Particular Continue to Flocculate and Grow in Size as They Settle

Flocculation

Flocculation is the process of transferring coagulated colloids into contact with each other to form larger aggregates (Klimpel and Hogg, 1991; Gregor et al., 1997).

From: Water Research , 2005

Introduction to the Physics of Cohesive Sediment in the Marine Environment

Johan C. Winterwerp , Walther G.M. van Kesteren , in Developments in Sedimentology, 2004

4.4.2 NON-EQUILIBRIUM CONDITIONS

From our analysis of the Lagrangean flocculation model in Section 4.3.5, we obtained the time scales for flocculation (e.g. equ. (4.35)). The flocculation time can vary between a few minutes to many days, and even more, depending on the hydrodynamic conditions and the suspended sediment concentration. As a result, flocs of cohesive sediment, observed in marine environments, are often not in equilibrium with the local hydrodynamic conditions.

Let us first compute the time evolution of floc size, as predicted with the Lagrangean flocculation model (4.31). The results are shown in Fig. 4.9, together with data obtained by Van Leussen (1994) in a settling column (for details, see Winterwerp, 1998). We observe that for two cases, equilibrium conditions are obtained within a few hundred seconds. However, for one situation, the flocculation time exceeds one hour.

Fig. 4.9. Floc growth as a function of time.

(after Winterwerp, 1998)

Fig. 4.10 presents the time evolution of flocs, as measured by Hogg et al. (1985) on a suspension of kaolinite clay, peptised with a polymeric flocculant. The clay concentration was about 80   g/l, and flocculation conditions are extremely favourable through the addition of the flocculant. Yet, the flocculation time was still about one hour.

Fig. 4.10. Floc growth measured by Hogg et al. (1985).

Also McAnally (1999) found flocculation times of many hours, depending on the turbulence intensity and sediment concentration.

These observations have important implications for the flocculation in marine conditions. Flocs settle continuously due to gravity and are remixed by turbulence. Hence, they experience varying hydrodynamic conditions (shear rates) during their journey through the water column. This implies that the ratio between flocculation time and residence time in a specific turbulent environment becomes an important parameter.

This can be quantified by considering a situation where the turbulence field is homogeneous over the water depth, as can be realised in a settling column, for example. The mean residence time T_r for all particles in the water column with initial height Z₀ above the bottom of the water column can be obtained from:

(4.37) ${\displaystyle {\int }_0^{T_r}{w}_s\mathrm{d}t={\alpha }^{″}{\displaystyle {\int }_0^{T_r}{D}_f\mathrm{d}t=Z_0}}$

in which w_s is the settling velocity and α″   = α′D_pgΔ/v(≈ 3   s⁻¹ for D_p   =   4   μm) (e.g. Section 5.2). The residence time T_r can be solved directly from this equation using the simplified flocculation model (4.31) to yield:

(4.38) $T_r=\frac{Z_0}{{\alpha }^{″}{D}_{f,e}}+\frac{D_f-D_0}{k_A\mathit{cG}{D}_f{D}_0}=\frac{Z_0}{w_{s,e}}+T^{\prime }\left[\frac{D_{f,e}}{D_0}-\frac{D_{f,e}}{D_f}\right]$

in which w _s,e is the equilibrium settling velocity. The maximum floc size, as a function of a limited residence time, follows from (4.37) and (4.38) (t  = T_r ):

(4.39) $D_{f,\mathit{max}}=\frac{D_{f,e}}{\left(\frac{D_{f,e}}{D_0}-1\right)exp\left\{-\frac{k_A\mathit{cG}{Z}_0}{{\alpha }^{″}}\right\}+1}$

The maximum floc size predicted with equ. (4.39) is plotted in Fig. 4.11 for the model parameters of Table 4.2, a suspended sediment concentration of 1   g/l, and Z₀   =   1, 2, 5 and 10   m, respectively. It is shown that for small values of G, the flocs cannot attain equilibrium conditions because of the limited residence time. If the concentration would decrease by a factor 10, the shear rate at which equilibrium would be possible, would increase by the same factor.

