Median Settling Velocity Research Articles

Direct measurements of settling velocities in the owen tube: A comparison with gravimetric analysis

Over the last two decades the Owen Tube or derivatives of the original design have been used extensively to determine the settling velocities of suspensions of cohesive material in the field. These gravimetric measurements have been used as elements of basic research programmes and also as important input parameters for predictions of siltation rates. During a two year research programme at HR Wallingford undertaken between 1989 and 1991 consideration was given to an alternative technique for analysing the settling of cohesive material. This technique is based upon obtaining high magnification video images of the processes occurring within the settling column. The video images are then analysed using a PC based image analysis system which enables settling velocities and floc dimensions to be determined. The technique has been applied on a number of occasions over the past four years and was included in the European Intercalibration Exercise that took place on the river Elbe in June 1993. Analysis of the processed field data indicates floc break-up, reflocculation and the development of significant circulations within the settling column during the withdrawal period required for the gravimetric analysis (typically a duration of about an hour). The interpretation of the visual data obtained by the video image analysis technique has indicated median settling velocities significantly greater than those obtained from the gravimetric technique. The paper describes the field and analysis methods and results from the recent intercalibration on the Elbe. During this exercise an experiment was undertaken whereby video image analysis was done on the results of filming a settling column whilst the standard gravimetric technique was applied. The paper also discusses the engineering applicability of the results obtained and raises the question as to which technique provides the most representative rate of settlement of flocs in the natural environment?

Settleable sewer solids at stormwater tanks with clarifier for combined sewage

A six-year study of CSO problems showed that in most cases the settleable sewer solids are responsible for poor water quality. This is the reason why the settling behaviour of settleable solids should be taken into account at stormwater treatment to a greater degree than it is done up to now. New efficiency measurements at stormwater tanks with clarifier overflows for combined sewage showed two important results: 1) The annual mean surface loading qA of a clarifier-type CSO tank is only about 25 % of the design surface loading of 10 m3/(m·h) recommended by the German guideline A 128. 2) The settling process in stormwater tanks is effective: - The median settling velocity vs of sewer solids is reduced from about 0,21 cm/s to vs = 0,035 cm/s. - The concentration of settleable solids is reduced about 80 %, that of total solids about 65% and COD concentration about 50 %. With this efficiency, well designed stormwater tanks will be a good measure in many cases to reduce significantly CSO impacts on receiving water quality.

Settleable sewer solids at stormwater tanks with clarifier for combined sewage

A six-year study of CSO problems showed that in most cases the settleable sewer solids are responsible for poor water quality. This is the reason why the settling behaviour of settleable solids should be taken into account at stormwater treatment to a greater degree than it is done up to now. New efficiency measurements at stormwater tanks with clarifier overflows for combined sewage showed two important results:1)The annual mean surface loading qA of a clarifier-type CSO tank is only about 25 % of the design surface loading of 10 m3/m·h) recommended by the German guideline A 128.2)The settling process in stormwater tanks is effective:-The median settling velocity νvs of sewer solids is reduced from about 0,21 cm/s to νs = 0,035 cm/s.-The concentration of settleable solids is reduced about 80 %, that of total solids about 65% and COD concentration about 50 %.With this efficiency, well designed stormwater tanks will be a good measure in many cases to reduce signi ficantly CSO impacts on receiving water quality.

Settling velocities of mud aggregates in the oosterschelde tidal basin (The Netherlands), determined by a submersible video system

A submersible video system was used for direct measurements of the form, size and settling velocity of mud aggregates in the Oosterschelde tidal basin. Continuous in situ measurements were obtained during one tide. They showed that even at relatively low concentrations (10 mg l −1 ) large aggregates are present. The largest aggregates and highest settling velocities were determined during high-water slack (for aggregates ≥ 50 μm: median aggregate diameter = 300 μm, median settling velocity = 1·74 mm s −1 ). The settling velocity of mud aggregates ( W ) depends on their diameter ( d a ) according to W ∼ d a ″. For the Oosterschelde an n -value of 0·92 was found for aggregates >50 μm. This value is within the range of 0·4-1 which is usually determined for mud aggregates. Likewise, the reduction of aggregate density as they grow can be related to the diameter of these aggregates (de) according to Δϱ ∼ d a m . The value of m for the Oosterschelde aggregates >50 gm ranged from − 0·69 to − 1·84.

