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Flocculation

[flok-yuh-leyt]

Flocculation is a process where a solute comes out of solution in the form of floc or flakes. The action differs from precipitation in that the solute coming out of solution does so at a concentration generally below its solubility limit in the liquid.

In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or can be readily filtered from the liquid.

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.

For emulsions, 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 aging of the emulsion (droplet coalescence and the ultimate separation of the phases).

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.

In biology the process is used to refer to the asexual aggregation of microorganisms, most commonly brewing yeast at the end of a brew.

Flocculation and sedimentation are widely employed in the purification of drinking water as well as sewage treatment, stormwater treatment and treatment of other industrial wastewater streams.

Flocculants

Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. Flocculants are used in water treatment processes to improve the sedimentation or filterability of small particles. For example, a flocculant may be used in swimming pool or drinking water filtration to aid removal of microscopic particles which would otherwise cause the water to be turbid (cloudy) and which would be difficult or impossible to remove by filtration alone.

Many flocculants are multivalent cations such as aluminium, iron, calcium or magnesium. These positively charged molecules interact with negatively charged particles and molecules to reduce the barriers to aggregation. In addition, many of these chemicals, under appropriate pH and other conditions such as temperature and salinity, react with water to form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping small particles into the larger floc.

Long-chain polymer flocculants, such as modified polyacrylamides, are manufactured and sold by the flocculant producing business. These can be supplied in dry or liquid form for use in the flocculation process. The most common liquid polyacrylamide is supplied as an emulsion with 10-40% actives and the rest is a carrier fluid, surfactants and latex. Emulsion polymers require activation to invert the emulsion and allow the electrolyte groups to be exposed.

The following chemicals are used as flocculants:

The following natural products are used as flocculants:

Chitosan
Moringa oleifera seeds
Papain
A species of Strychnos (seeds)
Isinglass

Coagulants

According to IUPAC, the terms flocculant and coagulant are often used interchangeably. However, some authors distinguish between the two, in the way that a coagulation implies the formation of compact aggregates,leading to the macroscopic separation of a coagulum, whereas flocculation implies the formation of a loose or open network which may, or may not, separate macroscopically. In water treatment, dissolved substances may be flocculated into microscopic particles, and then these particles may be clumped together for separation and removal via a coagulating agent. In general, coagulants will have higher net charge and a lower molecular weight than flocculants.

Thus, a distinction between the two terms could be made based on their traditional use as follows:

Coagulation: aggregation of colloidal particles in primary energy minimum, leading to an irreversible aggregation.
Flocculation: aggregation of colloidal particles in secondary energy minimum, leading to a reversible aggregation.

Deflocculation

A deflocculant is a chemical that is added to prevent a colloid from coming out of suspension.

Flocs

Particles finer than 0.1 µm (1 E-7 m) in water remain continuously in motion due to electrostatic charge (often negative) which causes them to repel each other. Once their electrostatic charge is neutralized by the use of coagulant chemical, the finer particles starts to collide and agglomerate (combine together) under the influence of Van der Waals's forces. These larger and heavier particles are called flocs.