Fig. 4.11. Effects of limited residence time on the relation between floc size and shear rate.

(after Winterwerp, 1998)

It is noted that this picture is qualitatively similar to the heuristic picture presented in Fig. 4.3 . Moreover, similar results have been observed during the amaseds-campaign, as shown in Fig. 4.12. However, one has to be careful with a quantitative interpretation of Fig. 4.12, as the various data points were obtained at different levels in the water column (so different residence times), and at different suspended sediment concentration.

Fig. 4.12. Relation between floc size and shear rate, measured by Berhane et al. (1997).

We can substantiate these observations further, and investigate when the classical picture of flocculation processes in the water column is correct. This picture was drawn by Van Leussen (1986) and is sketched in Fig. 4.13. It shows larger flocs higher in the water column, where turbulent shear is relatively small, and smaller flocs near the bed, where turbulent shear is high.

Fig. 4.13. Schematic picture of flocculation processes in the water column.

(after Van Leussen, 1986)

This picture can of course only hold when the flocculation time is small in comparison to the mixing and settling time of sediment. As the settling velocity of mud flocs is generally of the order of a few 0.1   mm/s, or smaller, the vertical mixing time is generally much smaller than the settling time. Moreover, the time scale for floc break-up is almost always smaller than the time scale for aggregation, as follows from equ. (4.35). Hence, we can compare aggregation time with settling time only, i.e. the time necessary to form larger flocs with size D_h higher in the water column through the aggregation of smaller flocs with size D_l lower in the water column, which are (assumed to be) in equilibrium with the hydrodynamie conditions near the bed. Also, we assume that the vertical gradient in suspended sediment concentration is small.

The flocculation time T_f for D_h   > D_l reads:

(4.40) $T_f\approx \frac{1}{k_{A}c{G}_u{D}_l}$

where G_u is the mean value of the shear rate in the upper 25   % of the water column. We assume that D_l is the floc size in equilibrium with the shear rate G_l in the lower 25   % of the water column. G_u and G_l are found from averaging equ. (4.4):

(4.41a) $G_u=\frac{1}{0.25}{\displaystyle \underset{0.75}{\overset{1}{\int }}G\mathrm{d}\zeta =0.36\sqrt{\frac{u_{*}^3}{\mathit{\kappa vh}}}}\mathrm{and}$

(4.41b) $G_l=\frac{1}{0.25}{\displaystyle \underset{0}{\overset{0.25}{\int }}G\mathrm{d}\zeta =3.82\sqrt{\frac{u_{*}^3}{\mathit{\kappa vh}}}}$

and the equilibrium floc size in the lower part of the water column D_l follows from:

(4.42) $D_l=\frac{k_{A}c}{k_B\sqrt{G_l}}$

The settling time of large flocs in the upper part of the water column T_s reads:

(4.43) $T_s=\frac{h}{W_s}=\frac{h}{{\alpha }^{″}{D}_u}$

where D_u is the floc size in equilibrium with G_u , and α″is defined in Section 5.2. If the relative flocculation time T_f /T_s   <   1, we expect that aggregation can take place and that flocs higher in the water column are larger than those near the bed. Substitution from equ. (4.40) through (4.43) yields T_f /T_s:

(4.44) $\frac{T_f}{T_s}=\frac{{\alpha }^{″}}{k_A\mathit{ch}}\frac{1}{G_u}\sqrt{\frac{G_l}{G_u}}=0.0012\sqrt{\frac{1}{h{c}^2{u}_{*}^3}}$

where we use the various parameter values obtained for Ems mud, e.g. Section 4.3.5. The relative flocculation time T_f /T_s is depicted in Fig. 4.14 for a variety of suspended sediment concentrations, showing under which conditions vertical gradients in floc size (i.e. D_u ≫ D_l ) are expected to occur.

