Flocculation Sedimentation Research Articles

A simple model for turbulence induced flocculation of cohesive sediment

The transport and fate of fine grained cohesive sediments in estuarine and coastal waters is influenced by the settling velocity of the sediment, which in turn is affected by flocculation effects. The flocculation processes depend on the physico-chemical properties of the sediment and the water, and on several physical mechanisms, of which turbulence is a major one. In the present paper the effects of turbulence are further analyzed. The floes are treated as self-similar fractal entities. As a result, the settling velocity is shown not to scale with the diameter D squared, as predicted by Stokes' law, but with Dnf-1 , where nf is the fractal dimension. Based on a collision frequency function and a floe breakup formulation, a simple flocculation model is proposed that can be solved analytically for uniform conditions. The flocculation and floe breakup coefficients were obtained from experiments described in literature. It is shown that the maximal attainable floe size and settling velocity are determined to a large extent, especially at low turbulence levels, by the available residence time, i.e. water depth.

Pilot-plant studies of non-steady-state continuous sedimentation: hydrodynamics and solids distribution profiles

A sedimentation—flocculation pilot plant has been designed and built, capable of operating under continuous and discontinuous conditions, incorporating a system to detect solids concentration profiles throughout the settler using electrical conductivity, and an automatic control device, supported by suitable computer equipment with its own software. The hydrodynamic behaviour of the continuous thickener has been studied through transient-response techniques. Results of experiments carried out with mineral suspensions in continuous operation are presented here.

Impact of a ship passage on bacteria levels and suspended sediment characteristics in the Buffalo River, New York

Ship traffic in freshwater rivers may impact water quality by resuspending contaminated bottom sediment. Water samples were analyzed from two different depths at three sites across a transect of the Buffalo River, NY, before and after passage of a lake-going ship. Samples from all sites were analyzed for fecal coliform (FC), heterotrophic plate count (HPC), and total suspended solids (TSS). The levels of all test parameters increased immediately after the ship passed with the largest increases observed at mid-channel sites. Epifluorescent direct counts of mid-channel samples showed that the bacteria in the water column increased significantly ( p < 0.001) at both 1 and 4 m below the surface and were positively correlated with TSS ( r = 0.84). Particle size distribution in the upper water column was determined by automated image analysis. The median particle size class, by volume, before and after the ship passed was 48.9–56.6 μm and 366.5–391 μm, respectively. A few large flocculated particles accounted for most of the volume of the resuspended sediment. Resuspension of flocculated, bacteria-laden particles affects bacterial transport dynamics in riverine systems. The characteristics of flocculated sediments in suspension therefore should be considered in water quality modeling.

Phytoplankton biomass and productivity in the Amazon River plume: correlation with seasonal river discharge

Phytoplankton biomass and primary productivity were assessed on the continental shelf in the plume of the Amazon River during a series of cruises conducted within periods of minimum, maximum, rising and falling river discharge. Chlorophyll concentrations were greatest (up to 25.5 μg l−1) in a zone located outside the turbid, high nutrient, low salinity riverine waters but shoreward of the clear, high salinity, low nutrient waters. Vertical distributions of chlorophyll further delineated the influences of these environmental regimes, with maximum chlorophyll concentrations occurring in the upper 5 m of water columns characterized by reduced salinities and elevated nutrients at the surface and low nutrient, high salinity water below. Fluorescence was elevated in the transition zone as a result of the phytoplankton standing stocks, and was also elevated in low-salinity waters influenced by the Amazon outflow. The residual fluorescence was coupled to salinity but not to chlorophyll, which suggested that it was related to dissolved organic matter which originated in the Amazon. Primary productivity on the continental shelf was greatest in the transition zone and occasionally exceeded 8 g C m−2 d−1. Productivity in the turbid, nutrient-rich waters and the clear, offshore regions averaged 2.18 and 0.81 g C m−1 d−1, respectively. Phytoplankton photosynthesis in waters influenced by the Amazon River appeared to be limited by low levels of available irradiance inshore, whereas offshore it was nutrient-limited. The narrow zone of high production was supported by the riverine input of nutrients and the dynamics of sediment flocculation. Removal of the inorganic sediment load was necessary to allow for adequate irradiance penetration to support photosynthesis.

