniform or stratified, both theory and experiment show that the particles that are lifted into the convection are smaller on average than those which settle at the base of the lower layer. Thus, when the lifted particles are eventually allowed to settle there is a discontinuity generated in the variation of the size distribution of particles with height in the final sedimented pile. This phenomenon may be an important mechanism for secondary layering in the deposits from turbidity currents and pyroclastic flows.

Dust entrainability and its effect on explosion propagation in elongated structures

AbstractTwo apparatuses have been developed to characterize the entrainability of dust deposits. The minimum aerodynamic wall shear stress required for dust removal (Critical Shear Stress), measured by the first apparatus, is an indicator of the ease with which dust particles can be removed from a layer. The second apparatus measures the settling velocity distribution of dust clouds, which is an important factor controlling the extent of dispersion as well as the time period during which the entrained dust cloud can remain suspended.Testing has shown that the critical shear stress depends not only on the nature of the dust, but on the way the dust deposit is formed. Therefore, a portable version of this apparatus was developed to allow field testing of dust deposits. This methodology has also been used to evaluate the effect of oil treatment sometimes used in the grain handling industry to reduce dust emission.Simulated explosion tests have demonstrated the importance of critical shear stress on dust entrainability. A simple model calculating unsteady dust concentration in elongated structures has been developed. The model shows that flammable dust concentrations induced in a gallery by a primary explosion exist over a finite length and time period. Therefore, the probability of a secondary explosion depends not only on the dust and layer properties, but on the details of the primary explosion and of the enclosure geometry. A simple criterion for transmission of a primary explosion into an adjoining elongated structure is discussed in the paper.

Density Currents and Shear‐Induced Flocculation in Sedimentation Tanks

A numerical model for the computation of turbulent flows in sedimentation tanks is presented. Based on a nonorthogonal grid formulation, it can handle fairly arbitrary two-dimensional geometries. It solves the full two-dimensional elliptic governing flow equations with the \Ik\N–ϵ\N turbulence model. Effects on momentum, as well as turbulent kinetic energy balances, due to sediment-induced density currents are taken into account. Solids with a broad settling velocity distribution are treated by considering different settling velocity classes. A simple flocculation model assuming only turbulent shear-induced flocculation is formulated, and the importance of this cause of flocculation is examined. Predicted velocity and concentration profiles and removal efficiencies are compared with observations in a tertiary settling basin in Sweden. Results suggest that the flow field is significantly changed by seemingly small density differences, that shear-induced flocculation is relatively unimportant, and that the influent settling velocity distribution is critical for accurately predicting removal efficiencies.

Laboratory methods for measurement of soil erodibilities (K-factors) for the universal soil loss equation

This paper reports a comparison of several methods for estimating the K (erodibility) factor for the Universal Soil Loss Equation (USLE) on the basis of laboratory measurements of soil properties. All methods used the nomograph of Wischmeier et al. (J. Soil Water Cons., 1971, 26, 189-93) to calculate K on the basis of laboratory data, but the data inputs ranged from: dispersed particle sizes as originally used in the nomograph; non-dispersed particle size, measured after shaking in water; and equivalent sand size distributions, based on settling velocity distributions of particles (aggregates and sand grains) at the soil surface under rain. A further method tested with the use of aggregated particle sizes resulting from rainfall wetting was a correction of the calculated K based on average density of wet sediment >0.100 mm diameter. Estimated K factors were compared with K factors derived from field measurements of soil loss for five soils. Use of dispersed particle sizes gave poor prediction of field K values for the three clay soils, and size distributions measured after rainfall wetting gave poor predictions of field K values for the four soils that had sediment of low density. Predictions of K from the use of non-dispersed particle sizes, and from the use of particle size at the soil surface under rain combined with a correction for sediment density were good for three soils, reasonable for one, and little different to that from the nomograph for the remaining soil. These latter two methods gave similar results, and were successful in predicting field values of K. As well, both methods appear to be sensitive to effects of cropping management on soil structure and erodibility. Either method could be used, depending on laboratory resources available and possible other uses of the data obtained. Simpler methods for measuring size distributions after rainfall wetting and for estimating sediment density are suggested.