Fig. 4.14. Relative flocculation time of mud flocs in water column; the diagonal lines represent the condition T_f  =   T_s.

This graph suggests that for estuarine conditions, with shear velocities of a few cm/s, vertical gradients in floc size do only occur for suspended sediment concentrations beyond a few 100   mg/l. Around slack water, suspended sediment concentrations should even be much larger for vertical gradients in floc size to occur (e.g. Winterwerp, 2002). It is noted that these results are obtained for the parameter settings obtained from laboratory experiments with Ems mud, and therefore cannot be considered to be universally valid. However, this analysis clearly indicates that vertical gradients in floc size, as depicted in Fig. 4.13, can only occur if the residence time of the flocs in the upper part of the water column is large enough.

If the flocculation time is large, the mean floc size throughout the water column can be estimated, assuming that the floc size is in equilibrium with near-bed hydrodynamic conditions (i.e. local G and c).

A limited residence time will also have implications on the floc size distribution. Fig. 4.15 presents a picture of the initial floc size distribution, i.e. prior to flocculation, and the floc size distribution for equilibrium conditions, as measured by Kranck (Kranck and Milligan, 1992). It shows a shift towards lager flocs in the equilibrium situation, as expected. However, if equilibrium conditions are not met, more or less any floc size distribution between the two curves of Fig. 4.15 may occur. Of course, this observation also holds for the median floc size. It is clear that this has major implications for the interpretation of floc size data obtained in the marine environment.

Fig. 4.15. Particle size distribution for initial and equilibrium conditions.

(after Kranck and Mïlligan, 1992)

A final remark on the implications of non-equilibrium conditions concerns the structure of the flocs. Fig. 4.16 presents the fractal dimension of flocs found in the Tamar estuary, as derived by Manning (e.g. Winterwerp et al., 2002). It is shown that the fractal dimension of flocs can be clustered in two groups. In group 1, prior to High Water, n_f   =   2 – 2.5, whereas in group 2, after High Water, n_f   =   2.6 – 3.0. It is hypothesised that the group 2 sediments contain many particles that were eroded from the bed, hence have a different structure than the particles of group 1 (e.g. Chapter 7), which have been residing in the water column for a longer time. The particles of group 2 apparently had not sufficient time to become adapted to their new flow environment.

Fig. 4.16. Variation in fractal dimension measured by Manning in Tamar estuary.

(after Winterwerp et al., 2002)

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Surfactant and Polymer-Based Technologies for Water Treatment

Li-Cheng Shen , ... Rajindar Singh , in Emerging Membrane Technology for Sustainable Water Treatment, 2016

10.2.3 Adsorptive Micellar Flocculation

AMF was originally proposed by Porras–Rodriguez and Talens-Alesson [32] as an intensive surfactant-based technology to treat 2,4-dichlorophenoxyacetic acid (2,4-D) by Al³⁺ -induced flocculation of lauryl sulphate micelles. During the AMF process, the organic pollutants bind with the highly positive charge of the Al ³⁺ ion via ionic and complex binding. The Al³⁺ as a flocculant is also bound to the micellar surface and neutralises the surface charge. The charge neutralisation leads to a rapid decrease of electrostatic repulsive force between micelles resulting in a microscopic aggregation of the colloidal micelles (flocculation). The resulting flocs are then separated from solution by a coarse filter or even gravity settling. After that, the retentate/sediment is easily dewatered because of the open and porous structure of flocs. Finally, the organic pollutants can be separated from the surfactant and flocculant via solvent extraction and then recycled back to the process [33]. The major advantages of AMF are fast kinetics and not using an expensive membrane separation. However, it still has some drawbacks such as 'back contamination' of surfactant and flocculant and inefficient removal for pollutants in dilute concentrations, as well as a relatively low substrate recycle ratio [34] and selectivity. A schematic of the AMF process is shown in Figure 10.3.

Figure 10.3. Schematic of adsorptive micellar flocculation.

