Flocculation

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"Flocculent" redirects here. For the galaxy type, see Flocculent spiral galaxy.

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IUPAC definition

Flocculation (in polymer science): When a sol is colloidally unstable (I.e., the rate of aggregation is not negligible) then the formation of aggregates is called flocculation or coagulation.[1]

Agglomeration (except in polymer science)
Coagulation (except in polymer science)
Flocculation (except in polymer science)
Process of contact and adhesion whereby dispersed molecules or particles
are held together by weak physical interactions ultimately leading to phase
separation by the formation of precipitates of larger than colloidal size.

Note 1: In contrast to aggregation, agglomeration is a reversible process.

Note 2: The definition proposed here is recommended for distinguishing
agglomeration from aggregation.

Note 3: Quotation from ref.[1][2]

Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flake; either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no formation of a cake, since all the flocs are in the suspension.

Coagulation and flocculation are important processes in water treatment with coagulation to destabilize particles through chemical reaction between coagulant and colloids, and flocculation to transport the destabilized particles that will cause collisions with floc.

Term definition

According to the IUPAC definition, flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters." Flocculation is synonymous with agglomeration and coagulation / coalescence.[3][4]

Basically, coagulation is a process of addition coagulant to destabilize a stabilized charged particle. Meanwhile, flocculation is a mixing technique that promotes agglomeration and makes particles settling down. The most common used coagulant is alum, Al2(SO4)3•14H2O.

The chemical reaction involved:

Al2(SO4)3•14H2O → 2Al(OH)3(s) + 6H+ + 3SO42- + 8H2O

During flocculation, gentle mixing accelerates the rate of particle collision, and the destabilized particles are further aggregated and enmeshed into larger precipitates. Flocculation is affected by several parameters, including mixing speeds, mixing intensity, and mixing time. The product of the mixing intensity and mixing time is used to describe flocculation processes.

Applications

Surface chemistry

In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid (creaming), settle to the bottom of the liquid (sedimentation), or be readily filtered from the liquid. Flocculation behavior of soil colloids is closely related to freshwater quality. High dispersibility of soil colloids not only directly causes turbidity of the surrounding water but it also induces eutrophication due to the adsorption of nutritional substances in rivers and lakes.

Physical chemistry

For emulsions, flocculation describes clustering of individual dispersed droplets together, whereby the individual droplets do not lose their identity.[5] Flocculation is thus the initial step leading to further ageing of the emulsion (droplet coalescence and the ultimate separation of the phases). Flocculation is used in mineral dressing[6]

Civil engineering/earth sciences

In civil engineering, and in the earth sciences, flocculation is a condition in which clays, polymers or other small charged particles become attached and form a fragile structure, a floc. In dispersed clay slurries, flocculation occurs after mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractions between negative face charges and positive edge charges.

Biology

Flocculation is used in biotechnology applications in conjunction with microfiltration to improve the efficiency of biological feeds. The addition of synthetic flocculants to the bioreactor can increase the average particle size making microfiltration more efficient. When flocculants are not added, cakes form and accumulate causing low cell viability. Positively charged flocculants work better than negatively charged ones since the cells are generally negatively charged.[7]

Cheese industry

Flocculation is widely employed to measure the progress of curd formation in the initial stages of cheese making to determine how long the curds must set.[8][9] The reaction involving the rennet micelles are modeled by Smoluchowski kinetics.[8] During the renneting of milk the micelles can approach one another and flocculate, a process that involves hydrolysis of molecules and macropeptides.[10]

Flocculation is also used during cheese wastewater treatment. Three different coagulants are mainly used:[11]

Brewing

In the brewing industry flocculation has a different meaning. It is a very important process in fermentation during the production of beer where cells form macroscopic flocs. These flocs cause the yeast to sediment at the end of the fermentation. Subsequently the yeast can be collected from the bottom (lager fermentation) or the top (ale fermentation) of the fermenter in order to be used for the next fermentation. It is not to be confused with sedimentation in colloidal dispersions as the mechanisms are different even though they appear to be pretty similar.[12]

Water treatment process

Flocculation and sedimentation are widely employed in the purification of drinking water as well as in sewage treatment, storm-water treatment and treatment of industrial wastewater streams. Typical treatment processes consist of grates, coagulation, flocculation, sedimentation, granular filtration and disinfection.[13]

Jar test

The purpose of this test is to select types of coagulant (alum) and also to estimate the optimal dose needed in removing the charged particles that occurred in raw water. Jar test is an experiment to understand the processes of coagulation, flocculation and sedimentation (AWWA, 2011).

Jar test apparatus consists of six batch beakers, and equipped with a paddle mixer for each beaker. In a standard practice, jar test involving of rapid mixing, then follow by slow mixing and later with sedimentation process.

Deflocculation

Deflocculation is the exact opposite of flocculation. Usually in higher pH ranges in addition to low ionic strengths of soil solutions and domination of alkali metal cations the soil colloidal particles can be dispersed.[14] The additive that prevents the colloids from forming flocs is called a deflocculant. According to the Encyclopedic Dictionary of Polymers deflocculation is "a state or condition of a dispersion of a solid in a liquid in which each solid particle remains independent and unassociated with adjacent particles. A deflocculated suspension shows zero or very low yield value".[14]

Deflocculation can be a problem in wastewater treatment plants as it commonly causes sludge settling problems and deterioration of the effluent quality.

See also

References

  1. ↑ 1.0 1.1 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "flocculation".
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Adamson A.W. and Gast A.P. (1997) "Physical Chemistry of Surfaces", John Wiley and Sons.
  6. ↑ Investigation of laws of selective flocculation of coals with synthetic latexes / P. V. Sergeev, V. S. Biletskyy // ICCS’97. 7–12 September 1997, Essen, Germany. V. 1. pp. 503–506.
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ 8.0 8.1 Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ 14.0 14.1 Lua error in package.lua at line 80: module 'strict' not found.

Further reading

  • John Gregory (2006), Particles in water: properties and processes, Taylor & Francis, ISBN 1-58716-085-4
  • John C. Crittenden, R. Rhodes Trussell, David W. Hand, Kerry J. Howe, George Tchobanoglous (2012), MWH's water treatment: principles and design, third edition, John Wiley & Sons, ISBN 978-0-470-40539-0

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