Floc Density Research Articles

Using ADV for cohesive sediment settling velocity measurements

Using an Acoustic Doppler Velocimeter (ADV) to simultaneously measure the turbulent diffusivities and Suspended Sediment Concentrations (SSC) at one point in the water column provides an in-situ approach to measure the settling velocity of cohesive sediments. This approach does not alter the ambient turbulence, SSC, salinity and biological constituents that are the major factors affecting floc density and size, and thus, the settling velocity. The settling velocity measured using this technique is much higher than that obtained from the traditional Owen Tube method, which eliminates the effects of ambient turbulence. Using the settling velocities measured from this new approach and the erosion rates from in-situ measurements in a numerical study of the dynamics of turbidity maximum in the York River, the simulated results were quite reasonable. This is encouraging because it was the first time that a modeling study used in-situ measured settling velocities and erosion rates together and correctly predicted the formation of turbidity maximum. Although this is a promising approach, there are improvements that should be included to establish the procedures of using an ADV for measuring the settling velocity of cohesive sediments.

In situ analysis of flocs

Research Article| February 01 2007 In situ analysis of flocs Rajat K. Chakraborti; Rajat K. Chakraborti 1CH2M HILL, Inc., 325 Hillcrest Drive, Suite 125Thousand Oaks, CA 93063, USA Tel: +1 (805) 371 7822 Fax: +1 (805) 371 7818; E-mail: rajat.chakraborti@ch2m.com Search for other works by this author on: This Site PubMed Google Scholar Kevin H. Gardner; Kevin H. Gardner 2Environmental Research Group, Department of Civil Engineering, University of New Hampshire, 336 Gregg Hall, Durham, NH 03824, USA Search for other works by this author on: This Site PubMed Google Scholar Jagjit Kaur; Jagjit Kaur 3CH2M HILL, Inc., 1000 Willshire Blvd, 21st floorLos Angeles, CA 90017, USA Search for other works by this author on: This Site PubMed Google Scholar Joseph F. Atkinson Joseph F. Atkinson 4Great Lakes Program, Department of Civil, Structural, and Environmental Engineering, State University of New York at Buffalo, 207 Jarvis HallBuffalo, NY 14260, USA Search for other works by this author on: This Site PubMed Google Scholar Journal of Water Supply: Research and Technology-Aqua (2007) 56 (1): 1–11. https://doi.org/10.2166/aqua.2007.063 Article history Received: March 15 2006 Accepted: August 02 2006 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Cite Icon Cite Permissions Search Site Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsThis Journal Search Advanced Search Citation Rajat K. Chakraborti, Kevin H. Gardner, Jagjit Kaur, Joseph F. Atkinson; In situ analysis of flocs. Journal of Water Supply: Research and Technology-Aqua 1 February 2007; 56 (1): 1–11. doi: https://doi.org/10.2166/aqua.2007.063 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex The physical properties of suspended particles and the relationship between particle size and structure were investigated. In situ properties of the aggregates in a coagulation–flocculation process were obtained using a non-intrusive image analysis technique. Derived properties, including density, porosity and the number of primary particles in a floc, were estimated from aggregate structure using a fractal approach, which better represents the distribution of mass in an aggregate, compared with a conventional Euclidean approach which considers uniform mass distribution in an assumed spherical shape. A spherical particle assumption overlooks the highly porous nature of real aggregates and underestimates volume, which subsequently influences coagulation and settling estimates in solid–liquid separation processes. The present results illustrate a strong inverse relationship between the fractal dimension and aggregate length, consistent with the idea that larger aggregates in general are more porous. In addition, correlations between the solids content, floc density and the number of primary particles that constitute a floc of a given size were established. It is suggested that the aggregation process produces flocs of constantly changing morphology and related physical properties. Overall, these findings can provide additional information for understanding and modeling suspended particle characteristics. aggregate structure, density, floc properties, fractal geometry, number of particles, porosity This content is only available as a PDF. © IWA Publishing 2007 You do not currently have access to this content.