Flocculation of sediment from the Tanshui River estuary

Experiments on the flocculation of fine-grained sediments from the Tanshui River estuary, Taiwan, were carried out. The effects of fluid shear, sediment concentration and salinity on floc growth were investigated. The ranges of these parameters were 12.5 to 400 s-1 for the shear, 50 to 600 mg L-1 for the sediments concentration and 0% to 100% sea water for the salinity. The results showed the following. (1) The time required to reach a steady state ranges from 2 to 44 h and median floc sizes range from 100 to 520 μm. (2) The steady-state median floc size decreases with the increase in sediment concentration. (3) The floc size increases as the shear increases. (4) Increasing the proportion of sea water from 0% to 50% decreases the floc size to one-fourth of that of freshwater flocs, and a further increase in salinity does not decrease floc size significantly. (5) The time required to reach a steady state decreases with the decrease in shear, the increase in sediment concentration and the increase in salinity. (6) The larger the steady-state flocs, the longer they take to form. Comparison of these results with those obtained in similar studies of Lake Erie sediments shows that the flocculation properties of Tanshui sediments are substantially different from those of Lake Erie sediments. Tanshui sediments are much more difficult to aggregate, but once flocs are formed they are larger. The reasons why different types of sediment behave differently should be studied further.

Flocculation of suspended sediment in rivers of southeastern Canada

Flocculation has been studied extensively within marine science and sanitary engineering and the controlling processes are generally well known. Less research, however, has focused on flocculation within the freshwater river environment. Consequently the occurrence, importance and controlling mechanisms are poorly understood. We examine the suspended sediment characteristics of six rivers in southeastern Canada in order to determine the nature and significance of flocculation in finegrained suspended sediment transport and the factors that may influence it. While flocs may not always comprise the majority of particles in transport, they always represent more than 90% of the total volume of sediment transported. Independent variables that may influence flocculation, such as temperature, pH, major ions, dissolved and particulate organic carbon, suspended solids concentration and bacteria were measured in conjunction with floc size for one of the rivers. Particulate organic carbon concentration, suspended solids concentration and attached bacteria may be important controlling factors of riverine flocculation.

Comparative Studies on the Primary Treatment of Municipal Wastewater for Ocean Disposal

The objective of this study is to evaluate the effectiveness of primary treatment on municipal wastewater before it is discharged into the ocean. A series of filter papers of specified pore size was applied to measure the removal of organics and solids in wastewater through various primary treatment processes. For plain sedimentation, the removal of suspended solids was always less than 50% while COD and BOD removals were 23-41% and 15-27%, respectively. For chemical coagulation with the addition of poly aluminium chloride (PAC), 70% SS removal was obtained with a PAC dose of 30 mg/l. If polyelectrolyte is added (about 1 mg/l), the dosage of PAC can be reduced to around 10 mg/l to maintain the SS removal efficiency at the same level. For air flocculation treatment by preaeration followed by sedimentation, the most achievable removal of solids from wastewater of more than 80% was obtained at an aeration rate of 0.5-1.0 Nl air/l. In particular it is more effective for smaller solids than for larger solids in wastewater. On organic removal, about 15-40% removal in terms of COD or BOD was obtained by treating wastewater with either plain sedimentation or air flocculation and sedimentation processes. The efficiency of organic removal from wastewater increased to about 60% by utilizing chemical coagulation and sedimentation treatment. Based upon the characteristics of tested municipal wastewater, in order to maintain the SS removal at 65% or above as regulated, the treatment processes of chemical coagulation or air flocculation followed by sedimentation should be employed.

Quantitative Impacts of Lake-Level Stabilization on Material Transfer between Water and Sediment in Newnans Lake, Florida

Spillways at lake outlets reduce water-level fluctuations but may accelerate sedimentation in the lake. In eutrophic Newnans Lake, Florida, a transect of sedimentary profiles, dated with 210Pb and 137Cs by γ-ray spectroscopy, showed threefold increases in accumulation rates of organic matter, total Kjeldahl nitrogen (TKN), and total phosphorus (TP) 1200 m lakeward of a spillway since its construction in 1967. Concentrations of TKN and TP increased 3.5 and 2.4 times, respectively, in sediments deposited since 1967. These increases were progressively less at stations farther from the spillway. Postspillway accumulation of TP was focused toward the dam whereas recent TKN deposition was similar lakewide. Flocculent sediment (&gt; 90% water) accumulated at 1.4 cm/yr. Dams designed to reduce water-level fluctuations may provide short-term benefits for lake access and navigation but in the long-term may accelerate deposition of nutrient-rich detritus, reduce lake volume, cloud the water, alter plant communities, and change lake productivity.

