Flocculation, in the field of
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, is a process by which
colloidal particles come out of
suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a
clarifying agent. The action differs from
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
in that, prior to flocculation, colloids are merely suspended, under the form of a stable dispersion (where the internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation) and are not truly dissolved in
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Solutio ...
.
Coagulation
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism ...
and flocculation are important processes in water treatment with coagulation aimed to destabilize and aggregate particles through chemical interactions between the coagulant and colloids, and flocculation to sediment the destabilized particles by causing their aggregation into floc.
Term definition
According to the
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
definition, flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters". Flocculation
is synonymous with
agglomeration
Agglomeration may refer to:
* Urban agglomeration, in standard English
* Megalopolis, in Chinese English, as defined in China's ''Standard for basic terminology of urban planning'' (GB/T 50280—98). Also known as " city cluster".
* Economies of ag ...
and coagulation /
coalescence
Coalescence may refer to:
* Coalescence (chemistry), the process by which two or more separate masses of miscible substances seem to "pull" each other together should they make the slightest contact
* Coalescence (computer science), the merging o ...
.
Basically, coagulation is a process of addition of coagulant to destabilize a stabilized charged particle. Meanwhile, flocculation is a mixing technique that promotes agglomeration and assists in the settling of particles. The most common used coagulant is
alum, Al
2(SO
4)
3·14H
2O.
The chemical reaction involved:
:Al
2(SO
4)
3 · 14 H
2O → 2 Al(OH)
3 + 6 H
+ + 3 + 8 H
2O
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.
Jar test
The process by which the dosage and choice of flocculant are selected is called a jar test. The equipment used for jar testing consists of one or more beakers, each equipped with a paddle mixer. After the addition of flocculants, rapid mixing takes place, followed by slow mixing and later the sedimentation process. Samples can then be taken from the aqueous phase in each beaker.
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
Settle or SETTLE may refer to:
Places
* Settle, Kentucky, United States
* Settle, North Yorkshire, a town in England
** Settle Rural District, a historical administrative district
Music
* Settle (band), an indie rock band from Pennsylvania
* ''S ...
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
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
due to the adsorption of nutritional substances in rivers
and lakes and even boats under the sea.
Physical chemistry
For
emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Alth ...
s, flocculation describes clustering of individual dispersed droplets together, whereby the individual droplets do not lose their identity. 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, but can be also used in the design of physical properties of food and pharmaceutical products.
Medical Diagnostics
In a
medical laboratory, flocculation is the core principle used in various diagnostic tests, for example the
rapid plasma reagin test.
Civil engineering/earth sciences
In
civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewa ...
, 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 structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...
, a floc. In dispersed
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
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
Biotechnology is the integration of natural sciences and engineering sciences in order to achieve the application of organisms, cells, parts thereof and molecular analogues for products and services. The term ''biotechnology'' was first used ...
applications in conjunction with
microfiltration to improve the efficiency of biological feeds. The addition of synthetic flocculants to the
bioreactor
A bioreactor refers to any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substance ...
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.
Cheese industry
Flocculation is widely employed to measure the progress of
curd
Curd is obtained by coagulating milk in a sequential process called curdling. It can be a final dairy product or the first stage in cheesemaking. The coagulation can be caused by adding rennet or any edible acidic substance such as l ...
formation in the initial stages of
cheese making to determine how long the curds must set.
The reaction involving the
rennet micelle
A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated coll ...
s are modeled by
Smoluchowski kinetics.
During the renneting of milk the micelles can approach one another and flocculate, a process that involves
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
of molecules and macropeptides.
Flocculation is also used during cheese
wastewater treatment. Three different coagulants are mainly used:
* FeSO
4 (
Iron(II) sulfate)
* Al
2(SO
4)
3 (
Aluminium sulfate)
* FeCl
3 (
Iron(III) chloride)
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 or rise to the top of a fermentation at the end of the fermentation. Subsequently, the yeast can be collected (cropped) from the top (
ale
Ale is a type of beer brewed using a warm fermentation method, resulting in a sweet, full-bodied and fruity taste. Historically, the term referred to a drink brewed without hops.
As with most beers, ale typically has a bittering agent to bala ...
fermentation) or the bottom (
lager fermentation) of the fermenter in order to be reused for the next fermentation.
Yeast flocculation is primarily determined by the calcium concentration, often in the 50-100ppm range. Calcium salts can be added to cause flocculation, or the process can be reversed by removing calcium by adding
phosphate to form insolubable calcium phosphate, adding excess
sulfate to form insoluble calcium sulfate, or adding
EDTA to
chelate the calcium ions. While it appears similar to sedimentation in colloidal dispersions, the mechanisms are different.
Water treatment process
Flocculation and sedimentation are widely employed in the
purification of
drinking water
Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, ...
as well as in
sewage treatment
Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding en ...
, storm-water treatment and treatment of industrial wastewater streams. Typical treatment processes consist of grates, coagulation, flocculation,
sedimentation, granular filtration and disinfection.
Deflocculation
Deflocculation is the exact opposite of flocculation, also sometimes known as
peptization.
Sodium silicate (Na
2SiO
3) is a typical example. Usually in higher
pH ranges in addition to low
ionic strength of solutions and domination of monovalent metal
cations the
colloidal particles can be dispersed.
The additive that prevents the colloids from forming flocs is called a deflocculant. For deflocculation imparted through electrostatic barriers, the efficacy of a deflocculant can be gauged in terms of
zeta potential. 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 (much like
emulsifier). A deflocculated suspension shows zero or very low yield value".
Deflocculation can be a problem in wastewater treatment plants as it commonly causes
sludge settling problems and deterioration of the
effluent quality.
See also
*
*
*
*
* (stability of colloids)
*
*
*
*
*
*
*
*
*
References
Further reading
* John Gregory (2006), ''Particles in water: properties and processes'', Taylor & Francis,
* 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,
{{Separation processes
Metallurgy
Chemical processes
Separation processes
Sewerage
Water treatment
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