In-situ measurements of particle settling velocity in the deep sea

Two instruments, the remote optical settling tube and the benthic autonomous settling tube, have been deployed on the Nova Scotian continental rise at a depth of 4820 m. The few results obtained suggest that optical and gravimetric methods give comparable results. Assumptions about particle density little affect the settling velocity distribution, but do scale the total magnitude. By using a distribution of declining density with increasing size indicating increasing water content, the total concentration inferred from the light attenuation by using Mie theory is quite close to the total found from gravimetric data. The coarser end of the size distribution changes more with overall change in concentration than the fine end. The settling velocity distribution shows the presence of several modes. The two principal ones can be matched with modal sizes of ∼3.3 μm and ∼60 μm in Coulter Counter data allowing inference of Stokes density contrasts of 1.5 and 0.061 g cm −3, respectively. At concentrations of about 200 mg m −3, more than 50% of the material has a settling velocity less than 2 × 10 −4 cm s −1 (probably < 3 μm in size), but this percentage appears to decrease with an increase in total concentration.

The effects of sediment concentration and tube-diameter on particle settling velocity measured beyond Stokes' range; experiment and theory

ABSTRACT The effect of initial sediment concentration on settling velocity distributions measured by the modified bottom withdrawal tube (MBWT) method is presented for a well-aggregated clay soil (a vertisol). Sediment concentrations in the experiments range from 10 to 100 kg m-3 of suspension. Particle Reynolds numbers in the polydisperse suspensions range from less than 0.1 up to 100. Settling velocities of all size classes in the sediment are reduced by an increase in sediment concentration. The empirical equations of Richardson and Zaki appear useful for predicting ratios of settling velocity at different polydisperse concentrations, but absolute velocities predicted are lower than measured. The discrepancy is attributed to the empirical equations being derived for ratios of particle Reynolds numbers much lower than are present in this natural sediment. Hydrodynamic wake capture occurs during this sedimentation, a process not considered in the empirical equations. Settling velocities of all size classes in the sediment are also reduced by a decrease in MBWT diameter, and an incorrect relation between settling velocity and sediment concentration is measured using the small tube originally recommended for this technique. A minimum tube diameter of 45 mm is recommended to avoid such errors.

Estimates of the seafloor area impacted by sewage sludge dumped at the 106-mile site in the mid-atlantic bight

Approximately 8 × 10 6 wet tons of sewage sludge is dumped annually at the 106-Mile Deep Water Municipal Sewage Sludge Disposal Site, 185 km off the New Jersey coast (Sano, 1989). A simple particle-tracking model is used to estimate the areas of sludge deposition using current meter observations made as part of the Mid-Atlantic Slope and Rise (MASAR) Study (Brown et al., 1987) and Lavelle et al.'s (1988) settling velocity distribution for sewage sludge. Model results indicate that sewage particles settle primarily to the southwest of the dump site due to the mean southwesterly flow throughout the water column. Only the coarser fraction (particles with settling velocities greater than 0·04 cm s −1), about 23% of the sludge discharged, is predicted to settle on the seafloor within 350 km of the site. The model predicts a maximum flux in the southwestern quarter of the site of 60 mg m −2 day −1 which decreases to about 25 and 1 mg m −2 day −1 at distances of approximately 50 and 350 km to the southwest, respectively. Little material is predicted to reach the shelf and the finer particles are assumed to be widely dispersed in the water column. These estimated fluxes compare with a natural particle flux of 100–150 mg m −2 day −1 as measured by sediment traps to the north of the 106-mile site (Biscaye et al., 1988). Field studies to assess the effects of sludge dumping on benthic organisms should initially focus on the region 0–50 km southwestward of the site.