Growth of wild rice ( Z. palustris L.) in fertilized flocculent sediments

Agriform 20-10-5 slow-release and urea fertilizers were used on highly flocculent lake sediments to determine if they would produce commercial yields of wild rice. A small pilot study demonstrated that these fertilizers stimulated wild rice growth, but the results were highly variable because of small plot size and low plant density. Individual yield components generally showed a trend towards increasing production with increasing fertilizer application rate. The rate of maturation of the plant increased with fertilization and may account for some of the variability observed. A model of individual plant response to fertilization was developed using the yield component data of the pilot-scale study. This was incorporated into an econometric model to determine the profit-optimizing fertilizer application rate for whole-lake fertilization. Subsequent fertilization of the experimental lake demonstrated that fertilization of flocculent sediments can result in the production of a commercial crop. Differential effects of fertilization on yield components are discussed and suggestions made for more efficient applications.

Sediment transport over salt‐wedge intrusions: Fraser River estuary, Canada

ABSTRACTA field investigation was conducted in the Fraser River estuary, Canada, to examine flow and suspended sediment concentration above salt‐wedge intrusions. The thickness of the upper layer above the intrusion decreased with distance seaward of the salt‐wedge tip, but salinity and velocity increased. A seaward decline in the suspended sediment concentration of the upper layer is believed to result from interference in the exchange of sediment between the flow and the bed, reduced turbulence, flocculation of fine sediment, and dilution of the sediment‐water mixture.Multiple regression models based on simple physical reasoning and easily obtained data provide relations for the suspended sediment concentration of the upper layer. Scaling the models did not produce notable improvements over the dimensioned results and introduced numerical instability. The models showed that sediment concentration was controlled by riverine processes and that the rate of seaward decline in concentration was exponential. These observations support a descriptive salt‐wedge sedimentary model.

Biogeochemistry of billabong sediments. I. The effect of macrophytes

SUMMARY. We examined the effects of an emergent macrophyte (Eleocharis sphacelata R. Br., Cyperaceae) and a submerged macrophyte (Vallisneria gigantea Graeb., Hydrocharitaceae) on the biogeochemistry of the sediments of a billabong in south‐eastern Australia. Sediments from an E. sphacelata bed had significantly lower concentrations of exchangeable phosphorus than did sediments from a nearby bare area or a V. gigantea bed, but neither macrophyte had a measureable effect on their sediment's exchangeable ammonium content. The redox potential in the upper 10cm of E. sphacelata sediments was about 100 mV higher than that of bare sediments, or of sediments colonized by V. gigantea. There were few consistent differences between vegetated and bare sediments in terms of the activity of extracellular enzymes, such as α‐amylase, protease, β‐d glucosidase, lipase or alkaline phosphatase. Rates of alkaline phosphatase activity (235–306μmol (g dry wt)−1 day−1) were markedly higher than those commonly reported for sediments or soils. Rates of gas release were higher from bare sediments (21–93 ml m−2 h−1) than from E. sphacelata or V. gigantea sediments (17–23 and 21‐24ml m−2 h−1, respectively). Gas bubbles consisted mainly of methane (26–66%) and nitrogen (15–68%). Rates of methane ebullition varied from 5 to 60ml m−2 h−1. In‐vitro methanogenesis was most rapid in samples of the upper flocculent sediment. Methanogenesis was slower in V. gigantea sediments than in bare area or E. sphacelata sediments, but was markedly accelerated by additions of acetate and/or H2/CO2 in all sites. Profiles of total extractable fatty acids and phospholipid fatty acids demonstrated that material derived from higher plants dominated the sediment organic matter in all sites. Bacteria were also a significant component of sediment organic matter, as fatty acids for which bacteria can be assumed the sole source accounted for 18–30% of total fatty acid content. Biomarkers for sulphate‐reducing bacteria (Desulfobacter spp.) were detected, and for type II methanotrophic bacteria.

Ecological relationships of wild rice, Zizania aquatica. 9. Production in organic–flocculent sediments

Season chemical trends and wild rice production were examined in unproductive organic–flocculent sediments from three lakes. Variations occurred in the level of organic matter and nutrient concentrations among these lakes. However, these among-lake variations were not as distinct as the differences that occurred between these lakes and a productive wild rice lake. The three organic–flocculent lakes exhibited similar poor wild rice production, retarded phenological development, and red colouration on wild rice stems and leaves. Although nutrient variations did occur among the organic–flocculent lakes, it was hypothesized that a common limiting factor was affecting wild rice production in this type of sediment. Key words: Zizania aquatica, nutrients, phosphorus, sediment, primary production, establishment.