Entropy analysis of in situ particle size spectra

Entropy analysis has been used to classify in situ particle (floc) size spectra of suspended particles into groups based on similar distribution characteristics. Results revealed that the in situ spectra sorted into groups that reflected different forcing conditions (e.g. variations in turbulence). The different forcing conditions were not necessarily reflected in other commonly used distribution measures such as median floc diameter. This suggests that entropy analysis may be an effective approach for investigating the effect of changes in forcing conditions on floc size. It is hypothesized that it may be possible to derive the average shape of floc size spectra from measurement of the forcing conditions alone and subsequently derive parameters such as floc fraction, floc density, floc settling velocity and the optical properties of the water column from the average spectra.

Seasonal and spatial variation of floc size, settling velocity, and density on the inner Adriatic Shelf (Italy)

Measurements of floc sizes, floc settling velocities and effective floc densities were obtained at three locations on the inner Adriatic shelf (Italy) in February and May/June 2003 using the in situ size and settling column tripod INSSECT (Mikkelsen et al., 2004. INSSECT—an instrument platform for investigating floc properties close to the seabed. Limnology and Oceanography: Methods 2, 226–236). Measurements were carried out in a water depth of 8–12 m and the final data set comprised 2491 flocs. Relationships of observed floc size vs. floc settling velocity and floc size vs. effective floc density were similar to those observed by other authors working in similar settings. The raw data showed significant scatter around mean trends, indicating that only a relatively small fraction of variability in floc settling velocity and effective floc density was explained by floc size. For bin-averaged data, however, much of the variability in settling velocity and density was explained by floc size. On the Adriatic shelf mean floc settling velocities varied from 0.48 to 1.35 mm s −1, while mean effective floc densities varied from 8.1 to 27.5 kg m −3; within the range reported by other authors. Analysis of variance showed significant differences in floc settling velocities, effective floc densities and floc size in space and time (seasons). Thus, floc settling velocities, effective floc densities and floc size on the inner Adriatic shelf could not be characterized by a common mean, but were more appropriately characterized by a range of values, varying in time and space.

Alkaline‐cell lysis through in‐line static mixer reactor for the production of plasmid DNA for gene therapy

A state-of-the-art in-line static mixer reactor (ISMR) was invented to lyse E. coli cells and neutralize the cell lysate continuously and efficiently for the extraction of plasmid DNA. It comprised two connected static dynamic mixers, each 0.01 m in diameter and 0.9 m in length, one for lysis and one for neutralization. Cells were lysed using concentrated alkaline with 1% SDS and the lysate was neutralized at feed rates of cell suspension:lysis solution:neutralization solution of 125:250:125, 250:500:250, and 500:1,000:500 mL/min. Distances for the mixtures to reach color homogeneity were dependent on feed rates. The higher the feed rates the shorter the mixing distances and times. However, complete cell lysis and neutralization were independent of color homogeneity. Lysate viscosity and neutralized floc size decreased and floc density increased, as distances and feed rates increased. High plasmid yields were obtained from both lysis and neutralization at feed rate ratios of 125:250:125 and 250:500:250 mL/min within mixing distances < or =0.6 m. Poor mixing performance and plasmid yield were obtained at a high feed rate of 500:1,000: 500 mL/min when residence and reaction times were less than 2 s and from mixing distances > or =0.6 m at all feed rates due to a longer exposure to strong alkali and shear flow. This invention showed excellent performance with scaleable potential for the commercial manufacture of plasmid DNA.

Models for effective density and settling velocity of flocs

New models to predict settling velocity and effective density of flocs are proposed. The models are based on the concept of fractal geometry, but with the assumption of variable fractal dimension with the floc size. The best results are obtained when the fractal dimension is estimated by a power law function of the floc diameter. The models are compared with observations from 26 published data sets relating floc size to settling velocity measured under various conditions and at different locations. The floc size covered by the data varies between 1.4 and about 25,500µm. Five commonly used models are also compared to these data and found to reproduce inadequately the full range of the observations. Sensitivity analysis shows that, with the proposed models, the spread in the data may be reproduced by varying the size of primary particles from about 0.05 to 20µm. The new models are proposed for integration into numerical models to simulate sediment transport of cohesive sediments, contaminants, and biological microorganisms such as phytoplankton.