Modified from Ref.[32]

To overcome the drawbacks, attention has been focussed on improving the selectivity and removal efficiency of AMF. With regard to the selectivity, it was enhanced by adding a monovalent salt such as NaCl over a narrow range of Al³⁺ concentrations [35]. The removal efficiency also improved by applying a multistage separation [33]. To minimise back contamination, marble dust as an adsorbent was used [33]. Apart from modifying the AMF process, finding a suitable application is equally important. AMF was employed to treat soil processing extract, which is a promising demonstration of its application [36].

Various combinations of surfactant and flocculant to treat organic pollutants at optimum operational conditions were investigated to extend the applications of AMF. Two surfactants (SDS and α-olefinsulphonates) [37], their mixtures [38], and two flocculants (Al³⁺ and Fe³⁺) [7,32] were examined to remove organic pollutants such as phenol, benzoic acid, 2,4-D, 2,4-DB (2,4-dichlorophenoxybutiric acid), phenylamine and catechol [39,40] and tetracycline [41]. In addition, the AMF was also robust in the presence of Zn²⁺ [42] and hydrocarbons [43]. For optimum operational conditions, the AMF worked effectively within the pH range of 5–8 and at flocculant and organic compound molar ratios of 6:1 and higher under certain mixing conditions [39,44,45]. Recently, a pilot plant study of AMF was conducted, which demonstrated the feasibility of scaling-up to a continuously operated unit [34]. Although there is little detailed work in the literature regarding the recovery of organic and recycling of surfactant and flocculant, it is important for the economical viability of the AMF process. In conclusion, AMF is a promising but not yet fully developed process to (pre-)treat charged organic pollutants in aqueous solutions.

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Assuring Purity of Drinking Water

C. Johnson , in Comprehensive Water Quality and Purification, 2014

2.4.2.3.2 Flocculation

Flocculation allows for the destabilized particles to agglomerate into larger particles that can be removed by gravity through sedimentation. Flocculation often utilizes a series of basins with different speed flocculation mixers to promote interparticle collisions and the production of a large floc for sedimentation. The mixing intensity of each mixer in series decreases as the water flows through the basin, with the mixer at the end of the basin having the lowest mixing intensity to build a substantial floc that can be settled during sedimentation. The mixing energy must be low enough to allow mixing and particle contact while not providing so much energy that the delicate floc particles shear apart. Detention times for this process typically range from 20 to 30  min (Kawamura, 2000).

Often, mixing can be quantified by mixing intensity (G) and time (t). The product of the mixing intensity and time (Gt) provides a parameter to quantify the different mixing systems in the coagulation and flocculation processes.

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Developments in Palygorskite-Sepiolite Research

Alexander Neaman , Arieh Singer , in Developments in Clay Science, 2011

6 Flocculation of palygorskite-containing clays

Flocculation of clays has received much attention because of its effect on the physical behaviour of soil. FV is the minimum electrolyte concentration necessary to flocculate a given colloid dispersion in a given time ( van Olphen, 1977).

Neaman and Singer (1999) investigated flocculation of standard palygorskites, palygorskite–montmorillonite mixtures and palygorskite-containing soil clays. The flocculation of the clays was determined visually by settling after 24   h of standing in a series of test tubes (Figure 9A ). The FV of Na-saturated palygorskites at near neutral pH was found to be significantly lower than that of Na-saturated montmorillonite from Wyoming. The FVs of four standard palygorskites of different origins were in the range from 0.2 to 2.5   cmol_c/L NaCl, and the FV of Na-montmorillonite was 13.3   cmol_c/L NaCl at near neutral pH values. The low FV of palygorskite is related to its low degree of isomorphic substitution and to its low negative surface charge (Figure 3).

Figure 9. Flocculation test shows the Florida palygorskite suspensions after 24   h of standing and effect of clay concentration on settling of clay particles. Clay concentration: (A) 0.066% and (B) 0.666%. NaCl concentrations: (1) 0, (2) 0.3, (3) 1.7, (4) 3.3, (5) 6.7, (6) 8.3   cmol_c/L.