Ecological relationships of wild rice, Zizania aquatica. 8. Classification of sediments

Cluster and discriminant analysis classified sediments from potential and existing wild rice lakes into three major and three hybrid types. The three major sediment types were described as clay, organic, or flocculent, and the three hybrid types as organic – flocculent, organic – clay, or organic over clay. The major differences among the six sediment types were the percent loss-on-ignition, bulk density, phosphorus, cation, and pH values. Organic sediments were characterized by high phosphorus, cation, and loss-on-ignition values, and lower pH values. Clay sediments had the highest bulk density values, but were low in phosphorus. Flocculent sediments exhibited the lowest bulk density, phosphorus, and cation values, and the highest loss-on-ignition and pH values. The three hybrid sediment types had values for these variables intermediate to their parent types. Flocculent, clay, and organic–flocculent sediment types produced the lowest dry weights of individual wild rice plants. Best production (in terms of dry weight) occurred in organic, organic–clay, and organic over clay sediments that had loss-on-ignition values ranging from 29 to 49%, were slightly acidic (pH values of 5.9 −6.2), and had high phosphorus (&gt; 1.0 g ∙ m−2) and cation values.

Suspended sediment transport across a tidal flat

Many investigations have shown that most of the suspended sediment supplied to the Wadden Sea originates from the North Sea. Based on measurements of current velocity, depths and suspended sediment concentrations in the Danish part of the Wadden Sea, computations indicate, however, that suspended sediment is in fact exported to the North Sea. The reason for this discrepancy is twofold: Firstly, measurements have been carried out mainly in tidal channels where the largest amounts of water and suspended sediment are in the main transported during ebb periods. Secondly, computation of net transport in an estuarine environment such as the Wadden Sea always suffers from the difficulty that net transport within each single tidal period is of the same order as the uncertainty of the transport rates during ebb and flood periods respectively. The first problem can be dealt with if measurements are carried out in both tidal channels and over the tidal flats. The second problem can be solved by collecting time series instead of discrete measurements because accumulation of the transport rates found for successive tidal periods will neutralise the relatively large random error. In this paper, time series of suspended sediment flux across a tidal flat covering a full spring neap tidal cycle are presented. They showed net transport of suspended sediment in a landward direction which is mainly caused by flocculation of fine-grained sediment during periods of high concentration level. During such periods the field settling velocities of suspended sediment are so large that the settling- and scour-lag processes can be effective.

A note on the seasonally shifting zone of high primary production in the Bay of Marajó, Pará, Brazil, 1983-1984

To understand the hydrological and limnological conditions during the annual regime of high and low river flow, as well as the action of the tides, a series of 18 collecting trips were conducted in monthly and bimonthly intervals across the Bay of Marajó during the years 1983-1985. The Tocantins River provides more than 80% of the inflow into this bay and shows a much greater difference in water volume flow between high and low water season than the Amazon. The annual displacement of brackish water influence is thus more extensive Marajó Bay than in the Amazon estuary. During the dry season of low river discharge (September-December), traces of seawater are found to penetrate up to 90 km upriver in the Guamá River. The high degree of turbidity of inner estuarine waters impedes light penetration and results in the near absence of primary production in spite of ample nutrients. Where these turbid river waters mix with brackish estuarine waters of 2 to 4‰ salinity, flocculation and subsequent sedimentation causes visibility to increase from a few to sometimes 200 cm. The water in this zone assumes a bright green color due to phytoplankton. About 90% of the biomass consists of a polyhalobic diatom species, Coscinodiscus. Concomitant great reductions in silica and other nutrient concentrations are noted. During low river flow (September to December), this high production zone is located in the central part of Marajó Bay, whereas it lies outside of the bay over the continental shelf during high river discharge (February to April).

Limnocorral Studies of Chemical and Biological Acid Neutralization in Two Freshwater Lakes

Twenty-nine 1 m diameter limnocorrals were installed in two Canadian Shield lakes to examine the relative importance of individual acid neutralization processes. Once the water column alkalinity was depleted, the within-lake alkalinity production processes were confined to the lake sediments. The flocculent organic-rich sediments of a shallow dystrophic lake (Lake 114) neutralized acid at a significantly greater rate than the sandy littoral sediments of an oligotrophy lake (Lake 302). Since the water column alkalinity of Lake 114 is much lower than Lake 302, ambient alkalinity is not a good predictor of acid neutralization capacity in the sediments. In both lakes, HNO3 was neutralized at a substantially higher rate than HCl or H2SO4. The rates of nitrate consumption, sulfate consumption, and abiotic acid neutralizing processes are independent of each other. Denitrification to an uncharged end-product is the primary mechanism of NO3− removal. The principal source of non-anion-specific buffering in the enclosure experiments was Ca2+ release from the sediments in Lake 302 and Ca2+ and NH4+ release in Lake 114. There is no significant difference between the organic-rich sediments of Lake 114 and the sandy sediments of Lake 302 in the concentration of exchangeable cations (μeq∙cm−3) for all cations except NH4+ and Mn2+. Alkalinity production in epilimnetic sediments is an important component of the resistance of softwater lakes to anthropogenic acidification.