The effect of polymeric flocculants on floc strength and filter performance

Polymeric flocculants are widely used throughout the water industry as flocculant aids, they are known to increase floc density and aid settlement in the clarification stage of the water treatment process. In this research, polymeric flocculants were used to improve floc strength prior to filtration on a dissolved air flotation (DAF) plant in an attempt to prevent filter breakthrough. A modified jar test procedure using a PDA (photometric dispersion analyser) optical flocculation monitor was developed in order to evaluate the system floc strength. Filtration trials were carried out on a pilot filter rig situated on a surface water treatment works in Yorkshire. The filter feed originated from the main plant filter channel. Filter performance was assessed by continuous online monitoring of effluent particle counts, turbidity and headloss over the period of the filter run. Results indicated that low doses of polymeric flocculants had a beneficial effect on filtered water quality, as measured by particle counts, turbidity, UV254 absorption and dissolved organic carbon (DOC). Polymeric flocculants also had the effect of extending filter run length. The modified jar test results indicated that the flocculants used improved the floc strength and enhanced reflocculation of the micro flocs present after the flotation process.

Structural Features of Biomass in a Hybrid MBBR Reactor

The structural features of biomass present in the hybrid MBBR (Moving Bed Biofilm Reactor) aeration tank were studied in two subsequent periods, which differed in hydraulic and substrate loads. The physical characteristics of attached-growth biomass, such as, biofilm thickness, density, porosity, inner and surface fractal dimensions, and those of suspended-growth biomass, such as, floc size distribution, density, porosity, inner and surface fractal dimensions, were investigated in each study period and then compared. The results indicated that biofilm always had a higher density, geometric porosity, and a larger boundary fractal dimension than flocs. Both types of biomass were found to exhibit at least two distinct Sierpinski fractal dimensions, indicating two major different pore space populations. With the increasing wastewater flow, both types of biomass were found to shift their structural properties to larger values, except porosity and surface roughness, which decreased. Floc density and biomass Sierpinski fractals were not affected much by the system loadings.

Single-grain, microfloc and macrofloc volume variations observed with a LISST-100 and a digital floc camera

Fine-grained (< 63 μm) particles in the aquatic environment flocculate into larger, porous entities (flocs). Flocculation models have been proposed wherein the particles in the water exist either as single grains or as part of flocs. This approach, however, contradicts the idea put forward by Eisma [Eisma, D., 1986. Flocculation and de-flocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183–199.] that a flocculated suspension would consist of large porous, fragile flocs, macroflocs (> ∼ 125 μm), made up of smaller, more sturdy flocs (microflocs, < ∼ 125 μm) and single grains. This paper combines results from a LISST-100 laser diffraction particle sizer and a digital floc camera in order to produce full-size spectra covering in situ particle sizes > 2.5 μm, the smallest size resolvable by the LISST-100. For the first time, this allows a detailed investigation of changes in floc volume during break-up and flocculation. In accordance with Eisma [Eisma, D., 1986. Flocculation and de-flocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183–199.], flocs are divided into macroflocs (here > 133 μm) and microflocs (36–133 μm). In addition, a single-grain fraction (<36 μm) is introduced as particles of this size are of importance for the optical properties of the water column. The variation of these three fractions over a range of forcing conditions is examined using a particle volume size range from 2.5 to 9900 μm. When stress increases, the volume occupied by macroflocs decreases while the volume occupied by microflocs increases. It is also demonstrated that when stress decreases, the volume occupied by macroflocs first increases and then decreases as flocs settle out during quiescent conditions. However, in general no overall relationship between stress and floc size was found because parameters other than stress influence floc size, most notably resuspension, settling, advection, and biological activity. On average, macroflocs make up 40–65% of the total suspended volume, whereas single grains and microflocs each make up 15–34% of the total suspended volume. Because an inverse relationship between floc size and density generally exists, this means that occasionally the majority of the suspended mass can be found in single grains and microflocs. This has implications for the optical properties of the water column as scattering is dominated by the smallest particles in suspension. The findings also have implications for existing flocculation models that would probably benefit from inclusion of the microfloc fraction.