After Neaman and Singer (1999).

FVs of palygorskite–montmorillonite mixtures at near neutral pH values increased with montmorillonite percentage in the mixture. At a specific concentration of montmorillonite in the mixture (in the range of 40–60%), the FV of the system attains the FV of pure montmorillonite and does not change with further montmorillonite addition. FVs of three palygorskite–smectite–kaolinite-containing soil clays (from which carbonates, iron oxides and organic matter had been removed) at near neutral pH values were the same as FV of standard montmorillonite.

Studies by Goldberg and Glaubig (1987) for kaolinite–montmorillonite mixtures as well as the above-mentioned results of Neaman and Singer (1999) indicate that smectite has a dispersive effect on both kaolinite and palygorskite. Thus, presence of palygorskite in the clay fraction of soils that also contain smectite does not influence the FV of the soil clay.

The industrial experience of less sensitivity to salts of palygorskite drilling muds in comparison to smectite muds (Galán, 1996) suggests that the FV of palygorskite is higher than that of smectite. Above-mentioned data indicate, however, that the FV of palygorskite at near neutral pH is significantly lower than that of montmorillonite. This apparent contradiction can be explained by the difference in the sedimentation rates of palygorskite particles at different clay concentrations.

Only very diluted suspensions (<   0.1%) can be used for FV measurements of palygorskite using flocculation series tests. Settling in the suspension of 0.066% was observed after 24   h of standing, but this did not occur in the 0.666% suspension (Figure 9). Likewise, Dixon and Golden (1990) described the problem of dewatering and sedimentation of palygorskite–smectite suspensions that are waste products in phosphate mining areas in Florida. The sedimentation of these clays proceeds at a very slow pace even in saline environments.

Two parts of the flocculation process can be distinguished: (1) association between the individual particles and formation of flocculi (coagulation) and (2) settling of flocculi formed, under gravity. Although coagulation also occurs in concentrated (>   0.1%) palygorskite suspension with salt addition, the flocculi formed cannot settle down under gravity (Figure 9B) due, most probably, to formation of a three-dimensional network structure or 'scaffolding structure' throughout the mass of the suspension (van Olphen, 1977).

In industrial applications, concentrated palygorskite suspensions are used. Electrolyte addition to such suspensions causes coagulation of the fibres that, in turn, increases viscosity of the suspension. At pH   <   7, the influence of electrolyte addition on the viscosity of palygorskite suspensions is slight (Figure 7), because the initial magnitude of the surface charge is low and electrostatic repulsion is weak (Figure 3).

Thus, low FVs of dilute palygorskite suspensions and low rheological susceptibility of concentrated palygorskite suspensions to salts known from industry are not contradictory. This apparent contradiction can be explained by the difference in the sedimentation rates of palygorskite particles at different clay concentrations.

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FLOCCULATION AND DISPERSION

I. Shainberg , G.J. Levy , in Encyclopedia of Soils in the Environment, 2005

Soil clay systems

Flocculation and dispersion behavior of soil clays differs significantly from that of pure clay systems, possibly because soil clays usually occur as mixtures and because of their association with other minerals and organic matter present in the soil. A few studies demonstrate that the FV values of soil clays is two- to tenfold higher than for pure clay systems ( Figure 8). Conversely, in the presence of hydrous oxides or sparingly soluble minerals such as CaCO₃, clay dispersivity is reported to be less severe. Hence, extrapolation from pure clay systems is of limited practical value.

Figure 8. Flocculation behavior of Na clays suspended in NaCl solution. (Reproduced with permission from Frenkel H, Fey MV, and Levy GJ (1992) Organic and inorganic anion effects on reference and soil clay critical flocculation concentration (CFC). Soil Science Society of America Journal 56: 1762–1765.)