Chlorine Dioxide Effects on THMFP, TOXFP, and the Formation of Inorganic By‐products

An investigation of raw and finished waters was conducted (1) to determine whether chlorine dioxide pretreatment lowers trihalomethane (THM) and total organic halide (TOX) formation, (2) to evaluate the rate of chlorine dioxide consumption and the corresponding rate and extent of chlorite formation, and (3) to investigate the stability of chlorite in treated water and its interaction with chlorine. Results of these experiments showed that chlorine dioxide can reduce THM and TOX precursor concentrations and that when added to raw water, chlorine dioxide will not persist long enough to maintain oxidizing conditions through flocculation–sedimentation basins. Utilities should be able to meet the recommended limit of 1.0 mg/L for the sum of the residual chlorine dioxide species by not exceeding a chlorine dioxide dosage of 1.2—1.4 mg/L.

The consolidation of concentrated suspensions. Part 1.—The theory of sedimentation

The concentration or consolidation of suspensions of fine particles under the influence of a gravitational field has been analysed. The rate and extent of consolidation depends upon a balance of three forces, the gravitational driving force, the viscous drag force associated with flow of liquid in the sediment and a particle or network stress developed as a result of direct particle–particle interactions. In the case of colloidally stable suspensions, this particle stress is the osmotic pressure of the particles; in the case of flocculated or coagulated suspensions, it is the elastic stress developed in the network of particles. A constitutive equation is suggested for irreversibly flocculated suspensions undergoing consolidation which embodies the concept of a concentration-dependent yield stress Py(ϕ). This is then used to analyse the sedimentation behaviour of flocculated sediments and to derive expressions for the initial sedimentation rate. The initial rate of change of sediment height with time in a uniform gravitational or centrifugal field is given approximately by: [graphic ommitted] where B=Δρgϕ0H0/Py(ϕ0), u0 is the sedimentation rate of an isolated particle, ϕ0 is the initial (uniform) volume fraction of solids, r(ϕ0) is a dimensionless hydrodynamic interaction parameter, Δρ is the difference in density between solid and liquid, g is the gravitational or centrifugal acceleration and H0 is the initial sediment height. The theory accounts correctly for the equilibrium consolidation behaviour of strongly flocculated suspensions, and preliminary experimental data suggest that it is not inconsistent with their dynamic behaviour. The estimation of the yield stress Py(ϕ) from a batch centrifuge experiment is also described.

Metabolic activities in flocculent surface sediments and underlying sandy littoral sediments1

Parts of the sandy littoral sediment of a Precambrian shield lake (Lake 302S) were intermittently covered with a layer of flocculent organic‐rich material 1–20 mm thick. Sandy sediments with flocculent surface sediment had higher rates of respiration (31–105%) and photosynthesis (37– 224%) than those without. Densities of invertebrate macrofauna were 10 times higher in the flocculent surface sediment than in the underlying sandy sediment, accounting for 17 vs. 5% of the total respiration. Microprofiles of oxygen concentration showed that almost all of the dissolved oxygen in the overlying water was consumed as it diffused through 5–7 mm of the flocculent surface sediment, so that the underlying sandy sediment was almost completely anaerobic in the presence of flocculent surface material. The presence of flocculent surface sediment also decreased the depth of penetration of sulfate into the sandy sediment since the zone of sulfate reduction was located closer to the sediment‐water interface. In this experimentally acidified lake, neutralization of H2SO4 via sulfate reduction was more rapid (2.04 × 10−1 mmol H+ m−2 d−1) in the presence of surface flocculent sediment than in its absence (1.57 × 10−1 mmol H+ m−2 d−1).

Mercury removal from petrochemical wastes

The precipitation of mercury as mercury-starch-xanthate is studied. The process is studied in terms of optimum mercury-xanthate molar ratio, of influence of flocculation sedimentation times and of interferences (aluminum, iron, silica, chloride, organic substances and suspended solids). The studied reaction is advantageously applied to mercury removal from petrochemical wastes. The optimized process is applied both to a simulated and to a real wastewater, coming from a petrochemical industry (ANIC-Ravenna, Italy).