Effects of coagulants on the surface forces of colloidal alumina under water treatment conditions

The effect of sulfate and selenate anions on the rate of coagulation of alumina colloids by Al(III) salts has been investigated under conditions typical of potable water treatment. Methods used included conventional light scattering and jar test coagulation experiments, determination of zeta potential and measurements of surface forces using an atomic force microscope. The presence of sulfate or selenate increases the initial rate of Brownian coagulation of the colloids, compared to an Al(NO 3) 3 coagulant by approximately two orders of magnitude. In jar testing, Al(NO 3) 3 coagulant produced fluffy, low density flocs with the greatest floc formation occurring at the highest pH (pH 8), whereas with Al 2(SO 4) 3, floc density was greater and optimum floc formation occurred at an intermediate pH (pH 6). AFM measurements reveal that sulfate (as Na 2SO 4) exhibits surface forces that cannot be modeled by simple DLVO theory, with a long-range (>10 nm) attractive force dominating, with hysteresis on the compression and decompression curves indicative of a bridging mechanism. The same behaviour was observed with the analogue Na 2SeO 4. With Al(NO 3) 3 electrolyte, simple forces that can be modeled using DLVO theory were observed, but the uniformly repulsive forces were independent of pH. The force curves in the presence of Al 2(SO 4) 3 were more complex than those with Na 2SO 4, showing van der Waals attraction dominating at small separation at the PZC and largely repulsive forces at other pH values. The overall results underline the unusual surface effects induced by sulfate (and selenate) with aluminium oxyhydroxide surfaces.

Recovery of Bacillus sphaericus spores by flocculation/sedimentation and flotation

The aim of this work was use flocculation/sedimentation and flotation for recovery of spores of the Bacillus sphaericus. Microorganism was produced batchwise using culture medium based skimmed milk, corn steep liquor and mineral salts. The best results of flocculation were obtained using CaCl2.2H2O, FeCl3.6H2O, Al2(SO4)3 and tannin as flocculating agents, with optimal flocculation concentrations of 1,500, 3,000, 2,000 and 1,700ppm, respectively. Flocculent suspensions were characterized based on floc diameter and density. Settling tests were performed in batch at different concentrations of the cellular suspensions and revealed high recovery of the solids in suspension in all cases. Flotation tests were accomplished using a mechanical agitated flotation cell and the process was favoured by the increase of the system agitation and for the presence of a cationic collector.

A COMPREHENSIVE NUMERICAL APPROACH TO PREDICT OIL-MINERAL AGGREGATE (OMA) FORMATION FOLLOWING OIL SPILLS IN AQUATIC ENVIRONMENTS

ABSTRACT Aggregation between suspended sediment grains and oil droplets, which leads to the formation of agglomerates commonly referred to as Oil-Mineral Aggregates (OMA), is widely acknowledged as a natural process that enhances dispersion of spilled oil in aquatic environments. A comprehensive numerical approach is developed to predict the contribution of OMA formation to the dispersal of spilled oil. The model comprises four modules to calculate maximum size of oil droplets, to predict formation of oil droplets from a slick, to predict formation of sediment floc, and to calculate density of oil-sediment flocs. The inputs of the model are environmental conditions, oil properties and concentration and grain-size distribution of suspended sediments. Sensitivity analysis performed using five crude oils covering a range of viscosities from 8 10−3 to 68 10−3 kg/ms, a kinetic energy dissipation rate from 10−3 to 102 m2/s3, a sediment grain size of 3 μm and a sediment concentration of 250 mg/l showed that formation of OMA is strongly dependent on the oil-water interfacial tension and the kinetic energy dissipation rate. Under breaking wave conditions, the contribution of OMA formation to the dispersal of spilled oil varies between 31 and 97 % depending on characteristics of the individual test oils, in particular oil-water interfacial tension. Results show also that OMA formation is enhanced when the Weber number approaches a value of 0.05.

Protocol to Measure Network Strength of Sludges and Its Implications for Dewatering

The ability to measure is important in sludge conditioning and dewatering applications. Previous research has mostly focused on determining sludge floc indirectly through measuring other parameters such as floc size, floc density, and dewaterability of sludge. Few methods were developed for direct measurement of floc but these methods are not very practical for sludge applications. For the purpose of sludge characterization, it is more appropriate to determine the overall of sludge rather than determining the of individual sludge flocs. This is because individual floc strengths will vary greatly, especially during conditioning, and therefore is a more meaningful measurement compared to floc strength. This paper refers to raw or conditioned sludge as network, and provides a protocol to measure network strength which can be used to evaluate the dewater- ability of water and wastewater sludges. The proposed protocol uses rheology, a fundamental character of sludge, to measure in terms of energy required to break up the structure of a certain volume of sludge. Two methods are presented to directly measure using either torque or concentric cylinder rheometers. A mathematical derivation showed that the area under a shear stress-shear rate or a torque-time rheogram is representative of the energy dissipation within the sludge system, and the total dissipated energy is related to the strength. This study does not intend to measure the absolute strength, rather a comparative of different sludges using the same instrument and measurement conditions.