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Interactions of Dissolved Organic Matter and Humic Substances

L.J. Tranvik , E. von Wachenfeldt , in Encyclopedia of Inland Waters, 2009

Sunlight

Flocculation has been observed during exposure of DOM to UV and short wavelength visible light, suggesting a photochemical stimulating effect on flocculation processes. Irradiation leads to cleavage of the DOM into a variety of photoproducts such as dissolved inorganic carbon, carboxylic acids, and a range of other carbonyl compounds, with implications for the bacterial utilization of DOM. Accordingly, photochemically produced carboxylic acids have been demonstrated to account for most of the carbon demand of bacteria in the epilimnion of a humic lake in summer. The background for the effect of light is unclear, but it is possible that the photochemical conditioning of DOM renders it increasingly hydrophobic, thereby increasing the potential for aggregation.

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Oil spill clean-up

P. Senthil Kumar , in Modern Treatment Strategies for Marine Pollution, 2021

4.9.1 Role of flocculation—interaction with inorganic matters

Flocculation of fine particulate matter into larger aggregates increases the settling rate of the fines which is applied to pollutant particulates as to natural sediments. Surface particulate matter has the potential to interact with oil and cause it to sink to the seabeds. Experiments were performed with different sands, such as clay and fresh sediments, and it was found that the presence of salinity interrupts flocculation. The role of salinity also has been studied in detail, showing that the lowest rates of clay–oil flocculation are found in freshwater, higher rates in marine environments and the highest rates at lower to intermediate salinity ranges. There are various mechanisms for flocculation in inorganic matter with oil. They are adsorption of oil onto suspended particle matter, oil attaches to particles as globules and adherence of particles to oil droplets, preventing further coalescence of the oil, and thus stabilizing the suspension at sediment concentrations up to 100  mg/L. Above this the suspension is destabilized and settles down. This process – "armouring" of individual oil droplets by fine clays – has been used to prevent spilled oil from adhering to shorelines and to enhance bioremediation. When sedimented clay–oil flocs reach the depositional area further degradation of oil takes place but at a slower rate. Also there is finding stating that oil–particle aggregates transported offshore have minimum toxicity due to the extent of their dispersion [20].

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Storage, Clarification and Chemical Treatment

Malcolm J. Brandt BSc, FICE, FCIWEM, MIWater , ... Don D. Ratnayaka BSc, DIC, MSc, FIChemE, FCIWEM , in Twort's Water Supply (Seventh Edition), 2017

8.10 Chemical Coagulation and Flocculation

Coagulation and flocculation are essential processes in the treatment of most surface waters; one exception being the application of slow sand filtration ( Section 9.13). The two processes, operating in conjunction with solid–liquid separation processes, remove turbidity, colour, cysts and oocysts, bacteria, biological matter, viruses and many other organic substances of natural and industrial origin. Some are removed directly and some indirectly through attachment or adsorption onto particulate matter.

The terms coagulation and flocculation are two separate processes, contrary to common usage. In coagulation the coagulant containing the aluminium or iron salt is mixed thoroughly with the water and various species of positively charged aluminium (3+) or iron (3+) hydroxide complexes are formed. These positively charged particles adsorb onto negatively charged colloids such as colour, clay, turbidity and other particles through a process of charge neutralization. Flocculation is the process in which the destabilized particles are bound together by hydrogen bonding or Van der Waal's forces to form larger particle flocs during which further particulate removal takes place by entrapment. Flocculation is usually achieved by a continuous but much slower process of gentle mixing of the floc with the water in one of numerous types of plant. In the theory of flocculation the rate at which it takes place is directly proportional to the velocity gradient (Camp, 1943) and equation (8.2) used in Section 8.9 for mixing is also used for determining the velocity gradient G for flocculation. If the residence time in a flocculation chamber is t seconds then the extent of flocculation which takes place, or the number of particle collisions which occur, is a function of the dimensionless expression Gt which is given by the equation:

$Gt=\frac{1}{Q}{\left(\frac{PV}{\mu }\right)}^{1/2}$

where the symbols have the same meaning as in equation (8.2).