Characterization of Suspended Solids in Urban Wet Weather Discharges

The objective of this study is to reach a better understanding of the main characteristics of urban stormwater discharge pollution, by providing information on:–paniculate pollution, its importance in relation to dissolved pollution and its abatement through decantation,–the grain size, the specific mass and the settling velocity of these solids. The first part of this paper is given over to the presentation of the sites chosen and of the experimental methods put into operation. In the second part we go on to analyze the results obtained for ten rainfall events over four different sites, according to their geographical position, to the type of network and to the size of the catchment basin. It is quite clear from the results obtained that the solids are the main vector of pollution. These solids are very fine (median diameter = 25 to 44 µn) and generally have a very high settling velocity (median settling velocity = 4 to 11 m/h). Thus it can be stated that decantation represents an efficient method to combat the pollution in urban stormwater discharge and this has been proved experimentally.

Settling speeds of sewage sludge in seawater

Laboratory analyses of sludges from four treatment plants were performed to determine solid content, size distributions, and settling spectra of sludge particulates in seawater. The settling experiments were conducted after partitioning samples by sieving into coarse and fine particulate fractions. For the fine fraction, experiments proceeded after controlled mixing of samples with filtered seawater to particulate concentrations low enough (approx. 10 mg/L) that flocculation was limited during subsequent measurement periods. Results show that 14.6-47.3% of the sludge particulates by weight had diameters greater than 63..mu.., with at least 5% exceeding 250 ..mu..m. Median settling velocities of this fraction ranged from 6 /times/ 10/sup /minus/2/ to 3 /times/ 10/sup /minus/1/ cm/s. The bulk of the particulates were smaller in diameter, and these flocculated in seawater. Resulting from aggregates had median settling velocities by volume ranging from 7 /times/ 10/sup /minus/4/ to 3 /times/ 10/sup /minus/3/ cm/s, with less than 10% of the particles settling more slowly than 10/sup /minus/4/ cm/s.

Flocculated suspended sediment in a micro-tidal environment

Silt and clay particles are unstable when suspended in saline waters. They coagulate into sediment flocs having settling velocities orders of magnitude higher than the individual particles constituting them. In the Danish Wadden Sea in-situ analyses were carried out to obtain the natural textural composition of the suspended sediment. A settling tube, Braystoke SK 110, was used; and the analysis revealed that the suspended sediment at the investigation sites was composed of an unflocculated population coarser than about 4.5 φ and of a flocculated population finer than this grain size. Median settling velocities from 10 −4 to 10 −3 m s −1 were determined within the concentration range 50–1000 mg 1 −1. It is concluded that suspended sediment concentration is a major determinant for the natural textural composition of flocculated sediment. The variation in concentration can account for about 80% of the variance in median settling velocity. Turbulent mixing within the water column is found to be the limiting factor for the growth of sediment flocs.

The Effects of Deposit Feeding Oligochaetes on Particle Size and Settling Velocity of Lake Erie Sediments

ABSTRACT Sediment collected from the western basin of Lake Erie has a median particle size of 1.5 µm and a median settling velocity of 0.0002 cm sec-1 measured by standard settling tube analysis. Microscopic examination of this sediment shows that most of the particles are bound into larger cylindrical aggregates by the feeding activities of tubificid oligochaetes (average pellet size = 280 µm length 70 µm diameter). Standard settling tube analyses destroy oligochaete fecal pellets and maintain particles in an unnatural disaggregated state. Laboratory experiments and field observations indicate that for most of the year the top 0.5-1 cm of western basin sediment is pelletized. Less destructive settling velocity m asurements of surface sediments yield median values of 1 cm sec-1 for tubificid fecal pellets, 0.06 cm sec-1 for the top 1 cm of western basin sediment, and 0.03 cm seca for degraded fecal pellets. No realistic measurement of the physical properties of Lake Erie sediments, of sediment erosion and transport, or of diffusion rates of pore water solutes can be made without understanding the biological processes active in fine grained sediments.

Differentiation of Beach and Dune Sands, Using Settling Velocities of Light and Heavy Minerals

ABSTRACT Marine foreshore and dune sands from New Jersey are distinguished by comparing settling velocities of two or more minerals of differing densities that occur together. Good environmental separation is obtained by plotting delta values (defined as log median settling velocity of the selected mineral minus log median velocity of associated quartz) against log median quartz velocity. A straight line drawn on such a scatter diagram containing delta values for hornblende from 48 dune and marine beach samples properly classified all but four samples. The technique requires neither paired sampling of adjacent environments nor statistical evaluation of groups of samples. The parameter should respond immediately to the depositional processes that are operating, and should therefore reflect the environment immediately responsible for deposition. Heavy minerals from marine beach and dune sands settle in water more slowly than would normally be predicted by equivalence with the associated quartz. This seeming anomaly is attributed to the difficulty of entraining small (heavy mineral) grains--a difficulty that must be compensated by a higher ratio of fluid drag to grain mass.