Structural controls on floc strength and transport

An examination of how floc structure (internal and external) influences floc strength and transport is described. Samples were collected from both a natural river and a combined sewer outfall to provide significantly different structures for comparison. Regardless of floc type, the internal matrix was observed to be mediated by a dynamic microbial community. Microbial secretions of fibrilar extracellular polymeric substances (EPS) were found to be interwoven within the floc forming a framework and providing structural stability. This continually changing framework (due to floc physical, chemical, and biological activity) is shown to have extending influence over floc transport by modifying floc size, stability, density, porosity, and biochemical processes. This paper uses correlative microscopy to assess floc structure and behaviour in terms of the transport of sediments and associated contaminants. A conceptual model is provided which links floc strength to sediment transport (deposition and erosion) in the water column and at the sediment water interface.Key words: flocculation, bacteria, extracellular polymeric substances, stability, transport, structure.

Practical design of flocculator for new polymeric inorganic coagulant - PSI

Physical characteristics of flocs formed by new polymeric inorganic coagulant, polysilicato-iron coagulant (PSI) were investigated using kaolinite clay suspended water as test water. Floc density was determined by measuring floc settling velocity and spherical equivalent floc diameter. The floc density function proposed by Tambo and Watanabe could be applied to PSI. Flocs formed by PSI showed higher floc density than those formed by alum at the same floc size. Floc size distribution after the rapid mixing as also determined. PSI could produce well-grown flocs at the stirring speed of 300 rpm. Stirring at 300 rpm for 7 minutes, residual turbidity after 15 minute sedimentation became lower than rapid mixing at 120 rpm for 5 minutes followed by slow mixing at 40 rpm for 25 minutes. Flocs formed by PSI was much stronger than those formed by alum. Using the data obtained, the design parameters of flocculator by use of PSI were estimated following Tambo and WatanabeÕs procedure. According to the estimation, it was suggested that rapid mixing process alone can make well-grown flocs and slow mixing process, which is essential for the conventional water purification system using aluminium coagulants, can be eliminated by use of PSI.

Effects of symmetrically alternative rotating flow on flocculation

A symmetrically alternative rotating flow pattern was designed for flocculation process in order to produce large and dense flocs. The special effects of a symmetrically alternative rotating flow on the diameter and density of flocs were investigated. The results show that under the new fluid conditions, the primary particles on the outer part of the formed flocs may be cut down and the flocs contract at the end of the original rotating direction; then fluid changes its rotating direction, an opposite shearing is imposed to the flocs and makes some primary particles slide along the floc surface, leading to a denser floc; meanwhile, the broken and unflocculated particles on the trajectory may have opportunities to penetrate into or cohere to the flocs. Compared with the conventional rotating flow, the new-designed flow pattern can not only keep the floc size (even enlarge the floc diameter if a suitable flow is chosen) but also increase the floc density effectively.

The contribution of flocculated material to shrimp ( Litopenaeus vannamei) nutrition in a high-intensity, zero-exchange system