For the common coagulants of aluminium and ferric salts the value of G for flocculation is usually in the range 20–100   s⁻¹ with the residence times in flocculation chambers varying from 10 to 40 minutes. However, there are cases where flocculation times approaching 60 minutes have been necessary for waters of extremely high colour and low temperature (Adkins, 1997). The value of Gt would be in the range from 20 000 to 200 000. The values of G and t depend on the raw water quality (e.g. colour, turbidity, algae), water temperature and the required floc size (inversely proportional to G). Typical target floc sizes are 2–5   mm for clarifiers in general; 2.5   mm–150   µm for dual media filters in direct filtration (Hudson, 1981) and 25–50   µm for dissolved air flotation (DAF; Edzwald, 2010). Therefore, each application should be individually evaluated by pilot trials unless adequate information is available for almost identical conditions. In direct filtration where the intention is to form a microfloc, G values of the order of 100   s⁻¹ and a residence time of about 10 minutes are used. For DAF the values used are typically: G about 50–70   s⁻¹ and residence time about 15 minutes for algal laden water and 20 minutes for waters with colour. Amato (2001) has suggested that 10 minutes is adequate for most UK waters. Flocculation for sedimentation including lamella settlers would require G value in the range 30–70   s⁻¹ and residence time between 20 and 40 minutes. For optimum flocculation, the coagulated water should be subjected to a decreasing level of energy with time; the so-called 'tapered energy' flocculation provided in two or three equal size compartments. The G values quoted above are the mean values for two or three stage flocculators. The G value applied in the last stage is about 10–30% of the first stage G value. For example, in DAF the first and last stage G values could be 100 and 25   s⁻¹. In the case of the high rate 'Actiflo' process (Section 8.17), in which microsand is used to ballast the floc, typical G values and residence times are 150–300   s⁻¹ and 4–8 minutes, respectively, usually applied in a single stage.

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Rare earth elements and radionuclides

Juliana A. Galhardi , ... Yuri J.A.B. da Silva , in Emerging Freshwater Pollutants, 2022

17.4.1.2 Coagulation/flocculation

Flocculation is typically applied before sedimentation and rapid sand filtration, in which charges are neutralized by adding coagulants (e.g., iron, titanium, and zirconium salts) following the combination of small particles into larger aggregates (flocs) ( MRWA, 2003). This process is also applied in combination with metal precipitation, which forms low-solubility metal compounds bound to carbonates, sulfides, and hydroxides. Kim et al. (2019) reported coprecipitation of radionuclides with hydrous ferric oxide (HFO) followed by a coagulation-flocculation system using HFO-anionic polyacrylamide (PAM) composite floc, which successfully removed radionuclides from radioactive wastewaters (removal of 99% for Mn-54, Co-60, Sb-125, and Ru-106).

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Handbook of Clay Science

G.D. Yuan , ... W.P. Gates , in Developments in Clay Science, 2013

5.1.6.7 Waste Removal and Disposal

Flocculation enables the pollutant, associated with the adsorbent, to be separated from water. However, since the clay minerals often remain in dispersion, the contaminant may be immobilized but is not removed from the system. In this case, other means of withdrawing the dispersed phase from the liquid phase need to be devised. Oliveira et al. (2003) have proposed the use of magnetic clay mineral–iron oxide composites for this purpose. These adsorbents are re-usable as Taylor and Churchman (1998) have shown for magnetized alumina. Adsorbed contaminants may be degraded by microorganisms, as already discussed. However, if the adsorbed contaminants are not bioavailable, recalcitrant, or highly toxic, their leaching may be prevented through solidification in cements. This approach seems feasible in the case of organoclays (Lo, 1996; Cioffi et al., 2001b). The relatively low cost and wide versatility of bentonite favour its use as an important component in barrier systems for waste disposal (Gates et al., 2009).

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