Thorium isotopes, particle cycling models, and inverse calculations of model rate constants

Generalized models of thorium and particle cycling, data from Station P, and an inversion technique are used to obtain rate estimates of important biological and chemical transformations occurring in the water column. We first verify the inversion technique using an idealized data set generated by a finite difference model, and then apply the inversion technique to data from Station P. With the Station P data, predicted rate constants for adsorption and release of thorium between the dissolved and small particle phases are consistent with the results from other workers. The predicted rate constants for the interaction between small and large particles are smaller than previous estimates. The predicted concentration of large rapidly sinking particles is greater than the concentration of suspended non‐sinking particles, whereas the reverse is usually assumed to be the case. The calculated sinking rate for the large particles is 20 m d−1. This sinking rate is an order of magnitude smaller than the large particle sinking rate inferred from sediment trap mass fluxes at two levels in the water column. The reason we predict a high large particle concentration and slow settling velocity has not been uniquely determined. Possible modifications of the current model that could help to reconcile the differences between observations and model predictions include: 1) two classes of rapidly sinking particles or rate constants that change with depth, 2) direct interactions between the large particle and dissolved phases, and 3) incorporation of a continuous distribution of particle size and settling velocity.

A new instrument for measuring settling velocities in situ

An in-situ settling cylinder for determining particle settling velocities has been deployed during a field experiment in San Pablo Bay, California. The instrument is a spring-loaded cylinder which traps approximately 31 of water and suspended sediment between two end plates. Mounted between the end plates is a vertical array of five miniature nephelometers, positioned 4–20 cm below the top end plate, which continuously monitor (5 Hz sampling rate) suspended sediment concentration at each level as particles settle. The resulting settling curve can be divided into components with characteristic settling velocities and a settling velocity distribution obtained for the suspension. The results are conservative estimates of settling velocities for the suspension. In-situ estimates of particle settling velocity are higher than Stokes settling velocities calculated for the disaggregated grain-size distribution of a suspended sediment sample, indicating that the sediment in suspension is in the form of aggregates. The aggregates have settling velocities as high as those of sand size quartz spheres.

In situ measurements of marine particle settling velocity and size distributions using the remote optical settling tube

A remote optical settling tube was deployed in the deep ocean to obtain the first ever in situ distributions of particle settling velocity. These distributions were extrapolated to particle size distributions assuming several possible fixed and variable (with size) particle densities. Cumulative slopes of the particle size distribution (with fixed particle densities of 1.1–2.0 g cm−3) were consistent with earlier measurements made shipboard in the same region. Cumulative slopes obtained using a more realistic variable density (as would be the case for populations including zero‐order and first‐order aggregates) were considerably lower, suggesting that the remote settling tube is detecting aggregated material more accurately than can be done with impedance‐based particle size analyzers. This is supported by the fact that the particulate relative real refractive index was also computed from the specific beam attenuation coefficient and yielded expected results only when the variable density size distribution was applied. These data and analyses demonstrate that the remote optical settling tube can be used effectively (even at low concentrations of suspended material) for in situ determinations of the particle size distribution, especially where relatively fragile aggregates may be present.

Measurement of soil aggregate settling velocities .II. Sensitivity to sample moisture content and implications for studies of structural stability

The importance to settling velocity of initial sample moisture content of non-immersed sediment is presented for a well-aggregated, cracking clay soil (a Vertisol). Eroded sediment was analysed immediately following deposition and after drying to various moisture contents. Settling velocity distributions were measured using the modified bottom withdrawal tube method. Initial aggregate settling velocities are not retained if sediment is allowed to dry to pore water suctions greater than 1.5 MPa prior to analysis, which involves immersion. Coarse aggregate velocities are reduced, a result attributed to slaking, while fine aggregate velocities are increased, a result attributed to binding of dispersed clay during drying with reduced re-dispersion upon immersion. Exposure of air-dry sediment to a saturated atmosphere for varying lengths of time can improve agreement with initial levels of aggregation, but the improvement varies between different samples of this Vertisol. The settling velocity distribution of a sediment is shown to be an indicator of aggregate stability to wetting. Combination of a useful pretreatment of air-dry sediment, which affords improved reproducibility, with the recognized low technique variability given by settling tubes, is demonstrated to be a useful method for comparative studies of structural stability.