High-intensity, zero-exchange shrimp ponds contain a high density of flocculated particles, rich in bacteria and phytoplankton, compared with flow-through systems. The flocculated particles provide a potential food source for shrimp. Short-term tank experiments were conducted to determine the retention of nitrogen (N) from natural biota, dominated by flocculated particles, in white shrimp ( Litopenaeus vannamei) at a high-intensity, zero-exchange shrimp farm in Central America (Belize Aquaculture (BAL)). There were two treatments: ‘floc’ and ‘floc+20%’ (3×1000-l replicate tanks each) based on two densities of flocculated particles. The floc density in the ‘floc’ treatment was typical of shrimp growout ponds at BAL, whereas the ‘floc+20%’ treatment had a 20% higher density of flocculated particles. Three consecutive experiments were conducted with 1, 5 and 9 g shrimp, respectively. At the start of the experiment, 15N-ammonium was added to the tanks and assimilated by the natural biota. Shrimp were maintained in these tanks for 48 h after the 15N-nitrogen enrichment. After this time, shrimp were found to be enriched with 15N-nitrogen. It was calculated that between 1% and 3% of the particulate nitrogen in the tanks, principally from the flocculated particles, was retained by the shrimp. The proportion of estimated daily nitrogen retention of the shrimp contributed by the natural biota was calculated to be 18% to 29% for 1 to 9 g animals in the floc treatment. There was a tendency for greater retention in the floc+20% treatments, but this trend was not consistent. This study suggests that natural biota, which in this system was largely flocculated particles, can contribute substantially to the nutrition of L. vannamei. There are, therefore, benefits for shrimp in the promotion of flocculated particles in L. vannamei ponds. Whether this translates into improvements in shrimp growth and production efficiency remains to be established.

Separation of flocs in hydrocyclones—significance of floc breakage and floc hydrodynamics

Improved understanding of the behaviour of flocs within a hydrocyclone is necessary to progress the use of hydrocyclones for the clarification and thickening of fine particles. This paper describes experimental and modelling work to investigate the separation behaviour of flocculated particles in a hydrocyclone to better understand the different mechanisms influencing separation. Flocculated pseudo-monodisperse and polydisperse alumina trihydrate in a 1 wt.% water slurry was separated in a 22-mm Mozley hydrocyclone. Floc structure properties, floc size distribution and also primary particle composition within those flocs were measured experimentally for all flows (feed, overflow and underflow). For the case study system at 100-kPa hydrocyclone-operating pressure, there was an improvement in reduced efficiency from 0.75 to 0.84 with flocculation. Contrary to the assumption of literature models, breakage was limited, with the effect of floc hydrodynamics determining the separation behaviour. With density being the most important hydrodynamic effect, the reduced density of flocs compared to primary particles is the main reason for the limited improvement flocculation could achieve in hydrocyclone separation and not, as often suggested, floc breakup. A micro-scale semiempirical model was developed to predict the separation performance of flocculated particles in hydrocyclones. The model represents the relationships between floc hydrodynamics and hydrocyclone classification as well as between floc strength and hydrocyclone shear. Comparison with the experimental results is used to highlight the areas where further work is required to progress understanding.

Dewatering of alumino-humic sludge: impacts of hydroxide

The paper draws together information on factors which influence the conditioning and dewatering behaviour of an alum sludge gained from the coagulation of a low-turbidity coloured water. A principal focus is the potential impact of aluminium hydroxide on the sludge character. Background information is provided on the composition of the source floc for the domain pH 6.0–6.5 and Al>2.0 mg/l. From this, there were many pointers to the presence of Al(OH) 3(s) within the floc. A series of comparisons were made between an alum sludge and a hydroxide suspension at a concentration equivalent to the coagulant fraction within the sludge. The parameters studied included floc size, floc density, polymer adsorption and dewatering behaviour at different time-scales. In all cases, there were strong similarities in the behaviour of the two suspensions—indicating the potential impact of the hydroxide. There was also evidence of common features being displayed by both the organic fractions and the hydroxide. It was suggested that some of the behavioural features might emanate from a common fractal structure within the source floc, the fractal dimension (≈1) being insensitive to composition.

Floc size estimation in iron induced electrocoagulation and coagulation using sedimentation data

The present work deals with the estimation of average floc diameter using sedimentation and optical data. This study was applied to a kaolin suspension destabilized by chemical dosing with iron salts (FeCl 3 or FeSO 4) or electrocoagulation (EC) with iron electrodes. Optimum pH and coagulant concentration were defined by turbidity measures on destabilized suspensions. The suspensions were subsequently settled in a vertical column. Critical heights of sedimentation delimiting hindered settling and consolidation ranges were estimated when plotting data of settling rate of the suspension–supernatant height as a function of time. Average floc diameter and density were assessed considering the Richardson and Zaki model in the hindered settling range. Floc images obtained by optical microscopy allows us to check the relevance of the floc sizing derived from sedimentation data. Eventually, the estimated size and density of flocs generated by EC and chemical dosing techniques were compared.